Journal of the
Proceedings of the American Society of Civil Engineers
GUIDE FOR THE DEVELOPMENT OF FLOOD PLAIN REGULATIONS
Task Force on Flood Plain Regulations
Committee on Flood Control, Hydraulics Division
Southwest i 'Florida. Water iJ/anagemeni 3&itict
This report attempts to provide an understanding of the flood plain prob-
lem in the United States and the technical, social, and legal guides necessary
for implementing an effective program at the Federal, state, and local levels
to regulate land use and development in flood-prone areas.
The limitations of the present (as of 1962) national flood control policy,
which is based primarily on the construction of flood control works, is rec-
ognized by this report, which emphasizes the need for flood plain regulations
as an essential part of a comprehensive plan for alleviating flood damages.
The report also presents the engineering principles and techniques involved
and considers the technical data required for establishing an equitable and
reasonable flood-plain regulations program. A brief discussion of the manner
in which regulations are adopted and implemented is also presented.
This study was made and the report was prepared by the Task Force on
Flood Plain Regulations which was established for that purpose by the Flood
Control Committee of the Hydraulics Division, American Society of Civil
Note.-Discussionopenuntil February 1, 1963. To extend the closing date one month,
a written request must be filed with the Executive Secretary, ASCE. This paper is part
of the copyrighted Journal of the Hydraulics Division, Proceedings of the American So-
ciety of Civil Engineers, Vol. 88, No. HY 5, September, 1962.
74 September, 1962 HY 5
The alleviation of flood damage has been accomplished almost exclusively
by the construction of protective works such as detention reservoirs, channel
improvements, by-pass channels, walls, and levees. Measuresto reduce flood
losses by human adjustment to the flood situation have been neglected, with
few exceptions, until the late 1950's. Because of this single-method approach
to the problem, the number of persons and value of property in the path of
flood waters is increasing at a rate more rapid than that at which protection
is being provided. Thus, in spite of the billions of dollars that have been spent
for the construction of well-designed and efficient flood control works, the
nation is losing ground in the battle against mounting flood losses.
Physical protection of many flood hazard areas is essential and must con-
tinue to be vigorously pursued. Many flood-prone areas, however, are unde-
veloped or have not yet reached the stage of development that would justify
the construction of flood control works. Eveninthose watersheds in which the
construction of protective works is warranted, complete protection of all
flood plains can seldom, if ever, be economically provided. Therefore, a new
look must be taken at the entire flood situation, and all additional tools fully
utilized in a comprehensive attack, not only to hold the line, but to gain and
eventually solve major flood problems. In order to alleviate flood losses, it is
necessary to recognize the flood damage situation and to utilize additional
measures such as flood plain regulations, flood forecasting, temporary evacu-
ation, permanent evacuation, flood proofing, and possibly flood insurance.
The comprehensive approach to flood damage abatement is emphasized in
the flood control literature. It has foundwidespread support among engineers,
planners, and public officials at all levels of government, and also from lay-
men throughout the nation. The Tennessee Valley Authority (TVA) assumed
national leadership by putting this new approach into action in the Tennessee
Valley in 1952. Following a conference in December 1958, the General As-
sembly of the States commended the principle and urged state officials to
take action for implementing such an approach. The Corps of Engineers, under
authorization of Public Law 86-645 dated 1960, compiles and disseminates
information on the flood hazard and provides engineering advice to local in-
terests for their guidance in avoiding flood damage. The Geological Survey,
United States Department of the Interior (USGS), also has authority to pre-
pare special maps for communities, showing areas inundated by selected
floods. The United States Senate's Select Committee on National Water Re-
sources in its report early in 1961 urged the policy of a comprehensive flood
damage alleviation program.
Flood Plain Regulations. -Flood plain regulations as an integral part of an
over-all program for community development are considered the most useful
of the preventive tools for reducing loss of life, property damage, and the ul-
timate cost of flood control. They involve the use of the police powers by a
state or community to guide and control the use and development of flood
hazard areas. Zoning, subdivision regulations, channel and other encroach-
ment statutues, and building codes are included.
Other Measures. -In addition to flood plain regulations, the measures ex-
amined briefly in the following paragraphs should also be considered as es-
sential features of any comprehensive flood damage alleviation program.
_ 11 ~
Flood Forecasting.-Flood forecasting and warnings for the larger streams
can save lives and property located in flood hazard areas. Because of the
highly technical nature of the work involved and the interstate factors that
must be considered, the Federal Government has provided the leadership in
developing and operating the major flood forecasting system.
Temporary Evacuation.-Temporary evacuation of persons and property
from the path of flood waters is another important part in alleviating flood
losses. After the imminence of high water is known, homes can be evacuated,
material raised above flood waters or removed to higher ground, emergency
protective measures can be undertaken, and the flood fighting and relief agen-
Permanent Evacuation.-Permanent evacuation of developed areas subject
to inundation involves the acquisition of lands by purchase, through the exer-
cise of the powers of eminent domain if necessary, the removal of improve-
ments, and the relocation of the population from such areas. Lands acquired
in this manner could be used for agriculture, parks, or other purposes that
would not interfere with flood flows or result in material damage from floods.
Open Spaces.-Great emphasis is being placed on the growing need for
vastly increased areas for recreational and other open-space uses. Develop-
ment rights, easements, or fee title to undeveloped flood-prone areas adja-
cent to rivers might thus be acquired by Federal, state, or local governments
to provide these needed areas at reasonable costs. A number of localities
throughout the country are using this method to remove the temptation for the
development of cheap lands and to put them to a needed useful purpose. Such
projects have been carried out largely at the local governmental level, some-
times with the assistance of benefactors.
Flood Proofing.-Flood proofing measures taken to render existing or
proposed structures, property, and grounds less vulnerable to flood losses
are an important and effective adjunct to flood protection.
Flood Insurance.-Flood insurance, if established on a sound and equitable
basis, could provide still another supplement to many flood damage allevia-
tion programs. In order that insurance of flood losses does not encourage
improper development of flood plains, however, rates should realistically
reflect the flood risk. Such risk would be reduced by regulation of use of the
flood plain. The Congress authorized a subsidized, experimental insurance
program through its Federal Flood Insurance Act of 1956, but has not appro-
priated funds to carry out its provisions.
Urban Redevelopment (Renewal).-This can be used inflood-blighted areas
that are a drain on the economic life and welfare of the community and that
do not lend themselves to other methods of regulation and control. The Federal
Urban Renewal Program, enacted by Congress in 1929 and considerably
amended and enlarged in scope since that time, provides substantial assistance
to municipalities burdened with such conditions.
Such a redevelopment program should include setting aside the lower flood
plain areas for parks, open spaces, and such other uses as are not subject to
substantial flood damages. Public parking areas may be designated, provided
adequate regulations or precautions are established. The upper areas can be
utilized for new structures so as to minimize flood damage.
Warning Signs.-The conspicuous display of signs indicating the specific
heights of flood waters and the dates of the events is useful in informing the
76 September, 1962 HY 5
inexperienced developer and the prospective purchaser of flood conditions in
the area. Signs bearing general flood information are also effective.
Tax Adjustments.-Tax adjustments for land dedicated to agriculture, rec-
reaction, conservation, or other open-space uses may be effective in pre-
serving existing floodways along streams. Unless such concessions are made,
rural farm land adjacent to communities will become more valuable each
year as residential or commercial developments move into parts of it, causing
the tax evaluation of all adjacent farm land to rise to the point that the land
no longer can profitably be used for farming or open uses.
Development Policies.-Resistance to the extending of utilities and to the
construction of local streets will deter development in flood plains, as will
many other wise day-to-day policy and action decisions. Street improvements
elsewhere, schools, and other public facilities wield a "soft-sell," negative
influence on flood plain exploitation and positive leadership toward the higher
Scope of This Report. -Flood plain regulations have been studied in order
to develop engineering guide criteria. The report is primarily for the guidance
and use of those engineers who are not familiar with the field of planning nor
with zoning and other types of regulations effected under the authority of the
police power. It outlines data required by planners or officials in considering
the most reasonable solution to flood problems. The relationships and re-
sponsibilities of the various levels of government are stated and the processes
involved in the preparation and adoption of regulations are considered.
Another purpose of this report is to assist planners and officials in a
better understanding of the sound technical basis that is indispensable in the
preparation of reasonable and acceptable regulations. It will also provide
laymen and officials with a brief explanation of the processes involved.
ENGINEERING DATA TO BE DEVELOPED
This section deals with the data that must be developed by the engineer to
permit an intelligent appraisal of the floodhazard. Such information is needed
by the property owner so that he can plan the best use of his lands consistent
with the flood risk, by the engineer to permit him to reach a sound decision
in regard to the economic feasibility of providing protection against floods,
and by planning agencies and public officials to guide them in determining the
need for placing restrictions, in the public interest, on the use of flood plain
The information that must be assembled is considered in detail in subse-
quent paragraphs. Not all of the data mentioned need be collected or derived
in all situations. The information to be obtained in each case will depend on
the local problem. In the interest of economy, it should be limited to that nec-
essary to permit sound supportable decisions, not only by the engineer but by
others who might use the data. For the same reason, field work and office
computations should not be carried beyond the degree of accuracy warranted
by the basic data.
Report Format. -The data and the results of studies should be assembled
in readable report form. Particular attention should be paid to the format and
language of the report because many of those using it will not be familiar
with engineering terms. Technical terms that are unfamiliar to the layman
should be defined. The report should follow logical presentation and full use
should be made of tables, charts, and maps for the sake of clarity. The visual
aids should be simple and uncluttered. Reference to such visual aids and to
pertinent conclusions that can be drawn from them should be made at related
places in the body of the report. In most cases, the text of the report can be
limited to a presentation of the problem, a statement as to the purpose of the
report, a discussion of the basic data used and the method of analysis fol-
lowed, an indication of the degree of accuracy obtained, and the findings of
the study. The detailed computations and other supporting technical data
should be retained in the files of the engineer for ready reference. If it is
felt that such supporting data will eventually be sought by users of the report,
it may be well to assemble it in reproducible appendix form as a supplement
to the main report.
Definitions.--A general understanding of certain technical terms is needed
by the reader of reports pertaining to floodplain utilization and related prob-
lems. Some of these have been selected for clarification because of their im-
portance in connection with discussions of flood problems and solutions. Def-
initions for these terms are given in Appendix I.
Sources of Flood Data.--Information on past floods and other engineering
data related to the flood problem in specific areas can usually be obtained
from many sources.
The principal Federal sources are the various offices of the Corps of
Engineers, the USGS, the Weather Bureau, United States Department of Com-
merce (USWB); the Soil Conservation Service, United States Department of
Agriculture (SCS); the Bureau of Reclamation, United States Department of
the Interior (USBR), in the western states; and the TVA in the Tennessee
River Basin. In Sec. 206 of Public Law 86-645, Congress directs the Corps
of Engineers to furnish the states and responsible local political subdivisions,
on request, flood hazard information and advice to assist in land-use planning
activities. That program was launched by the Corps of Engineers in 1961 and
much basic data (27)1 are available. The TVA has been conducting a similar
regional program since 1952 and makes available basic data in the Tennessee
River Basin. The USGS, through a cooperative program, is compiling data
and preparing overflow maps pertaining to selected floods in specific areas.
Other sources of information include the following:
State agencies Watershed associations
County engineers Power companies
City engineers Industries
Private engineers Newspaper records
Flood-control districts Technical magazines
Drainage or reclamation districts Private publications and diaries
Irrigation districts Local residents
There is no substitute for athoroughsearchof all records and discussions
with those familiar with the specific problem. Many times it will be found
that one source of information leads to another. County or city engineers can
frequently refer the investigator to "old timers" who observed a flood and can
1 Numerals in parenthesis refer to corresponding item listed in Appendix III.
recall the crest stage reached by reference to a landmark or to some known
flood stage. Such references must be checked by hydraulic computations or
correlation with other known data. The engineer, in certain cases, must
"select and eliminate" when the information obtained is conflicting. Such
decisions must not be made haphazardly or arbitrarily. The reliability of the
source must be considered and the manner in which the data fit into other re-
lated information must be taken into account.
Floods to be Considered. -The impact of possible future floods on man-
made activities and improvements is always measured by the public through
comparison with the impact of past floods. The flood history of the stream
under consideration must be analyzed in appraising the flood hazard. In addi-
tion, the flood potential, which may not be reflected in the record, must not
only be analyzed but emphasized in the study.
A brief description should be given of the subject basin and its flood char-
acteristics. The latter can best be accomplishedby describing specific floods,
preferably recent major floods. Aerial or ground photographs of past floods
and the damages they caused are especially useful. Fig. 1 is a good example
of effective photography.
The floods usually considered in establishing regulations for flood plain
uses will be examined.
Floods of the Past.-Historical as well as recorded floods should be con-
sidered in appraising the flood hazard. Even though the available data for
such floods may not be sufficient for analysis, they frequently assist in evalu-
ating the reasonableness of derived theoretical floods that exceed the maxi-
mum recorded flow. It is preferable to use maximum peak flows in place of
the maximum 24-hr discharge and, in cases of floods with sharp peaks, it is
essential. Fortunately, in most regions, sufficient data have been collected
on peak flows to make their use possible either from continuous records or
by correlation with maximum 24-hr discharges over gaps in the recorded
data. In certain regions of the United States, both rain floods and snow-melt
floods occur in the same drainage basin. Sometimes they occur in combina-
tion, but usually they can be expected in different seasons of the year. Be-
cause their characteristics differ, they must be analyzed and discussed as
separate groups of phenomena.
In addition to a listing of all known past floods, the flood history at se-
lected points should be shown graphically in the report. One effective method
of plotting is shown in Fig. 2. The occurrence of each flood is shown by both
month and year, flood crests are related to the flood stage, and the datum
used is indicated. Because of the use tobe made of the data, plotting of stages
is preferable to plotting of discharges, although one can be readily converted
to the other by means of the stage-discharge curve.
The engineer must adjust the recorded or derived data to reflect appre-
ciable changes that have taken place since occurrence of the flood event.
Such changes, which could markedly affect the shape and crest of the flood
hydrograph, may have resulted from the construction of upstream storage
reservoirs, natural or man-made channel modifications, the construction of
levees along the stream, or encroachments on the channel or flood plain. In
the same manner, the effect of proposed or authorized, but not yet constructed,
flood control works must be taken into account if their construction is immi-
nent. The effect of future developments and changes of land use is particu-
larly important on smaller watersheds. In describing the characteristics of
FIG. 1.-EFFECTS OF A 1936 FLOOD ON A NEW ENGLAND CITY
A| | | |I | I I 7
F I I I I
M I 2 I 5
o ZZI I I I I I I I I 2I ZI I 4 I o
DI _____ IIII IIiII L I 2 z
N I 0
301--i-i-- -- --[-i-i-i-----i---i----- --------2
Staffaff Periods of gage record
25 Data from newspaper files and historical documents Staff recorder 1650
20 i- 1645
10 Flood stage 1635
0 I I1625
1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960
FIG. 2.-GRAPHICAL PRESENTATION OF FLOOD HISTORY OF FRENCH BROAD RIVER
AT MARSHALL, N. C.
a flood, the engineer must explain whether they are based on recorded or
historical data or whether adjustments have been made to reflect changes in
conditions that have occurred since the event. Where appropriate, informa-
tion for both conditions should be included for one or more of the larger
The maximum known flood may have been a rare event because of its great
magnitude or it may have been only slightly greater than floods that occur
fairly frequently. The category into which it falls is determined by hydrologic
studies. Formerly, such determination was usually made by single stream or
regional frequency studies or, where data were lacking, by use of empirical
formulas. Such methods are still useful. However, since approximately 1935,
rapid advances in the field of meteorology and hydrology have made possible
the isolation and the separate analysis of the major independent but interre-
lated variables that cause floods. Transposing and combining such variables
in a logical manner permits the engineer to derive a Maximum Probable
Flood and a Standard Project Flood against which the maximum known flood
can be measured. It should be noted, however, that the science of developing
theoretical floods is not yet (as of 1962) exact and that judgment still plays
a part in their derivation.
Maximum Probable Flood.-The Maximum Probable Flood is the largest
that can reasonably be expected on the basis of existing knowledge. For that
reason it was, until about 1962, termed Maximum Possible Flood by most
agencies and in most literature. Although it would be catastrophic in nature
if it occurred in a populated and developed river valley, its size and rarity
are such that protection against it by engineering works can seldom, if ever,
be economically provided at the present time. Similarly, and again for eco-
nomic reasons, it has little bearing on the uses to which flood-plain lands are
put. It is principally used to determine spillway capacities on major dams
where failure during such an event would result in a major damage to the
structure. Accordingly, the derivation of the Maximum Probable Flood is
seldom necessary for the purpose of flood plain regulations.
Standard Project Flood.-The Standard Project Flood is the largest that
can be expected to develop with the coincidence of the most critical conditions
that have been experienced in a wide area. It has been experienced on many
streams and has been approached on others. It should be considered to be
well within the realm of probability. The Corps of Engineers, the Federal
agency generally responsible for flood control works on the major streams
in the country, and TVA determine the magnitude of the flood analytically.
The Corps utilizes it as the designfloodin highly developed areas where pro-
tection against it can be provided economically. TVA calls it "Maximum Prob-
able" and uses it in the design of the physical features of reservoirs, dams,
powerhouse, and local flood protection works, the failure of which might be
Regional Flood.-The Regional Flood, as used in the Tennessee River Ba-
sin, is comparable to the largest known floods on similar streams in the im-
mediate region. In developing this flood for any stream under consideration,
TVA plots peak discharges against drainage areas for similar streams in the
region, as shown on Fig. 3. Fig. 4 shows photographs used by the TVA in one
of its reports, illustrating the heights of both the Regional and Standard Proj-
ect Floods in relation to existing structures.
7 CC= =---.-- = -- ------
500 -- --- --- --- --- ----- --/ -- -- -- -- -
I -- = -- -- -- -- -- -- -- -
I I -- 6 I _7
A, 9 --8 / Little Pigeon River at Sevierville
300 1 10
__ __1 12~/_ ___
S 4 Little Pigeon River above West Fork
200 I I I
Sg16 17 T -West Fork Little Pigeon River at Sevierville
02 18 19
100 22 5 2 Numbers on chart refer to those on key map
-- o Estimated standard project flood
30 29 (b) Estimated regional flood
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Discharge, in thousands of cubic feet per second
FIG. 3.-DETERMINATION OF REGIONAL FLOODS ABOUT SEVIERVILLE, TENN.
FIG. 4.-ILLUSTRATION OF EFFECTS OF REGIONAL AND STANDARD PROJECT
Design Flood.-The term "design flood" as normallyusedis related to ex-
isting or proposed flood controlworks. It canbe described as the flood against
which protection is or will be provided. It is usually selected on the basis of
economic consideration; that is, against how large a flood is it economical or
most economical to protect. The answer varies with local conditions. High
construction costs and limited improvements in an agricultural area may re-
sult in selection of a design flood having a relatively frequent occurrence.
However, highly concentrated residential, commercial, or industrial develop-
ments in a community may warrant the selection of the Standard Project Flood
as the design flood.
Data to be Developed. -The information to be collected and developed falls
generally into two categories. First, topographic and related data giving in-
formation on physical conditions in the flood plain; and second, hydrologic
data describing the overflow hazard and the effects of floods on typical activi-
ties and properties. As a minimum, the report should incorporate a typical
flood hydrograph, typical stream and valley cross sections, water surface
profiles and overflow maps for selected floods, a tabulation and chart giving
information on known floods, and typical flood photographs.
Areas subject to inundation from various size floods must be determined
and outlined on maps. A search of the records of various Federal, state, and
other public and private sources will, in many cases, yield information on
such areas. If the study covers a region for which no recorded overflow data
exist, information on past floods can often be obtained by personal interview
with long-time residents in the area together with field plotting, from news-
paper accounts, and from other leads such contacts may develop. Such data
can be used along with topography and cross sections as a basis for inter-
polation or extrapolation for other floods of varying magnitudes. Extrapola-
tion is necessary for floods larger than those experienced, but it must be
performed with caution and with careful attention to the natural topography
and the effect of man-made changes on hydraulic flow conditions. This is
especially true in an area of rapid economic development in which major
changes may have taken place since preparation of the base map that is being
used or since the overflow data being used were recorded. If major changes
have taken place, the overflow and other data must be updated by field work
and hydraulic computations.
Stream bed and valley cross sections, bank profiles and other related
physical data can sometimes be developed from topographic maps of the USGS,
Army Map Service, or other sources with only a minimum of field work. In
other cases, extensive field work will be necessary. A tabulation should be
included showing pertinent physical features of bridges and other obstruc-
tions that influence flood heights. Such data can be obtained from the owners,
by field measurements, or by a combination of such procedures. Typical
bridge data are shown in Table 1.
A sufficient number of "control points" must be selected over the reach of
the stream under consideration to define fully its hydraulic characteristics
and permit the computation of profiles for large floods. Such control points
are usually selected at established stream gaging stations and at intermediate
points, such as bridge or topographic constrictions, where stable conditions
exist and where there is a break in the water surface profile. A typical ex-
ample of such an intermediate control point might be a solid highway fill with
a narrow waterway opening that creates upstream pondage during major
TABLE 1.-BRIDGES ACROSS MIDDLE FORK HOLSTON RIVER IN VICINITY OF MARION, VA.
Mile Stream Floor 1957 Above Below
aMie Identific n bed elevatflood Elevation, 1957 1957
above Identification elevation, crest, in feet flood, flood,
mouth in feet in feet in feet in feet
38.89 Virginia Highway 659 2049.1 2068.0 2059.5 2066.8 7.3
39.68 Private Road 2059.3 2069.9 2070.3 2068.4 1.9
40.24 Footbridge 2069.8 2076.8 2079.2 2076.2 3.0
40.36 Footbridge 2070.3 2079.8 2081.2 2079.6 1.6
40.48 Country Club Road 2075.3 2083.0 2083.2 2078.6 4.6
41.13 Sewer Line 2083.2 2109.2 2093.2 2105.2 12.0
41.87 N & W Railway 2099.2 2128.4 2110.7 2117.6 6.9
41.98 Sewer Line 2100.8 2113.5 2112.3 2109.5 2.8
42.02 North Church Street 2101.3 2123.1 2113.3 2120.1 6.8
42.19 Sewer Line & Footbridge 2107.0 2123.0 2117.9 2118.7 0.8
42.52 East Chilhowie Street 2114.6 2128.0 2124.7 2126.6 1.9
42.70 U. S. Highway 11 2118.3 2132.7 2127.8 2128.8 1.0
43.69 Footbridge 2140.2 2145.8 2145.7 2145.7 0 0
44.12 Footbridge 2144.9 2153.7 2153.0 2153.5 0.5
44.42 Private Road (low water br.) 2152.4 2155.0 2160.2 2154.2 6.0
44.62 Virginia Highway 691 2153.4 2169.8 2165.2 2168.2 3.0
44.90 N & W Railway 2161.6 2183.6 2172.2 2177.9 5.7
45.08 U. S. Highway 11 2167.2 2186.7 2177.6 2183.1 5.5
1: 1 I 1 1
floods, thus significantly changing the shape of the flood hydrograph and flood
elevations both immediately above and below it. Inorder to reflect accurately
the flood hazard in the vicinity of such a bridge, hydraulic data must be de-
veloped both immediately above and below it.
The various types of data and information frequently developed and pre-
sented in flood reports will be examined.
Flood Hydrographs.-A flood hydrograph should be included for the maxi-
mum known flood or some other large flood for which data are available. A
hydrograph for the Standard Project Flood, if developed, may be used effec-
tively. If both rain and snow-melt types of floods can cause overflow, hydro-
June 1928 July 1928
27 28 29 30 1 2 3 4
n | I 15
896 -1 -- h --
6 -1957 flood -
892 ------- -10
Flood stage 10
27 28 29 30 31 1 2 3
January 1957 February 1957
FIG. 5.-STAGE HYDROGRAPH, FLOODS OF JUNE 1928 AND
JAN.-FEB. 1957 ON LITTLE PIGEON RIVER
graphs for both types may be needed to illustrate the different characteristics.
Fig. 5 shows typical flood hydrographs. The hydrograph shows the relation-
ship between water surface elevation and time, the rate of rise and fall of the
water surface, the duration of flow above the flood stage, and the possible
hazard from double peaks. Also, both the bankful stage and the datum used
are shown on the chart. The shape of the hydrograph and its meaning should
be examined in the report. Wherever the shape varies appreciably at different
sites, the effect of such differences should also be discussed.
Flood Frequency.-The theory of flood frequency and the techniques of de-
veloping curves are extensively and adequately covered in existing engineer-
ing literature and will not be repeated here. Frequency curves can be devel-
oped either from the records of a single stream or on a regional basis, using
the records of adjacent streams with similar meteorologic and hydrologic
characteristics. The frequencies of various floods for a single stream analy-
sis can be plotted in absolute values, showing the expected average recurrence
interval for various peak flows in cubic feet per second, or stage, or in rela-
tive values, showing such recurrence intervals for various peak flows con-
verted to cubic feet per second per square mile or ratios of the mean annual
flood peak. In the regional analysis, the latter method of procedure is neces-
sary because it permits combining the records of streams in a homogeneous
region to produce a better basic relationshipbetweenfrequency of occurrence
and flood magnitude. A frequency curve plotted both in absolute and relative
terms is shown in Fig. 6.
When using frequency curves, it is important to note that their accuracy
is dependent not only on the techniques of their preparations but also on the
length of record and the portion of the weather cycle covered by such records.
Accordingly, it must be recognized that relationships on a frequency curve
are relative rather than absolute and that they may be subject to change as
the length of record increases. For these reasons, it is necessary to include
a full, though brief, explanation of the limitations of any frequency data shown.
Stage-Discharge Curve.-Stage-discharge curves are developed by actual
measurements of flow and stage over the range of recorded flows at estab-
lished stream gaging. stations and are available on request from the USGS.
For flows beyond those measured, the curves must be extended. Techniques
for preparing and extending such curves are adequately covered in engineer-
ing literature. A stage-discharge curve is shown in Fig. 7. The curve is usu-
ally plotted on rectangular coordinate paper in order to show best the rela-
tionship of the overflow depth to the size flood. The datum of the curve should
always be given and the flood stage should always be shown.
Although such curves usually are not included in the flood report, they, of
necessity, must be obtained for the purpose of the hydraulic studies.
Profiles.-Water-surface profiles for selected floods over the full reach
of the stream under consideration should be included in the flood report. The
floods should include those for which overflow areas are outlined on the maps,
but care should be taken to limit the number of flood profiles shown to only
those essential for a clear presentation of the problem. The profiles usually
selected for plotting should include those of the maximum known flood, the
Regional Flood, and the Standard Project Flood. If data on the maximum known
flood are not available, some major flood for which data are available should
be substituted. The datum for the profiles shouldbe the same as that used for
the overflow map.
In addition to data on specific floods, the profile should show other perti-
nent information. Such information should include location and identification
of pertinent control points used, profile of the top of bank, profiles of the top
of existing adjacent levees on both sides of the stream, profile of an identified
low flow in the stream, profile of the stream bed, elevation of the bottom
chord (clearance) and the roadway and the top of solid parapets on bridges,
pertinent elevations of obstructions such as fixed or movable dams, and perti-
nent identification ties to natural or man-made features shown in plan on the
map. When pertinent or controlling, the grade of adjacent parallel railroads
or highways should be shown, particularly if they influence flow conditions
and the height of floods.
1 Flood stage
0- ---- 1(-1-----
1.1 .0 3
5 6 7 8 10 20 30 40 5060 80 100
Recurrence interval, in years
FIG. 6.-FREQUENCY CURVE
12 ____ ____ ____
S/ Flood stage
o Determined from stream gaging
2 x---x Extended by hydraulic analysis
Zero stage=El. 1200.00 USC & GS
0 1 2 3 4 5 6 7 8 9 10 11 12 11
Discharge, in thousands of cubic feet per second
FIG. 7.-STAGE-DISCHARGE CURVE
1.. 1.5 2 3 4
Water surface profiles of past floods should be identified by their dates
and theoretical floods should be identified by their descriptive names. In
plotting, it is best to indicate measured and computed elevations by different
legends as well as to use different legends for recorded and computed water-
surface profiles. The scale used for the profile will depend on the amount of
information to be shown thereon, the slope of the stream, the variation in
heights of the floods, accuracy needed, and other factors. It is important to
attain clarity. Profile sheets need not be more than eleven inches wide unless
- Standard project flood-, -- I-- FI
~- -- ~~~~Left end dam
I (high par)
Top of rail
--! --- + A-
Regional flood tc! arance .e-R.
~ 11 \922
= p of bank -D -E
op o a~ --mE
-~ -i U --
-(USC & GS 13 up d.
Miles above mouth
FIG. 8.-PORTION OF A PROFILE SHEET
there are compelling reasons for larger sheets. Length is not so important.
A portion of a typical profile sheet is shown in Fig. 8.
Overflow Maps.-A map showing overflow is probably the most under-
standable and of the greatest interest of all the exhibits in the flood report.
The type of map to be prepared will depend on the available basic maps and,
if known, on the type of flood plain regulations contemplated. As the latter
information is not usually known at the time the map is prepared, the tendency
should be to show "too much" instead of "too little." The maps are usually
prepared at scales varying roughly from 1 in. = 500 ft to 1 in. = 2000 ft, de-
pending on the scale of available maps, the extent of development of the area,
1.-*fr s .^;:
ground slopes, and the degree of accuracy desired. The vertical datum for
the profiles and the map should be the same. The overflow maps need not be
more than eleven inches wide unless there are compelling reasons for using
larger sheets. Colors are effective and desirable for clarity, although they
will add to the cost.
The base for the overflow map may be USGS or Army Map Service topo-
graphic maps, or any other reasonably accurate public or private map that
might be available. If the base map needs updating, it can often be accom-
plished by means of aerial photographs and a nominal amount of field work.
In other cases, especially where contours are not shown"on available maps,
extensive field work may be necessary to showtopography, culture, and man-
made improvements. Aerial photographs can also be used, not only for such
updating, but as a base map. They are even more valuable if they can be tied
in with sufficient accuracy to existing horizontal and vertical controls. It
should be recognized and accepted that, in many cases, the overflow map will
have to be subsequently supplemented and refined by more accurate mapping
after tentative conclusions are reached by planning agencies or officials on
the floods to be considered and the type of flood-plain regulations to be for-
The overflow map is primarily for the purpose of showing the areas sub-
ject to inundation by selected floods for which the water surface profiles have
also been plotted. The overflow limits of such floods should be shown by dif-
ferent legends. In addition, the map should normally show contours, elevations,
depths or some other indication of the expected depths of flooding for the
various floods; man-made structures and improvements in the flood plain
and adjacent thereto; and the culture and type of land use. Maps without con-
tours may be used when data on flood outlines will suffice for the local plan-
ners. Distances along the stream should be shown and should be tied into the
profile sheet. In all cases, the source of the base map and the datum used for
the contours or indicated elevations should be noted on the overflow map. A
portion of a typical overflow map for an urban area is shown in Fig. 9.
Cross Sections.-Cross sections across the stream and flood plain are
helpful in showing the relationship of flood elevations to the ground surface
and the relationship of the main channel to the total flood channel or natural
floodway. Such sections, with pertinent flow line information, are particularly
desirable at natural constrictions and at bridges and obstructions such as
floodwalls and other man-made structures. The scale of such cross sections
normally must be distorted because of the great disparity between horizontal
and vertical distances in order to give a satisfactory visual description. Typ-
ical sections of report-page size can be included in the published report.
Typical cross sections of this type are shown in Fig. 10. The cross sec-
tions should be shown looking downstream. They should indicate the direc-
tion of sight and the river mileage and be properly identified. Where perti-
nent, the name of the bridge or other constriction or obstruction that justified
the inclusion of the section should be shown. The same vertical datum should
be used for the cross sections as for the profile and the overflow map. Water
surface elevations for the more meaningful floods plotted on the profile sheet
should be shown on the cross sections. In addition, pertinent natural and man-
made features should be indicated and identified and clearances of bridges
and important utilities shown.
- ir a oaal a
I I' '. ,.j,'.] 1 ..j~l l .l.,.t. ] 1] nllu l IplI
'*' >' '. '. I *' .' *.' : .jn~ *I 1 J )lu"
FIG. 9.-PORTION OF A TYPICAL OVERFLOW MAP
HY 5 FLOOD-PLAIN REGULATIONS 91
Velocities.-It is difficult to determine velocities during flood periods.
The best that can be done is to obtain such "spot velocity" readings as may
have been measured during past floods and then correlate and extrapolate
from mean velocities. Such spot velocity measurements are occasionally
taken during major floods by various Federal and state agencies. If measured
SECTION 1-MILE 28.55
200 0 200
Horizontal distance, in feet
30 old Highway 73
t Highway 73 I
1020- Floor -
1000 -(b) SECTION 4-
600 400 200 0 200
Horizontal distance, in feet
S Highway 73
1030 _.... ... ; _
(c) SECTION 7-MILE 30.63
1000 1 8
1200 1000 800 600 400 200 0 200 400
(USC & GS 1936 Supp. Adj.)
Horizontal distance, in feet
Horizontal distance, in feet
FIG. 10.-VALLEY CROSS SECTIONS, LITTLE RIVER VICINITY OF TOWNSEND AND
KINZEL SPRINGS, TENN.
velocity data are completely lacking, they can be estimated from hydraulic
computations by taking into consideration the velocity pattern as it may be
affected by channel configuration. In any case, the potential velocity hazard
should be appraised by the engineer in connection with his studies and if, in
92 September, 1962 HY 5
his opinion, an actual or potential hazard exists, it should be noted in the
Flood Hazard. -The report should not be limited to the presentation of the
technical data only. Such data must be interpreted and conclusions that can be
drawn from them indicated. Although many of the interpretations and conclu-
sions will be contained in the body of the report where specific data are pre-
sented and discussed, other important considerations may not be directly re-
flected in the data or their importance may be masked by the technical pres-
The effect of the terrain on the damage potential might not be evident with-
out detailed discussion. The main stream and, in certain cases, secondary
channels and swales that become effective only during flood periods normally
carry a large portion of the flood flow. Such secondary flood channels should
be identified and the expected frequency of their utilization considered. Simi-
larly, attention should be invited to possible high-velocity problem areas. A
typical case might be one in which flood waters could take a shorter course,
with a steeper slope, across river bends. In wide valleys of alluvial streams,
where the natural river banks may be higher in elevation than the valley floor
behind them, constrictions or obstructions of the natural drainage channels
on the valley floor may result in the ponding of flood waters long after the
flood crest passes. This may have a direct bearing on the type of flood plain
regulations that may be adopted for such portions of the flood plain. It often
happens that, because of natural or man-made conditions, there is a certain
sequence of overflow along a stream and various areas are inundated pro-
gressively until general overflow occurs. Such phenomena may have some
bearing on the land-use pattern to be adopted and, accordingly, should be
considered where pertinent. The foregoing localized effects all have a bearing
not only on the frequency of flooding but also on the depth and duration of
The flood report should contain all readily obtainable information on both
present and prospective future flood damages. Monetary estimates are im-
pressive and can frequently be obtained from various Federal and state agen-
cies that are involved in flood control work. If not available from these
sources, such estimates are seldom justified unless specifically directed by
those for whom the report is being prepared. Estimating flood damages, es-
pecially for future conditions of development, is highly specialized work, is
time consuming, and, if properly done, expensive. The flood hazard should in
all cases, however, be discussed in general terms, possibly by sub-areas.
Such discussion should include the effect of overflow, including its depth,
duration, and frequency, on existing and possible future land uses, improve-
ments, utilities, and possible loss of life. In agricultural areas, the period
of the year in which floods occur may have an important bearing on the use
to which lands can be put.
ENGINEERING CRITERIA TO BE ESTABLISHED
There are certain engineering criteria and guides that are applicable in
determining solutions to flood problems. Certain areas of the flood plains
must be made available to pass the large flood flows. Probabilities of flood-
ing at various sites must be known for determining risks. Velocities and ele-
vations are controlling factors in designing structures that will not fail and
in determining safe uses.
Floods Used in Developing Regulations.--The portion of the flood plain to
which land-use regulations are to be applied is determined primarily by the
flood hazard, which is related to specific flood occurrences. If only a desig-
nated floodway is to be established, a single flood can be adopted for regula-
tory purposes. If both a designated floodway and zoning or other controls for
the adjacent restrictive zones are being contemplated along the same reach
of the stream, then more than one flood may have to be used to fix their re-
quirements. As an example, the lines defining a designated floodway may be
fixed by the requirements of adesignfloodfor a future levee system, whereas
the limits of the adjacent restrictive zones may be selected by consideration
of a lesser or a greater flood.
The selection of floods to be used for regulatory purposes is one of public
policy and is dependent on many non-engineering as well as engineering con-
siderations. In the final analysis, they are selected and adopted by the elected
public officials who will be responsible for the enforcement of the regula-
tions. In fixing the magnitude of such flood, however, such public officials
may not only utilize the data and studies prepared by the engineer but may
call on him as well as on available planning agencies for further guidance
and advice. Because of the non-technical considerations involved, the engi-
neering report need not include any recommendations in regard to the size
floods that should be used for such purposes unless specifically requested by
those for whom the report is being prepared. In case recommendations are
made, the basis of the floods selected should be clearly and fully explained,
with emphasis on the reasons behind their selection.
Because of the many variable conditions encountered throughout the United
States, no specific guidelines can be established for the selection of such
floods. It is largely a matter of judgment based on local conditions. Probably
the most important engineering considerations in the choice of such floods,
between various alternatives being considered, are their relative expected
occurrences, the relative areas inundated, and the differences in the depths
of inundation. Some of the important non-engineering considerations that may
have an important bearing on their selection consist of economic pressures
for the use of overflow lands and available alternates to such use, the type of
flood-plain regulations contemplated, and the extent of resistance of property
owners in the flood plain to land-use controls. Some of these non-technical
matters are in the field of public planning, some are in the area of public
education, while others are in the realm of public policy. Because of the many
complex problems involved and in order to be equitable, selection of the flood
magnitude frequently necessitates sacrificing the engineering "ideal" inorder
to reach the "attainable."
Information on the Standard Project Flood considered previously in this
report should always be made available. Its elevation divides the upland areas
where the risk of flooding is nearly zerofrom the areas where the risk grad-
ually increases from the outside of the flooded area towards the main chan-
nel. Many government agencies, industries, businesses, and individuals would
prefer not to take any risk of flooding. The elevations and areas outlined by
such floods provide them with guides.
Floods that normally fall between the Standard Project Floods and the
maximum known floods should be determined for use of officials making local
94 September, 1962 HY 5
decisions. Because of the short period of record, the maximum known flood
in most instances could be expected to recur too frequently, and planning
based on it would involve considerable risk. At the upper end of the scale,
the large Standard Project Flood is so great and would occur so infrequently
that only in rare cases would it be either economical or practical to adopt it
for regulatory purposes. This is especially true in areas where intensive de-
velopments and land use have been or are expected to be experienced. Studies
of maximum known floods on other streams in the immediate region should
provide a practical indication of the size of flood that may reasonably be ex-
pected on the stream being studied. This may be termed a Regional Flood or
given any other name that may be-desirable. Experience in many regions has
proven that there is another factor that gives such a flood greater local value.
Local residents are familiar with the area within 50 miles to 100 miles of
their homes and are psychologically more likely to accept information based
on records in such a limited area.
It should be noted that such a Regional Flood may, in certain instances,
be smaller than the maximum known flood but will never be greater than the
Standard Project Flood. Its frequency may not be known, but the probability
of its occurrence is easily understood by local people.
Floodways and Encroachment Lines. -The limits and characteristics of all
natural floodways were originally developed by nature. Man's activities on the
flood plain and throughout the drainage basin as well as in the channel have
modified the natural conditions. Too often, the net effect of such changes has
been to increase stages and to extend the limits of the overflow area.
The main channel portion of the natural floodway with the adjacent low-
lands, and, in certain cases, secondary channels and swales that become ef-
fective waterway areas only during flood periods, carry the greater portion
of the flood flow. Shallow overflow areas and backwater areas, which may
form the greater portion of the flood plain, are relatively less effective in
their flood carrying capacity. The main channel portion of the natural flood-
way and frequently the effective secondary flood channels are usually the most
economical locations for designated floodways.
The objective of upstream flood storage is to reduce flood flows to the
capacity of either an unleveed or a leveed floodway. The objective of a levee
system is to confine floods within the limits of a floodway. Because of the
importance of a floodway for the passage of floods, either with or without
flood protective works, it would be well to define the limits of a designated
floodway on each stream and take such legal action as may be required to
guarantee that improvements either subject to flood damage or having a detri-
mental effect on the hydraulic capacity are not permitted in such areas. The
engineer should outline the floodway required for hydraulic flows, but in es-
tablishing the designated floodway limits, the engineering requirements must
often be modified by practical, economic, social, and related considerations.
Width of flood plains and elevations of the normal overflow areas may vary
considerably along any stream. The narrow sections normally control up-
stream water-surface elevations. Wide flood plains upstream from naturally
restricted sections act as ponding areas in whichthe water elevations are in-
creased to the degree necessary for developing ahead required for producing
higher velocities to carry the flow through the restricted sections. In these
areas, the entire flood plain is not required for flow and, therefore, portions
of it can be occupied without affecting hydraulic flow conditions except for the
relatively small loss of storage.
The designated floodway to be reserved by zoning or the establishment of
encroachment lines should be adequate for the passage of the major floods
or a flood of a specific size without unduly raising upstream water surface
elevations. Its size must be based on sound hydraulic and economic criteria
and on computations uniformly applied throughout the length of the stream
Floods to be considered in determining designated floodways should be
large enough to demonstrate the effect of existing and possible future en-
croachments. It is neither sound engineering nor effective area control to set
the flood standard so low as to produce regulations that condone existing en-
croachments and invite more. When the size of flood is set too low, there is,
in effect, little regulation and only small benefits can be expected. On the
other hand, if the flood selected is too large or insufficiently supported by
engineering facts, the regulation may be an uneconomical restriction of land
use and an unreasonable invasion of private property rights. Courts may rule
to that effect.
The design flood may not enter directly into the selection of the control-
ling criteria for establishing flood-plain regulations, but it may have an im-
portant indirect bearing on such decisions. As an example, if a designated
floodway is to be established through a presently unprotected area and if there
is a probability that early protection will be provided by means of levees or
similar protective structures, from a practical and economical standpoint,
the lateral limits required for the future levee system should be considered.
This requires the determination of the design flood for the future levee sys-
tem. If future protection is tobeprovidedby upstream storage, or a combina-
tion of such storage and levees or similar protective structures, the engineer
may have to determine both the design flood for the future reservoir and the
resulting regulated discharges from the reservoir before the designated flood-
way can be established intelligently.
Floodways or encroachment limits should not be established as a basis
for obtaining land that might be required for the location of protective works
at some future time. Encroachment lines and designatedfloodways are estab-
lished under the police power to protect life and property from the damages
resulting from uncontrolled constrictions. This differs from the setting aside
of lands for use in a later flood control program.
Almost all of the communities in the Tennessee Valley that have adopted
flood-plan regulations have established designated floodways that will pass
the Regional Flood and the Standard Project Flood without making flood heights
more than 1 ft higher than they would otherwise be. Two of them, however,
have restricted the designated floodway so that the effect on the Regional
Flood would be 1 ftor less, butthe effect on the larger Standard Project Flood
would approach 2 ft. In Tehema County, a rural area in California, however,
the designated floodway selected may permit a rise of 2 ft to 5 ft in the re-
gional flood profile. In computing these effects, itwas assumed that the entire
area outside the designated floodway would be filledor built up solidly so that
no water could flow or pond there, a condition that is highly improbable.
The State of Connecticut determines the location of encroachment lines
along its streams by computing the width of the designated floodway between
the lines necessary to carry a flood that is a multiple of the mean annual
flood. For most of the Connecticut streams, a flood with peak discharge five
times as great as that of the mean annual flood is used. For a few unusual
streams, the multiple is seven instead of five. The mean annual flood is the
arithmetic average of the annual floods and is equalled or exceeded on the
average of once every 2.33 years.
Final selection of the limits of a designated floodway will often be strongly
influenced by non-engineering factors. Flood plain lands are frequently quite
valuable to the future growth of a city because of their location in relation to
major developments. For that reason, it is often considered economical and
advisable to restrict the floodway and make a little more land available for
concentrated development, even though the action may result in somewhat
higher stages for any given flood flows. The engineers should determine and
advise concerning the effects on flood stages of certain sizes of floodways
so that the other effects can, in turn, be determined and evaluated in com-
parison with possible benefits.
Designated floodways should be kept open to obtain maximum flows. How-
ever, limited and judicial use of structures is often desirable to meet the
over-all requirements of an expanding social system. Small or even large
structures in medium or wide sections may not have appreciable effects on
flows. The percentage of the section that is blocked as well as the length of
the reach affected must be studied in hydraulic flow terms.
Water-use structures and many varied recreational structures may be
permitted if properly designed to withstand inundation and velocities. How-
ever, even small structures in small channels, such as those less than 100
ft wide, may have such great effects that they should not be permitted. In
sections where flood overflows are several feet deep and thousands of feet
wide, a structure such as an open-ended stadium might be constructed without
major effects on flood heights.
Elevations for Controls in Restrictive Zones. -Controls over development
in restrictive zones outside the designated floodways should be designed to
permit the most effective use of the land without undue risk of damage from
flooding. This may be accomplished by limiting the type of land use, filling
of land, elevating structures, or other measures.
The important criteria for use of the areas along both sides of the desig-
nated floodways (herein termed restrictive zones) are (1) minimum elevations
for floors, fills, and other improvements and (2) provision for local drainage.
The second of these is generally understood and may be provided for by fol-
lowing the many available texts or instructions for handling local drainage.
The first of these criteria is not as well understood and is sometimes difficult
Areas that are intensively developed, such as the hearts of some of the
cities, are quite different problems from rural areas in which probabilities
indicate little demand for other than open uses. Nevertheless, engineering
data should not involve land-use needs although subsequent studies by officials
should determine the most practical relation.
The regulations should establish minimum elevations below which floors
of structures in these restricted areas would not be permitted. The eleva-
tions should be those of a selected flood profile in order that the risk at all
points along the stream will be uniform and have the same probability.
From the engineer's viewpoint, this profile should be determined by con-
sideration of the local flooding probabilities. Basic considerations in deter-
mining the size of the flood most appropriate for this use are similar to those
for determining the size of the designated floodway. It is difficult, if not im-
possible, to set a fixed standard for such an engineering determination. How-
ever, an effort should be made to provide reasonable guidance to the non-
technical officials who will be making the final decision. Local officials may
adopt the profile suggested by the engineers or they may adopt one higher or
one lower after considering the over-all need of land and risk that would be
Years of experience have shown that the public can readily understand the
Regional Flood and will give it major consideration when designing land-use
controls and other flood-plain regulations. Because of its probability of oc-
currence as shown by actual experience on many neighboring streams, plan-
ning agencies and public officials should give the Regional Flood careful con-
sideration in selecting controlling elevations for regulations. Nearly all of
the many cities in the Tennessee Valley that have adopted such regulations
have used this flood as the controlling elevation in the restrictive zones.
Velocities and Depths.-Flood velocities are often animportant considera-
tion in appraising the need for or in establishing flood plain regulations and
related building codes. High velocities in main channel and in flood-carrying
secondary channels and swales can undermine bridges and floodwalls, erode
confining banks with a resultant loss of lands and improvements thereon, and
wash out adjacent unpaved levees and railroad and highway fills. High over-
bank and overflow velocities can cause both surface deposition and erosion,
resulting in damage to lands, crops, and improvements; can damage or destroy
growing crops, goods, materials, and improvements by their force alone;
and, combined with the buoyancy effect can float and carry structures down-
stream, thus compounding the flood hazard.
In urban areas, high velocities even with shallow depth of overflow are
hazardous to life. A typical area in such a category might be a small commu-
nity in the foothills, located on the banks of a stream that is susceptible to
great velocities during sharp flash floods that result from cloudburst-type
Information on damaging velocities is limited in current technical litera-
ture. In a report issued by the USGS (66) it was stated that:
"Average and maximum velocities of 1 and 4 feet per second, respec-
tively, for an overflow section would not be conducive to serious scour
in an unobstructed cross section. However, velocities of 4 feet per sec-
ond in depths of 3 feet or more might easily sweep individuals off their
feet, thus creating definite danger of drowning. Where the passage of
overflows is more seriously restricted, point velocities in the order of
7 to 10 feet per second could reasonably be expected. Velocities of this
magnitude could definitely cause scour leading to failure of building
Velocities and depths are required as guides in the structural design of
buildings. Pressures and buoyancy should be recognized in the design of any
structure and especially if velocities are great.
Rate of Rise.--Streams with small drainage areas or steep slopes, and
those fed by fan-shaped watersheds, may rise quite rapidly. The period of
rise and fall of these streams, as well as the length of time the flood stays
at its peak stage, are all short. The rate of rise, both within the main channel
and above the accepted flood stage, is a factor that must be recognized in
planning developments or improvements that willinvolve plans for evacuation.
Water in one mountain stream passing through a nationally famous vacation
city rose 15 ft in 12 minduring a medium-sized flood. Rises in other streams
are even greater than this. At the other end of the scale are large streams,
such as the Mississippi River, that rise and fall very slowly.
Extent of Rise,-Related to the rate of rise is the vertical difference be-
tween normal stages and stages during large floods on the streams. Extreme
floods on some streams may not reach stages greater than 10 ft above normal
flows. Compared with this is the record of almost 100 ft of rise in one reach
of the Rio Grande during a major flood in the 1950's. Variation in magnitudes
of rises affect different uses and structures in such a manner that each pro-
posed use should be considered individually.
IMPLEMENTATION OF FLOOD-PLAIN REGULATIONS
Methods. -Methods of regulating the use of flood plains must be based on
considerations of the limits of man's control over natural phenomena, on the
limits of regulating the use of privately ownedproperty in the public interest,
and on rational approaches to enforcement of regulatory measures. The in-
tensity and duration of rainfall and many other factors that produce floods of
various magnitudes cannot be controlled by man. Therefore, a combination of
assumptions and practical or realistic approaches must be a guiding factor.
Flood plain regulations involve the exercise of police power-a power inherent
in state governments or delegated by them to local levels of government and
their agencies. Practical and effective enforcement measures andprocedures
are obvious necessities.
The control of flood waters is usually bounded by practical and economic
limitations. The control of man's use of the flood plain is, by comparison,
far less easily bounded. The various methods available for establishing con-
trol measures are all directly related to the stream and its characteristics.
A sufficient waterway must be provided for the passage of floodwaters, and
restrictions over use of the remainder of the flood plain must be provided.
The cross section of the stream valley can be divided into two zones: (1)
The designated floodway and (2) the restrictive zone, as shown in Fig. 11. The
designated floodway is required for carrying or discharging the flood waters
that may be expected. Velocities are greatest and most damaging in that area.
Obstructions or encroachments that restrict the discharge capacity of this
designated floodway should be prohibited in the public interest although open-
type use may be permitted.
The restrictive zone is the remainder of the flood plain, in which inunda-
tion may occur but where depths and velocities are generally low. Those
areas do not contribute substantially to the passage of flood waters down the
valley, although they do permit varying amounts of valley storage. Such areas
can be used by man as long as developments are placed high enough to be
reasonably free from flooding. Obviously these two zones must be treated
HY 5 FLOOD-PLAIN REGULATIONS 99
Restrictive zone -- Designated floodway
1 I .1
100 September, 1962 HY 5
differently to minimize the hazards to life and property and thus reduce the
In those portions of the flood plain beyond the limits of regulations, de-
velopment is permitted without restrictions. However, those lands will be
subject to inundation when floods occur that are greater than that selected as
the basis of the regulations. The flood potential of this upper flood plain area
should be fully appreciated in order to assure that development is not under-
taken with a false sense of security. The Standard Project Flood will serve
as a guide for defining the extent of the upper flood plain area and the risk
involved in developing those lands. It should also be appreciated that regula-
tions established for the designated floodway and the restrictive zones are
based on a flood of a magnitude limited by practical and economical consid-
erations. Here too, the Standard Project Flood should be utilized to evaluate
the flood damage potential and warn developers in the areaof the risk involved.
Encroachment Lines.-Encroachment lines may be established to outline
the designated floodway. This is done in several states and is generally the
action of a state agency, although it also may be accomplished by local action.
In other states, the designated floodways are zoned as part of the zoning ordi-
nances. Encroachment lines are often established to preserve designated
floodways, even though controls for the remainder of the flood plain are not
adopted. Pertinent excerpts from typical state encroachment line statutes are
contained in Appendix II.
Zoning Ordinances.-Zoning is a universally accepted tool used by states
and municipalities to regulate the use and development of lands within their
boundaries or spheres of authority. Flood plain zoning merely adds another
set of provisions to the general zoning ordinances. State enabling statutes
generally authorize the adoption of regulations for the purpose of promoting
public health, safety, morals, comfort, general welfare, and the conservation
and protection of property and property values. Certain states have added the
word "flood" to the purposes of zoning or made similar changes in the enabling
statutes. Flood plain zoning seeks to protect life and reduce structural and
property damages and costs, including the damages at important and vital
industries where interruption of operations might have a great state and na-
Designated floodways may be zoned for the purpose of passing flood waters
and for other limited uses that do not conflict with that primary purpose. The
ordinance may also establish regulations for the restrictive zone. Those in-
clude controlling minimum elevations above which developments must be
constructed. Pertinent excerpts from typical zoning ordinances are contained
in Appendix II.
Subdivision Regulations.-The regulation of subdivisions provides one of
the most immediately effective means of reducing flood damages in generally
undeveloped areas. They too are the areas in which the municipality should
proceed early and rapidly to establish regulations because of the opportunity
of producing seemingly ideal developments not hampered by nonconforming
existing uses. Floodways can be established by subdivision regulations in the
same manner as with zoning ordinances, but restrictions for development of
the area are somewhat different. Pertinent excerpts from typical subdivision
regulations are contained in Appendix II.
Fig. 12 shows typical relationships of areasfloodedby various size floods,
restrictive zones, and designated floodway limits.
Building Codes.-It is not practical to prevent the location of buildings in
all areas subject to flooding. It is, therefore, advisable to include in building
codes necessary regulations for minimizing structural and consequential
damages resulting from flood velocities and inundation. Unfortunately, poten-
tial flood damages to structures have notbeen given much consideration in the
drafting of those codes. Certain states, however, have prepared codes with
FIG. 12.-PORTION OF MAP SHOWING DESIGNATED FLOODWAY
AND RESTRICTIVE ZONES
enabling legislation authorizing municipalities to establish regulations con-
trolling the design of buildings to protect the structures against damage from
One of the most important items that should be included in a building code
adapted to flood damage prevention concerns the foundation and the anchorage
of the superstructure to the foundation. Other items that should be included
are restrictions against the use of structural materials that deteriorate rap-
idly and materials and equipment that might be hazardous to life when sub-
merged. Pertinent excerpts from typical building codes are contained in
Implementation.-This section is to assist the engineers, planners, and
others in formulating and establishing flood plain regulations. It briefly re-
views the background and organization for all regulations and outlines the
action that should follow the collection of flood data, although it may be begun
while the flood report is being prepared.
With the flood data, the community's comprehensive plan for expansion
should be reviewed and considered in light of the local flood situation. Alter-
natives in a flood damage prevention program should be evaluated within the
context of the community's general planning activities. For sparsely devel-
oped or open flood plains, regulations controlling uses in the flood hazard
areas should be considered. In built-up areas, investigations should include
the feasibility of flood control projects or flood proofing measures for pro-
tection. The solution, whether it be either of these or a combination of the
two approaches, must be related to the community plan for development if
the greatest benefit is to be derived.
Flood plain regulations are not a separate and special set of regulations.
Rather, they are an integral part of the zoning ordinance, subdivision regu-
lations, building code, or other regulations adopted by the respective govern-
ing body for assuring the orderly development of the community for the
greatest benefit of all. Regulations for flood plains are those parts of
community-wide regulations that deal with the flood threat of the stream as
a physical hazard, similar to the manner in which steep and high hills or other
such terrain features are considered.
There are two objectives of flood plain regulations. First, they are to as-
sure and guarantee the retention of the required floodway area without unduly
raising flood heights, and to permit completed or proposed flood control
works to accomplish what they were designed to do. Second, they are to en-
courage sound land use in the flood plain that is consistent with the flood
hazard and the community land-use needs, including land for the passage of
flood waters. To accomplish these objectives most effectively, it is necessary
to consider the regulations as a part of a comprehensive flood damage pre-
vention program and design them accordingly.
It is desirable to establish a comprehensive set of regulations that includes
a designated floodway and controls for the restrictive zones. However, in
many instances, especially where designated floodways are established by
state governments and controls for restrictive zones are adopted by local
government, it will generally be more practical to accomplish the actions
separately. When such actions are taken by separate steps, the actions should
be coordinated. Also, adjacent political subdivisions along a stream should
coordinate their efforts in this field in order to attain consistency.
Authority for Regulations.-Regulation of land use and of development in
the flood plain is established through the use of the police power, which is
vested in the states. The states have, in turn, granted use of the power to
counties, cities, and certain other local governmental bodies. Therefore, only
the state and local officials can adopt such regulations and enforce them.
State Regulations.-Zoning specifies the type of land use that will be per-
mitted in various areas. Some states are now zoning for limited purposes.
Zoning for flood areas is as important and acceptable as for many other pur-
poses, such as state highway location, areas along highways to control access,
areas along highways to control the type of development, and others that have
become acceptable from necessity.
States may utilize zoning and other regulations to control development in
flood hazard areas. Some have used the channel encroachment approach and
others have used designated floodways. Also, limiting criteria may be estab-
lished at the state level for assuring that local action is reasonably safe.
Existing channel-encroachment statutes in several states specify open
areas for the flow of flood waters, but too many of them reserve this only for
the main channel, not recognizing the total problem. Such statutes control the
type and number of structures permitted, but do not specify the allowable
type of land use. Floodway statutes exist in several of the states, permitting
control of the type and number of structures and sometimes specifying per-
missible land use. However, in too many of those states, the statutes are not
being administered so as to make them effective.
Local Regulations. -Regulations that can be utilized by local communities
include zoning, subdivision regulations, building codes, and other related ac-
tions. Generally, zoning and each of the others can be adopted for the entire
area over which the local governmental body has jurisdiction. For example,
in many states, a city's governing body, often termed City Council, can adopt
zoning for all of the area inside the corporate limits of the city and a county's
governing body, often termed County Court, could adopt zoning for the re-
maining area within the county. These zoning ordinances control the type of
use that can be made of the land, such as residential, industrial, commercial,
floodway, and so forth. They also control the density of occupancy, the size
and types of buildings and other pertinent though general criteria.
Subdivision regulations are used by local governments for the entire area
for which they have responsibility. Most states, however, permit local munic-
ipalities to exercise this authority beyond the corporate limits (usually 1 mile
to 5 miles). These regulations specify the manner in which land may be sub-
divided for various uses. They may state the width of streets, requirements
for curbs and gutters, size of lots, elevation of land, freedom from flooding,
floodways, and other points pertinent to the welfare of the community.
Considerations in Determining Flood Plain Regulations. -The primary con-
sideration in determining flood plain regulations is the effect on the health
and welfare of the community. Unrestricted use of areas subject to undue
flooding could also take a toll of human lives and suffering as well as dollars,
if permitted. It is the intent of the regulations to deny uses that would cause
increased flood heights that would multiply the loss and suffering.
The establishment of land-use regulations on the framework of proper
planning is essential, but is not sufficient initself. Regulations must be based
on sound legal principals and must be supported by adequate technical data.
They must not be restrictive or limited to isolated areas but should be com-
prehensive in scope, treating all areas with a similar flood hazard alike. The
provisions of the regulations should be reasonable on the basis of precedent
and judgment so that, if tested in the courts, they can satisfy the requirement
of non-confiscation of property without due compensation. Actual flood ex-
periences and sound engineering studies form some of the principal bases
for tests of comprehensiveness and reasonableness. The intent of the regula-
tion, not only from a legal standpoint but also from the standpoint of effective-
104 September, 1962 HY 5
ness, must be satisfied by actual compliance. As an example, if an ordinance
states that flood damage is to be prevented it must accomplish such objective.
In most situations, flood plain regulations should include a designated
floodway. Such action recognizes nature's demands and also strengthens the
regulations in the eyes of the court, as few will debate the reasonableness of
providing for the natural flood flows. In restrictive zones, improvements and
structures would be permitted as long as filling or construction maintained
the elevations of the lower floors above the acceptable flood height or other
measures were taken to reduce flood damages.
Agency Responsibilities. -States are taking a more active part and must
assume even greater responsibility in this field. They are adopting controls
in regional areas in which it is not considered practicable to expect the local
governments to coordinate widely separated and diversified interests. Mini-
mum criteria as guides to local governments may be expected at the state
level. In some instances, action should be shared with the local officials. One
very important contribution in many states is that of local planning assistance.
Local governmental bodies are responsible for planning and for action on
the local level. Sometimes there may be some overlapping of city and county
and possibly regional agencies. This can always be handled when there is
coordinated effort. City, county, or regional bodies should plan for and adopt
The Federal Government is taking various actions to assist in this field.
Some Federal agencies have programs through which basic flood information
as well as technical guidance for interpretation and use are provided state
and local officials. Such help is made available only to responsible local
governments or groups that have declared their interest to use it. That re-
quirement encourages local and state action. The Federal Government may
require cooperation of the local or state officials in acquiring and evacuating
flood-prone areas or in adopting supplementary flood plain regulations as
part of the local assistance necessary for any flood control works that are
being financed or partly financed by Federal funds. It may assure that the
Housing and Home Finance Agency (HFA) program, involving Federal Housing
Administration (FHA), Urban Renewal Administration (URA), and Public Hous-
ing Administration (PHA), and the Veterans Administration (VA) program
recognize flood hazards and refuse financial assistance where they are too
great and unless regulations are adopted. There are also other fields in which
Federal financial or other assistance may require local action to reduce the
Action Required for Adopting Flood Plain Regulations.--The mechanics of
legislation varies throughout the country and no set method can be outlined.
However, for most areas, the procedures outlined herein are applicable. A
planning commission is generally utilized for studying conditions and pre-
paring proposals, although other specified groups do this in certain regions.
The proposals must then be acted on by the legislative bodies. Planning com-
missions are appointive bodies and are generally the designers, administra-
tors, and watch-dogs over zoning ordinances, subdivision regulations, and
related regulations. In most states, these commissions not only prepare but
also adopt and administer subdivision regulations after a public hearing. In
other states, subdivision regulations mustbe adoptedby the legislative bodies,
who should proceed in accordance with their standard procedures.
The planning commission can recommend, but only the legislative bodies
such as city councils, county courts, or state legislatures can adopt zoning
ordinances, building codes and other regulations, except for subdivision regu-
lations in some states. For these reasons, a planning commission must es-
tablish a close and healthy relationship and understanding with the chief ex-
ecutive and the legislative body.
Order of Action.--Normally, it would be well for the local planning com-
mission to obtain flood data as described previously herein. A request for
such data should go to the designated agency if Federal assistance is desired,
although some commissions may wish to make their own engineering study by
their own forces or through the use of private engineers. With the flood data
in hand, the planning commission, or whatever group is designated by the local
government to consider the problem, shouldmake planning study. This study
should consider the present and future land-use requirements as well as the
various methods of preventing flood damages. From the study should come
an outline of a comprehensive plan, including regulations, pertaining to the
flood plains. This plan should then be submitted to the respective legislative
body for action in accordance with the applicable laws and procedures.
The local legislative body then schedules a public hearing to provide an
opportunity for the public to state any objections or other reactions to the
plan. Local regulations often require three readings by that body, with stated
intervals between the readings. The public hearing must be held prior to or
at the time of the final reading. There will be some variations in the action
and the order of action as the rules of procedures will vary in diverse lo-
Regulations are normally printed and distributed to various officials as
soon as reasonable after they are adopted. For greatest effectiveness, copies
should also be made available to the public as guides to citizens planning use
Enforcement of Regulations. -Legally and technically sound regulations
must be adequately enforced in order to be effective. Experience shows that
non-enforcement of properly conceived and drawn regulations by public offi-
cials and administrators is a major and serious problem in the area of flood
plain regulations. Pressure from vested interests frequently results in "waiv-
ers" or "exceptions" from the requirements in many cases when not justified
or warranted. Another serious problem involves the establishment and fi-
nancing of a proper policing and enforcement organization. If a regulation is
to accomplish the objective for which it was established, such an organiza-
tion must be established with adequate power to take police action, if neces-
Any regulation must have public acceptance and support if it is to be suc-
cessfully applied. This requires giving the public an opportunity, by means of
hearings and news media, to review and comment on the proposed regulation
before formal adoption as well as patterning the regulation on public needs
and desires. It also requires continuous and constant education of the public
and special interests on the intent and purpose of the ordinance.
Finally, any regulation must have public acceptance and support if it is to
be applied successfully. This requires not only giving the public an oppor-
tunity, by means of hearings and news media, to review and comment on the
regulation before formal adoption but also requires continuous and constant
education of the public and special interests on the intent and purpose of the
This report is respectfully submitted by the Task Force on Flood Plain
Robert E. Cyphers
George R. Shankin,a 1958-1960
William S. Wise
James E. Goddard, Chairman
of the Committee on Flood Control of the Hydraulics Division, ASCE
Francis G. Christian
H. Alden Foster,a 1953-1958
Joseph F. Friedkin,a 1954-1958
Arno T. Lenz,a 1956-1960
Max L. Mitchell
Hu B. Myers
Joseph I. Perry,a 1955-1961
Walter G. Schulz
Arthur R. Luecker, Chairman
APPENDIX I.-TERMS AND DEFINITIONS
Efforts have been made to simplify definitions presented herein primarily
to meet the needs of the average reader of flood-plain information reports
and similar documents who is not a technical specialist in flood analyses.
For more detailed scientific definitions, reference is made to technical pub-
lications (27), (29), (34).
Building Code.-A collection of regulations adopted by a local governing
body setting forth standards for the construction of buildings and other struc-
tures for the purpose of protecting the health, safety, and general welfare of
Channel.-A natural or artificial watercourse of perceptible extent, with
definite bed and banks to confine and conduct continuously or periodically
flowing water. The top of the banks form the dividing lines between the chan-
nel and the flood plain. (See Fig. 11.)
Design Flood.--The selected flood against which protection is provided,
or eventually will be provided, by means of flood protective or control works.
It is the basis for design and operation of a particular project after full con-
sideration of flood characteristics, frequencies and potentials, and economic
and other practical considerations.
a Member during the period shown.
Designated Floodway. -The channel of a stream and that portion of the ad-
joining flood plain designated by a regulatory agency to reasonably provide
for passage of flood flows.
Encroachment Lines.-Lateral limits or lines beyond which, in the direc-
tion of the stream or body of water, no structure or fill may be added without
permission from the regulatory agency that established them. Their purpose
is to preserve the flood carrying capacity of the stream or body of water and
its flood plain. Their location should be such that the designated floodway be-
tween them, including the channel, will handle a designated flood flow or
Flood. -An overflow of lands not normally covered by water and that are
used or usable by man. Floods have two essential characteristics: The inun-
dation of land is temporary; and the land is adjacent to and inundated by over-
flow from a river or stream or an ocean, lake, or other body of standing
Normally a "flood" is considered as any temporary rise in stream flow
or stage, but not the ponding of surface water, that results in significant ad-
verse effects in the vicinity. Adverse effects may include damages from over-
flow of land areas, temporary backwater effects in sewers and local drainage
channels, creation of unsanitary conditions or other unfavorable situations by
deposition of materials in stream channels during flood recessions, rise of
ground water coincident with increased stream flow, and other problems.
Flood of Record. -Any flood for which there is a gage record or other
systematic or reliable record useful for technical analysis. Included are
USGS and other official gage records; records documented in diaries, news-
papers, and other reports; known high water marks; eyewitness accounts;
and other data that are reliable and complete enough to be useful to the hy-
Flood Control.-The elimination or reduction of flood losses by the con-
struction of flood storage reservoirs, channel improvements, dikes and levees,
by-pass channels, or other engineering works.
Flood Crest. -The maximum stage or elevation reached by the waters of
a flood at a given location.
FLjod Damage Stage.-The stage or elevation in a stream orbody of water
at which damage becomes significant in the reach or area in which the eleva-
tion is measured. It is generally comparable to "flood stage" and has been
commonly referred to as "flood stage" by the USWB and others.
Flood Duration. -Generally, the total length of time the stream is above
"flood stage," represented by hydrograph section (b) to (d) in Fig. 13.
Flood Frequency. -The average interval of time, based on the period of
record, between floods equal to or greater than a specified discharge or stage.
It is generally expressed in years.
Flood Hydrograph.--A graph or curve (see Fig. 13) representing stage or
discharge at a particular location on a stream, plotted against time during a
flood. Stage is usually expressed as elevation in feet above an arbitrary gage
datum or above mean sea level; discharge as rate of flow in cubic feet per
second; and time in hours or days.
Flood Peak.-The maximum instantaneous discharge of a flood at a given
location. It usually occurs at or near the time of the flood crest.
Flood Plain.--The relatively flat area or low lands adjoining the channel
of a river, stream or watercourse or ocean, lake, or other body of standing
water, which has been or may be covered by flood water.
Flood-Plain Regulations. -A general term applied to the full range of codes,
ordinances, and other regulations relating to the use of land and construction
within the channel and flood plain areas. The term encompasses zoning ordi-
nances, subdivision regulations, building and housing codes, encroachment
line statutes, open-area regulations, and other similar methods of control
affecting the use and development of the areas.
Flood Profile.--A graph showing the relationship of water surface elevation
to location, the latter generally expressed as distance above mouth for a
stream of water flowing in an open channel. It is generally drawn to show
surface elevation for the crest of a specific flood, but may be prepared for
conditions at a given time or stage.
Flood peak or crest
(b)/ X Flood stage (d)
Flood Proofing. -A combination of structural changes and adjustments to
properties subject to flooding primarily for the reduction of flood damages.
Flood Recession.-The phase of a flood during which the stage or dis-
charge is generally decreasing, as represented by the hydrograph section (c)
to (e) in Fig. 13.
Flood Rise.-The phase of a flood during which the stage or discharge is
generally increasing, as represented by the hydrograph section (a) to (c) of
Flood Stage.-The stage or elevation at which overflow of the natural banks
of a stream or body of water begins in the reach or area in which the eleva-
tion is measured.
Historical Flood. -Any known flood for which there is no gage record or
other systematic or usable technical record.
Maximum Known Flood.-The largest flood that has occurred within the
history of a region whether it is an historical flood or flood of record.
Maximum Probable Flood. -The most severe flood with respect to flood
peak that may be expected from a combination of the most critical meteoro-
logical and hydrological conditions that are reasonably possible on the drain-
age basin. The basis for estimating the discharge of such a flood is Hydro-
meteorological Report No. 33 of the USWB. The term is used by others, such
as TVA, with a different meaning.
This flood has previously been termed MlmImhA ~UI M l Fle f "
Mean Annual Flood. Approximtoly vui Rl tt the
arithmetic mean of the highest pea 41tn6ote -mperil
during each year of record at a given location. Tech-
nically it is the value of the graphical frequency at a
recurrence interval of 2.33 years.
miles but often is irregular in shape. extraordinarily large and rare 1o50 "
are not included in this determination.
Restrictive Zone.--The portion of the natural floodway between the limits
of the designated floodway and the limits of the flood selected for controlling
Stage-Discharge Curve.-A graph or curve showing the relation between
the gage height and the amount of water flowing in a channel, expressed as
volume per unit of time. This is also commonly termed "rating curve."
Standard Project Flood. -The flood that may be expected from the most
severe combination of meteorological and hydrological conditions that are
considered reasonably characteristic of the geographical area in which the
drainage basin is located, excluding extremely rare combinations. Peak dis-
charges for these floods are generally about 40% to 60% of the Maximum
Probable Floods for the same basins. Such floods as used by the Corps of
Engineers are intended as practicable expressions of the degree of protection
that should be sought in the design of flood control works, the failure of which
might be disastrous.
It is synonymous with the flood the TVA terms "Maximum Probable Flood."
Subdivision Regulations. -Regulations and standards established by a local
authority, generally the local planning agency, with authority from a state en-
abling law, for the subdivision of land in order to secure coordinated land
development, including adequate building sites and land for vital community
services and facilities such as streets, utilities, schools, and parks.
Zoning Ordinance. -An ordinance adopted by a local governing body, with
authority from a state zoning enabling law, which under the police power
divides an entire local governmental area into districts and, within each dis-
trict, regulates the use of land, the height, bulk, and use of buildings or other
structures, and the density of population.
APPENDIX II.-EXCERPTS FROM PERTINENT
ORDINANCES AND REGULATIONS
EXCERPTS FROM ZONING ORDINANCES
Kingsport. -From Kingsport, Tenn., Ordinance No. 1563, adopted 22, July
F-1 Floodway channel district regulations:
Within the F-1 Floodway channel districts, as shown on the Zoning map,
the following regulations shall apply:
(a) Uses permitted.
Any use permitted in the nearest adjacent zoning district on the same
side of Reedy Creek.
(b) Uses and improvements prohibited.
1. No building or structure shall be constructed, altered, or extended
in said flood area, and no building or structure shall be moved
within or into the F-1 district.
2. No landfill or dumping shall be permitted in said flood area.
3. No permanent storage of materials or equipment shallbe permitted
in said zone.
SECTION VI. Be it further ordained by the City of Kingsport, Tennessee,
that the Zoning Map of the City of Kingsport, adopted and enacted as part of
Zoning Ordinance No. 894, the same being Chapter 34 of the 1949 City Code,
as the same has been heretofore amended, be further amended so as to pro-
vide that the area and territory hereinafter described under this section be
declared to be and the same shall be designated as an F-1 Floodway Channel
District, and which said area is located in the City of Kingsport, and the llth
Civil District of Sullivan County, Tennessee, and being described and bounded
as follows, to-wit:
Beginning at a point on the corporate limits line, said point being forty
(40) feet from and radial or perpendicular to the centerline of Reedy Creek
and approximately two hundred (200) feet southofthe center of the Clinchfield
Railroad Bridge, measured along said corporate limits line; thence on a line
meandering easterly and northeasterly forty (40) feet from and parallel with
the centerline of said creek for a distance of about 2,200 feet to a point 2,000
feet upstream from the center of the Clinchfield Railroad Bridge, measured
along said centerline; thence on a line radial or perpendicular to the center-
line of said creek for a distance of 25 feet to a point 65 feet from said center-
line; thence on a line meandering in a generally northeasterly direction 65
feet from and parallel with the centerline of said creek .. thence southerly
along said corporate limits line to the point of beginning.
Tehama County.--From Tehama County, Calif., Ordinance No. 302, adopted
5, September 1961:
SECTION 24-A. "PF" PRIMARY FLOOD PLAIN DISTRICTS
Sec. 24-A.1.-This district classification is intended to be applied by the
County to properties which lie within a primary floodway, which for the pur-
poses of this ordinance shall be construed to be a stream channel and the
portions of the adjacent flood plain as are required to efficiently carry the
flood flow of the stream, and onwhichproperties special regulations are nec-
essary for the minimum protection of the public health and safety, and of
property and improvements from hazards and damage resulting from flood
The following specific regulations and the general rules set forth in Section
31 shall apply in all "PF" Districts.
Sec. 24-A.2. Uses Permitted:
(a) Crop and tree farming, truck gardening, viticulture, livestock grazing,
and other agricultural uses which are of the same or a closely similar
(b) Public utility wire and pipe lines for transmission and local distribu-
Sec. 24-A.3. Uses Permitted Upon The Issuance of Use Permits:
(a) The following uses, buildings and structures when it is found by the
Planning Commission that such buildings and structures will be so
constructed or placed, or will be so protectedby levees or other flood
proofing that they will not be appreciably damaged by flooding, will
offer a minimum obstruction to flood flow, and will resist flotation:
1. Buildings and structures accessory to agricultural uses for the
storage of goods and equipment and the shelter of animals and fowl,
but not including dwellings or buildings for human occupancy.
2. Public utility buildings and structures other than wire and pipe
3. Public parks and recreation areas and facilities, including boat
ramps, docks, parking areas, picnic tables and fireplaces, private
and commercial recreation developments and facilities, camp
grounds and trailer parks, provided that rest room facilities shall
be located and constructed in accordance with Health Department
(b) Commercial excavation of natural materials, filling of land areas;
construction of levees, dikes or other structures designed to divert
or obstruct the flow of flood waters.
(c) Single family residences and multiple family residences when it is
found by the Planning Commission that such buildings and structures
will be so constructed or placed, or will be so protected by levees or
112 September, 1962 HY 5
other flood proofing that they will not be damaged by flooding or con-
stitute obstructions to flood flow or hazards to life or property.
D. By the addition thereto of a new Sectionto be designated Section 24-B,
and to read as follows:
SECTION 24-B, "SF" SECONDARY FLOOD PLAIN DISTRICTS.
Sec. 24-B.1. This district classification is intended to be applied to prop-
erties which lie within areas inundated by overflow waters during the histori-
cal flood of 1958 of the Sacramento River and its tributaries, excluding areas
within established "PF" Districts, which properties are subject to occasional
flooding or inundation by overflow flood waters, and so require special regu-
lations for the protection of such properties and their improvements from
hazards and damage which may result from flood waters.
The following regulations and the general rules set forth in Section 31
shall apply to all "SF" Districts.
Sec. 24-B-2. Uses Permitted:
(a) Uses as permitted in "PF" Districts, (Sec. 24-A.2 and Sec. 24-A.3).
(b) Single-family dwellings and accessory residential and agricultural
structures located on agricultural properties of three or more acres,
provided that the ground floor level of such dwellings shall be above
the level of the 1958 flood waters upon the particular property.
(c) Residential subdivision improvements and dwellings, as definedherein,
within subdivisions approved and recorded in Tehama County after the
fourth day of March, 1957, and subject to conditions to such approval
and to ground floor level as specified in Sec. 24-B.2 (b) above.
Sec. 24-B.3. Uses Permitted Upon the Granting of Use Permits:
(a) Improvements, buildings and structures listed in Sec. 24-B.2(a) and
(b) above which may not meet the ground floor level specified, but
which will otherwise be adequately protected by levees or other ac-
ceptable flood proofing.
(b) Single family dwellings, motels, multiple family dwellings.
(c) Local service commercial uses and structures.
(d) Uses and structures for the storage, distribution and processing of
agricultural products, supplies and equipment.
(e) Public and private schools, churches, libraries, organization meeting
halls and other similar places of public assembly.
EXCERPTS FROM SUBDIVISION REGULATIONS
Clinton. -From Clinton, Tenn., Subdivision Regulations, adopted December,
Land which the planning commission has found to be unsuitable for sub-
division due to flooding, bad drainage, steep slopes, rock formation, or other
features likely to be harmful to the safety, health, and general welfare of the
future residents shall not be subdivided unless adequate methods approved by
the city engineer are formulated by the developer for meeting the problems
created by the subdivisions of such land.
Land within the Floodway shall not be platted for residential occupancy or
building sites. Other land subject to flooding may be platted for residential
occupancy or for such other uses which will not increase the danger to health,
life, and property. Fill may not be used to raise land in the Floodway areas.
In other areas subject to flood, fill may be used providing the proposed fill
does not restrict the flow of water and unduly increase flood heights.
In applying this provision land below the elevation of the 50,000 cfs Flood
shall be considered subject to flood. The elevation of the 50,000 cfs Flood
shall be determined from the chart "High Water Profiles, Clinch River, Vi-
cinity of Clinton, Tennessee" (Tennessee Valley Authority, November 1957),
which chart is made a part of these regulations. Areas included in the Flood-
way are as shown on the Zoning Map of Clinton, Tennessee.
No street shall be approved unless it is at an elevation within two feet of
the elevation of the 50,000 cfs Flood. The planning commission may require
where necessary profiles and elevations of streets for areas subject to flood.
Fill may be used for streets provided such fill does not unduly increase flood
heights. Drainage openings shall be so designed as to not restrict the flow of
water and unduly increase flood heights.
For each lot in a subdivision that does not contain a flood-free building
site as defined by these regulations, the subdivider must fill to the required
elevation and meet other requirements concerning fills as specified in the
introduction to Article II or include in the deed the restriction that the lower
floor of any structure be built at an elevation not lower than that of the 50,000
cfs Flood referred to in the introduction to Article II.
If any portion of the land being subdivided is subject to flood, as defined
in the introduction to Article II, the limits of such land shall be shown on the
final plat as recorded.
EXCERPTS FROM ENCROACHMENT STATUTES
Connecticut.--From Connecticut Public Act 364, as contained in Statutes
Pertaining to the Water Resources Commission, Hartford, Conn., compiled
Section 9, Powers (c).-Said commission shall establish, by order after a
public hearing and in accordance with sound engineering principles, on any
waterway under consideration for stream clearance, channel improvement or
any form of flood control or protection work, lines beyond which, in the direc-
tion of the waterway, no obstruction or encroachment shall be placed by any
person, firm or corporation, public or private, unless specifically authorized
by the commission. The provisions of this section shall not affect the provi-
sions of section 25 of number 13 of the public acts of 1957 authorizing any
town, city or borough to establish such lines within its jurisdiction prior to
the establishment of lines by the commission, provided the commission may
alter any lines, however established, upon finding such alterations necessary
to effectuate the purpose of this section and section N 209. Notice of any such
order of the commission establishing or altering any such line shall be given
by mailing notice thereof to all persons known to be affected thereby and by
publishing such notice three times in a newspaper having a general circula-
tion in the area involved. Any persons aggrievedby any order of the commis-
sion as to the location of any such line may appeal from such order to the
court of common pleas within thirty days of the giving of such notice. Service
of notice of appeal shall be made upon the chairman of the commission. When-
ever the commission shall find that existing encroachments within lines es-
tablished or to be established constitute a hazard to life and property in the
event of flood, it is empoweredto take land as provided by chapter 360. When-
ever the commission finds that the regulation and use of flood control meas-
ures at dams and reservoirs will constitute an effective part of a flood control
program it may negotiate and enter into agreements with the owners of such
dams and reservoirs to carry outthe purpose of this section and section N209
with due regard to water supply needs and reserves of communities in drought
Section N192, Study of Conditions Relating to Flood Control.-Said commis-
sion shall make a comprehensive study of all conditions, wherever located,
in any way relating to: (a) the control of flood waters, the establishment of
encroachment limits along waterways to provide reasonable flood discharge
capacity, the flood control features of existing and future dams and reservoirs,
the removal of stream obstructions caused by flood waters, and extent of
damage caused by flood waters to property of the state, its political subdivi-
sign, industry and agriculture and any necessary means or method by which
such damage may be repaired or provided against in case of future floods;
(b) river and harbor improvements, obstructions or encroachments in any of
the navigable waters or tributaries within the state; and (c) any matters kin-
New Jersey.--From New Jersey Revised Statutes (1929) C.1 Title 58:
Encroachment on Streams
58:1-26.-Construction and maintenance of structures within streams regu-
lated; penalty for violation. No structure within the natural and ordinary high
water mark of any stream shall be made by any public authority or private
person or corporation without notice to the commission, and in no case with-
out complying with such conditions as the commission may prescribe for pre-
serving the channel and providing for the flow of water therein to safeguard
the public against danger from the water impounded or affected by such struc-
ture, and this prohibition shall apply to any renewal of existing structures.
No such approvalby the commission shallimpair or affect any property rights,
otherwise existing, which might be invaded by the construction or mainte-
nance of any structure.
EXCERPTS FROM BUILDING CODES
Los Angeles County.-The Los Angeles County, Calif., Uniform Building
Code of 1956 contains the following statement concerning construction in
A permit shall not be issued for a group A, B, C, D, H, or I occupancy in
an area which is subject to flood hazards by reason of inundation, overflow,
or erosion, and is so determined to be by the County Engineer unless such
hazard is eliminated to the satisfaction of the County Engineer by providing
adequate drainage facilities, by protective wall, by suitable fill, by raising
floor level of the building, by a combination of these methods, or by other
National Building Code.--From National Building Code, Recommended by
the National Board of Fire Underwriters, New York, N. Y., 1955:
Sills shall be anchored to the foundation walls at intervals not exceeding
6 feet by anchors equivalent to bolts not less than one-half inch in diameter
with proper washers, embedded at least 7 inches in the foundation.
Basic Building Code.-From Basic Building Code, 1955 Edition, Chicago,
Building Officials Conference of America, Inc., 1955:
Section 874.3 Foundations.-Exterior walls below grade and the cellar
floors of all buildings for institutional and residential uses (use groups H and
L) enclosing habitable or occupiable rooms or spaces below grade shall be
made watertight, and when necessary shall be reinforced to withstand water
pressure as prescribed in sections 710 and 871. The basement walls of build-
ings in the residential use groups and the walls of all habitable and occupiable
rooms and spaces below grade shall be protected with not less than a one-
coat application of approved waterproofing paint, or a one-half (1/2) inch
pargeting coat of portland cement mortar or other approved damproof-
Wrightsville Beach.-From Wrightsville Beach, N. C., Ordinance of 17,
January 1955, amending the Building Code of the Town of Wrightsville Beach:
(5) Girders resting on masonry foundation walls or piers shall be anchored
thereto with not less than 1/2 inch bolts embedded at least 6 inches in
(6) Wooden columns and posts shall be securely anchoredto their founda-
tions and to the members which they support.
All new structures and all structures rebuilt or repaired where the struc-
ture required a new foundation or where rebuilding or relocating a building
on existing foundations is necessary, shall be built upon piles in accordance
with the following requirements.
(1) Height. Piles shall not be less than eight (8) feet in height measured
from the "Building Line" of the Town of Wrightsville Beach as estab-
lished by the North Carolina General Assembly and as shown on the
map of the Town of Wrightsville Beach dated September 4, 1941. The
height of the piles measured from the mean high water mark if estab-
lished by competent authority may be used in lieu of the "Building
Line" in measuring the required pile height. The average elevation of
the building lot may be obtained by averaging the known elevations
measured at the corners of such lot.
(2) Type of Pile ....
116 September, 1962 HY 5
(3) Required Depth of Piles. Piles should be sunk or buried to a depth of
not less than 100% of the required height of the pile.
(4) Size of Wood Piles. .
(5) Spacing of Wood Piles. -The maximum center-to-center spacing of wood
piles shall not be more than eight (8) feet on centers under weight
bearing sills. However, for two story or larger buildings or where
the load bearing requirements demand it piles may be required to be
spaced closer together by the building inspector.
(6) Tieing and Bracing of Wood Piles. Wood piles shall be tied to the
structure with bolts or galvanized strips at least four inches wide with
galvanized nails, or tied in some other approved manner. Each pile
shall be properly braced in an approved manner and when timber
braces are used the recommended size shall be 4" x 4".
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