Water Management Technology and Institute Excerpt

Material Information

Water Management Technology and Institute Excerpt
Harper & Row


Subjects / Keywords:
Groundwater ( jstor )
Water supply ( jstor )
Floods ( jstor )
Spatial Coverage:
North America -- United States of America -- Florida


Water Management Technology and Institute
General Note:
Box 9, Folder 7 ( SF-Safe Yield - 1956-1995 ), Item 6
Digitized by the Legal Technology Institute in the Levin College of Law at the University of Florida.

Record Information

Source Institution:
Levin College of Law, University of Florida
Holding Location:
Levin College of Law, University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.


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Technology and Institutions

Warren Viessman, Jr.
University of Florida

Claire Wety
Massachusetts Institute of Technology


Cambridge, Philadelphia, San Francisco,
London, Mexico City, Sio Paulo, Singapore, Sydney


groundwater is a major drinking water source, and it is the only source in
many localities. The usability of groundwater depends very much on its
quality. Factors affecting the quality of the groundwater in the United
States are discussed in Chapter 6.

Safe Yield
In the past, safe yield has been defined as the quantity of water that
can be withdrawn annually without ultimate depletion of the aquifer.
This term has been viewed with disfavor in recent years because it is felt
that the concept implies that it is desirable to avoid aquifer depletion.
This is not necessarily the case, because sometimes it is not practical to
operate in the safe yield mode. For example, where aquifer recharge
amounts to virtually zero, a decision to mine the water may be made
based on the lack of availability or on the quality of alternative water
supplies. Mining an aquifer should therefore not be viewed as a practice
to be avoided per se, but rather as a management option that should be
exercised with full awareness of the impacts on the aquifer and its ex-
pected useful life.
For aquifers that are readily recharged, the safe yield management
technique may be used for balancing aquifer recharge and depletion. One
method for calculating safe yield has been developed by Hill. In this
method, the mean annual draft is plotted against the mean annual change
in groundwater level for a number of years approximating the long-term
average water supply. For a basin receiving a fairly uniform water supply,
the plotted points can be fitted by a straight line. The safe yield is taken
as the draft that results in zero change in groundwater level. This method
is illustrated in Figure 7-24.

Darcy's Law and Governing Equations

The basic equation used for groundwater flow analysis is based on Darcy's
law, which states that the water velocity is proportional to the head loss
in the system. A basic assumption is that groundwater flow velocities are
exceedingly low-on the order of five feet per day to a few feet per year.
Darcy's law can be stated as:

Q = -KA dh/dx

where Q = total discharge across a cross-sectional area of the
permeable bed (vol/time)
K = hydraulic conductivity'of the material

.. :





-'" 5

Figure 7-24 Example c

A = cross-s(
x = length


h, the total head or
expressed as (see Fig
h = zA + hA

where ZA = elevat
hA = head i

A; ..J.;


,i -S--';




Figure 7-24 Example of the determination of safe yield with the Hill method.

is felt
cal to
ild be
ts ex-

i. One
n this

h, the total head or piezometric head at a selected elevation, can be
expressed as (see Figure 7-25):

h = zA + hA


where ZA = elevation above a selected datum
hA = head measured from selected elevation to water table

Figure 7-25 Definition sketch
showing hydrostatic pressures
in a porous medium.


ce in
)n its

A = cross-sectional area of the permeable bed
x = length of the bed under consideration

ad loss
ics are
r year.





Note that h is a negative value for elevations in the unsaturated zone, Grou
~ because zg exceeds P, under unsaturated conditions.
B Equation 7-20 may be rewritten in terms of velocity, which is the Grou
form of Darcy's law used in solving most groundwater problems. The galle
Darcian velocity is thus: eartl

q = Q/A = Kdh/dx (7-22) infilt
where q is also called the specific discharge, that is, the discharge per unit tratc
area, in units of velocity. near
Another expression for q is:
q = nV, (7-23) ing
where n = porosity of the permeable material fer.
Vp = pore velocity
The hydraulic conductivity K (also called the coefficient of permeabil- begi
* ity) may be expressed in a number of ways. The USGS defines K,, the resi.
Standard coefficient of permeability, as the number of gallons per day of The
water that will pass through one square foot of a porous medium under resti
Sa unit hydraulic gradient at 60*F. Afield coefficient of permeability, Kf, spree
may be derived from K, by adjusting for the dynamic viscosity of water in tl
under field conditions based on the relation: som
Kf = K, (uo/luf) (7-24) tion
where ueo = dynamic viscosity of water at 60*F
uf = dynamic viscosity at field temperature
Another useful coefficient in analyzing groundwater fow is the coefi-
cient of transmissivity. It may be expressed as:
T=Kfb (7-25)
where K = field hydraulic conductivity or coefficient of permeability
b = the saturated depth of the aquifer
It is important to note that Darcy's law applies only to flow that can
.-2 be characterized by a low Reynolds number (on the order of 1)-that is,
in flow regimes where viscous forces predominate. The Reynolds number
is an expression of the ratio of inertial forces to viscous forces acting on a
fluid. The Reynolds number is designated as:
NR = Vd v d" (7-26)
where V = flow velocity
d = mean grain diameter
v = kinematic viscosity Figt

-- .* *. . .. .- .
**, -- .

I- ..M


le special
her floors
Most resi-
o 50 psig

isis of the
om 3 to 5
ifies crite-
be main-

h a layout
has been
lows must
on of flows
ven sector
., :and if a
. period at
aks but the
flow at the
,,nts of the
ally, water
(c plus fire
which of the
11 excess of
c carefully
e historical
I costly.
iand) to be
at specified
i pipe sizes
n pressures
usually not
rmissible to

let them drop to about 20 psig. Main feeder pipes should be designed for
pressures in the 40 to 75 psig range unless local conditions indicate a lesser
pressure would be acceptable (1). Actual pipe sizing is accomplished using
standard hydraulic methods and network analyses (7-18).

Water distribution systems are costly to build, maintain, and operate. As
a result, designers must be careful to avoid overconservatism, while at the
same time providing for reasonable increases in water demands from the
system as the area expands in population or other growth activities. Fur-
thermore, economies may also be achieved through the development of
optimal operating policies. That combination of pipe sizes, storage, and
operating rules which leads to the minimum cost of providing the region's
water supply should be sought. A well-thought-out operating procedure
combined with a smaller pipe network might provide as much water as
a larger pipe system operated in a less efficient manner. Initial construc-
tion costs must also be weighed against long-term operation and mainte-
nance costs in determining the nature of facilities to be provided.

Water is stored for a variety of purposes. These include flood control,
water supply, water quality enhancement, recreation, navigation, and
hydroelectric power generation. Both surface and subsurface storage
may be used. Surface storage often requires the development of a reser-
voir whereas subsurface storage can take advantage of the storage capac-
ity of underground formations.
The purpose of providing storage works is to regulate stream flows so
that surplus waters can be retained for use during periods of shortage. In
this way the variability of the stream is reduced, and the effects of
droughts can be mitigated. Regulation is the amount of water that is stored
or released from storage during a time period. The degree to which a
given reservoir can regulate flow is determined by the ratio of its capacity
to the volume of streamflow encountered during the time interval. In the
United States, many rivers are regulated to one degree or another and
considerable amounts of water have been made available through the
development of storage (19). In some river basins there are opportunities
for substantial gains to be made in water supply development through
storage. Ultimately, however, such regulation follows a law of diminishing
Through regulation, the safe yield of a stream can be made to ap-
proach its average annual flow as the level of storage approaches full
development. While this ideal cannot be attained in reality, safe yields of
75 to 90 percent of the mean annual flow can be achieved through regu-

- .~--~L

L :*' ? *..- *-. ^


lation. Such increases have significant implications for regional water
Since the environmental movement of the 1960s, there has bet
widespread opposition to the construction of new reservoirs. Neverth
less, they are often the most efficient and cost-effective means for increm
ing water availability. Additional storage should thus be evaluated aloi
with other options when plans are being made for meeting water require

Reservoir Storage Allocations
Most modern reservoirs are multipurpose in nature. Storages are allocate
for a number of functional uses. These uses are often incompatible and
such require that compromises be made. The nature of the princip
functional uses is described here along with comments about the confli
that these uses generate.

Flood control was one of the earliest uses made of reservoirs. The stora;
allocated for this purpose is intended to be available during times
flooding and is expected to be depleted as soon after the flood event
possible in order to make space available for the next critical event. Su
an operation troubles those who seek to retain as much water as possil
for water supply and other purposes. Given the limitations on stora.
capacity at any reservoir site, it is easy to understand that those who wa
to store water for later use do not favor the retention of empty stora:
space. Although flood control storage is not compatible with other typ
of storage, recent advances in management techniques related to rest
voir operation and hydrologic forecasting indicate that some storage al'
cations for flood control might be modified to better accommodate oth
uses. The design floods used to set storage allocations for large reserve
are the probable maximum flood, the standard project flood, and t:
frequency-based flood (20). The probable maximum flood is the flood th
could occur as the result of the most severe storm that is consider,
reasonably possible to occur. The standard project flood is obtained frc
analysis of severe storms that were recorded in the general vicinity of t
drainage basin. Frequency-based floods are those based on studies of lo
records that can be expected to yield the probabilities of extreme even

Many reservoirs are designed to accommodate recreation as well as t
conventional uses of water supply. Most recreationists desire use of t
reservoir during the summer period, a period during which withdraw
uses are usually highest. For optimal boating, swimming, and associate
uses, the pool level should be maintained relatively constant. This conflict