• TABLE OF CONTENTS
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 Cover
 Table of Contents
 List of Figures
 Introduction
 General description of the study...
 Field investigations
 Analysis
 Summary of the state of the...
 Recommended improvement measur...
 Impact assessment of the proposed...
 References






Title: Ponce de Leon channel improvement studies Punta Gorda, Florida
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 Material Information
Title: Ponce de Leon channel improvement studies Punta Gorda, Florida
Physical Description: Book
Creator: Wang, Hsiang
Publisher: Coastal and Oceanographic Engineering Department, University of Florida
Publication Date: 1996
 Subjects
Subject: Coastal Engineering
Spatial Coverage: North America -- United States of America -- Florida
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Funding: This publication is being made available as part of the report series written by the faculty, staff, and students of the Coastal and Oceanographic Program of the Department of Civil and Coastal Engineering.
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Table of Contents
    Cover
        Cover
    Table of Contents
        Page i
    List of Figures
        Page ii
        Page iii
    Introduction
        Page 1
    General description of the study area
        Page 1
        Page 2
        Page 3
    Field investigations
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
    Analysis
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
    Summary of the state of the channel
        Page 23
    Recommended improvement measures
        Page 23
        Page 24
    Impact assessment of the proposed improvement plan
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
    References
        Page 36
Full Text



UFL/COEL-96/004


PONCE DE LEON CHANNEL IMPROVEMENT STUDIES
PUNTA GORDA, FLORIDA






by


Hsiang Wang
Lihwa Lin
and
Xu Wang


May, 1996


Sponsored by


CITY OF PUNTA GORDA, FLORIDA








Table of Contents


1. Introduction ..................................................................................................................... 1

2. General Description of the Study Analysis ............................................................. ............... 1

3. Field Investigations ................................................................................................................ 4
3.1 Bathymetric Survey ............................................................................................ 4
3.2 Tidal Elevation Measurements ................................................................ ................. 4
3.3 Current Measurement ............................................................. ........................ 8
3.4 Bottom Material Size Distribution ............................... ..... ...................... 8
3.5 Wave Characteristics .......................................................................................... 8

4. A analysis ................................................................................................................................16
4.1 Shoreline Changes from Aerial Photographs ..........................................................16
4.2 Evidence from Field Measurements .........................................................................16
4.3 Stability Analysis of Ponce De Leon Inlet-channel System ....................................... 20

5. Summary of the State of the Channel....................................................................................23

6. Recommended Improvement Measures.................................................................................23

7. Impact Assessment of the Proposed Improvement Plan.........................................................24
7.1 Impact on Adjacent Shores.......................................................................................24
7.2 Enviorment Impact...................................................................................................31

References............................................................................................................................36








List of Figures

Figure 1: Ponce de Leon Inlet and the associated canal network. ........................ 2

Figure 2: Locations of field data measurement stations ................................ 3

Figure 3: Field surveyed bathymetry of Ponce de Leon Inlet. .......................... 5

Figure 4: Field surveyed profiles of Cross Sections 1, la and lb. ........................ 6

Figure 5: Field measured tidal elevations and currents ................................ 7

Figure 6: Field measured flood tidal currents at three cross sections. ..................... 9

Figure 7: Field measured ebb tidal currents at three cross sections........................ 10

Figure 8: Field measured flood flow pattern. ..................................... 11

Figure 9: Field measured ebb flow pattern. ..................................... 12

Figure 10: Sand sample locations and measured median sand size in mm ................. 13

Figure 11: Wind rose diagram (Fort Myers International Airport) ...................... 15

Figure 12: Comparison of Ponce channel configurations from 1981, 1988 and 1992 aerial
photographic m aps............................................................................................. 17

Figure 13: Field surveyed bathymetric contours of Ponce de Leon Inlet (1991)........................18

Figure 14: Field surveyed bathymetric contours of Ponce de Leon Inlet (1995)........................18

Figure 15: Comparison between field surveyed bathymetries of 1991 and 1995/1996................19

Figure 16: Escoffier stability diagram for Ponce de Leon Inlet............................................21

Figure 17: A schematic of proposed plan................................................. .......................... 25

Figure 18: Design of north jetty structure.............................. ................ ............................26

Figure 19: Design of south shore riprap structure....................................................................27

Figure 20: Littoral drift environment near Ponce de Leon Inlet...........................................28








Figure 21: Typical flood current pattern for unimproved inlet....................................................29

Figure 22: Typical ebb current pattern for unimproved inlet.......................................... .....30

Figure 23: Typical flood current pattern for inlet with two jetties.............................. ...........32

Figure 24: Typical ebb current pattern for inlet with two jetties.................................... .....33

Figure 25: Typical flood current pattern for inlet with one jetty........................................34

Figure 26: Typical ebb current pattern for inlet with one jetty....................................................35








PONCE DE LEON CHANNEL IMPROVEMENT STUDIES
PUNTA GORDA, FLORIDA


1. Introduction

Ponce de Leon channel serves as a major boat access for the canal community in the City
of Punta Gorda. The channel which cuts through a tidal marsh land is flanked on the two sides by
salt vegetation mainly of mangrove community. Over the past number of years, falling mangroves
along the channel banks have caused concern.

In 1992, the Department of Coastal and Oceanographic Engineering, University of Florida
was commissioned by the City to conduct a study to determine whether Ponce de Leon Channel
suffers erosion and whether any remedial measure should be taken (Wang, et al, 1992).

The main conclusions of that 1992 study were:

1. Bank erosion is most visible in the lower reach of the Ponce channel near the Charlotte
Harbor entrance. It is determined that the combined wind wave and tidal current force is the major
cause. Of the two factors, wind wave appears to play an important role because of its dynamic
nature.

2. In the middle and upper reach of the channel, tidal current is the main erosional force. The
observed tidal current strength, in the range of 0.4 to 0.6 m/s, is a moderate erosional force. The
closing of the barge canal on the south side of the Ponce channel is concluded to contribute to the
increase in current strength in the channel. The Ponce channel is likely still in a state of adjustment.

The main recommendations from that study were:

1. To reopen the old barge canal to reduce the erosional stress of Ponce channel.
2. To improve channel entrance with stabilizing structure.
3. To defer taking measures for channel bank protection until further evidence suggest that
such protective measures are warranted.

The present study is commissioned by the City to re-examine the Ponce channel erosional
potential and propose remedial measures as deemed necessary.

2. General Description of the Study Area

The City of Punta Gorda is located at the east side of Charlotte Harbor on the south bank
of Peace River. Charlotte Harbor is a shallow tidal estuary subject to tidal action from the Gulf of
Mexico through Boca Grande Pass. The tide is a mixed diurnal and semidiurnal with the diurnal
being the more prominent component and an associated spring and nip tidal range in about 1.0 m








PONCE DE LEON CHANNEL IMPROVEMENT STUDIES
PUNTA GORDA, FLORIDA


1. Introduction

Ponce de Leon channel serves as a major boat access for the canal community in the City
of Punta Gorda. The channel which cuts through a tidal marsh land is flanked on the two sides by
salt vegetation mainly of mangrove community. Over the past number of years, falling mangroves
along the channel banks have caused concern.

In 1992, the Department of Coastal and Oceanographic Engineering, University of Florida
was commissioned by the City to conduct a study to determine whether Ponce de Leon Channel
suffers erosion and whether any remedial measure should be taken (Wang, et al, 1992).

The main conclusions of that 1992 study were:

1. Bank erosion is most visible in the lower reach of the Ponce channel near the Charlotte
Harbor entrance. It is determined that the combined wind wave and tidal current force is the major
cause. Of the two factors, wind wave appears to play an important role because of its dynamic
nature.

2. In the middle and upper reach of the channel, tidal current is the main erosional force. The
observed tidal current strength, in the range of 0.4 to 0.6 m/s, is a moderate erosional force. The
closing of the barge canal on the south side of the Ponce channel is concluded to contribute to the
increase in current strength in the channel. The Ponce channel is likely still in a state of adjustment.

The main recommendations from that study were:

1. To reopen the old barge canal to reduce the erosional stress of Ponce channel.
2. To improve channel entrance with stabilizing structure.
3. To defer taking measures for channel bank protection until further evidence suggest that
such protective measures are warranted.

The present study is commissioned by the City to re-examine the Ponce channel erosional
potential and propose remedial measures as deemed necessary.

2. General Description of the Study Area

The City of Punta Gorda is located at the east side of Charlotte Harbor on the south bank
of Peace River. Charlotte Harbor is a shallow tidal estuary subject to tidal action from the Gulf of
Mexico through Boca Grande Pass. The tide is a mixed diurnal and semidiurnal with the diurnal
being the more prominent component and an associated spring and nip tidal range in about 1.0 m












Colony Pt.


Shoal /
Spit /
I-
I/


Punta Gorda
City Park


CHARLOTTE
HARBOR


MangroVe
Point 1


Figure 1: Ponce de Leon Inlet and the associated canal network












CHARLOTTE
HARBOR


0 500m


Ponce De (
Inlet


K Station 1
+ Station 2
o Station 3
a Channel
Marker 8
x Channel
Marker 18


Figure 2: Locations of field data measurement stations.








(3ft) and 0.6 m (2 ft), respectively. The average depth of the Charlotte Harbor bay is only about 2.6
m (8 ft). Peace River is the largest river emptying into the estuary with an estimated annual runoff
of 85 m3/sec (3,000 cfs).

Ponce de Leon Inlet is one of the major boat accesses for the Punta Gorda community and
serves as the major outlet for a large canal network shown in Figure 1. With the closure of the old
barge canal, the only other active outlet is a canal opening at Colony Pt. due north.

3. Field Investigations

A field measurement study was carried out from November 30 to December 5, 1995. The
field data measurements include: (1) tidal elevation, (2) current, (3) tidal flow pattern, (4)
bathymetric survey, and (5) bottom material sampling. Tidal elevations were measured during the
study at two different locations, one near the inlet entrance at Channel Marker 8 in the Charlotte
Harbor and the other near the junction of inlet and canal network at Channel Marker 18 (Figure 2).
Current measurements include analog data collected by a Marsh McBurney current meter installed
just inside the inlet (Station 1) and one-minute average current data measured by the Oddmeter at
three cross sections designated as Stations 1, 2, and 3 in Figure 2. Tidal flow patterns were
measured in the inlet entrance area by tracking drogues during both flood and ebb tidal cycles.
Bathymetric survey and sand sample collections were also carried out in the inlet entrance area. A
second field trip was made on March 1,1996 to collect supplement data on bottom bathymetry,
channel cross section measurements and bottom material sampling.

The results from the field measurements are summarized here.

3.1 Bathymetric Survey

Figure 3 presents the inlet bathymetry that was surveyed on December 2, 1995 and on
March 1, 1996. The numerical numbers are the actual depths of soundings in meter with reference
to NGVD. A number of cross sections along the channel based on the soundings are constructed
and plotted in Figure 4. The stations identifications are given in Figure 3.

3.2 Tidal Elevation Measurements

Figure 5 shows the result of water surface level fluctuation measurements from two tide
gage locations, one near the inlet entrance at Channel Marker 8 in the Charlotte Harbor and the
other near the upper reach end of the inlet at Channel Marker 18. The variations of the measured
water surface levels are mainly tidal induced as there is no major storm or runoff event occurred
during the data measurement period. The patterns of water surface fluctuations at the two gage
locations are very similar, with almost identical shape and magnitude but with a slight phase shift.
The tidal elevation measured near the upper reach end of the inlet at Channel Marker 18 shows a
time lag of about 6 to 8 minutes to that measured just outside the inlet at Channel Marker 8.



















































Figure 3: Field surveyed bathymetry by soundings in meter (NGVD).




5































Station b


I I I I I
5 10 15 20 25

Horizontal Distance (m)



Figure 4: Field surveyed profiles of Cross Sections 1, la and lb.


0
z



0
- -2

LU

















































3
December 1995


Figure 5: Field measured tidal elevations and currents.


0.1 1


-0.1



-0.2



-0.3



-0.4



-0.5
0


- Tide at Channel Marker 8
- Tide at Channel Marker 18
..... Current at inlet entrance
X Oddmeter data


0.4-



0.3-








3.3 Current Measurement


Current measurements were collected by two kinds of instruments; an electric-magnetic
current meter manufactured by Marsh McBurey was installed next to a boat ramp just inside the
inlet (Station 1) and an Oddmeter hand held type was used to take one-minute averaged current
speed at three different channel cross sections (Stations 1, 2, and 3 as shown in Fig.2). The
stationary current data measured on the November 30 and December 1, 1995, are also shown in
Figure 5, along with the tidal elevation records. It can be seen that the current data are consistent
with the tidal differentials caused by the time lags between the upper and lower reaches of the inlet.
Figures 6 and 7 present the Oddmeter current data measured on November 30 from the three cross
sections at Stations 1, 2, and 3. The Oddmeter data from Station 1 are also plotted against the
Marsh McBurney current meter data in Figure 5. It is seen that the measured current values from
Oddemeter and from Marsh McBurney were very close during the flood cycle. However, current
values from Marsh McBurney current were smaller than that from the Oddmeter during the ebb
cycle. Since the Marsh McBurney current meter was installed close to the north bank just inside the
inlet entrance the current at this location is probably influenced by an ebb induced return flow (this
can be seen from the ebb current circulation patterns given in Figure 9). The Oddmeter data are
taken inside the channel and thus, are not influenced by the return eddies.

Flood and ebb current patterns in the inlet entrance area are determined by tracking
drogues. Figures 8 and 9 display the results of the measured flood and ebb flow patterns,
respectively. During flood tide, flow is more uniform as it enters the inlet like entering a funnel.
During ebb tide, flow returns to the bay like a jet causing flow entrainment from the sides, thus,
inducing return eddies. Currents are much stronger in the main channel than the shallower flats on
the side. The major axis of Ponce de Leon channel is nearly aligned with east-west. The dominant
flow directions for both flood and ebb near the entrance are turning slightly into a SWW-NEE
orientation.

3.4 Bottom Material Size Distribution

Sand samples were collected in the main channel and in the shallow water area outside the
inlet. Figure 10 shows the location of sand samples collected as well as the median size (in mm.)
obtained from the sand samples. Bottom material is found to be coarser in the main channel with
a mean diameter in the order of 0.3-0.35 mm, whereas, much finer outside the main channel with
sand size reduced to less than 0.2 mm in the shallows.

3.5 Wave Characteristics

Owing to the limited scope of the project, waves are not measured. Estimations are based
upon fetch and wind conditions.

Punta Gorda is hidden in the north end of Charlotte Harbor protected by a head land from









Station 1 Time=l 6:43


0.41


S0.21


Fo0.39 O28curr
e- S 0.37 1/30/9 5
Flood current(m/s)


Time=16:55


S0.42 0.51 0.54

S0.36 ."0. '
-B.39


074


Time=17:09


5 0.50


e 0.26


S0.54

@059 p.33


Horizontal Distance (m)


Figure 6: Field measured flood tidal currents at three cross sections


e 0.44


U




-2


Station 2


0
Z

E

0
c> -2
(,
w


Station 3


Time=1 6:43


Station 1


-2 L











Station 1Tie140


S0.32


00.31


Eb0.22 02 0.m26 '
SS ^- 11/30/95
Ebb current(m/s)


0.30


Time=1 4:34


8 0.30


S0.22 0.

0.23 .


Station 3
rn-


Time=1 5:02


0.23


a 0.23


V. 0.310

00.30 00.21


Horizontal Distance (m)


Figure 7: Field measured ebb tidal currents at three cross sections


0.25


0.




-2


Station 2


0.27


S0




0
S-2


Sr\ 4 0


-2L


Station 2


i I


Time=1 4:07


n oitatS 1

















0 50m









SPonce De Leo


in Inlet


0.5m/sec


CHARLOTTE
HARBOR


Figure 8: Field measured flood flow pattern.


















0


50m


Ponce De Leon Inlet


0.5m/sec


CHARLOTTE
HARBOR


Figure 9: Field measured ebb flow pattern.

12


A
















0


50m


0.19


+ 0.15

+ 0.35


+ 0.18 + 0.20
+ Ponce De Leon
+.2
0.15
.23

0.




IARLOTTE
ARBOR






Figure 10: Sand sample locations and measured median sand size in mm.


Inlet


CH
H


A








the main water body. Swells from offshore are unlikely to create heavy wave conditions at Punta
Gorda. Based on the wind information obtained from the record of the Fort Myers International
Airport from Oct.1994 to Sept. 1995, the wind rose diagram was constructed as shown in Figure
11. The most likely sources of waves are:

(1) Wind waves generated in the Charlotte Harbor due to southerly wind blowing along the
major axis of the Harbor (approximately oriented in N-S direction). These waves then
reach the Ponce de Leon Inlet site at a rather sharp angle but with reduced magnitude.
(2) Local wind waves generated due to westerly and northwesterly winds. The fetches are
shorter than that of the southerly wind, but waves from these direction will impinge
directly upon the inlet, particularly towards the south shoreline.

A crude estimation on the wave conditions due to both sources is made based on SMB
method ( Shore Protection Manual, 1994). The basic equations are:

gH=0.283tanh[0.0125( gF)042],
U2 U2

gT =1.2tanh[0.077( F)0.25],
27tU U2



Where, H is the significant wave height, T is the significant wave period, and U is the wind speed
and F is the fetch length.

(1) Waves due to southerly wind. Wind waves in the main water body of Charlotte Bay are
estimated approximately 24,000 m along the longitudinal axis which is broken into two
sections, the first section of 8,000 m is assumed to have an average constant depth of
3 m (10 ft) and the second section of 16,000 m is assumed to have an average constant
depth of 4.5 m (15 ft). The wind is from south with unlimited duration.
(2) Waves due to northwesterly wind. Waves due to northwesterly wind are estimated on
the assumption of a shallow basin of constant depth of 2 m (6 ft) and a fetch length of
4,800 m.

The computed wave conditions as estimated for the design purpose are given in Table 1.

Table 1: Estimated Wind Wave Conditions at Ponce de Leon Inlet
wind direction wind speed (m/s) wave height (m) wave period
(sec)

southerly wind 11.0 1.0 4

northwesterly wind 11.0 0.6 3















Wind Rose Diagram (Wind Speed >10 kt)
90


0 E


270


Direction




Figure 11: Wind rose diagram (Fort Myers International Airport, Oct. 1994 Sept. 1995).


a)
U



W .980
0

E
z








4. Analysis


4.1 Shoreline Changes from Aerial Photographs

Long term shoreline changes along the Ponce de Leon channel were traced from aerial
photographic maps of 1981, 1988 and 1992. They are compared in Figure 12. The horizontal scale
of the aerial photos used in the study is of 1 in. = 200 feet. The shorelines are determined from the
color differences between water and vegetation. Strictly speaking, they are actually vegetation lines
along the shore as oppose to the actual earth banks or sandy shorelines. Owing to the large
horizontal scale and the approximate nature of the shoreline tracing technique the comparisons are
not meant to be quantitative. Nevertheless, prominent features on shoreline changes are discernable.

From Figure 12 the following features are observed:

1. Bay front shoreline on the south side of the entrance has experienced the most
pronounced recession. The entrance has also become wider and bank recession gradually progressed
into the channel.
2. In the mid reach, the channel has a clear tendency to straighten its path. As a
consequence, narrower sections can become wider during the process of channel straightening.
3. This straightening process has a major effect around the bend. The covers of the bend
were significantly eroded away and accretions also occurred along the outer bend, which result in
reducing the curvature of the bend (see detail around the bend in Figure 12).

4.2 Evidence from field measurements

Bathymetric survey in the vicinity of the Ponce channel entrance was also conducted in
August, 1991 (Wang, et.al., 1992) and the results are reproduced in Figure 13. Figure 14 shows
the new bathymetric survey of 1995/1996. Figure 15 shows the comparisons between the two. It
clearly reveals that the shoreline adjacent to the south side of the inlet suffered steady erosion. The
average rate of this shoreline retreat between 1991 and 1995/1996 is about 0.5 m per year. The
erosion also propagates into the channel entrance causing bank retreats on both sides and the
resulting channel widening. However, the overall bank erosion appears to be mild with overall bank
recession being no more than 1.0 m on both sides over the four years.

The bottom bathymetry of Ponce channel in the vicinity of the entrance remains stable. This
is clear as seen from the bathymetry comparisons in Figure 15. From the cross section plots given
in Figure 4, it is seen that the main channel is about 2 m deep and this depth has largely been
maintained since last survey in 1991. The position of the main channel also remains rather stable.
However, the main channel near the entrance does not exactly aligned with the bank but forms an
angle with the centerline as shown in Figure 15. This is mainly because the ebb flow from the
channel is being deflected towards SW by the hardened shoreline protrusion at the tip of the
















N


0 6m
0 60m


Figure 12: Comparison of Ponce channel configurations from 1981, 1988 and 1992 aerial
photographic maps.





























Figure 13: Field surveyed bathymetric contours of Ponce de Leon Inlet (1991).


Figure 14: Field surveyed bathymetric contours of Ponce de Leon Inlet (1995).
18



























120 -

\ -- 1995/1996 Survey
-10 -1991 Survey
0 Contour in m (NGVD)
160/
350 300 250 200 150



Figure 15: Comparison between field surveyed bathymetries of 1991 and 1995/1996.


Ponce de Leon park. This current deflection can also be detected from the measured current
patterns shown in Figures 8 and 9. To correct the channel alignment, it is necessary to realign this
protruding tip.

Based on the present bathymetric measurements, shorelines on both sides of the entrance
are found to be erosional. The shoreline on the north side in the Ponce de Leon park is hardened
by a section of seawall; therefore, it can not physically retreat. The erosional stress, however, is
evident from the exposed bare rocks at the toe of the seawall. The south shoreline, on the other
hand, is unprotected and shoreline retreat is clearly observed from both field data and aerial
photographic analysis. As shoreline retreats the channel entrance becomes wider. More wave energy
will be admitted into the channel, consequently causing the bank erosion expand into the channel.








From shallow water dives and the material samples taken from the sea bed, there is no
evidence of bottom vegetation around the inlet vicinity both inside and outside the entrance. Bottom
material is found to be mainly sand and shell fragment. The material is coarser in the main channel
with a mean diameter in the order of 0.3-0.35 mm, whereas, much finer outside the main channel
with sand size reduced to less than 0.2 mm in the shallows.

4.3 Stability analysis of Ponce de Leon inlet-channel system

Jointly, the vast canal network and salt water ponds shown in Figure 1 behave like a tidal
reservoir that exchanges bay water through connecting inlet openings. Before 1983, the exchange
mainly took place through two outlets, the Colony Pt. outlet on the north end and the old barge
canal entrance on the south west comer. Ponce de Leon Channel was plugged at the end, thus, only
served as outlet of the local tidal lagoons but had little exchange with the canal network. In October
1983, the end plug of Ponce de Leon channel was removed. In exchange, the old barge canal was
blocked at both ends. The canal network now connects to the bay through Ponce inlet and Colony
Pt. In the 1992 study, the analysis showed that the exchange between Ponce channel and the canal
network was about 15 to 16 m3/s on a tidal-cycle time averaged basis. This increase in discharge
would naturally cause the Ponce channel to adjust.

A relevant question to ask now is that whether the Ponce channel is still undergone adjust.
And, if it is whether the channel will be stable or whether it has eventually reached a new state of
equilibrium. These are difficult questions particularly for as complex a hydraulic system as the
present canal network. So far only semi-empirical approach of approximate nature is feasible. Such
an analysis will be carried out here.

An Escoffier diagram is prepared for the Ponce inlet as presented in Figure 16. This diagram
plots the relationship between maximum velocity in the channel, U,, and the throat cross section
of the channel, A, based on energy and flow continuity considerations. Thus, this relationship has
a solid theoretical basis although a number of coefficients such as friction coefficient, entrance and
exit losses have to be estimated empirically. For the application to Ponce inlet, an equivalent tidal
prism value also has to be determined from the canal network. This was accomplished by estimating
the surface area covered the ponds and canals. The Escoffier curve consists of two branches, a
rising branch on the left and a falling branch on the right separated by a peak value U.r The ac value
corresponding to the peak U, is usually treated as the minimum cross sectional area required to
maintain a stable inlet. The present maximum flow condition of Ponce inlet plotted on this diagram
is on the right hand side of the branch. Thus, the inlet is hydraulically stable. In the 1992 study, it
was shown that if the old barge canal were open, the velocity in Ponce channel would have been
reduced by nearly a half. The corresponding Escoiffer curve for this hypothetical condition is also
plotted in the figure by the dotted line. This curve is seen to fall below the Escoffier curve of the
present hydraulic condition.

To assess whether the inlet is erosional, accretional or in equilibrium, the inlet's sediment
transport capability due to the tidal current must be examined. If this transport capability is less








maximum velocity vs. entrance cross-section area
1.5. I I




unstable -r -- stable .-





S/ present condition



E 0.5 -1 te

no etties


Old Barge Canal Open


0
100 101 102
cross section area, m^2



Figure 16: Escoffier stability diagram for Ponce de Leon Inlet.

than the rate of sediment supply, the inlet will be accretional; conversely, if the inlet transport
capacity is more than the rate of sediment supply the inlet will be erosional. For a channel that is
hydraulically stable, the above process will eventually bring the inlet to a state of equilibrium such
that the rate of transport equals the rate of sediment supply. To estimate the equilibrium state for
Ponce channel, an empirical formula on sediment balance as proposed by O'Brien (1966) is used.
It takes the form:

a =bPN







where P, is tidal prism and b and N are empirical coefficients. For Gulf coast inlets, the following
values are suggested:

* For natural inlets:

b = 6.3 x0-4, N = 0.85

* For inlets with jetties:

b = 9.0 x 10-4, N = 0.85.

The sediment curves corresponding to the above two cases are also plotted on Figure 16. If the
present condition is above the sediment curve the inlet's sediment transport capacity is larger than
the rate of supply. Thus, the inlet cross section needs to be enlarged and current velocity reduced
to reach an equilibrium, or, the inlet is presently in an erosional state. On the other hand, if the
present condition falls below the sediment curve, the inlet cross section needs to be reduced to reach
an equilibrium, or the channel is presently in an accretional state. The associated inlet hydraulic
change has to follow the Escoffier curve. Therefore, the intersect of the Escoffier curve with the
sediment curve represents the equilibrium condition.

At present, the inlet has no jetty structure. This condition is seen to be above the sediment
curve of no-jetty; or the inlet is erosional. To reach equilibrium, the channel cross section has to
increase and the channel velocity has to decrease. The present mean hydraulic conditions are: U.
= 0.75 m/s andA = 50 m2. To reach an equilibrium in a natural state with no jetty the channel cross
section will be further enlarged to, say, around 60 m2. The present hydraulic condition, however,
is much closer to the equilibrium condition with jetties. Therefore, an addition of jetty will bring the
inlet closer to equilibrium without further enlarging the cross section.

From Figure 16 the influence of opening the barge canal is evident. Under that condition,
the equilibrium cross section is in the order of 40 m2 with a corresponding maximum channel
entrance velocity in the order of 0.45 m/s. Thus, opening the barge canal will cause the present
Ponce channel to reduce its cross section as well as current velocity.

It should be noted that the above analysis is largely empirical and imprecise. The present
knowledge on inlet hydraulics and sediment transport is grossly inadequate. The results presented
here could only be viewed as qualitative to serve as trend indicators and not to be overly relied
upon in the actual design. Nevertheless, the results clearly demonstrate that the removal of Ponce
channel plug coupled with the old barge canal closure increased the erosional stress in Ponce
channel. Accordingly, the channel apparently has not yet reached a new equilibrium condition.
However, the channel is also not overly stressed. With the exception of a few local spots the
channel as a whole has not experienced major changes over the last four hears. In all likelihood, the
channel will gradually reach a new equilibrium with a straighter alignment and an enlarged cross
sectional area in the neighborhood of 60 m2.








5. Summary of the state of the channel


Based on the analysis presented in the previous sections, Ponce de Leon Channel, although
suffers mild bank erosion in the past years, is judged to be overall stable. There are two local
segments where shorelines and bank alignment have undergone significant changes, one near the
entrance and the other around the bend.

Near the entrance, shorelines on both sides are under erosional stress. Since the north side
is hardened with seawall, actual erosion takes place only along the south shore. As the shoreline
retreats, the entrance becomes wider and erosion extends into the channel, but not at an alarming
rate.

The channel apparently is experiencing some natural adjustment of straightening and
widening; both are in response to the increase in current velocity. The channel straightening process
is detectable along the entire channel but is most evident near the bend. The comers on both sides
of the bend retreated appreciably as a consequence of this adjustment. They clearly have the
appearance of severe erosion. This adjustment will slow down and will stop at certain point.

From the surveys and the aerial photos, the channel is seen to be widening over the years.
The exact extent of bank erosion is difficult to determine because of the heavy growth along the
bank. Based on the surveys of 1991 and 1995/1996, the total bank erosion near the entrance is
estimated to be around 1 m on both sides during this period. Evidence from aerial photos and visual
observation suggest that the channel proper also sustained mild erosion. The extent of erosion is
more difficult to establish so as its cause. There exists no definite bank and bank protection material.
The soil could simply be loosened by the growth of roots and then carried away by the current.

In the 1992 study, it was concluded that the erosion near the inlet entrance, both inside and
outside, is mainly caused by wave action. The erosional stress inside the Ponce channel can be
relieved by opening the old barge canal.

Based on the hydraulic analysis, the present channel condition is shown to be stable.
However, it probably has not yet reached equilibrium. In its present natural state, the channel cross
section most likely will be further enlarged; the magnitude of further enlargement, however, would
not be possible to predict precisely. According to the empirical diagram presented in Figure 16 the
channel cross section may have to increase from the present 50 m2 to about 60 m2 before reaching
equilibrium. The same figure also reveal that construction of jetties will put the present channel
condition closer to equilibrium without further enlargement.

6. Recommended Improvement Measures

In the 1992 report, a number of improvement options have been proposed including
reopening the old barge canal and install inlet entrance stabilization structures. The former was ruled
out as impractical based on environmental concern. The latter was adopted by the City but








5. Summary of the state of the channel


Based on the analysis presented in the previous sections, Ponce de Leon Channel, although
suffers mild bank erosion in the past years, is judged to be overall stable. There are two local
segments where shorelines and bank alignment have undergone significant changes, one near the
entrance and the other around the bend.

Near the entrance, shorelines on both sides are under erosional stress. Since the north side
is hardened with seawall, actual erosion takes place only along the south shore. As the shoreline
retreats, the entrance becomes wider and erosion extends into the channel, but not at an alarming
rate.

The channel apparently is experiencing some natural adjustment of straightening and
widening; both are in response to the increase in current velocity. The channel straightening process
is detectable along the entire channel but is most evident near the bend. The comers on both sides
of the bend retreated appreciably as a consequence of this adjustment. They clearly have the
appearance of severe erosion. This adjustment will slow down and will stop at certain point.

From the surveys and the aerial photos, the channel is seen to be widening over the years.
The exact extent of bank erosion is difficult to determine because of the heavy growth along the
bank. Based on the surveys of 1991 and 1995/1996, the total bank erosion near the entrance is
estimated to be around 1 m on both sides during this period. Evidence from aerial photos and visual
observation suggest that the channel proper also sustained mild erosion. The extent of erosion is
more difficult to establish so as its cause. There exists no definite bank and bank protection material.
The soil could simply be loosened by the growth of roots and then carried away by the current.

In the 1992 study, it was concluded that the erosion near the inlet entrance, both inside and
outside, is mainly caused by wave action. The erosional stress inside the Ponce channel can be
relieved by opening the old barge canal.

Based on the hydraulic analysis, the present channel condition is shown to be stable.
However, it probably has not yet reached equilibrium. In its present natural state, the channel cross
section most likely will be further enlarged; the magnitude of further enlargement, however, would
not be possible to predict precisely. According to the empirical diagram presented in Figure 16 the
channel cross section may have to increase from the present 50 m2 to about 60 m2 before reaching
equilibrium. The same figure also reveal that construction of jetties will put the present channel
condition closer to equilibrium without further enlargement.

6. Recommended Improvement Measures

In the 1992 report, a number of improvement options have been proposed including
reopening the old barge canal and install inlet entrance stabilization structures. The former was ruled
out as impractical based on environmental concern. The latter was adopted by the City but








application of permit was denied by the Florida Department of Environmental Protection requesting
further study to reaffirm the need of such improvement structure and, in particular, the impact of
such structure on down drift erosion and other environmental concerns.

The present study reaffirms that channel entrance condition is clearly deteriorating and will
need immediate attention. Shoreline restoration and entrance stabilization and improvement
measures are again recommended here.

The proposed plan is shown schematically in Figure 17. The measure contains: (1) Restore
south shoreline with dredge fill and ripraping the restored beach; (2) Realign the tip of the north
shore seawall and construct a section of shore connected jetty. These measures will restore the
shoreline to its natural configuration, reduce the penetration of wave energy from northwest
direction (winter waves) and straightening the access channel to avoid potentially troublesome cross
channel shoaling.

This plan is in variance with the original plan proposed in the 1992 study but serves the same
general purpose. The 1992 plan consisted of a main jetty along the south side of inlet and a smaller
jetty on the north side. The new plan retains and lengthens the north side jetty but replaces the south
side jetty with shoreline restoration and riprapings. Both plans will serve the same function to
stabilize the channel entrance. The present plan is likely to cost less and immediately restores portion
of the eroded south side beach. The lengthening of north side jetty offers better entrance sheltering
from northwesterly and winter storms. The eventual net difference of these two plans are small.

It is also recommended here that no action be taken in the inner channel beyond the
proposed section. As stated earlier, the channel is in an adjusting period and any hasty action at this
moment is unwarranted. It would, however, be extremely useful to set a simple monitoring program
to document further changes and to observe whether the channel is approaching a new equilibrium
as predicted in this report.

Ponce inlet is located in a relatively mild environment. Low cost simple rubble structures are
recommended. Design details are given in Figures 18 and 19 for the north rock jetty and south
shoreline ripraps, respectively.

7. Impact Assessment of the Proposed Improvement Plan

7.1 Impact on Adjacent Shores

Florida Department of Environmental Protection raised specific concern on the impact of
down drift erosion by the proposed structure. This concern is addressed here.

Ponce de Leon channel is located in the middle of a marsh head land shown on the map in
Figure 20. Based on wind and wave analysis presented earlier, one can see that during winter
season, waves are mainly north west and the corresponding littoral transport direction is toward








application of permit was denied by the Florida Department of Environmental Protection requesting
further study to reaffirm the need of such improvement structure and, in particular, the impact of
such structure on down drift erosion and other environmental concerns.

The present study reaffirms that channel entrance condition is clearly deteriorating and will
need immediate attention. Shoreline restoration and entrance stabilization and improvement
measures are again recommended here.

The proposed plan is shown schematically in Figure 17. The measure contains: (1) Restore
south shoreline with dredge fill and ripraping the restored beach; (2) Realign the tip of the north
shore seawall and construct a section of shore connected jetty. These measures will restore the
shoreline to its natural configuration, reduce the penetration of wave energy from northwest
direction (winter waves) and straightening the access channel to avoid potentially troublesome cross
channel shoaling.

This plan is in variance with the original plan proposed in the 1992 study but serves the same
general purpose. The 1992 plan consisted of a main jetty along the south side of inlet and a smaller
jetty on the north side. The new plan retains and lengthens the north side jetty but replaces the south
side jetty with shoreline restoration and riprapings. Both plans will serve the same function to
stabilize the channel entrance. The present plan is likely to cost less and immediately restores portion
of the eroded south side beach. The lengthening of north side jetty offers better entrance sheltering
from northwesterly and winter storms. The eventual net difference of these two plans are small.

It is also recommended here that no action be taken in the inner channel beyond the
proposed section. As stated earlier, the channel is in an adjusting period and any hasty action at this
moment is unwarranted. It would, however, be extremely useful to set a simple monitoring program
to document further changes and to observe whether the channel is approaching a new equilibrium
as predicted in this report.

Ponce inlet is located in a relatively mild environment. Low cost simple rubble structures are
recommended. Design details are given in Figures 18 and 19 for the north rock jetty and south
shoreline ripraps, respectively.

7. Impact Assessment of the Proposed Improvement Plan

7.1 Impact on Adjacent Shores

Florida Department of Environmental Protection raised specific concern on the impact of
down drift erosion by the proposed structure. This concern is addressed here.

Ponce de Leon channel is located in the middle of a marsh head land shown on the map in
Figure 20. Based on wind and wave analysis presented earlier, one can see that during winter
season, waves are mainly north west and the corresponding littoral transport direction is toward





































Figure 17: A schematic of proposed plan.


south. This drift direction can also be seen from the breaker patterns identifiable from aerial
photographs taken during winter time. During the summer season, the littoral drift reverses its
direction from south to north as the dominant winds are now shifted from the south. However, the
supplies of littoral material from both direction are very limited. The length of shoreline north Ponce
inlet as indicated by A-B-C in Figure 20 is rather short. Moreover, the littoral material in reach A-B
is unlikely to feed towards Ponce inlet owing to its orientation. Although shoreline on the south side
is long, the break at Mangrove Point will also keep the northerly littoral transport largely limited
to a short reach between Mangrove Point and Ponce inlet. Therefore, the setting of this marsh head
land is entirely different from an open sandy coast environment. The scale of the littoral drift
environment is so small that the problem of down drift sediment transport disruption does not
actually exist. Any shoreline perturbation along this headland is purely local.

However, shoreline erosion on both sides of the entrance is evident. This erosion is clearly
local and is caused by the combined tidal and wave -induced currents. This flood and ebb current


















+1.58m (4.5ft)


existing seawall
S approximate sea bottom












400-1000 Ib Armor Stones
S1.4m
W 4 t) +1.58m (+4.5ft)

01.5 000
0 0 0 00

O 00 'O v O OO
00 OO 0 0o O~ O 0 0 o 0 o 0 0 o0 OO -0.53m (-1.5
S 3'-8' quarry

7,7m (22Ft)

A-A Cross Section















Figure 18: Design of north jetty structure.
















50-300 tb riprap


+0.0 m


' /graded surface fill


1 ft gravel blackket


Figure 19: Design of south shore riprap structure.


patterns for the present inlet condition are illustrated in Figures 21 and 22, respectively. As a
consequence, shoreline on both sides of the inlet recedes in nearly symmetrical form. There is no
evidence of a dominant littoral drift component; its existence usually will be manifested in ebb
channel re-orientation, updrift/down drift shoreline off set and/or the development of a skewed
shoal; none of the above is observed. The erosion is at the expense of local shoreline retreat. As the
entrance becomes wider, more wave energy is admitted adding erosional stress to the south
shoreline. This erosional stress gradually propagates into the channel along its entrance.

The improvement plan proposed in this study will achieve the followings:

1. Partially restore the south shoreline that has suffered noticeable erosion in the past and
stabilizes it with ripraps This restoration is expected to relieve erosional stress to the inlet entrance
and recover portion of the wet land. It will have negligible effect on littoral transport for reasons
stated above.

2. A north jetty is added to serve for two major proposes: (a). Provide shelter for inlet
entrance and reduce wave attack on south shoreline due to northwesters. (b). Intercept material
eroded from local beach and rebuild the north shore through natural process. One of the main
effects of jetties is the creation of stagnation zones behind them and the interception of longshore


+1.2m (+3.5ft)









Colony Pt.


Shoal /
Spit /
I/\


CHARLOTTE
HARBOR


B
A51-
-e B


Punta Gorda
City Park


V.'


MangroVe
Point I
I


Figure 20: Littoral drift environment near Ponce de Leon Inlet.


-- Winter Drift
-D- Summer Drift


)dl













4


S - - I "








Ponce De






0.3m/sec

-OTTE
OR





Figure 21: Typical flood current pattern for unimproved inlet.


1


Len Inlet


CHARL
HARB


Ak



















% % % 0 50r








- - - Ponce De- -
- - - -

Ponce De








0.3m/sei


OTTE
Figure 22: Typical ebb current pattern for unimproved inlet.







Figure 22: Typical ebb current pattern for unimproved inlet.


n


Leon Inlet


CHARL
HARBC


Ak


c








littoral transport as illustrated in Figures 23 and 24 for two jetties case and Figures 25 and 26 for
one jetty case. On an open sandy coast, this often is the cause of down drift erosion and eventually
shoreline offset. For the present situation the jetty only aids in retaining material eroded from the
local shoreline on the north side. The shoreline line on the south side near the inlet will be sheltered
from the northwesterly waves as shown, thus, will subject to less erosional stress.

3. A minor adjustment of boat channel alignment to eliminate the potential of cross channel
shoal formation.

For the particular environmental setting, these restoration measures are beneficial to
shoreline restoration and stabilization for the entire reach along the head land, in addition to its main
function of reducing erosional stress to the channel entrance.

7.2 Environmental Impact

The sea bottom of the proposed north jetty location is mainly covered with fine sand mixed
with shell fragment. There is no detectable patch of sea grass or other vegetation. Approximately
500 m2 (5,000 f ) of bottom will be covered. In exchange, a 850 ni (30,000 fi ) inter tidal rock
mound environment will be created.

Along south shore, approximately 1,000 m2 (10,000 if) marsh headland will be created
together with some intertidal rock environment.

It is always difficult to quantify environmental impact. However, in view of the small scope
of the project and the exchanges nature of aquatic environment and the recovery of marsh land, one
may safely conclude the net effect is environmentally beneficial.












V


4.


Ponce De Le
LOTTE .
BOR


0.3m/sec




ift




Figure 23: Typical flood current pattern for inlet with two jetties.
^LOTTE----------- /


on Inlet


CHAF
HAR






































CHARLOTTE

HARBOR













Littoral Drift


O 50m




.-%;. -- %

-----------m
---- ------- ----
--- ----- --- ---

- -- --- /__----
---0.3 rn/s
- - / '^ Ponce De Leon Inlet










y/ /0.3m/sec


Figure 24: Typical ebb current pattern for inlet with two jetties.


33














NW Waves


. .. . . . I

.0 50m





`---
vl* .* -.-.- ----- [-----


- - -- / ^^. t - - -


"'.- ..- Ponce De Leon Inlet
- , , -

/ Fill



I


0.3m/sec
)K/


CHARLOTTE

HARBOR


Figure 25: Typical flood current pattern for inlet with one jetty.


A


--
--
--





c
c










NW Waves


. ...S. .......
. . I . . .
. 50* .



------------dlaw Zqne-

S- - - ----- --

- - - - Ponce De Leon Inlet

Fill


I
I S I I I I
O.3m/sec

~RLOTTE
RBOR


Figure 26: Typical ebb current pattern for inlet with one jetty.


CH/
HAI


~


Ak








References


1. O'Brien, M.P., 1966. "Equilibrium Flow Areas of Tidal Inlets on Sandy Coasts," Proceedings
of the 10th Coastal Engineering Conference, Vol. I. pp. 676-686.

2. Escoffier, F.F., 1977. "Hydraulics and Stability of Tidal Inlets," U.S. Army Coastal Engineering
Research Center, Department of the Army, Corps of Engineers. GITI Report 13. Fort Belvoir,
Virginia 22060.

3. "Shore Protection Manual," 1984. Department of the Army, Corps of Engineers. Washington,
D.C.

4. Wang, H., J.L Lee, and L Lin, 1992. "Ponce de Leon Channel Shoaling and Erosion Studies,
Punta Gorda, Florida," Coastal and Oceanographic Engineering Department, University of Florida.
UFL/COEL-92/013.




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