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Ponce de Leon channel improvement studies Punta Gorda, Florida

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Ponce de Leon channel improvement studies Punta Gorda, Florida
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Wang, Hsiang
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Coastal and Oceanographic Engineering Department, University of Florida
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Coastal Engineering
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North America -- United States of America -- Florida

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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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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 Bathym etric Survey ................................................................................................. 4
3.2 Tidal Elevation M easurem ents ................................................................................. 4
3.3 Current M easurem ent .............................................................................................. 8
3.4 Bottom M aterial Size Distribution ........................................................................... 8
3.5 W ave Characteristics ............................................................................................... 8
4. Analysis ................................................................................................................................ 16
4.1 Shoreline Changes from Aerial Photographs ............................................................ 16
4.2 Evidence from Field M easurem ents ......................................................................... 16
4.3 Stability Analysis of Ponce De Leon Inlet-channel System ....................................... 20
5. Sum m ary of the State of the Channel ..................................................................................... 23
6. Recom m ended Improvem ent M easures .................................................................................. 23
7. Im pact Assessm ent of the Proposed Improvem ent Plan .......................................................... 24
7.1 Impact on A djacent Shores ....................................................................................... 24
7.2 Enviorm ent Im pact ................................................................................................... 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 maps ........................................................................ 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. 1
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-

Punta Gorda City Park

CHARLOTTE HARBOR

Mangrove Point I

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




CHARLOTTE HARBOR

0 500m

0
Ponce De
Inlet

X Station 1 + Station 2 0 Station 3 0 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 Mnet 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 l b

Horizontal Distance (m)

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

0 1-2
E
0
w




3
December 1995

Figure 5: Field measured tidal elevations and currents.

0.4

0.1 F

-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 ) Oddmeter data




3.3 Current Measurement

Current measurements were collected by two kinds of instruments; an electric-magnetic current meter manufactured by Marsh McBurney 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 Oddmneter 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 mmn 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=1 6:43

S0.41

( 0.21

e0.39 e 0.2_8- S e037 11/30/95
Flood current(m/s)

Time=16:55

0.42 ( 0.51 E 0.54
0.36 ."0.39

Station 3

@ 074

Time=17:09

@050

e 0.26

. .. ......26

-2-

Horizontal Distance (m)

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

S0.44

U .
2-

0
Z
IC.
E
0
4
>-2
(w

Station 2

Time=1 6:43

Station 1




Station 1Tme140

00.32

00.31

EbbS0m11130/95
Ebb current(m/s)

Time=14:34

0 0.30

00.2 00.23 0

Station 3

, "l .4

Time=1 5:02

00.23 00.23

U. L.10
00.30 p.21

) 5 10 15 20 25 3
Horizontal Distance (m)
Figure 7: Field measured ebb tidal currents at three cross sectiions

00.25

-2 -

Station 2

S0.27

0 0
Z
tf
O
0
>-2
a)
w

0 0.30

Station 2

Time=1 4:07

Station 1




0 50m
Ponce De Leon Inlet

0.5m/sec
.5m/sec

CHARLOTTE HARBOR

Figure 8: Field measured flood flow pattern.




0

I
50m

Ponce De Leon Inlet

0.5m/sec

CHARLOTTE HARBOR

Figure 9: Field measured ebb flow pattern.
12




0 50m

0.19

+ 0.15 + 0.35

+ 0.18 + 0.20
+ Ponce De Leon
+2
0.15
IARLOTTE ARBOR
Figure 10: Sand sample locations and measured median sand size in mm.

Inlet

CF HJ

L
0




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:
gLH -0.283tanh[O.O 125( gF )0.421,
U2 U2
gT =1.2tanh[0.077(gF).25],
27cU 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).

W 0
(U
0
w 80
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 corners 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




I
0 60m

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




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
140 1995/1996 Survey
/ --1991 Survey
Contour in mn (NGVD)
160 0
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 corner. 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 Escoffler 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.. The ac value corresponding to the peak Urn 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




maxium velocity vs. entrance cross-section area
1.5 . ,. .
unstable .. stable.-'
Ixes
C) 1. / "
0 /\.- prsn odto
E\.. i
E
E 0.5- /
Old Barge Canal Open ..
0 -I
100 101 102
cross section area, mA2
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 x104', N = 0.85
" For inlets with jetties:
b = 9.0 x 10'. 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: Un = 0.75 rn/s andA = 50 in2. To reach an equilibrium in a natural state with no jetty the channel cross section will be further enlarged to, say, around 60 in'. 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 m' 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 in'.




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 in 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 in' 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




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 seawaLt [ A approximate sea bottom
400-1000 lb Armor Stones
1. 4r
(4Ft) "+1.58m (+4.5ft)
0 0-" 0 r- 0 0 0 0 .. .
11 0
0000 O OO O 0 000 0 0 0 00 0 0 -0.53 (-.5
3'-8' quarry
7,7n (22Ft) A-A Cross Section
Figure 18: Design of north jetty structure.




50-300 lb riprap

+0.0 m

* graded surface Fill
1 Ft gravel bltackket

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

+1.2r) (+3,5f t)




Colony Pt.

Shoal / Spit /
I\

CHARLOTTE HARBOR

Punta Gorda City Park

/
V

MangroVe Point I

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

- Winter Drift
E> Summer Drift

A5
.01 *B




K]

. . . I I
0 50n
-
- - - - -- --
.......- PonceDe
0.3r/sec
-OTTE
OR
Figure 21: Typical flood current pattern for unimproved inlet.

I1

Leon Inlet

CHARL HARB

A




% % 0 50
- - - - - Ponce De
0.3m/se
OTTE
OR
Figure 22: Typical ebb current pattern for unimproved inlet.

n

Leon Inlet

CHARL HARB

.4k

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 rn (5,000 fe ) of bottom will be covered. In exchange, a 850 n] (30,000 f? ) inter tidal rock mound environment will be created.
Along south shore, approximately 1,000 m2 (10,000 ff ) 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.




4d

* .*0 50m
- N- -- - -
Ponce De Le CHARLOTTE......
HARBOR
0.3mr/sec
oral Drift
Figure 23: Typical flood current pattern for inlet with two jetties.

'on Inlet




CHARLOTTE
HARBOR
Littoral Drift

0 50m
-- --------- -- ---- -- - - --- --.. Ponce De Leon Inlet
0.3m/sec

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




NW Waves

0 50m
I....... \ ,
-.
-- -. -. - - -.
Ponce De Leon Inlet
0.3mr/sec
...,

CHARLOTTE HARBOR

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

N




NW Waves

0.. 50m
- .S I
-~ ~ ~ ~ ~ - -- -- (t~dwZ~f
- - -. -.- .- -- -- - - - ,- , Ponce De Leon Inlet
' / Fill
0.3m/sec
I
RLOTTE RBOR

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

CHA HA

.4k




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.