2.0
GATOR SLOUGH WATERSHED AND AREA HYDROLOGY
2.1
Area Hydrology
The Gator Slough canal system watershed is located in the northwest area of Lee County and covers an area of approximately fifty-eight square miles in the counties of Lee and Charlotte. An additional thirty-three square miles are within Charlotte County mostly within the Cecil Webb Wildlife Management Area. As agreed in the planning meeting with SFWMD, this modeling study focused mainly on the Lee County portion of watershed (starting west of U.S. Highway 41). Data from the U.S. Geological Survey gauging station at U.S. Hwy 41 were used to represent runoff water entering into Lee County from the Webb Wildlife Area. The east boundary in Lee County is along portions of an abandoned railroad and U.S. 41, the south boundary is Pine Island Road, the west boundary is the Cape Coral perimeter canal. This perimeter canal outfalls to a mangrove marsh, which in turn flows to Matlacha Pass [1]. The Lee County portion of the main Gator Slough canal watershed, utilized for the calibration of the model, is relatively narrow, varying from one to three miles in width. The complete drainage basin includes Gator Slough plus three other main canals, Horseshoe, Hermosa, and Shadroe, and their secondary branches. It is about six miles in width.
The watershed is channelized for its complete length from the
Lee-Charlotte County Line to the perimeter spreader waterway system in Cape
Coral adjacent to the salt marsh/mangrove estuary.
This spreader system was built by the developer of Cape Coral to help
reduce the impact of direct flow out of the canals to the estuary in the
1970’s [2].
The spreader system is a canal that parallels the salt/marsh mangroves
zone with the purpose of causing the water to sheet flow into the wetlands.
It also provides additional salinity control for the canals which
discharge into it.
Most of the Cape Coral area in this watershed has been previously cleared
for future residential area. More
water can be expected to enter the system as northern Cape Coral builds out.
This study’s simulation has been kept consistent with current
conditions. All of the existing
natural wetlands on the west side of U.S. 41 are north of the main Gator Slough
canal flow-way. The City of Cape
Coral’s canal system could be used to divert and store fresh water from the
main flow-way. The final purpose of
this study is to verify and quantify the opportunity of raising two weirs in the
main flow-way (Gator Slough) as well as three weirs in the other main canals to
retain additional runoff water. Diverting
water into more of the Cape Coral canal system by raising weir elevations would
be consistent with several Lee County Plan policies and would likely increase
estuarine productivity, decrease the impacts of excessive fresh water discharge
and increased fresh water storage.
The type of development and the general canal conditions affects the
hydrology of the basin. The watershed has experienced growth in the eastern portion
of its Lee County area since the late 1970’s, and is currently estimated to be
about 30% developed. The majority
of the watershed in Lee County is just north or within the City of Cape Coral.
The area within the city consists entirely of single–family residential
development. The remainder of the
watershed in Lee County is shown in the Lee County Comprehensive Plan as “Open
Lands” with some “Resource Protection-Transition Zone“ and “Suburban”. The majority of the unincorporated area within the watershed
is sparsely developed. Most of the
single-family residential development within the watershed has little or no
surface water detention [3].
The topography within the watershed varies in elevation from about +7
feet NGVD at the western boundary of Cape Coral to about +10 feet NGVD at
Chiquita Boulevard, then to about +17 feet NGVD at Andalusia Boulevard, with
about +24 feet NGVD at the north end of the Lee County portion of the watershed.
Certain areas around U.S. 41 experience over bank flooding in the medium
to large storm events. The average
ground slope in both Lee and Charlotte Counties is about one foot per mile.
The majority of the upper reaches of the watershed must convey runoff via
overland flow. Runoff from the remainder of the watershed is over large
sheet flows areas or from residential areas noted above, directly to the
channel. The runoff from the
developed area is directed to the conveyance via overland flow or through
ditches and/or culverts.
There are several tributary canals directly discharging to Gator Slough.
These canals convey much of the northern portion of Cape Coral’s
surface water runoff. There are no
water control structures on most of these tributary canals; therefore runoff is
fairly rapid from developed areas. Canals
that connect further downstream have some controls [3].
Depending on the canal water levels, water can flow in or out of the main
canal. Therefore the links created
to mimic the canal system include no flap gates.
Bypassing of water control structures in the main canal is also possible.
The watershed boundary was set to be reasonably consistent with the canal
system design. Uneven rainfall
pattern and/or other hydrological events and conditions could cause flow
differences in the main canal. It
was assumed that runoff would not be diverted in or out of the watershed.
In order to fully analyze the hydrologic characteristics of this
watershed, it is necessary to consider the portion of watershed in Charlotte
County. It extends into the Cecil
Webb Wildlife Management Area north of Tucker’s Grade.
This area is the headwaters for the Gator Slough Watershed.
By its very nature, the Webb Wildlife Management Area has not been and
will not be developed. It is a
natural flat prairie area that historically has sheet flowed south into Lee
County and Gator Slough [1]. Monthly
discharge coming from this portion of the watershed located north and east at
Hwy U.S. 41 has been statistically evaluated and established as inflow data for
the Gator Slough at its most upstream point (node 31) of the simulated canal
system, just before the quadruple 10 feet x 6 feet box culvert under U.S. 41.
Data obtained from United States Geological Survey gauging station
located 0.5 miles west of U.S. 41, named “Gator Slough at U.S. 41 near Ft.
Myers, FL” were used to represent runoff water entering into Lee County.
Figure 2. Gator Slough Watershed showing portions west and east of U.S.
41.
The conveyance elements within Lee County for the Gator Slough Watershed
consist of a main channel length of a little over ten miles, with an associated
upstream sheet flow area. There are
several culverts, bridges and weirs along the primary conveyance.
During the development of the City of Cape Coral, the canal was
constructed to replace the broadsheet flow that was the original cross section.
The conveyance is an excavated channel from its outflow into the City of
Cape Coral’s western perimeter canal to about 4,500 feet upstream of U.S. Hwy
41. The Gator Slough main channel is approximately 200 feet wide in the reach
from Burnt Store Road to a point one-half mile west of U.S. 41.
In this last one-half mile it narrows considerably to about 30 feet wide
with several with several shallow areas in this vicinity.
In its downstream reaches, Gator Slough is interconnected with the City
of Cape Coral’s canal system. The
canals connected to Gator Slough convey water to and from it. Some of the
connecting canals have no water control structures at their confluence with the
Gator Slough channel, others are culvert connection and some have weirs for
control of elevation and quantity of flow [1].
At its downstream origin, Gator Slough has a bottom elevation of about
– 6.5 feet NGVD. This elevation rises to about – 4 feet NGVD at Burnt Store
Road. From Burnt Store Road east, the bottom elevation rises from – 4 feet
NGVD to about +4 feet NGVD at Nelson Road.
The channel bottom remains at a relatively constant elevation up to a
point about one-half mile west of U.S. 41.
At that point, the bottom rises sharply to about +11 feet NGVD.
The bottom elevation at the upstream end of the Lee County portion of the
watershed is approximately +21 feet NGVD.
Downstream of Burnt Store Road, the conveyance is controlled by tidal
conditions. The Burnt Store Road
weir was installed in 1972 and provides a salt-water barrier.
It also helps to maintain higher groundwater elevations upstream during
the dry season.
Most of the channel downstream of Burnt Store Road is relatively clear of
debris and aquatic vegetation. The
side slope vegetation varies depending on location.
There are very few seawalls along the conveyance. Most of the channel
side slopes are vegetated; some being maintained by the property owners while
other areas are covered with Brazilian pepper and scrub vegetation [4].
The excavated channel upstream of Burnt Store Road to Chiquita Boulevard
is of a fair uniform cross section. The
channel from Chiquita Boulevard to Juanita Boulevard is about half the width of
the channel downstream and upstream.
The channel contains excessive amounts of vegetation along most of its
length from Burnt Store Road weir to the Lee County / Charlotte County line. In
many places, this vegetation completely extends the complete width of the
channel. The side slopes are
covered with scrub brush and sporadic pepper trees.
Most of the vegetation in the channel consists of cattail and other
plants attached to the bottom [1].
Thirty percent of the non-Charlotte County portion of this watershed consists of single-family residential development within the City of Cape Coral. About half of the watershed in Lee County is undeveloped with the remainder in mobile home parks or sparse residential. Since most of the residential development along the channel occurred prior to current regulations, much of the flood plain in Lee County has been encroached upon and filled in the developed areas. There has been very little development in this watershed within Charlotte County. Most of the Charlotte County area is within the Cecil Webb Wildlife Preserve.
Gator Slough is currently being enhanced to provide additional water to
the City. A description of the
major canal components in Gator Slough and the modifications currently in
progress or proposed is presented as follows [4]:
Gator Slough has been dredged to clear weeds and promote flow. The
profile from where the Slough enters the City to 8 feet below the top of Weir #9
has been cleared of rock to provide unimpeded drainage flow. This has increased
the flow to Cape Coral from the Slough. However, flow records show that there is
little flow in the Slough during the dry season. Therefore, the change in
profile would only allow some water into the Cape Coral system during the rainy
season or after rain events.
Basin 1, the first canal basin that the Slough enters, contains a
substantial amount of canal surface area in addition to the Slough. Weir #19
(elevation 10.37 feet) discharges from Basin 1 to Basin 2, which comprises only
Gator Slough canal. The water from
Gator Slough in Basin 2 can flow over Weir #9 (elevation 8.5 feet) continuing
over Gator Slough or flow over Weir #58 (elevation 8.35 feet) entering Basin 4.
Weir #9 is a rectangular weir that zigzags diagonally across the channel.
Hence it is know as the Zigzag weir.
Downstream from Basin 2, a balance structure in Gator Slough transfers
water by gravity flow through four 36-inch culverts from Basin 4 to Gator
Slough. Gator Slough flows through
Basins 1,2,4 and 6, eventually discharging to North Spreader Canal System
Structural
details
The following is a brief synopsis of each structure along the Gator
Slough conveyance as given by Johnson Engineering, Inc., Camp Dresser &
McKee Inc., Hole, Monte & Assoc., and W. Dexter Bender & Assoc. (1991)
[1]. Appendices C and D give the respective diagrams for some of these
structures.
The most downstream structure is located at Old Burnt Store Road, east of
the perimeter canal outfall. The
structure consists of a pair of concrete bridges, each 102 feet long with a load
elevation of +12.3 feet NGVD. There
are two sets of concrete support pilings at this structure.
This is a 156 feet long concrete bridge at Burnt Store Road.
The road elevation is about +10.8 feet NGVD.
There are five sets of concrete support pilings in the channel.
This is the Burnt Store Road weir. It is a reinforced concrete weir,
which has a crest elevation of +2.4 feet NGVD with a length of 175 feet.
There is no notch in the crest. There
is one slide gate on the south end of the weir.
See Appendix C1.
El Dorado Boulevard crosses over a 27 feet x 14 feet concrete box
culvert. The top of road elevation
over the structure is about +13.4 feet NGVD.
The invert elevation of the culverts is – 3.1 feet NGVD.
There is no center column.
This is the Chiquita Boulevard weir.
It is a reinforced concrete weir that has a crest elevation of +6.3 feet
NGVD with a length of 230 feet. This
is a polygonal weir resembling interconnected boxes.
The bank-to-bank distance perpendicular to the flow is 140 feet. There is no notch in the crest.
There are no gates at this structure.
It is modeled as ‘user defined’ exponential rating curve with length
230 feet at depth 0.0 feet to length 120 feet at depth 2.0 feet.
Two 27 feet x 9 feet concrete box culverts provide the Chiquita Boulevard
crossing. The top of road elevation over the structure is about +12 feet NGVD.
The invert elevation of the culverts is +0.4 feet NGVD.
This is the Nelson Road weir. It
is a reinforced concrete weir that has a crest elevation of +8.5 feet NGVD with
a length of 220 feet. The weir
shape resembles a set of stairs across the canal.
The weir length is the wetted perimeter along the channel.
The bank-to-bank distance perpendicular to the flow is 120 feet.
There is no notch in the crest. There
are no gates at this structure. It
is modeled as ‘user defined’ exponential rating curve with length 220 feet
at depth 0.0 feet to length 140 feet at depth 2.0 feet.
This is the structure at Andalusia Boulevard.
It is a double 24 feet x 10 feet concrete box culvert, which incorporates
a 46 feet long (92 feet of total length) weir structure located diagonally
inside each box culvert. The flow
over the weir crest is at an angle to the stream flow.
The weir crest elevation is +10.37 feet NGVD.
The top of road elevation over the box culvert is approximately +16.7
feet NGVD. It is modeled as a
double ‘user defined’ weir with length 46 feet at depth greater than 5.4
feet (crest of 10.1 feet minus invert of 4.7 feet) and 0.0 feet length at depth
less than or equal to 5.4 feet. See
Appendix C11.
A single 20 feet x 14.6 feet concrete box culvert at West Gator Circle is
the next structure upstream from Andalusia.
The top-level elevation over the culvert is about +21.0 feet NGVD.
The invert elevation of the culvert is about +4.4 feet NGVD.
This is a single 15 feet x 14 feet concrete box culvert at East Gator
Circle. The top of road elevation
over the culvert is about +20.4 feet NGVD.
The invert elevation of the culvert is +4.5 feet NGVD.
The Garden Boulevard crossing is a single 15 feet x 14 feet concrete box
culvert. The top of road elevation over the culvert is about +20.6 feet NGVD.
The invert elevation of the culvert is +4.1 feet NGVD.
This is a quadruple 10 feet x 6 feet concrete box culvert at U.S. Hwy 41.
The top of road elevation over the culvert is about +23.1 feet NGVD.
The invert elevation of the culvert is about +13.1 feet NGVD.
Three water control structures are located
downstream of Weir #19: Weir # 9, 4 and 11.
Downstream of Weir #11 there is one culvert and one bridge (# 94).
Upstream of Weir #11, there are two other bridges (#74 and BS).
Bridges and culverts are located on both the main channel and the branch
canals.
 | Weir
#9 stair steps across the canal. It is represented as a rectangular weir without end contraction [2]. |
|
Weir
#4 is a reinforced concrete rectangular weir without end contraction [1]. |
|
Weir
#11 is a reinforced concrete rectangular weir without end contraction. There
is one slide gate on the south end of the weir [2]. |
|
Weir
#19 is a rectangular weir with end contraction enclosed in two box culverts,
one upstream and the other downstream. |
For flow rate in cubic feet per second, length and head in feet, the
discharge equation for a rectangular weir without end contraction is:
