2012 FDOT Mitigation Plan - Southwest Florida Water Management ...

duration of surface water inundation of this habitat, there is more ground cover vegetation including dwarf palmetto
(Sabal minor), and various low panicums and sedges where the canopy shade is not as prevalent. The highest grade
elevations are within a hardwood hammock located in the southeast portion of the wetland. This transitional habitat
has dominant overstory coverage provided by laurel oak, water oak, scattered large live oak, loblolly pine, cabbage
palm, and dwarf palmetto provides minor to moderate ground coverage. There is minimal coverage of non-forested
wetland habitat within the project area, primarily limited to five borrow pits (each covering less than 0.5 acre) dredged
to provide the necessary fill material for the original berm construction. These ponds have predominant coverage of
spatterdock, duckweed, and floating pennywort, and they provide a valuable dry season water source for wildlife in the
vicinity. The depth of berm fill material for the roadway portion crossing the hardwood hammock averages 1-2 feet
above natural grade, compared to the lower elevation obligate zone where the berm material ranges 2-4 feet above
grade (photos). Six of the 10 culverts were installed within a 500 ft. long segment of the road that crosses the obligate
zone. The berm diverts and concentrates the contributing upstream flow from the east to the lower elevation obligate
zone. Then four culverts located within a 50 ft. length of the berm (photo) concentrate the outfall into a meandering
creek that discharges into the Withlacoochee River. Historically the contributing basin flow from east of the berm
would include more ground water seepage and wider sheet flow characteristics to the wetland floodplain west of the
berm, versus the concentrated creek channel. This same but reverse groundwater and sheet flow condition existed
when the river would overflow the banks and contribute flow to the wetlands east of the berm. Now that the flood
waters are blocked by the berm and concentrated through the four main culverts, it limits important and valuable flood
waters from reaching and attenuating in the wetland area east of berm.
C. Brief description of proposed work: Prior to nominating Halpata to the FDOT mitigation program in 2007, an
extensive hydraulic and hydrologic analysis was necessary to determine if a restoration project could be constructed to
benefit the wetland floodplain and confirm no potential of any off-site drainage alterations. This analysis was
conducted in 2006-2007 to evaluate the degree of wetland hydrologic impacts caused by the berm and culverts, and
alternatives to restore flow conditions to benefit the wetland habitat while still maintaining a modified access road. The
results of the modeling effort found that wetlands could hydrologically benefit from removing at least portions of the
berm and the majority of culverts. The final design includes removing 2,600 cubic yards of berm material at three
separate locations to match adjacent natural grade for a total distance of 1,000 feet (Figure D). After berm removal, an
additional 4-6 inches of material will be excavated below grade, followed by installation of Geoweb fabric and 6-8
inches limerock base material. The Geoweb and rock will provide a stable access road while allowing water to sheet
flow over the road; thus restoring hydrologic connectivity to slightly higher wetland elevations during normal seasonal
high water levels as well as flood events. This includes an isolated cypress dome within the northwest portion of the
project area that doesn't receive the historic flood waters due to the berm. A segment of berm material will be retained
through the obligate zone however the associated six culverts will be replaced with three wedge-shaped breaches
lined with geotextile fabric and filled with rip-rap rubble to match the original berm height. Replacing the culverts with
rubble rip-rap will slow the rate of surface water discharging from the east side of the berm to the creek channel. This
will result in extending the hydroperiod for the wetland east of the berm, thus enhancing the habitat and provide more
water for wildlife use. The remaining 4 culverts will have sumps and riprap placed at each end to reduce water velocity
and minimize scouring. Seeding of winter rye or brown-top millet will be placed on exposed soil after grading, followed
by any necessary supplemental herb plantings such as maidencane. Figure D depicts the 103 acres of wetland habitat
that are anticipated to receive enhancement by the proposed construction activities. An additional 110-150 acres of
the same wetland will also receive secondary enhancement by the project. However the degree of enhancement for
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