LOCAL
HYDROLOGY
The Big Cypress Watershed
 The
largest feature of a natural drainage system
is the drainage basin, or watershed.
The boundary of the basin is called the
basin divide and is created by an upper
elevation feature so that the natural land
elevation directs runoff from the basin
divide (illustrated with black arrows) toward
a major drainage feature, such as a stream,
lake, bay, or ocean.
The Big Cypress Watershed includes all of Collier County and
significant parts of Lee, Hendry, and Monroe Counties.
Collier
County has divided the Big
Cypress Watershed into sub-basins,
also defined by basin divides, as a tool
for studying
and managing drainage in particular
areas.
Topography
The
topography of an area defines the natural
drainage system. Collier County has
a natural terrain that is extremely flat.
From a high point near Immokalee
the drainage pattern is south and southwesterly
toward the coast (yellow arrows in watershed
figure above) with an average slope of one
foot per mile (0.0002%).
Drainage Pattern
Stormwater
runoff can be defined as the water that
flows overland during and immediately following
a storm event. Because of the natural
flat terrain of Southwest Florida, stormwater
runoff historically follows a sheetflow
pattern within a very shallow but wide depression
called a slough.
Climate
Southwest
Florida is subtropical with a distinct wet
season and dry season. The Big Cypress
Watershed receives an average of 55 inches
of rain annually, with 44 inches in the
wet season (June thru November).
Storm events can be intense during the wet
season with rain sometimes falling at rates
of 6-inches in 1-hour.
Groundwater
Stormwater that does not runoff either
sits at the surface and evaporates or soaks into the ground. The sandy soil of
Southwest Florida allows surface water to be readily absorbed
into the ground where it then becomes groundwater for use by
plants or recharging the aquifer system. The slow movement
of runoff across sandy vegetated soil is ideal for capture of
stormwater into the ground.
Under natural conditions then, Collier
County has an abundance of groundwater with 99% of groundwater
originating as surface water. As the wet season progresses
the ground becomes full of water and not able to absorb more
runoff.
Prior to intervention
by man, the run-off flowed very slowly through
extensive cypress forests and other wetlands
developed in sloughs. In developed
areas, this natural condition must still
be somewhat preserved, so when groundwater
levels are high stormwater still sits at
the ground surface in man-made wetlands,
ponds, canals and ditches.
Tidal Influence
Tidal influence on surface water flow
is significant here even though tide levels only vary 2-3 feet. With
the flat coastal topography and mild tide, the interface between
marine water and fresh water under natural conditions on the
flat Southwest Florida coast stretched for miles inland, creating
extensive estuarine habitat near the coast and helping to slow
the progress of stormwater run-off toward the Gulf of Mexico.
Land within a few miles of the
coast often cannot drain at all during periods
of high tide. Thus, developments
in these areas must have additional capacity
for storing stormwater until high tide conditions
pass.
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