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Mapping the Zone: Improving Flood Map Accuracy
of acres in the Special Flood Hazard Area, providedthe stream location and topographic information areproperly aligned.
In some regions, drainage is dominated by water flow from one ponded area to the next. Rivers still exist in such landscapes, but the mechanisms by which water reaches them are different than in the normal dendritic stream and channel systems that carry flow downstream. Ponding landscapes are common in Florida, where surficial sedimentary deposits overlie limestone formations. Dissolution within the limestone causes pitting, subsidence, and in some cases, collapse of the surface to form sinkholes.
The land surface terrain in these landscapes has low slope, so watershed delineation becomes an exercise in determining the drainage area surrounding each depression (Figure 4.18), rather than the drainage area of a point on a stream network. During severe storms, water accumulates in each land surface depression until it reaches the lowest elevation on its drainage divide with a neighboring depression and flows into the next downstream pond. This process continues until a developed stream or river is reached, at which point the flow dynamics become similar to those in dendritic drainage landscapes.
The committee’s frequency analysis of stage heights included 10 stream gages with long-term flow records in southwest Florida (Figure 4.3). No significant differences in the sampling uncertainty of the 100-year flood stage were found for the Florida gages compared to the 21 gages that were studied in North Carolina.
Finding. Despite the difference in landscape flowprocesses between the dendritic stream river systemsof North Carolina and the ponding landscapes inFlorida, the resulting river base flood elevationsdetermined at USGS gage sites have a similar sampling uncertainty.
FEMA guidelines do not specify procedures for dealing with the hydrology and hydraulics of ponded landscapes. The Southwest Florida Water Management District (SWFWMD) has developed some sophisticated tools for delineating drainage areas in
FIGURE 4.18 Drainage areas (red lines) of a ponded landscape in Florida. SOURCE: Southwest Florida Water Management District. Used with permission.
pitted landscapes. The InterConnected Pond Routing model (ICPR) uses broad-crested weir equations to compute the hydraulics of flow between ponds. These equations determine the flow over a berm between one pond and the next as a function of the elevation of water above the berm. The interaction of one pond with the next is treated like upstream and downstream flow through a culvert—if the water elevation in the downstream pond is high enough, it can affect the discharge from the upstream pond. Other factors that are important include the volume of the water temporarily stored in the depressions, the duration of the critical design storm, and the rate of percolation of floodwaters through the base of the ponds or pits. Surface sediments can absorb significant quantities of water during a long design storm, but hydrologic methods that account for percolation have not yet been incorporated into FEMA flood mapping guidelines. Significant work remains to lay the scientific foundation for flood modeling of