Forested Wetland Values Along the Black and Waccamaw Rivers, South Carolina

This study funded by The Nature Conservancy, Open Space Institute, and Southern Environmental Law Center details the degree that the forested and palustrine forest floodplains along the Black, Waccamaw and the Pee Dee Rivers are contributing to lower flooding for crops and infrastructure in the basins. The approach assumes that the floodplain will provide (1) passive flood storage of ponding above the land surface, and (2) active storage from soil saturation (filling pores). The existing river gage data is used to define the “with floodplains” condition. The “without floodplains” condition is computed using the gage heights vs discharge relationships and assumes that the river volume is increased by the storage values computed for each level of daily flooding with a concomitant increase in river height during a 1-day period. The rate of river rise (and thus floodplain storage) is an important variable in defining the value of floodplains for flood reduction potential. This technique differs with how FEMA generally interprets the loss of floodway fringe (i.e., floodplains). FEMA’s method concentrates on the hydraulic loss of cross-section and thus the ‘damming’ of water upstream as opposed to looking at the downstream increase focused on in this report. The total story is, of course, a combination of the two. The primary intent of this study is to provide an appreciation of the benefits that are now being provided but may have not been conceptualized or described with specific accounting based on the flood damage reduction that currently exists.

Example of flooding during 5-year event without the forested floodplain storage.