Measurement of Floodplain Value for Downstream Flood Reduction

This pilot study examined the water storage/flood resiliency value of lands upstream of urban/suburban centers under riverine dominated flooding. The goal was to develop a process that could be applied to other areas in the Southeast US.

This study’s intent was to quantify the ability of the floodplain to reduce flooding and assign a defensible dollar value to those benefits. The technique uses a series of 2D GIS based-models as opposed to a dynamic hydraulic model. The GIS approach provides the ability to expand the use to other areas without complex modeling requirements. The technique assumes that the flood plain will provide (1) passive flood storage of ponding above the land surface and (2) active storage from soil saturation (filling pores) above the shallow groundwater table, and (3) that those waters reduce the flow of the river at the population center under study.

The basic approach uses the flow during Florence as a real-world example of a 1% or above riverine dominated flood and how the system reacts to this water input. The empirical data have been used in conjunction with previously developed Northcoast (TNC) flood products to define spatial and temporal relationships and algorithms that describe the flood/discharge aspects of the Conway pilot study area.

Value of floodplains in 2019 dollars/acre

One acre parcels were used to sum the flooding reduction values for multiple flood conditions that were scaled according to yearly return period. The 1-acre boxes in the study area comprise about 19,400 acres with a summed flood reduction value of $5,215,573 per year. The average flood-damage-reducing value per/acre is about $270 per year. In 2035 the projected value of the same area is $6,696,117 (2019 dollars) or about $345 per acre/per year.

Example use; assuming the development (highlighted in gray) would preclude flooding in the area, the total lost value of floodplains per year is $14,570.

For more information - the entire report can be accessed here