Optimizing Regulatory Floodways: Dynamic Programming Approach

Abstract

The United States National Flood Insurance Program (NFIP) relies on floodway and encroachment line standards to regulate floodplain development and minimize flood damage while ensuring public safety. A regulatory floodway includes a watercourse’s main channel and adjacent floodplain areas critical for safely conveying a specified flood flow (the base flood discharge) without raising water surface elevations beyond permitted levels. The flood fringe, the remaining floodplain area inundated by the regulatory flood, can be developed under specific conditions. This paper introduces an optimization-based method to set encroachment limits while minimizing the value-weighted land area within the regulatory floodway, thereby maximizing the developable land in the flood fringe. The procedure organizes the complex decision process into a series of individual choices made at each cross section of a stream reach. These choices affect water surface elevations while meeting specific criteria, focusing primarily on limiting water level increases. Furthermore, floodplain encroachments are required to reduce a specified cross-section property on both sides of the floodplain equally. Two cross-section properties are considered in this study: conveyance and flow area. The NFIP’s required approach is equal conveyance reduction. However, the cross-section flow area is more equitable for defining regulatory boundaries with long-lasting consequences. The computational method utilizes dynamic programming to minimize the value-weighted land area within the floodway. Additionally, it reduces the occurrence of irregular floodway boundaries and the need for discretionary modifications to encroachment limits, potentially eliminating such adjustments.