Encroachments in floodways of the Mississippi River and Tributaries Project

Abstract

The Mississippi River and Tributaries (MR&T) Project aims to protect the land within the lower Mississippi Valley from floods and to assure the navigability of the channel. One of the main flood risk mitigation features comprised in this project are floodways, or flood bypass channels, which divert excess flows in order to lower critical stages in the river. However, encroachments, i.e., buildings and other structures constructed within the floodways, can jeopardize their proper functioning and therefore compromise the functioning of the entire system. The presence of encroachments and resistance to flooding them delayed activation of the Birds Point-New Madrid Floodway in the flood of 2011. However, there has been no prior comprehensive study of encroachments on this and the West Atchafalaya floodways, the two largest in the MR&T system. Using historical maps and aerial imagery, we systematically measured the number, type, and spatial distribution of encroachments in the two floodways since the late 1960s. Encroachments in the Birds Point-New Madrid Floodway decreased by half, while the number of encroachments in the West Atchafalaya Floodway tripled, mainly concentrated along highways, such that they create linear bands of higher hydraulic resistance. The presence of encroachments clearly discourages use of the floodways from their designed purpose, but we are not aware of any studies that have analyzed whether the presence of these structures would measurably affect flow hydraulics through the floodways.