Flood management along the Lower Mississippi and Rhine Rivers (The Netherlands) and the continuum of geomorphic adjustment

Abstract

Flood management alters fundamental fluvial processes that have geomorphic consequences for rivers and floodplains. The Lower Mississippi and Rhine Rivers (The Netherlands) are two important examples of intensively regulated large rivers. Understanding the magnitude and direction of change caused by flood management requires a long-term perspective. This is particularly true of large lowland fluvial systems because of substantial lag-times required for adjustment to be manifest in the floodplain geomorphology. This study is a historical analysis and synthesis of the impacts of flood management on the Lower Mississippi and Rhine Rivers (The Netherlands), and investigates the interrelations of flood management with floodplain geomorphology. Although flood management varies between the Rhine and Mississippi on many accounts, the actual techniques of flood (and river) management are somewhat similar, and primarily include dikes, groynes, cutoffs, and bank protection. The implementation and history of these specific types of activities, however, varies considerably. Historical flood management along the Lower Mississippi can be characterized as abrupt, with the major options imposed within about five decades, while historical flood management along the Rhine River in The Netherlands is characterized as incremental and adaptive, with the major options imposed over about eight centuries. Conversely, modern flood management plans are implemented much more promptly along the Dutch Rhine than the Lower Mississippi. Changes to the Lower Mississippi include channel adjustment (width and incision) caused by meander bend cutoffs. The majority of the knickpoint incision in response to cutoffs occurred by 1963. Channel adjustment in some reaches is likely constrained by the presence of resistant alluvium and lithology. Floodplain geomorphic changes include the creation of new oxbow lakes within an embanked floodplain. Embanked floodplain sedimentation of oxbow lakes created from the 1928 Mississippi River & Tributaries Act have rapidly infilled, with 67% of the lake area converted to wetlands. In comparison, older oxbow lakes located outside of the embanked floodplain have undergone much lower amounts of infilling, averaging 37% of oxbow lake area converted to wetlands. The floodplain geomorphology is further modified by numerous large floodplain borrow pits and the selective removal of fine-grained deposits, primarily created for dike (levee) construction and maintenance. The Dutch Rhine has been managed for flooding for over eight centuries and exhibits specific types of humanized embanked floodplain geomorphology that require a greater period of adjustment. Dike breaches create ponds (wielen) and sandy splay-like deposits, which represent distinctive anthro-geomorphic environments along the margins of embanked floodplains. Channel stabilization by groynes and dikes has resulted in the formation of new floodplains along Rhine distributaries. The trapping of flood sediments within the embanked floodplain has resulted in aggradation that has reduced the inundation capacity of the embanked floodplain. This geomorphic alteration reduced the effectiveness of the existing flood management infrastructure and has stimulated a change towards a new flood management approach designed to "work with the river". The major conclusions are placed within a conceptual model, and illustrate that; 1. in many instances specific flood management options were constrained by the type of floodplain deposit; 2. geomorphic adjustment to flood management occurs along a time–space continuum; 3. flood management initiates positive feedbacks with unintended geomorphic consequences that require further management options to minimize flood risk.