The Journal of the American Water Resources Association (JAWRA) has just published an OpenAccess paper by former M.S. student Forrest Williams (M.S. Environmental Science, 2019) and other AGL members and collaborators. The article, entitled Sediment and phosphorus contributions from eroding banks in a large intensively managed watershed in western Iowa, United States, outlines the implementation of the AGL-developed tool AIMM in quantifying net erosion associated with channel migration in the river network of the Nishnabotna River.
Net erosion in this context means total erosion minus total deposition. In AIMM, erosion and deposition are quantified using remote sensing and geomorphometry algorithms described in Forrest's earlier work. AIMM's output is a map of volumetric change polygons representing either erosion or deposition along the channel boundaries. In practice, we often use field and laboratory measurements to convert these volumetric changes to mass changes. On a reach or network scale, erosion minus deposition should correspond to the net contribution of lateral channel migration to the sediment (and any sediment-bound substances, including P) budget of the watershed.
The Sankey diagram (constructed in R with the package PantaRhei) on the right is an attempt to illustrate that relationship based on the results presented in this paper. Each arrow in the Sankey diagram represents a source or sink in the simplified watershed budget. AIMM helps to quantify Bank Erosion and Lateral Accretion components, but the remaining sources and sinks must come from other sources. Fortunately, in the Nishnabotna River basin, we have estimates of Export from the Iowa Geological Survey and IIHR, as well as a way to estimate Delivery using the Daily Erosion Project. The remaining unknowns in the watershed budget are vertical accretion of sediment and P, which as a result are assumed to be the residuals. Since Delivery is relatively poorly known (due to the uncertainty tangled up in the Sediment Delivery Ratio), I present this result with relatively low confidence. Even so, it points to a need to quantify vertical accretion, meaning the deposits on the floodplain surface that are left behind following major floods. This is one of the tasks of current AGL graduate student Kelvin Adu Baah.