How can floodplain lowering and sediment nourishments help mitigate channel bed incision?

Engineered rivers are frequently associated with channel bed incision, which hampers navigation, flood safety, and ecology. Larger moderate-to-high discharges due to climate change are expected to enhance channel bed incision. To mitigate channel bed incision, river managers consider the deployment of different measures. Yet intervention planning and design does not tend to account for neither the large-scale effects of such measures nor the potential effects of climate change. In this paper we focus on the lower Rhine River (Bonn, Germany- Gorinchem, the Netherlands, 300 km), as it is a heavily engineered river that incises in response to past channelization works in the 18th-20th centuries, which is expected to continue to incise by up to 1.5 m in the up coming 50 years due to both past human intervention and future climate change.

Considering a 50-year timescale, our goal is to assess (1) the potential of two types of engineering measures, namely sediment nourishments and floodplain lowering, to mitigate large-scale channel bed inci sion, (2) the potential large-scale side effects of such measures, in the form of additional incision, and (3) the influence of climate change on the efficacy of the measures.

To this end, we use a schematized one-dimensional numerical model, as it captures the primary component of large scale channel response. We subject the model to different scenarios of sediment nourishments (with independently varying grain size, spacing, and volume), and floodplain lowering (by different heights and at different locations). In all cases, we consider the effects of a changed hydrograph and sea level rise due to moderate and high-end climate change. We find that floodplain lowering is not able to significantly mitigate channel bed incision (reduced incision by 0-0.20 m). This is likely due to the large timescale of channel adaptation to floodplain lowering and to a decreased floodplain inundation over time associated with channel bed incision. On the other hand sediment nourishments of sufficient volume (order of 150’000-200’000 m3/a) may be able to reduce incision by 0-1 m.