Erosion mitigation testing for a small brook system under human influences in South Limburg, the Netherlands: a field and HEC-RAS modelling approach

Abstract

Riverine ecosystems with a surface water gradient of > 0.01 m/m have the tendency to be of erosional character, with the potential to greatly alter surface and subsurface hydrological fluxes. Primarily, the degree of erosion is strongly related to the bed material and the frequency and magnitude of peak flow discharges. However, given the increasing environmental risks associated with climate change and land use changes, erosional processes are expected to intensify. Thus, investigating these processes in hydrologically and ecologically complex areas remains critical, as their susceptibility can be particularly high.

This paper studies the Noor brook and the adjacent upward-seepage-dependent Natura 2000 ecosystem in the south of the Netherlands, to explore channel erosional processes and mitigation practices in hydrogeologically highly complex environments. This small brook is one of many in the area facing peak flow discharges associated with heavy rainfall and sewage water overflow. When the brook would completely cut through the poorly permeable clayey valley filling, it is expected that more seepage water will flow directly into the Noor brook. In turn, this will reduce the calcium carbonate-rich seepage water for the Noorbeemden causing a detrimental impact on the ecological value of the area.

Extensive field studies have been conducted regarding the local geomorphology of the brook and the local hydrogeology, including hydrological experiments on seepage, sewage and rain water. Here, we conduct a sediment transport and modelling study to deepen the understanding of the impact of erosion mitigation practices on the hydraulics and morphodynamics of the Noor brook. This way, the response of the Noor to further bedload erosion was assessed through shear stress analysis. Gravel armouring and gabion dams were incorporated into 1D steady and quasi-unsteady simulations through the Hydrologic Engineering Center’s River Analysis System (HEC-RAS) modelling software. Lastly, modeling results regarding sediment transport dynamics were used to estimate the response of deep and shallow groundwater levels on mitigation measures.

For erosion mitigation purposes, the identification of areas susceptible to deep incision was found to be closely linked to the magnitude of the peak discharge and the morphology of the brook. The river gradient was found to have major contributions to accelerating deep incisions in the Noor brook. Gabion dams and gravel armouring showed contrasting results in erosion dynamics based on the magnitude of their application. Yet, such practices could potentially benefit groundwater levels at the riverine scale when properly implemented. All in all, the combination of field and HEC-RAS modelling exercises showed that, within reasonable limits, the erosional behaviour of small riverine ecosystems can be understood, as well as the impact of mitigation practices.