Water retention through restoration of the sponge function of drained soils

Urbanisation and agriculture in the Rhine basin have driven loss of water storage capacity in floodplains and marshes and soils of smaller valleys and have caused ecosystem degradation. Changing precipitation patterns due to climate change are expected to lead to increased peak flows as well as critically low water levels. The combination of both developments increase the chance of severe impacts on society. Precipitation falling on the plateaus and slopes in the Middle Rhine region comes down as throughflow (water flowing through the soil) and passes the valley floor before entering the stream. Drained valley floors accelerate this process, i.e. artificial drainage “empties” the hydrological column of entire slope and plateau, contributing to flood peaks in the tributaries to the Rhine.

Developed by

Wetlands International

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Natural Water Retention through restoration of the ‘sponge’ function of currently drained soils in the middle-mountains of the Rhine basin is a locally applied nature-based solution to flood mitigation with potential impacts at basin scale. We argue that the benefits of the proposed solution are not only local, but will also favour end-users further downstream. This solution fits a systems approach and contributes to achieve water, agriculture and nature policy objectives as well as delivering societal benefits such as recreation and carbon capture.

TRL 5

Technology validated in relevant environment.
Fidelity of breadboard technology increases significantly. The basic technological components are integrated with reasonably realistic supporting elements so they can be tested in a simulated environment. Examples include “high-fidelity” laboratory integration of components.

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Testing plan completed

The testing plan and the BRIGAID’s Testing Innovation Framework (TIF) has been rightly applied and finished. The TRL of the innovation has been effectively reached.

Whereas the terms “technical readiness” and “prototype” allow a fairly clear guideline on how to describe the stage of a technical project, they are less applicable to describe the level op development of a nature-based solution. That being said: we strongly feel our concept is at Technical Readiness Level 4 and perhaps even higher because the following crucial elements to allow a scale-up are available: - several field projects in which the positive effects are demonstrated on a local scale (several Waterboards are implementing the concept on local scale for local aims) - a conceptual framework, GIS research and first hydrological calculations which, at the very least, make it plausible that upscaling of the approach will not only translate into more local effects, but also has a positive impact (on nature, climate resilience, flood control) basin-wide. Being able to better demonstrate the latter is crucial to harness financial, policy and political support on a national or even international level. In fact, our project proposal aims to reach this tipping point: from local application for local purposes to (multiplied) local application for (inter)national purposes.

How does it work?

The most suited location for restoring the natural sponge function is at the foot of slopes in U-shaped valleys. Removing drainpipes and ditches would slow down the runoff response of a much larger area than the space needed for the measure itself. Calculations for local catchments in the Mosel basin (the case study area) shows potential for local peak reductions of 5 – 8 % in the tributaries to the Rhine, and provide partial evidence for the hypothesis that natural retention can result in substantial reduction of flood peaks. Critical to bridging the gap between our innovative approach and the end-users is to provide more clarification regarding the location and scale of implementation as well as the expected effects of the measure.

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