Using HEC-RAS 2D to investigate nature-based approaches

Challenge

The RSPB has taken over the management of Swindale near Shap in Cumbria. A range of nature-based approaches to reduce surface runoff are being tested.

We have developed broad-scale whole-catchment modelling and mapping techniques with JFlow® to simulate the effect of these very distributed measures. We utilised the facility of the HEC-RAS 2D solution to tabulate sub-grid hydraulic tables to model conveyance more accurately, which leads to accurate outputs.

Solution

We aim to model the impacts of existing and proposed future nature-based interventions on the 23 km² Swindale Beck catchment in Cumbria. We will then understand the effects on the hydrographs in terms of size and timing, as far downstream as the Eamont Bridge.

The Eden 2009 CFMP estimates 51-150 properties in Flood Zone 3. The key modelling scenarios break down into the following:

  • Intensification
  • Business as usual
  • Mature RSPB Natural Flood Management (NFM) interventions:
    • Runoff attenuation
    • Grip-blocking
    • River restoration
    • De-stocking
    • Tree-planting
    • ‘NFM-Max’ scenario

RAS Mapper was first used to import a Digital Terrain Model (DTM) incorporating some high resolution <0.5m drone-based photogrammetry. A 25m resolution grid was initially set up, relying on the pre-computation of hydraulic tables to pick up the detailed sub-grid storage and conveyance characteristics in the detailed DTM.

The CORINE Land cover data was used and different covers were assigned appropriate roughness for the baseline case.

HEC-RAS 2D allowed use of fine-scale surveyed topography despite the large grid size. The model showed peak attenuation and delay in some scenarios such as tree planting and floodplain reconnection.

Benefit

Initial findings suggest that HEC-RAS 2D is very easy to set up to model some aspects of NFM, and it has advanced visualisation and GIS tools to support this. It is easy to use and introduce changes to the roughness and boundary conditions for a single Flow Area, but it would be useful to be able to set up grids for effective rainfall. Then we could vary hydrological losses, and have monitoring cross sections where we can plot flow hydrographs anywhere.

Want to know more?

For more information on this project contact Barry Hankin. You can also find out more on our Catchment and River Restoration web pages.



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