Project Details - Natural Flood Management

Client: Thames 21

Categories: Natural Flood Management, SuDS

When: March 2020 - May 2021

Location: Enfield, London, UK

Edenvale Young Associates ltd. were appointed by Thames21 to undertake hydraulic modeling of the Salmons Brook catchment in order to test Natural Flood Management (NFM) options. The Salmons Brook is a tributary to the Pymmes Brook and the River Lea flowing through the North London Borough of Enfield.

Natural Flood Mangement

Approach & Challenges

An ESTRY-TUFLOW fully-distributed direct rainfall model of the catchment was built to characterise the baseline conditions and calibrate the baseline model to a number of observed events. Following calibration a series of NFM options were tested against a key event, the October 2000 event, as well as the 5% and 1% annual exceedance probabilty events.

Key challenges encountered during the modeling process include the representation of the surface water drainage network and the sensitivity of soil characteristics in order to achieve better calibration. Many of the NFM measures presented technical challenges and modeling assumptions such as the creation of rural SuDs (permeable bunds) and the optimisation of the storage. The conclusions highlight the significant potential impact that proposed NFM measures in the Salmons Brook catchment could have at reducing downstream flood risk.

Outcomes

• Up to 23% reduction in the peak flow of the October 2000 event as a result of rural river restoration.
• Up to 35% reduction in the peak flow of the October 2000 event as a result of rural SuDs (permeable bunds). With a reduction of up to 120mm in peak flooded level at properties in Bush Hill, a key location identified at the outset of the project.
• Between 10% (200ha young woodland) and 70% (639ha more mature woodland) reduction in the peak flow of the October 2000 event as a result of woodland planting. With a reduction of up to 400mm in peak flooded level at properties in Bush Hill.
• Up to 80% reduction in the peak flow of the October 2000 event as a result of a maximum benefit combined options scenario (415ha more mature woodland, 90% rural river restoration and 46 permeable bund features). With a reduction of up to 300mm in peak flooded level at Bush Hill.
• A 75% reduction in the peak flow of the 5% annual exceedance probabilty event and a 65% reduction in the peak flow of the 1% annual exceedance probabilty event as a result of the maximum benefit combined options scenario. With a reduction of up to 600mm in peak flooded level at Bush Hill.

Salmons Brook

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