Curtailment of muddy floods in the Sompting catchment, South Downs, West Sussex, southern England

Evans, R. and Boardman, J., 2003. Soil Use and Management

Original research (primary data)
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Muddy floods due to runoff are a widespread problem on the South Downs of southern England and arc associated with increased growing of autumn-sown cereals over the last two decades. The 10.6 km(2) Sompting catchment has been monitored over the 12-year period 1990/91-2001/02. A housing estate at the lower end of the catchment that was frequently inundated by muddy floods in the late 1980s and early 1990s has not been flooded since the winter of 1993/94, even during the heavy rainfall events of October and November 2000. This is a result of the ameliorative measures put in place in the early 1990s, primarily the reversion of some winter cereal fields to permanent grassland (set-aside). Other land management changes helped, for example, some parts of the catchment were put down to short-term grass leys and small dams were constructed to impound runoff. Flooding of the housing estate occurred when more than 30% of the catchment was covered by eroded fields, which contributed runoff to the valley floors leading down to the housing estate. The length of continuous down-valley flow was greater in the early 1990s compared with later years. The introduction of grassland reduced the risk of flooding not only by reducing the area contributing to runoff, but also by stopping valley floor flows linking up. Such measures to alleviate runoff, erosion and flooding fit well with policies proposed in the recent report by the UK Policy Commission on the Future of Farming and Food.

Case studies

Basic information

  • Case ID: INT-143-1
  • Intervention type: Combination
  • Intervention description:

    farmers converted to grassland some of the land used for autumn-sown cereals as ‘set-aside’ – land allowed after cereal harvest to revert naturally to grass, funded by European Union Common Agriculture Policy – or as re-seeded grazed grassland paid for by an Environmentally Sensitive Area scheme funded by the UK Government In November 1992 it was noted that some fields, or parts of fields, had not been ploughed and drilled after cereal harvest but left to be colonized by shed cereal seed, grasses and weeds. These fields were in the lower part of the Stump Bottom sub-catchment, on both sides of the valley and on its eastern watershed (Figure 2), as well as a strip of land in the field below New Hill Barn. This was permanent set-aside land – allowed to revert to grassland on which a subsidy would be paid under regulations of the European Union’s Common Agricultural Policy. Also, strips of land adjacent to the valley floors in the upper Stump Bottom catchment (Figure 4) were put under grass as part of an Environmentally Sensitive Area (ESA) scheme. Altogether, about 100 ha of land were put down to grass, mostly on slopes vulnerable to runoff and erosion, about 15% of the arable land. Later, permanent set-aside would also be put in three other locations, all flanking the lower part of Lychpole Bottom (Figure 2)

  • Landscape/sea scape ecosystem management: Yes
  • Climate change impacts Effect of Nbs on CCI Effect measures
    Freshwater flooding  Positive No specific measures, but authors use a variety of observations to paint an overall picture of the extent of runoff and flooding severity across the watershed. They link this to the % of set aside covering arable fields. They also assess changes to susceptibility to generate flood by “relat[ing] the average October and November rainfall to (i) the area of catchment from which runoff could be generated, and (ii) the longest length of continuous flow, for both the early and late 1990s. “ • Change in rainfall threshold that generates runoff from 40% of the catchment • Extent of runoff (observed) from fields • Properties flooded • Water level observations • Extent of dam breaching
    Soil erosion  Positive • Site visits to assess erosion throughout the watershed on a yearly basis • Evidence of runoff captured through visual observation and photographs to trace ‘flow routes’ • Photographs compared year to year to rank severity of erosion
  • Approach implemented in the field: Yes
  • Specific location:

    Sompting Catchment, south downs

  • Country: United Kingdom
  • Habitat/Biome type: Temperate grasslands |
  • Issue specific term: Not applicable


  • Notes on intervention effectivness: The below highlights the effectiveness of set asides. The paper also focuses on the effectiveness of dams and eba-relevant agricultural interventions, but the impacts of these are discussed separately thus we can code for the outcomes driven by the NBS separately. Fields in the ‘core’ area that did not suffer erosion or eroded only a few times (i) were those under set-aside (Figure 2) (iii) had gentle valley floor and valley side slopes that were not conducive to erosion the reduction in land area contributing to erosion and the breaking of the continuity of flow from one field to the next. Set-aside, particularly, and grass strips have played an important role in this Set-aside not only cuts down the volume of runoff generated in comparison with that from a bare field – as shown in Stump Bottom and the eastern tributary to Lychpole Bottom – but also the vegetated valley floors more readily absorb the runoff generated up-valley Although there was flow in adjacent fields, except in the upper Lychpole Bottom valley the flow in the lower field was not augmented by flow from the field above. This may be explained in the main valleys by the reduction in runoff due to set-aside Primarily because of the introduction of set-aside to the Sompting catchment, the risk of the housing estate being flooded has been greatly reduced In the Sompting catchment, set-aside covering 15% of the arable land has greatly reduced the risk of a flood carrying sediment out of the catchment via the housing estate. Disruption in the continuity of flow by the set-aside is such that since 1993 a much greater amount of rain in October and November has been required to flood the housing estate
  • Is the assessment original?: Yes
  • Broadtype of intervention considered: Not applicable
  • Compare effectivness?: No
  • Compared to the non-NBS approach: Not applicable
  • Report greenhouse gas mitigation?: No
  • Impacts on GHG: Not applicable
  • Assess outcomes of the intervention on natural ecosystems: No
  • Impacts for the ecosystem: Not reported
  • Ecosystem measures:
  • Assess outcomes of the intervention on people: Yes
  • Impacts for people: Positive
  • People measures: observed reduction in flooding events to a housing estate. Primarily because of the introduction of set-aside to the Sompting catchment, the risk of the housing estate being flooded has been greatly reduced In the Sompting catchment, set-aside covering 15% of the arable land has greatly reduced the risk of a flood carrying sediment out of the catchment via the housing estate
  • Considers economic costs: No
  • Economic appraisal conducted: No
  • Economic appraisal described:
  • Economic costs of alternative considered: No
  • Compared to an alternative: Not reported

Evaluation methodology

  • Type of data: Quantitative
  • Is it experimental: No
  • Experimental evalution done: Not applicable
  • Non-experimental evalution done: Empirical case study
  • Study is systematic: