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

Abstract. Muddy floods due to runoff are a widespread problem on the South Downs of southern England and are associated with increased growing of autumn-sown cereals over the last two decades. The 10.6 km² 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.