DETERMINING A SUSTAINABLE APPROACH FOR MANAGING AN ESTUARY MOUTH – A CASE STUDY

Abstract

Estuaries and their floodplains represent locations where significant population, industry and environmental interests exist. Sand spits located at estuary mouths are one factor in influencing both hydrodynamics and geomorphology. As such, their management is important as regards managing both flood and coastal risk, and the evolution of designated flora and fauna. This paper describes a study to develop a way forward for managing Dawlish Warren sand spit, located at the mouth of the Exe Estuary in Devon, UK. Dawlish Warren sand spit is important as it influences geomorphological evolution of the mouth and wider estuary, provides storm sheltering for up to 2,900 properties and national transport infrastructure in the estuary, is an internationally designated Special Area of Conservation (SAC), and influences the wider Exe Estuary Special Protection Area (SPA). The Exe Estuary is located in Devon, UK (see Figure 1). It is a spit enclosed drowned river valley (Defra, 2007), which has been subjected to marine inundation caused by a rise in sea level at the end of the most recent glaciation (which ended c. 12,000 years ago). The Exe Estuary has a shoreline length of 40km, channel length of 16km, valley width 2km and a mouth width of 380m. It is classified as macrotidal with a range of 4m, whilst the River Exe (the main tributary) has a mean flow of 23m 3 /s, and a maximum of 371m 3 /s. At the mouth of the Exe Estuary, the sand spit of Dawlish Warren covers approximately three quarters of the estuary mouth width, and consequently potentially shelters the estuary from the coastal swell wave climate, as well as influencing the propagation of extreme tide levels. The in-estuary extreme wave climate is consequently limited to significant wave heights of less than 1.1m, with extreme tide levels between 3-4mAOD. The estuary exhibits high tidal velocities through its mouth, with flood/ebb tidal deltas and offshore banks present. The Exe Estuary has been significantly modified over centuries. In the 12 th century the majority of the west bank floodplain was reclaimed, with what was the adjacent mobile sand spit being fixed with seawalls and developed as the town of Exmouth in the 15 th century. The mainline railway was built in the 18 th century, reinforcing and extending the west bank floodplain reclamation. Dawlish Warren sand spit itself has a long history of being damaged and breached in storm events, with around 13 such events in the last two centuries. In response to this, engineers applied increasingly extensive engineering methods to manage the sand spit, currently including groynes, gabions, rock armouring, concrete revetments and wave recurve walls. The assets managing flood and erosion risk have allowed the development of villages and towns to occur within the floodplains, along with their associated infrastructure. Over the last 50 years the Exe Estuary has also become a heavily designated environment. The mudflats, saltmarsh and wetlands in and around the estuary, and the bird population supported by these, are Ramsar and SPA designated features or sites. Dawlish Warren sand spit itself is designated as a SAC for the physical processes (dune system) and flora and fauna dependent on these. The management of Dawlish Warren sand spit is therefore of particular importance, as it will influence:  Flood risk for 2,900 properties in the present day, and up to 7,000 properties by 2110 due to climate change, in the wider Exe Estuary.  Flood risk to the nationally important mainline railway, as well as regional and local infrastructure.  Physical environments (both saline and freshwater) that support the Exe Estuary SPA designated