The Course of History
How did the River Sid's valley originally form, and how has its course changed naturally over time?
The Sid valley likely formed from melt waters during the Pleistocene epoch, leaving a broad valley with a "misfit stream". As the climate warmed, the river became a meandering stream with a low gradient. Coastal changes also played a significant role; a shingle ridge grew eastwards due to longshore drift, pushing the river's outlet to the east over post-glacial times. Historically, the river's mouth fluctuated east/west, and a major storm in 1322 shifted the lower reaches significantly eastwards.
Human intervention has significantly modified the River Sid. These changes, primarily for flood protection and historical land use, have resulted in a river that is straighter, armoured in places, confined by walls, and impeded by weirs. This has altered its natural erosive processes, flow patterns, and the movement of aquatic life. Steep urban gradients also contribute to rapid surface runoff, impacting the river during rainfall. Straightening and armouring, particularly at locations like The Ham, have further exacerbated erosion due to the river's naturally steep gradient.
Sidmouth town is built on the river's flood plain, necessitating significant flood protection measures. Within the town boundary, the riverbanks are "highly modified" and confined by walls. Weirs have been constructed to slow the flow, with the 2.5m high School Weir, built in 1974 for flood alleviation, being a notable example. These structures, while providing flood protection, also act as barriers, preventing the movement of migratory fish and eels.
The lower reaches of the Sid valley were initially formed by post-glacial sea level rise. A shingle ridge, built by long shore drift, pushed the river's outlet eastwards. Following the significant eastward shift caused by the 1322 storm, the lower reaches were re-routed in the early 1700s with a new straight section, establishing the current course. Sidmouth town centre's eastern parts are built on land that was originally the river's estuary and floodplain, which silted up behind the shingle ridge.
Coastal protection structures have significantly impacted the river mouth. The construction of groynes and a small timber sea wall in 1824 led to a significant reduction of the Main Beach. Today, a 100-year-old training wall exists along the western side of the river outlet, alongside the East Pier Groyne. The western river wall was extended seawards in 1921 to join the training wall, reducing the bridge from three to two spans and creating a hardened area that was previously part of the river.
Weirs constructed on the River Sid, such as the School Weir, are significant barriers to the natural movement of aquatic life. These structures prevent the passage of migratory fish and eels, negatively impacting the river's biodiversity by fragmenting habitats and limiting the ability of these species to complete their life cycles.
The River Sid naturally has a steep gradient, giving it significant energy for erosion. However, human interventions like straightening and armouring have exacerbated this erosion. By confining the river and altering its natural course, these modifications disrupt the natural erosive processes and sediment transport, leading to increased erosion in modified areas while potentially reducing deposition in others.
The steep gradients of the surrounding hills, particularly in urban areas, contribute to rapid surface water runoff during rainfall events. This quick runoff flows into the drainage system and indirectly impacts the river's flow dynamics, potentially leading to faster and more intense flow responses during periods of precipitation.