Case Study
Digswell Chalk Stream Restoration
Location
River Mimram, Digswell Lake Nature Reserve
Date
February - March 2018
Client
Environment Agency
Stakeholders
Digswell Lake Society
Tasks
- Tree works to facilitate access and to provide material for the woody berms
- Tree thinning to increase light in the channel and to stimulate macrophyte diversity.
- Constructing low-level berm to narrow the channel and accentuate variation in the geomorphology of the river.
- Whole trees placed in the channel to kick start both scour and depositional processes.
- Deflectors placed in the channel to promote the formation of a pool and subsequent riffle through natural processes following completion of the project.
- Serious of offline scrapes excavated into the floodplain to increase water storage capacity and habitat heterogeneity.
Key remarks
- Works carried out on a sensitive local wildlife site. A badger sett was identified, thus, an exclusion zone was erected.
- Some trees present on site categorized as “medium potential” for bats were checked by a qualified ecologist prior to commencement of works.
- Japanese knotweed and Himalayan balsam were present on site. Therefore, fencing was installed 7m from the closest plant to ensure the risk of spreading was reduced to a minimum.
- The needs of all stakeholders, including the EA, members of the public and the local fishing club were balanced to ensure a successful outcome.
Effects
- Restored an over widened chalk stream by increasing its sinuosity, local flow velocities and habitat complexity.
- The berms were loaded with soil from the offline ponds and topped with site won existing marginal vegetation. This method created new habitats for juveniles fishes and invertebrates.
- Utilizing the existing vegetation meant that the habitat is more resilient to grazing activities from riparian species (e.g. Water Voles).
- The generation of woody debris and offline ponds promotes an healthy spawning environment for fish.
