Removal of in-channel structures

Description of technique

Weirs are used within river channels to impound water upstream of a structure, control water levels and conveyance of water downstream. In-channel structures have often been constructed in association with flood defence and agricultural activities but many have reached the end of their useful life or are no longer required for their original function. This measure relates to the complete removal of weir structures that are considered obsolete from within a river channel.

Application

A clear decision making process is needed when seeking to remove structures from the watercourse (click Remove Obsolete Structure to see the decision making process that should be applied). In some cases, site specific constraints (see below) may mean that complete removal of the structure is not feasible in the short-term (see Modify or Enhance Structure / Structural Operation). In-channel structure removal can be undertaken in different river environments, although the associated risk of instability following removal of a structure is greater in higher energy environments than low energy environments.

Potential limitations include high capital works costs (depending on the nature of the structure), engineering constraints that prevent the removal of the structure (e.g. downstream flood risk, risk of instability whilst the river re-adjusts to removal of the structure) and disturbance of contaminated sediment. Some structures in transitional and coastal waters are also vital in maintaining the balance between freshwater and saline conditions, and their removal may adversely impact on associated floodplain and estuarine habitats if immediate removal were to be implemented. Other structures may have intrinsic landscape and ecological value and may isolate genetically rare fish populations upstream (Environment Agency, 2004).

Benefits for Water Framework Directive

This measure can help deliver objectives of the Water Framework Directive. Key benefits of removing dams and weirs are:

  • Re-establishing the natural continuity of hydromorphological processes (reducing impoundment, upstream siltation and downstream erosion) along the river system.
  • Re-establishing natural habitat diversity.
  • Reducing barriers to natural migration and movement of fish and other aquatic species.
  • Re-establishing a self-sustaining river system, without the need for further maintenance.
  • Re-establishing natural continuity in temperature, oxygen balance, pH and salinity.

The presence of in-channel structures creates slow or ponded flow conditions, which consequently encourages deposition of fine sediment on the channel bed. Such conditions are less favourable for many plants, invertebrate and fish species and are associated with reduced habitat diversity. In-channel structures also disturb the sediment, nutrient, hydrologic and temperature regime downstream of the structure. Depending on the size of the in-channel structure, its influence can persist for a considerable distance upstream and downstream. Flow over the obstruction can also cause localised scouring of the bed and banks downstream. In-channel structures can act as a barrier to fish migration (particularly weaker swimmers) and hinder the mobility of other aquatic species.

Other benefits of in-channel structure removal may include:

  • Avoidance of health and safety risks associated with the existing structure.
  • Reduced costs and manpower associated with day-to-day maintenance and repair of the structure.
  • Reduced flood risk upstream of the existing structure.

To read more about the effectiveness of the measure within academic literature please click here: Effectiveness for Biological Quality Elements

prev   1  2  3  4   next