The most suitable restoration option for a weir will depend on a number of factors. These include the structural make-up of the weir, the condition of the river upstream and downstream, the aims of the catchment plan, the surrounding land use, funding, and the presence of protected structures and habitats. No single option is suitable for all situations.
There is, however, a general order in which measures should be considered. This is because some measures provide greater benefits to restoring natural processes and allowing free passage. For example, whilst a fish pass will focus on passage for some fish species, the impoundment remains. Whereas weir removal will provide free access up and downstream for all biota, as well as restore hydrological connectivity and sediment movement. Other case studies and guidance can be found in CIRIA’s River Weirs - Design, maintenance, modification and removal guidance.
High working with natural processes
Moderate Working with natural processes
Low working with natural processes
1. If the river has been realigned and there is space
Fish passage benefits - High
Hydromorphology benefits - High
Biodiversity benefits - High
2. If the weir can be removed
Remove a low head weir
Remove a high head weir
Fish passage benefits - High
Hydromorphology benefits - Moderate
Biodiversity benefits - Moderate
3. If the weir cannot be removed
Nature-like bypass
Rock Ramp
2012 / Step-pool cascade fish pass between flood walls
2010 / Weir lowering and rock ramp construction
Modify the weir
Fish passage benefits - Moderate
Hydromorphology benefits - Low
Biodiversity benefits - Low
4. If the weir cannot be modified
Install a fish pass
Fish passage benefits - Low
Hydromorphology benefits - None
Biodiversity benefits - None