This presentation contains 20 slides and note pages on CIRIA C763 River weirs: design, maintenance, modification and removal. Research contractor: JBA Consulting, South Barn, Broughton Hall, SKIPTON, BD23 3AE, 01756 799 919. Prepared by: Amanda Kitchen, lead author, August 2016.

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Weirs typically perform the following functions: • water level management: impound water for navigation, or manage water levels

for land drainage or flood risk management; • flow measurement: provide a unique stage-discharge relationship; • channel stabilisation: dissipate energy at a defined location or manage water levels

for the stability of riverside structures; • environmental enhancement: divert water to wetlands, provide habitat diversity or

improve water quality and ecology, provide recreation, or enhance the landscape; • commercial and other: impound water for abstraction or hydropower, provide fish

counting or fishing for human use. Weirs may be of historical significance, either on their own, as part of an historic water management system (e.g. mill) or by contributing to the aesthetic setting or soundscape of a heritage asset. Weir may also form part of a designed landscape, as an architectural feature, by impounding water or by altering the quality of water as it flows through a landscape. Case study: Pulteney Weir, Bath This weir is an attractive curved weir and part of a World Heritage Site.

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The Environment Agency River weirs – good practice guide was published in 2003. The guide was aimed at all involved in the planning, design and improvement of weirs, as well as operation and maintenance. It also briefly covered demolition of weirs. The guide aimed to ensure that mistakes were avoided and opportunities were not missed. The guide gave advice on law and planning, safety, engineering and environmental. It also give many photographs and case studies to illustrate the points made. Although comprehensive, lessons have been learned in terms of operational safety since the first publication, and the implementation of the Water Framework Directive (WFD) 2000 has led to a greater focus on weir removal.

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Weirs may impact on of water users, land users, inspection and maintenance staff. The primary hazards are: • hydraulic: deep water and submerged hydraulic jump downstream, aeration

leading to loss of buoyancy, strong currents, sudden changes in conditions , esp sluice gates;

• physical: submerged hazards, vertical wingwalls, steep or slippery surfaces and riverbanks, movable gates, mechanical and electrical equipment;

• chemical: contaminated sediment due to historic land use may be mobilised during works;

• biological: Weil’s disease (Leptospirosis) – impounded water ideal for retaining and transmitting.

Case study: weir safety improvements, River Tryweryn, Gwynedd • flow measurement weir near town centre; • quiet with submerged hydraulic jump; • dogwalker drowned attempting to rescue dog caught in re-circulating flow; • modified to reduce hazard following numerical and physical modelling; • low flow crest shortened and high flow crest extended inwards and lowered to

increase flow velocity whilst avoiding an increase in flood risk; • ramp added to reduce deep recirculation; • islands added downstream to increase flow depth and flush objects through.

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Weirs may impact on of water users, land users, inspection and maintenance staff. The primary hazards are: • hydraulic: deep water and submerged hydraulic jump downstream, aeration

leading to loss of buoyancy, strong currents, sudden changes in conditions , esp sluice gates;

• physical: submerged hazards, vertical wingwalls, steep or slippery surfaces and riverbanks, movable gates, mechanical and electrical equipment;

• chemical: contaminated sediment due to historic land use may be mobilised during works;

• biological: Weil’s disease (Leptospirosis) – impounded water ideal for retaining and transmitting.

Case study: weir safety improvements, River Tryweryn, Gwynedd • flow measurement weir near town centre; • quiet with submerged hydraulic jump; • dogwalker drowned attempting to rescue dog caught in re-circulating flow; • modified to reduce hazard following numerical and physical modelling; • low flow crest shortened and high flow crest extended inwards and lowered to

increase flow velocity whilst avoiding an increase in flood risk; • ramp added to reduce deep recirculation; • islands added downstream to increase flow depth and flush objects through.

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Weirs may impact on of water users, land users, inspection and maintenance staff. The primary hazards are: • hydraulic: deep water and submerged hydraulic jump downstream, aeration

leading to loss of buoyancy, strong currents, sudden changes in conditions , esp sluice gates;

• physical: submerged hazards, vertical wingwalls, steep or slippery surfaces and riverbanks, movable gates, mechanical and electrical equipment;

• chemical: contaminated sediment due to historic land use may be mobilised during works;

• biological: Weil’s disease (Leptospirosis) – impounded water ideal for retaining and transmitting.

Case study: weir safety improvements, River Tryweryn, Gwynedd • flow measurement weir near town centre; • quiet with submerged hydraulic jump; • dogwalker drowned attempting to rescue dog caught in re-circulating flow; • modified to reduce hazard following numerical and physical modelling; • low flow crest shortened and high flow crest extended inwards and lowered to

increase flow velocity whilst avoiding an increase in flood risk; • ramp added to reduce deep recirculation; • islands added downstream to increase flow depth and flush objects through.

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Weirs have potential impacts on fisheries and ecology: • delay or obstruct migration • even with fish pass, reduces likelihood of reproductive success • more likely to have isolated and genetically impoverished populations • increase in species adapted to still waters (such as carp) Water Framework Directive aims to maintain or improve the chemical and ecological status of watercourses and to restore surface waters to a more natural state where technically feasible. This is, in order to protect human health, water supply, natural ecosystems and biodiversity. There is also a legal duty to avoid obstructing fish and eel passage under the Salmon and Freshwater Fisheries Act 1975. In waters frequented by salmonids, fish passage must be provided if a weir taken down to the extent of one-half of its length is rebuilt or reinstated. Case study: weir on River Wharfe, West Yorkshire • impassable to fish except in very high flows; • during lower flows, fish jump a couple of metres, attempt to swim, then are

washed backwards as the spillway gets steeper towards the crest; • a fish pass is under construction as part of a hydropower scheme.

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Weirs can also impact on geomorphology, water quality and habitat. Impacts on geomorphology include: • change in flow regime both upstream and downstream; • increased water level, depth and flow width, and reduced flow velocity, leading to

deposition of sediment upstream; • reduced sediment transport downstream and potential sediment starvation; • change in floodplain wetting frequency.

Potential impacts on water quality: • low dissolved oxygen upstream due to reduced re-aeration, increased retention

time and increased oxygen consumption by accumulated sediment (especially if polluted);

Potential impacts on habitat: • reduced marginal vegetation and diversity, leading to invasive species colonisation; • differing conditions in velocity and depth upstream and downstream; • loss of connectivity between upstream and downstream habitats.

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New river weirs guide will replace Environment Agency 2003 River weirs – good practice guide. It is due December 2016. Digital issue only to start with, free to download from CIRIA website. Although it is acknowledged that the majority of work on weirs is still carried out to maintain their current function(/s), the new guide leans heavily towards the alteration of weirs to benefit ecology, reflecting an industry need for greater guidance on topics such as geomorphology, environmental issues, alternatives to weirs and weir removal. The new guide covers weirs on rivers, canal and canalised rivers in the United Kingdom (the old guide covered England and Wales). It does not cover dams, reservoir spillways, estuarine barrages or structures regulating natural lakes. The new guide is aimed at those involved in the planning, design, construction, maintenance, modification or removal of weirs, including: • regulatory authorities; • professionals: engineering consultants and contractors, architects; • weir owners and preservation societies, navigation authorities, heritage bodies,

abstraction licence holders, hydropower promoters, land agents and farmers; • other stakeholders: fisheries owners, angling clubs and recreation bodies.

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The work was funded by: • DARDNI; • Environment Agency; • Scottish Government. The work was guided by a UK-wide project steering group, including: • River Restoration Centre; • government bodies; • environmental bodies; • recreation bodies • heritage bodies • consultants • hydropower promoters The research team was led by JBA Consulting with the support of Mott MacDonald and FAS Heritage, giving a broad range of interests and experience.

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The guide aims to lead the reader through the process of managing weirs and is presented in three parts: • Part 1 Overview: for everyone interested in weirs; • Part 2 Essentials of weir management: for those responsible for policy and

decision-making; • Part 3 Detailed technical guidance: for those responsible for project delivery and

asset management. Examples and case studies are given throughout.

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The need to intervene will depend on: • the legal and policy framework; • the function of the weir and any operational constraints; • the performance of the weir, its condition and the rate of change; • impacts of the weir on natural processes; • site context, land ownership, existing land use and future development plans; • opportunities due to funding or other works nearby. Broadly speaking, if weir performs well, has acceptable impacts and there are no opportunities to address, there is no need to intervene. Otherwise, consider doing something. Case study: partial replacement of paddle and rymer weir, River Thames • historically significant weir with boards (paddles) manually inserted into river

between vertical timber posts (rymers) and lifted to manage water levels and/or allow navigation;

• development of a flash lock, which was used to allow river navigation before the introduction of pound locks;

• operatives required to lift weights three times above recommended limits; • weir was partially replaced with a modern structure and a section of the historic

weir retained.

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Alternatives to weirs can mitigate geomorphological, environmental and safety impacts. They should be considered in preference to a new weir, or if replacing or removing an existing weir. The choice of approach depends on the objectives, catchment processes and river type. Methods should be chosen to suit the natural processes of the catchment. Water level management measures such as rock ramps or morphological features emulate the natural river channel. Non-intrusive flow measurement methods such as ultrasonic gauges or flumes can be installed on open channels, but have limitations in accuracy compared with weirs. Research and development is ongoing to overcome functional drawbacks. Non-intrusive abstraction may use bankside, river bed or submerged intakes, or create deeper zones of water by modifying the channel width or introducing morphological features; Natural flood management improves connectivity between river and floodplain (where this does not impact on people, property or valuable land). It is often used to reduce the severity of smaller, more frequent flood events rather than eliminate flood risk associated with more extreme events.

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Weir removal: • generally beneficial; • improves fish passage, geomorphological condition and processes, habitat quality,

availability and connectivity; • reinstates natural river processes; • river response strongly dependent on local conditions and variables; • assessments of impacts should be undertaken by experienced geomorphologist. Case study: weir removal on Afon Aran, Snowdonia • weir on a salmon, sea trout and eel fishery removed as part of a flood risk

management scheme; • confined site in busy town, with services present in bridge and beneath weir; • weir removal increased diversity of habitat; • additional baffles were needed below the bridge to facilitate fish passage; • erosion checks were installed at the toe of riverside walls to prevent undermining; • weir removal reduced noise and this was seen as a benefit by residents; • the timing of the works important – a traffic light scheme was used to identify the

best timing; • consultation with residents, councillors and other stakeholders was essential.

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Weir lowering, partial removal or breaching: • can be viable where the risks associated with full removal are high; • can maintain acceptable stability upstream of the weir; • can maximise improvements in natural geomorphological processes (i.e. partial

removal of the impoundment to allow some recovery in the natural flow and sediment regime upstream);

• may be undertaken alongside fish pass construction where the lowering does not satisfy fish passage requirements.

Weir lowering may be used as a compromise solution: • to give a moderate increase in water surface gradient upstream; • to allow some sediment transport; • to improve geomorphological condition of the channel upstream. Partial removal or breaching involves removing portion of weir laterally (rather than vertically as for weir lowering): • this can maintain channel stability over one side of the channel (e.g. where critical

infrastructure is located).

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Weir infilling may be viable if weir removal or lowering is not possible: • involves placing fill across and downstream of the weir using suitable bed material

(e.g. rock ramp) to remove the ‘step’ created by the presence of the weir; • gradually reduces head difference through design of a suitable slope; • improves river continuity and fish passage.

Case study: rock ramp at Sharpsbridge, East Sussex • solid concrete bridge footing acted as a weir; • drop between concrete slab and downstream water level acted as a barrier to fish

passage; • rock ramp installed and boulders embedded in concrete; • during low flows, flow velocities between perturbation boulders and water depth

are good for fish migration; • at high flows, river is slower over high-flow channel and sufficiently deep for fish

passage.

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Weir infilling may be viable if weir removal or lowering is not possible: • involves placing fill across and downstream of the weir using suitable bed material

(e.g. rock ramp) to remove the ‘step’ created by the presence of the weir; • gradually reduces head difference through design of a suitable slope; • improves river continuity and fish passage.

Case study: rock ramp at Sharpsbridge, East Sussex • solid concrete bridge footing acted as a weir; • drop between concrete slab and downstream water level acted as a barrier to fish

passage; • rock ramp installed and boulders embedded in concrete; • during low flows, flow velocities between perturbation boulders and water depth

are good for fish migration; • at high flows, river is slower over high-flow channel and sufficiently deep for fish

passage.

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Weir infilling may be viable if weir removal or lowering is not possible: • involves placing fill across and downstream of the weir using suitable bed material

(e.g. rock ramp) to remove the ‘step’ created by the presence of the weir; • gradually reduces head difference through design of a suitable slope; • improves river continuity and fish passage.

Case study: rock ramp at Sharpsbridge, East Sussex • solid concrete bridge footing acted as a weir; • drop between concrete slab and downstream water level acted as a barrier to fish

passage; • rock ramp installed and boulders embedded in concrete; • during low flows, flow velocities between perturbation boulders and water depth

are good for fish migration; • at high flows, river is slower over high-flow channel and sufficiently deep for fish

passage.

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Weir infilling may be viable if weir removal or lowering is not possible: • involves placing fill across and downstream of the weir using suitable bed material

(e.g. rock ramp) to remove the ‘step’ created by the presence of the weir; • gradually reduces head difference through design of a suitable slope; • improves river continuity and fish passage.

Case study: rock ramp at Sharpsbridge, East Sussex • solid concrete bridge footing acted as a weir; • drop between concrete slab and downstream water level acted as a barrier to fish

passage; • rock ramp installed and boulders embedded in concrete; • during low flows, flow velocities between perturbation boulders and water depth

are good for fish migration; • at high flows, river is slower over high-flow channel and sufficiently deep for fish

passage.

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The impacts of weir failure may include: • reduced upstream water levels; • increased stream power and erosion; • increased sediment transport (including possibly contaminated fine sediment); • knick point recession, leading to lowering of water levels; • potential river bank collapse; • disconnection between floodplain and river channel. Mitigation may be required to prevent environmental harm (e.g. flooding or mobilisation of contaminated sediment). Repair or replacement may also be necessary if the weir is still needed to perform a function. Case study: weir failure on River Browney, Co.Durham • immediate and longer-term impacts of weir failure observed; • within a fortnight: rapid acceleration of flow in the near-bed zone, bed

mobilisation, knick-point migration and bed lowering upstream, bank collapse due to bed lowering and flow acceleration, sediment release downstream;

• after twelve months: modest channel adjustments, further bank adjustment downstream of knick-point, encouraged by trees loading destabilising the riverbank, knick-zone similar location, less obvious, shallower in gradient.

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The impacts of weir failure may include: • reduced upstream water levels; • increased stream power and erosion; • increased sediment transport (including possibly contaminated fine sediment); • knick point recession, leading to lowering of water levels; • potential river bank collapse; • disconnection between floodplain and river channel. Mitigation may be required to prevent environmental harm (e.g. flooding or mobilisation of contaminated sediment). Repair or replacement may also be necessary if the weir is still needed to perform a function. Case study: weir failure on River Browney, Co.Durham • immediate and longer-term impacts of weir failure observed; • within a fortnight: rapid acceleration of flow in the near-bed zone, bed

mobilisation, knick-point migration and bed lowering upstream, bank collapse due to bed lowering and flow acceleration, sediment release downstream;

• after twelve months: modest channel adjustments, further bank adjustment downstream of knick-point, encouraged by trees loading destabilising the riverbank, knick-zone similar location, less obvious, shallower in gradient.

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The impacts of weir failure may include: • reduced upstream water levels; • increased stream power and erosion; • increased sediment transport (including possibly contaminated fine sediment); • knick point recession, leading to lowering of water levels; • potential river bank collapse; • disconnection between floodplain and river channel. Mitigation may be required to prevent environmental harm (e.g. flooding or mobilisation of contaminated sediment). Repair or replacement may also be necessary if the weir is still needed to perform a function. Case study: weir failure on River Browney, Co.Durham • immediate and longer-term impacts of weir failure observed; • within a fortnight: rapid acceleration of flow in the near-bed zone, bed

mobilisation, knick-point migration and bed lowering upstream, bank collapse due to bed lowering and flow acceleration, sediment release downstream;

• after twelve months: modest channel adjustments, further bank adjustment downstream of knick-point, encouraged by trees loading destabilising the riverbank, knick-zone similar location, less obvious, shallower in gradient.

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The impacts of weir failure may include: • reduced upstream water levels; • increased stream power and erosion; • increased sediment transport (including possibly contaminated fine sediment); • knick point recession, leading to lowering of water levels; • potential river bank collapse; • disconnection between floodplain and river channel. Mitigation may be required to prevent environmental harm (e.g. flooding or mobilisation of contaminated sediment). Repair or replacement may also be necessary if the weir is still needed to perform a function. Case study: weir failure on River Browney, Co.Durham • immediate and longer-term impacts of weir failure observed; • within a fortnight: rapid acceleration of flow in the near-bed zone, bed

mobilisation, knick-point migration and bed lowering upstream, bank collapse due to bed lowering and flow acceleration, sediment release downstream;

• after twelve months: modest channel adjustments, further bank adjustment downstream of knick-point, encouraged by trees loading destabilising the riverbank, knick-zone similar location, less obvious, shallower in gradient.

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A bypass channel around a weir may suitable if no modification to a weir structure is possible to improve fish passage or geomorphological condition (e.g. due to heritage value): • improves ecological and geomorphological condition; • nature, length, slope and geometry depends on surrounding land use and

conditions at weir; • may retain some flow over weir and divert remainder through bypass channel.

Case study: replacement weir on Kennet & Avon Canal, West Berkshire • sheet pile and concrete weir constructed to replace a failing gabion weir; • weir was constructed on same line as existing weir; • safety boom installed to prevent craft from reaching the weir; • weir boards included to provide a means of preventing water flow over sections of

the weir for future inspection and maintenance, and to allow the weir level to be adjusted slightly if unintended issues with the design level are found in use, preventing expensive remedial works;

• fish bypass channel was constructed for migratory salmon and trout; • lessons learnt included the need to start landowner negotiations early and consult

stakeholders.

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The useful life of a weir foundation usually exceeds that of operational equipment, hence the serviceable life of weir can often be extended by rehabilitation and repair. Repair and refurbishment are more common than new build or replacement schemes, and some weirs see multiple refurbishments before being decommissioned. Repairs make good damage that may have occurred over time. Rehabilitation is a more fundamental intervention to rejuvenate elements of the weir, typically back to the original design standard. Improvement enhances the ability of the weir to deal with contemporary requirements. Key issues are to understand the structure, assess its condition (not always easy if it is submerged or there are no drawings), consider fish and eel passage, operation, maintenance, public safety. De-watering and temporary works can be tricky. Case study: Horrabridge weir, Devon • former mill weir was re-engineered for flow gauging as part of a flood alleviation

scheme; • improved efficacy of existing fishpass and provided an eel pass; • repaired the existing structure in the same style as the original, using reclaimed

stone on site to clad the new concrete fish pass; • undertook de-silting.

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Often there are conflicting demands on weirs and their owners, and a weir which meets functional objectives may present an operational safety hazard and difficult rescue conditions. A hierarchy of measures to improve safety at weirs is recommended: • eliminate or reduce hazard by removing or modifying the weir; • reduce likelihood of harm by installing warning signs, fencing or safety booms to

discourage access, providing a safe egress point and portage around a weir for canoeists, providing water safety education for water users and information for professionals and stakeholders;

• facilitating rescue installing equipment to facilitate self- or assisted rescue (as a measure of last resort).

Case study: warning, danger and restricted zones, USA • warning, danger and restricted zones are defined upstream and downstream of

weirs to control access to the hazardous areas; • standards specify the location and format of signage, which should be directed

towards water users as well as land users.

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Reasons for a new weir: • replace an existing weir that is no longer economic to repair; • replace an existing weir that does not cater for current requirements and is not

economically adaptable (e.g. excessive manual force is required to operate the weir);

• new functional requirement has been identified (e.g. a new abstraction). Key issues: • site selection: consider topography, geology, geomorphology, environment, flood

risk, hydrology, ease of river diversion (amongst other things); • design flows: consider acceptable afflux, topography of upstream river banks and

value of flood receptors present. The design life of new weirs can be 50 to 200 years, so decisions made during design may last several generations into the future. In making the decision to build a new weir, it is important to: • be aware of all of the issues and potential conflicts; • consult interested parties in order to seek the best solution; • involve engineering, ecological and geomorphological expertise; • to ensure opportunities for best practice are taken.

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Many weirs perform useful functions and may be historically significant or contribute to the landscape. However, they can impact on natural environment and safety , for example: • obstruct or delay fish and eel migration; • disrupt sediment transport along a watercourse; • affect water quality, habitat and geomorphological condition; • affect flora and fauna; • can present hydraulic, physical, chemical and biological hazards. Weirs which no longer perform a function may be removed to reduce maintenance costs, restore fish passage and support the achievement of Water Framework Directive targets. The decision to remove a weir will depend on function and whether removal will cause loss of economic function or increase risk to property, structures, aquatic ecology or dependent wetland habitats. If full removal is not feasible, partial removal, lowering or by-passing may be possible. Consider alternatives to weirs. If these are not possible, opportunities for improvement should be identified. If constructing a new weir, the location should be chosen carefully and the weir designed to minimise impacts.

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