Siting an intake weir for hydropower
The most effective way of reducing the environmental impact of an intake weir is to choose the right location for it in the watercourse. Some conventional designs can often have adverse environmental impacts simply because they are in the wrong place and basic design principles are overlooked. It is also important to recognise that some rivers or streams do not have suitable locations for the construction of a weir.
Benefits of well sited structures may include:
- enabling a smaller hydraulic structure to be built
- minimising the need for bank or bed stabilisation work
- increasing structural stability
- reducing maintenance needs
- reducing construction effort and costs
Intake weirs must be designed to suit the specific characteristics of each site with the intention of replicating naturally occurring channel features nearby. They should be designed to consider a ‘whole life approach’ from installation, through operation and maintenance to decommissioning. A design can incorporate several good practice design principles, but there is not a ‘one size fits all’ solution.
Opportunities for low-impact intake weir sites can be identified by looking at:
- the natural features within a river
- local geology
- bed material (size, shape & quantity)
- bank profiles and elevations,
- channel gradient and the position within the overall long profile
- bank side habitat (e.g. avoidance of bank side trees or other valuable features).
Bed gradient, weir height and backwater length
New intake weirs should be sited in reaches with a steep stream bed gradient and have a low crest height relative to the upstream level of the natural stream bed. They should be sited on or immediately downstream of natural steps, cascades or falls in the river channel. This will reduce the depth and length of the backwater created upstream of the weir and minimise the space to trap and retain sediment. Small backwaters are likely to quickly fill up with sediment and allow natural transport of sediment over the weir to begin again.
Sediment transport is likely to be maintained if the backwater is no greater than other naturally formed backwaters or pool features nearby where the river reach has similar channel characteristics to the weir site.
Reaches with low bed gradient can occur in upland areas, particularly in plateau or moorland where the impacts of impoundments on sediment transport can be significant. Many low gradient channels in headwater catchments also have peat soils. These locations are unsuitable for weirs as their backwaters may change local hydrology leading to instability of the stream bed and banks.
The impact of a new weir can be reduced by filling the upstream backwater with boulders or coarse sediment after construction to just below the weir crest. This will reduce the space available to trap downstream-moving sediment and enable onward movement of sediment to quickly begin again. It is important however that stone used for backfilling should not be excavated from other parts of the river bed, banks or channel. Backfilling should not be used as a substitute for poor siting of a weir.
An intake weir with low crest height and short backwater is likely to be more acceptable to neighbouring landowners and means that the size of any fish pass or easement will also be kept to a minimum.
For most high-head, micro-hydropower weirs with an overspill intake, sediment should be allowed to accumulate upstream and does not need removal. It is unlikely to affect abstraction and allows sediment to naturally continue downstream over the weir and prevent any further upstream retention.
Bank and bed material
Weirs should be constructed on solid ground, including bedrock and boulders, which is stable and resistant to erosion. They should also be located where banks consist of stable, consolidated materials that have not been disturbed during the construction process. Construction at a location of unconsolidated bed and bank material risks erosion through piping and wash out under high flows, leading to ground and structural failure.
Weirs should be located on straight sections of river channel. Straight sections are more stable than bends which are prone to the natural processes of erosion and sediment deposition.
Deposition on inside bends can lead to large, upstream accumulations of sediment on one side of a weir structure. This could block or divert stream flow away from intakes, low flow notches and fish passes, potentially compromising scheme operation and needing a high level of scheme maintenance.
Weirs should be set perpendicular to the channel and stream flow direction. Use of angled or V shaped weirs should be avoided as they have a higher risk of changing local flow patterns and altering geomorphological processes. Exceptions to this include larger sites where weir design may need to be accommodate site specific fish pass requirements.
Some watercourses are not suited to the construction of any form of weir.
If it is necessary to locate an impounding weir on a bend (by agreement with our Specialists), then the residual flow weir crest and low flow notch should be located centrally or to the outside of the bend to avoid accumulation of sediment. This will ensure that there is less risk of sediment blocking these components, allowing the residual flow to pass to maintain ecological continuity.
Bank profiles and elevations
Weirs are best sited where the river channel is contained within naturally high banks. This will reduce the risk of water, under high flow conditions, passing around the sides of the weir nd prevent the adjacent banks from eroding and affecting structural stability. Remedial works to stabilise banks from erosion is costly and would require additional permissions and consents.
Upland streams with steeper bed gradient are more likely to have an incised channel and higher bank elevations that will allow high flows to be contained.
During construction, developers should use a method of temporary coffer dams, overpumping or fluming to create a dry working area for the in-river construction of a weir. The practice of excavating a diversion channel should be avoided as it will be highly damaging to the river environment. It also risks creating a preferential flow path in unconsolidated backfill through which water can infiltrate, thereby reducing the stability of reinstated channel banks and bed material which may subsequently be reopened during flood conditions.
The natural width of the river channel at a weir site should not be altered. Streams and rivers naturally develop a channel width that is a combined reflection of the flow regime, catchment topography, land use and geology. Where changes to river width are made there is a risk of destabilising existing natural bed and bank material. Widening a stream channel causes deposition which may compromise scheme operation and geomorphology of a reach. Conversely narrowing the channel, or infilling of natural flood routes, can disrupt the natural stream energy balance, causing excessive erosion that can damage the scheme, geomorphology and habitats of a reach.
Siting the intake to reduce impact on flows
Intakes should be located upstream of inflowing tributaries to partially mitigate the effects of reduced flows caused by abstraction. The incoming flow from the tributary will introduce more flow volume and variability into the depleted reach and help to maintain some natural flow characteristics.
Careful consideration should also be given to sediment load of tributary inflows. Whilst it may be attractive to site an intake immediately downstream of a tributary inflow to maximise the amount of water for a hydropower scheme, a confluence is also the point at which the sediment load from a tributary catchment enters the main river. This often creates conditions where a high level of sediment deposition and accumulation occurs.
Existing boulders within river banks and beds should be retained in their natural positions and not disturbed during the construction process. Large boulders and accumulations of coarser sediments, such as rocks and cobbles, are in most cases established geomorphological features that provide stability to the river channel under conditions of high flows. Disturbing these ‘anchor’ features risks destabilising a river channel, potentially leading to increased erosion while the geomorphological processes at the site seek to rebalance and return the channel to a natural condition.
Where to locate intake weirs - key principles
- Locate intake weirs in reaches with a steep bed gradient that result in short backwater lengths (backwater length should be no greater than natural backwater lengths in the same reach.)
- Locate intake weirs at naturally occurring step features in the river channel.
- Keep the weir crest height as low as possible.
- Avoid low gradient, plateau or moorland sites.
- Locate intake weirs at sites with stable bed and bank materials and no evidence of previous erosion, undercutting or channel migration.
- Locate intake weirs in narrow valley sections with an incised channel and high banks.
- Locate intake weirs on straight, stable sections of rivers.
- Locate impoundments upstream of inflowing tributaries.
- Retain the natural width of a river channel.
- Design weirs to be perpendicular to the river flow – avoid use of angled or V shaped weirs.
- Minimise disturbance to the construction site, especially river banks and bed.
- Retain existing boulders, rocks and cobbles in their natural positions in the river channel and banks outside the footprint of the weir structure.
- Backfill the backwater upstream of a new weir with boulders and rocks from outside the river channel.