Routing a penstock for a hydropower scheme
Penstock is the term given to the pipeline that conveys water from the intake to the turbine house. Once the impacts to geomorphology have been addressed by appropriate siting and design of the intake structure, the next step is to minimise the impact of the penstock on the channel and riparian zone through appropriate routing.
The environmental design of the penstock and its construction is not normally regulated through Water Resource licences but is a requirement of any Town & Country planning consent and also Flood Risk Activity Permits or Ordinary Watercourse consents for works within or adjacent to river channel
The penstock should be routed to leave the river channel as close to the intake structure as possible. This reduces its impact on geomorphological processes and prevents it from obstructing migrating fish. Construction of the penstock must be carried out with minimal disturbance to the river bank and bed to retain ground stability and reduce the future risk of erosion.
Routing the penstock within the river channel should be avoided. In-channel routing is only be acceptable where it enables better siting of an intake structure and the penstock can be securely mounted out of the main flow channel.
The penstock route should be planned to avoid river crossings whether as above ground structures (pipe bridges) or buried across a stream bed, unless their use is advantageous in enabling a lower impact siting of the intake structure. If a river crossing is necessary, then an above-ground pipe bridge should be considered first, with abutments set back from the channel to avoid blockage of flood flow routes.
Construction works involved in buried pipeline crossings are likely to involve major disturbance to bankside and in-channel ground conditions and habitats. They could also lead to unstable geomorphological conditions if bed and banks are not restored to their previous state. In most cases buried pipelines will need to be buried deeply to avoid future exposure caused by stream bed erosion.
If a buried stream crossing is necessary then the bed and banks of the channel at the crossing must be reinstated to their original cross sections, elevations and profiles. Where buried pipelines are constructed in a flood plain area or valley bottom the ground must be reinstated to its original levels to prevent disruption of natural flow routes for overland flood flows. Alternatively, any above ground pipeline should be sensitively routed and mounted to avoid any disruption to flood plain flow paths.
Penstocks should be laid following good practice for pipeline construction. Trenching for the penstock and poor backfilling can inadvertently lead to the penstock route acting as a buried land drain. Where this happens, it can provide a preferential surface or sub-surface flow pathway that alters local drainage patterns and can lead to scour and instability around the penstock itself or adjacent ground. The penstock route should also be constructed to protect local surface drainage pathways and avoid blocking drainage channels or creating embankments that change flow direction, especially in flood plain or wetland areas.
Laying penstocks above ground may be appropriate in some locations where trenching is likely to cause environmental harm, such as damage to protected woodlands or wetland sites vulnerable to changes in natural drainage patterns. Conversely above ground penstocks have a visual impact on the landscape and are likely to require special permissions from Local Planning Authorities.
Best practice construction
Adopting best practice construction methods to reinstate previous ground conditions should minimise disturbance when laying the penstock, enable quicker site recovery and reduce the need for unnecessary hard engineering to stabilise banksides and channel bed. Use of techniques such as placing boulder stone on a bankside or concreting the downstream channel bed is strongly discouraged and would need to be included in any licence application. Hard engineering can disrupt local patterns of flow and sediment transport and can inadvertently lead to increased erosion while geomorphological conditions rebalance. The use of bioengineering techniques for reinstatement of river bed and banks should be implemented wherever possible.
Routing a penstock - key principles
- Route the penstock out of the river channel as close to the intake as possible
- Avoid routing the penstock within the river channel
- Avoid river crossings in the penstock route
- Protect existing drainage patterns when routing and constructing the penstock
- Employ best practice methods for pipeline construction to avoid preferential flow pathways through trenching
- Employ best practice construction methods during construction to minimise impact on the river banks and channel bed
- Reinstate river channel bed and banks to original cross sections, elevations and profiles
- Minimise the use of hard engineering techniques to stabilise river banks and channel bed.