Assessing flood risks for Goring and Streatley hydro9 August 2010
James Heptonstall and Simon Palmer from Peter Brett Associates provide an interesting insight into the methodology used for flood risk modelling on the River Thames in the UK
In March 2009 Dave Holt, hydro project leader of the Goring & Streatley Sustainability Group (GSSG) wrote the article ‘Power to the people’ for IWP&DC. This gave readers an insight into the innovative hydro scheme that is planned for the River Thames at Goring and Streatley in the UK.
The scheme comprises of three Archimedes screws driven by the head difference of the Goring weir complex resulting in an estimated 246KW peak output. This renewable energy will be sold to the National Grid to generate an income that will fund other local sustainable projects in the area.
Since March 2009 Dave Holt and the GSSG have progressed with the project and commissioned Peter Brett Associates (PBA) to assess the flood risk associated with the scheme. Flood risk is an inherent issue with such an installation and needs to be evaluated and mitigated against accordingly. The evaluation of the flood risk therefore lends itself to a comparative exercise comparing the current or base line flood risk against the flood risk with the proposed design in place. This is best evaluated by quantifying flood levels for both scenarios with a solution showing no difference between the two.
Through close liaison with the Environment Agency, PBA agreed a methodology to assess the scheme using the powerful tool of hydraulic river modelling.
Goring and Streatley weir
Historically, water levels within the non-tidal section of the River Thames have been heavily modified and managed through the construction of weirs and locks for navigation and for powering numerous mills for industry.
These weir complexes are still used today for managing water levels for navigation and also for managing the flood risk to the towns and villages that have expanded into the Thames floodplain.
Typically a weir complex consists of fixed crest weirs with additional sluice or radial gates to manage water levels during times of high or low flows.
The Goring and Streatley weir complex consists of two separate areas divided by Withy Eyot. On the western (Streatley) side, the weir complex incorporates a mill channel, two gated weirs, a Paddle and Rymer weir, and a series of overflow weirs. On the eastern (Goring) side, the weir complex consists of three gated weirs, a series of overflow weirs, a lock island, a lock and a mill channel.
The proposed hydro power scheme is to be situated on the 16m long overflow weir adjacent to the lock keeper’s house on the Goring side of the Thames. The screws would effectively obstruct 13m of weir from the Goring and Streatley weir complex.
Flood risk assessment
English government Planning Policy Statement 25 (PPS25) requires that new development does not create an increase in flood risk to third parties.
The complexity of the Goring and Streatley weir complex and flood hydraulics of the Thames along this reach required a hydraulic modelling approach to satisfy the requirements of PPS25.
The Environment Agency, as regulators, worked closely with the GSSG to produce a modelling specification that utilised their existing strategic flood risk mapping ISIS (1 Dimensional) hydraulic model.
Using this model, PBA were commissioned by the GSSG to first provide a review of the existing hydrology and hydraulics of this ‘baseline’ model to assess the suitability of the model for assessing flood risk at Goring.
Secondly PBA were requested to produce a design model that included the three Archimedes screws, associated housing and control building. Each model would be run for three increasingly greater magnitude flood events (1:5 year (222m3/sec), 1:20 year (284m3/sec) and 1:100 year (355m3/sec). The predicted increase in flows over the lifetime of the development from climate change was also accounted for by increasing the 1:100 year flow by 20%.
A comparative exercise would be then carried out to assess how the hydro power scheme impacted on flows across the weir system and how this might impact on flood risk to third parties.
Stage 1: model review
The Environment Agency Strategic ISIS hydraulic model covers the area between Sandford on Thames, 32km upstream of Goring, and Whitchurch on Thames, 8km downstream of Goring.
The model represents the channel and floodplain using either extended cross-sections or via a quasi two-dimensional approach using lateral spills into reservoir units that represents large flood storage areas within the Thames floodplain.
The strategic nature of the model meant that in rural areas, where the channel and floodplain are uniform, model detail was low with cross-sections spaced anywhere up to 700m apart. However around the weir complexes such as at Goring and Streatley, the model schematisation was found to be detailed and provide a good representation of the weir hydraulics and therefore suitable for the flood study.
The hydrology for the model was originally based on the Dunsmore technique that estimates design flows from a flow-catchment area relationship that can be derived from the long gauging records on the Thames. The method was originally derived in 1991. For this project PBA reviewed the results from the method using the additional 17 years of extra flow gaugings made available by the Environment Agency, which had been recorded on the Thames gauge network since the method was derived. The method was found to still provide a reliable estimate of design flows and so the hydrology was left unchanged.
Stage 2: hydro power modelling
The main components of the scheme include three, 3.6m diameter Archimedes screw turbines, a control building and an additional gated weir for mitigation.
The control building is to be designed to flood with any sensitive plant set above the 1:100 year flood level including an allowance for climate change and freeboard. The building itself will be small enough not to impact on flood flows.
The three turbines are expected to occupy a 13m section of the 16m wide fixed crest weir adjacent to the lock house. The additional gated weir is then proposed to occupy the remaining 3m of the fixed crest weir.
A conservative assumption was made that the three turbines would create a complete blockage over 13m of the existing fixed crest weir. This section of the weir was therefore removed from the model.
Initial analysis of the model showed that the turbines had greatest impact at low return period flood events, redistributing flow across the other structures. The results also showed that the weir rapidly became drowned out with increasing flows, caused by backing up from a downstream railway embankment and bridge. As the weir became drowned out, the impact from the turbines reduced to displacing less than 4% of the total flow in the river, causing no change in flood level.
Due to the sensitivity of some property in Goring and Streatley to relatively frequent flood events, it was important that mitigation was provided at the low return period events (1:5yr to 1:20yr design events) to ensure no increase in flood risk to these receptors.
A gated weir was therefore proposed and modelled with a lower fixed crest than the existing weir. This effectively allowed an increase in flow across the structure at the lower return period flood events, fully mitigating for the hydropower scheme.
Stage 3: approval
A flood risk assessment has been written based on the modelling results and submitted to the Environment Agency, alongside the hydraulic model for approval. The project has been run in close liaison between the Environment Agency, GSSG and PBA which has thus far proved effective in moving the project forward in a consistent direction. It is expected that this will continue with planning permission being sought later this year.