Penwhirn reservoir enlargement

Client: Black & Veatch
Scale: 1/20th

The Penwhirn Reservoir impounds raw water for public supply, with treatment being undertaken at the works immediately downstream behind an earthfill embankment approximately 240 m long and 16 m high at a crest elevation of approximately 159 mAOD. The existing ogee spillweir is approximately 27 m long. The spillweir discharges into a tumble bay channel, prior to a stepped cascade spillway channel approximately 6 m wide and 150 m long. The spillway chute falls 10 m to discharge to a rock lined channel, discharging into a natural watercourse. It was proposed to construct a new labyrinth weir in place of the existing ogee spillweir.

The existing ogee overflow arrangement is of concrete construction, faced with dressed masonry. An access bridge crosses the tumble bay with four, central piers, located on the weir crest. It is proposed that the existing ogee weir is to be replaced by a labyrinth weir arrangement.
The purpose of the model study was to determine the hydraulic characteristics of the proposed Labyrinth overflow weir arrangement and, were necessary, develop modifications to ensure that the works can safely pass the specified PMF flows.

Additionally, flow must be adequately contained within overflow channel and out of channel flow directed away from the reservoir embankment, in order to maintain a satisfactory degree of reservoir safety.