Swindon CSO Chamber

Client: MWH (Optimise)
Scale: 1/10th

The proposed Swindon CSO chamber will be constructed on the main Swindon Trunk Sewer at the point where four culverts combine at a common confluence within the network.  The CSO chamber will include an 8.10m Longwood screen capable of screening a storm overflow of 2500 l/s which equates to a 1 in 5 year storm event.  The CSO chamber also includes a 17.0m long relief weir to accommodate flows in excess of 2500 l/s.

The purpose of the model test was to verify the design over the full range of inflows and water levels to establish the operating characteristics of the proposed arrangement.  Provided operating characteristics were considered acceptable no significant modification to the structure was anticipated.

Initial testing of the Swindon CSO chamber demonstrated a primary area of concern, which was evident when operating under the 1 in 5 year scenario.  There was evidence of flow over topping the relief weir immediately upstream and downstream of the overflow weir and screen.  The overtopping was attributed to turbulence and super-elevated water levels produced as flow passed underneath the screen.  It is critical to the Client that all flow must pass through the screen at the 1 in 5 year event, hence some development work will be required to ensure fully compliant conditions.

In addition a low velocity region immediately downstream of the screen, at the head of the spill culvert was evident.  This area of low velocity was subject to accumulation of solids and very little clearance was noted, irrespective of water level and flow.  This region would require re-benching to minimise solids deposition and promote a self-cleansing chamber.

Final testing determined that the introduction of raised sections to the relief weir local to the upstream and downstream end of the Longwood Screen, prevented the overlapping/topping of the relief weir while operating under the critical 1 in 5 year 50% blinding scenario.  In addition, the proposed infill benching within the spill culvert reduced low velocity areas and hence reduced the potential for accumulation of negative buoyancy material.  Final Testing highlighted that positive and negative buoyancy material passed through the Swindon CSO chamber with very minimal areas of accumulation.