Improving the flood resistance of sewer systems through strategic positioning and design of flow controls

• Main Author: Newton, Christopher John
• Other Authors: Memon, Fayyaz Ali ; Butler, David ; Jarman, Daniel Stuart ; Andoh, Robert Yaw Gyamfi
• Published: University of Exeter 2016
• Subjects: 628.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.693728

Sewer systems are designed and installed to convey stormwater and wastewater from areas where it is collected to a treatment works or a suitable discharge point into a natural watercourse. Over time, however, changes in climate, population growth and urbanisation increase the flows these sewer systems need to convey. This thesis discusses a method, and developed assessment framework, to propose solutions to address flood risk. The method involves iteratively simulating a sewer system and designing vortex flow controls and orifice plates into the sewer system. These passive flow controls are designed into the sewer system, whilst accounting for sub-catchment flood vulnerability scores, to attenuate potential flood volumes and make use of available storage capacities in the upstream sections of the sewer system. The flow control positioning method uses the SWMM 5 program to simulate the sewer system, a developed hyetograph generator for the rainfall inputs and a flow control design tool. The whole assessment framework was programmed in Microsoft Excel and VBA. Vortex flow controls and orifice plates were chosen as they require no power, have no moving parts and are self-activating. Vortex flow controls are primarily designed into the sewer system, over orifice plates, due to their higher mean flow-rate and reduced blockage risk due to the larger outlet diameters. Four sewer system models were used to demonstrate the application of the assessment framework, in which one model was hypothetical and three were anonymised. The smallest model consisted of 14 nodes and collected surface runoff from 1.5 hectares. The largest model consisted of over 280 nodes and was a combined sewer system from a town in the South-East of England. In the application of the assessment framework, it was shown that strategically positioning and designing passive flow controls into the sewer system can have a beneficial effect by reducing flood risk. In one case study, the flood resistance level of a sewer system was increased from a 1 in 3 year return period to a 1 in 108 year return period. In the largest case study, which was of a combined sewer system, the flood resistance level was increased from a 1 in 1 year return period to a 1 in 71 year return period. The results of the analyses also found that installing vortex flow controls, instead of orifice plates only, tended to achieve a greater increase in the flood resistance level. Comparison of the proposed solutions from the method to alternative flood alleviation methods shows that the strategic positioning of passive flow controls to be a competitive and feasible solution to reduce flood risk.