Hydraulic design and engineering aspects of combined sewers

Several cities of India were developed in the colonial era and provided with a combined system for sanitation and disposal of storm run-off. Further extension of the sanitation infrastructure had to involve a combined system in order to be consistent or address other constraints, generally congestion. For example, Kolkata receives > 1600 mm of average yearly rainfall, mostly from July to September. Storm run-off from any catchment is estimated to be about 80 to 120 times the corresponding sewage flow. Hence, only a small portion of the conduit's capacity is used during the dry period constituting the majority of the time (nine months). This paper summarises the rationale behind the general hydraulic design philosophy pertaining to the expansion of such a combined sewerage system. The approach uses differential values of Manning's coefficient (n) dependent on flow depth and restricts adoption of the q_actual/Q_full ratio to 1·0 to address potential surcharge condition. A reasonable and approximate procedure of hydraulic design using a circular conduit is described thereafter utilising basic design charts and simple logic functions. Lastly, a set of design evaluation parameters are suggested for comparison and optimisation of such hydraulic design. These parameters, once calculated for similar designs, can be mutually compared and used as indicators to assess the level of optimisation achieved in any particular design.