Sewsan Contributor Hydrograph Theory

Contributor hydrograph theory was originally developed by Shaw. The concept is very similar to using unit hydrographs for storm water system design. Each contributory unit (normally a single pipe in the sewer network) "contributes" a 24-hour sewer flow hydrograph to the system.

This hydrograph consists of:

• Local Inflow, contributed by the land parcels serviced by the pipe

• Leakage from plumbing into the system

• Storm water ingress into the system

• Ground water infiltration into the system

• User supplied hydrographs representing the flow where other networks join the network being analysed, where pumps discharge into the network, or where a large system has to be fragmented

The local inflow to a pipe can be very accurately determined in Sewsan if the water consumption of the land parcels connected to the pipes is known. Sewsan then used the annual average daily demand (AADD) inflow calculation method. Alternatively the unit hydrograph peak inflow calculation method can be used.

The two methods differ in the calculation of the volume of the inflow hydrograph. The unit hydrograph peak method uses the expected peak flows associated with the landuse types to calculate the volume of the input hydrograph where as the AADD method bases the volume of inflow on the annual average daily water demand of all the land parcels serviced by the pipe.

The 24-hour contributor hydrograph for each pipe is routed down the network to the outfall manhole using time-lag routing. The time-lag is calculated using full-flow velocities in the pipes according to the Manning formula.