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City of Morgan Hill
Sanitary Sewer Flow Monitoring and Inflow/Infiltration Study
12-0248 AEG CofMorganHill FM Rpt.docx
Page 43 of 46
Inflow / Infiltration: Synthetic Hydrographs
In order to model design storms, synthetic hydrographs were developed to approximate the actual
RDI hydrograph shape in terms of the time to the peak and the recession coefficient. The actual RDI
hydrograph was best matched with a synthetic hydrograph by separating the synthetic hydrograph
into seven volume components (R1 through R7). The seven components represent different
response times to the rainfall event and, therefore, different infiltration or inflow paths into the sewer
system. R1 is characterized by a short response time and is assumed to consist of mainly inflow. R7
represents slower response and longer recession times and consists of mostly infiltration. Levels of
soil saturation are also considered. Using synthetic hydrograph analysis, appropriate time and
recession parameters were estimated by a trial-and-error procedure until a good match was obtained.
For example, the hydrograph and its component hydrographs for Storm Event 1, for Site 3 is shown in
Figure 31.Figure 31. Site 3, Storm Event 1: Synthetic Hydrograph
Rain
Realtime I/I Hydrograph
Synthetic Hydrograph
R1 Component
R2 Component
R3 Component
R4 Component
R5 Component
R6 Component
R7 Component