Several methods have been used to estimate rainfall derived inflow and infiltration (RDII), but there is little consensus on either how to apply these methods or which are most appropriate for particular catchments. A review of the literature demonstrates the merits and drawbacks of the various RDII estimation techniques examined, including the constant unit rate, percentage rainfall, percentage streamflow, return period, synthetic streamflow regression, rainfall/flow regression, synthetic unit hydrograph (SUH) methods, and software implementations of these methods.
A review of case studies shows that the most commonly applied methods are the constant unit rate method, percentage rainfall method and SUH method. The most commonly applied SUH method is the RTK method, which is available in the U.S. EPA’s Storm Water Management Model (SWMM). The RTK method uses 3 triangular unit hydrographs with 3 parameters each yielding 9 parameters in total. The number of parameters allows the method to better fit the data, but leads to calibration problems. The U.S. EPA’s Sanitary Sewer Overflow Analysis and Planning (SSOAP) Toolbox is meant to simplify the calibration process and its merits are discussed.
Lengthy flow monitoring records coupled with rainfall records are needed to use SSOAP effectively. A discussion of flow monitoring practices and perceptions of monitoring devices illustrates the difficulties that arise when obtaining data to calibrate the RTK parameters.
As RDII is generated by stochastically occurring rainfall events, the corresponding infiltration/inflow rates are also stochastically determined. Ideally, the estimation of RDII flow rates should be performed in concert with their associated frequencies of occurrence. Various methods to associate occurrence frequencies with RDII magnitudes are reviewed and discussed, and an assessment of current challenges is presented.