ASTM Rainfall Weighting Factors

This paper outlines a new ASTM method that has been developed to define regional rainfall intensity weighting factors to assess annual pollutant removal in areas of differing hydrology based on verified pollutant removal efficiencies from testing protocols such as the ETV Canada OGS laboratory protocol and NJDEP HDS laboratory protocol. Stormwater control measures (SCMs) are typically sized by converting local rainfall intensities into flow rates and assessing the performance of the SCM over the range and frequency of flow rates anticipated on an annual basis. This analysis can be done using a continuous simulation computer model that converts local rainfall data into a continuous series of flow rates. The computer model must include a method of calculating the mass of pollutant during each flow rate based on some form of pollutant build-up during dry days (exponential, power-linear, etc.) and a form of wash-off during periods with rainfall (power-exponential, rating curve). In its simplest form, an unlimited supply of pollutant can be modeled in each flow as a constant concentration (EMC).  In addition, the model must include a method of calculating pollutant removal performance which has been calibrated to actual field or laboratory testing removal results. The analysis can also be done using a spreadsheet calculation of annual performance instead of a computer model by conducting a frequency distribution analysis of the rainfall data to characterize the annual hydrology combined with pollutant removal performance data from laboratory testing.  The percentage of total rainfall recorded for each rainfall intensity frequency is defined as the weighting factor for that rainfall intensity. Each rainfall intensity is then multiplied by the site area and runoff coefficient to determine a corresponding flow rate. Independently verified pollutant removal rates corresponding to each flow rate are then multiplied by the correlated rainfall intensity weighting factor to generate the percentage of pollutant removal attributed to the rainfall intensity. This calculation is repeated for all of the rainfall intensity frequencies to determine an overall annual pollutant removal result.

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