Design guidance for permeable pavement systems advise constructing the storage layer subgrade as flat as possible or with a series of cells if the subgrade is sloped greater than 2% (Wisconsin DNR Technical Standard 1008). What is the best way to design the storage layer if there is a significant slope and how can one analyze system performance if there is a significant slope?
The City of Wauwatosa in Wisconsin requested the Milwaukee School of Engineering (MSOE) look into the matter by investigating the hydraulic conductivity and porosity of potential subgrade materials, creating a spreadsheet-based tool to analyze the performance, and developing design guidance for proposed sloped green alleys within the city.
MSOE constructed a 12-foot-long variable slope flume into which potential subgrade materials could be loaded. Based upon water level measurements along the length of the flume, hydraulic conductivity was computed using the Hele-Shaw solution for unconfined flow in a sloped aquifer. A sieve analysis determined the gradation of the aggregate. Porosity was measured using standard ASTM methods. Locally available drainage stone aggregate sizes of 3/8” chips, No. 1, No. 2, and No. 3 stone were characterized.
The analysis spreadsheet has two steps. The first computes the subsurface water table profile, maximum depth, and storage at different flow rates for a two-layer subgrade. The second uses storage routing to route the inflow hydrograph through the subgrade.
Analysis results turns the typical design upside down! It is most efficient to place smaller aggregate below larger aggregate. Simulation results for storm sizes ranging from 1-year to 100-years indicate a 4 to 9 fold reduction and 24 to 54 minute delay in peak discharge rate to the storm sewer system with greatest effectiveness during small storms. The Excel spreadsheet design tool is available free-of-charge to interested parties.
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