Any standards to create Stormwater Report for subdivision when build-out is unknown?

[annsera]

Hi everyone,
In my state, subdivisions receive approval that create only the lot lines and the roadway. Most designs have infrastructure for the roadway and stubs to bring utilities to each lot, but they do not have final grading, landscaping, building footprint, impervious surface amounts like driveways/walkways, etc. Some municipalities require that the developer create stormwater systems that will accommodate the entire subdivision, including the build-out. The municipality will evaluate the proposed stormwater system largely through examination of the stormwater report, which will include the HydroCAD analysis. The engineer must design a storwmater system and present a hydroCAD analysis when they do not know what the proposed conditions really are.
What are the typical assumptions that engineers make in this situation?

 

[TerryScan]

From your description, Residential lots are not an unknown use of the land as you imply. One may not know the exact build but there is certainly enough to go with. TR-55 lists CN values for Residential districts by lot sizes and lists the assumed impervious percentages for each category. If zoning in your application allows for a higher impervious on the lot, one could be conservative and assume max. impervious allowed on the lots by code.