On occasion, a special hydrologic method may need to be used. For example, if a city is funding the surface drainage facilities, that city may insist on using its own specific hydrologic method. Usually, such special methods are similar to the Rational Method with some minor variations.

Some situations may require the use of some variation of Natural Resources Conservation Service (NRCS) hydrologic estimating methods such as the NRCS TR-55 or TR-20 procedures. (See for information on contacting this agency.) In other situations, the use of a unit hydrograph procedure may be in order. Refer to s in Chapter 4 for detailed information on the NRCS methods.

Where considerable storage is required in the storm drain system or detention is being designed, hydrologic routing methods should be employed to accommodate peak flow attenuation. Refer to Chapter 4 for information on s.