When a requirement exists to match runoff rate or volume to “pre-development conditions,” there is a range of options that could be applied to define land cover conditions. This range goes from pre-settlement, which assumes land is in an undeveloped condition, to the land use condition immediately prior to the project being considered, which assumes some level of disturbance in the natural landscape has already occurred. Interpretations of this variation from Scott County, Project NEMO, Dane County (WI), and the USDA-NRCS ere used to lay out the range of approaches that local units can use when applying this criterion. Please note that selection of a pre-development definition should occur only after an evaluation of the hydrologic implications of the choice is performed.
The most conservative assumption for pre-development conditions is the assumption that the land has undergone essentially no change since before settlement. In this case, a meadow or woodland in good condition is commonly used to portray a “natural” condition. The following table shows the curve numbers used when this situation is applied using TR-55. Similar hydrologic characteristics would be applied when using other models.
Curve number for use with pre-settlement conditions.
Link to this table
|Runoff Curve Number*|
|Hydrologic Soil Group (HSG)||Meadow||Woods|
* Curve numbers from USDA-NRCS, Technical Release 55
On the other end of the pre-development definition is the assumption that land disturbance has previously occurred with the land use in place at project initiation. This is the definition used under most circumstances by the MPCA in the Construction General Permit (CGP). Under this scenario, runoff assumptions after construction need to match those of the land use prior to the development using matching curve numbers or runoff coefficients. The new project could possibly improve runoff conditions, if the prior land use did not accommodate any runoff management. That is, implementation of good runoff management to an area that had previously developed without it would likely reduce total runoff amount compared to existing development. Note that the MPCA could alter its definition of pre-development under certain circumstances, such as a TMDL established load limit.
NRCS (TR-55) notes that heavily disturbed sites, including agricultural areas, curve numbers should be selected from the “Poor Condition” subset under the appropriate land use to account for common factors that affect infiltration and runoff. Lightly disturbed areas require no modification. Where practices have been implemented to restore soil structure, no permeability class modification is recommended.