Assessing Hydrologic Change in Surface-Mined Watersheds Using the Curve Number Method

The U.S. Surface Mining Control and Reclamation Act of 1977 requires that mine operators minimize disturbances to the prevailing hydrologic balance of surface-mined sites and associated off-site areas through land reclamation. The hydrologic evaluation to support this requirement is often performed using the curve number (CN) rainfall-runoff method. Accurate application is limited by a scarcity of tabulated CN values for reclaimed mine lands, and assumptions regarding the hydrologic behavior of reclaimed mine lands may be inaccurate. Four watersheds (three reclaimed surface coal mines and one forested reference) in the Georges Creek basin of western Maryland were instrumented for rainfall and runoff. CNs calculated for the reclaimed mine land watersheds using rainfall and runoff data (range=68–92) were generally higher than CNs estimated by prevailing engineering methods (range=65–79). The general agreement of these results with other studies indicates that a more conservative hydrologic approach may be warranted in engineering design and hydrologic analysis of surface-mined lands. These results also indicate that surface mining and reclamation may have similar hydrologic impacts as urbanization. Whereas urbanized landscapes typically feature engineered storm water management to mitigate hydrologic change, these features are typically absent on reclaimed mine lands.