Initial Hazard Potential Classification and Streamside Legacy Impoundments

Many legacy CCR impoundments are situated adjacent to streams and creeks. The Environmental Protection Agency has determined that the initial hazard potential classification for legacy impoundments (low, significant, or high) must be assigned by May 8, 2026. Hazard potential classification is used under the CCR Rule to determine the appropriate design storm event for the impoundment and the need for an Emergency Action Plan. This presentation discusses hydrologic and hydraulic modeling used to assess the initial hazard potential classification for two streamside legacy impoundments. The criteria used to develop these models included direct watershed draining to the impoundments, available storage, discharge mechanisms, and initial conditions. Of particular importance was the magnitude of flow in adjacent waterbodies and naturally flood-prone areas downstream of the impoundments. After modeling, GAI Consultants assigned the legacy impoundments a significant hazard impoundment classification rather than the high hazard classification that had initially been recommended.