Hydrologic Issues for Dams - Association of State Dam Safety Officials

Current State and County PracticesAs noted above, all County-owned dams are regulated by the Dam Safety Standards ofthe NJDEP’s Dam Safety Section. Similar to the FERC’s Engineering Guidelines for theEvaluation of Hydropower Projects, these state standards classify dams by Hazard Classbased upon the threat posed by a dam failure to downstream structures and roadways. Asummary of the various NJDEP Hazard Classes is presented in Table 1 below.Table 1Summary of Dam Hazard Classes and Spillway RequirementsNJDEP Dam Safety StandardsHazard Class Downstream Failure Threat Spillway Design StormI – High Loss of Life or Extensive Property Damage PMPII- Significant Significant Property Damage One-Half PMPIII – Low Minor Property Damage 100-Year NRCS Type IIIIV – SmallDamsHeight < 15 Feet, Storage < 15 Acre-Feet,Drainage Area < 150 Acres** Class IV Dam criteria only applicable if Class I or II threats do not exist.100-Year + 50 PercentNRCS Type IIIAccording to discussions with NJDEP Dam Safety personnel, PMP rainfall amounts aretypically obtained from HMR No. 51. For the majority of projects, spatial distribution ofthe rainfall is not necessary and the rainfalls are temporally distributed in accordancewith a variety of storm distributions, including the NRCS Type III distribution and theU.S. Army Corps of Engineers PMP distribution as contained in HEC-1 FloodHydrograph Package. One foot of freeboard is required above the routed SDS watersurface. The NRCS Runoff Equation and Dimensionless Unit Hydrograph are typicallyused to compute losses and convert the rainfall to runoff. Antecedent MoistureConditions are typically assumed to be either average (AMC II) or high (AMC III).Sources of dam failure parameters vary, with those published in the FERC EngineeringGuidelines or derived from the Froelich equations utilized most frequently.Downstream discharges and water surface profiles are typically computed using steadyflow assumptions with HEC-1 used for discharge and failure computations and eitherHEC-2 Water Surface Profiles or HEC-RAS River Analysis System used for profilecomputations. Unsteady flow computations using either DAMBRK or FLDWAV areused on approximately 10 percent of the projects, which typically involve large dams anddrainage areas. Downstream failure impacts are typically assessed based upon thedifference in water surface with and without a dam failure, with a difference greater thantwo feet considered excessive. A minority of projects utilizes the Downstream HazardClassification Guidelines published by the Bureau of Reclamation.As noted above, the Somerset County dam safety criteria, which were originallyestablished in 1975, apply to all structures subject to County Planning Board approvalunless superceded by the NJDEP regulations described above. Hazard classification forPaper 9 - Skupien 40