A Dive into DeHart Dam: How PMP Studies and CFD Modeling Can Reshape a Dam’s Rehabilitation Alternatives

DeHart Dam, located on Clarks Creek in Dauphin County, Pennsylvania, is a 105-foot-high earthen embankment dam with a concrete chute spillway constructed in 1940. It has a contributing watershed of 21.15 square miles and serves as the primary water supply reservoir for the City of Harrisburg. Previous work was performed to determine the extent of rehabilitation required to allow the passage of the Probable Maximum Flood (PMF) through the spillway at DeHart Dam. The previous studies included hydrologic and hydraulic (H&H) studies using Hydrometeorological Report (HMR) 51 that indicated that the spillway crest structure and chute did not have adequate capacity to safety pass the PMF, which is the spillway design flood for this high hazard dam. Based on these studies, replacement of the spillway crest structure and significant modifications to the spillway chute were recommended.
AECOM performed an updated H&H analysis of the DeHart Dam watershed in support of a rehabilitation of the service spillway. The analysis included developing input parameters for the watershed model and estimating the PMF using Probable Maximum Precipitation (PMP) depths and distributions resulting from the 2019 statewide PMP Study for Pennsylvania. Application of the Pennsylvania PMP study to this watershed resulted in a significant decrease of the peak inflow discharge to the dam during the PMF event compared with previous studies.
A Computational Fluid Dynamics (CFD) model was also developed to study the hydraulic performance of the existing spillway at DeHart Dam. A particular focus of the study was to provide chute wall water profiles to be used as part of the evaluation of the adequacy of the existing spillway. The CFD model indicated that flow is generally contained within the existing spillway chute but with minimal freeboard in several locations. The top of right chute wall is shown to be overtopped by 6 inches approximately two-thirds of the way down the chute, and the flow is shown to exceed the top of the existing stilling basin walls by up to four feet. The model also indicated significant contraction of the flow over the spillway crest primarily caused by the unfavorable training wall geometries.
Based on the results of the updated H&H analysis and CFD modeling, the existing spillway configuration is considered to be generally acceptable with some minor modifications to improve the hydraulic performance, with a focus on addressing other structural and geotechnical concerns.