Modeling Interconnected Reservoirs with HEC-ResSim

The USACE Hydrologic Engineering Center (HEC) reservoir simulation software, HEC-ResSim, is being used by reservoir modelers world-wide. HEC-ResSim can be used to simulate reservoir operations for multiple objectives including flood risk reduction, navigation, hydropower, and environmental support. ResSim uses a map-based schematic to represent the river and reservoir system. Reservoir operation is modeled through a set of operation rules that describe the operating goals and constraints. The Tioga-Hammond reservoir system, located in the Chemung River watershed at the headwaters of the Susquehanna River Basin, contains two parallel reservoirs, Tioga and Hammond, whose pools are joined by a connecting channel. The connecting channel contains a control structure with two gates and an overflow weir. Under normal operations, Hammond’s pool is kept 5 feet higher than Tioga’s and Hammond’s excess inflows are released into Tioga by utilizing the gates in the connecting channel. During flood operations, the pools may rise and the 5 foot elevation difference may not be maintained. Under extreme events, one or both pools may exceed the elevation of the connecting channel’s weir resulting in an uncontrolled exchange of flow between the reservoirs. HEC-ResSim was designed to represent only dendritic river systems so modeling the bi-directional flow of the Tioga-Hammond connecting channel posed a significant challenge. The solution involved the use of some special modeling techniques and the powerful scripting capability within ResSim. This presentation will describe the intricacies of this modeling effort within ResSim including how the network was constructed and how scripting was used to make it work.