HEC-ResSim Computational Strategy

HEC-ResSim is a link-node model. Instantaneous storage and water surface elevations are computed at the start and end of every time step for reservoir elements, and time step-averaged flow is computed through reaches and diversions. For each reservoir, a priority queue of operational rules (specifying, for example, downstream minimum flow, hydropower generation, release rate of change, etc.) is evaluated to determine flow through each reservoir outlet or diversion. Operational rules may include system rules, coordinating the operation of multiple reservoirs within a watershed. Reaches utilize hydrologic routing methods (for example, the Coefficient or Muskingum method) to predict downstream flow given upstream conditions. Because of the complex and non-linear properties of the system, the computational strategy of ResSim is to run iteration loops to find an acceptable solution. At a given time step, operation decisions are made and the solution is predicted a number of steps into the future, in order to see the outcomes of those decisions at downstream locations. If rules are violated, the operations are adjusted, and a forward simulation again projects those results into the future. When the operational rules are met to the extent possible, the operations of the current time step are accepted, and the solution moves forward to the next time step. An intrinsic assumption in ResSim is that flow moves only in the downstream direction. (An exception is pumpback rules, where downstream re-reg reservoirs pump water back upstream during off-peak energy demand hours; this operation is not currently supported in the water quality calculations).

The forward-looping, iterative calculations of ResSim may be computationally expensive when performed on large watersheds. To streamline calculations, large watersheds are broken up into sub-watersheds based on dependencies within the system. For example, two reservoirs utilizing system operations must be included in the same sub-watershed. A reservoir with a downstream control rule and another reservoir whose releases impact the flow at that downstream control point must be included in the same sub-watershed. But a reservoir located high in the watershed with only rules governing its local releases can be separated from the rest of the watershed and solved independently. In this manner, upstream sub-watersheds can be solved for the entirety of the simulation in a first pass. Their outflows are saved during that compute and then utilized as boundary conditions to the downstream sub-watersheds. This avoids iteration on sections of the watershed that aren't necessary. Large watersheds may be broken into many sub-watersheds. The number of forward-looking routing steps is calculated based on the longest time for information to travel from the decision point (e.g., a reservoir outlet) to a downstream control point.

The state of the sub-watershed (flows, storages) is saved in memory, so it can be restored after the forward simulation is completed and a decision is made. After that decision is made, the sub-watershed is updated through the time step and another set of forecast steps is calculated to determine operational decisions for the next time step. This looping progression through time continues until all time steps have been solved for all the sub-watersheds. In order to support this computational strategy, the HEC-WQ Engine runs each water quality subdomain (corresponding to an individual reach or reservoir) independently during a time step. Computations are ordered from upstream to downstream, so that the flux of material into the downstream junction is aggregated over the time step and later applied as an upstream boundary condition to the downstream adjacent subdomain.

Reservoir Hydrologic Computations

HEC-ResSim passes inflows, outflows, seepage rates, and evaporation and precipitation rates to the WQ Engine at every time step. The WQ Engine allocates the inflows and outflows to individual layers and calculates the vertical velocities as described in Water Quality Transport Numerical Methods. Concentrations of dissolved gases and water temperature are assumed not to be concentrated by evaporation or diluted by precipitation; all other constituents are.

Reach Hydrologic Computations

The computation and utilization of reach hydraulic variables from HEC-ResSim into the WQ Engine is more complex than the reservoir hydrology. Cross-sectional geometry files and steady flow simulation results from HEC-RAS are required to define the channel geometry for stream reaches. Depending on the hydrologic routing method chosen for the reach, the ResSim-computed flows maybe incompatible with the finite volume method used in the transport equation numerical solution. Each of these two points is explained him more detail below.

HEC-RAS Geometry and Steady Flow Results Table

For accurate transport of material through river channels, hydraulic properties (average depth, flow, velocity, conveyance area, top width) are needed at a reasonable spatial resolution along the channel. Channel top width is needed to compute surface heat fluxes and gas exchange. ResSim was designed as a reservoir and reservoir systems operation model. Flows are computed through river reaches between junctions using hydrologic routing methods (e.g., Muskingum routing, Coefficient routing). ResSim does not natively have any knowledge of the geometry of the reach or the water volume in the reach (except to the extent it is calculated in some routing methods). However, the HEC-RAS model is ideally set up for users to input 1D Channel geometry in the form of cross-sections and use them to calculate hydraulic properties in steady and unsteady flow simulations.

Instead of replicating these features in HEC-ResSim, two files that can be output from HEC-RAS are used as input to the ResSim water quality computations. The first is a shape file of RAS cross-section locations. As described in Computational Geometry this allows each reach to be discretized, with face locations corresponding to cross-section locations, where the hydraulic properties are known. The second is a summary output table from a series of steady flow runs. For each flow rate, HEC-RAS calculates the hydraulic properties at each cross section using the Saint-Venant equations for open channel flow. A series of steady flows are run, spanning the expected range of flows for the channel, and hydraulic properties are output for each cross-section and each flow rate. The resulting table serves as a lookup where flows calculated in the ResSim model can be translated into hydraulic properties needed for water quality transport and transformations.

Hydrologic Routing with the Finite Volume Method

Hydrologic routing methods are designed to predict the attenuation of a flow pulse as it travels downstream over relatively coarse time intervals. They work well on the daily time step used by many HEC-ResSim models. While these methods conserve volume over a sufficiently long time interval, they can lead to problems when their flows are applied to finite volume transport calculations on an individual reach. Two example cases are given below.

Null routing For short reaches and long time steps null routing is appropriate. The flow at the upstream and downstream ends are approximately equal at each time step. Flows that are passed into the WQ Engine each time step are equal for the upstream and downstream faces of every cell in the reach. The consequence is that the volume in the water quality computational cells never changes during the simulation. Since the travel time of the constituent is related to the cell volume divided by the face flow, transport speeds are incorrect for every time step other than the first.

Coefficient routing One example of coefficient routing is to apply a lag of one day. Flows at the downstream end of a reach are set to the upstream end flow from the previous day. For the rising limb of a hydrograph on a moderately sized river (20 miles long, 100 feet wide), a flow increase of 1,000 cfs over a day would lead to an eight foot rise in water depths. Similarly to the null routing case, the incorrect cell volumes again result in large incorrect travel times.

Some more advanced hydrologic routing methods are better designed for smaller length and time scale routing calculations. This includes the Modified-Puls method supported by HEC-ResSim. When used with the appropriate parameters, these methods are expected to conserve volume, and their use with the WQ Engine will maintain appropriate cell volumes and transport material with expected travel times.

When simple hydrologic routing methods are used there is a computational option in HEC-ResSim to "Preserve concentration (at the expense of mass conservation)". This option instructs the WQ Engine to update water quality cell volumes at the start of each step. Volumes are adjusted to reflect the volume expected based on the HEC-RAS steady flow file results. When adjusting the volumes, the concentrations for all water quality constituents are held constant. A consequence, the water quality simulation no longer maintains mass conservation. However, material travel times are much more realistic. For constituents such as water temperature, which are heavily influenced by surface heat fluxes, this is the recommended method when utilizing simplified routing methods.