Gate Time Series

Inline structures with gates can be included in a quasi-unsteady sediment model. However, gate and reservoir operations are ill posed in a quasi-unsteady model. Since the quasi-unsteady approach uses a series of steady flows, it will compute a reservoir elevation based on the head required to reach steady state equilibrium, to 'push' the reach flow through the gates. This can lead to dramatic fluctuations in reservoir stage from time step to time step and does not conserve mass.

To replicate historic reservoir stages with gates in quasi-unsteady flow, a modeler could use the orifice equation to back calculate the orifice opening that will produce the historic head for the historic flow. But recent versions of HEC-RAS include an Internal Stage Boundary Condition

Unsteady sediment transport and in HEC-RAS version 5.0 should make quasi-unsteady gate operation mostly obsolete (with the exception of flushing studies, see section below on Interaction Between Internal Stage and BC Gates), which is why the quasi-unsteady gates have only one boundary condition while there are four different ways to operate gates in an unsteady flow simulation.

However, if an inline structure has gates, HEC-RAS requires gate openings (even if the model does not use them – see the section on Interaction Between Internal Stage BC and Gates) and the quasi-unsteady editor will generate a mandatory internal boundary condition for the structure (see the figure below). If the user does include gated inline structures in a quasi-unsteady model, however, the T.S. Gate Openings boundary condition in the Quasi-Unsteady Flow editor will control them. The T.S. Gate Openings boundary condition is required. Therefore, if the project includes an inline with gates the structure will show up in the Quasi-Unsteady Flow Editor.

If the model includes multiple gate groups, the boundary condition will require gate openings to cover the entire simulation time for each gate group. A drop-down menu in the upper right-hand corner allows users to specify gates for each gate group in a structure.

Modeling Note: Specify Openings for ALL Gate Groups

A common error involves only specifying gates for the gate group that is active when the editor opens, and not realizing there are others that need gates.

Warning: Quasi-Unsteady Flow Does Not Route Through Reservoirs or Gates

Instead it simply computes the backwater stage for each time step based on the flow through the gates, leading to unrealistic reservoir stage fluctuations. Quasi-unsteady gate operations are NOT RECOMMENDED for most applications. Consider running reservoir models in unsteady flow or use one of the quasi-unsteady reservoir approaches described in the reservoir modeling video series like the Internal Stage Boundary Condition (next heading).

Internal Stage BC

Recent versions of HEC-RAS include an internal stage time series boundary condition that can control stage within the model. The most common use for this is to control reservoir stages. By defining a stage series at the cross section just upstream of the inline structure representing a dam, modelers can control reservoir stages directly throughout the duration of the model. This allows modelers to use the Quasi-Unsteady mode to simulate reservoir operation without routing the flows (if they know the reservoir stage for the entire simulation).

The model will still require Gate Openings if the inline structure has gates. However, the quasi-unsteady model does not use the gates to compute flow through the structure (it is a backwater calculation, so the flow is specified), only the reservoir stage. Therefore, if the user specifies the stage upstream of the dam, HEC-RAS does not end up using these Gate Openings and the modeler can fill them with place-holder values.

Modeling Note – Transition Between Stage and Gate Control

Sometimes it can be useful to switch between stage control and gate control in a reservoir model. The most common application of this is for a periodic reservoir flush. The figure below shows a simplified, quasi-unsteady approach to simulating an annual flush over an extended, period of record, simulation. The Internal Stage Boundary Condition holds the reservoir stage for most of the year (8567 hours is 51 weeks). But then, for 1 week per year (168 hours) the stage field is blank. When HEC-RAS encounters a blank stage in the Internal Stage Series, it reverts to the backwater computation. In this example, the blank internal stages correspond to fully opening the gates (20m). This does not gradually drain the reservoir. A modeler would have to gradually decrease the internal stages to simulate a gradual drawdown. But it does switch the reservoir between a specified pool and a run-of-river flush.

Modeling Reservoirs with Unsteady Sediment

Reservoir modeling is the most common application of Unsteady Sediment Transport. In addition to routing and conserving flow, unsteady flow has a range of tools that make reservoir modeling easier and more powerful. In particular, the Operational Rules feature in the unsteady flow editor is an exceptionally powerful tool to model reservoir sediment management. The operational rules include concentration and bed elevation triggers, so users can write simple, but powerful, rules to operate their reservoir based on sediment output. For example, Gibson and Boyd (2016) used the very simple concentration rules in to re-operate a reservoir flushing event to maintain a proposed TMDL (result in the right pane in the Figure below).

There are several published examples (with sample code) of simulating reservoir management alternatives with operational rules in HEC-RAS sediment simulations, including:

Gibson, S. and Boyd, P. (2016) "Designing Reservoir Sediment Management Alternatives with Automated Concentration Constraints in a 1D Sediment Model," River Sedimentation: Proceedings of the 13th International Symposium onon River Sedimentation, ed edited by S. Wieprecht, et al.

Gibson, S. and Crain, J. (2019) Modeling Sediment Concentrations during a Drawdown Reservoir Flush: Simulating the Fall Creek Operations with HEC-RAS, RSM Tech Note ERDC/TN RSM-19-7. Vicksburg, MS: U.S. Army Engineer Research and Development Center. http://dx.doi.org/10.21079/11681/33884
Gibson, S. and Boyd, P. (2014) "Modeling Long Term Alternatives for Sustainable Sediment Management Using Operational Sediment Transport Rules," Reservoir Sedimentation –Scheiss et al. (eds), 229-236.