Hydraulic Boundary Conditions
Boundary conditions, where flow enters or exits a model domain, can have a consequential impact on the calculated flow field and if it accurately simulates the real-world condition. A river reach for example, extends beyond the model domain. –The domain downstream of the model usually influences the water surface elevation at the downstream end of the model. The velocity distribution entering the model is usually the result of river geometry beyond the domain. Specifying these boundaries correctly is necessary for the model to produce accurate water surface profiles, capture hydraulic jumps and backwater effects, and respond realistically to flood wave propagation.
FLOW-3D HYDRO includes a set of hydraulic boundary conditions designed for open channel, river and coastal modeling that incorporate waves, rating curves, natural inlet treatments, mass sources, and flow regime controls that hydraulic engineers work with in practice.
Rating Curve Boundary Condition
A rating curve describes the relationship between water surface elevation and discharge at a river cross-section. Rating curves are the standard tool for characterizing flow conditions at downstream model boundaries. Rating curves can be measured in the field when possible or calculated from a calibrated river model, such as HEC-RAS.
In transient FLOW-3D HYDRO models, rating curve boundary conditions can be applied at both inlet and outlet boundaries. At an inlet, the rating curve defines the inflowing water surface elevation and the model flow is an artifact of the model geometry and associated headloss. At an outlet, the rating curve provides a physically consistent downstream water surface elevation that responds to the simulated discharge or time. Allowing the water surface to vary with flow eliminates the need for a fixed pressure boundary that may not reflect actual downstream conditions.
Rating curve boundary condition example showing a flood wave simulation. Time histories of fluid elevation and volume flow rate at the inlet (xmin) and outlet (xmax) boundaries are shown alongside the Froude number distribution along the channel. The inlet rating curve forces a supercritical flood wave; the outlet uses a subcritical rating curve, producing a hydraulic jump in the domain.
Volume Flow Rate Boundary Condition
At an inlet where only the flow is known and no water surface elevation data is available, the Volume Flow Rate boundary condition allows the inflow depth to vary with conditions (regime) at the boundary of the domain. A minimum and maximum fluid elevations can be provided, to limit water level range and prevent inadvertent super-critical flow when subcritical conditions are expected.
The shallow water volume flow rate boundary condition offers a spatially varying velocity profile at the inlet. This reduces the need for additional upstream domain length to allow the flow profile to develop before reaching the area of interest. Users can choose between a uniform, parabolic and depth-depending velocity profiles. The last option is particularly useful for river applications where the velocity is high in deep water and low at shallow water. A user defined lateral velocity variation is not available.
The same flood wave scenario with the inlet changed to a subcritical volume flow rate boundary condition. Without supercritical forcing at the inlet, the domain maintains subcritical flow throughout, as shown by the lower Froude numbers across the channel.
Natural Inlet Boundary Condition
For natural channels where neither rating curve data nor a fixed water surface elevation is available, the Natural Inlet boundary condition sets the water depth at the inlet using the depth in the adjacent cells immediately inside the domain and/or the depth in the previous time step. This eliminates the potential for sharp discontinuities in water surface elevation at the inlet that could cause numeric instabilities in transient simulations.
As the water surface inside the domain rises or falls in response to changing flow conditions, the inlet adjusts automatically to match. The flow regime at a natural inlet can be specified as subcritical, supercritical, critical, or automatic – where the automatic option allows the flow category to change during the simulation based on computed Froude number. Minimum and maximum elevation limits can also be set to prevent the inlet water surface from dropping below terrain level or exceeding a physical upper bound.
A rating curve relaxation time option is available for natural inlet boundary conditions, to improve the response to transient flow conditions and reduce numerical oscillations when the inlet flow rate changes rapidly.
Applications
Hydraulic boundary conditions in FLOW-3D HYDRO are required for all simulations but are notable for their ability to include the following cases:
- Flood wave propagation modeling for hydraulic structures in 3D and shallow water model
- River reach simulations where time dependent upstream and downstream gauge data is available as rating curves
- Models where discharge is known but stage data is unavailable
- Hydraulic jump and backwater analysis where accurate downstream boundary conditions are critical
- Inlet boundary setup for models where flow profile development length would otherwise require a large upstream domain
- Time-varying flood scenarios where inlet discharge follows a hydrograph and outlet conditions respond dynamically