This video will cover how to represent hydraulic controls in 2D model mesh geometries.
The mesh geometry, developed from the terrain data, has the most significant impact on model results.
The features of the terrain having the largest impact on hydraulics are referred to as hydraulic controls.
Representing hydraulic controls in the mesh is important for creating an accurate and reliable hydraulic model.
Hydraulic controls are topographic, or manmade features,
that have a significant impact, or control, on how water is conveyed through an area.
Typical hydraulic controls include; channels,
roadway embankments, channel banks, berms and levees,
natural channel or floodplain constrictions,
natural or constructed grade control structures,
and other project specific terrain features.
Hydraulic controls vary depending on the model application, site features, and discharges analyzed.
Hydraulic controls on large river systems with larger discharges will be different than controls on smaller systems and smaller discharges.
Also, terrain features controlling hydraulics at low discharges may not have any hydraulic impact during high discharges,
and similarly features controlling hydraulics for larger discharges may have no impact for small discharges.
It is always important to understand what the model application is,
so an appropriate mesh resolution can be created to provide useful results.
One benefit of flexible mesh geometries is the ability to vary the mesh density to capture important topographic features.
Element sizes can be reduced to represent specific hydraulic controls,
and then elements sizes can be increased over large areas with little to no controlling features.
Mesh densities can also be varied to represent areas with rapid changes in hydraulic conditions,
or when detailed hydraulic results are needed at a particular location.
When developing mesh geometries, remember that only one water surface and velocity are computed for each element,
and then interpolated to nodes.
The most important aspect of representing hydraulic controls in mesh geometries, is to align element edges with a topographic feature.
For example to correctly represent a roadway embankment in a mesh, at a minimum
one line of element edges should be located along the crest of the roadway.
If the element edges are not aligned to the roadway crest,
the mesh does not completely represent that topographic feature and the model will not "see" the feature correctly.
This could result in the water flowing through the embankment,
or the embankment being poorly represented and not impacting the flow as much as it should.
The element edge can be created along specific features using arcs or breaklines in the mesh creation process.
If a breakline is added along the top of the feature, the mesh elements align sufficiently to represent the terrain.
Then the results reflect the mesh representing the hydraulic control.
Representing hydraulic controls with element edges is necessary for all types of 2D models including meshes that use sub-grid resolution.
Sub-grid resolution is a great approach to reduce mesh sizes,
but the sub-grid resolution only accounts for the volume in the sub-grid features
and does not account for potential hydraulic controls within the element area.
The geometry with respect to hydraulic controls is only represented at the element face.
Controls within the element are not "seen" by the model.
In this example the roadway embankment terrain is shown in the two element edges that cross the roadway.
However, the model does not see the roadway as an obstruction to flow, because there is not an element edge along the crest.
In this case the flow in the model would see the terrain along the toe of the embankment,
and flow would be conveyed through the element.
Sufficiently representing hydraulic controls in the mesh geometry is one of the most important steps to developing a reliable model.
You can download a one page summary with links to additional resources here.