Surface Water Routing:Overland/Channel Interaction
Each overland flow cell may contain part of, a single, or multiple channel nodes. For overland cells with stream nodes, water in the stream cell may flow into the channel depending on options selected by the user. The default is for water to flow into the channel based on the broad crested weir equation. Unless other options, as described below, are included in the project file, water will flow from the overland flow cell into the channel regardless of the relationship between the water surface elevations in the channel and in the overland flow cell. This is the normal condition for simple upland watershed studies. In this case the relationship between the channel thalweg and top of bank elevations with the overland cell elevation is irrelevant and the user is not required to spend a great deal of time ensuring the proper relationships, unless this is important for other purposes, such as groundwater interaction with the stream. In addition to this simple representation of overland and channel interaction GSSHA includes two optional levels of complexity.
Including the OVERLAND_BACKWATER card in the project file indicates that the user wishes to include backwater effects on the overland flow plane due to restricted flow into the channel. If the elevation of water in the channel exceeds the overland cell elevation, flow from the overland flow cell to the channel is restricted. If the water surface in the channel is higher than the overland cell elevation but lower than the water surface elevation in the cell flow into the channel is computed with the overland flow equation. If the water level in the channel is higher than the water level in the overland flow cell, no flow occurs. In versions previous to 571 the top of bank elevation was not considered in these calculations, only the relationship between the stream water surface elevation and the overland flow water surface elevation. For versions 571 and beyond, the top of bank is an additional restriction. Water in the overland flow cell must be higher than the stream top of bank before water will flow into the stream. The top of bank is defined as the thalweg elevation plus the stream depth. For trapezoidal channels the user specifies the stream depth and the thalweg elevations. For natural cross sections the top of bank is taken from the XY point series inputs.
Including the OVERBANK_FLOW card increases the level of connection between the overland and channel by allowing water to spill from the channel back onto the overland flow plane. For water to spill from the channel to the overland flow, the water in the channel must be higher than the channel top of bank and higher than the water surface in the overland flow cell. If the water on the overland is lower than the top of bank, flow from the channel to the overland is computed with the broad crested weir equation; submergence effects are included in these calculations. If the overland water surface elevation is higher than top of bank, flow is computed with the overland flow equation.
It should be noted that the OVERBANK_FLOW card will not cause backwater effects on the overland flow plane. If the user wishes to see both backwater effects and overbank effects then both cards should be included in the simulation.
Whenever OVERLAND_BACKWATER and/or OVERBANK_FLOW are included in the project file the relationship between the channel thalweg, top of bank and the overland flow grid cell becomes critical to the computation. In this case the user is required to ensure that these relationships are accurate and appropriate. This may require adjustments to the channel thalweg elevations, channel depths, and stream cell elevations.
In addition, inclusion of either of these cards in the project file imposes tremendous computational complexity into the model. This may require the user to reduce the model time step or increase channel node lengths to reduce numerical instability. Despite these actions, numerical instability may persist. The OVERBANK_FLOW card is more likely to cause fatal instability than the OVERLAND_BACKWATER card. The user should keep in mind that these are advanced features that should really only be employed by advanced users with a solid understanding of the numerical considerations involved. Similar information can be obtained by extrapolating the channel water surface elevations out onto the overland flow grid. Essentially 100% of all FEMA flood studies are performed in this manner.
