GSSHA water elevation in channels

[Hamish Asmath 0]

Hi. I was wondering if anyone could clarify an aspect of the water elevation in the channel when using GSSHA.

In the development of the channel input file for GSSHA, firstly stream arcs are generated from the DEM in WMS for GSSHA; then the thalweg profile (vertices) are then assigned elevations and corrected using the Smooth GSSHA Streams dialog box; and finally channel properties (channel depth, bottom width and side slope, and Manning's N) are assigned (using the Feature Objects -> Attributes). Figure 1 (attached) illustrates the cross-section of a river, where the brown line is a 40m resolution DEM that was used as the DEM input to GSSHA, and used to generate the stream network. The orange lines are the actual channel bed derived from field observations, and the blue area is water in the channel. A was the water level in the channel when the cross-section was recorded, When the thalweg profile (at one data point in the form of a vertex for this location) is assigned using the Smooth GSSHA Streams option, the elevation extracted from the data for this location would be A (1m). Then the channel properties are assigned, in this case a trapezoidal channel with a channel depth of 0.5m. This would mean that the streambed elevation would be 1-0.5 = 0.5m. My question is, if the channel depth (CHANNEL_DEPTH) project card is included in the simulation, and the water elevation rises above A, what processes/computation does GSSHA use to calculate the channel depth, considering that what GSSHA thinks is the top of the river channel is reached? Would this only of consideration if the OVERLAND_BACKWATER and/or OVERBANK_FLOW project cards are included in the simulation?

Thank you in advance for any information.

Hamish Asmath

Institute of Marine Affairs, Trinidad and Tobago

[Chris Smemoe 1]

Hamish,

There are 2 ways for water to enter streams in GSSHA. Water can either enter the streams from the 2D overland flow plane or from the groundwater levels. If the groundwater process is not turned on, water can only enter streams from the 2D overland flow.

The default in GSSHA is that all the water goes into the stream and no water leaves the stream. This means if a stream node is in an overland cell, water enters the channel using the broad-crested weir equation and the water depth on the overland flow plane. Flow into the channel occurs regardless of the relationship between the water surface elevation in the channel and in the overland cell. The elevation of the stream thalweg and banks in relation to overland cell elevation is irrelevant in this default case.

If the OVERLAND_BACKWATER card is used, if the water surface elevation (WSE) in the channel is higher than overland cell ground elevation, flow into channel is restricted. If the channel WSE > the overland cell ground elevation but the channel WSE < the overland cell WSE, flow into channel is computed using the overland flow equations. If the channel WSE > the overland flow WSE no flow occurs. Flow does not occur from the channel to the overland cells. Thalweg and bank elevations in relation to overland cell elevations are important in this case.

If the OVERBANK_FLOW card is used, flow out of the channel occurs if the channel WSE > the bank elevation and > the overland WSE. If the overland WSE < the channel bank elevation, flow from the channel to the overland cell is calculated using the broad-crested weir equation. If the overland WSE > the channel WSE flow from channel to the overland cell is calculated using the overland flow equations. In this case, the thalweg and bank elevations need to be considered closely.

In all cases, WMS helps you modify the thalweg and bank elevations by selecting the streams and using the GSSHA stream editor. It is recommended that you edit the stream elevations before creating the 2D overland flow grid for your model. Then, when you build the grid, the grid elevations for cells that intersect your streams are set based on the stream elevations and the estimated depth of flow in the streams. There is a toggle box when you generate your grid that tells WMS to use the stream elevations when creating your 2D grid. This toggle box defaults to ON if streams exist.

Hope this helps,

Chris

[Hamish Asmath 0]

Hi. First I must say thank you very much for your response. It was very comprehensive and clarified many of the aspects that I was fuzzy on. Apologies for taking so long to say thanks for that. 

You mentioned the GSSHA stream editor, and editing the stream elevations. I thought that you were referring to the Smooth Streams/Pipe Arcs option, is that correct?

Thanks again. 

-Hamish

 

[Chris Smemoe 1]

Yes I was referring to the Smooth Streams/Pipe Arcs menu item.