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Hamish Asmath

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    Trinidad and Tobago

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  1. 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
  2. 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
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