yanglong09

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  1. Hi Chris, Thanks for your follow up. I have frozen this work for a while, and pick it up again this week. As I mentioned earlier, I'm using GSSHA for the simulation of an extreme flood event, but somehow the model breaks down at certain point. It might be due to the abrupt transitions across the cross sections of channels within the model, which is at least what we concluded weeks ago. After many trials and errors, I'm now 'unifying' the cross sections, meaning that all the channels are sharing the same cross-section profile (also same Manning). But, the model still breaks down. I guess there might be some other tricks that we did not pay attention. Cheers, Long
  2. Thanks, Chris. The resolution of DEM is 30m, I guess the channel bottoms are not represented in the DEM. I'll do some trial and errors. Long
  3. Thanks, Chris. I tried to use the same cross section for all the channels (ensuring no transition), and the problem was perfectly solved. The model ran with no warnings or any error messages. I was just wondering if there are alternative ways to solve the 'abrupt transition' issue, cause sometimes transitions in the channels are physically present. The way I can think of would be to add a structure link in between the channels with sharp transitions. However, I'm not sure if it is entirely feasible, cause I do not really know what kind of transition does the model regard as too 'sharp' to solve. It seems that I could not locate the sharp transitions by simply looking up warning messages in the summary file, cause the nodes that broke down are not always the ones with sharp transitions. Any suggestions? Thanks, Long
  4. Hi Chris, This is exactly what I'm asking. Thanks! I'm currently trying to simulate extreme flood events using GSSHA. I tried not to turn on OVERBANK_FLOW, cause the simulation just cost too much time to finish. However, if I did not turn on OVERBANK_FLOW option, the model crashed at certain nodes, and I got the error message as "The channel routing time step is getting really small less than 1/1000 of a second. Reduce the model time step by a factor of 2 and try again. Program stopped." I know there might be some abrupt transitions of flow volume around the nodes, due to which the model stability could not be satisfied by reducing time steps. The time step is 1s for my case. I then tried to reduce the time step to 0.5s. The new simulation passed the crashing nodes of the 1-second simulation, but crashed over other nodes anyway. I'm using the natural channels (cross sections extracted from DEM). I know there might be sharp longitudinal transitions of cross sections over the channels, but not quite sure if this would crash the model. Any suggestions? Thanks, Long
  5. Hi Chris, Thanks for your swift response. I've checked the page before. According to your understanding, the model just picks up the largest Y values in the XY point series as the TOB (I'm using natural cross sections)? Is that what it is? Another follow-up question, if we assume no interactive flows from channel to grid cell, what would happen in the model when the water depth in the channel exceeds TOB? Thanks, Long Yang
  6. I was trying to simulate interactive flow between the channel and the corresponding grid cell. I turned on the OVERBANK_FLOW on so that flow from the channel will spill onto the grid cell when surface elevation exceed the top of bank for the channel. My question is: how top of bank (top) is determined in the model. I'm using channel cross sections extracted from DEM. Does the model use the highest point in the elevation, or there are more sophisticated ways of doing that? Thanks!
  7. Hi Chris, I noticed on the GSSHA wikipage: it says" it is recommended to define lakes as point attributes in a GSSHA coverage." I guess I could just define a node as a detention basin hydraulic structure without creating polygons as the embankments. However, this seems not working. An error message pops up while progressing to "number of coords for lake #1's search order". Is the polygon feature necessary here? Thanks, Long
  8. I have three questions regarding detention basins in GSSHA: 1) Does anyone know how to determine min/max and init water surface elevation for detention basins in GSSHA? Should we just assign min elevation based on the background elevation of the points? 2) While developing rating curves for the detention basins, is GSSHA still using elevation instead of water depth? 3) If we assign the same elevation for the detention basin as the channel, probably water would back up instead of filling the detention basin. How to deal with this. I'm using WMS 10.0 Thanks, Long