[ckatsoulas]

without looking at your HMS setup, it would seem you have not properly considered the stream flow in HMS that is likely causing the faster response time and higher peak in RAS. Also you need to make sure that the mannings roughness used in RAS is a reflection of your HMS catchment setup. I have done many rain-on grid RAS runs and found very close matches to hydrologic packages such as DRAINS that we use here in OZ.

The other thing to check is the volumetric runoff, this will tell you if you have setup HMS properly to match RAS.

[ckatsoulas]

Hi Cameron, how is the development of HECRAS 5.1 going? do you think we will see it by end of this year?

[ckatsoulas]

have you tried reducing the cell size, for a dam break I assume you will get large flows coming down steep lopes at the dam interface that could be causing problems. The other thing to do is check the cells where the instabilities are forming to see the problem spots.

[ckatsoulas]

To lower the terrain you need to use a GIS software like QGIS. You create a polygon add an elevation field and set the required elevation. Then you burn the new level of the shape into the terrain using the ‘Mosaic raster’ option.

[ckatsoulas]

I don’t think it’s incorrect. Your first picture shows some flows escaping the channel as overflows and then re-entering the channel further down.

The second picture which is probably your main concern is showing spilling from the south. It then hits a wall/mound and due to the reverse crossfall of the berm it creates eddies that spill backwords and return to the channel upstream.

You will notice in the second picture that where it hits the wall/mound right on the left of the bend there is splitting of overflows, some go forward and others hit the wall and form eddies and come back.

The same thing is happening in the third picture to the north as it hits that small mound.

I think due to the momentum of the main channel, the overflows are bouncing off the mounds and heading backwards.

[ckatsoulas]

How wide is the river?, 100m cells seems way too big and you may need to introduce breaklines along the banks to align the cells along the river edges.

[ckatsoulas]

From what I can see the cell (#18491) is too high for the rating curve you have provided. You are giving HECRAS a starting water level below the ground level. You rating curve needs to have higher elevations or your cell where it connects to the SA 2D connection needs to be lowered so that the starting water level has some positive depth.

[ckatsoulas]

The only time you would use it manually is if you have a SA/2D connection say for a culvert and you wanted to set the weir to the existing ground levels using the new automatic feature. You need to compute the hydraulic table before you can use this feature to match the weir levels with the ground levels.

[ckatsoulas]

Scott I’m pretty sure the header is self explanatory
ie Type RM Length L Ch R = 1 ,750 ,25,25,25

Type=1
RM=750
L=25
Ch=25
R=25

the 25’s are the left bank, main channel and right bank distances to next cross section
750 is the XS chainage

[ckatsoulas]

for DV, you need to calculate the DV map by right clicking on the results layer and selecting “manage results map”. You can then select the DV for either a certain time or maximum value and then save the new DV map.

with regard to individual maps, it is the same process as above for depth but you need to do them one by one.

[ckatsoulas]

you need to follow the correct order when creating the cross sections. HECRAS has a default order, you add cross sections from upstream to downstream and the station data is entered from left to right looking downstream. The best way to remember it is to draw the cross section as if you are the water heading the river and writing left to right.

[ckatsoulas]

To create a mannings layer you can import multiple shape files for each landuse type as follows:
– right click on map layers and select “add new Mannings n Layer”
– hit the plus key and browse and select all your mannings shapefiles
– in the bottom left box type in your description for each landuse
– in the bottom right box for each landuse, enter the mannings n
– save your mannings layer

you can also copy a shapefile imported into the “Map Layers” folder to the Mannings n “override region” layer in the geometry folder. this will override any created mannings layer.

[ckatsoulas]

The best way is to create a shapefile in a GIS package such as QGIS which is free software. You create the polygon shapefiles and then import them( you can drag them) into HECRAS. Then you associate the mannings n areas with a geometry.

[ckatsoulas]

I ran your model and it worked fine. You need to associate the terrain with the geometry and plan. In RAS mapper, right click on the Terrains heading and select “Geometry associations”. then select the terrain to be associated with the plan (it may do this automatically for you as you only have one plan and terrain). Then run the model and the inundation will then be shown. As you have only done a 1D model with cross sections the terrain is not being used. to use the terrain where the channel spills over you need to create a 2D flow area and then connect the 1D cross sections to the 2D area so that water can spill between the 1D and 2D areas.

[ckatsoulas]

You could try changing the near and far spacing values until they provide a good match with surrounding grid so that the errors are reduced

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