Cell Volume Vs Elevation

[robdir]

Regarding the table “Cell Volume VS Elevation”,
how are calculated the values?
Starting from the Terrain (DEM), what is the Max Elevation-Volume reported in the table(last – higher point of the plot)?
Is it the elevation inside the cell corresponding to the max volume stored?
(that is the max volume “ponding” in any area of the cell…)

[Anonymous]

Each cell contains a cross sectional face. The volume is calculated by the cells terrain which is represented by each cross sectional face. You can view the cross sectional face of any cell within RAS Mapper by selecting the face and clicking the plot cross sectional face option

[robdir]

Hi Luis, I don’t think so. If I have the higher (or the lower) elevation inside the cell (not on the face) how could it be considered?
If HECRAS takes only the cross sectional face, this point shouldn’t be taken in account.
I think the program calculates the volume below the plan at different Z heights (until Zmax).
It should be something like this:

[Anonymous]

I think i am misunderstanding your question then. Im not sure what you are asking about now. I was talking about the cell vs elevation tables that RAS calculates within its hydraulic tables which are referenced for the solution of each cell

[robdir]

I was talking about Elevation-Volume Table (and Plot) referred to the single cell.
If I read the values from the RasMapper in any cell I find a table like this:

I was asking for the criteria used to evaluate the values reported on the table and graph.
I checked some cells and I found that the Max Volume (last point in the table-plot) was referred to the max depth inside the cell (in some cases located in the middle of the cell or in other positions away from the faces).
So I think that the Elevation-Volume points are extracted from the DEM inside the cell like a Surface-Volume calculation used in GIS application.

[cameron]

Each cell is a mini storage area with a volume elevation curve generated by the terrain under the cell. HEC-RAS solves the water surface for each cell (only 1 wse per cell) and the velocities are computed at each node of a cell.

Each cell face is a cross-section and has currently 1 roughness value per cell face (will change if future versions) and is what is used to determine which way the flow goes.

If your cell face cross-section cuts through a levee or high ground as in your picture, the model does not see and water appears on the other side just as would happen in a 1D cross-section model. This is called leaking and is why it is important to use breaklines to force the cell faces to go along high ground or levees so the entire cell face cross-section picks up the high ground.

Previous versions of 2D HEC-RAS used a lot of points in the volume curve which slowed the model down and took up a lot more space. There is now a filter that is applied to limit how many points are in the volume curve.

If you are asking what the max height is used for the volume elevation curve, that is calculated internally by HEC-RAS.

[robdir]

Thanks Cameron for your response.
This is what I’m talking about.
My picture was an example (simplified in 1D direction) referred to the evaluation of the Volume-Elevation points reported in the property table.
I believe that HEC-RAS reports the couples points extracting the elevation values from DEM and evaluating the Volume for each of it.
Just something like this command used in ArcGIS (or others GIS softwares) :http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=How%20Surface%20Volume%20%283D%20Analyst%29%20works

I checked it on the single cell and I obtained similar results 😉

[Author]

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