Stage-Storgae Calculations 1

To determine the stage-storage relationship for your basin you can use the average-end area method whereby you determine the surface area at various elevations within the facility, take the average of the areas between adjacent stages and multiply by the distance (elevation difference) between them.

1. Determine the stage increments that you need.

2. Start at the bottom (depth = 0) of the facility and for each stage (elevation) determine the surface area

3. Determine the incremental volume between each stage. Incremental Volume = (Average Surface Area between adjacent stages) x (difference in elevation)

4. Once all incremental volumes are determined, do a cumulative summation to determine the stage-storage relationship.

Set-up a spreadsheet and this becomes easy as pie. When you get to it, your spreadsheet can also include some columns for the discharge calculations.


I typically determine my S-S relationship in 0.100 m (4") increments. With the help AutoCad, this is what I do:

1. Start with an AutoCad drawing of the facility in which the outline of the bottom (depth = 0) of the facility is a PLINE (a closed polygon with an associated area)

2. OFFSET the PLINE to the depth = 0.100m (4") level. For example: If your pond side slopes are 3:1, then you would offset the PLINE 0.300m (12"). If the pond side slopes vary, then you may have to make some adjustment of account for the different slopes. Determine the area at this stage.

3. Repeat step 2 for depth = 0.200 (8") by either offsetting the bottom by 0.600m (24"), or offsetting the 0.100m depth contour by another 0.300m (12"). [These numbers are still assuming 3:1 pond side slopes]

Hope this helps.

This is the method we use for retention basins (zero outlet, usually sink holes). We use an excel spreadsheet and get our contour areas from Terramodel.

For detention basins, we use Hydraflow, which calculates our in- and out-flow hydrographs using either the SCS or Rational method (we use SCS as it's more conservative). Hydraflow is simple software to use, but it's not free.

I believe that SWMM can do this task and a whole lot more, but I haven't had much chance to play with it yet. SWMM is free software distributed by the EPA.

HydroCAD can be used for easy stage-storage calculations. It includes the "average area" method described above (which is strictly accurate only for prismatic shapes), plus other options for conic ponds (sloped slides) and various geometric shapes. You can do these calculations with the free HydroCAD Sampler at

I use XPSWMM for complex modeling including interconnected ponds but it is hard to get started on it. I use Hydra flow for everything else and it is the best ive ever spent on software