HydroCAD® Stormwater Modeling - Since 1986
Using the Pond Sizing Report
(adapted from the HydroCAD help system)
The pond sizing report indicates the approximate storage volume required to attenuate the (proposed) inflow peak to a specific target level, such as the pre-developed flow. This information is compiled from a large number of simulated pond routings, using the actual inflow hydrograph with various hypothetical pond sizes.
To use the pond sizing report:
2) Construct a model for the proposed conditions, including a pond with the appropriate inflows. The pond itself does not require any description at this point, but it must have the actual inflow(s) that you are designing for.
3) Open a report window for the pond. (Right-click the pond and select Report.)
4) Click on the "Sizing" tab. This will display a graph of required storage vs. target discharge.
5) Locate the desired discharge on the left axis. Then come across to the curve and down to the bottom axis. This is the approximate storage required to attenuate the inflow to the desired level.
Pond Design Tips
Designing a pond generally starts with an initial estimate of the storage and outlets. This description is then refined based on the results of the pond routing calculations. This iterative process allows the pond's design to be optimized for the specific purposes it must meet. Although there are techniques intended to directly determine required storage or outlet capacity, they generally involve a number of assumptions that limit their applicability and may compromise their accuracy. Furthermore, pond design is a multi-variable problem, and the final specifications usually involve a number of modifications to the storage characteristics and outlet devices.
It is strongly recommended that you examine the stage-discharge plot to verify your selection and configuration of outlet devices. Also check the stage-storage plot. Unexpected routing results are often the result of incorrect outlet configuration or stage-storage data. Verifying these items is the easiest way to detect and correct most problems.
If your actual pond routing fails to achieve the attenuation predicted by the sizing report, or your pond overflows, your outlet device(s) may not be permitting enough discharge early in the storm. For optimum storage utilization and maximize attenuation, you need to maximize the discharge early in the storm, so that adequate storage will be available at the time of the peak inflow. This typically entails the use of a low-flow outlet, such as a culvert or orifice near the bottom of the pond, in combination with a weir or other device to handle larger events. Using a weir alone will often cause the pond to fill up early in the storm, causing the peak flow to spill over the weir with minimal attenuation.
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