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HydroCAD Stormwater Modeling - Since 1986

Pond Sizing

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:

Click for more slides1) Determine the target (allowed) discharge.  This is typically done by modeling the existing site and recording the peak discharge for the 100-year event.

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.

Usage Tips

bulletFor more precise viewing, switch to the 2D view and zoom in to the region of interest.
bulletSizing estimates are also available in tabular form by clicking the "Table" button.
bulletFor a new pond, the sizing report will use large or small units, as specified on the Settings|Units screen. Once the pond is described, the report will use the same units as the pond description.

Other Notes

bulletThe pre/post projects are not linked or connected in any way.  The estimate is based on the actual inflows in the proposed scenario, along with the target (existing) discharge.  Other than the peak flow, the existing hydrograph does not enter into the calculations.
bulletThe sizing report contains estimates only. It is intended as a guide for establishing initial trial parameters to be used in an actual pond routing. The estimates may not be reliable in cases involving multiple peaks, base flows, or other uncommon conditions.  The final pond size will be determined as you complete the design of the actual pond and optimize it to meet your design requirements.
bulletThe sizing estimates assume that the pond discharge is proportional to the storage.  Actual routing results will vary depending on the exact configuration of your outlet devices.

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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