HydroCAD® Stormwater Modeling - Since 1986
If the defined pond storage becomes completely full, the routing will normally continue by applying additional head (pressure) to the outlet devices. In essence, the pond is treated as a closed volume without any overflow or temporary storage. HydroCAD will always generate a warning message to alert you to this condition. In addition, you may see an exceptionally high water surface elevation. This represents the head (pressure) that is required for the pond to discharge the inflow hydrograph without utilizing any additional storage.
If the storage volume is physically closed (like a tank), and you intend it to operate under pressure, then no further action is required. However, if any additional storage is available inside the pond at the reported peak elevation, the model must be adjusted in order to accurately define this volume:
(1) In the case of a custom storage definition for an open-air pond, one or more additional stages should be added. This typically involves defining the surface area at additional contour elevations. Include only the area (volume) that lies within the sides of the pond and the outlet devices. You should also have at least one defined stage above the highest outlet device.
(2) If the primary storage is defined using one of the common shapes (such as an underground vault or prefab chamber), then additional storage definition(s) should be added to describe the overflow volume. This might include secondary storage chambers, a feeding pipe, or even above-ground areas that would be inundated when the primary storage overflows, such as a parking lot.
(3) When using a weir outlet, be sure to define the temporary pond storage above the crest elevation, representing the additional volume that will be temporarily detained behind the weir. Make sure you define enough storage to suppress the "overfilled storage" message. This applies to all outlet devices, but the oversight is most common with a weir.
In each case, you don't need to "make the pond bigger" or enter "fictional" storage. Just supply complete information about the available storage so that an accurate routing can be performed. For the weir shown in this photo, you must specify the storage to at least the 3.60 foot WSE shown on the gauge. Without this information, additional head will be applied to complete the routing, rather than using the actual storage available in the pond.
After adding the extra storage, re-check the peak elevation and make sure this value is reasonable. If the value seems too high, you're probably omitting storage and/or overflow devices that would store and/or discharge the excess volume.
What if I don't want the water to go that high?
You still have to provide additional storage information in order to accurately assess how high the water will actually rise. After you provide the storage information and get an accurate routing, then you can adjust the model to reduce the peak elevation.
Why do I need enter storage data above a weir crest?
In order for water flow over a weir, the water surface elevation must exceed the crest elevation. And that head, however slight, corresponds to a specific volume of water stored in the pond. For example, if you have 1/10 foot of head over a weir with an average pond surface area of 10 acres, you have 1 acre-foot of temporary storage above the crest. This volume contributes to the pond's detention effects, and must be specified in order to get an accurate routing.
How do I calculate storage above the weir crest?
Construct an imaginary vertical wall above the weir and describe the volume on the upstream side of the wall. This is the storage volume being controlled by the weir. Do not include any volume below the weir, since this is not part of the pond's level pool.
What if the water overflows in several places?
Define each overflow as a separate weir. Or use a custom weir to define a more complex weir crest. Define the storage volume that is controlled by the weirs, using an imaginary wall above each weir to define the edge of the pond.
What if the water overflows evenly on all sides, like a depression or an open tank?
Define the entire perimeter as a weir, or use a horizontal orifice with the weir-flow option enabled. Define the storage to some point above the crest, using an imaginary wall above the crest to define the edge of the storage volume. For an overflowing tank with vertical walls, this can be done by enter a storage height that is slightly greater than the tank height. e.g. If the tank is 10 feet high, your would set the weir invert at 10 feet, but set the top of the storage at 11 feet. This allows the volume to be properly calculated as the tank overflows.
For additional information, read about pond storage calculations.
Note: HydroCAD-6 (and earlier) considered all storage definitions to be open to the air, and would automatically extrapolate from the storage curves whenever they where overfilled. This allowed an approximate routing to be performed until exact storage data was supplied. In contrast, HydroCAD-7 (and later) considers each storage volume to be closed, and never exceeds the defined storage. This allows HydroCAD to model compound storage arrangements and pressurized ponds. Note that in the absence of a storage warning, pond storage calculations are comparable in all versions. For details see SSB 104.
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