Overfilled Storage
What Happens
If the defined pond storage becomes completely full, the routing will generally continue by applying additional head (pressure) to the outlet devices. In essence, the pond is treated as a closed volume without any "overflow" 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.
What
to do
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 physically available 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. You should also have at least one defined stage above the highest outlet device. For a weir outlet, specify the area (or volume) on the upstream side of the weir. This is the storage that is controlled by the weir.
(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.
In either case, you don't necessarily need to "make the pond bigger", you just have to supply complete information about the available storage so that an accurate routing can be performed. Without this information, additional head will be required to complete the routing, as reflected in the peak water surface elevation.
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.
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.