HydroCAD® Stormwater Modeling - Since 1986
Pond Storage Calculations
HydroCAD provides many options for defining pond storage. In addition to entering stage-storage data manually, HydroCAD can automatically calculate the stage-storage relationship for many common shapes, as listed below. (If you don't see all these options in your program, please update to the latest version.)
Although many ponds can be modeled with a single storage definition, HydroCAD allows any number of definitions to be used within a single pond. When using multiple volumes, the total storage is determined by adding the storage provided by each volume. (This assumes that all volumes are properly interconnected such that the water surface elevation is the same in each volume.)
Storage definitions can also be embedded within each other, such as a storage chamber within a gravel bed. When entering data for these situations, enter the outermost storage definition first, along with the appropriate voids. Then define each of the enclosed volumes and set its embedding as required. For further details, read about chamber storage and the chamber wizard..
When using area-based exfiltration, HydroCAD automatically uses the total area of the outermost (non-embedded) storage volume(s). It is assumed that water can flow freely between the inner and outer storage volumes, so they can be modeled as a single level-pool.
Interconnected ponds are generally modeled by routing one pond directly into the next, using a tailwater-sensitive routing procedure. However, if the connection between two ponds is sufficient to keep the water level equalized at all times, consider the option of modeling the two storage volumes as a single "pond", thereby avoiding the tailwater calculation. Just use a separate line on the Storage tab for each volume. If you wish, you can also include the storage contribution of the interconnecting pipe or channel. When describing the outlets, you should list only the final controls that effect both ponds, excluding the interconnection.
Modeling Off-Line Storage
Off-line storage refers to any arrangement where the storage is placed beside the main flow path, and water is diverted into the storage volume by an adjacent control structure, such as a catch basin and weir. The intent of this arrangement is generally to store all the inflow until the pond level reaches the weir or other overflow device. Any excess volume will flow over the weir and continue downstream. As the inflow subsides, the storage may be emptied by exfiltration, and/or a low-flow outlet device.
To avoid the complexity of using two ponds (the diversion "pond" and the storage volume), plus a possible reversing flow, this situation can often be modeled as a single "pond", with the upstream flow routed directly into the pond. To include all the storage components, just use a separate line on the Storage tab for each volume. The outlet control devices (exfiltration, low-flow outlet, and/or overflow) are part of the same pond, as described under Interconnected Storage, above.
To help you estimate detention pond storage requirements, HydroCAD provides a special pond sizing report.
In most cases, storage should be defined up to some point above all outlet devices. For example, when using an overflow weir, the software must know how much storage is available when the water is flowing over the weir. This requires knowledge of the storage that lies above the spillway. If you fail to provide this information, the routing cannot utilize the (substantial) storage that may lie above the spillway.
When entering stage-area data, this volume is easily defined by entering an additional stage at some point above the spillway. For this stage, specify the area (or volume) that lies upstream of the weir (inside the pond.) This is the volume that is controlled by the weir.
Special attention is required if the pond storage fills completely. For details read about overfilled storage.
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