Software Status Bulletin 104

HydroCAD^® Stormwater Modeling - Since 1986

Software Status Bulletin # 104

Subject: Changes in Pond Routing Calculations

Applies to: HydroCAD-7.0 and later

Description

When the defined pond storage is exceeded, HydroCAD-7.x (and later) may produce different results than earlier versions.

Explanation

If a pond routing calculation exceeds the defined storage of a pond, HydroCAD-6.x (and earlier) would automatically extrapolate from the pre-calculated rating curve. This allowed the routing to continue using an approximate discharge and storage volume. A warning message [91] was issued to bring attention to the situation. The message specifically recommended that additional storage be defined in order to prevent extrapolation and ensure accurate results.

Beginning with HydroCAD-7.0, pond storage is no longer extrapolated. When the available storage is filled, the routing continues using only the defined storage. This may cause the peak elevation to increase, as additional head is required to complete the routing. A warning message is issued, recommending that any additional storage be explicitly defined.

Recommendation

Observe all warning messages! With open-air ponds, always supply enough stage-storage information to prevent the storage range from being exceeded. This will ensure consistent results between different versions. In the absence of a specific storage warning, comparable results can be expected without any changes to the project.

Comments

This is an intentional change in HydroCAD-7.0 and later. It allows the modeling of closed storage chambers, where automatic extrapolation would produce incorrect results. Providing adequate storage information for open-air ponds will ensure consistent results between different versions of HydroCAD.

Technical Details

The HydroCAD 6.x extrapolation was based on the pre-calculated Storage-Indication routing curve, and not on the individual storage and discharge curves. This approach does not attempt to consider the exact storage or discharge characteristics at higher elevations, and therefore will produce different results than when additional storage is explicitly defined, as recommended by the warning message.