The Performance of Hydraulic Software Solutions Through Sudden Velocity Changes

The Auckland Council 2011 Stormwater Flood Modelling Specifications recommends modelling software based on the St Venant equations, but warns “Performance of hydraulic structures should always be cross-checked with manual calculations or other software”. The New Zealand Standard NZS 4404:2010 goes further and states “Stormwater systems shall be designed by calculating or computer modelling backwater profiles” using the Bernoulli solution. Verification Method E1/VM1 published by the Department for Building and Housing also specifies the Bernoulli solution. It seems hydraulic modellers are faced with two incompatible standards, but on closer examination the incompatibility is resolved if “manual calculations or other software” is taken to involve the Bernoulli equation. Further, the Auckland specifications associate “energy losses due to turbulence” with “any sudden changes in velocity (magnitude and direction) e.g. at transitions, junctions, bends, entrances, exits, and obstructions.” Since the St Venant equation has nothing to do with energy, this expands recommended use of the Bernoulli equation beyond engineered structures to natural rapid changes in crosssection, such as at lake and swale exits and channel overfalls. Examples are discussed of errors associated with the misuse of the St Venant equations in such cases.