Hydraulic Modelling of Urban Catchments

With the development of powerful micro-computers, hydraulic modelling of drainage systems is possible for most existing and proposed drainage systems. Modern software packages such as Hydroworks and Infoworks by Wallingford Software provide a full solution to the flow equations, and can cope with branching and reverse flows in a drainage system.

We use these systems every day to design new drainage systems, analyse existing systems and to calculate the runoff and storage required for flood risk assessments.

The modeller constructs a representation in the computer of the actual drainage system, with its pipes and manholes, pumps and overflows. Simulated rainfall events are applied to the model to gauge the response of the system to increasingly severe events. The analysis of the new and existing systems use the same tools, but with a different end.

For a new system, the object will be to comply with design criteria, usually Sewers for Adoption, which requires that the pipes should cope with a 2-year storm, and that there should be no flooding in a 30-year storm. Flow balancing using oversized pipes, tanks and open water storage can be simulated.

For an existing system, the model is constructed to represent what is actually there, and the model is verified using flow monitoring and rain gauges. The predicted flooding is compared with actual records as part of the verification process. When the model is calibrated, increasingly severe rainfall events can be applied to locate any weaknesses and to test any changes to the system before they are constructed.

Hydraulic modelling has transformed drainage design over the last 10 years, and is an essential part of both sustainable drainage system design and the management of our existing sewerage network.

AutoCAD output from Hydroworks model prepared by Fenland Hydrotech