Sewer Flooding Analysis Using Digital Terrain Models

By Alan Hymers

The current investment period in England and Wales for water companies includes for over £1 billion (about USD $1.9 billion) in capital expenditures to solve property flooding arising from sewer systems.

Although available sewer network modeling software allows for predicting the response of the underground sewer network to storm conditions, what happens to the flow once it has escaped from the sewer system via a manhole cover is less clear. Interaction with the ground surface can mean that adjacent properties can become at risk of flooding since flow can travel over land, especially where property is located in low-lying areas. While a number of improvements are being developed in the industry, Mott MacDonald has developed techniques to help predict the risk associated with flooding and aid the design engineer in fully understanding the mechanism of flooding and hence deriving a cost-effective solution.

Digital Terrain Models

Digital terrain models (DTM) are built to allow the ground surface to be replicated, particularly in an area known to be at risk of flooding. Ground elevation data, usually captured by aerial laser scanning techniques (LIDAR), is used to generate the DTMs. Custom-designed software then reproduces a model of the ground surface. Geo-referenced data (that is, referenced to a common datum) can be overlain onto the DTM to display a number of views relevant to the study being undertaken.

The modeling tool creates a 2D mesh over the area of interest and routes flood water over ground surface.

The DTM is now ready for use and the underlying data can be queried in a number of ways. Height data can be readily displayed anywhere on the DTM, hence potential routes for new pipelines can easily be checked for viability in terms of potential construction depths. Potential new construction sites can initially be identified from aerial photography and areas to be avoided, such as nature-sensitive sites, can also be mapped onto the DTM and displayed visually.

Predicting Surface Flow Paths

Additional 2D modeling software is now also used that, when combined with sewer modeling software, allows a better understanding of the flooding mechanism. The 2D modeling tool creates a 2D mesh over the area of interest and routes flood water over ground surface held in the 3D model. As shown in the figures, early in the storm, floodwater on the surface is routed across the 3D ground model (shown in blue). In the middle of the storm, flow is routed across the surface from north-west to south-east. By the end of the storm, flow has been routed to south-east corner of the site and is filling a depression on the surface (shown as dark blue).

About the Author:

Alan Hymers is an associate with Mott MacDonald, based in the company’s Newcastle-Upon-Tyne office. He is a chartered civil engineer with experience in the project management, design and site supervision of civil engineering works.