The use of InfoWorks CS brings benefits when modeling urban drainage design according to work that has been carried out by the River Engineering & Urban Drainage Research Centre at the Universiti Sains Malaysia. The work has shown that urban drainage modeling can help account for drainage failures and identify the backwater effects and shortcomings that contribute to urban flooding.
Drainage modeling is essential for better and more economic hydraulic designs, and InfoWorks CS provides a good platform for model building, analyses and preliminary design. In particular, it gives crucial information to design engineers, namely the interaction between the drain and receiving water, the indication of flooding and the cause of any drainage failure.
However, the key to a good model is not only down to the software, it also depends on the modeler. Correct results come from a combination of the technology, human factors and having adequate data and information available. Uncertainties have to be addressed, including making decisions about boundary conditions, and most importantly, tackling the calibration of ungauged catchments.
Urban areas alter the hydraulic processes in an area, with features such as SUDS or flood control facilities having a marked impact on the catchment’s hydrograph.
Much information can be gained from urban drainage modeling. The interaction between the drain and receiving water can be captured by creating a detailed model that splits the catchment into a series of areas instead of treating it as one. Comparing the catchment’s runoff and that from the drainage can highlight discrepancies.
Most drainage systems do not have gauges and so calibration is very important. Model results can be out by as much as 250% if an uncalibrated model is used with 100% radar-generated rainfall.
InfoWorks was utilized in studying an extensive urban area in a sub-catchment of Malaysia’s River Melaka. Data preparation took account of both existing and future infrastructure for 50 and 100 year annual recurrence intervals. The work examined the situation both with and without a barrage in operation.
The model was kept as simple as possible to aid calibration. Like most urban catchments, there were no gauges in the system but the modelers developed an approach that allowed them to create their own gauging records. This involved studying both flood and tidal marks as well as gathering extensive verbal information from both local people and the relevant authorities. All the information was analyzed to create logical relationships that gave credibility to the resulting estimated records.
The decision was taken to calibrate the resulting model by comparing it with an annual event. The modelers checked the model’s accuracy by comparing predictions of flood locations with actual results. Results of the simulation checked against observed data where it was available, and the resulting matches of which areas flooded or remained clear of water gave confidence in the model.
In total, eight existing and eight future scenarios were modeled, covering major and minor rainfall events for two different boundary conditions with and without the barrage in use. The model was able to demonstrate the effectiveness of a proposed solution.
This article is by Leow Cheng Siang of the River Engineering & Urban Drainage Research Centre at the Universiti Sains Malaysia. It is based on a presentation he gave at the 2008 Wallingford Software User Conference (Asia).