Sizing stormwater detention basins as part of a Water Sensitive Urban Design can be a difficult and timeconsuming process. Fortunately Bentley has a software solution that can make the designerís life easier.

The concept of Water Sensitive Urban Design (WSUD) is well known and widely recognized in Australia and throughout the world.

And at a high level, WSUD is generally a matter of applying some common-sense principles at the initial stages of a development process to ensure that the natural water cycle is not negatively impacted by urban development.

However, as with most engineering projects (and life in general) the devil is often in the details. One example is in the hydraulic design of stormwater detention basins, where determining the correct size of the basin has traditionally been very challenging. But, armed with the right tool ñ such as Bentleyís CivilStorm, a comprehensive stormwater modelling and analysis solution ñ this task can be dramatically streamlined.

Stormwater detention basins are an important part of WSUD. These facilities capture stormwater runoff and typically detain the runoff for some time to reduce the peak runoff rate, while allowing some (or all) of the runoff to infiltrate slowly into the ground to reduce the total runoff volume. This helps to maintain the natural hydrologic behaviour of the surrounding waterways.

However, it is critical to size these basins correctly. If they are too small there is a chance that they could overtop in a severe storm ñ potentially damaging the basin structure, or causing flooding in nearby urban areas. But if they are too big, they may be unsightly (think of a large water feature that is permanently dry), expensive to construct, and may also retard any natural environmental flows.

So how does the designer determine the right basin size?

It turns out that the calculations required for this are extremely complex. Rainfall data is used to compute a stormwater runoff hydrograph (a curve showing the runoff rate versus time), which is then routed to the detention basin (possibly through a series of pipes, channels or swales). Then, the flow out of the basin is computed as a function of the basin water level and the configuration of the basin outlet structure (whether that is a weir, culvert, riser, orifice, or several of these devices combined together).

Fortunately, while this problem is practically impossible to accurately solve by hand, it is ideally suited to a purposebuilt computer application.

CivilStorm from Bentley is one such application. It takes input such as rainfall data, catchment area and perviousness, basin size and outlet structure configuration, computes runoff amounts, and then solves the full partial differential hydraulic equations (known as the St. Venant equations) to determine flow rates, basin water levels and hydraulic grade line elevations. It can even compute water quality parameters such as the transportation of pollutants.

CivilStormís powerful scenario management features also allow designers to compute the peak stormwater runoff rate and total runoff volume for their sites prior to development. Then, they can construct another scenario for the postdevelopment condition and compare the two. If the post-development calculations indicate that the detention basin overtops, its size can be increased and the scenario computed again. Similarly, if the CivilStorm results indicate that the peak runoff rate from a site after development exceeds the peak rate from that same site prior to development, the basin outlet structure can be modified to restrict more flow - thus reducing the peak runoff rate.

Since CivilStorm can handle extremely complex hydraulic systems (comprising of basins, pipelines, natural or manmade channels, pumps, wet-wells, drainage pits and outfalls) under any sort of flow condition, it is flexible enough to tackle any type of urban development project.

CivilStorm can run as a stand-alone windows application, or from within the MicroStation or AutoCAD platforms. Regardless of the platform used, it is possible to build a model from a wide array of existing data (GIS Shapefiles, DXF files, Excel spreadsheets, Access databases, etc.), as well as export hydraulic results back out to those same data formats.

So if you or your organization is involved in WSUD, be sure to investigate CivilStorm to see how it can help you streamline and improve your design process. For more information on CivilStorm or any other software solutions from Bentley:

• Tel: 1800 500 227 (toll free from within Australia) or 0800 500 874 (toll free from within New Zealand)
• Email: sales.haestad@bentley.com 
• www.bentley.com/civilstorm/NRWM