Computer Simulations for Stormwater Management and Design

Editor’s note: This article first appeared in the June 2015 issue of Stormwater.

Computer simulations for stormwater management and design make it possible to try out new ideas without the risk of disrupting communities or ecosystems, to evaluate how a watershed would respond to acts of nature or engineering. Jeff Shiner believes computer modeling gives engineers and decision makers a set of powerful tools to make better decisions for the wellbeing of the community and the health of the watershed. They also make it possible to envision nature’s potential for good or for ill, and to be ready before extraordinary events occur.

Shiner, a civil engineer with the Metropolitan St. Louis Sewer District (MSD) in Missouri, notes that MSD has developed an ongoing relationship with the software provider Vieux & Associates to perform important computer modeling tasks for its stormwater program. According to Shiner, the company’s RainVieux software is a workhorse of the stormwater program. The agency uses the software package to obtain the highly accurate rainfall data it needs for planning. RainVieux computationally combines radar rainfall data with rain gauge measurements tabulated by instruments the county MSD owns and operates. The application allows users to download, display, and query rainfall in near real time.

Using RainVieux, MSD crosschecks its rain gauges against radar data and receives bias-corrected rainfall reports on a daily basis. This allows managers to pinpoint, repair, replace, or calibrate any malfunctioning or errant rain gauges whose readings fall outside the bounds of probability, ensuring data that are as accurate as possible.

Shiner says that about a decade ago, hydrologists performed a study to develop rain depth, duration, and frequency (DDF) curves for St. Louis County to help engineers envision the effects of precipitation from storms the region would be likely to dish out. More recently, MSD updated the data for its DDF curves, using the capabilities of RainVieux to perform an analysis of thousands of storm cells that had passed over the county. These studies were used to verify the rainfall statistics that the municipality can use as a reliable basis for capital improvement planning.

A Stream of Knowledge
Jared Barr, an engineer with MSD, says MSD has embarked on a few additional modeling exercises using a different Vieux & Associates software package called Vflo. According to Barr, it works together seamlessly with RainVieux, incorporating Vflo’s hydrology model with RainVieux’s highly accurate rainfall data to simulate runoff during storms.

Barr says MSD maintains a high-resolution light detection and ranging (LIDAR) dataset for the county’s topography, which is updated about every two years, along with digitized impervious area data from aerial photography, both of which can be easily entered into the Vflo program. In addition, he supplies the model with infiltration parameters from various sources such as the US Geological Survey soil data sets or the National Land Cover Dataset.

After running the model, Barr says, the results become very visible. “When you’re looking at the model, you see a bunch of grid cells, and then you’ll see arrows indicating flow path. Obviously, you can add background images. Without doing any of the computations, you can select one of the pixels and the program will highlight all of the other pixels that are upstream of that area, so it gives you an indication of what the drainage area is immediately, or the size of the drainage area. When you perform your computation for whatever event you’re looking at, it will generate hydrographs for that outlet or for a set of outlets. You can also choose other observation areas within the watershed—it will generate a hydrograph for that area, and it can give you an indication of depth of flow for those areas as well.”

Barr says part of the problem with stormwater in urban areas is “you get overland flooding that can sometimes surcharge a sanitary or combined sanitary stormwater sewer.” He adds that Vflo can help analyze these situations as well.

“The advantage of Vflo is not only is it able to give you a hydrograph based on rainfall, but it can also give an estimate of what the flow path is, and what the flow depth is,” allowing engineers to evaluate the potential impacts of overland flooding. Barr says that to model overland flooding with other software such as 1D modeling software, “you have to have some estimate of flow path.” In contrast, he notes, “You don’t have to, outside of the program, determine your drainage path, so that makes it quicker to use. Vflo takes the whole 2D surface and it will compute for each grid cell which way the water is flowing, what’s the depth of that grid, and where the water is coming from.”

Shiner observes that the ability to run simulations is having a big influence on how engineers think about stormwater, helping them see beyond the old Band-Aid solutions of building concrete channels to treat water as an inconvenience or a hazard to be dispatched as rapidly as possible. He believes models like Vflo help practitioners take a more holistic approach to stormwater, looking at “the overall health of the watershed, and trying to determine where the source of the problems are.”

With Vflo, he says, “You can see flood waves move down the system and be confident that it’s based on good science and good input data, and that you’re going to get a reasonable and accurate result. That does help you do a better job of planning the improvements you need on a watershed-wide basis.”