October 2011

Stormwater Modeling and Analysis Software

Municipalities take advantage of the latest tools.

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In one part of the United States, municipal officials discover the original mapping of a floodplain is incorrect, setting the stage for a potential crisis following a significant weather event. Elsewhere, an engineer seeks answers to help mitigate frequent flooding. Sizing storm systems correctly is a challenge for another engineering firm.

To that end, they are all using stormwater modeling and analysis software to help solve those challenges in a way that could not have been done in previous years.

Remapping the Floodplain
In the late 1990s, when the University of Colorado was doing due diligence while looking at a parcel of land, “they discovered that the flood plain would actually overtop U.S. 36 and spill through the city,” says Kurt Bauer, the engineering project manager for Boulder, CO. “The original mapping done in the 1980s was incorrect.”

In conducting the South Boulder Creek Flood Analysis, Boulder is using one-dimensional and two-dimensional hydrological and hydraulic numerical modeling and GIS inundation mapping.

The South Boulder Creek Flood Mapping Study was initiated to more accurately identify the floodplain and assess the flood hazard in east Boulder. It will determine the intensity and volume of rainfall and flooding that could occur during several different storm events and define the extent and depth of the resulting flooding for each.

The study will reconcile information from previous studies using a more technologically advanced approach than used in the earlier studies.

The hydrological model is a continuous soil moisture accounting model that is calibrated to historical flood events using the embedded auto-calibration module.

The hydraulics of the system were described using MIKE FLOOD from DHI Software, a coupled one-dimensional and two-dimensional hydrodynamic model. The hydraulics model is an unsteady state implicit solution that includes the entire system with dams, reservoirs, and bridges in one single consistent model. The combined model is calibrated to a series of historical events.

The calibrated hydraulics model is also applied for inundation mapping. Several historical events and design storms are simulated, and the resulting flood inundation is mapped in GIS. The modeling environment also provides animations of flood wave propagation.

“It combines the one-dimensional for the creek, channel, and ditches and two-dimensional for overbank flooding,” says Bauer. “We remapped and submitted that to FEMA, showing that U.S. 36 would overtop and approximately 700 structures would be in the 100-year floodplain in what we call the West Valley, which is everything west of South Boulder Creek and east of the Foothills Parkway.”

Additionally, a risk study indicated approximately $215 million in damage could occur during a 100-year weather event.

“We initiated the flood mitigation study in 2010 and selected CH2M Hill as a consultant team to help us develop a set of alternatives and go through the public process,” says Bauer. “They are using the two-dimensional MIKE product to develop conceptual alternatives, but they’re using eight-meter grid cell, which runs three days instead of two weeks to run the model, instead of the four-meter grid cell. The final selected alternative will be evaluated using the regulatory model, which takes about two weeks to run.”

There have been a series of public meetings and reviews by various boards to examine nine alternative solutions in an effort to narrow the options down to five, and then eventually to present the preferred solution to the city’s council by early 2012.

Of those options, one is to take no action, says Bauer. Another is high-hazard mitigation.

“There is a multifamily structure that’s in a high hazard zone, which the city of Boulder defines as a condition when depth and velocity poses a threat to life and safety,” says Bauer.

Another alternative is to perform limited site work to eliminate the high hazard for the multifamily structure and then floodproof a number of critical structures in the floodplain, such as assisted-living facilities.

Another approach entails examining large regional detention sites to detain the water and meter it out to eliminate the overtopping of U.S. 36 and the flooding through the West Valley.

A fifth option is the construction of a pipeline, which comes with a “high price tag,” notes Bauer.

Bauer says the city could not use a traditional, one-dimensional, HEC-RAS model because of the many complicated split flows in the region. “It wouldn’t simulate the hydraulics appropriately, so we had to use a two-dimensional model, because so much of the flooding leaves the actual channel,” he says. “The channel overtops at approximately 2,500 cfs [cubic feet per second], and we’re trying to deal with 7,000 cfs.”

Although a two-dimensional model offers good technical information, “It requires a lot of horsepower and a consultant team that really is on top of hydraulic modeling,” says Bauer. “The regulatory model takes two weeks to run, so anyone using that has to keep that in mind. HEC-RAS will run in seconds or minutes, so you can evaluate alternatives and tweak them very quickly, whereas with MIKE FLOOD, a coarser grid model takes three days to run. Your costs and time frame are going to go up substantially.”

The updated mapping study results that were sent to FEMA replace the current regulatory mapping based on a 1986 US Army Corps of Engineers study. The city is already using the new maps to review, regulate, and issue permits for all properties within the South Boulder Creek basin.

FEMA is slated to officially adopt the study in late 2011 to coincide with the adoption of the Digital Flood Insurance Rate Maps for Boulder County. The 700 structures identified in the 100-year floodplain encompass 240 more structures than currently exist in the 100-year floodplain.

CSO and Flood Modeling
HDR in Kansas City, MO, uses XP Software for its stormwater modeling and analysis.

“We use XP Software’s XP-SWMM model frequently for our municipal stormwater projects,” says Raziul H. Mollah, P.E., project manager for water resources and GIS coordinator for HDR. As an engineer working with stormwater, Mollah faces ongoing challenges in modeling, including the level of detail, accuracy, efficiency, and cost of the model development.

“We look for more robust hydrology in the model for accurate hydraulic responses, therefore better simulation,” he says. “XP-SWMM is a one of the top tools for urban stormwater modeling.

“I’ve worked with this model for more than 10 years. XP Software is constantly improving the tool and adding more features. I use other models, too, but I always prefer to use this when there is a need for sewer network system modeling.”

Some of the functions for which his company uses the software include CSO/SSO [combined sewer overflow/sanitary sewer overflow] modeling, stormwater master planning, storm or sanitary sewer analysis and design, and highway drainage design.

“XP-SWMM is a robust software package that works well with the pipe network system,” says Mollah. “We use this model to develop hydrology and hydraulics of the sewer system. It has lot of flexibility, using several options of hydrologic methods and hydraulics. It is the most state-of-the-art full-equation solver with adapted time step technology, therefore it is a very stable solution engine with high accuracy.”

Mollah has found that while there are similar software products in the United States market, “the XP-SWMM is one of the top models in the stormwater/sanitary practice area. XP-SWMM has a very good and easy-to-use graphical user interface, external database connection such as an Excel spreadsheet, and easy to use geographic information system data. Model building using the external database and GIS makes it easy and efficient to develop even complex models.”

Mollah also likes using the software’s two-dimensional modeling capability.

“Basically, you can easily develop an underground sewer network and overland two-dimensional surface model for hydraulics as a combined model. The stormwater flooding—like sewer overflow that creates street or property flooding—modeling can be very accurately modeled by using the one-dimensional/two-dimensional interface of this software. It also has a water-quality component, which is very useful for stormwater master planning projects.”

Recently, HDR developed an XP-SWMM model for a stormwater flooding problem in a commercial area of Lenexa, KS.

The City of Lenexa’s 108th and Pflumm Drainage Improvement Project was jointly funded by the city and Johnson County, KS.

“The project goal was to identify and solve frequent flooding problems at B/E Aerospace, then develop plans and specifications to alleviate the frequent flooding to this property,” says Mollah.

The watershed primarily encompasses a commercial and piped sewer system. “But where the flooding occurs, the sewer system daylighted to a concrete-lined channel system,” he says. “The one-dimensional/two-dimensional interface helped us to accurately model the scenario. In this project, we developed open channel and topographic surface as two-dimensional overland flow underlying the one-dimensional sewer network.

“The one-dimensional and two-dimensional parts of the model run simultaneously; therefore whenever flow exceeds capacity of the pipe, it overflows to the ground and develops overland gravity flow to follow the surface topography.”

 The XP-SWMM model HDR created was calibrated to a real storm event, to simulate the flooding created by the storm.

“Using this model, we designed alternatives to alleviate the flooding and finally designed a system for construction,” says Mollah.

Mollah also created a YouTube video using a two-dimensional simulation of flooding. The software helps him meet his goals as a stormwater engineer, Mollah says. “My first goal is to identify existing conditions by understanding the present condition of the system—if we can simulate one or more real storm conditions—then find appropriate solutions to the problem that presently exists,” he says. “Like other stormwater modeling software, XP-SWMM has the capability to simulate actual storm events efficiently, which is very important for consultants to be cost-effective, then develop alternative solutions to the flooding.”

Modeling for a Racetrack
Stormwater modeling plays a major role in the work conducted by Paulus, Sokolowski & Sartor, a Warren, NJ-based multidiscipline engineering and architecture firm offering building design, land design, and environmental services to a wide spectrum of clients.

The firm does site development, including obtaining all necessary local, state, and federal permits, and maintains stormwater management regulations.

In stormwater modeling, “We’re trying to mimic real life as close as possible,” says Adolph Montana, project engineer. “The hardest part is to make sure your assumption mimics the real world.”

Paulus, Sokolowski & Sartor primarily uses Bentley System’s StormCAD to size pipe systems. “It seems to be the one that is most straightforward,” says Montana. “It’s very user-friendly. In terms of all of the programs that are out there, it’s the one that we feel does the best job.”

StormCAD’s comprehensive modeling for the design and analysis of storm sewer systems uses a peak flow (Rational Method) approach. It includes automated constraint-based design to size pipes and determine invert elevations, scenario and data management, and reporting capabilities.

Montana recently used the software for a job at the Monmouth Park Racetrack in Oceanport, NJ.

“We needed to upgrade the stormwater system,” he says. “They were having issues with contamination going into the stream, and we needed to separate the dirty system and the clean system, so we modeled the entire site using StormCAD. It helps in getting a good idea on how the entire system acts together.”

Montana favors Bentley software products because of the technical support provided, he says. “They are always available if you have questions, so if I have a problem or if there’s something wrong with the model, they’re available to help with that 24/7,” he says. “Usually when we have a problem, I will e-mail them, and they will have an answer for me fairly quickly.”

Computational Hydraulics International (CHI) introduced stormwater modeling software more than 25 years ago. Its present PCSWMM—a software program to model water systems—is designed to work with the EPA-created SWMM (Stormwater Management Model).

PCSWMM is a spatial decision support system with all hydrologic and hydraulic calculations done by the official version of the EPA SWMM, notes CEO Rob James.

The software can be used to design and size drainage system components for flood control, size detention facilities, and design control strategies for CSOs and SSOs, among other functions. It can also generate nonpoint-source pollutant loadings for wasteload allocation studies.

Its real-time capabilities is what CHI emphasizes, James says. “That’s where the system is running in real time, downloading, processing radar rainfall data, forecasting conditions into the near future, modeling for the highly detailed hydraulic model, and forecasting water levels throughout the watershed,” he says.

Presently, low-impact development (LID) modeling has become a “hot topic” for CHI, he adds.

“We have a nice LID tool kit that was just released last year. It allows you to model virtually any LID device, but it models it very physically, so it breaks down the compartments within LID—the different processes going on, surface storage, infiltration, and water balance within the LID itself,” says James.

“Because it’s so physically based, our approach to LID modeling can model very specifically LIDs at a lot level, and also there are tools for abstracting that to much larger models where you wouldn’t have the time or budget to get into the lot-level specifics for LID implementation,” he adds.

That’s applicable for master planning in cases where there are large models of the entire city or drainage area.

“You may want to see [what would happen] if you put so many rain barrels into these neighborhoods, and this many green roofs in commercial areas, or high-infiltration swales. You can effectively do that in the model,” James says. “You can get very specific and do a lot-level study or you can scale it to do very large overall water balance model.” 

James says his company’s mandate is to encourage the use of the EPA’s SWMM5 model.

“That’s one of the things that sets us apart,” he notes. “We don’t offer our own customized version of SWMM. We encourage the official use of the EPA’s. By increasing the user group and encouraging EPA’s involvement, we move that engine forward.

“We think it’s important to standardize on an engine that produces similar, reliable results that people can standardize and know what to expect,” he adds. “The nice thing about the EPA SWMM engine is that it’s public domain open source, and it’s very well-written code and easy to get into and see what’s going on. There is a very vibrant users’ group. By encouraging its adoption, the users’ group expands and the software moves forward.”

CHI also does consulting and training as a small part of its business. “We as a company try to focus on technology transfer,” says James. “We much prefer to educate our clients and help them to the work rather than for us to do the work. To that end, we’re putting a big emphasis on developing very powerful, very scalable software that’s also very easy to use and intuitive. People can focus less on the
intricacies of the interface and more on the engineering work.”

CHI offers workshops, including self-paced e-learning. “As much as we would like this to be an intuitive product, there’s a lot to learn about this type of modeling, especially as you get into the more advanced topics,” notes James.

Modeling has some inherent challenges, he concedes. “With water-quality modeling, there is a lot of uncertainty,” he points out. “We’re trying to improve what we do in that regard.”

Software and routines used for modeling for hydraulics and hydrology are quite sophisticated now, James notes.

“The client is usually data starved,” he says. “It’s a very capable piece of software, but they need to get good-quality data to put into the model. We’re trying to develop tools that allow you to bring in very large data sets and to do quality control on them very quickly, to create a model from the existing data sets the cities may have.”

Data collection for calibration models also is “a very big issue,” notes James. “Ultimately when you’re modeling existing systems, you’re trying to tackle existing problems and improve existing infrastructure,” he says. “You really need a lot of good-quality data to make sure your model is producing reasonable results.”

CHI’s software incorporates many quality assurance and quality control tools.

“People are more and more using very large data sets,” says James. “Twenty-five years ago, we did modeling that was small and simple because there wasn’t enough computational capability to handle anything complex. You were modeling with 100 pipes, and it was very easy to go over that data set and hand check it.”

Today, people are modeling more on a one-to-one ratio of what’s in the ground to what’s in the model, he adds.

“They’re not simplifying things as much,” says James. “Our model is very scalable. We have the ability to model hundreds of thousands of pipes and roads and manholes. It’s next to impossible to manually go through that data. We need tools to automate that process.”

Visualization techniques help in checking data. “It’s a lot easier than going through tables and tables of data,” points out James. “There also is a lot of missing data and we have the tools for helping fill in that missing data in an effective way.”

Author's Bio: Carol Brzozowski specializes in topics related to waste management and technology.