Flood fight in Fargo

CE May 2010 » PROJECT CASE STUDY
Technology triumphs in the effort to forecast and plan response to spring floods.
Jon Thorp
The Red River crested at a stage of 40.82 feet on March 29, 2009. This flood-inundation polygon shows the downtown Fargo-Moorhead area at a 41-foot stage.

Spring is typically a pleasant time for the Midwest region of the country. Cold, harsh winter weather is replaced by warmer temperatures, melting snow, and sunny skies. However, for Fargo, N.D.; Moorhead, Minn.; and surrounding areas (collectively referred to as the F-M area), the season also marks the time of year for flood concerns. Even for an area that is no stranger to flooding, the spring of 2009 was perhaps one of the most frightening and uncertain times the tight-knit communities had ever experienced.

Project
Flood management, Fargo, N.D. and Moorhead, Minn.

Civil engineer
Houston Engineering, Inc.

Product application
HEC-RAS software, a Flood Forecast Display Tool, GIS, LiDAR data, and GPS data collectors improve the accuracy of flood models and community preparation.

On March 28, 2009, the Red River at Fargo crested at a stage of 40.82 feet, officially marking the area’s highest known river level. This bested the previous historic record crest stage of 40.1 feet, recorded in 1897. It also exceeded the highest river crest in recent history, when the Red River reached 39.57 feet in 1997. To give a better idea of the severity of these river levels, flood stage in the F-M area is 18 feet; moderate flood stage is 25 feet; major flood stage is 30 feet. (Editor’s note: In late March 2010, the Red River crested at 36.99 feet in Fargo.)

This event could have gone down in history as the F-M area’s greatest disaster, but it didn’t. In fact, one could have visited the area this past summer and found a vibrant, thriving community showing almost no signs that a historic flood occurred only two months earlier. The area was able to avoid a catastrophe because city leaders and residents were well informed of the impending danger and prepared for the worst possible scenario. Drawing on their experience from the recent flood event in 1997, the area’s citizens acted as first responders and worked together to save their cities and homes.

While most people look back on the flood fight and remember filling, slinging, and stacking sandbags, many groups were supporting the effort behind the scenes. One Fargo-based company, Houston Engineering, Inc. (HEI), worked around the clock to provide support to the National Weather Service and the cities of Fargo and Moorhead and surrounding areas to assist in flood-fighting efforts. The firm, which is experienced and skilled in water resources management and GIS technology, was asked to help with several projects.

“Water resources and flood planning and response are a big part of our company’s services, so we’re always involved with those types of projects,” said C. Gregg Thielman, P.E., vice-president of HEI. “And we ended up putting a lot of tools and experience from past projects to work during the spring flood fight of 2009.” Also a Certified Floodplain Manager, Thielman’s background and extensive knowledge allowed him to become a leader and major contributor to the forefront of the flood fight.

Super modelers
While experts recognize that every flood event is unique, they still look at past events to calibrate models and help predict how a current flood is going to play out. But in the case of the 2009 F-M flood, the National Weather Service was having difficulty accurately predicting the movements of the Red River.

“The Red River, especially through Fargo and Moorhead, is a difficult area to model and forecast,” said Thielman. “And with this particular flood, the river wasn’t behaving the way the weather service’s flood wave [FLDWAV] models were showing.”

Experts knew that any mistakes in the modeling would lead to incorrect forecasting and crest predictions, resulting in an insufficient preparation plan. This uncertainty in its models prompted the weather service to enlist the help of a few modeling specialists. After working with HEI on flood-related projects in the past, weather service experts at Grand Forks, N.D., and Chanhassen, Minn., knew the company had the experience and expertise to provide modeling assistance.

HEI utilized a different model — Hydrologic Engineering Centers River Analysis System (HEC-RAS) — and used those results to provide input for the forecasting and crest predictions. The results from the HEC-RAS models helped the weather service forecast the river crest date with complete accuracy and closely predict the river crest level. Experts have found the benefits and accuracy of the HEC-RAS models to be of such value that the National Weather Service is currently making the switch from FLDWAV to HEC-RAS for future flood forecasting on the Red River.

To further help the communities with flood preparations, the weather service used a tool created with the help of HEI in 2005. The Flood Forecast Display Tool (FFDT) was developed to display National Weather Service forecasts graphically. The FFDT maps flood predictions in a time series, every six hours for a seven-day forecast period. A seven-day forecast is animated into a time/date sequence, and looks similar to an in-motion weather map. The graphic also shows the estimated arrival time for the flood peak and flow conditions in and around the F-M area for the forecasted flood period.

Accurate forecasting and crest predictions from the weather service were crucial to the flood fight for several reasons. They kept city leaders informed and able to advise the public on what was happening and what types of precautionary measures to take. They also gave residents a clear date for when to expect the river crest, allowing them ample time to take the recommended precautionary actions or evacuate their homes if necessary.

HEI’s work with the National Weather Service wasn’t the only place its modeling experience came into play during the 2009 flood fight. After working with them in the past, the city of Fargo also looked to the firm for assistance on a major project.

In March 2009, floods inundated large portions of the Briarwood neighborhood in South Fargo, N.D.

The city of Fargo wanted the flood inundation polygons hosted on its website to be updated because new models and light detection and ranging (LiDAR) data were available. HEI had previously conducted a lot of the hydraulic modeling for the river throughout the F-M area for the ongoing Federal Emergency Management Agency flood insurance study update. Knowing this, the city of Fargo contracted the company to develop the flood-inundation polygons update.

Using the new LiDAR data, HEI took the hydraulic models that were developed for the flood insurance study and referenced them to flood stages as measured at the U.S. Geological Survey reporting station located near the center of the cities. They then interpolated intermediate profiles between the events that were modeled. This allowed them to develop a water surface profile tied to flood stage at every half-foot increment. Next, water surface triangulated irregular networks (TINs) were created to account for the profiles. The results were brought into GIS, which was used to map flood-inundation areas.

The flood-inundation polygons offered several benefits. For residents, it kept them informed of the potential risk to their property at the current forecasted flood stage. The tool was hosted on both Fargo’s and Moorhead’s websites. It was also available on www.inforum.com, the website of the local F-M area newspaper, The Forum of Fargo-Moorhead. The accessibility and user-friendliness of the tool made it convenient and fast for residents to assess the risk for flooding in their neighborhoods. The maps showed areas that would be protected by existing levees and areas that would experience ponding. They also showed areas that would flood if temporary protection methods failed, which was necessary for evacuation planning.

Also valuable to city leaders, the flood-inundation polygons tool visually depicted what flooding would look like at any river stage, in half-foot increments. Because of extreme, changing weather conditions during the lead up to the flood, the crest prediction level varied from 37 to 43 feet. This made it imperative to have a tool that could instantly show what would happen at the current crest prediction level.

The tool also assisted city staff with establishing flood protection levels throughout the city. A significant part of the flood fight involved construction of emergency sandbag dikes around homes, in backyards, and near water-filled ditches — an estimated 3.5 million sandbags were used. Local surveyors staked and verified heights so volunteers knew how high to build these dikes. HEI’s surveyors used the water surface TINs that were created from the hydraulic models and converted them for use in GPS data collectors.

“It allowed our surveyors to select the forecasted flood stage elevation and allowed field staking of protection elevations that followed the water surface profile generated by the hydraulic models,” said Thielman.

This GPS surveying technique had several advantages. First, the surveyors received updated forecast information immediately, and were able to obtain water stage elevations at the click of a button. This highly efficient method was especially valuable because the changing forecast required staking multiple times. The continuous flow of updated information also allowed the surveyors to keep working around the clock, imperative because the flood fight was a 24/7 effort.

Ultimately, winning the battle against the 2009 flood came down to strong community leadership, volunteers, state and federal support, and the fact that the community was well prepared and willing to fight. But accurate forecasting was key to this readiness because, without it, citizens would have had to go into the fight blind, not knowing what to expect or how to prepare.

Moving forward
Future flooding provides a significant risk to the residents of the F-M area. To ensure the community doesn’t have to go through a flood fight of this magnitude again, F-M community leaders must plan for permanent flood protection.

Through assistance on several options and support projects, HEI is continuing to put its modeling expertise to work. As part of an ongoing study by the U.S. Army Corps of Engineers, HEI has developed HEC-RAS models to study the Red River and its tributaries. The Corps is using this information to help analyze potential flood control projects.

HEI has also developed a stormwater management model (SWMM) to better understand interior drainage within the city. Thielman explained the purpose of the SWMM: “If a levee goes in, you have to deal with runoff water from the interior of the city. So we’re doing modeling to help design those components.”

While the community’s readiness and response was key to keeping the river waters from completely destroying the area, one cannot forget about the supporting efforts that allowed for sufficient preparation. “The newer hydraulic models, LiDAR, GIS, Flood Forecast Tools, and other decision-support technology were tremendous assets to the 2009 flood fight,” said Jeff LeDoux, president and CEO of HEI. “This science and technology is rapidly changing and becoming more accurate … and in turn, it’s becoming more valuable to community leaders in making informed and timely decisions.”

Dikes in downtown Fargo, N.D., near the Red River kept floodwaters at bay during spring 2009.

Jon Thorp is public relations director for The Promersberger Company. He can be contacted at jon@promersberger.com.


Upcoming Events

See All Upcoming Events