Turning the Tide in Tulsa18 Sep, 2006 By: Kenneth Wong
Army Corps Uses River Analysis Technology to Mitigate Flood Damage
Memorial Day 1984 was a muggy Sunday in Tulsa, Okalahoma. It also proved to be one of the darkest hours in the city’s history. While many in the rest of the country enjoyed barbecued ribs and chicken wings, people in Tulsa watched nearly 7,000 of their homes and other buildings washed away by a devastating flood. It killed 14, injured 288 and cost the city $180 million.
Flood is a recurring theme in Tulsa’s history, going all the way back to 1908. On the first anniversary of Oklahoma’s statehood, the Arkansas River overflowed, costing Tulsa $250,000. In 1970, Mingo and Joe Creeks swept away property worth $163,000. In 1974, Bird Creek unleashed its fury twice, in April and May. Later on, in June, its destructive example was followed by Joe, Fry, Haikey and Mingo creeks. In September, Mingo Creek flooded once more to close out the year. In 1976, yet another flood destroyed 3,000 homes and left the city with a $40-million damage bill.
After all of this destruction, why is the NFIP (National Flood Insurance Program), a FEMA (Federal Emergency Management Agency) offshoot, offering Tulsa residents federally backed flood insurance at a 40% reduction in premiums? Russell Wyckoff, a hydraulic engineer for the U.S. Army Corps of Engineers, Tulsa District, has an explanation.
Keeping Tulsa Dry
After the 1984 Memorial Day drench, Tulsa residents decided to pour their resources into understanding and controlling the erratic rivers and creeks in the vicinity. They invested $10.5 million in flood control works and set aside $2.1 million for master drainage plans. Funded mostly by local capital sources, flood insurance claim checks and federal funds, the total capital program reached $30 million.
Its test came in 1986, when the Arkansas River flooded again. Between September and October, to relieve the bloated Keystone Reservoir, the Army Corps was forced to dump water at the rate of 310,000 cubic feet per second. Damage from the downstream flooding was limited to 64 buildings, totaling $1.3 million of damage.
FEMA’s NFIP uses a CRS (Community Rating System) that, as FEMA defines it, is “a voluntary incentive program that recognizes and encourages community floodplain management activities that exceed the minimum NFIP requirements. As a result, flood insurance premium rates are discounted to reflect the reduced flood risk resulting from the community actions meeting the three goals of the CRS:
reduce flood losses;
facilitate accurate insurance rating; and
promote the awareness of flood insurance.
“For CRS participating communities, flood insurance premium rates are discounted in increments of 5%; i.e., a Class 1 community would receive a 45% premium discount, while a Class 9 community would receive a 5% discount …” In NFIP’s May 2006 CRS scorecard, Tulsa was the only community ranked Class 2, which qualifies for a 40% discount.
A River Runs through It
The Arkansas River, the Mississippi River’s major tributary, cuts across Tulsa’s wooded terrain and rolling green hills. So any time the Arkansas swells up, the surrounding rivulets and creeks are likely to discharge, threatening nearby picnic areas, playgrounds, public parks and waterfront shopping districts. To study and predict the Arkansas’s flow, Wyckoff and his colleagues at the Army Corps used RiverCAD, an AutoCAD-based river modeling software from BOSS International.
RiverCAD, available as either an AutoCAD module or a standalone version, allows hydraulic engineers at the U.S. Army Corps, Tulsa District, to better understand the flood risk of the region.
“Here, in our office, we do lots of floodplain studies to design and evaluate flood-control structures,” Wyckoff explains. “We’ve used RiverCAD extensively for those areas for close to 10 years.” Wyckoff was partly responsible for modeling a 230-mile section of the Arkansas River.
RiverCAD supports HEC-RAS and HEC-2, the standard flood analysis programs developed by the Army Corps’ HEC (Hydrologic Engineering Center). Available either as an AutoCAD module or a standalone CAD program, RiverCAD computes water surface profiles for modeling bridges, culverts, spillways, levees, bridge scour, floodway delineation, floodplain reclamation, stream diversions, channel improvements and split flows.
RiverCAD XP, the latest version, offers improved automated GIS features for generating ArcExplorer, ArcView and ArcGIS project files, water surface elevation data, edge of floodplain, contours of flow depth and uncertainty band (based on the amount of mapping error from the underlying terrain data).
Divide and Conquer
River modeling, as described by Wyckoff, is similar to putting together a jigsaw puzzle. Because topographical data usually is available in cross sections, the river-modeling program has to reconcile various chunks of data to develop the larger picture, or the profile of the floodplain under scrutiny.
“If you put enough cross sections close together, you start to get a pretty good picture of the river’s profile,” he says. “So if you have a tool like RiverCAD that makes it easier to extract the required data, such as the elevation models, contours and topologies, you can put more cross sections together. That’s a very time-consuming thing to do by hand.”
Part of the knowledge hydraulic engineers bring to the table, Wyckoff points out, is the understanding of certain river characteristics that can affect its flow. That enables them to put the correct cross sections together at the correct spacing so the water model accurately reflects actual conditions. The heavy computation work, however, can be left to RiverCAD. The software comes with a number of built-in tools:
FlowCalc, for computing normal depth, normal discharge, critical depth, critical discharge, critical slope, flow area, average velocity, hydraulic radius, wetted perimeter and other hydraulic properties;
ScourCalc, for computing and plotting out the total scour at a bridge cross section due to contraction scour, pier scour and abutment scour; and
GRreduce, for cross-section geometry ground point reduction.
“In the old days,” ponders Anthony Veroeven, a senior account manager at BOSS, “engineers would go out to the field, gather elevation data for river cross sections, come back and enter it manually into the HEC-RAS program and then run the analysis. To do a floodplain study, they’d go into a CAD program, like AutoCAD or MicroStation, to stitch together the results [of the analyses on several cross sections].”
“There are lots of programs that can extract the data you need to do backwater modeling from a set of topologies, so you can later import it into something like HEC-RAS to generate the model,” says the Army Corps’s Wyckoff. A typical workflow might be to extract the topographical data from a CAD file or GIS files, conduct the analysis in HEC-RAS software and then export the result back into a CAD or GIS program for additional work. “The good thing about RiverCAD is that it does everything in one package. If we have a topology set, we draw a cross section. [RiverCAD] does the computation for us, then plots it immediately afterwards. There’s no importing or exporting, because all the programs are rolled up into one.”
RiverCAD can extract the necessary data and construct a cross-sectional terrain from a combination of 3D digital contour maps, TINs (triangulated irregular networks), DEMs (digital elevation models), 2D digital contour maps, hard-copy contour maps, hard-copy cross-section plots, on-screen digitizing, manual data entry, complete or partial HEC-RAS or HEC-2 input files, station-elevation and northing-easting or x,y,z coordinate data. The software can automatically stitch together a terrain model by layering and consolidating the various types of data imported. Up to 500 ground points can be used to define each cross section. This all-embracing environment proves to be a tremendous timesaver in conducting analyses based on limited data, Wyckoff points out.
For highly developed metropolitan areas, it’s fairly easy to obtain accurate elevation and topographical data from the wealth of geospatial data already collected, but for rural areas, it’s quite a different story. “For those areas, you may not have recent survey data. You may only have an aerial photo taken from a high altitude or, worse, a satellite photo that you’ll have to convert into an elevation model. It’s generalized data, so when you convert it, there’s a higher rate of errors, but if you don’t use that, what else can you use?”
Wyckoff’s office uses MicroStation in addition to AutoCAD. “BOSS has been very good at helping us convert our MicroStation files so we can use them in RiverCAD,” he remarks. At present, Wyckoff and his staff convert MicroStation data into AutoCAD data before importing it into RiverCAD. With BOSS’s help, they hope to be able to import MicroStation files directly into RiverCAD in the near future.
Wyckoff says he now knows too much about rivers to take them lightly, but that doesn’t deter him from an occasional fishing excursion along the Arkansas.