Stormwater | Project Profile

By Darren Baird, Steve Jencen, Elizabeth Krousel,
and Raja Srinivasan

The Cobb County (Georgia) Stormwater Management Division hired AMEC Earth and Environmental to apply its Automated Floodplain Generator (AFG) tool to create floodplains and elevations for compliance with the Model Floodplain Management/Flood Damage Prevention Ordinance. The Metropolitan North Georgia Water Planning District (MNGWPD) created this ordinance to protect public safety, to minimize both public and private losses due to potential flooding from stormwater runoff, and to protect the floodplain functions that manage water quantity and quality. A pilot area was selected for study and includes streams within the Olley Creek, Rottenwood Creek, and Nickajack Creek watersheds (referred to as Priority Area 3 by Cobb County).

Figure 1. Cobb County Priority Area 3

The model ordinance was designed for adoption by communities within the MNGWPD (including Cobb County and incorporated areas within Cobb County, such as the City of Marietta), with regulation occurring during the land development review process. Several key components within the ordinance extend beyond the typical Federal Emergency Management Agency (FEMA) regulatory requirements for floodplain management. These components include the following:

Establishing a "Future Condition Flood," which is based on future-condition flows as defined in the community's land-use plan. FEMA typically requires that existing-condition flows be used to define floodplains.
Applying flood hazard determinations to streams with drainage areas of 100 acres or more. FEMA guidance defines streams for flood hazard determinations as anything greater than 0.5 square mile for urban areas and 1 square mile for rural areas.
Establishing elevations for the regulation of residential and nonresidential structures in and adjacent to the flood hazard areas. Typical Zone A or "non-detail studied" flood zones do not have flood elevations associated with them.

Figure 2. Impervious area coverage

The scope of the pilot study includes data collection, hydrologic analysis, hydraulic analysis, mapping, quality assurance/quality control (QA/QC), and training.

For approximately 8 miles of Zone AE (detailed hydraulic study) streams, the building restriction elevations for residential and nonresidential structures were mapped. In addition, for approximately 11 miles of Zone A (approximate hydraulic study) streams and 14 miles of unmapped streams, the regulatory flood (future conditions) were mapped.

Challenges of Meeting Ordinance Requirements
Cobb County, along with many of the other MNGWPD communities, faces challenges of meeting the more stringent requirements of the ordinance, because traditional hydrologic and hydraulic analyses in support of floodplain mapping are costly and stormwater management budgets are limited. To assist Cobb County in meeting this challenge, AMEC applied the AFG on approximately 33 miles of primarily urban streams within Cobb County's Priority Area 3 (Figure 1), demonstrating the cost-effectiveness of this tool. The ordinance requires that municipalities within the county determine the regulatory, future-conditions floodplain based on their land-use plan; map all streams draining 100 acres or more; and establish flood elevations for residential and nonresidential structures in and adjacent to flood hazard areas defined by a 3- and 1-foot increase in the 100-year future floodplain elevations, respectively.

Figure 3. Floodplain comparison with FEMA Q3 data

Because of the unusual requirements of this ordinance as compared to traditional FEMA flood studies, and because of county budget constraints, a flexible approach was needed to perform the analysis accurately and cost-effectively. The AFG was well suited for this type of application because it was designed to use the best available data without ties to any particular data source. Hydrology was easily calculated by incorporating Cobb County's existing land-use plan from the flood insurance study update performed for Cobb County by AMEC in 2004 into the Georgia United States Geological Survey (USGS) regression equations. The USGS regression equations use percent impervious area (Figure 2) and contributing drainage area to determine peak flows. Hydraulic cross sections and floodplain maps were generated using Cobb County's 5-foot contour coverage rather than a standard USGS Digital Elevation Model.

The AFG automated the following processes:

Delineating contributing drainage areas
Performing regional regression equation calculations
Creating three-dimensional cross sections
Generating HEC-RAS input files necessary to perform steady-state floodplain analysis
Simulating HEC-RAS models
Delineating floodplains for each modeled stream

Figure 4. AFG floodplain comparison with topography and FEMA Q3 data

Geographic information system data were stored in an ESRI-compliant geodatabase, along with supporting files (such as HEC-RAS input files) in a directory unique to that stream name to facilitate distribution of the stream models and floodplain boundaries.

Cobb County Pilot Study Results
The AFG was used to determine flows at major confluences and stream headwaters, to automatically lay cross sections at a user-specified distance and width, and to delineate the floodplains utilizing the same algorithms in HEC-GeoRAS. In addition, the AFG was used to locate streams draining more than 100 acres, ensuring that these streams were included in the analysis as required by the ordinance.

The last phase of the pilot study involved mapping the residential and nonresidential structure elevations in and near flood hazard areas. As stated, these elevations are defined as being 3 feet above the 100-year future floodplain for residential structures and 1 foot above nonresidential structures. Once again, the AFG was used to define floodplain extents through the use of its user option to specify a certain rise above the computed floodplain. Although these floodplains were developed using the regulatory future floodplain as the baseline, engineers reviewed the results to ensure that floodplain boundaries were technically sound with respect to backwater and hydraulic connectivity as an additional QA/QC measure.

After generating the three sets of floodplains (future conditions, future conditions plus 1 foot, and future conditions plus 3 feet), AMEC used its floodplain cleaning methodology used in FEMA Zone A flood studies to ensure that the floodplains were one smooth coverage free of jagged edges. The final result of this analysis was a set of floodplains for Priority Area 3 that compared very well with existing Cobb County digital elevation data and was a major improvement in terms of accuracy over existing FEMA Zone A floodplains (Figure 3). Figure 4 illustrates the consistency between the AFG-produced floodplains and topography based on the contour line agreement and also illustrates the extension of the AFG-generated floodplain farther into the headwaters beyond the FEMA Q3 data. Note that these floodplains were generated for a fraction of the cost of traditional hydrologic and hydraulic analyses in support of floodplain mapping.

Last August, the Atlanta Regional Commission organized a floodplain mapping workshop to showcase mapping efforts to local governments subject to the requirements of the Metro North Georgia Water Planning District.

The authors thank William Higgins, head of the county's Stormwater Management Division, and Cobb County for their support of the project.

Darren Baird, P.E.; Steve Jencen, P.E., CFM; Elizabeth Krousel, P.E.; and Raja Srinivasan are with AMEC Earth and Environmental Inc.

SW January/February 2006

Home + About + Subscribe + News + Calendar + Glossary
Talk + Images + Advertise + Contact Us + Search + Email + Services