Jerrell R. Ballard, Jr. and Margaret Rose Kress

Cost-Distance Flood Surface Generation for Economic Impact Assessment

This paper describes a spatially explicit approach to estimating flood-related economic damage in the Lower Mississippi River Valley. Through an iterative process, cost-surface concepts are applied to the problem of determining the flow path of flood water over an elevation grid of the floodplain. The resulting flood surface representation properly accounts for the presence of man-made flood protection structures and natural barriers to overland flow. Using surveyed first floor structure elevations, the water depth in each structure is calculated and the damage, in dollars, is determined from depth-damage relationships.


Introduction

In the past, methods for estimating economic damage from floods in the Lower Mississippi River Valley provided limited decision-support products for managers (Ballard and Kress, 1998). U.S. Army Corp Engineer (USACE) Waterways Experiment Station has developed a new approach that uses a geospatial-based (GIS-assisted) flood damage estimation methodology.

Requirements

To provide reliable economic impact assessment of flood events, a detailed overland flow flood surface model is required. This model must be able to represent a wide range of flood stages specifically for low to moderate flood events, operate from stage data derived from point measurements or river cross-sections, and be able to route overland flow for backwater flood events.

The data requirements are a detailed flood elevation model designed for overland flow modeling (Kress et al., 1998); stage data representing the water surface elevation; cross-section data representing the water surface elevation; structure data that contains the location of the structures, ground elevation, first floor (sill) elevation, description, and value of the structure.

Flood Surface Generation

The flood surface is developed in a four-step process that iterates until a flood surface meets predefined criteria. The process is initiated by interpolating the stage data to derive a stage surface extended across the study area extent.

The first step uses the stage surface which is compared with the digital elevation surface to produce a first approximate flood surface. If the elevation of the stage surface is greater than or equal to the ground surface, then the grid cell is flooded; else the grid is not flooded. The resulting flood surface does not contain water surface elevations. It only indicates all grids that could be flooded under these stage conditions. It does not consider effects of man-made or natural levees nor the actual flow of water across the flood plain.

The second step in this process considers the effects of topography on overland flow. The initial flood surface is modified to eliminate all flooded grid cells that have no direct connectivity to any of the input stage data locations. This is accomplished in ArcView Spatial Analyst Extension by using the function CostDistance(). The function CostDistance() calculates the least- accumulative-cost distance over a surface to a set of source points and also produces a direction surface for each cell to its closest source point. In this application the source points are the stage locations in the main river channel and the direction surface shows the overland flow path from each cell to the closest flood water source.

The third step uses the first approximate flood surface and the direction surface to determine the flood surface elevation in each flooded cell. The direction surface is sampled at a user defined regular intervals and analyzed to define the shortest water flow path from a cell back to the source point across the flood surface. The water surface elevation of the source point is assigned to the cell and all cells traversed.

In the fourth step a second approximate flood surface is interpolated using the water surface elevation of the sample cells, all cells in the flow path, and the original stage points in the main river channel. This surface is generated without regard to topography.

This secondary approximate flood surface becomes the input for step one and the process is repeated until step two is completed. The remaining flooded cells from step two are assigned their corresponding water surface elevations from the second approximate flood surface.

Calculation of Economic Damage

Economic damage is estimated on a per structure basis for residential and commercial structures. Requirements for the structure database are described by Ballard and Kress, 1998.

For every structure in the area, the depth of the flood water in the structure is computed using the flood water surface elevation at the location of the structure and the first floor elevation of the structure. The depth of water in the structure and the type of structure are used as indices in structure and content damage tables. These tables estimate damage as a percent. The percent is applied to the assessed value of the structure to calculate economic damage in U.S. Dollars.

Acknowledgments

This study was funded by the U.S. Army Engineer District, Vicksburg.

References

Ballard, J. R., Jr. and Kress, M. R. (1998). Flood Impact Support Tool (FIST) User's Manual and Technical Documentation. Instructional Report EL-98, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS. (In Publication)

Kress, M. R., Ballard, J. R., Jr., and Graves. M. R. (1998). Elevation Grid Enhancements to Support Flood Impact Assessments. Proceedings for 18th Annual Esri User Conference, San Diego, California, July 27-31, 1998.


Jerrell R. Ballard, Jr.
USACE Waterways Experiment Station
Environmental Laboratory
Vicksburg, MS 39180-6199
Email: ballard@zen.wes.army.mil
Voice: 601.634.2946
Fax: 601.634.3726

Margaret Rose Kress
Physical Scientist
USACE Waterways Experiment Station
Environmental Laboratory
Vicksburg, MS 39180-6199