NNRC has developed an interactive process that takes advantage of Arc's GRID, ARCEDIT, and ARCPLOT modules, integrated with watershed and river modeling software, to develop flood-prone area maps. The process allows for rapid and efficient, limited-detail floodplain mapping over large rural areas where there is limited existing development and little potential for future development. For the Nuckolls County pilot project, NNRC's interactive process extracted critical hydrologic information from digital elevation data using GRID. Discharges were calculated at user-specified locations using regional regression equations. Flood depths were calculated in watershed modeling software using the calculated discharge. The flood depths were converted to a flood surface in GRID, which was compared to the digital elevation data to determine the flood-prone areas. Arcedit was used to review and edit these boundaries. The resulting flood-prone area coverage was published with a DOQ background. This process provides a cost-effective method of identifying and mapping flood-prone areas.
Flood-prone area maps developed in the 1970's and 1980's cover approximately 33% of Nebraska. The vast majority of the state remains unmapped, and in some cases, the mapping that exists is of poor quality. Mapping flood-prone areas is expensive both in terms of time and money. Costs to conduct a detailed floodplain study are as high as $8250 per stream mile. In areas where there is little development and little potential for development, these costs are not justified. Approximate methods for delineating floodplains are far less expensive, but the manual techniques involved are cumbersome and time-consuming. With the availability of digital elevation data statewide, NNRC sought to accelerate and enhance flood-prone area identification and delineation using approximate methods by automating the time-consuming tasks.
Objective
The objective of this project was to utilize GIS to automate flood-prone area identification and delineation.
Initial Mapping Area
The first area mapped using the process described in this paper was Nuckolls County, Nebraska. Nuckolls County encompasses an area of 576 square miles. Three hundred seventy eight square miles drain into the Little Blue River Basin. The rest drains into the Republican River.
The Process
Inventory existing flood stage data. Existing flood-prone area information was identified, collected and compiled from all known sources. These sources included the Federal Emergency Management Agency (FEMA), the Army Corp of Engineers (USACE), the Natural Resources Conservation Service (NRCS), the United States Geological Survey (USGS), the Nebraska Department of Roads (NDOR), and the Nebraska Natural Resources Commission (NNRC). Data collected included locations, 100-year discharges, 100-year flood elevations, and 100-year flood stages. This data was used to augment and verify the data developed subsequently.
Hydrology. The purpose of the hydrology step was to determine the 100-year discharge and channel slope for a minimum number of cross-sections located throughout the watershed. Most of the traditional GRID hydrologic functions were used in this step to extract hydrologic information from a variety of digital elevation data. The hydrology step was implemented on a small watershed basis. Watershed boundaries from a modified NRCS 11-digit Hydrologic Unit (HU) coverage were buffered and used to clip out digital 10-ft contour data (assembled from 1:24,000 USGS hypsographic separates) and digital hydrographic data (1:100,000) which were primary inputs for this method. Watershed Digital Elevation Models (DEMs) were created in topogrid using a 30-meter horizontal grid spacing. The hydrographic layer was "burned" into the DEM to improve the subsequent channel delineation and channel slope calculations. Streams draining sub-watersheds of at least one square mile were identified and delineated. In Nuckolls County, approximately 530 miles of drainage were identified for flood-prone area mapping.
Representative cross-sections were interactively selected throughout the watershed to capture the stream and watershed characteristics. Cross-sections were established at the uppermost points in the watershed and where the discharge was expected to change significantly, usually near confluence of tributaries to a main stem.
100-year discharges were calculated using 1993 revised USGS regional regression equations. Parameters for the regional regression equations such as contributing drainage area and channel slope were developed from DEM data.
The 100-year discharge was calculated at each cross section in GRID. Contributing area and the upstream channel slopes were extracted from the DEMs. The energy gradient (channel slope in the immediate vicinity of the cross-section) and the elevation of the invert were determined at each cross-section from the hydrographic and digital contour data. A coverage of cross-sections was created with discharge, channel slope, and invert elevation attributes and was exported in .dxf format. The vector hypsography for the watershed was also exported as a .dxf file. This information was required input for the hydraulics step.
Hydraulics. The purpose of the hydraulics step was to develop flood depth (to channel invert) data at regular intervals for all stream reaches. Flood depths were computed using normal depth calculations in Boss RiverCAD at the identified cross sections and were then densified using Watershed Management Systems (WMS) software.
The cross-section point and digital contour .dxf files were imported into Boss RiverCAD. Each cross-section was visited and a valley cross-section was "cut" by digitizing a line from bluff to bluff perpendicular to the flow direction. Station and elevation data were collected at each point where the digitized line intersects the digital contours. Flood depths were calculated by using the 100-year discharge, the channel cross-section, Manning's n value, and channel slope.
The calculated flood depths were imported into Watershed Management Systems (WMS) software. Additional flood depths were determined by linear interpolation at regular (240 meter) intervals along the channel. The horizontal (X, Y) and vertical (Z) coordinates for all of the flood depths were exported as a text file to conclude this step.
Delineation. The purpose of the delineation step was to determine the area inundated by the 100-year flood using the calculated flood depths and to create a coverage representing this area. A flood surface was created in GRID using the XYZ text file from the hydraulics analysis. Flood elevations were obtained by adding the flood depths from WMS to the elevation of the channel invert from the DEM. The point flood elevations were converted into a flood surface in GRID with a 10-meter horizontal grid spacing. This surface was compared to the elevations on a high-resolution (10-meter) DEM. The flood-prone area was defined as the area where the flood surface elevation was greater than the land surface elevation on the DEM. The flood-prone area coverage defines the outer limits of the flood-prone area.
Quality Control. The purpose of the quality control step was to identify and correct inconsistencies between the flood-prone area delineation and the calculated flood depths. Anomalies in the DEM degrade the accuracy of the delineation. Abnormally high invert elevations from the DEM were observed to widen the floodplain delineation. The digital contour information in combination with the flood depths provides a way to assess the accuracy of the delineation. The floodplain coverage was compared to this reference and edited to ensure that the delineation conformed to the calculated flood depths and the digital contours. The edited coverage is similar to the information depicted as approximate A-Zones on the FEMA Flood Insurance Rate Maps. Publication options include paper products with a minimum amount of information to a variety of digital publication formats. The coverage is geo-referenced and can be layered with any other spatial data in the same projections.
Conclusion
Wide spread availability of digital elevation data and GIS software permit the automation of the time-consuming tasks associated with flood-prone area delineation using approximate methods. The analytical capabilities of the GIS significantly speed the calculation of 100-year discharge information and ultimately flood-prone area delineation. Preliminary estimates indicate that a typical Nebraska county can be delineated by this method in less than two months-using 1.25 FTEs. The 100-year discharge information is developed for entire watersheds and includes areas outside the county boundary. Further timesaving can be realized by working on adjacent counties.
References:
Cordes, K.E., and R.H. Hotchkiss. 1993. Design Discharge of Culverts. Nebraska Department of Roads Research Project Number RES-1 (0099) P466, Transport Reach Studies.
Maidment, David R., 1995. GIS & Hydrology Workshop: Hydrologic Data Sets and Tools for Their Interpretation. A Workshop of the 15th Annual Esri Users Conference, Palm Springs, CA, May 21, 1995.
Beckman, Emil W., 1976. Magnitude and Frequency of Floods in Nebraska. Water-Resources Investigations 76-109. U.S. Geological Survey.