Sam U. Shamsi, Ph.D., P.E.
For years, floodplain management studies have been expensive and unwieldy, with much of the analysis performed “by hand” using paper maps. Today, new technologies, such as GIS, GPS, and remote sensing are helping the floodplain managers to create accurate and current floodplain maps with improved efficiency and speed at a reasonable cost. Accurate floodplain maps are the key to better floodplain management. This paper describes GIS applications for developing floodplain models and maps. Examples and case studies are presented to illustrate the applications.
1. List of Acronyms
2. Introduction
3. Floodplain Analysis Steps
3.1. Data Collection
and Assembly
3.1.1. Digital Elevation Data
3.1.2. Remote Sensing Data
3.2. Model Development
and Execution
3.2.1. Three Methods of GIS Linkage
3.2.1.1. Interchange Method
3.2.1.2. Interface Method
3.2.1.3. Integration Method
3.3. Floodplain Mapping
4. Software Examples
4.1. DHI MIKE Products
4.2. HEC-GeoRAS
4.2.1. Inundation Mapping
4.3. ArcGIS Hydro Data
Model
4.4. GIS Stream Pro
4.5. RiverCAD
5. Case Study
6. Useful Web Sites
7. References
8. Author Information
DEM Digital Elevation Model DFIRM Digital Flood Insurance Rate Map DTM Digital Terrain Model FEMA Federal Emergency Management Agency FIRM Flood Insurance Rate Map HEC Hydrologic Engineering Center NFIP National Flood Insurance Program TIN Triangular Irregular Network
Accurate and current floodplain maps can be the most valuable tools for avoiding severe social and economic losses from floods. Accurately updated floodplain maps also improve public safety. Early identification of flood-prone properties during emergencies allows public safety organizations to establish warning and evacuation priorities. Armed with definitive information, government agencies can initiate corrective and remedial efforts before disaster strikes (Chapman and Canaan, 2001).
GIS is ideally suited for various floodplain management activities such as, base mapping, topographic mapping, and post-disaster verification of mapped floodplain extents and depths. For example, GIS was used to develop a River Management Plan for the Santa Clara River in Southern California. A GIS overlay process was used to further plan efforts and identify conflicting uses along the river and areas for enhancing stakeholder objectives. A 1 inch = 400 ft (1 cm = 122 m) scale base map was created to show topography, planimetric features, and parcels. Attribute data were entered into a separate database and later linked to the appropriate map location. Six layers were created for flood protection related work: 100-year floodplain, 100-year flood way, 25-year interim line, existing facilities, proposed facilities, and flood deposition. The lessons learned from this mapping project indicate that GIS is useful in capturing and communicating a vast amount of information about the study area and the river. While the use of GIS and the process to gather and record data were not without problems, the overall value of GIS was found to overweigh those challenges (Sheydayi, 1999).
Typical floodplain analysis involves three major steps (Dodson and Li, 1999):
1. Data collection and preparation
2. Model development and execution
3. Floodplain mapping
GIS can help in all of these steps as described below.
Typical floodplain analysis data requirements include
Hydraulic Data: loss coefficients and hydraulic boundary conditions
These data can be obtained from a variety of sources including the following.
The latest GIS technology allows the users to draw lines perpendicular to a waterway on
a DTM or contour map to extract floodplain cross section data. Using the latest automated
floodplain mapping software, DEM, DTM, and TIN data can be used for computing floodplain
elevations and mapping floodplain boundaries. TIN data structure has the ability to
precisely represent linear (banks, channel bottom, ridges) and point features (hills and
sinks), which are critical to accurately define the channel and floodplain geometry.
In GIS, a line is a series of connected points having a beginning and an end. In
ArcInfo, the beginning and end point of each line (arc) is called a "node" and
the intermediate points are called vertices. Attributes of each line provide the
descriptive information, such as length, direction, and connectivity (Cameron, et al.,
1999). GIS has excellent capabilities for storing and manipulating a 3D surface as a DEM,
DTM or TIN. GIS can create line features from a TIN of the channel and adjacent floodplain
area. These line coverages can be used to create the input data for HEC-RAS.
The U.S. Army Corps of Engineers has developed a new method and incorporated it into an
ArcInfo program called CHANNEL to automate the generation of bathymetric (or channel)
surfaces along a river reach, requiring only a limited number of channel sections as
input. This method is used to develop underwater terrain representation from HEC-2
cross-section input data. The underwater data can be merged with the rest of the terrain
representation to form a seamless terrain model that can be directly used for automated
geometry extraction for hydraulic models (Long, 1999).
Although for many legal requirements it is necessary to map flood-prone areas from
high-resolution aerial photography, remote sensing data provide initial conditions for
flood forecasting, monitoring flooded areas and conducting flood damage assessment.
Floodplains have been delineated by using remotely sensed data to infer the extent of the
floodplain from vegetation changes, soils, or some other cultural features commonly
associated with floodplains (Rango and Anderson, 1974). Low resolution digital Landsat
data have been used for producing flood and flood-prone maps at scales of 1:24,000 and
1:62,500 (Sollers et al., 1978). Medium resolution Thematic Mapper (TM) and SPOT satellite
data and high resolution IKONOS data can be reasonably expected to produce more accurate
delineation of flood prone areas.
Mississippi River flooding created havoc in the spring of 1997. Figure 1shows 25-meter resolution before and after Landsat images of the flooding. The photos show the Mississippi just below its confluence with the Ohio River in areas south of Cape Girardeau, Missouri. The before flooding image shown at left was taken on July 3, 1996. The post-flooding image shown at right was taken on March 16, 1997. A visual comparison of the two images clearly indicates the extremely large extent of the 1997 flood. Images like this can be effectively used in flood damage assessment and developing flood relief activities (Civil Engineering News, 1997).
Figure 1. Before and After Landsat Imagery for the Mississippi Flood of 1997(Photo
Courtesy of Space Imaging EOSAT)
Floodplain modeling involves two aspects: hydrology and hydraulics (H&H). Hydrologic analysis determines peak flood flows and hydraulic analysis determines peak water surface elevations. A hydrologic model, such HEC-HMS, can be used to model stormwater runoff. This calculation is based on physical characteristics of a drainage area that can be estimated from a GIS database. The runoff information from the hydrologic model can then be combined with stream cross-section information in a hydraulic model, such as HEC-RAS, to determine the depth of flooding.
The integration of a GIS with floodplain computer models allows users to be more
productive. Integrated models enable users to devote more time to understanding flooding
problems and less time to the mechanical tasks of preparing input data and interpreting
the output.
According to a literature review of GIS applications in computer modeling conducted by
Heaney et al. (1999) for the U.S. Environmental Protection Agency (EPA), Shamsi (1998,
1999) offers a useful taxonomy to define the different ways a GIS can be linked to
computer models. The three methods of GIS linkage defined by Shamsi (2001) illustrated in
Figure 2 are:
1. Interchange method
2. Interface method
3. Integration method
Figure 2. Three Methods of GIS Linkage
The interchange method employs a batch processing approach to interchange (transfer)
data between a GIS and a computer model. In this method, there is no direct link between
the GIS and the model. Both the GIS and the model are run separately and independently.
The GIS database is pre-processed to extract model input parameters, which are manually
copied into a model input file. Similarly, model output data are manually copied in the
GIS to create a new layer for presentation mapping purposes. This is often the easiest
method of using a GIS in computer models, and it is the method used most at the present
time. Using GIS software to extract floodplain cross-sections from DEM data or runoff
curve numbers from land use and soil layers are some examples of the interchange method.
The interface method provides a direct link to transfer information between the GIS and the model. The interface method consists of at least the following two components:
The interface method basically automates the data interchange method. The automation is
accomplished by adding model-specific menus or buttons to the GIS software interface. The
model is executed independently from the GIS; however, the input file is created, at least
partially, from within the GIS. The main difference between the interchange and interface
methods is the automatic creation of a model input file.
U.S. Army Corps of Engineers HEC-GeoRAS software is a good example of the interface
method. Developed as an ArcView GIS extension, GeoRAS allows users to expediently create
input data for their HEC-RAS models. Additional GeoRAS information is provided below.
GIS integration is a combination of a model and a GIS such that the combined program
offers both the GIS and the modeling functions. This method represents the closest
relationship between GIS and floodplain models. Two integration approaches are possible:
Because development and customization tools within most GIS packages provide relatively
simple programming capability, the first approach provides limited modeling power. Because
it is difficult to program all the GIS functions in a floodplain model, the second
approach provides limited GIS capability. Applications are being developed to connect
HEC-HMS and HEC-RAS models in a single ArcView GIS environment that would allow users to
move easily from a DEM to a floodplain map within a single program (Kopp, 1998).
The latest GIS technology allows the users to drape the modeled floodplain boundaries for various design storms on a base map. The modeled inundated areas can be shown as 3D flythrough animations as shown in Figure 3 or in an Internet compatible format for Web browsers as shown in Figure 4.
Figure 3. 3D Flythrough Animation of Modeled Floodplain
Figure 4. Interactive 3D Flythrough Maps of Modeled Floodplain in a Web Browser
The modeled water surface profiles (elevations) can be imported in a GIS and overlayed
upon the terrain surface to create flood maps and determine which areas will be inundated.
These maps can be used to develop flood related emergency response procedures. A recent
study (Dodson and Li, 1999) compared the results of floodplain mapping for an actual
stream channel using the traditional (manual or paper based) and automated (GIS and TIN
based) methods. The GIS method used the GIS Stream Pro software described later in this
paper. A careful record was maintained of the tasks performed and time required for each
task. The results indicated that GIS-based floodplain mapping software provided
significant improvements in efficiency for many of the tasks involved in floodplain
computations and mapping. Approximately 2/3rd of the effort required to perform a
floodplain study was eliminated using the GIS approach. Even more dramatic improvements
should be expected when revisions or corrections are required to existing data because
recomputing floodplain elevations and remapping floodplain boundaries is fully automated
and can be redone almost instantly.
The study also concluded that the elimination of almost all manual data entry through
the use of automated floodplain mapping software should result in significantly fewer
human errors in the hydraulic analysis and floodplain mapping procedures. Whereas human
errors may be expected 1-10% of all manually computed normal cross-sections (more in
longer cross-sections), GIS-based method practically eliminates human errors. Therefore,
the floodplain boundaries and profiles produced using the automated procedures should be
more accurate under most normal conditions, provided that the TIN model available for use
in automated computations is derived from the same topographic data source used for the
manual data entry.
Another study conducted in the 3.42 square mile portion of the Mill Creek watershed
located in the Lufkin, Texas, indicated that the 100-year floodplain boundaries created
using the GIS (Stream Pro) method were different from the effective FIRM boundaries
(Kraus, 1999). Some reaches had wider floodplains, while other areas showed distinct
reductions in floodplain widths. This study also noted a reduction in time required to
manually code the cross-section points into the HEC-RAS model and elimination of human
errors due to typographical mistakes. The GIS approach was also found to improve the
plotting of floodplains. Before GIS, floodplain plots between cross-sections were subject
to interpolation of contours. With GIS, the floodplain is plotted continuously according
to the terrain TIN and no interpolation between cross-sections is required.
Some floodplain mapping and modeling software examples are presented below.
DHI Inc. (http://www.dhi.dk) has three floodplain
modeling packages that have GIS linkage capabilities: MIKE 11, MIKE 21, and MIKE FLOOD.
MIKE 11 models floodplain hydraulics. MIKE 11 GIS is a spatial decision support system for
river and floodplain management. It is an ArcView GIS extension for Mike 11 models. The
2001 release of MIKE 11 includes a new floodplain encroachment model to assess the
hydrodynamic impacts of floodplain encroachments on the water and energy levels. Figure 5
shows a MIKE 11 GIS screenshot illustrating how a DEM grid can be used in ArcView to
create floodplain cross-sections for input to program's hydraulic engine. Today, many
flood studies require detailed spatial resolution which can be achieved through the
application of 2D techniques. MIKE 21 is DHI's preferred 2D engineering modeling tool for
rivers, estuaries and coastal waters. DHI's latest floodplain modeling package, MIKE FLOOD
combines the best features of 1D and 2D flood modeling technology. MIKE FLOOD is assembled
from the components taken from MIKE 11 and MIKE 21. This combination allows users to model
some areas in 2D detail, while other areas can be modeled in 1D. Like MIKE 11 and MIKE 21,
MIKE FLOOD also has GIS linkage capabilities which can be used, for example, to produce
inundation maps as a result of levee or embankment failures.
The HEC-RAS system is intended for calculating water surface profiles in a full network
of channels, a dendritic system, or a single river reach. HEC-GeoRAS for ArcView is an
ArcView GIS extension specifically designed to process geospatial data for use with
HEC-RAS. The extension allows users to create an HEC-RAS import file containing geometric
attribute data from an existing DTM and complementary data sets. GeoRAS automates the
extraction of spatial parameters for HEC-RAS input, primarily the 3D stream network and
the 3D cross-section definition. Results exported from HEC-RAS may also be processed in
GeoRAS. ArcView 3D Analyst extension is required to use GeoRAS. Spatial Analyst is
recommended.
Free download of GeoRAS program is available from the HEC software website http://www.hec.usace.army.mil/software/.
While the GeoRAS program was developed for HEC-RAS; it is not exclusive to HEC-RAS. It can
be applied in the floodplain analysis and modeling using other river analysis programs. An
ArcInfo version of GeoRAS is also available. To use this version, a knowledge of ArcInfo,
ARCEDIT, and ARCPLOT is advantageous, but not necessary.
HEC-RAS modeling results are exported to a data exchange text file that includes the
locations of the cross-section cut lines along with water surface profile data and a set
of polygons that describe the extent of the modeled floodplain. A line coverage of
cross-section cut lines is created and attributed with water surface elevations. For
inundation mapping, a TIN of the water surface is then generated from the water surface
elevations. Background coverages may be displayed along with the inundation data to
determine flood prone areas. Figure 6 shows an example inundation map created by GeoRAS.
It shows a flooding depth grid with ArcView's Identify Results window for water depth.
Deeper water is indicated by darker blue color.
Figure 5. Mike 11 Screenshot Showing How to Draw Lines on a DEM Grid to Generate
Floodplain Cross-Sections for Model Input
Esri's ArcGIS Hydro Data Model (http://www.Esri.com/software/arcgisdatamodels/arcgishydromodel/index.html) can be updated to display purely cartographic data, such as the types of data used in creating NFIP floodplain maps. A typical geodatabase model for floodplain mapping may include the following data:
This geodatabase includes more information than the required data for a standard DFIRM
database. However, a DFIRM database may contain much more information than listed here.
The parcels, buildings, and spot elevations classes are all examples of data that are
neither required in DFIRM database nor shown on the floodplain maps, but can provide
useful information.
Figure 6. Inundation Map Created by HEC-GeoRAS
Many utility programs are available to create the input data for floodplain models.
These programs use an approach similar to the interchange GIS linkage method described
above. For example, GIS Stream Pro from Dodson & Associates (http://www.dodson-hydro.com) is an ArcView
Extension that runs with ArcView 3D Analyst Extension. It simplifies HEC-RAS input by
extracting 3D stream network and the 3D cross-section data from a terrain TIN and
automates floodplain delineation based on the HEC-RAS Geographic Data Export File and the
original terrain TIN used to create the geometry import file. It uses 3D spatial features
to identify the stream network and HEC-RAS model layout. Once identified, GIS Stream Pro
generates the HEC-RAS Geometry Import File. GIS Stream Pro imports terrain data in ArcInfo
format from various data sources, such as, DEM, DTM, field survey, and LIDAR. It can
define base 2D spatial features, such as, stream centerline, left and right bank lines,
center of flow paths for the channel and overbanks, and cross-section layouts as ArcView
shapes over the terrain TIN. Attributed 3D spatial data for 3D stream centerlines and 3D
cross-sections can also be generated.
RiverCAD is a floodplain mapping software from Boss International (http://www.bossintl.com). RiverCAD uses its own
built-in AutoCAD compatible CAD system which allows creation of 3D CAD drawings of HEC-RAS
(or HEC-2) models. It displays HEC-RAS results directly on top of a contour map, showing
the extent of water surface with regard to the ground topography. Reportedly, RiverCAD
also creates scaled cross-section and profile plots that are ready for FEMA submittal. It
will read and display FEMA Q3 floodplain maps, and can generate ArcView GIS shapefiles of
HEC-RAS input data and analysis results. It allows creation of cross-section data from a
contour map, TIN, or DTM by simply drawing a line. It can directly read survey files, USGS
DEM files, and ArcInfo GIS files.
Many communities are using the latest Information Technology (IT) tools to develop
"Watershed Information Systems." For example, the City of Charlotte and
Mecklenburg County of North Carolina, which are one of the fastest growing metropolitan
areas of the United States have developed a watershed information system called WISE that
integrates data management, GIS, and standard stormwater analysis programs like HEC-1,
HEC-2, HEC-HMS, and HEC-RAS. Using this method, the existing H&H models can be updated
at a fraction (less than $100,000) of the cost of developing a new model (more than $1
million). WISE data management system emphasizes data storage and access using pre- and
post-processing techniques. Pre-processors funnel appropriate data in the correct format
to industry standard H&H models. Post-processors extract and assimilate model results
inside a GIS. WISE consists of several modules. The terrain module of WISE effectively
manages large amounts of digital terrain data and can merge terrain data from various
sources into a single, seamless terrain model. This module creates TINS and raster grids
for use in other WISE modules. The hydrology module uses data from previously existing
models, GIS coverages, and surveys to generate hydrologic data sets, including curve
numbers, time of concentrations, and hydrographs. The hydraulic module generates working
hydraulic models, flood profiles, floodplain mapping, and hydraulic results from WISE data
sets such as total stations or GPS data survey data and digital terrain data (Edelman et
al., 2001). The basic model data consist of elevation contours and H&H factors, such
as the rainfall, soil characteristics, slope contours, creek characteristics (size, shape,
and roughness), physical features (culverts and bridges) and past, current, and
anticipated land use information.
Hurricane Floyd was probably the most powerful hurricane in the Atlantic Basin since
Hurricane Andrew. It was definitely the most intense of the 1999 Atlantic Basin Hurricanes
when its winds reached 155 mph. It started out as a depression in the Central Atlantic on
September 7, 1999. Severe riverine flooding occurred in Eastern North Carolina due to
heavy rainfall associated with Hurricane Floyd, causing an $800 million damage to
property. This extensive damage required more than $100 million in post-disaster
mitigation funding from the government. Because detailed flood data were not available for
specific waterways, FEMA and North Carolina Department of Emergency Management requested
that hydraulic studies be performed to establish approximate base flood (100-year)
elevations to assist in proper floodplain management. Flood Hazard Mitigation Plans were
prepared for the four major watersheds located in the central portion of Mecklenburg
County. Extensive GIS data were utilized to perform the hazard evaluation and risk
assessment including past flooding complaints tracked by the City of Charlotte and
Meclkelnburg County, flood insurance policy and claim data, GPS elevations and locations
of floodprone structures. Remarkably, there were more than 2,500 structures in the
100-year floodplain. A GIS layer of the existing floodplain and floodway boundaries was
created to facilitate the flood hazard evaluation.
ArcGIS Hydro Data Model - http://www.Esri.com/software/arcgisdatamodels/arcgishydromodel/index.html
DHI's Mike 11, Mike 21, and Mike-Flood - http://www.dhi.dk
FEMA - http://www.fema.gov/
GIS Stream Pro Software - http://www.dodson-hydro.com
HEC-GeoRAS Software - http://www.hec.usace.army.mil/software/
NFIP - http://www.fema.gov/nfip
RiverCAD Software http://www.bossintl.com/
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Uzair M. ("Sam") Shamsi, Ph.D., P.E.
Senior Technical Manager
USFilter Engineering & Construction
250 Airside Drive, Pittsburgh, PA 15018, USA
Phone: 412-809-6618
Fax: 412-809-6611
E-mail: shamsiu@usfilter.com
Web Site: http://www.GISApplications.com