This paper documents a process we have used to complete a flood hazard management plan using a Geographic Information System (GIS) as a decision-support tool. Its purposes are to show results of applying the technology to an existing planning process; and to put forth how that process may be enhanced by what we have observed.
The paper begins with an overview of regulatory and physical environments within which our plan is based. The overview includes summary results of the plan's existing conditions analysis conducted using ArcInfo and ArcView. This is followed by a brief explanation of the multiple-software client-consultant environment in which the study takes place. Our issue identification and planning methodology are presented, followed by examples of indicators obtained from the GIS that assist identification of mitigation options. Our conclusion focuses upon observed strengths and weaknesses in composition of the study environment, and upon additional enhancements GIS has to offer the flood hazard management planning process.
A background of the flood hazard management process in Washington State and an overview of the natural, riverine, and built environments in Yakima County are presented in this section. The process background summarizes regulatory requirements for preparation of comprehensive plans addressing flooding activity. The environmental overview presents results of theme-based selection techniques from ArcView 2.1 applied to ArcInfo coverages obtained from Yakima County, and references data obtained from external sources.
Comprehensive Flood Hazard Management Plans (CFHMPs) are used to plan for needed flood control and maintenance facilities, as well as to influence the land use planning and zoning process in Washington State jurisdictions. They represent a comprehensive, proactive approach to floodplain management. The plans are based on analysis of realistic alternatives to determine what flood hazard management projects and programs are needed within a particular jurisdiction, as well as to rank multiple projects.
Preparation of CFHMPs is authorized in Washington State by State Participation in Flood Control Maintenance (Revised Code of Washington 86.26) originally enacted in 1951 and revised in 1994. Preparation of a CFHMP is required prior to receipt of grant funds under Washington's Flood Control Assistance Account Program (FCAAP) for construction of structural flood control facilities or funding of non-structural mitigation activities.
The CFHMP planning process lends itself to the use of GIS. CFHMP planning begins with a comprehensive examination of the existing planning area characteristics. The following planning area description elements are useful in developing CFHMPs (WAC 173-145-040):
Yakima County is one of many jurisdictions in Washington that uses ArcInfo to store and manage much of their planning data. The quality of the County's GIS, combined with the consultant's ability to use the data with ArcView, allowed efficient compilation of existing planning area characteristics, and led to a quick understanding of resource (biological, water, mineral, scenic/historic/cultural), infrastructure (roads, drainage), locations of past damage, land use, and zoning distributions in the floodplain.
The study area for the CFHMP lies within the Yakima River Basin, a major drainage basin of the Columbia River. It includes the Yakima River from the base of Yakima Canyon to Union Gap, and the Naches River from its mouth to State Route 12. Affected jurisdictions include Yakima County, the Cities of Selah, Yakima, and Union Gap, and the Yakama Indian Nation. (Figure 1)
Figure 1: Yakima County CFHMP Vicinity Map
A preliminary Flood Insurance Study (FIS) identified an 11-square-mile 100-year floodplain (1) which was overlaid upon coverages from Yakima County's ArcInfo database using theme-based selection in ArcView. Combining these results with other reference sources provides the following descriptions of existing conditions in the study area.
Annual daily average flow on the Yakima River is approximately 2,500 cubic feet per second (cfs). Alluvial fans, landslide hazards, and steep slopes are present within the study area, as are critical habitat areas for deer, elk, Canada goose, mallard teal, and many other species. Primary soil types are Yakima loam (2), sandy loam, and gravelly sand loam. There are 472 identified wetlands in the study area, ranging in size from 0.7 to 315 acres, with an average size of about 5 acres. The river currently supports populations of spring and fall Chinook and coho salmon as well as steelhead. The number of these fish has declined drastically due to low stream flows, impassable dams, loss of habitat, and poor water quality. Water quality trends indicate increases in stream temperature, specific conductance, and nutrients.
The Yakima River Basin originates high in the Cascade Mountains east of the Stampede and Snoqualmie Passes and runs for over 200 miles to the Columbia River. The river is used extensively for irrigation water supply. Six storage reservoirs, 14 diversion dams, approximately 2,000 miles of irrigation canals, numerous pump stations, and three hydroelectric plants have been constructed to service approximately 500,000 acres of irrigated agriculture. About 60 percent of total water use is attributed to agriculture.
Flooding is common in the Yakima Basin. Since 1894, the flow in the Yakima River has exceeded flood stage 45 times. Since 1970, Yakima County has been declared a federal disaster area six times, most recently in February, 1996. Peak flood flows typically occur during the winter months due to warm temperatures, melting snowpack, and heavy rainfall. Within the planning area, the Yakima River exhibits a steep gradient, with easily erodable alluvial channels. In undeveloped areas, the floodplain is wide, with meandering and braided channel patterns; in developed areas the floodplain is narrow, with linear channels due to levees.
Within the floodplain, current land uses are predominantly agricultural and rural. (See Figure 2.) Agricultural and General Rural zoning predominates, and land ownership is primarily private, with minor government ownership in the upper and lower reaches of the study area. Parcels in the floodplain have a total estimated assessed value of $90.2 million, of which improved parcels with structures account for 86 percent. The average assessed value per parcel is about $82,000. Of the more than 1,100 parcels in the floodplain, 42 percent are classified resource production and extraction, 32 percent residential, and 25 percent undeveloped. A total of 737 structures are located within the floodplain. Thirty-four (34) percent of the land area of the floodplain is currently within the County's Open Space Tax program, which results in reduced property taxes to landowners in exchange for long-term conservation of priority open space resources.
Figure 2: Land Use Distribution in the 100-Year Floodplain
BOSS HEC-2 for AutoCAD (enhanced version 2.0) was used to perform one-dimensional, steady-state hydraulic modeling of the Yakima River. HEC-2, originally developed by the U.S. Army Corps of Engineers, is the standard computer model used to define regulatory floodplain boundaries. BOSS HEC-2 for AutoCAD was selected due to its enhanced floodplain mapping capabilities. Various river hydraulic conditions were simulated to predict resulting water surface elevations along the Yakima River. Predicted water surface elevations were then intersected with digital topographic data to define floodplain boundaries for various hydraulic conditions. Predicted floodplain boundaries were then imported into ArcView in a .dwg format for overlay analysis.
ArcInfo GIS software is used at Yakima County, in conjunction with Informix software used to manage the county's Assessor's database. Yakima County's GIS consists of three Unix workstations with peripherals such as a CD press, a large format black-and-white scanner, a large-format electrostatic plotter, a 36-inch color inkjet plotter, an 11x17- inch color copier and an 11x17-inch black-and-white laser printer. Under the client-consultant arrangement between KCM and the County, these resources were available to assist the project.
To assess existing conditions in the study area, geology, wetlands, wildlife habitat, jurisdictional boundaries, land use, zoning, assessed property valuation, and open space were evaluated using GIS. The analysis allowed the quantification of information through automated queries that previously was either qualitatively reported or compiled by hand through tedious methods such as planimetering and referencing manuscript versions of an assessor's file. A principal benefit of this process is that once created, the routines for collecting this data may be stored either as a manual process or an automated script, and may be repeated quickly in response to a change in one or more of the existing conditions.
Concurrent with collection of data for existing conditions, flood hazard reduction goals were developed and key flooding issues were identified. Short- and long-term goals and objectives were developed to provide the framework for flood hazard management. With assistance from Advisory Committee members, planning goals became the broadest expression of the County's desire for flood hazard management. Objectives defined specific targets to be achieved to reach stated goals. Various flood hazard alternatives were selected to fulfill planning objectives.
Flooding issues and concerns were identified by examining historical flooding patterns, reviewing previous reports, examining floodplain information, and collecting input from Advisory Committee members and county staff. In addition, GIS results from evaluating existing conditions were also used to identify flooding issues. For example, overlaying the floodplain on various coverages revealed flooding issues such as inconsistent land use and zoning, geographic distribution of highly assessed properties that could potentially require flood protection, geographic areas that experience repetitive flood damage, and critical habitat areas that require protection.
Alternative solutions for each issue were developed, and alternative approaches were evaluated against planning objectives. This CFHMP is unique in that the majority of alternatives developed to address flooding issues were non-structural, such as land use and zoning recommendations, promotion of regulatory mechanisms such as open space taxation, public education, participation in the Community Rating System (CRS), and additional use of GIS to assist preparedness and response activities. However, the study also addressed a classic structural flood control question -- increasing the level of flood protection offered by an existing levee.
A preferred alternative was selected for each flooding issue following an evaluation of costs, impacts, and benefits. The preferred alternative was either one of the initial solutions put forward for analysis, or a combination of strategies present in multiple alternatives. In all cases, the selection of a preferred alternative was supported by the conclusions offered using GIS. Preferred alternatives for all 39 identified issues are listed in an action plan, which implements the CFHMP.
Selected indicators are linked to the goals and objectives developed for the CFHMP, and may include such items as protection of property value, limitation of residential development in the floodplain, promotion of open space and agricultural uses, and preservation of priority habitat. The indicators result from a battery of inventories and overlays that were run to provide a 'snapshot' of floodplain management results for existing conditions, creating valuable tools that relate to the plan, the local environment, and the law. Table 1 lists several indicators derived from the GIS for existing conditions. Indicators measured within the 100-year floodplain include assessed value of improvements, number of structures, and priority species and habitat areas.
Table 1.
Indicators of Existing Floodplain Management Conditions in Yakima County
Indicator CFHMP Goal or Objective Existing Condition Relative Proportion
Total value of Prevent damage of $77.5 million 86% of total
improved parcels public and private assessed value in
in the study property. the floodplain.
area's 100-year
floodplain.
Number of Prevent damage of 609 structures N/A
structures in public and private
100-year property.
floodplain.
Floodplain acres Adopt flood control 7,593 acres 79% of total
within priority measures that preserve floodplain acres
species and or enhance existing
habitat areas. fishery, wildlife, and
other natural uses of
the riparian zone.
The indicators used to examine mitigation strategies
are discussed below.
For this flooding issue, four levee improvement alternatives were compared to the existing regulatory floodplain. River hydraulic conditions were modeled for various levee alternatives and the results were compared to existing conditions to determine the flooding and socioeconomic impact of each strategy. Using hydraulic modeling in conjunction with GIS allowed a quick evaluation of alternative costs and assessed value of land and property protected. The preferred levee alternative provided the greatest property protection, measured by current assessed value; minimized environmental impact and financial requirements; and showed consistency with floodplain planning objectives.
Once generated, the alternative innundation area .dwg files related to each levee alternative were transferred to ArcView and placed in a project alongside a parcel coverage containing the assessed value data described in Table 2. All the innundation areas were assembled and displayed in a single view, along with the parcels. This view was used to create the map layout included as Figure 4.
Figure 4: Assessed Value Protected by Levee Alternatives
For each levee alternative, the assessed value contained within predicted floodplain boundaries was determined by performing overlay analysis with ArcView. The existing FEMA regulatory floodplain represented the existing condition and floodplain boundaries resulting from each levee alternative were used to determine changes in assessed value protected, and the number of structures and the priority species and habitat area within the floodplain.
Table 2.
Incremental Change in Assessed Value for Various Levee Alternatives
Enhance
FEMA Existing Setback 2000 10,000
Indicator Floodplain Levee Levee foot foot Levee
Levee
Value of improved parcels $0 -$3.75M -$4.1M +2.0M -$3.5M
in 100-year floodplain.
As part of the CFHMP, a floodplain survey was conducted within the study area. The survey consisted of field reconnaissance and reviews of aerial photographs to identify the number of structures in the floodplain, their location, current use, type of construction and foundation, and height of the first floor above ground level. The survey identified, by quarter section, 609 structures within the floodplain.
The coverage with predicted floodplain boundaries for various levee alternatives was used to determine what structures would be removed from the floodplain. Table 3 shows the number of structures removed from the existing FEMA floodplain for each alternative. The 10,000-foot levee removed the largest number of structures from the floodplain, followed by the setback levee. However, protection of 260 additional structures would come at the expense of losing potential fish habitat.
Table 3.
Incremental Change in Structures in Floodplain Downstream for Various Levee
Alternatives
Enhance
FEMA Existing Setback 2000 10,000
Indicator Floodplain Levee Levee foot foot Levee
Levee
Number of structures in 0 -195 -222 -91 -260
100-year floodplain.
In this paper, we have shown results of applying GIS technology to an existing planning process used for flood hazard management. Furthermore, we have offered suggestions based on what we have observed. In presenting our findings, we hope that others may learn from our experiences and that our observations will inform the future development of analysis software.
(2) Yakima loam is characterized by stratified layers and beds of permeable gravel and sand that occur at shallow depths.