Stephen W. Long
Jason F. Miller, P.E.
The U.S. Army Corps of Engineers is developing an enhanced Flood Warning and Response System (FWRS) for use on the Susquehanna River in the Wyoming Valley area of Pennsylvania. The stated objective is to provide accurate and timely warnings in order to maximize response time for flood plain residents and emergency managers and to provide a powerful planning tool for flood prone areas.
The Wyoming Valley is located in the northeast to north-central portion of Pennsylvania approximately 90 miles northeast of Harrisburg. The FWRS is limited to about 110 miles of the main stem of the Susquehanna River, specifically the flood prone areas of Luzerne, Columbia, Northumberland, Snyder, and Montour Counties. The upstream limit of the FWRS is the northern corporate boundary of Luzerne County. The downstream limit is the southern boundary of Northumberland and Snyder Counties. A total of five counties with over 50 local municipalities are included in the FWRS project area.
The Wyoming Valley area has had its share of damaging flood events over the years, including the flood of 1936, which prompted the authorization of the original levee system in the project area (USACE, 1995). The worst flooding experienced was in June 1972 and was brought about by the heavy rainfall associated with Tropical Storm Agnes. The flood event of 1972 overtopped the levees in place at the time and caused estimated damages of $1 billion (Times Leader, 1998). Since 1972, several high water events have reached peak stages close to the levee crests, most recently in January of 1996. In the Spring of 1997, the Baltimore District of the U.S. Army Corps of Engineers, through the Wyoming Valley Levee Raising Project, began work to complete raising and improvements to the levee systems in the area by between three and five feet (USACE, 1995). The purpose of the $175 million project is to supplement the existing flood control projects to provide protection against a recurrence of a storm equivalent to Tropical Storm Agnes and, in addition to the levee raising, includes structural and non-structural mitigation measures for adverse flood impacts (USACE, 1995). The Mitigation Plan includes $23 million to complete property acquisitions, structure raisings, structure flood-proofing and small-scale public works projects. Also, included is the development of Hazard Mitigation Plans, CRS applications, and the Flood Warning and Response System. The Philadelphia District is developing the FWRS to be part of the non-structural improvements to the overall flood control system for the area at a cost of approximately $2 million.
The objective of the FWRS is to provide a comprehensive, ease-of-use, warning and response system that provides accurate and timely flood forecasts that maximize the response time for flood plain residents and emergency management response officials. In addition, the FWRS will provide local officials with a powerful management and planning tool for flood prone areas. This paper will examine the level of detail and extent of the data collected for use in the FWRS, the methodology behind the FWRS, and the potential benefits and uses of the FWRS.
Detailed mapping data was collected within the estimated 500-yr flood plain area to include building footprints for all structures, spot elevations for all corners of structures and for each bridge, the transportation network, hydrography based on river channel surveys, and digital ortho-photographs. Detailed demographic data for structures within the 100-year floodplain was collected and includes, where available, the property owner’s name, address, phone number, digital photos, first floor elevation, a notation as to the existence of a basement, the total number of stories, assessed value, and a description of the building’s use and the type of business, as applicable. The mapping data was organized into coverages and shape files for use in ArcView and the demographic data was brought into Microsoft Access tables and linked to ArcView using the Object Database Connectivity (ODBC) feature.
A Digital Terrain Model (DTM)is the backbone of the flood inundation mapping data utilized by the FWRS. The DTM was developed to meet accuracy requirements for a two-foot contour interval and encompasses the approximate 500-year flood plain area by elevation for the entire study area, which is approximately 115 square miles. Utilizing the DTM and the most current channel, river gage and hydrologic information, a hydraulic model was developed using the US Army Corps of Engineers (USACE) river analysis software, HEC-RAS. The hydraulic model is run for an incremental range of discharge profiles accounting for flood events up to and including the 500-year flood. These individual discharge profiles are then digitally mapped using the DTM and the USACE’s HEC-GeoRAS mapping software, creating flood inundation layers for each discharge profile. Since the flood inundation layers are based on a specific discharge profile, each inundation layer also corresponds to a unique point on a stage-discharge rating curve at known locations on the reach length. These known locations are the four existing river gages at Wilkes-Barre, Bloomsburg, Danville, and Sunbury. The stage used as input for the system is the value forecasted by the National Weather Service (NWS) at each of the river gages and the resulting flood inundation layer is a prediction of the flooding associated with that forecasted stage. The process of associating the stage input to select a flood layer is simplified to the user by the use of an ArcView Extension. This extension will contain buttons, tools and menus, allowing the inexperienced ArcView user the ability to run the system while maintaining the functionality of ArcView for the more experienced user.
When the system is run it, will prompt the user for an input of a NWS forecasted stage at one or more of the four river gages. The system will then determine the flood elevation based on the hydraulic analysis rating curves and retrieve the appropriate flood layer. This flood layer will then be available for a variety of analysis purposes. Once a flood layer is determined, individual buildings and streets affected by the flooding can be queried. Now knowing the affected areas, field maps and reports can be produced to assist emergency action teams in their response efforts. Also, by using the detailed data collected for structures within the 100-year floodplain, various economic analyses can be performed. Along with the assessments, percent structure and content damage versus depth of flooding ratios have been assigned based on building type and use. The difference between the depth of flooding and zero damage elevation provides an interior depth of flooding for a specific building. This interior depth of flooding used in conjunction with the calculated economic damage ratios, provides an estimate of damage to both structure and content. The system will have the capability to report damages on a study-wide, county, community, or individual structure basis. This information has many advantages in terms of expediting damage relief and developing flood damage reports. Similar capabilities exist in terms of the transportation network in that depth of flooding determinations can be made at road intersections and on bridge spans. By determining which roads will be impacted and the extent of impact, emergency officials can make informed decisions in evacuation route planning.
The capabilities described to this point are all based on actual stage or discharge predictions from the NWS. Many significant benefits of the FWRS will arise from analyzing hypothetical flooding situations. Planners will have the ability to enter ranges of flooding scenarios to identify areas that require special attention in terms of possible removal, relocation, and flood-proofing. Emergency managers will have the ability to plan evacuation routes in advance of a flooding event based on hypothetical river stages. In addition, individual communities can use the FWRS to develop flood action response tables. By running the FWRS with different high water event scenarios, planners can determine appropriate safety and emergency actions that need to take place at corresponding river stages.
The purpose of this paper is to provide an introduction to the FWRS, describe the data that is required to produce a system of this nature, and mention some of the many benefits the system will bring to the project area. In addition to providing the communities with a representation of the extent and potential damages associated with a particular flooding scenario, the system is a planning tool that can be maintained and used to assist in preparations for hypothetical situations. The FWRS in conjunction with the structural protection provided by the Levee Raising Project will provide local residents and officials with a highly sophisticated means of dealing with future high water events. By keeping the system easy to use and update, we feel the FWRS will be effective in the long term.
The Times Leader. 1998. “Valley’s past swamped with disaster, recovery.” April 25.
U.S. Army Corps of Engineers-Baltimore District. 1995, Revised 1996. Susquehanna River Basin, Wyoming Valley Levee Raising Project, Final General Design Memorandum.
Stephen Long joined the Floodplain Management Branch of the U.S. Army Corps of Engineers in Philadelphia in 1991. Mr. Long was responsible for the creation of a geographic information system to assist FEMA in its flood-mapping program. He developed and assisted in the development of several applications, using ARC/INFO software’s AML programming language, for automating the production of FEMA’s Digital Flood Insurance Rate Maps (DFIRM). He is also involved in other GIS projects within the USACE Philadelphia District, including projects in civil and site engineering, hydrology, and surveying and mapping. In addition, he serves as a member of the Civil Site Field Working Group under the Federal Geographic Data Committee Geospatial Advisory Group.
Jason Miller, P.E.Jason Miller joined the Floodplain Management Branch of the U.S. Army Corps of Engineers in Philadelphia as a Hydraulic Engineer in 1999. Mr. Miller is responsible for the coordination, project management, and technical preparation of Type 19 and Limited Map Maintenance Program studies involving riverine and coastal flooding. He has a wide range of technical experience in the areas of surface water hydrology, open channel and closed-conduit hydraulics, flood plain management, storm water management, and site design engineering. Mr. Miller is a registered Professional Engineer in the Commonwealth of Pennsylvania.