|GIS Utilizes Elevation Data to Defend Moorhead in the 2009 Flood Fight
By Brad Anderson, City of Moorhead GIS
The months of March and April, 2009 were a stressful time for residents of the Red River valley. A two-day period of warm weather and heavy rain fueled a rise in the Red River of more than 23 feet in less than eight days, at times rising five feet per day. This rapid climb of the Red River left the cities of Moorhead, Minnesota and Fargo, North Dakota with little time to prepare for a flood that would soon become the flood of record with a crest of 40.82 feet on March 28th.
Emergency Protection Measures and GIS Analysis
GIS analysis of flood-related data enabled the city to plan emergency protection efforts for unprecedented river levels. Houston Engineering, using ESRI’s Spatial and 3D Analyst Extensions, derived water depths using a Digital Elevation Model (DEM) and a new river surface profile. The DEM was delivered as part of the 2008 LiDAR collection in the greater Moorhead and Fargo Metropolitan area. The finished product for Moorhead was a series of polygons that define areas in the city that would be affected by flooding at each river stage in ½ foot increments up to 44 feet. Engineering & GIS staff at the city worked with these flood stage polygons to determine where actions were required to protect critical city infrastructure (see Figure 1).
Figure 1: Emergency flood protection measures in place for the predicted 43 foot river stage.
Over the previous winter, the storm and sanitary sewer feature classes were analyzed to determine lift station and storm water outfall service areas. Each parcel in the city was attributed with its lift station or storm outfall identification number. Storm water outfalls without gates were a concern as the storm water system would have to be plugged to prevent the river from backing up through the system and entering the city. Protection of utility manholes and lift stations was critical to prevent inundation of the city’s underground infrastructure.
The results of the underground utility infrastructure analysis also assisted to define voluntary evacuation areas and provided information for the reverse 911 system in case a Code Red phone call was necessary to advise citizens of a flood-related emergency in their neighborhood.
Emergency Flood Preparation Manual
Moorhead maintains a comprehensive “Emergency Flood Preparation Manual” to ensure the effective and coordinated use of all resources available to the city to maximize protection of life and property in the event of a major flood. The manual was adequate to prepare for river levels similar to the previous record in 1997 of 39.57 feet, but major revisions were needed to plan the protection for the expected 43 foot crest. GIS enabled multiple “what-if” scenarios to be evaluated, providing new perspectives of the potential problems and helped to find solutions that had not previously been apparent.
Documenting the knowledge and experience of personnel who have been involved in fighting previous floods along with the GIS analysis of the newly acquired elevation data expanded the steps in the Manual’s Summary of Action Table from 60 to 180 steps (see Figure 2).
Figure 2: Sample from Moorhead’s Emergency Flood Preparation Manual “Summary of Action Table”. (Stage numbers are measured in feet.)]
Future GIS Flood Management Applications
Plans for a new and improved GIS flood management application are already in the works. New GIS tools will be available to city staff to help track flood management and protection efforts by integrating the “Emergency Flood Preparation Manual” with the GIS. This application will facilitate better communication of critical flood-related information to city staff. Residents of Moorhead will also benefit when a new Internet mapping application with enhanced capabilities is available in the near future.
GIS played an important role as a tool to help the city plan and prepare for the record crest, and documentation of the steps necessary to protect at these high river levels will be a guide in future flood fights – until permanent flood protection for the area is realized.