Floods can be defined as the accumulation of too much water in too little time in a specific area (USGS). Types of floods typical in the region of Northeastern Mississippi include regional floods and levee-failure floods. Extended rainfall creates saturated soil conditions and rain begins to runoff directly into streams and rivers, until their capacities are exceeded. Regional floods are often associated with slow-moving, low-pressure or frontal storm systems including decaying hurricanes or tropical storms. Persistent wet meteorological patterns are usually responsible for very large regional floods such as the MississippiRiver Basin flood of 1993 wherein damages were $20 billion (“TWC”). The Mississippi River has had many levees constructed to help avoid flood damage. Flooding from man-made levees can either be due to water levels exceeding theelevation, called overtopped, or they can be the result of engineering failure creating a flash-flood scenario. This type of event can be catastrophic as a large amount of water quickly rushes into the area being protected resulting in property damage and the loss of life.
The Mississippi River flows over 2,300 miles.It is the longest river in the United States and fourth longest in the world with an average discharge of 572,000 cubic feet per second (Wikipedia, Miss.). The stretch from Greenville to Natchez, MS has a population of approximately 80,000 inhabitants (sum of the 4 largest cities or towns) and approximately 200 miles of riverfront. This area undergoes frequent regional flooding.Historically the river at Greenville has reached Major Flood status twice and Moderate Flood status 8 times since 1922 (“NOAA”). According to the National Oceanic and Atmospheric Administration’s- National Weather Service-Advanced Hydrologic Prediction Service (NOAA-NWS-AHPS) the term moderate flooding is when there is “some inundation of structures and roads near stream. Some evacuations of people and/or transfer of property to higher elevations are necessary.” Major flooding is defined by NOAA as having “extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations”.
The following fictitious scenario illustrates the use of GIS in the event of a flood much like what has occurred in the past. A scenario of heavy rains contributes to the river overtopping its levees as well as the failing of levees being breached due to poor construction and lack of proper maintenance.
Step one is to prepare flood hazard and vulnerability maps, and to identify flood risk areas. GIS staff load watershed, drainage, elevation, and infrastructure data. Officials use this to develop watershed parameters for hydrologic modeling. The model will determine stream flow rates, which in turn are used to generate flood elevations. River gauges are monitored to the north to determine the amount of water approaching. Weather is also monitored. Snow water equivalency maps put out by NOAA show the potential for flooding as snow in the northern tier of the country melts.The model is adjusted to incorporate any changes in water levels.
Data layers required include elevation, infrastructure,landuse maps, and cadastralmaps. Cadastral maps show the number of stories in the structure and building materials.
Flood risk maps are created with flood zones clearly marked.All data is backed up and burned to DVDs.
Within an eight-hour time frame, emergency workers are brought up-to-date with the worst-case scenarios. Workers are all given mobile GIS consisting of a palmtop computer with a GIS including a database, or handhelds with internet access. They are trained to use the screen shots they need to complete their assigned tasks. They are also given directions for updating data with the emergency central.
Maps and tables are created that show the areas that need to be evacuated. Information includes the location of all evacuation sites by neighborhood, routes that will be open, alternate routes as roads close. The homes of all handicapped residents who will need assistance evacuating are marked. Routes are laid out that show which buses will be going where to help with evacuations. Non-profit organizations such as Salvation Army are notified of the potential damage and instructed to set up evacuation sites in predetermined locations.
Within the eight hour framework, local media will be asked to announce evacuations based on the worst case scenario. These announcements include GIS maps showing the movement of water and anticipated flood zones. First responders are sent to the anticipated flood zone to ensure evacuations are underway. They will notify the central command as each area is cleared. Buses will be routed from zone to zone.
A person is assigned to deliver all backed up data to the neighboring command centers.When the power goes down, the neighboring command centers will take charge.
Prepared maps include:
·Routes with anticipated road closings
·Medical facilities – evacuation and overflow
·Power plants and potential utility outages
·Sites with toxic or hazardous materials
Flood 0 hours
When the water level has reached ground level and the flood begins GIS technicians will create a mashup site. This site would allow all the identified evacuation sites to be identified on a public website and allow them to change their status from vacancy to no vacancy. Over the next few hours it will be important to create maps that show the direction the water is flowing. This will ensure correct, updated data.As the flooding continues, the evacuation sites fill up and road hazards begin to develop, the GIS routing capability will provide the public with the correct evacuation instructions. If live traffic is available it would also help in keeping routes open.
At this point in the flood you hope that everyone has evacuated and has safely made it out of town. A map showing the current water movement, the range of standing water and water levels should be created. These maps will help rescue teams know how to reach people that may still be trapped in their homes. Also at this time it may become a concern that radiological sources have been disseminated and could be a danger to the public. The Department of Energy is available on call to deploy to the flood region and perform a radiological survey of the area and provide additional maps to make sure no threats exist.
16 hours after the flooding had reached its peak, the rescue operations are still in full swing and relief efforts are in place. Now we begin to see GIS shift from response to full recovery.As the transitional period phases out, additional people and property may still need to be rescued.With the aid of remote sensing we can help to map the areas affected by the flood.Imagery from portable unmanned aerial vehicles could provide rapidly deployable and inexpensive tools for assessing the damage and finding victims in the remote regions that are currently inaccessible to rescuers.
Not only is GIS useful for situational awareness during the response, but it helps in determining the extent of the damage and evaluate the need for additional resources in the recovery process. GIS can be used to locate breaks in the levees where the Army Corp of Engineer construction crew needs to bring in pumps and sandbags to initiate repairs.The GIS can keep track of repairs as they are done, and prioritize the most critical breaches.The GIS can also be used to track the door-to-door efforts of rescue crews.Handhelds will enable workers to mark sites as they visit them and note any action that was or needs to be taken.
This flooding scenario of the Mississippi River from Greenville to NatchezMississippi is a story that has been told throughout history. Floods are not new to the Mississippi River, the people that live along its banks have grown accustomed to them. One such flood was The Great Flood of 1927.It flooded 27,000 square miles and killed 246 people in seven states (“Wikipedia”). These types of flood catastrophes are unnecessary and unacceptable, especially when we have technology tools that bring us so much information.The tools GIS provides are useful in all aspects of this flood emergency from preparation to recovery. These tools, if used properly, can save lives and reduce the loss of property.
As manmade levees age, they become weakened to the point of breaching. The manmade levees require constant maintenance to keep this system functioning safely. The people living along the banks of the Mississippi will always be at risk of flooding and in danger of levee breaching. New legislation requiring mandatory risk-based flood insurance for the people of these communities is necessary. Encouraging governments to invest in levee maintenance or investing in green spaces along the river can reduce the cost of relief in the event of the next flood.