On Wednesday, the Delta Council released the following flood report.
As floodwaters make their way south down the Mississippi River, the Flood of 2011 has left its mark along the way. The Helena gauge crested at 56.48, Arkansas City at 53.06, and Greenville appears to have crested 64.24. Vicksburg is near crest now and is projected to crest at 57.1 on May 19 (1.1 feet over 1927 stage).
Although the Mississippi River has reached (or is nearing) crest elevations throughout the region known as the “Mississippi Delta”, the road ahead is still relatively long.
At the present, there is a tremendousvolume of water flowing down the Mississippi River, approximately 2 million cubic feet per second. Although that number is relatively hard to rationalize, analogies from swimming pools to the Super Dome have been employed. Two million cubic feet per second (in a perhaps more locally relative figure) would fill up Lake Washington once every 6 minutes, or the Ross Barnett Reservoir once every 2.5 hours.
It will take time for this water to pass. River stages are projected to hold at near crest elevations for three to four days before falling out slowly. Again, to provide perspective, the Mississippi River at Vicksburg is projected to remain above 2008 flood levels for nearly 5 weeks.
This extended period of high water will continue to place a strain on the Mainline Mississippi River Levee. Therefore, the Levee Boards will continue to keep traffic to a minimum until it is absolutely certain travel can be safely allowed. Sand boils and seepage will likely remain a common occurrence throughout the coming weeks. However, as it has been demonstrated the Levee Boards and the U.S. Army Corp of Engineers are well equipped and very capable of handling these situations.
The economic disruption that has occurred due to the mainstem Mississippi River system is still ongoing. In addition to the loss and degradation of private property, homes, and both farm and timber land, the economic loss due to minimal port activities, inundated and/or stranded manufacturing and distribution facilities, and the loss of revenue and job interruption to the resorts in Tunica, Lula, Greenville, and Vicksburg located adjacent to the Mississippi River.
In Tunica, the first two resorts are scheduled to be opened (May 18), but there will be a long period of time before full economic recovery in all of these sectors can be achieved.
Yazoo Backwater area
The projected Vicksburg crest of 57.1 translates to 103.3 ft in relation to ground level (i.e. feet above mean sea level).
One of the most common questions regarding this may be: How can the Mississippi River overtop the Backwater Levee which is built to 107 feet?
The simple answer is the Backwater Levee is located approximately 12 miles upstream of the Vicksburg gauge. At high stages, the slop of the water surface on the Mississippi River over this length is approximately 3 feet.
The Yazoo River’s headwater effects are overshadowed by the volume of the Mississippi River backwater that is moving back up the Yazoo. Near the Steele Bayou structure, the projected crest elevation at the Vicksburg gauge with the additional slope added for the upstream location yield an approximate elevation of 106.7 feet. This would not overtop the Backwater Levee.
Fortunately, below average rainfall throughout the Yazoo River Basin and timely management of upstream flood control reservoirs have reduced Yazoo River headwater and possibly prolonged (if not decreased) projected crest elevations. Below average rainfall has also greatly benefited the Backwater Area.
Given no overtopping of the Backwater Levee and below average rainfall it is likely the Yazoo Backwater Area crest will be lowered from 95 feet with normal rainfall from this point forward until Steele Bayou can be open it will probably crest about 92 to 92.5 feet.
However, in the event of a levee failure in the Backwater Area, which is very unlikely, the flood elevation is estimated to reach 106 feet. It is projected that if such a failure occurs, the waters of the Mississippi River would take three days to reach Rolling Fork and approximately five days to cover more than one million acres in the South Delta extending to Highway 12 between Belzoni and Hollandale.
Satartia, Wolf Lake, Carter, and Silver City vicinity
Delta areas on the east side of the Whittington Auxiliary Channel (Satartia, Wolf Lake, Carter, and property extending to the upper end of the auxiliary channel in the vicinity of Silver City) are not protected by levees. Therefore, these areas are projected to experience flood elevations of 106.5 feet.
Currently, water surface elevations are 105.8 feet near Satartia, Miss.; 105.1 feet near Highway 149 and the Whittington Auxiliary Channel; and 104.1 feet near Carter, Miss. There is a noticeable slope in water surface elevations moving northeast as it takes time for backwater to fill available storage and equalize.
Given prolonged crest elevations of the current flood event, it is likely that backwater flooding will “level out” from the Steele Bayou Structure through the Wolf Lake/Carter area.
As storage areas (i.e. low spots) are being filled and the amount of slope in water surface elevation is reduced, the rate of floodwater rise will likely be reduced as well. For example, on May 12, the Wolf Lake area was experiencing approximately 2 feet of rise per 24 hours. This was due to roughly 4 feet of head pressure (i.e. higher slope) backing up from the Steele Bayou Structure. As water levels equalize, head pressure is less and we are observing water level rises of approximately 0.25 feet/day currently.
Preliminary projections for water fallingin the Wolf Lake area:It could be June 10-12 before water levels drop below 102 feet. This will be revised as the Mississippi River crests at Vicksburg later in the week.
The Silver City/Belzoni area east of the Yazoo River (Tchula/Bee Lake) is projected to see floodwaters up to 107.5 feet. Current water surface elevation of the Yazoo River at Belzoni is 106.53 feet.
Of particular interest is Highway 49W. This projected closure date has been pushed back to May 20 currently and Mississippi Department Of Transportation (MDOT) crews – with help from all partners involved in this effort – are constantly evaluating road conditions and making forecasts on the best information they can.
A message from MDOT about how they are going about this process:
“MDOTsurvey crews are monitoring water surface elevations near our flood-threatened roadways throughout the day every day. We are in consultation with the U.S. Army Corps of Engineers daily and together we use this information to make predictions about when roadways will be inundated and report that to the public here immediately as changes are made. These dates can and will change as flooding patterns change. These dates are merely estimates based on the most current information and should be used for planning purposes only. The actual road closures will only be performed when the water encroaches onto our roadways and make them unsafe for travel. MDOT will continue to keep our highways open as long as possible to serve the traveling public.”
For more, see current roadway information.