The Importance of Regulating the Discharge of the Reservoir
at the End of a Heavy Rainfall Period

It has been established that the banks of the Pearl River below the Ross Barnett Reservoir Dam have had increasing instances of sloughing (cave-in) since the reservoir has been in operation. (Note that this sloughing is in addition to the normal sloughing and erosion that is a part of any river)

One of the prime causes of bank sloughing is river bank saturation by a high river level, and a subsequent rapid drop of the river. This saturation will occur quickly when the river overflows the bank. If the drop of the river is faster than the water can flow out of the bank, a pressure imbalance is created, and failure can occur.  Reservoir operation has the capability of both aggravating or reducing this condition.

Bank sloughing continues to threaten structures that lie along the banks of the river today, as an example, a bridge near Monticello is to be re-located (at a cost of two million dollars) because of bank sloughing on a straight portion of the Pearl River. Adjacent to the bridge, numerous structures are in danger of falling into the Pearl.

The Corps of Engineers has an established that if the drawdown rate is two feet per day or less, bank sloughing is greatly reduced. The Corps also noted that falls greater than two feet per day were much more common after the reservoir was built.  This increases the occurrences of bank sloughing downstream.  Their recommendation is that the reservoir control the rate of flow decrease so that the fall is limited to two feet per day at the highway 80 gauge. This allows time for the water to seep from the bank, decreasing the pressure that causes the bank to slough. This recommendation is to take effect when the highway 80 gauge has reached 28 feet, a level at which the river has overflowed the banks.

The Corps blamed the increased occurrences of high drawdown rates on lack of rainfall absorption in the land occupied by the lake, channel improvements upstream, and increased development around the reservoir.   However, upon formulation, we are unable to find these courses to be significant.  The Two Lake Foundation believes that the dominant reason for the change is that the reservoir eliminated 30 miles of river reach and adjacent flatwoods, which served to store water during floods and slowly let it back in as the river fell. Keeping the reservoir at a constant level eliminates this beneficial process.

The following graph shows the complete cycle of rise and fall during the April 2000 rain event. Note the rapid falls at each location, which is primarily due to a sharp decrease in flow from the reservoir.

As can be seen in the above graph, the highway 80 gauge experienced a rate of fall of ten feet over a 48 hour period, clearly in excess of the two foot per day maximum, and at the expense of downstream communities. It is possible that the reservoir, which experienced very low levels last year because of abnormally dry weather, has decreased emphasis on sloughing to assure that sufficient water is available in the reservoir for recreation.

The Two Lake Plan has developed a sloughing procedure (based on the Corps guidelines), that will both maintain a desired reservoir water level, and prevent a rapid fall and subsequent sloughing of river banks downstream of Jackson. The following graph also shows how a two foot per day fall could have been achieved without danger of leaving the reservoir too low at the end of the procedure.

 

Sloughing Results for February 1999

In February 1999, the reservoir performed a sloughing operation by maintaining an almost constant rate of discharge decrease as shown in the graph below. Note the shape of the fall at highway 80. Since the stage is not directly related to the rate of discharge, it has a curved shape, which caused a fall of greater than two feet per day to occur.

The next chart shows the same time period, and how the reservoir also dropped too low at the end of the procedure. This could have been prevented with the Two Lake sloughing plan. The low water level at the end of the procedure was an unfortunate start to the very dry season that followed, where the reservoir never recovered to the normal level. The Two Lake plan could have added a foot or more to the water level at the end of the procedure, which would have kept the water at safe levels for longer during that dry season.  The failure on the part of the reservoir to successfully apply this procedure in February 1999 could be the reason that they did not try to do it in April 2000.

 

Sloughing Results for April 2000

The following graph shows the rate of fall after the April 2000 rain event, and how the Two Lake procedure differs.  Note that the highway 80 stage with Two Lake sloughing is a straight line with a fall of two feet per day.  This is achieved by a sloughing discharge that releases a greater volume at the start of the procedure than at the end.  The accuracy of inflow projections along with the ability to make changes during this procedure should give results fairly close to these projected results.  The basic principle of the Two Lake sloughing plan is simple:  

     -To perform the procedure requires about 8.5 billion cubic feet (BCF) of water

     -To the 8.5 BCF, add or subtract the volume required for any desired change to reservoir level

     -When the reservoir watershed has target volume of water remaining, start the procedure. 

Note that 8.5 billion cubic feet has an allowance for errors.

 

Need for an Independent Specialist to Regulate the Reservoir
(Only during and immediately after large rain events)

The Two Lake Foundation has proposed that a specialist be retained with a demonstrated ability in flood control and reservoir volume management.  Unique conditions control the interaction between the reservoir, the Jackson metropolitan area, and the downstream communities.  This makes the solving of Jackson's flooding situation into a "custom" job which does not lend itself well to solutions on generalized computer programs such as that used by the Corps, or national/international engineering firms.  For this reason, the Two Lakes Foundation believes that a local person, whose prime interests are:

     -The reduction of flooding in all communities below the reservoir

     -The reduction in bank cave-ins below the reservoir

     -The maintenance of normal reservoir water levels except in extreme conditions

This person should have independent control over the functions of the reservoir during times of significant rainfall.  This specialist would also assure that accurate rainfall and streamflow data are available to make the timely assessments during a rain event, and to be able to react in a timely manner.  That person would openly communicate the status and actions to all parties by publishing it to the internet.  It is critical that this person be free from all constraints, and provide reservoir regulation that would be in the best interest of both all parties.  

Those responsible for regulation of the reservoir might argue that the plan offered by the Two Lake Foundation would cause extreme fluctuations in reservoir levels. The Two Lake Foundation has shown that reservoir regulation does not have to be at the expense of those living on, or using the reservoir.