THE  RAIN EVENT OF FEBRUARY 2003 AND ITS FLOOD POTENTIAL
Updated on March 10, 2003

SUMMARY

The rainfall data available to the Two Lakes Foundation at noon, February 24, 2003, showed that there was adequate reservoir capacity to impound the peak flow from the reservoir watershed, and prevent major flooding in Jackson.  Even if the maximum projection were increased by 25% above what is shown below, only 40% of the current holding capacity will be needed to accomplish this. 

Reservoir officials increased the flood control capacity of the reservoir by releasing water prior to runoff reaching the reservoir, which had the desired effect of keeping the reservoir discharge low enough to prevent major flooding in the Jackson area.  Although the Two Lakes Plan would have released water earlier, the net effect would not have been significant. 

CREATION OF LAKES BELOW SPILLWAY WILL REDUCE FLOODING

Although this event only caused street flooding, the creation of lakes below the spillway will reduce flooding of all types. (The lakes are designed to eliminate all flooding for a 100 year flood event.)  For a flood of this size, the level of the lakes would rise less than one foot.  In the Hanging Moss creek area, the maximum stage was 16.3 feet or 276.3 feet above sea level.  The normal level of the lake in this area will be 270 feet.  This storm would cause the lake to rise less than one foot, reducing the flooding by more than 5 feet.  

LeFLEUR'S LAKE INFLOW PROJECTION

The following chart shows the inflow projection (the flow on the reservoir side of the dam).

The rise in reservoir level using the simulated inflow will be minimal.  The small rise after the initial drop in reservoir level is due to the volume of water "impounded" by the reservoir as the excess inflow (the inflow above 50,000 Cubic Feet per Second) is held by the reservoir until it is safe to discharge it. 

The actual reservoir level is shown in green at the top of the chart.  It shows how the reservoir discharge was increased to lower the reservoir in response to the runoff expected by the rain.  The Two Lakes procedure makes a more aggressive drop (the red line at the top of the graph), to increase the reservoir's ability to absorb the runoff. 

(A more detailed explanation of the Two Lakes procedure can be found in the discussion of the April 2000 Flood through the following link  :Additional Information on the Two Lakes Flood Control Plan)

Bank Soughing Procedure for the February 2003 Flood Event:

Once the water level in the Pearl River below the dam drops to the level of it's bank, a restriction is placed on the rate of fall to keep the bank from sloughing into the river. It has been accepted that a maximum fall of 2 feet per day is acceptable. The river is within its banks when the discharge flow drops to approximately 16,000 CFS.  From this point on, the discharge must be controlled to maintain a drop of not more than 2 feet per day.

As can be seen on the graph above,  the natural run-out (the blue line) from the rainfall is too steep, and would cause a drop over 2 feet per day.  If the sloughing procedure were started when the natural run-out is 16,000 CFS, there would only be about  4.2 BCF (billion cubic feet) left in the reservoir watershed.  Since the total volume required is 6 BCF, the extra 1.8 BCF would cause the reservoir level to drop, unless there was additional rainfall.

The Two Lakes Discharge Plan accommodates this by planning for the sloughing procedure in advance, taking in account the volume of water required to perform the procedure.  For an event such this, when the inflow equals 26,000 CFS, the discharge would be gradually dropped to 16,000 CFS (this would take about one day). 

Dropping the discharge to 16,000 CFS when the inflow is 26,000 will cause a temporary rise in the reservoir level, which is storage for the latter portion of the procedure. The rise in the reservoir will continue until the discharge and inflow are equal. At this point, the stored volume would be used to supplement the inflow, thereby maintaining the desired discharge flow rate. The end result is the desired reservoir level of 296 feet.

Using Flood Waters to Supplement Volume Needed for Sloughing:

The graph below shows the approximate volume of water "impounded" between Highway 80 and the dam.   Since for the same flow, the water level drops between these two points, the volume is roughly estimated by averaging the water level at those two points for a specific flow condition.

The sloughing procedure calls for a drop at the maximum rate of 2 feet per day.  The initial 10,000 CFS drop from 26,000 CFS to 16,000 CFS in one day would cause a theoretical drop at Highway 80 of 3 to 4 feet in the first day, if it were not for the watershed volume "impounded" between Highway 80 and the dam.  This volume, about 0.7 BCF, contributes to the flow from the reservoir (by an average of 8000 CFS over 24 hours), thereby decreasing the fall rate at Highway 80 to less than 2 feet per day.

After the initial fall to 16,000 CFS, the sloughing flow procedure is designed for a theoretical drop of 2 feet per day, which with the additional impounded flow, will result in an actual drop of less that 2 feet per day.  Additional rainfall, which often occurs this time of year, will further reduce the fall rate at Highway 80.

Additional Information on bank sloughing

INTERESTING RESERVOIR FACTS:

Reservoir Watershed Area: 3050 square miles

Main Reservoir Area: 50 square miles

Effect of one inch of rainfall runoff over reservoir watershed:

    1 inch over 3050 square miles       =  7.1 Billion Cubic Feet (BCF)
    Reservoir rise if 100% retained     =  5.4 feet

Average percentage of runoff for April = 80% of rainfall

The law which enabled the creation of the Ross Barnett reservoir (the Pearl River Water supply District Act 5956-51) intended that it be used for flood control, recreation and water supply.

The reservoir (if maintained strictly for flood control) would have 15 BCF of holding capacity (one third the total volume of the 1983 flood).

During the 1983 flood, 44 billion feet of runoff accumulated in the reservoir watershed (66 BCF in 1979).

During the 1979 flood, 6 BCF (approximately 10% of the total flood volume) was available in the reservoir for downstream flood reduction. By holding 6 BCF during the period of peak inflow, downstream flooding was reduced 1 foot from what it would have been had the reservoir not existed.

The reservoir presently maintains a level of 296.0 feet during winter. This provides 5.5 BCF of capacity for use in reducing future floods. (This represents one-third of its empty capacity.)

Prerelease of the reservoir to 292.0 feet (during and immediately after an extreme rain event) would provide an additional 4.6 BCF of holding capacity. This would give the reservoir a total of 10.1 BCF of capacity for use in reducing future flood crests. (This represents 67% of its empty capacity).

Perfect use of the reservoir's present and prerelease holding capacity (10.1 BCF) would reduce future downstream flooding by an additional 1.5 feet in major flood events.

Following the recommended procedures, the reservoir would be expected to be lowered to an extreme level (below 295.0 feet and as low as 292.0 feet) once every 15 years. During this 15-year event, recreational users of the reservoir would be required upon to loosen moorings and move boats.   

Following these procedures will result in the reservoir level being high enough for recreational use 2000 days for every one day that the level will be too low as a result of its flood control mandate.