THE  RAIN EVENT OF APRIL 2000 AND ITS FLOOD POTENTIAL
Revised April 25, 2000

Note: This document replaces the original dated April 4, 2000, and the April 7 revision.  The April 7 revision changed the inflow projection based on more accurate rainfall data.  This revision provides additional corrections and discusses the regulation of the reservoir discharge in more detail.

The following is a discussion of the events leading up to rainstorm of April 2000. The initial rainfall for this event occurred on March 27, with periods of rain through April 4.

This event will not cause major damage to structures as was seen in 1979 and 1983, however, the potential for damaging floods similar to those years was very real. Total rainfall varied from 3 inches to 7 inches over this event. The average rainfall was about 4.6 inches. If the rain had not stopped on Tuesday, April 4, rainfall levels could have approached the levels that caused major damage in 1979 and 1983. In fact, the rain that fell around Grenada, in an area adjacent to the Ross Barnett watershed and equivalent in size, had a 6-8 inch rainfall. Without effective pre-release of water from the reservoir, this rainfall would likely have caused significant damage to structures below the reservoir dam.

First, it must be recognized that the reservoir alone does not have the capacity to eliminate all flooding in Jackson.  However, it will have an effect on the magnitude of the flooding by retaining the peak flows entering the reservoir. The Two Lakes for Mississippi Foundation has formulated a proactive procedure that uses rainfall data to prerelease water from the reservoir when there is significant rainfall in the reservoir watershed - rainfall quantities that would cause damage to structures in Jackson.  It is not necessary to have the two lakes in place in order to obtain a reduction in flood levels. Using the reservoir's capacity, this procedure can be applied at the present time.

The ability to control damaging floods will be significantly increased if the Two Lakes project is implemented.  In addition to the economic and recreational benefits, the lakes below the dam will prevent additional flooding by increasing conveyance of water downstream and, to a lesser extent, absorbing some of the volume. This will eliminate 96% of the damage of another 1979 flood, lowering the water level by 10 feet. The lakes would also allow a more aggressive reservoir pre-release plan than was discussed above.

RESERVOIR PROCEDURES

The following is a discussion of the operation of the Ross Barnett reservoir as a response to heavy rain events. (A partial summary of the Ross Barnett operating procedures for the reservoir "pool" can be viewed in the box  below.)

Procedure items 1 and 2 indicate a winter pool stage of 296.0 and an upper limit of 296.4 feet. At the start of the rainfall leading up to the April 2000 event, the reservoir pool stood at 296.6 feet, 0.6 feet above the normal winter pool stage.  Any excess volume in the reservoir detracts from the ability of the reservoir to assist in controlling the floods in Jackson and communities below the dam. A level of 296.6 feet is not excessive, however, the reservoir should have been lowered to 296.0 at the first significant rainfall.

In response to the initial rainfall of March 28th, the reservoir was allowed to rise to a peak of 296.75 feet. This is opposite of the desirable reaction of lowering the reservoir in anticipation of additional rains. In fact, item 11 notes that when an inflow of 30,000 cubic feet per second (CFS) has been forecasted, the reservoir should be pre-released to reduce the level. Unless there is an extreme event, this target level is 295.0 feet. The Two Lakes plan is designed to anticipate flood events and proactively lower the reservoir to achieve the maximum flood control benefit, while minimizing the impact on those that live on the reservoir or utilize it for boating.

There are those who have a concern about decreasing the level of the reservoir to 295 feet (which would occur on average about once every two years). The concern is that the reservoir will not refill. This will never happen with the Two Lake Plan. A one foot drop is equivalent to about 0.2 inches of water in the watershed. With 3 inches (or more) of water in the watershed, there is enough volume to increase the reservoir level by a minimum of 15 feet.

RESERVOIR OPERATING PROCEDURES DEVELOPED BY THE TWO LAKES PLAN

The Two Lakes Plan has developed a set of operating procedures that can be used today. These procedures are termed "constrained" because without the benefit of the two lakes below the dam, the reservoir discharge is limited by the ability of the current Pearl River channel to absorb the flow.

Once the two lakes are in place the reservoir discharge will be much less limited due to the ability of the lakes to allow water to pass through and the ability to pool, or absorb excess volume.

The procedures are simple to use, but allow the reservoir to be used as an effective flood control tool in response to minor and extreme flood events. The criterion used is the volume of water (from a rain) remaining in the watershed. This can be calculated by maintaining a running total of rainfall runoff, less the dam inflow. This value is significant, since the dam must eventually discharge all runoff. The volume that is "waiting" in the watershed to be discharged is an excellent indicator. It has been shown that over the 30+ year history of the reservoir, the time from the mid-point of the rain event to the completion of the flood event (when the flow to the dam falls below 25,000 cubic feet per second) has been a very predictable 8 to 9 days.

The operating procedure is as follows. The figure in the left-hand column is the volume of runoff remaining in the watershed, in inches and in billion cubic feet (BCF). Note that this is not the rainfall, but only the runoff portion of the rainfall that is currently in the watershed.

Prerelease Procedure (Constrained) Based Upon Runoff Remaining in the Watershed

Note that a significant lowering of the reservoir level will be a very infrequent occurrence, and will be only in response to rainfall on the ground in the watershed. The chart below shows 50 years of recorded rainfall in the reservoir watershed, in 4-day rainfall totals, to show the relative magnitude of multi-day events.

The 4.6 inch rainfall of the April 2000 event is shown by the horizontal red line. Note that the frequency of this rain averages once in seven years.

Reservoir Capacity

The accepted practical limits of the reservoir level are a minimum of 292.0 feet and maximum of 299.7 feet. The normal level during the spring is 296.0 feet. Based on these levels the following may be defined:

Standing Capacity: The volume between 296.0 (normal level) and 299.7 (maximum level). This volume is 5.5 BCF (Billion Cubic Feet)

Pre-Release Capacity: The volume between 296.0 and 292.0 (minimum level for extreme events). This volume is 4.6 BCF.

The total reservoir capacity to store flood flow is 10.1 BCF, which is far short of having total influence on the flood waters, but this capacity can reduce the peak flow from each flood, which is where the damage will occur. As a reference, the 1979 flood had maximum of 66 BCF in the watershed; in 1983, 44 BCF; and April 2000, 20 BCF.

Rainfall data is available in hourly or daily increments from the National Weather Service within minutes after it is recorded. This rainfall is allocated to areas of the reservoir watershed, and a percentage is applied to the measured rainfall to determine the runoff, or the portion that actually ends up flowing downstream to the reservoir.  It may take up to several days for the runoff to travel down the creeks, streams and rivers from the edge of the watershed.

A computer program calculates when the runoff from each area is expected to arrive in the reservoir. Once it is in the main body of the reservoir, it flows quickly to the dam. The computer program makes a projection of both the rate of water reaching the dam (the "inflow", measured in cubic feet per second), and the time that it will arrive. This projection is made from the rainfall alone, but can be corrected (at a time later in the event) by comparing the projection to the actual data recorded at USGS stream flow gauges at various locations in the watershed.

Once the rainfall-based projection is made, an established procedure has been proposed to prerelease a volume of water from the reservoir in anticipation of the volumes to come. The prerelease volume and rate of discharge is based on the volume of water remaining in the watershed.

The peak flow to the reservoir occurs about four days after the median time of the rain event, and most of the runoff will have flowed out of the watershed and into the reservoir about eight days after the rain. These times will vary somewhat, depending on the concentration of the rain event, but it has been consistent over the years (30 years of data support this). For extreme events, such as the 1979 flood, the maximum benefit from the reservoir will require that the reservoir be at minimum level one day after the end of the rain. Since this drop will be accomplished over a period of time, the decision to prerelease must be made while the rain is in progress. The only way to accomplish this is an early response based on the volume of water in the watershed, which is established by rainfall.

INFLOW PROJECTION FOR MINOR FLOOD OF APRIL, 2000

Reservoir inflow is the volume of water reaching the dam, and is expressed in cubic feet per second (CFS).  For normal reservoir operation, the intent is to maintain a steady reservoir level; therefore the dam gates are adjusted to make the discharge equal to the inflow. If for some reason it is desirable to cause the reservoir to rise, then the discharge is set to a value less than the inflow. To cause a reservoir fall, the opposite would occur.

By utilizing National Weather Service data (11 rain volume measurements, spread over the 3050 square mile reservoir watershed), a computer simulation using the rainfall and the Ross Barnett watershed characteristics was used to produce a reservoir inflow projection.

Note that the Two Lake Plan would include a minimum of 25 rain volume sensors to adequately assess the rain in the reservoir watershed. For the current projection, only 40% of the rainfall data has been available. It is imperative to accurately assess the volume of rain as it falls to assure that sufficient advance notice available to determine the optimum reservoir operation in a flood event.  

Additional information is available from existing stream flow sensors maintained by the USGS in the watershed.  This data is used to verify that the rainfall and runoff projections are accurate.  Corrections to the inflow projections could be made based on this data.  The Two Lake Plan would increase the number of sensors, and assure that the sensor calibration is maintained.  It is important to realize that this data develops too late to be of value in the early stages in a potentially damaging flood, and early release must be based on rainfall alone.

It is obvious that the magnitude of the potential damage from flooding is more than sufficient justification to expand and improve the ability to make accurate inflow projections.

The following is a graph of the inflow to the dam, based on the available rainfall data for the April 2000 Flood, showing the details of events, and how the Two Lake Plan would operate the reservoir. 

The next chart is a revision of the April 7 chart, showing the same inflow projection, but revising the recommended discharge.  By error, the total runoff from rainfall (not the remaining runoff) was used for the original projection.  This was the wrong amount, and it exceeded 21 BCF, which resulted in a recommendation that the reservoir be lowered to 295.0 feet.  Actually, the original projection (based on rainfall data at the end of the rain event) showed the watershed runoff volume remaining in the watershed to be 20 BCF.  Under these conditions, the Two Lake Procedure calls for the reservoir to be taken down to a level of 295.5 feet.  Even if the reduction to 295.0 had been made, the water in the watershed was adequate to re-fill the reservoir several times.

The April 25 chart also shows the actual inflow that occurred.  The recommended discharge would normally be modified as required based on actual inflow and stream flow data.  Even if no changes were made, there was adequate water to refill the reservoir to its normal operating level.

Notes concerning the above chart:  The Projected Inflow to Dam curve was based on data at the end of the initial rain event (4/4/00).  The increase in Actual Inflow on 4/11/00 was likely due to additional rains that occurred after 4/4/00.

Prerelease and Reservoir Level Control

When significant rainfall quantities occurred in the reservoir watershed in April of this year, the reservoir (which was already standing above the normal maximum of 296.4 feet for this time of year) was allowed to rise instead of fall. This is opposite of the desired action, which would be to drop the level in anticipation of the volume of water to come. Near the end of the rain event, reservoir operation provided no prerelease capacity and actually stood 296.75 feet, or 0.75 feet higher than the desired winter pool. This eliminated over 1 Billion Cubic Feet (18 %) of the standing capacity of the reservoir, and provided no prerelease capacity.

By comparison, the Two Lakes Procedure would have lowered the reservoir in stages to 295.5 feet. Near the end of this rain, the entire 5.5 BCF standing capacity plus 0.65 BCF (14%) of the pre-release capacity, would have been available to reduce flooding.  (Coincidently, this is what would have happened if the reservoir officials had followed their own procedure)  

Had the rain continued, additional reductions in reservoir level would have been made. Since the additional rains missed the reservoir watershed, the level would have been allowed to rise back to the normal level of 296 feet.

The April 2000 event is very similar to the flood of 1983.  In 1983, reservoir officials had allowed the reservoir to rise to 297 feet. When they realized that a major flood was materializing, a level reduction was attempted, but the lowest level reached was 295.8 feet. At that point the reservoir officials made the best effort to reduce the downstream flooding, but when the upper limit of the reservoir was reached, they were forced to discharge additional water at a high rate, causing extensive flooding in the Jackson area.  The 1983 flood damaged over 800 homes and businesses; this number could have been significantly reduced with a more pro-active prerelease procedure, such as has been proposed by the Two Lake Foundation.

To emphasize this point, even though the reservoir procedures require a target level of 296 feet, as this report is being written (April 25, 2000) the reservoir stands at 297 feet.  This one foot excess is identical to the situation that occurred in May, 1983 when extensive flooding occurred in the Jackson area.

Hindsight is 20/20, but it should teach a lesson: Impending flooding requires a proactive plan to prerelease the reservoir. Two Lake Procedure does this. Development of the procedure utilized the past floods and computer simulation to develop an approach that decreases the reservoir level as the volume of water builds up in the watershed. Our procedure is a response that:

      -Is proportional to the magnitude of the event

      -Occurs as the event is in progress

If the 1983 flood should re-occur, and the Two Lake Procedure used, the peak discharge from the reservoir could be reduced by 30%, which would reduce the flood levels in Jackson by 1.5 feet.

Sloughing Procedure:

Once the water level in a river drops to the level of the 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, the natural run-out from the rainfall is too steep. If the sloughing procedure were started when the natural run-out is 16,000 CFS, the reservoir would have to provide additional volume to complete the procedure. (The reservoir level would drop)  In the April, 2000 case, subsequent rainfall occurred, and the procedure would have been modified to accommodate this.

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 as the April 2000 event, the discharge would be dropped to 16,000 CFS when the inflow equals 28,000 CFS. This 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 lines cross. At this point the stored volume would be used to supplement the inflow, thereby maintaining the desired discharge flow rate of drop. The end result is the desired reservoir level of 296 feet. 

INTERESTING 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).

The reservoir's design lends itself well to flood control.  Its maximum capacity cannot be used unless the reservoir is emptied, which would disallow its use for water supply and recreation.

Following the procedures recommended in this plan would enable the reservoir to function for flood control with up to 67% of its empty capacity.

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 procedures recommended in this plan would require the reservoir to be lowered to a level of 295.0 feet for a period of three days every two years.

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.