• EFFECTIVENESS OF PROCEDURES
• USEFUL FACTS
  • PEAK STEADY DISCHARGE vs. MAXIMUM INFLOW
• GETTING THE RESERVOIR AND DOWNSTREAM COMMUNITIES INVOLVED
• DEVIATIONS FROM CURRENT RESERVOIR DISCHARGE PARAMETERS
• DISCHARGE CORRECTIONS
• DEFINITION OF CONDITIONS FOR DISCHARGE STUDY
  • PRERELEASE PROCEDURE (CONSTRAINED) BASED UPON RUNOFF REMAINING IN THE WATERSHED
    • SIMULATION WITH EXPANDED SCALE TO ILLUSTRATE PRE-RELEASE PROCEDURE CONSTRAINED
  • PRERELEASE PROCEDURE (UNCONSTRAINED) BASED UPON RUNOFF REMAINING IN THE WATERSHED
    • 4 DAY TOTAL RAINFALL FOR RESERVOIR WATERSHED

EFFECTIVENESS OF PROCEDURES

More study will be done to refine the reservoir inflow simulation. At the present time it appears that errors will not exceed 5% in cumulative volume and 6 hours in timing. Discharge procedures in the very early stages will need to be based upon rainfall alone. Later in the flood event, these procedures can be modified as the record of inflow proceeds and as stream flow data in tributaries becomes significant. Additional corrections can be made on the backside of the storm due to the consistency of the run off pattern.
    Perfect hindsight reveals that the reservoir has the ability to lower major floods by an additional 1.5 feet. At this stage in our study, it appears likely that better inflow simulation and discharge procedures will yield at least one dependable foot of less flooding. This could be done at the present time under discharge conditions that are restrained by flooding of low lying structures in Metropolitan Jackson. This improvement would be significant. For example: Most homes and businesses that flooded in 1983 received less than 1 foot of water. Using these procedures at the present time would prevent flooding in up to 500 of the 750 homes and businesses that flooded in Jackson in 1983.
    After construction of the flood control lakes, unconstrained prerelease of the reservoir should result in a reduction of at least 1.2 feet in flooding downstream of Jackson. When combined with the 0.4 foot increase caused by capacity loss due to the lakes, the net result should be approximately 0.8 feet less flooding downstream.

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USEFUL FACTS

The law which enables 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 maintain strictly for flood control would have 15 bcf of holding capacity (almost 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 because, being empty, would disallow its use for water supply and recreation.

Following the procedures recommended in this plan will 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 if the reservoir had 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).

The graphs (facing page) may be used to determine the additional benefits to downstream flow rates that may be derived from present capacity and prerelease capacity in the reservoir.

Perfect use of the reservoir's present and prerelease holding capacity (10.1 bcf) would deduce 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 imposed 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.

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GETTING THE RESERVOIR AND DOWNSTREAM COMMUNITIES INVOLVED

During the entire 35 year history of the reservoir, its officials have been taking criticism from all manner of people whose only contribution has been to criticize reservoir management of the flood gates without giving any constructive help. The latest conspiracy theory (read in the Clarion Ledger) has been that reservoir officials have been keeping the reservoir at record low levels for their own purposes, while telling the public that they do not have sufficient rainfall. Actually the reservoir is performing a very valuable function for downstream communities by using its storage capacity to maintain necessary downstream flow. Up until this time, these officials have had every right to place us in the same category as any other detractor who may wish to supply only criticism without assuming any responsibility.

We must prove to the communities both above and below us that we can provide a simple, accurate and effective way for the reservoir to reduce downstream flooding and bank sloughing while, at the same time, operate reasonably within the parameters defined by the Corps of Engineers for reservoir operation in their 1999 study (see Addendum Section page 78). Deviations from the Corps plan are as follows:

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DEVIATIONS FROM CURRENT RESERVOIR DISCHARGE PARAMETERS

The discharge plan is in compliance with the U. S. Army Corps of Engineers September 1999 list of parameters (see Addendum page 78) for control of reservoir operations with the following exceptions:

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DISCHARGE CORRECTIONS

In order to perfect a discharge plan, we have found it necessary to first correct the reservoir discharge records. During minor floods discharge has been over reported (as much as 10%) and during major floods discharge has been under reported (as much as 12%) (see Addendum Section pages 79 - 88 (These changes match approximately with USGS readings at Highway 80.) Correction is necessary because all data essential to predict flooding is derived from discharge data.

For example: Reservoir inflow is calculated by adding the rise in the reservoir to the amount of water discharged. Errors in discharge may account for the reason that the Corps has been unable to effectively predict reservoir inflow from rainfall data. Persons familiar with the old data need to remember that the values have been changed in this study. For example, maximum discharge during the 1979 flood has been changed from 124,000 cfs to 137,000 cfs (see graph page 15).

Once these corrections were made, inflow volumes calculated from rainfall became predictable enough to allow for greatly improved downstream flood control through prerelease of the reservoir. Analysis also revealed that, because the reservoir watershed is uniquely simple, (a single bowl with short, steep tributaries that converge in the reservoir: see map page 14) errors in volume and timing of reservoir inflow are smaller than expected. Additionally, simulated use of storage capacity revealed that volume and timing of reservoir inflow errors are not as critical to effective flood reduction as previously believed.

Also, the complicated functions that control the volume and timing of reservoir inflow actually combine to produce simple, consistent results. For example: the three flood studies presented here are very different. The total volumes existing in the watershed vary from 66 bcf in 1979, 44 bcf in 1983 and 27 bcf in 1980 (see Calculated Inflow Graph). The rain event lasted only one day in 1979 but was spread out over 2-1/2 days in 1983. Peak inflow in 1979 was 179,000 cfs; it was 126,000 cfs in 1983 and 52,000 in 1980. And yet, in all three storms, all but eight bcf passed into the reservoir in very close to the same period of time (8-1/2 days). The remaining eight billion cubic feet ran out of the watershed in a remarkably consistent ten days (see graphs " see pages 19, 21, and 23).

This tells us that the timing of major floods in the Barnett Reservoir will not be difficult to simulate. Also the volumes of major floods are not so difficult to derive from rainfall. For example: in 1979, 90% of the rain ran into the reservoir. This is because the rain was so large that only 10% soaked in the ground or evaporated. Predicting runoff volumes in a major storm such as this creates less probability of error because you have only 10% of the rainfall with which to make error (see pages 19, 21, and 23). Predicting the minor floods, particularly those which overlap each other such as in 1980 is another matter. These floods tend to spread out and merge with other floods, the volume lost to absorption can vary from 20% to 40% (see page 23).

Since we only have two good major floods to practice upon, the 30 minor floods which have been recorded of the last 35 years will provide the test with which we can know the degree of accuracy we have obtained to apply to Jackson’s next major flood event (see page 23 ). In the future we hope to have enough ability to run the program through all of the minor floods with a degree of accuracy which relates to accuracy attainable in major floods, such as the 1979 simulation (see page 15).

Early simulations of the 1983 flood did not take enough into account that this flood occurred late in the year (May and June) when more rain is lost to absorption and evaporation (see 1983 Chart page 52). Rather than correct this simulation we based our discharge upon this erroneous simulation to determine what it would cost us. The results show that, even with a 5% volumetric error, prerelease and capacity management (that is built into the procedure) produced a reduction from 1983’s peak discharge of 85,000 cfs (corrected) to 71,000 (see page 52). (A savings of 14,000 cfs). With perfect simulation based on hindsight, this savings would have been 19,000 (see pages 34 and 35). So with an 5% error in volume 74% of the benefits were still there.

This plan also offers a bank sloughing procedure (see Addendum Section pages 90 - 94 at the back of this report).

Much work will be required in order to perfect the inflow calculations and discharge procedures that we are showing you at this time. The present work is a "work in progress" which reveals errors at which time you can perfect the model. The work is accurate enough to see the light at the end of the tunnel and the quicker that errors are called to our attention the quicker we will correct them.

The discharge graphs in this study have been constructed using the conditions stated in the following two sections.

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DEFINITION OF CONDITIONS FOR DISCHARGE STUDY

Prerelease will be studied under two opposing conditions:

1. Prerelease Constrained: by flooding of low lying streets and structures (present condition).
2. Prerelease Unconstrained: by downstream flooding concerns (after the lakes have been constructed.)

Flood reduction procedures will be studied under three qualities by inflow projection.

1. Perfect inflow (using hindsight of past floods).
2. Simulated inflow (using the inflow projection model with its best, to date, projections).
3. Simulated Error (using maximum errors in timing and volume to determine what effect this will have on end results).

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

(The graph below shows what would have happened, following the best simulation) using this procedure in 1983.

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

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