October 19, 2000

ROSS BARNETT RESERVOIR BULKHEAD SURVEY

The Two Lakes Foundation's simulation has shown how the rapid drawdown of the reservoir (up to 2 feet per day) will prevent much of the flooding in Jackson in the event of another 1983 magnitude storm.  After reviewing the condition of the reservoir bulkheads, we believe that this rate of drawdown is unlikely to cause general damage to bulkheads.  Any damage that might occur would small compared to the corresponding loss that would result in downstream homes without adequate pre-release.

Details of the Two Lakes Survey:

Note:  All photos were taken at a reservoir elevation of 293.4 feet.

The photo below is an example of a wooden bulkhead (with tie-backs) in Audubon Point  (one of the older subdivisions).   The water level is 1.5 feet below the bottom of the bulkhead.  

If a 1983 magnitude storm event occurred, the Two Lakes plan constrained procedure (the procedure that could be used now, without the two lakes in place), would drop the reservoir level to about 293 feet and prevent much of the flooding in Jackson.  The rate of drop could vary from 1 to 2 feet per day.  Dropping the level to 293 feet (which is close to the level it is today) would save 400-500 homes from damage.  A bulkhead such as the one below, which shows no evidence of sagging, should be safe from rapid drawdown from either the current recommended winter pool of 296 feet, or the Two Lakes recommended pool winter pool of 297 feet.

 

This view taken in Forrest Point is typical of the conditions from Bay Park to Audubon Point:

Newer areas, such as this photo of of a lot in Windward Oaks, shows a steel bulkhead, partially in shallow water, with areas of ground elevation almost 2 feet above the water level:

The following is an older steel bulkhead that some evidence of rust, but appears to have good structural integrity:

Purpose of the Survey:

The Two Lake Foundation has proposed a rapid drawdown of the reservoir to aid in flood reduction in the event of another 1979 or 1983 event.  One concern is the possible damage that might occur to reservoir bulkheads (seawalls) because of the rapid drop in water level.   Bulkhead loading will increase as the water level drops, until it drops to an elevation where the ground is exposed on the water side of the bulkhead.  At this point a further drop of the water level will have little or no effect on bulkhead loading.

This survey has been taken to provide an estimate of the elevation of the base of the reservoir bulkheads.  The base is defined as the intersection of the ground and bulkhead on the water side.  An estimate of bulkhead condition is also noted.  

Measurements:

There are approximately 600 waterfront residential lots below highway 43.  55% of these lots were evaluated by boat, and 25% by land.  The last 20% were inaccessible, and were evaluated by using data from the closest available lots.  Most of the inaccessible lots were in the driest portions of the reservoir.

Elevation data was estimated using the water line that is at approximately 297.4 feet on all bulkheads, and the current reservoir level.  The difference in these two values (4 feet during the observation period) was used as a reference to determine the elevation of the bulkhead base for each lot on the reservoir.  Dimensions were determined by both visual observation and analysis of photographic data, and the average elevation for each lot was used.  The intent was to obtain a rough estimate of the elevation of the ground (if exposed) or the depth of the water if no ground was exposed.  The elevation error is estimated at +/- 6 inches or less.  

The following is a graph of the data, showing the percent of bulkheads that will be exposed at various reservoir levels:

The above chart shows that by the time the reservoir reaches the current level of 293.4 feet, 80% of all bulkheads bases will be exposed.

Construction Data:

Steel:  31%

Wood:  29%

Block Construction (primarily cinder block):  16%

No Method of Retention (no bulkhead):  10%

Concrete:  8%

Rip Rap:  6%

Bulkhead Condition:

The general condition of the bulkheads is good.  The newer bulkheads on the north shore of Pelahatchie Bay are made of steel, as are the bulkheads east of Audubon Point.  Older bulkheads on the south shore of Pelahatchie Bay from Bay Park through Audubon Point are primarily wood or cinder block construction.  Most of these older bulkheads are in reasonably good condition, and their stability is enhanced by the high ground elevation on the water side of these bulkheads.  Eight of the bulkheads evaluated showed definite signs of age (cracks, leaning, etc.), and four more showed signs of collapse of portions of the bulkhead.  

Note that without an complete engineering assessment, the ability of a bulkhead (regardless of the visual condition) to withstand a rapid drop in water level is conjecture.  However, the visual evidence does show that the vast majority of bulkheads are in reasonably good condition and the oldest (and presumably the weakest) bulkheads will generally be the first to have the bases exposed, thereby reducing the loading.  Also, most reservoir draw downs will be slower during the first few days, further reducing the loading on all bulkheads.

About 15% of the residential shoreline is either rip rap or has no bulkheads at all.  These lots may have some soil sloughing due to the rapid drawdown, but since the magnitude of the drop will be small, it is unlikely that significant damage will occur. 

Damage Assessment:

Bulkhead damage costs should be evaluated on the depreciated value of the structure, taking in account the age, condition and how the bulkhead was built.  Twelve bulkheads were noted as having some structural problem.  These have only been exposed to normal reservoir conditions, and would probably have to be repaired at some time in the future in any case.  In any case, the cost to repair bulkhead damage (should it occur) would have to be compared to damage to homes in Jackson.

Summary:

At this time of record reservoir low levels, it is apparent that most (if not all) bulkheads will be completely exposed well before the minimum reservoir level of 292 feet is reached.  A further drop from the current reservoir level should not have an affect on the bulkheads.  For a 1983 storm, the minimum reservoir level is less than one foot lower than the current level.  

The least stable (and oldest) bulkheads are in areas that have the highest ground level on the water side of the bulkhead (the first picture is a good example).   The older, weaker bulkheads are generally in the first 40% to be uncovered, and by the time the reservoir reaches the current level of 293.4 feet, 80% of all bulkheads bases will be exposed.