A discussion of the individual features of the plan follows:

MAIN STEM LEVEE SYSTEM

The main stem levee system of the Mississippi River in its alluvial valley extends from just south of Cape Girardeau, Mo., almost to the Gulf of Mexico except where major tributaries enter the Mississippi or where high ground makes them unnecessary. This system, including the levees on both banks, is approximately 1,700 miles long and is substantially complete except for strengthening and for minor increases in height at scattered locations.

Additional work needed to complete the authorized levee plan.-The present levee system now provides a high degree of protection against flooding, having withstood the major floods of 1937, 1945, and 1950 without failure. However, in numerous locations the levees were, of necessity, constructed on poor foundations and, in other locations, dangerous underseepage develops during floods. Measures for strengthening the levee system have been under study by the Mississippi River Commission for a number of years and the results of the studies up to 1956 are contained in the report. Studies have been continued since 1956 and the work required to guarantee the integrity of the system during times of extreme flood has been reevaluated to reflect 1965 conditions and price levels. Basically this work consists of reinforcing as necessary either the landside or the riverside of the levee to provide stability and minimize seepage.

The total cost of the authorized plan of levee improvement is $294,800,000. New work requiring authorization of plan and cost.-It may be said that the main stem levees, which include those on the south bank of the Arkansas and Red Rivers, are the backbone of the project. Under the presently authorized plan, they are designed to pass the project flood with a 1-foot freeboard or safety factor over the computed flow line. The design or project flood developed for the lower Mississippi River is based on the runoff from a series of storms over the watershed that have a reasonable chance of occurrence with the resulting flows adjusted to take advantage of the flood control storage in all tributary reservoirs. The design flow line was reevaluated in the report. The new flow line, which is based upon larger flows modified by more extensive reservoir control, ranges from the same as that previously established to about 4 feet lower. This lowering is due largely to improved channel capacity as a result of channel improvement and stabilization. The reservoirs considered in the development of the new design flow line contained 91 million acre-feet of flood control storage. The reservoirs completed or under construction at this time contain approximately 90 million acre-feet of flood control storage, leaving only about 1 million acre-feet of storage to satisfy the conditions used in the computations. Reservoirs in the planning stage at this time which should be completed by 1970 will more than satisfy this requirement.

The discharge capacity of flood channels in the alluvial valley may be expected to vary from day to day and season to season due to erosion, formation, and migration of bars, and growth or destruction of vegetation. Allowances also must be made for temporary local conditions, such as wave action, that could cause local stages in excess of a computed flow line. These factors, plus the necessity of keeping abreast of the state of development of the alluvial valley, warrant the establishment of a uniform 3-foot freeboard on both the east and west bank main stem levees. This will involve raising and enlarging levees minor amounts at scattered locations. The estimated cost of this work at 1965 condi tions and price levels and which requires congressional authorization is $2,785,400,

FLOODWAYS

When major floods occur, the capacity of the Mississippi River leveed channe is exceeded at certain locations, and this excess floodflow is carried by relief outlet. or floodways.

Birds Point-New Madrid Floodway.-This floodway is on the west side of the river just downstream from the junction of the upper Mississippi and Ohio Rivers It provides protection for the city of Cairo from floods that would endanger th levee system protecting the city. The floodway includes an area of 205 squar miles with a population of about 10,000 between Birds Point and New Madri and is bounded on the west by the main line Mississippi River levee and on t east by a substandard levee following the course of the river. The floodway designed to become operative by overtopping of sections of the substandar levee near the upper end when stages reach 55 feet at Cairo with a further ris in sight.

The report shows that the project can function properly if the floodway is not operated until a stage of 58 feet or higher is anticipated at Cairo. Therefore, the report recommends strengthening the substandard levee that forms the east boundary of the floodway and modification of the operation plan to include artificial breaching of the levees at selected locations during the floods that reach 58 feet and threaten to exceed 60 feet at Cairo. This work is included in the Mississippi River levee feature just presented for which additional authorization is sought.

Atchafalaya Basin Floodway.-The safe flood-carrying capacity of the leveed Mississippi River below the general latitude of Old River is 1,500,000 cubic feet per second, whereas the project design flood at this latitude is on the order of 3 million cubic feet per second. In order to insure the integrity of the Mississippi River levees below Old River, it is necessary therefore to divert all flow in excess of 1,500,000 cubic feet per second from the Mississippi River at this general latitude. This diversion is accomplished through the Atchafalaya River and floodways adjacent thereto. Plate 4 of the report shows the various feature of the floodway.

The Atchafalaya River is a natural outlet or distributary of the Mississippi River. About 100 years ago, the Atchafalaya River was a small unimportant stream, generally filled with log rafts. As local interests began to remove the rafts and later to build levees along both sides of the river, it began to enlarge and deepen until it is now the third largest river in the United States on the basis of annual discharge. The Old River control works, now essentially complete (see table 1) were designed to regulate flows from the Mississippi to the enlarging Atchafalaya.

Improvement of the Atchafalaya River was authorized in 1936 so that it could be made to carry its share of the design flood. Although much improvement has taken place in the Atchafalaya River, its leveed channel does not have and is not planned to have sufficient capacity to carry the entire 1,500,000 cubic feet per second that must be diverted from the Mississippi River. The plan contemplates passing 600,000 cubic feet per second of this excess floodflow through the Morganza Floodway on the east side of the Atchafalaya River; 250,000 cubic feet per second through the West Atchafalaya Floodway; and 650,000 cubic feet per second down the Atchafalaya River itself. Flows from these threeroutes converge in the lower Atchafalaya Floodway and enter the Gulf of Mexicothrough Berwick Bay and an artificial outlet west of Morgan City. Flows through the floodway are confined by the East and West Atchafalaya Floodway protection levees. Although the leveed Atchafalaya River can now safely carry its design flows, the lower Atchafalaya Floodway cannot carry its design flows because of physical changes that have taken place in the lower portion of the basin,

Physical changes in the basin.-The increase in flow in the leveed Atchafalaya River at all stages has resulted in deposition of large quantities of sediment throughout the lower Atchafalaya Floodway which once contained large expenses of open lakes. Studies indicate that since 1932, about 2 billion cubic yards of material have been deposited in the lower floodway area. Deposition of sediment is continuing, and it is inevitable that the remaining lake areas will be filled. This deposition of sediment has resulted in a deterioration of the floodway capacity by raising the floodway floor about 3 feet, thereby necessitating the raising of floodway protection levees by the same amount.

During the period in which deposition was taking place, the floodway's main channel downstream of the Atchafalaya River levees did not develop at a ratesufficient to offset this loss in floodway capacity. This situation is further complicated by the fact that the floodway protection levees in this reach are substantially below grade because of poor foundation conditions. Consequently, the lower part of the floodway cannot safely pass its portion of the project flood.

Additional work required to make the plan effective.-Studies in progress but mcomplete at the time the report was prepared have been continued including the development of a stabilized flow line for the floodway based on changed condi1ts which have taken place in the floodway and anticipated future changes, such the filling of lake areas previously mentioned. The problem is to obtain a resonable balance between heights for the floodway levees and the dimensions of angle main channel which together will provide adequate flood carrying capacity n the floodway. The stabilized flow line in the lower Atchafalaya Floodway downstream from the Atchafalaya River levees is about 21⁄2 feet above the previous flow line. To construct the levees to a grade that will contain this flow line and provide a 2-foot freeboard, it will be necessary to raise the existing levees as much as 15 feet on the east side and 10 feet on the west side. Any additional

increase in the height of these levees would be exceedingly expensive. Therefore, it is vitally important that the main central channel be enlarged and developed to prevent further deterioration of the floodway.

Also required are bank stabilization works in the Atchafalaya River and other works of a minor nature.

Enlargement of the Atchafalaya main channel.-Extensive dredging will be required to enlarge the lower Atchafalaya main channel to the size required under the revised plan. To assist in obtaining this enlargement and to prevent further deterioration of overbank areas by deposition of silts, normal flows will be confined to the main channel by closing distributary channels to less than bankfull flows in the lower portion of the floodway. Control structures have been added so that fresh water can be introduced into the parts of the basin where fish and wildlife resources will be adversely affected by reduced overbank flows.

Floodway protection levees.-Construction of the floodway protection levees was started in 1929 and work has continued since that time. Levees on both sides of the floodway above the approximate latitude of the lower end of the Atchafalaya River levees are up to grade. However, downstream of this latitude the levees were of necessity constructed on soft spongy foundations because there was no other place to build them. In this lower reach, the weight of the levees has caused the foundation to sink as much as 20 feet, the average settlement being about 13 feet.

A

The past procedure has been to raise the levees in small increments ranging from 6 inches to 2 feet, thereby gradually loading the foundation. After this was done, considerable time was required for consolidation of the foundations to take place. Under this procedure, the levees could eventually be brought to grade but judging by past experience, at least 30 more years would be required. delay of this magnitude would be intolerable since the floodway would not be capable of carrying its portion of the project flood, and should the project flood or even a flood approaching this magnitude occur, the extensive development that has taken place and is still taking place on each side of the floodway would be severely damaged.

Many studies and tests to find ways of safely constructing the levees at a faster rate have been conducted over a considerable number of years. Results of these studies and tests indicate that by adding considerable base width to the existing levees, the weight of the levee can be distributed over a larger area without danger of overstressing the foundations and the settlement of the foundation will be limited. Under this plan, it is estimated that the levees can be completed in about 10 years provided adequate funds are made available.

Atchafalaya River revetments. In the Atchafalaya River itself, revetments are required to preserve the favorable alinement and to protect levees vital to developed areas. This work has been added to the plan.

Estimated costs.-These changes are necessary for the Atchafalaya Basin Floodway to operate effectively in safely carrying its share of the project flood. The estimated cost of these changes is $104,669,800 greater than the cost in the report. A breakdown of this increase in cost follows:

Lands and damages for levees and channels...
Inspection and maintenance roads on levees.
Enlarge channel area to 100,000 square feet.

Enlargement and raise in grade of levees and construction of stabili

zation berms...

Control structures for fresh water diversion..

Recreation facilities_..

Bank stabilization..

Engineering and supervision..

Total...

$1,967, 900

1,428, 900 42,096, 70

41, 119, 200 1,561, 000 290,000

10, 270, 30 5, 935, 80

104, 669, &ə

The total estimated cost of all work planned for the Atchafalaya Basin includ ing the increase of $104,669,800 is $284 million.

Bonnet Carre spillway.—Of the 1,500,000 cubic feet per second to be carrie 1 in the leveed main channel of the Mississippi River below the latitude of Olc. River during a project flood, 250,000 cubic feet per second are to be diverted to Lake Pontchartrain and the gulf through the Bonnet Carre spillway located about 25 miles above New Orleans. The remaining 1,250,000 cubic feet pr second will continue down the river to the gulf. The Bonnet Carre spillwa was completed in the early 1930's and has been successfully operated three times. The cost of this feature is $14.2 million and is included in the "Main stem completed separable unit" in table 1.