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Flood Control
   

Major Flood Events

Four (4) major floods have occurred since the 2.6-mile, 500-year earthen levee was completed in September, 1972. A major flood is when the water level in the Missouri River is at a minimum of 10 feet above flood stage for at least one week.

During the four (4) major floods, the District's flood control system sustained minimal damage that was quickly repaired.

Spring 1973 and fall of 1986: Crest elevations were under the 50-year flood level. The 1973 flood stage lasted about 75 days. This is significant as at this time, the 500-year levee was only six months old. The 1986 flood was higher than the 1973 flood but of a relatively short duration.

August, 1993: During this record level flood, the Missouri River crested at 14.6 feet above flood stage on August 2, and remained above flood stage for about 110 days. It has been estimated that at its August 2 crest, the Missouri River was at a 200-year flood level. The levee and the other components of the District's flood control system successfully resisted the flood.

May, 1995: the Missouri River crested at 11.7 feet above flood stage but the flood duration was relatively short.

Since the 500-year levee was completed in September, 1972,in addition to the four major floods, the Missouri River has been over flood stage numerous times --usually at a level less than 5 feet over flood stage. These are normal events.

The St. Louis District of the U.S. Army Corps of Engineers maintains a Missouri River gage station at river mile 27.2 (know as the St. Charles gage). The flood stage at this gage is 25 which is 438.59 NVDG. Anytime the Missouri River is above stage 25 -- the river is above flood stage.

Flood Control System

The 1,891-acre District is protected from flooding by an interrelated flood control system. A brief description of the major components of the system follows:

 

Levee: The District is protected by 3 reaches of levee

1. A 2.6-miles, 500-year rated earthen levee constructed in 1972 to U.S. Army Corps of Engineers standards. The levee extends from Interstate Highway 70 on the south to the Norfolk Southern Company's railroad trestle embankment at the Earth City Expressway on the north.

The 2.6-mile earthen levee is a sand core levee. A typical cross section of the levee has a 10-foot wide top with 1-foot vertically to 3-foot horizontally sloped sides. The riverside face of the levee has a 5-foot thick soil cover intended to protect the levee from seepage penetration during a major flood event.

2. A 1.4-mile section of Interstate Highway 70 extending eastward from its connection on the south end of the 2.6 mile levee section (e.g. east end of the Blanchette Bridge).

The flood protection provided by this section of the District's levee was enhanced in 1986 by the construction of a 500-year rated levee protecting the Riverport Levee District located immediately south of Interstate Highway 70.

3. A 1.6-mile section of the Norfolk Southern Company's railroad embankment beginning at the Missouri River crossing trestle extending eastward to Taussig Road.

Relief Wells

The District has a total of 83 relief wells of which 67 relief wells are located along the landside toe of the 2.6-mile levee. These 67 wells were installed between 1988 and 1992. The wells are 60 feet deep and the bottom 20 feet is a stainless steel screen. The top 40 feet is PVC pipe but some wells have stainless steel pipes. These are gravity flow wells with a designed discharge capacity of 780 gallons per minute.

Another 10 relief wells, installed in 1972 to a depth of 90 feet, are located along the forebay area immediately east of the pumping station and discharge structure built into the 2.6-mile levee.

Another 6 relief wells, installed in 1972 at a depth of between 60 feet and 90 feet, protect the rail underpass on St. Charles Rock Road.

Relief wells allow ground water under the levees to gravity flow to the ground surface while keeping underground soils from flowing out with the water on the the landside of the levee (e.g. sand boils). Without the relief wells, the soil moved by the underground water flow could create a void under the levee. The levee could collapse into the void area.

The 83 wells are basically dormant operating only during a major flood. As part of the District's maintenance program each year, 15 relief wells are pump tested to determine if they can perform to design standards. The District uses the U.S. Corps of Engineers standards to evaluate testing results. A relief well testing below 80% of design capacity is either rehabilitated or replaced.

Underseepage Protection Berm

A landside underseepage protection berm extends the entire length of the 2.6 mile levee up to a distance for 625 feet east of the toe of the levee. Narrower berms extend along the other two sections of the levee.

The purpose of the berm is to contain excess groundwater pressure. The berm area consists of a layer of heavy clay soil that counteracts groundwater pressure under the levee. (The soil berm pushes down against the upward pushing water).

Interior Storm Water Drainage System

The interior storm water drainage system for 82% of the District's 1,891 acres is handled through an interconnecting system of ditches, channels and lakes, all of which ultimately gravity flow to the District's pump station and discharge structure.

The storm water in the Rock Industrial Park and Northwest Industrial Park areas of the District contain storm water within their areas in retention ponds. The sole method of discharge from the ponds is percolation and evaporation.

1. Pump Station

The District's pump station and discharge structure, a vital component of the District's flood control system, penetrates the 2.6 mile levee about 500 feet south of St. Charles Rock Road. As mentioned earlier, 82% of the District's storm water is tributary to the pump station and discharge structure.

The pump station was completed in 1972 and rests on top of the discharge structure. The reinforced concrete pump station contains 3, 150 hp electric pumps each capable of discharging 22,500 gpm -- even during a major flood event. A diesel generator operates the pumps in the event of a power failure.

In 2000, the District modified the pump station and discharge structure installing a fourth diesel operated pump with a capacity of 45,000 gpm.

The pump station is fully automated.

2. Discharge Structure

This structure, also completed in 1972, consists of a three barrel, 10-foot high x 12-foot wide reinforced concrete box culvert which extends 310 feet through the earthen levee to the storm water discharge channel on the riverside of the levee.

Fabricated into two of the three box culverts are 10-foot x 12 foot steel sluice gates which can be raised to permit the gravity flow of storm water out of the District to the Missouri River. The third box culvert was modified in 2000 and now discharges water from the 45,000 gmp pump to the Missouri River.

Of the two methods of discharging excess storm water in the District's interior storm water drainage system,pumping the storm water is the method used most of the time as high river level often does not permit raising the sluice gates.

As with all components of the District's flood control system, all the pumps, the backup diesel generator and other equipment are routinely inspected and serviced by qualified contractors.

Maintenance

The District has developed a comprehensive and ongoing maintenance program whereby the entire levee system, relief wells, pump station and other mechanical and electrical systems are inspected at least annually by qualified independent contractors. The U.S. Army Corps of Engineers inspect the levee and pump station normally on an annual basis.

The District's levee and the pump station have qualified for participation in the Corp's rehabilitation assistance program for flood control projects (e.g. Public Law 84-99). As a result of such participation, the Corps will pay 80% of the construction costs incurred in connection with rehabilitation of the levee or pump station resulting from flooding. Costs such as dirt are not covered by the Corps' assistance program.

 

 

 
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