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Our levee system is composed of a number of features, some of which are seldom realized to be an important part of the levee.
The sketch below illustrates the most common features and their physical relationship to the levee proper.
The Design Levee is a continuous earthen dam rising 30 to 45 feet above the natural ground elevation, having a crown width of 10 feet, and side slopes of 1 on 3½ or flatter depending upon the material (sand, silt, clay) of which the levee is built. The design levee is the basic defense against flood water, and the other features either protect the levee or help it do a better job of controlling particular problems.
Roadways Sections is a fill added to the land side crown to provide a 25-foot wide area on top of the levee for traffic. This feature is not noticeable after it is built and appears to be part of the design levee.
Graveled Roadways is a means of providing all weather traffic for surveillance and flood fighting even during the wettest periods of time without damage to the levee crown.
Protective Sod is a means of preventing loss of dirt from the levee and to other features due to erosion by rain, wind, or river water. The sod is planted immediately after construction and carefully nurtured throughout the life of the levee structures.
Approach Ramps are provided at frequent intervals along the levee line to permit traffic to reach the levee roadway for crossing without damage to the sod on the slopes. The ramps are not cut into the levee, but rather, are added to it so that the ramp could be removed without changing the design characteristics of the levee.
Riverside Berms are short, narrow fills at the base of the levee to stabilize the relatively long, steep riverside slopes of the levee and minimize the entrance of seepage at the levee toe.
Riverside Borrow Pits are initially acquired as a source of material for building the levee and are subsequently used as a source of repair materials and a site for riverside timber stands.
Riverside Timber Stands provide wave wash and scour protection to the levee during high water. Most of the stands are natural, but in some reaches we have planted cottonwoods and other species to achieve the desired protection.
Traverses are narrow, road-like spaces of natural ground left between borrow pits at regular intervals along the levee. Their primary purpose is to prevent overflow water from flowing fast enough to cause scout in the borrow pits, but they also serve as means of access to other riverside lands.
Land-side Borrow Pits – In years past, particular conditions of river alignment made it desirable to obtain borrow material from the land side of the levee. This was advantageous when considerable work had to be done during periods when riverside lands were overflowed. Now such areas serve as sites for land side berms or for clear observation areas.
Land Berms are low, wide fills to reinforce the base of the levee and provide weight to counteract the uplift pressure of seepage waters during high water times.
Collection Ditches – As a policy, land-side areas are kept clear of brush and trees to permit ready observation of seep and boil areas during high water. Collection ditches along the outer edge of the cleared area drain off excess rain and seepage water so that these areas can be mowed during the summer months to prevent regrowth of bush and trees.
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