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e EM 1110-2-1913 31 Mar 78 levees and 3 ft for urban levees. Experience in actual flood and results of hydraulic model studies have indicated the need for addi- tional freeboard in the following locations. (1) The upstream end of a levee sell:lllent. Overtopping of levees near their upstream ends during the rising stage of a flood when the river level at the downstream portion was still a few feet below levee crest generally has caused greater damage than in the downstream reach as a result of higher initial current velocities and greater depths and durations of flooding. .An additional freeboard of 0.5 ft is commonly specified at the upstream end tapering to zero at the downstream end. . . (2) Drainage structure locations. tion against overtopping in the vicinity additi,onal freeboard of 1 ft is commonly either side of a structure. To provide additional protec- of structures in levees, specified to extend 100 ft on (3) Near bridges and other constricted areas. Overtopping can occur at these areas because of debris accumulations. .An additional 1 ft of freeboard at these areas extending 50 to 100 ft either side should be specified. e (4) Wave action. .An additional freeboard allowance may be needed to protect against wave action during design flood stage if severe wave action is likely. c. Crown Width. The width of the levee crown depends primarily on roadway requirements. To provide access for normal maintenance opera- tions and floodfighting operations, minimum widths of 10 to 12 ft are commonly used with wider turnaround areas provided at specified inter- vals; these widths are about the minimum feasible for construction using modern heavy earthmoving equipment. Where the levee crown is to be used as a higher class road, its width is usually established by the responsible agency. 6-2. Standard Levee Sections. & a. Many districts have established standard levee sections for particular levee systems, which have proven satisfactory over the years for the general stream regime, foundation conditions prevailing in those areas, and for soils available for levee construction. For a given levee system, several different standard sections may be established depending on the type of construction to be used (compacted, semicom- pacted, uncompacted, or hydraulic fill). The use of standard sections . 6-2 e , ,