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e . i e . . e :EM lllO-2-l9l3 31 Mar 78 and U. S. Army Engineer Waterways Experiment Station TM No. 3-424 (ref. A-3b(2)). Many combinations of well spacing and penetration will produce the desired pressure relief; hence, the final selected spacing and penetration must be based on cost comparisons of alternative com- binations. After the well spacing for a given reach of levee has been determined, the location of each well should be established in the office and field to ensure that the wells will be located at critical seepage points and will fit natural topographic features. c. Design of Individual Wells. The design of the well involves the selection of type and length of riser pipe and screen, design of the gravel pack, and design of well appurtenances. A widely used well design that has given good service in the past is shown in fig- ure 5-7. (l) Riser pipe and screen. The well screen normally extends from just below the top of the pervious stratum to the bottom of the well, with solid riser pipe installed from the top of the pervious strata to the surface. In zones of very fine sand or silt, the screen is replaced by unperforated (blank) pipe. The type of material for the riser and screen should be selected only after a careful study of the corrosive properties of the water to be carried by the well. Many types of metals, alloys, fiberglass, plastics, and wood have been used in the past. At the present time, wood and stainless steel are the most widely used, primarily because of their corrosion-resistant properties. Figure 5-7 shows a typical well using a wooden riser pipe and screen. Wood will not deteriorate as long as it is permanently submerged but will deterio- rate when subjected to alternate wetting and drying. For this reason that portion of the riser above the lowest expected water table should be surrounded with concrete. (2) Filter. The filter that surrounds the screen must be designed in accordance with criteria given in Appendix E using the slot size of the screen and the gradation of surrounding pervious deposit as a basis of design. No matter what size screen is used, a minimum of 6 in. of filter material should surround the screen and the filter should extend a minimum of 2 ft above the top and 4 ft below the bottom of the well screen. Above the filter to the bottom of the concrete or impervious backfill, sand backfill may be used. (3) Well appurtances. In selecting well appurtenances, considera- tion must be given to ease of maintenance, protection against contamina- tion from back floOding, damage by debris, and vandalism. To prevent wells from becoming backflooded with muddy surface water (which greatly impairs their efficiency) when they are not flowing, an aluminum check 5-9