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3.2.8 A 75-foot weir at elevation 3920.0 feet ms1 was designed to pass flow from Pioneer Ditch to the detention pond. The weir is located at the upstream end (west end of the south side) of the detention area. The weir itself would be part of the rock drop structure into the detention pond. It would be 7S feet wide and S feet long. Sides10pes would be 1 V:3H from the crest elevation of 3920.0 feet ms1 to elevation 3922.5 feet ms1 and 1V:SH to elevation 3923.5 feet msl. Riprap should be placed on the Pioneer Ditch bank in the vicinity of the weir entrance. It would be protected by a riprap layer coinciding with the slope chosen for the drop structure. Plate 7 shows the section at the crest of the weir, and Plates 8 and 9 show the profile in relation to the channel and the drop structure alternatives. 3.2.9 Several drop structures into the detention area were analyzed--alternatives with a lo-percent slope, a 5-percent slope, and a 2-pereent slope. The drop structure was designed based on guidance from references listed in paragraphs 2.7 through 2.11. The lO-percent slope alternative resulted in very high velocities, and the size of rock needed for riprap protection was large. The 5- and 2-percent slopes were more reasonable and are both presented here so that the City may choose the alternative that best suits its resources. Plates 8 and 9 show the general profile of the rock drop structures. The cross section is the same as that shown for the weir, except it should flatten out at the downstream end. Note that the tota11ength of the structure for a 2-percent drop is approximately 150 feet and the riprap layer thickness required is 18 inches. The tota11ength of the structure with a S-percent slope is approximately 75 feet and the required riprap layer thickness is 24 inches. Because turbulent flow will be a factor during a flood event, riprap gradations are based on 1.5 times the normal maximum rock size (D.,,). Table 3 shows the gradations that should be used. A filter may be required under the riprap layer. TABLE 3 OVERFLOW WEIR DROP STRUCTURES RlPRAP GRADATIONS * 2% DROP STRUCTURE 5% DROP STRUCTURE 1S-INCH RIPRAP LAYER 24-INCH RlPRAP LAYER % LIGHTER BY LIMITS OF STONE % LIGHTER BY LIMITS OF STONE WEIGHT WEIGHT WEIGHT WEIGHT (percent) (pounds) (percent) (pounds) 100 86-35 100 205-82 50 36-17 50 86-41 15 18-5 15 43-13 * Based on a specific weight of 165 pounds per cubic foot and a thickness 1.5 times the DlOO' 7