Grays Landing Spillway and Stilling Basin, Monongahela River, Pennsylvania; Hydraulic Model Investigation.

Continued)capability of various stilling basins to pass ice without impacting and abrading the apron, baffle piers, and end sill.The size, gradation, and extent of riprap required for adequate protection of the downstream channel were also determined.The hydraulic model investigation of the stilling basin revealed the general adequacy of the design of the spillway with satisfactory performance of the proposed spillway crest.From the hydraulic performance of various types of energy dissipaters investigated, the most appropriate stilling basin was the type 5 design.Shorter basins produced higher velocities downstream (up to 15.5 fps) and required larger downstream riprap protection.The type 5 design stilling basin allowed the baffle blocks to be located farther downstream to prevent the direct attack of ice against the baffles and stilling basin apron.The recommended type 5 stilling basin design is a conventional hydraulic-jump type energy dissipater which would perform well for both the initial construction of one lock, a 576-ft-wide spillway crest and stilling basin, and the ultimate construction of two locks, a 460-ft-wide spillway crest and stilling basin.The recommended downstream riprap protection, plan 2, consists of a 72-ft length of d100 = 30-in.riprap with a maximum stone weight of 1350 lb, followed by a 108-ft length of d 100 = 18-in.riprap with a maximum stone weight of 292 lb.Pressures obtained for a range of flow conditions indicated that no serious negative pressures should be encountered on boundaries of the prototype.Additional analysis of model • data was conducted to develop a rela~ tionship between headwater, tailwater, and minimum pressures on the upstream quadrant of the spillway crest.For spillway designs subject to submerged flow conditions, the plot presented as Plate 12 herein should be used to select an appropriate design head so that the minimum pressure, considered sufficient to induce cavitation, will not be less than -20 ft • .