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e e e DRAINAGE CRITERIA MANUAL (V. 2) HYDRAULIC STRUCTURES 2.3.5 JumD and Basin Lenath. The un-submerged jump length is typically between 3.6 and 6 times the tailwater depth, depending on the Froude number. For most cases, a basin length of 5 to 6 times the tailwater depth is the most advisable. A longer basin length is advisable for erosive soils or depending on the nature of the jump. Typically, at least 60% of the jump length is rock lined or otherwise reinforced. For baffle chute drops and vertical riprap, basin dimensions are empirically derived. In the trickle or low-flow channel alignment, the jump will tend to wash further downstream of the toe, and additional mitigation is recommended such as extending the basin length andlor providing baffles or large boulders that will break up the jet and dissipate energy. 2.3.6 SeeDaae Analysis. Subgrade erosion caused by seepage and structure failures caused by high seepage pressures or inadequate mass are of critical concern. These factors are important in the design and must be analyzed; otherwise, the structure might fail. Seepage analysis can range from hand-drawn flow nets to computerized groundwater flow modeling. Advanced geotechnical field and laboratory testing techniques may be used to confirm the accepted permeability values where complicated seepage problems are anticipated. Several flow net analysis programs are currently available that are suitable for this purpose. A minimal approach is Lane's Weighted Creep method. It can be used to determine dimensions or cutoff improvements that would provide an adequate seepage length. It should only be used as a guideline and, when marginal conditions or complicated geological conditions exist, a more precise analysis should be used. The involvement of a geotechnical engineer will often be necessary. Lane's method is given later in this section. 2.3.7 Force Analvsis. Each component of a drop has forces acting upon it that require evaluation. This subsection describes the general forces, except forces on riprap for which the reader is referred to Isbash (1936), Oliyer (1967), Smith (1975), Smith and Strung (1967), Stevens (1967), Taggart (1984), Abt (1986 and 1987), Wittler and Abt (1988), Maynord and Ruff (1987), Richardson (1988), and LSA (1986 and 1989). It is worth noting that riprap is subject to all of the usual forces plus the hydrodynamic forces of interflow through voids and related pressure fluctuations. A complete presentation of forces acting on riprap is not presented herein. Forces are described here as they would apply to sloping grouted boulder and reinforced concrete drops. Additional information on forces on baffle blocks is presented in the baffle chute subsection, and this information may also be useful to extrapolate for large boulders used as baffles in grouted boulder drops. The various criteria for structural slab thicknesses given for each type of drop have generally taken these forces into consideration. It is the user's responsibility to determine the forces involved. 06/2001 Urban Drainage & Flood Control District HS-15