COACHELLA VALLEY WATER DISTRICT GUIDELINE K-2 SLOPE (BANK) PROTECTION DESIGN GUIDANCE

Transcription

1 COACHELLA VALLEY WATER DISTRICT GUIDELINE K-2 SLOPE (BANK) PROTECTION DESIGN GUIDANCE K.2.1 General Requirements The design and construction of slope protection works may be required for several types of projects, as summarized below: Where a development is planned within 300 feet of one of the CVWD s stormwater facilities, is at risk of inundation or erosion or at the direction of CVWD (Section 8.4). Where a proposed project alters hydraulic conditions within a channel so that bank erosion or overtopping might result K.2.2 Recommended CVWD Slope Protection Design The preferred slope protection design for CVWD regional stormwater facilities is shown on the typical sheets that follow this guideline. Essentially, it consists of a concrete revetment laid at a 1.5:1 slope, from the design water surface profile (design water surface elevation plus freeboard) to the design elevation of the channel bottom, with a steeper cutoff wall extending to the maximum scour depth. Where the depth of scour below the bed extends below feasible or practical excavation depths (more than 10 to 15 feet below the bed) the preferred design is modified by adding a launchable toe of riprap to the bottom of the excavation that has sufficient volume to launch and protect the slope to the maximum scour depth. K.2.3 Alternative Slope Protection Designs CVWD will accept alternative slope protection designs, such as reinforced turf, grasses or other bioengineering designs, where design velocities or design shear stresses are less than critical or stable values for these products, bank slopes are suitably low, and a maintenance plan can be developed to maintain their effectiveness. These alternative designs may be used in combination with the concrete revetment (for instance, on the upper bank where velocities and shear stresses are lowest) or as a cover over the revetment (Bechtel Corporation 1995). CVWD requires appropriate hydraulic modeling to establish design velocities and shear stresses at the project site. Note that local conditions, such as constrictions, confluences, road crossings, bends or channel encroachments are likely to raise shear stresses and velocities above typical reach values and adjustments of design values will be required in their vicinity. Standards for stability thresholds for various alternative slope protection designs are described in Fischenich (2001), NRCS (2007), other publications, and CVWD unpublished reports such as Bechtel Corporation (1995). There is considerable uncertainty in the upper limits for stability for different soil and vegetation combinations and they also depend on flow duration. The maximum permissible shear stress for un-reinforced turf (well maintained grass) is about 3 lbs/ft2. K.2.4 Detailing the Typical Slope Protection Design The following sections provide detailed methods for estimating the various elevations required for implementing slope protection design. Page 1 of 9

2 Top of Slope Protection The top of slope protection is set to the higher of the SPF design water level plus one foot of freeboard or the 100-year design water level plus three feet of freeboard. CVWD will provide design water levels from its hydraulic model, where these are available, or from unpublished reports, such as SPF design water levels for the Coachella Valley Stormwater Channel from Bechtel Corporation (1995), where these remain appropriate. Where design water levels are not already available, CVWD will provide a preliminary hydraulic model suitable as a basis for further hydraulic studies. Hydraulic modeling will meet the standards discussed in Section 8.4. Top of Cutoff Wall Elevation The change from the concrete slope protection to the top of the cutoff wall occurs at the channel design invert elevation. For the Coachella Valley Stormwater Channel, these elevations are provided on as-built drawings of the CVSC or in Bechtel Corporation (1995). These elevations can be adopted for design if they concur with existing channel conditions. In the absence of design studies or other information, CVWD recommends using the lowest surveyed bed elevation in the project vicinity. Such an elevation requires recent channel surveys and the CVWD also recommends reviewing historical bed surveys, where these are available. If historical elevations are lower than recent elevations, we recommend adopting the historical ones. Bottom of Cutoff Wall Elevation The bottom of the cutoff wall extends to the maximum scour depth or the minimum scour elevation, calculated by subtracting the maximum scour depth from the design flood elevation. As discussed earlier, the bottom of the cutoff wall may extend to feasible excavation depths with a launchable that provides protection to the minimum scour elevation. The prediction of maximum scoured depths is very complex and is discussed in detail in Guideline K-3. Launchable Toe of Riprap The design of the launchable riprap toe requires specifying 1) the median size and distribution for the riprap appropriate for the particular situation and 2) the volume of rock required to protect the slope to the minimum scour elevation. In general, we recommend the procedures described in Engineer Manual (USACE 1991) for estimating the median size and distribution of riprap. Riprap sizes are calculated from the observed channel characteristics and velocities for the design discharge (100-year flood for bank protection) as described in Chapter 3 of the manual. The required volume of launchable riprap is then calculated from the procedures described in Chapter 3-11, assuming that the scour depth is the difference between the planned elevation of the toe of the concrete cut-off wall and the minimum scour elevation. A general layout for the launchable riprap toe is included on the drawing sheets at the end of this guideline. K.2.5 Design Drawing Requirements The design plan set should indicate the extent of the works and show the keys or tie-ins or other features at the upstream and downstream ends of the protection, particularly where the revetment joins existing slope works. Specific guidance to define the required extent of slope protection works is provided in USACE (1991) and other publications. The following provides specific information to be included on the drawings: Page 2 of 9

3 Plan, profile and typical sections of the slope protection works with stationing numbers according to CVWD designations, design water levels, top of slope elevations, cut-off bottom elevations, minimum scour elevations, channel bottom elevations, etc. Elevations will refer to CVWD vertical datum or NGVD 1929 vertical datum.. Cut-off wall depth should be referenced to the design invert. Details relative to tie-in with existing slope protection works, if applicable. Details of the wrap-around or key-in to the bank of the channel at the termination of slope protection works, if applicable. Details of the design of the launchable riprap toe or berm, if applicable. Details of protection works at the channel bottom at storm drain outlets through or in the vicinity of the proposed slope protection works which could adversely impact the integrity of the proposed works, if applicable. Show all utilities within District s right-of-way. K.2.6 Submissions The Hydraulic Design Report (see Guideline K-1) will be submitted with the design drawings. K.2.7 References Fischenich, C Stability thresholds for stream restoration materials. EMRRP Technical Notes Collection (ERDC TN-EMRRP-SR-29). US Army Engineer Research and Development Center. Vicksburg, MS. 10 pp. May, R.W.P, J.C.Ackers and A.M.Kirby Manual on scour at bridges and other hydraulic structures. CIRIA, London. Maynord, S.T Toe-scour estimation in stabilized bendways. J. Hydraulic Engineering (ASCE) 121 (7). Melville, B.W. and S.E. Coleman Bridge scour. Water Resources Publications, Colorado. Neill, C.R (editor). Guide to Bridge Hydraulics. University of Toronto Press for Roads and Transportation Association of Canada, Ottawa. Natural Resource Conservation Service (NRCS) Part 654 Stream Restoration Design National Engineering Handbook. Chapter 8: Threshold Channel Design. US Department of Agriculture. 43 pp. Richardson, E.V. and S.R. Davis Evaluating scour at bridges, third edition. U.S. Federal Highway Administration, Hydraulic Engineering Circular No.18. TAC Guide to bridge hydraulics, second edition. Thomas Telford, London, for Transportation Association of Canada, Ottawa. USACE 1991 (with revisions 1994). Hydraulic design of flood control channels. U.S. Army Corps of Engineers, EM Page 3 of 9

4 USACE Channel stability assessment for flood control projects. U.S. Army Corps of Engineers, EM USACE Whitewater River Basin Feasibility Report for Flood Control and allied Purposes. San Bernardino and Riverside Counties, California. Appendix 1 Hydrology. Los Angeles District Page 4 of 9

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