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and maximum permissible velocities. Maximum velo- cities 4. Total depth of channel lining will include shall be based upon design flow quantities. design water depth and freeboard. Manning Maximum Unlined Channels "n" 5. Attention shall be given to construction de- Velocity(fps) Side Slopes tails of linings such as thickness, reinforce - Rock,smooth and uniform 035 15 2: I ment, expansion and construction joints, cut - Rock, jagged and irregular .040 15 1: I Ledge coral orlime stone off walls, water -tight joints, placement of .025 10 I : I Earth, no vegetation 5 reinforcement, etc. Where the channel dis- .025 I: I Earth, grass, some weeds 5 charges into streams or other channels out - .030 1 2: I Earth, dense weeds side of the limits of a development, velocity .035 5 I Lined Channels reducing and transition structures shall be constructed to minimize erosion and over - Conc., trowel finish 013 No limitation topping of banks and subsequent flooding of Conc., smooth wood forms .015 No limitation downstream areas. Gunite 020 20 6. Where velocities are supercritical, rectangular Grouted Rip -rap & CRM channels shall be used, unless otherwise (Cement Rubble Masonry) .025 20 permitted by the Chief Engineer. Asphaltic Concrete .015 20 7. Earth channels shall be planted with vegeta- Corrugated Metal Flumes tion, such as grass of a species not suscep- Part- circle Sections .021 25 tible to rank growth. ® FREEBOARD. 1. Maximum design velocity for channels cut in earth shall not exceed 5 feet per second. The In designing open channels, freeboard must be pro - velocity shall be determined by using the vided to allow for surface roughness, wave action, tural exi f�e nasting slope o waterway with- air - bulking-,_and- splash - and - spray. The -se phenomena out utilizing grade transition structures to depend on the energy content of the flow. For water control the maximum slope for a given unlined flowing at velocity v and depth d, the energy per foot channel cross - section and design flow. of width per second is equal to (wvd) (v2 /2g) = wdv3 /2g, 2. Velocities betwee 5 f where w is the unit weight of water. e n et per second and 15 feet per second will be permitted in materials such as cemented gravel, hard pan, or mud rock depending upon its hardness and resist- ance to scouring. Borings and samples shall be submitted for evaluation before velocities exceeding 5 feet per second will be permitted. G CHANNEL LINING I. Earth channels shall be fully lined when ve- locities exceed 5 feet per second, unless otherwise permitted as noted in Section C -2 above. 2. All fill sections shall be lined. This lining shall be a complete lining including side slopes and invert with appropriate cut -off walls. If the invert.of the channel is in a cut section the invert slab may be omitted and appropriate cut -off walls provided at the toe of the side slope lining. . 3. Where linings are required or used, the linings shall be continuous. Lining of fill sections without continuing the lining out through cut sections in a channel will not be allowed un- less adequate- provisions are made to reduce the velocity from the lined section to meet the allowable velocity for the unlined section. Lue Thus, this kinetic energy can be converted to potential energy to lift the water surface when flow is stopped or changing direction as a function of depth and velo- city of flow. The U.S. Bureau of Reclamation has developed an empirical expression to express a rea- sonable indication of desirable free board in terms of depth and velocity as follows: Freeboard in feet = 2.0 + 0.025.3 d where v is the velocity in feet per second and d is the depth of flow in feet. The velocity of flow can be computed by dividing the design discharge by the cross - sectional area of flow. For convenience of ap- plication, the above expression is shown graphically in Plate 7 (pg. no. 21 ). For discharges less than 30cubic feet per second use channel size design of 100% greater capac- it than the design capacity. JUNCTIONS Junctions shall be designed to channel both flows as nearly parallel as possible to reduce velocity and momentum components, deposition of debris and ero- sion of banks. 0BENDS AND SUPERELEVATIONS Changes in the direction of flow shall be made with smoothly curved channel walls allowing for super - elevation in water surface. Curves will nearly always require additional depth. Trapezoidal channels for