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It shall be determined starting at the downstream end of the proposed drainage system and proceeding up- stream by adding the friction losses and manhole losses of the system. The hydraulic gradient for the design flow shall be at least one foot below the top of the manhole cover, or 1 foot below the invert of catch basin inlet opening. 1. Beginning Elevation The elevation of the hydraulic gradient at the downstream end shall be selected according to the following conditions: a. Connection to existing drainage system — determined from the hydraulic gradient computations of the existing drain; b. Discharge into a stream — determined from the flow conditions of the stream; c. Submerged tailwater condition — begin at the tailwater elevation; and d. Freefall condition (conduit) — begin at the crown of the proposed drain. 2. F-ritti h f = S{ (L), where: hf = head loss due to friction Sf = friction slope from Manning's formula, (n V)2 2.208 R 4/3 L = length of pipe or channel The friction loss shall be calculated for the condition of the design flow, that is, pipe flowing full or partially full. 3. Manhole Losses Manhole losses shall be as shown on the charts, "Head Losses in Manholes ", (Plate 17 & 18, pg. 32). The losses shall be eval- uated with pipes flowing full in the vicinity of the manholes; and therefore the velocity shall be for the pipe flowing full. The curves on the charts show the various losses: a. A curve — loss due to entrance and exit b. B curve — velocity head (1) Where the downstream velocity ex- ceeds the upstream velocity, the head loss shall be difference in velocity heads. velocity head loss shall be zero. c. C curve —loss due to change in direction, taking the worst case for branches at a manhole. d. D curve — loss due to incoming volume. O SPECIAL DETAILS The following structures shall be installed where re- quired: 1. Headwalls, aprons and cut -off walls at drain inlets and outlets. 2. Energy dissipators at outlets. 3. Debris — control structures. 4. Guard rails at headwalls and inlets, where they present a hazard to vehicular traffic or pedestrians. 4. OPEN CHANNELS O CHANNEL SIZE Use the Manning's Formula to determine the required waterway areas where uniform flow can be assumed. 1.486 R 2/3 S v2 Q - AV and V= n A = area of flow, in square feet V = velocity, in feet per second n = roughness coefficient ( Manning's) R = hydraulic radius, in feet S = slope of the energy gradient, in feet per feet The channel depth shall include design water depth and minimum freeboard allowances. Design water depth shall include rise in water surface caused by curves and junctions. OCHANNEL RIGHT -OF -WAY The channel width shall be sufficient to provide the required waterway area for the design storm as deter- mined bythese standards. The total right -of -way shall include a 15 -foot wide maintenance road along both banks where the top width of channel exceeds 50 feet, and along one bank where the top width is 50 feet or less. The maintenance road along the channel shall be topped with 6 inches of crushed coral or base course and treated with bituminous material. In lieu of a maintenance road, for normally dry channels, access ramps or other suitable alternative measures to facili- tate maintenance may be provided. (2) Where the downstream velocity is OPERMISSIBLE VELOCITIES AND "n" VALUES less than the upstream velocity, the Following is a list of "n" values for open channels 9