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<br />Page 14 <br /> <br />The channel or conduit flow time (t) is to be determined from <br />the velocity of flow computed for tbe hydraulic properties of <br />the channel, ditch, gutter, pipe or sewer. Manning's equation <br />for channel flow is useful for these calculations. Figure 4 <br />may be used to determine the velocity in the following formula <br />for tt: <br /> <br />tt = <br /> <br />L <br />~ <br /> <br />Where: <br />L = distance of flow in hydraulic structure (ft) <br /> <br />v = velocity of flow (fps) <br /> <br />For grassed swales or shallow gutter flow, Figure 5 may be used <br />to determine t . <br />t <br /> <br />b) Colorado Urban Hydrograph Procedure (CUHP) - Runoff for basins <br />larger than 130 acres must be determined using the Colorado <br />Urban Hydrograph Procedure. This procedure is detailed in <br />USDCM. The incremental design rainfail for use with the CUHP <br />method is given as Table IV. Figure 6 may be used for effec- <br />tive rainfall computation. <br /> <br />4) Offsite Flows: Flows entering the proposed development from outside <br />the property are offsite flows. The offsite storm runoff shall be <br />determined and included in the drainage system design. Available <br />drainage reports for offsite developed areas affecting the property <br />shall be reviewed and considered in the drainage system planning and <br />design. <br /> <br />Runoff entering the site from offsite shall be computed using runoff <br />coefficients based upon existing development or, for undeveloped <br />land, based on values in Table III for offsite flow analysis, which- <br />ever is greater. The initial system shall be designed for this <br />runoff rate or Q=.2A, whichever is greater. <br /> <br />5) Irrigation Ditches: Irrigation ditches frequently intercept natural <br />drainage. Although they may be adequate to convey the discharge <br />from frequent storms, this ability is dependent upon the operation <br />of control structures. Thus, a storm drainage system that relies on <br />irrigation ditches to convey storm runoff may not adequately convey <br />even the initial storm under all conditions of operation. Consider- <br />ing an irrigation ditch as part of the drainage system may result in <br />redirection of storm waters possibly resulting in damage to down- <br />stream property owners. For the purposes of routing the initial and <br />major storms, irrigation ditches are to be assumed flowing full at <br />all sections, and to not intercept any storm drainage. Wherever <br />possible, stormwaters are to be diverted from irrigation ditches. <br />