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April, 2004 <br />4.0 HYDRAULIC DESIGN <br />A nomograph study was applied to size the culverts and to determine the number of <br />barrels necessary to pass the design flow of 500 cfs. A preliminary examination of the <br />existing topography for the stream bed indicated that an upstream invert elevation of <br />4933 could be achieved with minor excavation along with a downstream invert elevation <br />of 4932. These inverts returned a slope of 0.015 ft/ft for the geometry and pipe length <br />described above. Visual inspection of the existing channel width indicated that 6-48" <br />diameter pipes could fit into the opening. <br />The design Q (500 cfs) was split between the six barrels with each barrel having an <br />individual design Q of 83 cfs. Figure 46, Culvert Capacity for 48-inch Diameter Pipe <br />was entered at the culvert discharge of 83 cfs. A vertical line was drawn to intersect <br />both the inlet control line as well as the outlet control line for a culvert 65-feet long. <br />Headwater depth was determined for each type of control. Since headwater depth was <br />larger for the inlet control pipe, the flow is considered inlet controlled. Six (6) culverts <br />with a diameter of 48" will pass the design Q of 500 cfs with a headwater depth of 4.1- <br />feet. In other words, the culverts would be completely inundated with a water depth <br />0.1-foot above the pipe crown. <br />Further analysis of Figure 29, Headwater Depth for Circular Pipe Culvers with Inlet <br />Control, shows that when the Boulder Creek water surface reaches elevation 4939, <br />(HW/D = (4939-4933)/4 = 1.5) the discharge through a single barrel will be <br />approximately 112 cfs or 672 cfs for the entire structure. If the water surtace of Boulder <br />Creek was to rise to the level of the roadway surface (elev. 4940) without failure of the <br />embankment structure, the discharge through a single barrel could reach approximately <br />130 cfs or 780 cfs for the entire structure. If the water surface elevation in Boulder <br />Creek rises to the overtopping elevation of 4940, failure of the low water crossing <br />structure is imminent and backwater effects due to the structure would be limited to the <br />Turnpike Pit property. Damages to property upstream of the low water crossing <br />resulting from the low water crossing being in place shall be the full responsibility of <br />Asphalt Specialties. <br />Turnpike Pit Low Water Crossing Design Report <br />Page 4 of 6 <br />