2019-09-20_PERMIT FILE - C1981019 (5)

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Jubb Creek Crossings For the Jubb Creek Culvert, the upstream drainage area consists of the combined streamflows from both the West Fork and East Fork of Jubb Creek. Map 12B shows the boundaries of this drainage basin. At the proposed crossing point, the upstream drainage area is about 4,665 acres (7.29 square miles). Rule 4.03.1(4)(e) requires that crossings be designed for a 10 year-24 hour event if the average end area of the culvert is expected to less than 35 square feet (an 80-inch diameter culvert). Nevertheless, for a more conservative approach, the culvert at Jubb Creek has been designed for the 25 year-24 hour event. Trihydro predicts the peak flow from the 25 Year storm to be 131 cfs and the total runoff volume to be 70 acre-feet(see Figure 3). Figure 4 presents a generally accepted FHWA nomograph for flow through culverts operating under the entrance control condition. The nomograph is marked up to illustrate acceptable pipe diameters for the Jubb Creek crossing. Acceptable solutions have been limited to diameters that can deliver the design flow with a Headwater/Diameter(HW/D) ratio of 1.5 or less. At Jubb Creek, Figure 4 indicates that a single 5-foot diameter culvert can pass the 131 cfs design flow with a HW/D ratio of 1.3. A 60 inch diameter culvert was included in the final design. General Culvert Design For the remaining smaller culverts under the Collom Haul Road SEDCAD was used to determine the design flow. Table 1 provides a listing of all lengths, diameters, and drainage areas of the culverts involved. As noted, all of the culverts under the Collom Haul Road have been specified to be 36 inches in diameter except where otherwise noted on Table 1 and 2. The larger size is mainly for road maintenance reasons, as the hydraulic design would typically allow a smaller diameter culvert. This design will ensure better results should future cleaning or maintenance be required. Culverts numbered 5, 6, 53, 54, and 55 are specified as 24 inch diameter and associated with minor drainages and road crossings not under the main Collom Haul Road. Table 2 provides a listing of the various vegetation communities and associated curve numbers for each structure's drainage area. These vegetative communities are shown on Sheet 2 of Map 25E. As noted in Table 2, most of the culverts also have some contribution from direct runoff from the road surface and other disturbed areas as well. Four representative SEDCAD analyses are attached. These are all based on a 10 year, 24 hour design event. By inspection, it can been seen that the largest runoff volumes from these 29 small watersheds would be expected to occur at culverts 01, 10, 14, and 34, as these have the largest drainage areas or associated disturbed areas. For this reason, only these four watersheds were modeled. As shown of the attached SEDCAD analyses, the culverts 01, 14, and 34 installations show that a 15 inch culvert is suggested, while the culvert 10 installation recommends a 6 inch culvert. Exh. 24-1-2 Revision Date: 7/30/19 Revision No.: TR-133