2015-10-06_PERMIT FILE - C1981019A (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. Pipe ends mitered to conform to the slope have been assumed, and the pipe will have riprap placed at the sides and inlet to a height 4 feet about the top of the crown. 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. General Culvert Design For all of the remaining smaller culverts, Culverts A through T, SEDCAD was used to determine the design flow. Tables 1 and 2 provide a listing of the relative percentage of various vegetative communities and associated drainage areas for each culvert. 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 as well. Two representative SEDCAD analyses are attached. These are both based on a 10 year, 24 hour design event. By inspection, it can been seen that the largest runoff volumes from these twenty small watersheds would be expected to occur at culverts D and N, as these have the largest drainage area and somewhat higher curve numbers. As shown of the attached SEDCAD analyses, the D culvert installation shows that a 10 inch culvert is suggested, while the N installation recommends a 24 inch culvert. All other culvert installations would be expected to predict similar or smaller culvert installations. As a conservative design element, all culverts A through T will be 36 inches in diameter. This size will ensure better results should future cleaning or maintenance be required. Culvert installation lengths will vary as shown on the plans but typically range from 165 feet to 335 feet. All culverts shall have riprap erosion protection at the outlet points. All culverts shall be installed at a minimum barrel slope of 1.5 percent. Exh. 24-1-2 Revision Date: 09/21/15 Revision No.: TR -106