2021-09-03_PERMIT FILE - C1981019 (6)

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Page Exh. 25F-2 Revision Date: 6/16/21 Revision No.: TR-149 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, SEDCADTM was used to determine the design flow and appropriate culvert size. Table 1 provides a listing of all the culverts numbers and diameters for the Collom Haul Road. Locations of the culverts can be found on Map 25E Sheet 1. . The hydrologic methodology used for Collom Haul Road culverts is somewhat different than the methodology presented in Volume 2D, Exhibit 7. Runoff curve numbers assigned to the undisturbed portions of the Collom Haul Road watersheds are based on pre-mine vegetation communities presented on Map 4. Curve numbers (CN) utilized for the general culvert design for the undisturbed watersheds are as follows: Bottom Land – CN 57 Grassland – CN 57 Juniper/Scrub – CN 73 Sagebrush – CN 63 Mountain Shrub – CN 57 Curve numbers for disturbance areas were selected in accordance with Exhibit 7 in Volume 2D, Table 1. A representative SEDCADTM analysis is attached which provides a general culvert sizing for all culverts installed on the Collom Haul road. This design is based on a 10 year, 24 hour design event. The largest contributing watershed to an individual culvert is the watershed contributing to culvert 16. Please see Figure Exh. 25F-1 for the delineation of the watershed contributing to culvert 16. The design flow and corresponding size for culvert 16 is the basis for all the culverts under the Collom Haul Road.