Laserfiche WebLink
The revised drainage calculations for the permanent Coal Gulch Channel are <br />• included in the appendix to this exhibit. The revised drainage area, slope, <br />curve numbers and runoff calculations were used in the design of the permanent <br />channel. Four (4) segments were evaluated for determining depth of flow, <br />velocity and subsequent riprap requirements. The four segments correspond to <br />uniform slope segments as identified on Map 16, Coal Gulch Permanent Channel <br />Profile. <br />In designing the channel, a two (2) stage configuration was chosen as the most <br />feasible for carrying both the 10-year and 100-year recurrence interval flow <br />events. Due to the steep slope conditions and restricted location for <br />positioning the channel, a riprapped channel was necessary. A uniform bottom <br />width of six (6) feet was chosen for use along the entire channel length, <br />after both narrower and wider widths were evaluated and determined less <br />feasible based on depths and amounts of riprap required. <br />The design for the 10 year event with one foot of freeboard can pass the 100 <br />year event. Although no freeboard is included in the 100 year design, <br />adjacent overbank areas can be utilized for large flow events. These overbank <br />areas will be revegetated following regrading. The majority of overbank flow <br />will occur along the east bank where slopes adjacent to the channel (3H:1V) <br />are sufficient to contain the flow within a relatively narrow flood plain. <br />All of the 100 year flow in the steepest slope segment (Segment 3, 14.5%) will <br />be contained within the riprapped channel. By utilizing the vegetated <br />overbank flow area, a stable channel for both frequent and infrequent events <br />results which uses a minimum of riprap and channel width. <br /> <br />Revised 9/12/86 <br />11 <br />