Laserfiche WebLink
<br />e <br /> <br />Design Flow Rate <br /> <br />~ <br /> <br />The design flow rate for roadside and median drainage channel <br />linings is usually one having a 5- or IO-year recurrence <br />interval. If a vegetative lining is feasible, and a temporary <br />lining is to be used during the establishment period, a lower <br />recurrence interval flow might be considered for the design <br />of the temporary lining than for the vegetative lining. <br /> <br />i <br /> <br />This is because the risk of damage to the temporary lining <br />during this relatively short time is quite low, and if the <br />lining is damaged, repairs are usually inexpensive. <br /> <br />Selection of the design flow rates should be based on analysis <br />of the cost of the lining and the damages incurred should the <br />channel lining fail. If available channel linings are found <br />to be inadequate for the selected design flow rate and inlet <br />spacing, it may be feasible to decrease the inlet spacing to <br />intercept more flow and reduce the flow rate to,a manageable <br />level. <br /> <br />e <br /> <br />Channel Bends <br /> <br />Flow around a bend in an open channel creates secondary <br />currents which impose higher shear stresses on the channel sides <br />and bottom than are found in straight chann<<ll reaches. According <br />to Anderson (6), the location of the maximum shear varies to such <br />an extent that it is not possible to define the exact points <br />where additional erosion protection is required. <br /> <br />. <br /> <br />In order to protect the bend against scour, it may be necessary <br />to use another lining material or, if rock riprap is used, adjust <br />the rock size. To determine the modifications necessary, a <br />correction factor varying linearly from I to 4 as a function of <br />v2/~ (Chart 33) is applied to the design depth of flow in the <br />straight channel reach to obtain an adjusted depth of flow, <br />dadj. V is the mean upstream channel velocity and Rd is the <br /> <br />average radius of the outside bank of the bend as shown in <br />Figure 2. <br /> <br />" <br /> <br />e <br /> <br />11 <br />