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duration. Since storm events in the Durham region have a mean duration of <br /> about 8 hours, the maximum duration of turbulent streamfiow conditions <br /> lg <br /> which may preclude settling of suspended pollutants is 12 to 16.hours. It <br /> Figure 3-2 illustrates the use of this method to develop pollutant delivery <br /> ratio contours for Lake Michie and Little River Watershed. The travel time <br /> and pollutant delivery ratio contours were based upon the mean storm event <br /> statistics (mean duration of about 8 hours and mean rainfall volume of 0.44 <br /> inches) . Channel slopes and cross-section characteristics were defined for <br /> the main stem channel system in each watershed. Based upon rational <br /> formula calculations for existing land use conditions, flow depths for the . . <br /> mean storm event were found to be on the order of one-third of the bankfull <br /> depth throughout the watershed. Based upon the Manning equation, stream <br /> channel velocities (typically 1.0 to 1.5 ft/sec) were calculated for <br /> specified reaches and travel time contours were developed for each <br /> watershed. As maybe seen in Figure 3-2, -maximum travel times to the _ <br /> reservoirs from headwater areas are on the order of 24 to 36 hours. <br /> Settling velocities for suspended pollutants were assumed to be on the <br /> order of 0.1 ft/hr (fine/very fine silt particles) . Under the assumption <br /> that turbulent streamflow conditions (i.e. , periods of no sedimentation) , <br /> occurred for 12 to 16 hours after the start of rainfall, sedimentation <br /> rates were calculated for transport from different sections of the <br /> watershed. The sedimentation calculations were based upon the ratio of <br /> depth settled by a particle following the turbulent flow period to the <br /> total depth of flow. For example, for a location 18 hours from Lake Michie <br /> which is subject to instream sedimentation for 6 hours (18 hours travel <br /> time minus 12 hour duration of turbulent flow) , the sedimentation rate <br /> (20%) is the ratio of settling depth, 0.6 ft (based upon 0.1 ft/hr times <br /> 6 hours) , to the average flow depth (3 ft) within a 6-hour travel time from <br /> the reservoir. The corresponding pollutant delivery-ratio for a 20% __._ _ ' <br /> - sedimentation rate is 80%. ----- <br /> The pollutant delivery ratios shown in Figure 3-2 were used to calculate <br /> average annual lake nutrient budgets for each reservoir. The delivered <br /> nutrient load was calculated_by multiplying the delivery ratio by the <br /> suspended fraction of the nutrient load and adding the product to the - - -- <br /> 3-21 <br />