HYDRO24135 - Laserfiche WebLink

Home Browse Search

bags and sediment deposited on dte forest floor above the diversion dam. The procedure ensured collection of neazly 100% of all sediment passing through treatments. The treatments were then compazed on the basis of the total exported sediment. Inflow and outflow runoff concentration data were analyzed in a paired sample design. Mean reductions between inflow and outflow concentrations were determined and used in analysis of vaziance (ANOVA). The hypothesis for this comparison was that treatments were equal in capacity [o fdter sediment-laden runoff. Mean reductions in runoff wncen[mtion and sediment yield were used as dependent variables for investigation. Independent variables considered in the ANOVA were treatment method, road grade, and mnoff volume. Where ANOVA indicated signifi- cant differences, individual treatment means were compazed using Duncan's Multiple Range Test. RESULTS Fifty-three sampling events were monitored during this initial phase of the study, which covered a period from 28 August 1997 to 27 December 1998. A sampling event consisted of an individual storm. Storm intensities ranged from 1.0 to 31.0 mm/hour with an average intensity of 4.4 mrn/hour during the lCrmonth study period (fig. 3). Inflow and outflow mnoff concentrations for each associated erosion control treatment were collected for each rain event. Sedunent deposifion was measured on four occasions during the study period by collecting sediment bags and measuring sediment deposited on the forest floor. 140 120 ~ 100 ~ 80 60 .Q .d 40 a 20 RUNOFF CONCENTRATION REDUCTIONS The reductions in runoff concentration deviated from normality and were normalized with a logarithmic trans- formation. ANOVA detected significant treatment effects (p> 0.001) in the concentration reductions. No signif-scant differences were detected in concentration reductions be- tween vegetation, sediment fence, and settling basin treat- ments (table 2). Settling basin and rip-rap treatments were statistically similaz in capacity to filter sediment laden mnoff. The settling basin and rip-rap treatments were less effective than other treatments a[ filtering sediment during large storm events (>40 tnm). Runoff during these storms exceeded the design capacity for the settling basins and flushed trapped sediment from the rip-rap treatment. The settling basin was 100% effective on all but 10 events, but yielded a mean reduction of only 85 over the study period. TOTAL ERPORTED SEDA~NT Sediment export was expected to be least below treat- ments with the highest reductions in runoff concentrations. No significant treatment differences in mean sedunent export or runoff volume were found during ttte 16-month study period (table 3). Sediment export for vegetation, sediment fence, rip-rap, and settling basin treatments were 43.4, 43.0, 30.1, and 23.0 kg, respectively. In an attempt to obtain a better understanding of the sediment export data, a detailed look was taken at the two components measured: sediment deposited on the forest floor, and sediment collected in sediment bags. On average for the four treatments, sediment deposited on the forest floor accounted for more than 93% of total sediment export for all four treatments. Sediment collected in sediment bags ^ 5 4, ^^ ^e ^a ,lry tih ti0 ,~^ ~°' ~~ eP e'? e~ a9 bti Storm Event # ~ Precipitation ®Stonn Intensity Figure 3. Oluerved predpifadon and intensity at 1Lskegee site during study period. 0 5 ~ t 10 £ 15 ~ 20 M C 25 30 E 35 y 40 TRANSACRONS OP'ni8 ASAE