WSP00684 - Laserfiche WebLink

Home Browse Search

debris fans was reworked by dam releases between 1966 and 1996. Reworking of these debris fans occurred during three dam releases that created discharges in Grand Canyon greater than powerplant capacity of 890 m3 Is. Historically, Glen Canyon Dam has released one significant discharge above powerplant capacity per decade. Therefore, we assume that for the post-dam era, the average volume reduction of aggraded debris fans is 25 percent per decade. Sediment-Yield Model The expected value of total annual sediment yield by debris flow for a given tributary is estimated as; E[Q,,qI=0.02'F'V(A)'R, (12) where E[Q,4jl = the expected value of annual sediment yield from debris flow, F = the frequency factor (the expected number of debris flows per century), ViA) = the maximum (V nlQ.,) or average (Va.g) volume-<lnveloping curve, R = the fraction of debris fans reworked by the Colorado River (either I orO.25/decade), and 0.02 is a volume-to-mass and century-to-annual conversion factor. Debris-flow occurrence varies considerably from year-to-year, both in terms of numbers of events and the volume of sediment delivered (table 14, fig. \3). The expected value of debris-flow sediment yield is computed using a conversion factor to convert the frequency information, which has a temporal unit of per century, to an annual unit that is compatible with streamflow sediment yield. Debris flows contribute between 141,000 and 295,000 Mg/yr of sediment to debris fans in Grand Canyon (table 17). Reach B (Marble Canyon) contributes the greatest amount of debris-flow sediment, which is consistent with both the empirical observations on where historical debris flows have occurred in the last century (fig. II) as well as the mapped distribution of logistical probabilities in Grand Canyon (Griffiths and others, 1996). Depending on the volume model used and the amount of debris-fan reworking, computations indicate that debris flows yield 6,440 to 53,700 Mg/ yr of sand to the river corridor. The debris-flow sediment-yield model requires a number of important assumptions. We assume that all debris flows from a given tributary are the same size, which means our model does not realistically depict a true magnitude-frequency relation. The only magnitude-frequency relation for Grand Canyon debris flows is for Prospect Canyon, a tributary that arguably is not representative of most of the ungaged tributaries (Webb and others, I 999b). The sediment-yield model produces an expected value of debris-flow sediment yield; therefore, extreme events not included in our historical record are not accounted for and small events are inadequately represented. Some of these problems could be resolved using a fully stochastic model of debris-flow frequency, but objectively detennining model constraints based on the limited data from the ungaged tributaries would be difficult. TOTAL SEDIMENT YIELD OF UNGAGED TRIBUTARIES We combined sediment-yield estimates for streamflow (data regression equation; table 6) and debris flow (table 17) to estimate total annual sediment yield from the ungaged drainage areas (table 18). Depending upon the assumptions of the debris-flow sediment-yield model (whether V ma.t or Va.g is used), sediment yield by debris flow ranges from about 4 to 23 percent of total sediment yield. The total sediment yield is highest in Reach F (river miles 156.8-226.6), reflecting the high streamflow sediment yield. The percent contribution of debris- flow sediment yield is highest in Reaches B, C, and o because of the high frequency of debris flows in those reaches (Griffiths and others, 1996). We calculated a range of possible sand yields given the assumptions of percent sand content (15, 50, and 75 percent) and debris-flow volumes (V ma.t or Vavg) and report low, average, and maximum sand delivery by the ungaged tributaries (table 18). We then added the variable of reworking of debris fans to simulate the impacts of Glen Canyon Dam on sediment storage in debris fans. The sand delivery rate from fully reworked debris fans, which reflects pre-dam conditions, averages about 1.3'106 Mg/yr for all ungaged tributaries. In Reaches A, B, and C (Glen and Marble Canyons, TOTAL SEDIMENT YIELD OF UNGAGED TRIBUTARIES 33