WSP05649 - Laserfiche WebLink

Home Browse Search

TOPPfNG ET AL: COLORADO RIVER SEDIMENT TRANSPORT, 1 in which sand would generally accumulate over 9 months and erode during 3 months of a year 10 one in which substantial storage of newly input sand cannot be demonstrated for more than 2 months per year. Appendix A: Estimation of Ungaged Tributary Sediment Input We estimated 1he monthly sediment loads of the ungaged tributaries by a two-step process. First, a long-term supply rate of 7.2 million t per decade for the ungaged tributaries between the Lees Ferry and Grand Canyon gages was estimated for the period of sediment-data overlap in the Paria and Little Colo- rado Rivers (i.e., sediment years 1949-1970). This value is equivalent to an annual fine-sediment yield of 220 t/km' (t indicates metric ton) and was determined by Griffiths el al. (1998] using the method of RerumJ (1972]. On the basis of USGS sediment-transport data and a 20% uncertainly, the annual sediment yields for the Paria and Uttle Colorado River basins are respectively 780 :!: 160 and 130 :!: 26 t/km'. Because the geology in the ungaged tributary basins is more similar to that in the Little Colorado River basin than that in the Paria River basin and because the fine-sediment yield of the Paria River basin is known to be amongst the highest on the Colo- rado Plateau, the fine-6Cdiment yield of 220 t/km2 computed by Griffiths el al. [1998J is appropriate. After determination of the long-term supply rate, because the hydrology of the ungaged tributaries is somewhat similar to that of the Pari3 and Little Colorado Rivers, the fine-sediment load of the ungaged trib- utaries each month within sediment years 1949-1970 was set as proportional to the combined monthly fine-sediment load of the Paria and Little Colorado Rivers. On the basis of the comparison of predictions of fine- sedimeTIt yield by the Renard [1972] method and field data from the Little Colorado River basin [Griffiths el ai., 1998], the uncertainly associated with the estimate of the fine-sediment loads in the ungaged tributaries was determined to be approx- imately a factor of 3. Though this approach may 6Cem primi- tive, the sediment budget is not extremely sensitive to the uncertainly in the estimated fine-sediment input from the un- gaged tributaries because the mean-aonual input from this source (i.e., 0.72:!: (a factor of 3) million t/yr) is comparable to the mean-annual uncertainty associated with the measured fine-sediment inputs. For example. given a 5% uncertainty in the load of tbe Colorado River at Lees Ferry and a 20% uncertainty in the loads of the Paria and Little Colorado Riv- ers, the mean-annual uncertainty associated with the measured inputs is 5.3 million t/yr during the predam era and 2.3 million tlyr during the postdam era. Appendix B: Sources of Sediment-Load Measurement Error There are six major sources of measurement error inherent in the daily sediment loads used to construct the sediment budget for Marble Canyon and upper Grand Canyon. The first and most easily quantifiable source of error in daily sediment loads arises from the computation of the discharge of water. This error is greatest in rivers with poorly defined stage- discharge rating curves, like the hydrologically flashy Paria River. For the main stem Colorado River the rating curves and shifts in ratings were well defined; thus the discharge errors reported by the USGS were only 2% (unpublished USGS 539 annual technical files from the Lees Ferry and Grand Canyon gages, 1948-1970). For the Little Colorado River the dis- charge errors reported by the USGS were typically 5% (un- published USGS annual technical files from the LCR near Cameron gage, 1948-1970), though the discharge errOrs ex- ceeded this during higher flows. However, for the Pacia River, because of a general lack of discharge measurements at higher flows. the errors were much larger and greatly exceeded 10% during periods of high flow. During the period for which a sediment budget can be constructed (October 1947 through September 1970), 50% of all discharge measurements on the Paria River were made at flows less than 0.45 m'/s, 97% of all discharge measurements were made at flows less than 15 m3/s, and the highest discharge measurement was made at a flow of 76 m'/s [after Topping, 1997J. During October 1947 to Septem- ber 1970, 13 floods had peaks higher than the highest discharge measurement made duriDg this period (Topping, 1997]. Be- cause of these problems, the discharge errOrs reported by the USGS for the Paria River were typically 5% at 80ws less than 14 m'/s and were greater than 8% at higher flows (unpublished USGS annual technical files from the Paria River Lees Ferry gage, 1948-1970). Prior to the first use of the modem slope- area method for estimating the peak discharge during a flood (method described by Dalrymple and Benson [1967]) in Sep- tember 1963, errOrs in Paria River peak 800d discharges some- times exceeded 50% [Topping, 1997]. The five major sources of error related to determining sed- iment concentration are much harder to quantify and are typ- ically larger than those associated with computing the dis- charge of water. The first of these is best tenned a "sediment- station location error." This error has to do with whether a suspended-sediment measurement station is in a good location to measure sediment loads. In the Grand Canyon region this source of error was significant (but hard to quantify) only on the Little Colorado River (LCR), where discharges and sus- pended-sediment concentrations were measured at different locations. On this river, discharges (and ultimately 6Cdiment loads) were computed a1 the LCR near Cameron gage, but suspended-sediment concentrations were measured upstream at the highway 89 bridge at Cameron, Arizona (Figure 1). Importantly, a major sediment supplier to the LCR, that is, Moenkopi Wash, enters the LCR downstream from the sus- pended-sediment measurement station and upstream from the gage. The limited number of USGS suspended-sediment mea- surements made on Moenkopi Wash indicate that in similar flows, sediment concentrations in this stream are typically a factor of 5 or more higher than in the LCR. Indeed, in some years, more of the sediment supplied to the Colorado River by the LCR is from Moenkopi Wash than from the LCR above Moenkopi Wash [e.g., Graf el al., 1996, Figure 25]. Memo- randa circulated between the USGS and the Bureau of Rec- lamation (the agency who provided funding for the sediment- load measurements on the LCR) in 1949 indicate that the USGS was aware of this problem but because of funding lim- itations could not add a daily sediment station on Moenkopi Wash (unpublished USGS permanent technical file from the LCR at Cameron water-quality station (located at the highway 89 bridge at Cameron)). Therefore, during October 1947 through September 1970, the USGS attempted to account for (he sediment inputs from Moenkopi Wash either by (1) sub- lracting the discharge from Moenkopi Wash and using the concentrations as measured at the LCR at Cameron, Arizona, water-qu::llI1Y station or (2) increasing the measured COllcen-