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Peter G. Griffiths

Publications and source records attributed to Peter G. Griffiths.

29 records · Page 2Linked to original sources
USGS2001

Techniques for estimating sediment yield of ungaged tributaries on the southern Colorado Plateau

Numerous regional sediment transport data are used to evaluate three techniques for estimating streamflow sediment yield from ungaged tributaries of the Colorado River in Grand Canyon. These techniques include: (1) a regression equation relating drainage area to sediment yield for all relevant sediment-yield data from northern Arizona, (2) an empirical relation developed by Renard (1972) selected from 8 potentially relevant methods, and (3) a new procedure that combines regional flood-frequency analysis with sediment-rating curves. Results based on techniques (1) and (2) are not significantly different. The third technique requires numerous assumptions, most notably that sediment yield on a decadal average can be described by several floods of recurrence intervals of 2 yr, 5 yr, and 10 yr described by regional flood-frequency relations. Using data collected at gaging stations, we develop a relation between peak discharge and total-event sediment yield derived from hydrographs and sediment-rating curves. This third technique produces sediment yield estimates comparable to those of the regional data regression and Renard (1972) relations and may be a more robust technique for estimating sediment yield when streamflow data are available.

Arizona↗
USGS2000

Sediment delivery by ungaged tributaries of the Colorado River in Grand Canyon, Arizona

Sediment input to the Colorado River in Grand Canyon, Arizona, is a valuable resource required to sustain both terrestrial and aquatic ecosystems. A total of 768 ungaged tributaries deliver sediment to the river between Glen Canyon Dan and the Grand Wash Cliffs (river miles - 15 to 276). The 32 tributaries between the dam and Lee's Ferry produce only streamflow floods, whereas 736 tributaries in Grand Canyon produce streamflow floods and debris flows. We used three techniques to estimate annual streamflow sediment yield from ungaged tributaries to the Colorado River. For the Glen Canyon and Marble Canyon reaches (river miles -15 to 61.5), respectively, these techniques indicate that 0.065 . 10 6 and 0.610 . 10 6 Mg/yr (0.68 . 10 6 Mg/yr of total sediment) enters the river. This amount is 20 percent of the sediment yield of the Paria River, the only gaged tributary in this reach and a major sediment contributor to the Colorado River. The amout of sand delivered ranges from 0.10 . 10 6 to 0.51 . 10 6 Mg/yr, depending on the sand content of streamflow sediment. Sand delivered in Glen Canyon is notably coarser (D 50 = 0.24 mm) than sand in other reaches (D 50 = 0.15 mm). A relation is given for possible variation of this sediment delivery with climate.

Arizona↗
USGS1999

Lava Falls Rapid in Grand Canyon: Effects of Late Holocene debris flows on the Colorado River

Lava Falls Rapid is the most formidable reach of whitewater on the Colorado River in Grand Canyon and is one of the most famous rapids in the world. Debris flows in 1939, 1954, 1955, 1966, and 1995, as well as prehistoric events, completely changed flow through the rapid. Floods cleared out much of the increased constrictions, but releases from Glen Canyon Dam, including the 1996 controlled flood, are now required to remove the boulders deposited by the debris flows.

Arizona↗
USGS1997

Reworking of aggraded debris fans by the 1996 controlled flood on the Colorado River in Grand Canyon National Park, Arizona

Debris flows from 600 tributaries in Grand Canyon periodically deposit poorly sorted sediment on debris fans along the Colorado River between Lakes Powell and Mead. Before regulation, stable fans and rapids along the river resulted from the interaction of tributary debris flows and large, mainstem floods. Floods in the Colorado River maintained fans and rapids as highly-reworked deposits of boulders and cobbles. After the closure of Glen Canyon Dam in 1963, decreases in stage and stream power associated with reduced annual peak flows drastically reduced the amount of debris-fan reworking. Previous research has shown that modest powerplant releases from Glen Canyon Dam, particularly in combination with tributary floods, can significantly rework aggraded debris fans. These limited flows have entrained boulders up to 1 mo in diameter, although discharges greatly exceeding the maximum powerplant release (946 m 3 /s) would be required to completely remove most aggraded fans. From 1987 through 1995, debris flows constricted the Colorado River at the mouths of at least 25 tributary canyons in Grand Canyon National Park, Arizona, creating 2 new rapids and narrowing at least 9 existing riffles or rapids. The highest peak discharge on the Colorado River in Grand Canyon between 1986 and 1996 was 960 m 3 /s in January 1993. In March-April 1996, we studied the effects of a 7-day flood release that peaked at 1,370 m 3 /s on 18 recently aggraded debris fans downstream from Glen Canyon Dam. The largest changes occurred at Badger and Lava Falls rapids, 38 and 312 km downstream from the dam, respectively; several other aggraded debris fans were only slightly changed. Areas of aggraded debris fans decreased by 2 to 42 percent; only the debris fan at Bedrock Rapid, which is controlled by a large bedrock outcrop, increased in area owing to deposition of reworked sediment on the downstream margin. Volumes decreased on 7 of 9 debris fans by 3 to 34 percent. The distal margins of most recently aggraded debris fans became armored with a lag of cobbles and boulders, and the width of the reworked zone on most debris fans increased by 4 to 30 m. Constriction of the river decreased at 11 of 18 debris fans, although some rapids, such as Tanner Rapid, became slightly more constricted at low discharges owing to changes in stage-discharge relations. Velocities on the left and right sides of Lava Falls Rapid decreased by about half, but velocities increased in three other rapids (e.g., Badger Creek Rapid). Stream power per unit width decreased in 9 of 10 rapids because of decreases in water-surface fall and widening of the rapids. Changes in the sizes of upper pool sand bars were inconsistent, although separation bars downstream from the reworked debris fan generally increased in size. The amount of stream power generated by the controlled flood greatly affected the variability of reworking among the 18 debris fans. For a given discharge, mainstem reworking is expected to vary with channel and debris-fan geometry, the initial particle-size distribution of the deposit, and distance from the dam. The elapsed time between debris flow and the controlled flood also was important because larger particles at older deposits became interlocked, imbricated, and (or) sutured together during smaller dam releases combined with tributary floods in January 1993 and August 1994. The effectiveness of future floods of similar magnitude in reworking debris fans will depend in part on the release history and extent of armoring in the period between the debris flow and the flood. If reworking of debris fans is a criterion for design of future controlled floods, our data support release of a higher peak discharge of shorter duration shortly after constricting debris flows occur.

Arizona↗
USGS1997

Debris flows in Grand Canyon National Park: Peak discharges, flow transformations, and hydrographs

Direct measurements of debris-flow hydrograph and flow behavior in remote drainage areas are rare. We infer hydrographs and flow behavior for recent debris flow in bedrock tributaries of the Colorado River from preserved stratigraphic relations, sedimentology and surface morphology of debris fans and evidence of flow-surface elevations. We propose that 3 types of debris-flow hydrographs occur in Grand Canyon: Type I flows have a single debris-flow peak followed by recessional 'hyperconcentrated flow' or streamflow; Type II flows have multiple debris-flow peaks with intervening 'hyperconcentrated flow' and (or) streamflow phases; and Type III flows begin as either Type I or Type II flows, but late-stage recessional streamflow is higher than the stage(s) of the debris-flow phase(s) and extensively reworks debris-flow deposits of buries them beneath streamflow sand and gravel. Field evidence shows that debris-flow peaks last for seconds to minutes, while recessional flows have durations of several hours to a day.

Arizona↗
USGS1997

Initiation of debris flows in tributaries of the Colorado River in Grand Canyon, Arizona

Debris flows are initiated in tributaries of the Colorado River in Grand Canyon when intense rainfall causes failures in colluvium and (or) bedrock. Most debris flows occur in the summer during localized convective thunderstorms with rainfall intensities as high as 40 mm/hr. Rarer and larger debris flows occur during unusually warm frontal storms in winter. Hourly precipitation data suggest that storms that cause debris flows terminate with a period of intense rainfall, a characteristic that complicates the use of daily rainfall records in assessing debris-flow hazard. Recurrence intervals for 1-day rainfall associated with 37 recent debris flows range from <1 to >50 years, with most <10 years. Recurrence intervals for the multi-day rainfall of storms associated with debris flows range from <1 to 158 years, but most were >10 years. The low recurrence intervals of debris-flow producing rainfalls, compared with the 10-50 yr recurrence intervals for most debris flows, underscores the co-dependence of debris-flow initiation on geologic factors, including bedrock type and antecedent soil-moisture conditions. The primary geologic factor influencing debris-flow initiation in Grand Canyon is the exposure of shale units at heights >100 m above the river. Exposed shale bedrock fails readily, either producing debris flows directly or contributing source material to wedges of colluvium that may fail later. Shales also provide silt- and clay-size particles that in part determine the rheological properties of debris flows.

Arizona↗