Evolution of coarse gravel bed forms: Field measurements at flood stage

Abstract

Field measurements to investigate the origin and growth of mesoscale gravel bed forms in deep flows were made in the North Fork Toutle River, Washington. Sonar observations of the gravel streambed at a stationary point were recorded during two storm flows in December 1989 and January 1990 with concurrent bed load sampling and continuous velocity measurements. Mean diameter of bed load was about 3 cm, flow depths were 1.4–2.4 m, and bed shear stresses were 2–5 times the critical stress of mean bed load diameter, as computed from the depth‐slope product. These records document the hydrodynamic conditions under which dunelike coarse gravel bed forms were observed. Coarse gravel dunes (height, 20 cm; length, 6–15 m) evolved more than 24 hours after peak stage, primarily by accretion, as inferred from bed form changes revealed in dual sonar records. Dune heights increased to 40 cm as mean trough elevation rose about 50 cm over several hours. Smaller dunes (wavelength, 1–3 m), transitional from bed load sheets, migrated on the backs of the large dunes. The superposed dunes finally became indistinguishable from the large dunes, which diminished in height by increasing the mean level of troughs. Gravel deposition occurred at the observation point in conjunction with migration of gravel dunes. The direct comparison of known bed form regimes and gravel bar facies provides alternative interpretations of gravelly deposits.

Bedform movement recorded by sequential single-beam surveys in tidal rivers

“…Bedforms still migrated and distorted too rapidly at high discharge to allow signi®cant correlation of pro®les, even with only 4 days between surveys. Repeated bedform surveys on a single day or continuous depth soundings at a stationary point can improve estimates of bedform celerity for transitional regimes (Dinehart, 1992). As described earlier for Eq.…”

“…However, bedform-transport rates may not be directly applicable to bedload-transport questions. The reliability of bedload-transport rates derived from bedform migration has been evaluated in¯umes and natural channels extensively (Hubbell, 1964;Simons et al, 1965;Crickmore, 1967Crickmore, , 1970Engel and Weibe, 1979;Engel and Lau, 1980;van den Berg, 1987;Kostaschuk et al, 1989;Dinehart, 1992). These studies have found a narrow range of equivalence between bedform-and bedload-transport rate that limits general use of the method.…”

“…If two bed elevation records are collected simultaneously, but separated by a streamwise distance on the order of the bedform length, then a bedform speed, or celerity, is calculable from the two records (Crickmore, 1967;Willis, 1968). This method is especially suitable for¯umes (Engel and Lau, 1980) and for rivers with high transport rates (Dinehart, 1992). It is less applicable on wide channels because the observed points at near-bank installations may not represent the rest of the bedform ®eld.…”

Sediment transport at gaging stations near Mount St. Helens, Washington, 1980-90. Data collection and analysis

“…Velocity profiles were measured with three vertically mounted meters throughout a range of discharge conditions for up to 8 hours an episode. Records of this type showed velocity pulsations of several minutes duration (Dinehart, 1992a).…”

“…Wavelengths of gravel bedforms were at least as long as the platform. Transport rates of gravel bedload, sampled adjacent to the platform during stationary measurements of bed elevation, corresponded with bedform migration (Dinehart, 1992a).…”