Principal Investigator: William C. Little (Georgia Institute of Technology)
Principal Investigator: Paul G. Mayer (Georgia Institute of Technology)
Sponsor: GWRI
Start Date: 1972-05-01; Completion Date: 1972-05-01;
Keywords:
Description:
The objective of this study was to systematically investigate the effects of sediment gradation on channel armoring. The primary variable was gradation of the sediment material. A geometric mean diameter of 1.00 millimeter was used for all sediments with geometric standard deviations, ago, of 1.12, 1.50, 2.00, 2.50, and 3.00. These mixtures of crushed quartz were placed in a recirculating flume with a sediment bed 1.97 feet wide, 40.0 feet long, and approximately 0.15 foot deep.
Arbitrarily selected flows, to purposely induce armoring, were kept constant throughout each experiment. Bed slope was initially set at 0.002. Depth of flow was held constant throughout each experimental run by a sluice gate. The armoring process was considered to be stable when the final
sediment transport rate was not more than one percent of the initial transport rate. The surface layer of particles was then removed by the wax method and the distribution of the armored particles determined. An empirical equation was developed through dimensional analysis relating the sediment properties of the original and armored distributions to the flow properties, when the sediment bed had become stable because of the armoring process.
The equation is
[3 -I 0.353u*c = 0.908 v(s-l)g dgo ° go (1)
where dga and dgo are geometric mean diameters of the armored and original sediment mixtures, respectively, σgo is the geometric standard deviation of the original sediment mixture, u* is the bed shear velocity defined by u* = gRS where g is the acceleration of gravity, R is the hydraulic radius and S is the slope of the energy grade line, ν is the kinematic viscosity of the water, and s is the specific gravity of the sediment (2.65). From the equation, for the given flow conditions and original sediment properties in a channel, the geometric mean diameter of the armored material, dga, can be calculated. The armored diameter calculated from this equation is applicable only if the channel would armor.
Another criterion was developed to determine, for the given sediment and flow properties, if the sediment bed could armor. If the calculated geometric mean diameter of the armored surface material was between the d05 and d95 (that size for which 5 and 95 percent, respectively, by weight is finer) size of the original material, the original bed material would armor for those flow conditions.
With the broadly graded materials, σgo >= 2.00, where channel armoring occurred, the bed degraded uniformly in depth along the length of the bed. However, for uniform materials, σgo < 1.50, little or no armoring could be induced, and the bed did not degrade uniformly in depth but degraded more at the beginning of the reach and less at the outlet end resulting in a reduced bed slope.
Dunes formed initially and as they moved off, armoring was immediately observable. Armoring of the surface had no significant change on the average bed shear stress throughout the armoring process.
After an armor coat had developed, a very low sediment transport rate continued for long periods of time. This transport was by local scour of fine material around larger particles. Fine material could be observed hiding in the wake or zone of separation of the large partides. Turbulence caused shifting of the zone of separation and sporadic movement of fine material. The results of this study were compared with the calculated armored distributions by a method developed by Gessler. The geometric mean diameters calculated by Gessler’s method were consistently lower than the measured values of this study and differed from 3.5 to 29.0 percent with an average difference of approximately 20 percent.