Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries Report and Symposium Proceedings (1987) / Chapter Skim
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Session A: Sediment Sources and Transport Processes
Session A: Sediment Sources and Transport Processes
Pages 31-126
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From page 33... ...
Lawrence. Correspondingly, the annual amount of sediment discharge would be the water discharge multiplied by the sediment concentration, and it can be seen that these values would show even wider variations.
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From page 36... ...
In 1 percent of tar . J of year 10050 WAUR "" In hull ret FIGURE ~ Annual suspended-sediment discharge of three rivers showing the frequencies of suspended-sediment discharges within individual years and the importance of infrequent heavy storms in producing large sediment loads.
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From page 37... ...
To put this into perspective, the total suspended discharge for the Eel River was 168 million tons that year, which compares with the 184 million tons carried by the Mississippi River past St. Louis during the same year.
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From page 38... ...
composition and particle size at the mouth of the Amazon River, and shows a mean size of about 4 ~m with a logarithmic distribution about this mean (Gibbs, 1967)
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From page 40... ...
SOURCE: After Johnson and Kelley (1984) COAGULATION PROCESSES As the particles discussed in the previous section encounter the first traces of seawater in an estuary, the double layer around each particle is compressed by ions in the seawater (especially by di~ralent ions)
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The first coagulation process is thermal agitation (Brownian motion or molecular diffusion) in which the random motion of small particles is brought about by thermal effects.
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, which for turbulent flow can be expressed as (E/V) l/2 , where E is the total energy dissipated and V is the kinematic viscosity.
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The overall Delaware Bay coagulation process is depicted in Figure 9, which shows particle size as very small in the river (< 10 ~m) , increasing to about 150 Am at salinities of less than 1 ppt, then decreasing farther down the estuary.
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47 ~RIVER ~STATIC / 51 - - - -- it 5 ~8 ·716~-.- ~ 718t r ...i2~.\ ~ ~ q __p~ 18 JU 21 am,, \ ~22 15 ~ OCEAN ~TRAnON at)
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From page 50... ...
The primary difficulty with field research is the fragile nature of floes, especially when subjected to sampling and analytical techniques . However, additional research is needed on floe size distribution, settling velocity, and resuspension of coagulated material in estuaries if we are to adequately understand the mechanisms of transport, deposition, and resuspension.
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From page 51... ...
1985. Suspended sediment transport of the Gironde Estuary, France (Abstract)
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From page 55... ...
Sediment of size greater than 60 Am is considered to be coarse, and less than this size to be fine-grained. There appears to be some confusion in sediment transport-related literature between this mode of classification, and one that considers sediment as either cohesionless or cohesive.
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From page 59... ...
This type of settling occurs when the sediment forms a nearly continuous network through which pore water must escape slowly upwards for settling to continue. A high density suspension characterized by hindered settling is commonly referred to as fluid mud (Krone, 1962~.
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From page 60... ...
At higher values of CO (ranging from 300 - 10,000 mg/liter) , Ad ~ 0.078 N/m2 was obtained, indicating the influence of continued aggregation under turbulent flows on the settling process at higher concentrations.
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As the sediment continues to be brought closer together and the upward flux of pore water lessens, the weight of this near-surface sediment gradually turns into an effective stress (i.e., the difference between total hydrostatic pressure and pore water pressure) , which is transmitted by virtue of particle-to-particle contact.
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From page 62... ...
the susceptibility to erosion of a consolidating soil decreases with time due to the continual increase in bed shear strength, and (2) the accompanying density change in turn changes the flux of eroded sediment per unit bed thickness.
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From page 68... ...
Under waves, both longitudinal and vertical diffusion is typically much more significant in the near-bed layer than in the upper portion of the water column. Near the bed, particularly if it is soft, the wave boundary layer, wave-induced bed interface undulations and associated vortex shedding contribute to high diffusion rates.
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From page 70... ...
Density profiles Bed erosion resistance balances bed shear stress; influenced by the time-dependent bed structure: related to rate of deposition. Bed ~roper-~;~=s prevent passage of ship's hull concept and successfully applied to dredging management in some ports; relates to bed properties such as density or rigidity.
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From page 71... ...
description, which must therefore be characterized and simulated with a good degree of accuracy to avoid gross errors in prediction, e.g., of zones of sedimentation or scour. With regard to field data collection, particular attention must be paid to the need to obtain settling velocities in situ, as well as density profiles from the water surface down to depths at which a stable bed is encountered, i.e., bed which is not periodically eroded either under spring tides or under seasonally dependent episodic conditions.
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From page 73... ...
1978. Erosion rates of cohesive soils.
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From page 74... ...
1977. The physical characteristics and environmental significance of fine sediment suspensions in estuaries.
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The generally low energy in the Rhine estuary results in a generally low-concentration, suspended solids regime. Both estuaries are the sites of major ports with large mud maintenance dredging requirements.
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An indication of just how much more severe are the problems of investigating fine cohesive sediments can be gained by a brief comparison with noncohesive sediments. In cohesive sediment regimes visibility is often low or nil ; as a result, visual observation or
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From page 80... ...
Some stationary suspensions are detectable by echo sounders and are commonly known as "fluid mud" (Inglis and Allen, l9S79. Settled mud is deposited cohesive sediment in which individual particles are supported by the surrounding particle framework.
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From page 83... ...
The expense incurred in collecting this large data set is justified by the increased knowledge of the processes controlling cohesive sediment behavior, and by the new scientific phenomena that have been recognized. The concentrations of dense stationary suspensions are outside the calibration range of optical systems, and combinations of gamma backscatter or gamma transmission gauges' together with pressure sensors, must be used (Figure 4~.
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From page 85... ...
RESULTS OF STUDIES OF LARGE-SCALE PHENOMENA The most important feature of continuous monitoring is that, unlike discontinuous sampling, integrating, or averaging techniques, which appear to show smooth transitions in sediment concentrations, the realtime suspension structures are typified by discontinuous distributions in both the vertical and horizontal planes. In the S eve rn and Rhine estuaries significant proportions (- 60 percent in the S eve rn and - 30 percent in the Rhine)
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_ .. l 1 ; l Suspended solids mg 1- 1 FIGURE 6 Continuous vertical profile of suspended solids concentration versus depth in mobile suspension phase showing a typical layered profile in the Seve, `~ estuary.
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From page 87... ...
On spring tides at the maximum ebb and flood velocities in the Severn, cohesive sediment is entrained from stationary suspensions, with a uniform bed-to-surface concentration. As velocity decreases toward low or high water, settling commences high in the water column and one or more steps may be generated.
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From page 88... ...
One type are those regularly created due to the nonuniform rate of vertical mixing up from the bed on semidiurnal time scales. Vertical mixing on a semidiurnal spring tide results in
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From page 90... ...
JO ~ :~. ~7 o c I, i ;r ID to or -- 1 clue ~ cot to ID In c =~-.~+ o~ ~o of lo ~o ~ o -cz E ~ ~X E~ C
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From page 92... ...
They provide the first complete description of the evolutionary sequence of suspension structures from the time of entrainment, through the various stages of transport, until final deposition. They show why it is inevitable that in all areas experiencing rapid, fine sediment deposition and having a high maintenance dredging requirement, deposition can only arise in the presence of stationary suspensions, which can themselves only have evolved from dense, near-bed mobile suspensions.
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From page 93... ...
1974. Seabed density measurements related to echo sounder records.
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From page 94... ...
discusses some bottom boundary layer models and sediment models in greater detail. THREE-DIMENSIONAL HYDRODYNAMIC MODEL Various estuarine hydrodynamic models can be classified according to the following important physical features: 1.
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From page 96... ...
(3) where ~ is surface elevation, U is vertically integrated velocity, T is vertically averaged temperature, superscript denotes contravariant vector component, subscript denotes covariant vector component, H is total depth, gO is determinant of the metric tensor gij between the prototype grid xi and the transformed grid Yi, i denotes a covariant spatial derivative, !
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From page 99... ...
BOTTOM BOUNDARY LAYER MODELS Bottom boundary layer dynamics play a predominant role in affecting both the erosion and deposition of sediments. When the free-stream velocity at some distance above the bottom and the bottom roughness is known, bottom boundary layer models are needed to estimate bottom shear stress and other mean and turbulent flow variables.
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From page 105... ...
If one performs ensemble averaging on Eqs. S and 6 and further assumes that all particles completely follow the turbulent eddy motion, the following equation is obtained: sac + U ec _ W ac But D B2C S where C and c are the ensemble-averaged and fluctuating sediment concentration, respectively, U.'s are the mean fluid velocities computed from a three-dimensioJal hydrodynamic model, Ws is the settling velocity of the sediment particles relative to the fluid.
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From page 106... ...
Within each of the partially consolidated layers, an exponential erosion relationship was proposed. As an example, an erosion relationship determined from laboratory experiments for Mississippi Sound sediments (Sheng, 1983)
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~0 FIGURE 8 (a) Erosion rate of the Mississippi Sound sediments in salt water as a function of bottom shear stress.
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Sublayer Resistance The deposition of sediment particles depends primarily on the particle diameter and particle density. The effects of particle density can be incorporated into the particle size by increasing the equivalent particle size.
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From page 111... ...
FIGURE 10 Sublayer deposition velocity as a function of particle size for various q values (cm/sec)
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From page 114... ...
(20) where m is the mixing ratio, n is the number density, subscript f refers to the fine particle groups and subscript c refers to the coarse particle group, Nff ~ Nfc represents the loss of fine particles due to self-collisions among fine particles, Nfc and Mfc represent the production of coarse number density and mixing ratio due to selfcollisions among fine particles, Ncf and Mcf are the rate of loss of fine particle number density and the rate of production of coarse mixing ratio by collisions between coarse and fine particles.
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1968. ~ model of the bottom boundary layer in water waves .
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On the effect of suspended sediment Concentration on bottom boundary layer dynamics. Sheng, Y
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From page 122... ...
HYDRODYNAMICS Accurate computation of hydrodynamics is, of course, essential to accurate computation of sediment transport, but the hydrodynamic calculations are not covered here. STUDH, SEDIMENT8, and the dump models require that water levels and current velocities either be specified or computed by another model.
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From page 124... ...
The next phase will involve the physical model and SEDIMENTS in analyzing the salt wedge portion of the estuary, where three - dimens tonal currents and transport mus t be reproduced . RESEARCH NEEDS To improve numerical and hybrid modeling of fine sediment bans port, research is needed to provide a better description of the physical processes, to provide better input and verification data _ _ the models, and to provide better solution techniques.
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