ENGR 691, Fall Semester 2010-2011 Special Topic on Sedimentation Engineering Section 73 Coastal...
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ENGR 691, Fall Semester 2010-2011 Special Topic on Sedimentation Engineering Section 73 Coastal Sedimentation Yan Ding, Ph.D. Research Assistant Professor,
ENGR 691, Fall Semester 2010-2011 Special Topic on
Sedimentation Engineering Section 73 Coastal Sedimentation Yan
Ding, Ph.D. Research Assistant Professor, National Center for
Computational Hydroscience and Engineering (NCCHE), The University
of Mississippi, Old Chemistry 335, University, MS 38677 Phone:
915-8969 Email: [email protected]
Slide 2
Outline Introduction of morphodynamic processes driven by waves
and currents in coasts, estuaries, and lakes Initiation of motion
for combined waves and currents Bed forms in waves and in combined
waves and currents Bed roughness in combined waves and currents
Sediment transport in waves Sediment transport in combined waves
and currents Transport of cohesive materials in coasts and
estuaries Mathematical models of morphodynamic processes driven by
waves and currents Introduction of a process-integrated modeling
system (CCHE2D-Coast) in application to coastal sedimentation
problems
Slide 3
Near-bed Orbital Velocities Applying linear wave theory, the
peak value of the orbital excursion (A ) and velocity (U ) at the
edge of the wave boundary layer can be expressed as H = wave height
h = water depth = angular frequency = 2/T k = wave number
Wave Boundary Layer (2) z u UU ww The wave boundary layer is a
thin layer forming the transition layer between the bed and the
upper layer of irrotational oscillatory flow (Fig.). The thickness
of this layer remains thin (0.01 to 0.1 m) in short period wave (
1, van Rijn proposes to use
Bed-form Roughness Ripple-related roughness s = ripple presence
factor(=1.0 for ripples alone, = 0.7 for ripples superimposed on
dunes or sand wave Dune-related roughness Symmetrical Sand Wave:
The leeside slopes of symmetrical sand waves are relatively mild.
Hence, flow separation will not occur. Therefore, the form
roughness of symmetrical sand waves is assumed to be zero.
Slide 65
Example and Problems A wide channel with a depth h = 8m has a
bed covered with dunes. Ripples are superimposed on the dunes. The
dune dimensions are d = 1.0m, d = 50.0m. The ripple dimensions are
r = 0.2m, r = 3.0m. The bed material characteristics are d50 =
0.3mm, d90 = 0.5mm. What is the effective bed roughness, the
Chzy-coefficient, and the Mannings n? Solution: Grain roughness
(lower regime) : =0.0015m Ripple form roughness ( s = 0.7): =0.187m
Dune form roughness =0.303m Effective bed roughness:=0.492m
Chzy-coefficient: =41.3 m 1/2 /s Mannings n: = 0.0342
Slide 66
Wave-related Bed Roughness The effective wave-related bed
roughness also consists of two components: In which k s,w =
wave-related grain roughness height (m) k s,w = wave-related
bed-form roughness height (m) The wave-related friction factor (f w
) for rough oscillatory flow is Time-averaged over half a wave
cycle bed shear stress is
Slide 67
Wave-related Grain Roughness A number of empirical formulations
based on experimental and field data on non- movable and movable
bed. Van Rijns approach is introduced as follows: According to van
Rijn, the effective grain roughness of a sheet flow bed is of the
order of the sheet flow layer thickness or the boundary layer
thickness (k s,w w ). The sheet flow layer is a high-concentration
layer of bed material particles. Van Rijn (1989) proposed the
following values to calculate the grain roughness: inwhich for
friction factor in transition regime m = kinematic viscosity of
fluid-sediment mixture in near-bed region ( m 10) The grain
roughness equations have to be solved iteratively. Typically, this
approach yields a value in the range of 3 ~30 d 90 for = 1 ~
10.
Slide 68
Wave-related Form Roughness Ripples are the dominant bed forms
generated by oscillatory flows. Ripples may be present on a
horizontal bed or superimposed on large sand waves. Large-scale
sand waves have no friction effect on the water waves, because the
water waves experience the sand waves as a gradual bottom
topography. When the nesr-bed orbital excursion is larger than the
ripple length, the ripples are the dominant roughness elements for
the wave motion in the sea waters. Apparently, bed-form roughness
depends on the bed form height and length. There are a number of
empirical formulations for estimating the ripple roughness. They
can be described as Van Rijn (1989) proposed s = ripple presence
factor(=1.0 for a ripple covered bed, = 0.7 for ripples
superimposed on sand wave s Raudkivi (1988)
Slide 69
Bed Roughness in Combined Currents and Waves The most important
bed form regime created by currents and waves: Ripples in case of
weak (tidal) currents and low waves Sand waves with ripples in case
of (tidal) current and low waves Plane bed with sheet flow in case
of strong (tidal) currents and high waves (surf zone) Sand waves
with sheet flow in case of strong (tidal) currents and high waves
(outside surf zone) More complicated! No universal solutions
Slide 70
Grain Roughness (k s )in Combined Currents and Waves Grain
roughness is dominant for both the wave-related and current-related
friction when the bed is plane. When bed forms are present and the
peak orbital excursion at the bed is smaller than the bed form
length (i.e. A < ), the grain roughness is also dominant for the
wave-related friction. In that case the bed forms act as
topographic features for the waves. For wave motion: For current
motion: Note that the calculation of the mobility parameter for
current are different from that for wave motion
Slide 71
Form Roughness (k s )in Combined Currents and Waves When the
bed is covered with ripples, the ripple roughness is dominant for
the current-related friction. Ripple roughness is also dominant for
the wave-related friction when the peak value of the orbital
excursion at the bed is larger than the ripple length (i.e. A <
r ). The ripple roughness is calculated by When sand waves with or
without (mega or mini) ripples are present, the large- scale sand
waves act as topographic features for the waves motion because the
sand waves have a length much larger than the orbital excursion at
the bed. Thus, the wave-related friction factor is not determined
by the large-scale sand wave dimensions, but by the small-scale
ripples (if present) on the back of the sand waves.
Slide 72
Dune on Mars ? Three pairs of before and after images from the
High Resolution Imaging Science Experiment (HiRISE) camera on
NASA's Mars Reconnaissance Orbiter illustrate movement of ripples
on dark sand dunes in the Nili Patera region of Mars. Image Credit:
NASA/JPL-Caltech/University of Arizona/International Research
School of Planetary Sciences
