coastal engineering

1. Onshore offshore sediment transport

2. Offshore transport
Movement of sediment or water away from the shore.
Occurs during the storm events
Onshore transport
Movement of sediment or water toward the shore.
Dominate during mild wave activity
Onshore-offshore transport
Transport perpendicular to the shore

4.  To predict the environmental quality and impact
 To understand the effect on habitat stability
 To be aware of public health risks
 To prevent marine hazards such as ship grounding
 To maintain a sound access to ports
 To comprehend the siltation of harbours, infill of reservoirs and
artificial lakes
 For coastline protection.
Study about the sediment transport processes are
important

5.  Beach and dune response to storms
 Equilibration of a beach nourishment project that is placed at slopes steeper
than equilibrium
 Shoreline response to sea level rise
 Seasonal changes of shoreline positions, which can amount to 30 to 40 m,
 Overwash, the process of landward transport due to overtopping of the
normal land mass due to high tides and waves
Scour immediately seaward of shore parallel structures
The three-dimensional flow of sand around coastal structures
Cross-shore sediment transport is relevant to a number of coastal
engineering problems,

8.  Changes in water level
 Tides
Waves
 Currents
Stream outflow
Processes responsible for initiating sediment
transportation

9. Within the surf zone, cross-shore transport may be
predominant due to sediment in suspension.
Dean (1973) noted that suspended sediment can move
either onshore (constructive) or offshore (destructive),
depending on how high a sand grain is suspended off the
bottom.
Gravity is the most obvious destructive force, acting
downslope and in generally seaward direction for a
monotonic profile
Often during major storm events, strong onshore winds will
be present in the vicinity of the shoreline. These winds
cause a shoreward-directed surface flow and a seaward-
directed bottom flow Destructive and constructive
forces for sediment movement

10. Coastal response
• The areas, most directly affected by the forces of the sea are the
beaches, the gulfs, and the nearshore regions that experience the full
impact of the sea’s energy.
• Normal condition prevails most of the time, and the wave energy is
easily dissipated by the beach’s natural defence mechanisms.
• When storm conditions generate waves containing increased amounts
of energy, the coast must respond with extraordinary measures, such
as sacrificing large section of beach and dune.
• Alternate erosion and accretion may be seasonal on some beaches;
the winter storm waves erode the beach, and the summer waves
rebuilds it.

11. Onshore-offshore transport
Cross shore transport is a result of the water motions due to the
waves and the undertow.
Sediment transport is perpendicular to the shore.
Important factors in determining the cross-shore transport of
sediment are
The ratio of wave height to wavelength
Physical parameters of the beach;
grain-size distribution, cohesiveness, beach
slope also play an important role.

12. There are two general types of dynamic beach
response to wave motion:
Response to normal condition
Response to storm condition
Normal wave conditions
offshore sediment transport and typically be deposited in a bar
resulting in an overall flattening of the slope of the shoreface.
During the following periods the bar will travel very slowly towards
the coastline again, practically rebuilding the original coastal profile.

13. CROSS-SHORE SEDIMENT TRANSPORT
Once the sediment is brought up into the water column it becomes available
for transport by the various hydrodynamic processes
In the cross-shore dimension the transport is considerably more complex.
The net sediment transport at a given point in the profile is often a balance
between an onshore transport caused by skewed incident short wave
motions, an offshore transport caused by mean currents and a transport
caused by long waves which can be either onshore or offshore directed
Consequently shore-normal sediment transport gradients can become large
and morphological changes created by such transport gradients are often
considerable, spatially as well as temporally.
Accretion occurs in zones of sediment transport convergence and erosion
occurs in zones of divergence

14. As surface gravity waves propagate from deeper waters to the
shore, their shape changes, primarily due to nonlinear wave
interactions and further on due to breaking. The nonlinear
effects amplify the higher harmonics and cause the oscillatory
flow to transform from nearly sinusoidal in deep water,
through velocity-skewed in the shoaling zone, to velocity
asymmetric in the inner-surf and swash zones

15. As for natural beaches, energetic (moderate)
wave climates mostly induce offshore (onshore)
sediment fluxes.
For a given significant wave height, an increase
(decrease) in the wave climate peak period is
associated with an increase (decrease) in wave
skewness and leads mostly to offshore (onshore)
sediment fluxes.
Skewed waves- Peaked
narrow crests and wide flat
troughs

16. The net sediment transport rate under strongly
skewed waves is either offshore directed due to
phase lag effects or onshore directed when the wave
‐
asymmetry is large enough.
Both these mechanisms probably largely contribute
to bar formation and migration.
Several conditions exhibit phase lag effects where the
‐
sediment is mobilized by the wave crest and
transported by the following trough, which produces
a net offshore transport even for a weak undertow

17. Wave shapes inducing velocity skewness (for sharp, high
crests and broad, shallow troughs) and velocity
asymmetry (for forward pitched of saw tooth type
‐
waves) are usually responsible for sediment transport in
the direction of wave propagation (onshore).
Typical skewed waves in the shoaling zone induce high
crest velocities in onshore direction that mobilize and
transport more sediment than the offshore directed
‐
trough velocities.
Additionally, the strong fluid acceleration induced by the
steep front faces of asymmetric waves enhances
sediment mobilization by the crests, further favoring the
onshore sediment flux

18. The undertow is usually considered to be the main
mechanism distributing sediment offshore, in correlation
with wave stirring that can be
enhanced by breaking wave turbulence
The net sediment transport associated with purely
skewed waves, however, may also be offshore directed,
due to phase lag between the mobilization and the
transport of sediment. In this case, the sediment
mobilized by the crest is transported by the following
trough before it settles.

20. Location A (deep water)
• Symmetric waves
• Inactive bed
• Transport is zero

21. Location B
• Skewed waves
• Ripples on sea bed
Example of wave ripples in the shoaling zone

22. Location C
• Skewed waves
• Bound infragravity waves
• Sheet flow (flat bed)
Infragravity waves - Surface gravity
waves with lower frequencies
(higher wave periods) than the
wind waves. Consisting of
both wind and sea swells
Skewed waves- Peaked narrow crests and wide flat troughs

25. Location D
• Asymmetric waves
• Undertow
Shoaling zone (1)
• Skewed waves stir AND transport sediment
• Near the bed- onshore transport
• Higher up in the vertical- No or small offshore transport
• Overall effect: onshore transport

26. Shoaling zone (2)
• Bound infragravity waves transport sand stirred by gravity
waves
• Large concentrations under high waves in the group coincide
with bound infragravity trough (offshore infragravity orbital
motion)
• Overall effect: offshore transport
Shoaling zone (3)
• Skewed waves: onshore transport
• Bound infragravity waves: offshore transport
• onshore >>> offshore

27. Breaking wave zone
• Breaking, asymmetric gravity waves stir sediment
• Large concentrations (breaking-induced turbulence)
• Sediment transport:
onshore by asymmetric waves
offshore by undertow
• In general:
few breaking waves onshore
many breaking waves offshore

28. Location E
• Infragravity waves
• Undertow
Swash zone (during storms)
• Water motion dominated by infragravity waves
• Infragravity waves stir AND transport sediment
• Large concentrations (breaking-induced turbulence)
• Sediment transport:
unclear
field experiments: onshore and offshore
Potential offshore contribution by undertow

31. Model theories for the calculation of Onshore-offshore transport
drift are
Simple cross shore transport model
Open loop models
Fall time model
Traction model for cross shore transport
Energetics model
Ripple model

32. • This model was first proposed by Moore and later modified by Kriebel
and Dean.
• The basic concept is that, for a uniform sand size across the profile and
an equilibrium beach, there is a constant energy dissipation rate per unit
volume.
• It is assumed that the amount of sediment moved will be dependent on
the difference between the actual energy dissipation rate and that for an
equilibrium profile .
is a dimensionless transport coefficient
is actual wave energy dissipation per unit volume of water
is the equilibrium wave energy dissipation per unit volume of
water
• where is the volumetric cross shore sediment transport rate per unit
width in the offshore direction.
Simple cross shore transport model

33. • If D is greater than equilibrium value D* there is a
greater turbulence level in the surf zone than that of for
the equilibrium profile. If qs is positive then there will be
a sediment transport in the offshore direction. On the
other hand , for values of D, less than equilibrium value,
onshore transport will occur. The value D can be
obtained as,
• which is dependent on the water depth and bottom
slope, which has stronger effect.

34. Open loop models
• Open loop Models usually relate cross shore
sediment transport to the detailed physics of the
flow field such as sediment concentration, fluid
velocity and bottom shear stress. There are
different formulae falls under this category
• The net time averaged flux of suspended sediment
past a section in the nearshore zone as
Where and are averaged horizontal velocity and
sediment concentration at level Z respectively.

35. • The total suspended sediment transport over a
wave period is given by
• Where is velocity, is concentration, denotes the
Lagrangian drift , represent the time averaged
concentration, is the wave period.

36. Equilibrium Beach Profiles (EBF)
• Various models have been proposed for representing equilibrium
beach profiles (EBP). Some of these models are based on examination
of the geometric characteristics of profiles in nature and some attempt
to represent in a gross manner the forces active in shaping the profile.
• One approach that has been utilized is to recognize the presence of
the constructive forces and to hypothesize the dominance of various
destructive forces.
• This approach can lead to simple algebraic forms for the profiles for
testing against profile data.
• Dean (1977) has examined the forms of the EBPs that would result if
the dominant destructive forces were one of the following:
(a) Wave energy dissipation per unit water volume.
(b) Wave energy dissipation per unit surface area.
(c)Uniform average longshore shear stress across the surf
zone

37. • using linear wave theory and a simple wave breaking model, the
EBP could be represented by the following simple algebraic
(1)
- Sediment Scale Parameter
- water depth
- offshore distance
- exponent
depends on the sediment size D .
• Brunn (1954) found the exponent is equal to 2/3
• Dean (1977) found the theoretical value of the exponent to be 2/3
for the case of wave energy dissipation per unit volume as the
dominant force (a) and 0.4 for the other two cases (b,c)

38. The following expression is recommended for use in
describing equilibrium beach profiles
(2)
Wave energy dissipation per unit water volume causes
destabilization of the sediment particles through the
turbulence associated with the breaking waves.
Thus the dynamic equilibrium results when the level
of destabilizing and constructive forces are balanced.

39. The sediment scale parameter A and the equilibrium
wave energy dissipation per unit volume are related
by Dean in 1991
(3)
Moore (1982) and Dean (1987b) have provided
empirical correlations between the sediment scale
parameter A as a function of sediment size D and fall
velocity

43. Computation of equilibrium beach profiles
• The most simple application is the calculation of
equilibrium beach profiles for various grain sizes,
assumed uniform across the profile.
• Lets try to determine the EBF for the sediment
grain sizes of 0.3 mm and 0.66 mm.
• From Figure in slide 46 and/or Table in slide 47,
the associated A values are 0.125 m 1/3
and 0.18 m
1/3
• Then using equation profiles can be plotted.

45. Application of equilibrium profile methods
to nourished beaches
• In the design of beach nourishment projects, it is important to estimate
the dry beach width after profile equilibration.
• Most profiles are placed at slopes considerably steeper than
equilibrium and the equilibration process, consisting of a redistribution
of the fill sand across the active profile out to the depth of closure,
occurs over a period of several years.
• In general, the performance of a beach fill, in terms of the resulting gain
in dry beach width relative to the volume of sand placed on the beach,
is a function of the compatibility of the fill sand with the native sand.
• Based on equilibrium beach profile concepts, it should be evident that
since profiles composed of coarser sediments assume steeper profiles,
beach fills using coarser sand will require less sediment to provide the
same equilibrium dry beach width ∆y than fills using sediment that is
finer than the native sand.

46. • Three types of nourished profiles are possible,
depending on the volumes of sand added and
on whether the nourishment is coarser or
finer than that originally present sand on the
beach
• These profiles are
Intersecting
Nonintersecting
Submerged

48. Quantitative relationships for nourished profiles.
• In order to investigate the conditions of profile type
occurrence and additional quantitative aspects, it is
useful to define the following non-dimensional
quantities:
N- Native sediment ; F – Filled sediment (4)
; , (5)
• Where the symbol V denotes added volume per unit
beach length, B is the berm height, and is the depth to
which the nourished profile will equilibrate.
• In general, this will be considered to be the closure
depth.
• - non-dimensional equilibrium dry beach width

49. • It is important to note that the width is based
on the native sediment scale parameter as
given (6)

50. • The non-dimensional equilibrium dry beach width
can be presented in terms of three non-
dimensional quantities
(7)
Given that the fill sediment scale parameter is
greater than or equal to the native sediment scale
parameter,
(8)

51. • The critical volume of sand deciding the
intersection and non-intersecting profiles is
(9)
• This applies only for , since for, the profiles will
always be nonintersecting.
• Even though it should be recognized that
nonintersecting profiles can also exist for.
• If, but , then the profile will be nonintersecting

52. • The critical volume of sand that will just yield a
finite shoreline displacement for the case of
sand that is finer than the native ()
(10)
• For intersecting profiles, the non dimensional
volume required to yield an advancement is
(11)

53. • For non-intersecting but emergent profiles the
non dimensional volume required to yield an
advancement is
(12)

54. • For a submerged beach profile )
non dimensional volume required to yield an
advancement is
(13)

55. • Equations 11 and 12 can be displayed in a
useful form for calculating the volume
required for a particular equilibrium additional
dry beach width.
• Since only two independent variables can be
displayed on a single plot, three are presented
in the following slides, one each for 0.333 and
0.25

59. • Determine the equilibrated additional dry beach width ∆y
due to cross-shore transport
• Data Given:
,
The sediment scale parameters can be determined :
:
The effective wave height is determined from
The closure depth is determined from
(1.57) (5.11) =8 m

60. The reference width of effective motion is
calculated using
Determine the non-dimensional quantities:
; ,
1

61. Using the figure in slide 63 ( which is corresponding to )
and can determine
Thus
Results of the graphical solution are below

62. • The solution is next carried out with the appropriate
equations for comparison with the graphical
procedure.
• Apply Equation 9 to determine whether the profiles
will be intersecting or nonintersecting
compared with the applied of 0.28
the solution is an intersecting profile.

63. • Applying Equation 11, for an intersecting profile
can be determined.
• This equation can be reduced to
• Solving the equation
= 0.0955 or

64. • is the answer from graphical method. when equations are used.
• Thus, for this example, the graphical solution is reasonable.
• The intersection distance for these two profiles can be
determined using the following argument
Rearranging the terms
By comparison, the corresponding values from the graphical
solution are = 495 m and = 6.25 m