Nearshore coastal processes

1
Nearshore coastal processes
2
Wave transformation and breaking

• Wave shoaling
• Wave refraction
• Wave breaking
• Wave diffraction

3
(No Transcript)
4
Breaker types

• Surf similarity parameter (Iribarren number)
• ?b lt 0.4 spilling breakers (dissipative
  beaches)
• 0.4 lt ?b lt 2.0 plunging breakers (swell breaks
  on flat sandy beaches)
• ?b gt 2.0 collapsing breakers (swell breaking on
  steep beaches)
• Surging breakers (very steep beaches, almost no
  breaking)

5
Wave diffraction

• Is concerned with transfer of wave energy across
  wave rays when the waves hit an obstruction (e.g.
  breakwater, groyne)
• Separate from wave refraction even though they
  happen simultaneously
• Wave energy is transferred from wave zone to
  shadow zone
• Calculation is rather complicated and so we use
  diffraction templates

6
Diffraction coefficient

• KD HD/Hi
• where
• HD local wave height
• Hi incident wave height at the structure tip
• water depth is assumed to be constant
• when they all happen together
• H/H0 KsKrKD

7
Tombolo or salient formation due to wave
diffraction

• Salient
• Tombolo

8

• Coastal sediment transport general
  characteristics
• Basic shore processes further details
  equations

9
Coastal sediment transport

• Most important aspect of coastal zone management
  because 90 of the sandy shorelines around the
  world are eroding
• Two main components (moved by both waves and
  wind)
• Longshore transport
• Cross-shore transport
• Can cause erosion or accretion

10
Types of beaches

• Granular shores non-cohesive (sand, gravel,
  shingle)
• Cohesive shores (soft rock, till, clay (mud))

11
Dynamic beach profile

• Beach shape
• Responds to environmental conditions
• Remains in equlibrium if the conditions are
  constant
• Annual storm-calm cycle (bar-berm)
• Dynamic equlibrium

12
Beach erosion

• Four main causes
• Decrease in sediment supply
• Comminution
• Submergence
• Human inteference

13
Cross-shore transport

• Dune-beach utopia
• This is complicated by
• 1. alongshore sediment transport
• 2. offshore bar formation
• 3. Canyons, etc

14
Dune-beach disturbance
15
Modern engineering design coastal management
should

• Not disturb the existing dune-beach systems
• Encourage the growth of them
• Emulate dune-beach systems wherever possible

16
Soft protection

• Beach nourishment
• Whole dune system would be better if possible
• Nourishment can be reinforced by structures
• In beach nourishment the most important parameter
  is the grain size (D50)

17
Hard Protection

• Structures
• Groynes
• Breakwaters
• Seawalls
• Revetments

18
Tombolo or salient formation due to wave
diffraction

• Salient
• Tombolo

19
Alongshore transport (littoral transport or
littoral drift)

• Waves at an angle
• Two mechanisms
• 1. Beach drifting
• 2. Transport in the breaking zone
• Difficult to quantify

20

• Gross sediment transport
• Net sediment transport

21
Measurement of littoral transport

• Using tracers
• By measuring differences in deposited volumes of
  sand
• By integration of suspended measurements
• Very difficult, expensive contain large
  uncertainities

22
Modes of sediment transport

• Bed load
• Suspended load

23
Computation of littoral transport

• Numerical modelling
• Bulk expressions (based on few easy to measure
  parameters)

24
Complications

• We assumed
• 1. unlimited amounts of sand
• 2. alongshore transport takes place only in one
  direction
• 3. short term variations (storms)

25

• Potential and actual sediment transport rates
• Actual rate can be determined by sediment budget
  calculations including all the sources and sinks
• Potential rate is approached for shorter
  durations when there is large supply of material
• Sediment transport rates should be expressed over
  short time spans of hours or days rather than
  years
• Short term rates can approach potential values
  but long term rates are much lower

26

• Transport in two directions
• Short term littoral transport (storms)

27
Cohesive shores

• Soft Hard shorelines
• Soft shorelines
• unconsolidated cohesive material, recently
  deposited in river deltas, tidal flats, and
  coastal wetlands.
• Valuable unique habitat

28

• Hard shorelines
• Consolidated materials deposited 1000s of years
  ago.
• Foreshore, toe of bluff, bluff
• Sand acts as an abrasive in erosion
• Eroded fine materials never return

29
Basic Shore Processes

• Nearshore current patterns
• 1. Longshore currents
• 2. Undertow (caused by wave set-up)
• 3. rip currents

30
Wave set-up and set-down

• When the waves break on a beach, they produce a
  set-up, a rise in the mean water level above the
  still-water elevation of the sea
• Set-up occurs shoreward of the point of initial
  wave breaking
• Wave set-down occurs just offshore of the
  breaking point, where waves undergo rapid
  transformations in wave height and energy

31

• Regular rip currents
• Transient rip currents

32
Rip currents
33
Rip currents
34
Longshore current velocity

• There are several expressions to calculate
  longshore current velocity
• For an infinitely long, straight beach,
• Shore Protection Manual (CERC, 1984)

35
Littoral materials

• Vary in size from boulders to clay
• Classified based on median grain diameter (D50)
  or
• Wentworth Classification

36

• Mean grain size (MF)
• Standard deviation of size distribution
• Skewness

37
Sediment fall velocity (wf)

• for 0.13x10-3 D50 1.6x10-3 m
• for 1.6x10-3 D50 8x10-3 m

38
Beach slope

• Related to grain size
• Beach slope through the breaker zone,
• Steeper beach coarser grains

39
Beach Profile

• Ap profile coefficient
• Bruun (1954) Dean (1977)
• for 0.1x10-3 D 1.0x10-3 m
• for 0.1x10-3 D 0.2x10-3 m
• Dean (1983)

40
Closure depth

• Hallermeier (1981) CUR (1990)
• Hs,12 significant wave height which occurs 12
  hrs/yr on average

41
Cross-shore sediment transport

• Takes place when an existing beach profile
  changes
• If the profile is close to equilibrium with the
  existing conditions, little x-shore transport
  occurs
• If the conditions change, x-shore transport picks
  up and brings the beach to corresponding
  equilibrium shape

42
Cross-shore sediment transport

• The rate of cross-shore transport is assumed to
  be proportional to the difference between the
  existing beach profile and the equilibrium
  profile that matches the new environmental
  conditions.
• Annual berm-bar cycle.
• - critical
  condition
• gt 1 sediment moves offshore (bar)
• lt 1 sediment moves onshore (berm)

43
Cross-shore sediment transport

• If in the long term, the material that is moved
  offshore does not come back onshore, the beach
  will erode
• Erosion causes recession (landward movement of
  the beach)
• One classic example is beach recession due to sea
  level rise
• Higher water levels allow larger waves to come
  closer to the shoreline
• Beach recession due to sea level rise can be
  estimated by assuming that same waves conditions
  the beach profile are retained

44
Calculation of beach recession(Bruuns rule)

• this is very approximate
• xc is very sensitive to dc.
• AB will be very flat heance the area of ABC
  contains lot of sand, which is ignored.
• No offshore movement of sand due to currents,
  tides gravity.
• Eroded volume is expected to produce the same
  volume when deposited.

45
Alongshore sediment transport rate

• Caused by beach drifting and transport in the
  breaker zone
• Coarser grains settle close to the shore are
  moved by beach drifting
• Finer materials further offshore are moved along
  the shore by longshore currents
• Alongshore sediment transport rate can be
  computed by using a detailed method or a bulk
  sedi. Transp. Expression
• Detailed computations need computer programs and
  much data to calibrate them

46
Alongshore sediment transport rate

• Since such data are not normally available, bulk
  expressions are usually used for practical
  engineering solutions
• Bulk expressions simply relate the total
  transport rate to some easily measured wave
  beach parameters

47
Alongshore sediment transport rate

• (m3/yr)
• (m3/hr) CERC (1984)
• (m3/yr)
• (m3/hr) -
• Kamphuis (1991)

48
Bulk expressions

• Gives maximum theoretical rates, which might not
  be achieved because they assume unlimited amount
  of sand
• Calibration of models is important
• Over-predict for gravel beaches because they do
  not include a critical shear stress

49
Actual alongshore sediment transport rate

• is calculated by examining the various inflows,
  outflows, sources, and sinks of sand such a
  calculation is called Sediment budget
• Sources rivers, dune or bluff erosion
• Sinks offshore losses, onshore losses due to
  wind, due to manmade construction, dredging
  sand mining
• Fig. 12.8
• Short-term transport rate differs from long-term
  rate and that is why we need to take both into
  account

50
The littoral cell

• Defined as a reach of shoreline in which all
  sediment transport processes are related
• Qin Qout 0
• Fig. 12.9
• May contain several sources and sinks
• Understanding the dynamics of a littoral cell
  means that the engineer or manager knows about
  how much sediment moves, where it moves, what the
  influences of the foreshore offshore conditions
  are, where the sources and sinks are

51
Sediment budget

• Purpose to identify relevant processes to
  estimate design volume rates of EROSION or
  ACCRETION
• Point source or sink acts over a limited area
• or
• Line source or sink acts over an extended
  segment
• or
• For a complete sediment budget