Title: Nearshore coastal processes
1Nearshore coastal processes
2Wave transformation and breaking
- Wave shoaling
- Wave refraction
- Wave breaking
- Wave diffraction
3(No Transcript)
4Breaker 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)
5Wave 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
6Diffraction 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
-
7Tombolo or salient formation due to wave
diffraction
8- Coastal sediment transport general
characteristics - Basic shore processes further details
equations
9Coastal 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
10Types of beaches
- Granular shores non-cohesive (sand, gravel,
shingle) - Cohesive shores (soft rock, till, clay (mud))
11Dynamic beach profile
- Beach shape
- Responds to environmental conditions
- Remains in equlibrium if the conditions are
constant - Annual storm-calm cycle (bar-berm)
- Dynamic equlibrium
12Beach erosion
- Four main causes
- Decrease in sediment supply
- Comminution
- Submergence
- Human inteference
13Cross-shore transport
- Dune-beach utopia
- This is complicated by
- 1. alongshore sediment transport
- 2. offshore bar formation
- 3. Canyons, etc
14Dune-beach disturbance
15Modern engineering design coastal management
should
- Not disturb the existing dune-beach systems
- Encourage the growth of them
- Emulate dune-beach systems wherever possible
16Soft 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)
17Hard Protection
- Structures
- Groynes
- Breakwaters
- Seawalls
- Revetments
18Tombolo or salient formation due to wave
diffraction
19Alongshore 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
21Measurement of littoral transport
- Using tracers
- By measuring differences in deposited volumes of
sand - By integration of suspended measurements
- Very difficult, expensive contain large
uncertainities
22Modes of sediment transport
23Computation of littoral transport
- Numerical modelling
- Bulk expressions (based on few easy to measure
parameters)
24Complications
- 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)
27Cohesive 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
29Basic Shore Processes
- Nearshore current patterns
- 1. Longshore currents
- 2. Undertow (caused by wave set-up)
- 3. rip currents
30Wave 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
32Rip currents
33Rip currents
34Longshore current velocity
- There are several expressions to calculate
longshore current velocity - For an infinitely long, straight beach,
- Shore Protection Manual (CERC, 1984)
35Littoral 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
37Sediment fall velocity (wf)
- for 0.13x10-3 D50 1.6x10-3 m
- for 1.6x10-3 D50 8x10-3 m
38Beach slope
- Related to grain size
- Beach slope through the breaker zone,
- Steeper beach coarser grains
39Beach 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)
40Closure depth
- Hallermeier (1981) CUR (1990)
-
- Hs,12 significant wave height which occurs 12
hrs/yr on average
41Cross-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
42Cross-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)
43Cross-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
44Calculation 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.
45Alongshore 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
46Alongshore 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
47Alongshore sediment transport rate
- (m3/yr)
- (m3/hr) CERC (1984)
- (m3/yr)
- (m3/hr) -
- Kamphuis (1991)
48Bulk 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
49Actual 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
50The 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
51Sediment 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
-