Transcript Διαφάνεια 1

Beach modelling II: Beach erosion diagnosis
and forecasting
Adonis F. Velegrakis
Dept Marine Sciences
University of the Aegean
Synopsis
1 Beach Characteristics
1.1 Beach morphology
1.2 Beach sediments
1.3 Beach energetics
2 Beach Dynamics
2.1 Changes in different scales
2.2 Sea level rise and coastal erosion
3 Beach erosion diagnosis and forecasting
3.1 The need for beach morphodynamic models
3.2 Models and reality
1. 1 Beach Characteristics: Morphology
Beach (cross-shore) profile is differentiated into: (i) a sub-aerial and (ii) a
submarine section.
The major morphological features of the submarine beach are
(a) the ‘longshore’ bar and
(b) the ‘longshore’ trough.
Under particular energetic conditions may form none or more than one pair of
the above features
The major morphological features of the sub-aerial beach are
(a)
the beach berm, i.e. a beach sediment deposit the position/height of which
is controlled by beach hydrodynamics and
(b)
the backshore, which is mostly controlled by beach aerodynamics
(occurrence of dunes)
The offshore boundary of the beach is defined by the ‘closure depth’
1.2 Beach Characteristics: Sediments
Beaches can form on any kind of sediments, in terms of texture
and composition.
For a beach to be in equilibrium, inputs and outputs should be
balanced
It must be also stressed that (submarine) beach sediments may
host benthic assemblages (e.g. seagrasses, algae) that can
attenuate the incoming wave energy, affect sediment mobility
and ultimately affect beach morphodynamics
1.3 Beach Characteristics: Energetics
The energetic control on beaches is (mainly) by waves and waveinduced currents.
Coastal waves and wave–induced currents are controlled by water
depth
Other hydrodynamic controls include
• Coastal tidal currents (meso- and macro-tidal beaches)
• Wind-generated and density currents
• Relaxation currents, following storm surges
Note: In meso-and macro-tidal environments, the distribution of the
wave energy is controlled by the tidal cycle.
2.1 Beach Dynamics: Changes in different scales
Beaches show large spatio-temporal variability
Adjacent beach sections can show different profiles due to spatial variability in the incoming
wave energy. There are also significant morphological changes in various temporal scales
Changes in beach morphology are precipitated by changes in the impinging wave energy and
sediment budgets. These are characterised by periodicity (daily, seasonal, interannual,
decadal etc) and/or randomness.
Periodic changes can be very substantial, with the most common being the seasonal
(transient) changes:
Winter (high energy density Ε=1/8ρgH2),
waves of high steepness γ = H/L
Summer (low energy density Ε=1/8ρgH2),
waves of low steepness (γ = H/L)
Erosion at the sub-aerial (berm/dune) and
deposition at the sub-marine beach with
construction of bars and troughs
Erosion at the sub-marine (inshore bars and
troughs) and deposition at the sub-aerial beach
(berm construction).
If, however, the beach suffers a large winter erosion, it might not recover fully in the following
summer, with the erosion becoming permanent
2.2 Sea level rise and beach erosion
One of the most potent forcing of beach erosion is sea level
rise
Sea level rise can operate in various temporal scales, i.e.
• can be gradual and permanent (ASLR) and/or
• rapid and reversible (due to storm surges)
Both can have devastating effects
3 Beach erosion diagnosis and forecasting
There must be a differentiation between long-term beach erosion and coastal
assets/infrastructure inundation/destruction risks due to short-term beach
erosion from storms/storm surges
In order to identify/quantify long-term beach retreat/erosion, long and
comprehensive beach morphological time-series must be acquired, due to
high short-term variability of beach morphology
Snapshots of shoreline positions/beach morphology can be misleading (even
if tidally-corrected).
3.1 The need for models
Morphodynamic models are necessary tools to get a diagnosis of beach
erosion, as they can test different forcings and controlling parameters
(e.g. sea level, wave characteristics, bed slopes and forms, sediments
etc)
Moreover, morphodynamic models are the only available tools to perform
predictions on future beach response
Nevertheless, it must be kept in mind that as the ‘map is not the terrain’,
‘models are not reality’
3.2 Models and reality
In order morphodynamic models to be manageable in terms of algorithm
simplicity/computation time, forcing options, etc they must deal with a
simplified version of reality
Although this is not necessarily bad, it may ignore very important processes
(e.g. the presence/distribution of various seabed roughness elements
such as ripples and sea grass meadows, bed armouring effects, storm
group effects, etc) that may change significantly model outputs
For example, beach retreat/erosion has been, in many cases, significantly
underestimated by models
Thanks you and have a nice evening!!
Fig. 1 The beach profile: Morphological features and wave zones
(Open University, 1997).
Fig. 2 Position of the thalweg of the longshore trough relative to the location of the wave
breaking zone (lt). (So, beach slope; Hb, wave breaking height; T, wave period, L∞,
offshore wave length). The bar crest is located at a distance lc from the thalweg of the
trough lc = 0.18 Hb(lt/ Hb )3/2(Komar, 1998)
Fig.3 Multiple pairs of
longshore bars and triughs
in the US Gulf coast
(Komar, 1998)
Fig.4 Berm scarp in S. California beach in late September. The scarp evidences
seasonal erosion due to storm event of the berm developed in summer.
Fig. 5 Berm height according to the wave characteristics. Hb, wave
breaking height; T, wave period, L∞, offshore wave length. (Komar, 1998)
Fig. 6 Sand dunes at the Christchurch beach backshore (New Zealand).
These dunes are controlled by the beach aerodynamics and have been
stabilised by vegetation SEPM, 1996
Fig. 7 The huge coastal dune of Arcachon, W. France (Dune de Pyla) (J-P. Tastet,
2005
Fig 8 Coastal sand dunes may migrate (transgress) . (SEPM, 1996).
Fig. 9 Dunes can, however, be
eroded by large storms (a)
storm-induced dune erosion (b)
dune erosion in N. Carolina due
to storms (USGS, 2003)
The closure depth
The closure depth forms the beach offshore boundary, i.e. the offshore
limit of the beach sediment reservoir.
Beach sediments that are transported beyond this depth are lost
forever, i.e. they can never come back to replenish the beach.
The closure depth hc can be estimated through:
hc = α He –β(He2/gTe2)
Where He και Te are the height and period of larger storm waves of the
year, and the α and β are 1.75-2.28 and 58-68.5, respectively
(Komar, 1998).
This expression has been calibrated in N. Carolina beaches and, thus,
should be used with care in other beaches.
(a)
(c)
(b)
Fig. 10 (a) Gravel beach in N. Zealand (b) shell beach (c) black sand beach
in N. Zealand, rich in ilmenite (Ti mineral) and magnetite (Fe mineral) (d) Green
Island beach, S. Alaska, US, the beach polluted by the Exxon Valdez oil spill (SEPM,
1996)
Beach sediment budget
‘Income’
•
•
•
•
•
Rivers
Coastal/beach erosion
Aeolian transport to the beach
Marine sediments
Beach replenishment
‘Expenditure’
• Winter (reversible) offshore
transport of beach sediments
• Storm (permanent) offshore
transport to the shelf and beyond
• Sediment entrapment in the backbarrier environments (lagoons)
• Beach sediment abstraction
• Longshore sediment transport to
adjacent beaches
Without meadows
With meadows
Fig. 11. Reef and seagrass distribution over the Negril (Jamaica) nearshore, and
modelled bed shear stress (force per unit area) by the SBEACH model (Larson and
Karuss, 1989) along a profile. Offshore wave height (H) 1 m, period (T) 6s). The
seagrass meadows spread the wave force on wider area.
Fig. 12. Wave spectra for two close stations (distance 9 m) along a flat part of a
beach profile (water depth 1.3 m) at Vatera beach , E. Mediterranean, showing the
attenuation of the wave energy due to the presence of vegetation; modelled
attenuation due to the distance covered by the incoming waves is much less.
Solid, blue line corresponds to the offshore station. (Vousdoukas et al, submitted)
Fig. 13. Waves (a) and waveinduced current components
(b,c,d) at a beach (depth 1.4
m). Vousdoukas et al.,
submitted
Fig. 14 Coastal wave zones. Longshore transport in the coastal zone occurs mainly in
the surf and swash (wave run up) zones (After SEPM, 1996). Key: h, water depth; H,
wave height; L, wave length.
Fig. 15. Wave spectra for the same station on a South Wales sand beach (no
vegetation), taken at different stages of a tide. As the water level increases, the
characteristics of wave energy affecting the seabed also change (Voulgaris, 1992)
Fig. 16 Beach changes in different spatio-temporal scales (Sunamura,
1989?)
Fig. 17 Beach dynamics in
Christchurch Bay, S.
England
Fig.18 Seasonal changes in beach morphology (profile) (Komar,
1998)
Fig. 19 Schema showing the beach response to sea level rise. For a level increase
α, sediments from the shoreface are eroded and transported to the submarine
section of the beach, resulting to a coastal retreat s.
Fig. 20 Beach response to storms/storm surges. Beaches respond with retreat, sub-aerial
beach erosion, profile flattening and deposition at the sub-marine beach. This response
results in the gradual attenuation of the incoming wave energy, due to the reduction of the
nearshore water depths (remember the Le Chatelier principle?). Nevertheless, the effects on
coastal assets/infrastructure close to the coastline can be devastating. After Pilkey & Thieler
(1992) and USGS (2003)
Fig. 21 Beach profiles at 5 monitoring stations see inset for location) along an
E. Med beach, showing high seasonal variability. (Velegrakis et al., 2008)
Fig. 22 Results of morphodynamic models Leont’yev (1996) και SBEACH (Larson and Krauss, 1989) for
the upper part of a linear and a natural cross-section (‘mean΄ section of Delilah experiment, US Army
Corps of Engineers) for 3 m waves and sediments with d50 = 0.2 mm and sea level rise of 0.10, 0.22 και
0.50 m. (A) Leont’yev model (linear profile, slope 1/10), (Β) SBEACH model (linear profile, slope 1/10) (Γ)
Leont’yev model (‘mean’ profile Delilah) και (Δ) SBEACH model (‘mean’ profile Delilah). (Monioudi et al.,
2008). x-axis, distance from the origin of the model application; y-axis, elevation
Coast/Beach
Reference
ASLR
(m)
Observed
retreat
(m)
Predicted
retreat
(m)
Potential causes of
underestimation
Herschel
Island
(Canada)
Lantuit and
Pollard,
2008
0.08
13.5
0.6 – 7.8
Ice, permafrost melting
N. Carolina
(US)
Morton and
Miller,
2005
0.25
> 300
2-12
Frequent storm surges,
high storm waves and
storm grouping
Texas (US)
Morton et
al, 2004
0.25
238±172
0.8-9
Subsidence, badlydesigned coastal works
Negril
RiVAMP,
2010
0.16
Up to 50
m
Up to 6 m
Reduction in bed sediment
supply, reduced natural
protection
Table 1. Observations versus model predictions (Monioudi, 2011)