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Investigation of Sediment Budget and Mechanisms of Dynamic - - PowerPoint PPT Presentation

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Investigation of Sediment Budget and Mechanisms of Dynamic Morphology changes along the West coast of Sri Lanka Yoshimitsu TAJIMA Coastal Engineering Lab. The University of Tokyo GEOSS APS2014 Ryogoku, Tokyo 2014 May 27th Local erosion

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Investigation of Sediment Budget and Mechanisms of Dynamic Morphology changes along the West coast of Sri Lanka

Yoshimitsu TAJIMA Coastal Engineering Lab. The University of Tokyo GEOSS APS2014 Ryogoku, Tokyo 2014 May 27th

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severe erosion

Groin & seawall protect beach Erosion. sand rock protects beach Local erosion & accumulations

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2001 Mar.9

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2005 Oct.27

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2010 Aug.9

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Contents Satellite-based observations of the Coast Coupling monitoring techniques for better understandings of the coastal environments

  • Shoreline extractions
  • Bathymetry estimations
  • Thermoluminescence (TL)
  • Numerical Model

Preliminary findings of coastal morphological characteristics on the west coast of Sri Lanka

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Contents Satellite-based observations of the Coast Coupling monitoring techniques for better understandings of the coastal environments

  • Shoreline extractions
  • Bathymetry estimations
  • Thermoluminescence (TL)
  • Numerical Model

Preliminary findings of coastal morphological characteristics on the west coast of Sri Lanka

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severe erosion

Groin & seawall protect beach Erosion. sand rock protects beach Local erosion & accumulations

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What are the primary factors of erosions around the north of Kalpitiya?

  • Decreasing sediment supply from the south?
  • Local unbalance of the longshore sediment transport rate?
  • Temporal shoreline retreat due to stormy waves?

We need more frequent monitoring

  • f the shoreline change
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2007.1 2009.12 PALSAR Good Relatively high resolution Shoreline is clearly detected Monitoring frequency Not so good

  • coarser resolution
  • Relatively low intensity of signal

from the lower land like sand spit

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Comparisons of GPS-log and PALSAR image

  • GPS-log data recorded

in July 2010 (red lines) were compared with the PALSAR image.

  • Sandy beach sometime

is “darker” than the swash zone.

  • PALSAR image can be

a useful data sets to monitor shoreline changes.

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x y (xc,yc,zc)

φ v

u

σ

Shoreline Extraction Procedures

Shoreline locations in pixel coordinates Shoreline locations on fixed XY-horizontal coordinate system Comparisons of shoreline locations at different time

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Extracted Shorelines

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distance along the shoreline Shoreline change since 2007 Jan 14th time-series of shoreline change

  • severe erosion around

X~20km whereas significant accumulation at X~15km

  • Relatively stable shoreline

at X<14km

  • Rapid shoreline change in

each beginning of the year?

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Contents Satellite-based observations of the Coast Coupling monitoring techniques for better understandings of the coastal environments

  • Shoreline extractions
  • Bathymetry estimations
  • Thermoluminescence (TL)
  • Numerical Model

Preliminary findings of coastal morphological characteristics on the west coast of Sri Lanka

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Estimated Bathymetry

sand accumulation?

  • Wide shallower area is developed on the north side of Kalpitiya.
  • These accumulated sand can be a source of longshore sediment supply to the north
  • Further monitoring is essential for future estimation of the sediment budget
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Contents Satellite-based observations of the Coast Coupling monitoring techniques for better understandings of the coastal environments

  • Shoreline extractions
  • Bathymetry estimations
  • Thermoluminescence (TL)
  • Numerical Model

Preliminary findings of coastal morphological characteristics on the west coast of Sri Lanka

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Temporal variation of natural grain TL signal

Natural residual TL signal ~ Solar exposure ~ Sediment transport 1. Travelling duration 2. Moving distance 3. Source identification

Nearshore Sediment Movement

Time Luminescence signal Initial TL Initial OSL bleaching (short time) Sand buried under the ground (long period) Deposition TL OSL Erosion Nearshore Sediment transport Laboratory Measurement Natural residual TL Sunlight Bleaching Luminescence signal decrease No light exposure Natural irradiation Signal accumulation Fast stage Slow stage

Thermoluminescence

Sand grain emits light (luminescence) when exposed to light (OSL: Optically Stimulated Luminescence) or heat (TL: Thermo Luminescence). Intensity of the luminescence depends on how long the grain was buried under the ground and how long it has been exposed to the sunlight.

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Clear peak around Kalu river > sand supply from the land Gradual decay toward north > Northward transport No major peak in the north of Nigombo > No major sand supply from land

Results and Findings

Small peak at the Kelani River mouth > relatively small sand supply from land

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Contents Satellite-based observations of the Coast Coupling monitoring techniques for better understandings of the coastal environments

  • Shoreline extractions
  • Bathymetry estimations
  • Thermoluminescence (TL)
  • Numerical Model

Preliminary findings of coastal morphological characteristics on the west coast of Sri Lanka

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Numerical Models for....

  • better understandings of the physical processes
  • Future predictions

Wave

  • Phase-averaged EBM
  • Time-dependent non-linear wave model

Current

  • Wave-induced nearshore currents
  • Tidal currents
  • Wave-current interactions
  • Tsunami inundation
  • Storm surge inundation

Sediment transport / Topography change

  • shoreline model
  • 3D beach evolution model

Observed shoreline/bathymetry change wave observation Future change of the LSST can be estimated.

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wave breaking y

θb

Shoreline Model

qy x

∫

∞

=

s

x y s

dx q Q

( )

( )

α θ − ∝

b b g

EC sin

xs

        ∂ ∂ = y xs arctan α

QsIN QsOUT Ds Ds +Ru Δxs Δy

New Shoreline Old Shoreline

y

  • Cross-shore beach profile stays the same.
  • Sand moves only where the water depth is

shallower than the critical depth, Ds.

  • Qs changes with the angle of shoreline, α.
  • Qs is semi-empirically determined based
  • n the assumption of long straight beach.
  • No circulation current is accounted for.

dy dQ R D dt dx

s u s s

+ − = 1

shoreline

Assumption

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Wave information we need for the shoreline model Wave energy and Wave Directions along the Wave Breaking Point Phase-averaged energy balance equations are preferred accounting for both computational costs and minimum requirements of the wave information for the shoreline model.

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x y θ

wave crest line

g f

EC E =

θ cos

g fx

EC E = θ sin

g fy

EC E =

x y y E E

fy fy

∆         ∆ ∂ ∂ + 2 1 x y y E E

fy fy

∆         ∆ ∂ ∂ − 2 1 y x x E E

fx fx

∆         ∆ ∂ ∂ + 2 1 y x x E E

fx fx

∆         ∆ ∂ ∂ − 2 1

∆x ∆y

Energy Balance Equation

Momentum equation + Continuity equation

time-average

( )

  • ut

in g

E E C E t E − + −∇ = ∂ ∂

wind wave breaking friction loss

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Wave Field Computation

Aerial photo & Satellite Thermoluminescence Shoreline model 0.1~1km 陸 Offshore

Computed wave field Coastal Structures Shoreline Coastal Structures Northward LSST Block of LSST at Colombo No sand supply from land in the north

South North Land

wave height (m)

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Model representation of the S.L.-change from 1956 to 2000

  • Coral reef around Kalpitiya caused local accumulation / erosion
  • While the shoreline is stable owing to coastal structures in the south of

Chilaw, predicted water depth in front of the shoreline is kept increasing.

sea wall HL HL

0 Hs(m) 1.6

2km (b) (c) (d)

N S

meas. comp.

Colombo Nigombo Chilaw Kalpitiya

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Extracted Shorelines

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Rough estimation* of future budget of LSST

* Numbers are subject to change with further detailed analysis

100km 20km 10km 1km year after 1955

Predicted time-series of LSST

LSST [m3/year]

カルピティア北部 カルピティア南部 チラウ ニゴンボ

Nigombo : rapid decay in first 50 yrs. followed by nearly no LSST Chilaw : gradual decay is accelerated after 50 yrs.

  • S. Kalpitiya: rapid decay after 100 yrs.

Kalpitiya : No significant change

1956 1988 1988(cal.) 2000(cal.) 2050(cal.) 2100(cal.) 2000 Nigombo

  • S. Kalpitiya

Chilaw Kalpitiya Kalpitiya

  • S. Kalpitiya

Chilaw Nigombo

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Summary Satellite-based observations of the Coast Coupling monitoring techniques for better understandings of the coastal environments

  • Shoreline extractions
  • Bathymetry estimations
  • Wave observations
  • Thermoluminescence (TL)
  • Numerical Model

Preliminary findings of coastal morphological characteristics on the west coast of Sri Lanka

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Thank you for your attention!

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