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

severe erosion Groin & seawall protect beach

Erosion. sand rock protects beach

Local erosion & accumulations

2001 Mar.9

2005 Oct.27

2010 Aug.9

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

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

severe erosion Groin & seawall protect beach

Erosion. sand rock protects beach

Local erosion & accumulations

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 of the shoreline change

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

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.

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

Extracted Shorelines

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?

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

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

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

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.

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

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

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.

wave breaking

y

θb

Shoreline Model

qy

x

∫∞

=sx

ys dxqQ

( ) ( )αθ −∝ bbgEC sinxs

∂∂

=yxsarctanα

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

on the assumption of long straight beach. • No circulation current is accounted for.

dydQ

RDdtdx s

us

s

+−=

1

shoreline

Assumption

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.

x

y

θ wave crest line

gf ECE =

θcosgfx ECE =

θsingfy ECE =

xyy

EE fy

fy ∆

∆

∂∂

+21 xy

yE

E fyfy ∆

∆

∂∂

−21

yxx

EE fx

fx ∆

∆

∂∂

+21

yxx

EE fx

fx ∆

∆

∂∂

−21

∆x

∆y

Energy Balance Equation

Momentum equation +

Continuity equation time-average ( ) outing EECE

tE

−+−∇=∂∂

wind wave breaking friction loss

Wave Field Computation Aerial photo & Satellite

Thermoluminescence

Shoreline model 0.

1~1k

m

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

)

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

Extracted Shorelines

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

LSS

T [m

3/ye

ar]

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

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

Thank you for your attention!