Sam.S.L HETTIARACHCHI1, Saman. P SAMARAWICKRAMA1, Harsha RATNASOORIYA1, Joe FERNANDO2 1University of Moratuwa, Sri Lanka, Democratic Socialist Republic of; 2Universiy of Notre Dam, USA

1. Investigating the performance of coastal
ecosystems for hazard mitigation
Sam Hettiarachchi H.J.S.Fernando
S.P.Samarawickrama University of Notre Dame
A.H. R. Ratnasooriya Indiana
University of Moratuwa USA
Sri Lanka

2. Hazard Impact- Tsunami Hazard

3. Satellite
Images
Impact of the tsunami on the
coastline.
(South west coast of Sri Lanka)
Indian
Ocean Earthquake
– Tsunami 2004

4. Indian
Ocean Earthquake
– Tsunami 2004

5. Indian Ocean
Earthquake – Tsunami
2004

6. Indian
Ocean Earthquake –
Tsunami 2004

7. Erosion and Deposition caused by tsunami waves
Crest
Trough
Long waves of high amplitude
High velocity profile from surface to sea bed

8. Tsunami – Erosion and Deposition

9. Measured currents offshore of Colombo
80
Current Speed
70
Velocity Magnitude (cm/s)
60
50
2.5
km/hour Tsunami
40
30
20
10
0
12/26/04 0:00 12/26/04 6:00 12/26/04 12:00 12/26/04 18:00 12/27/04 0:00
Time
360
315
270
Current Direction (deg)
225
180
135
90
45
Current Direction
0
24-Dec 25-Dec 26-Dec 27-Dec 28-Dec 29-Dec
Time

10. Sediment Transport by the 2004 tsunami at Sri Lanka
Ongoing Study (Tohoku University)
Sounding surveys were conducted before
(December 2004) and after (December

11. Sand Dunes
February 2002
Dunes
River entrances
Natural features
Sand Dunes

12. Sand Dunes and their destruction
Overtopping
January 2005
Vicinity of opening for rivers

13. Natural features
Coastal Vegetation

14. Destruction of vegetation in Sri Lanka
Mangrove destruction in Banda Aceh--
Dr.Subandano

15. Natural features of the neashore
Coral Reefs

16. Coral reefs are severely
affected and damaged
by the debris and sand
transported during the
inland and shoreward
movement of the
tsunami waves.
From Prof. H.Fernando

17. Natural Methods for Mitigation
Coral Reefs & Sand Bars
Sand Dunes
Coastal Vegetation and Mangrove Forests
Hybrid Solutions
Combination of Natural /Artificial Methods

18. Coral Reefs
Submerged natural breakwaters
Hi
H
t
h τb ub
Submerged depth (h)
d
Coral Reefs –
act as submerged
natural breakwaters
L Length (L)

19. Small submerged depth (h)
Significant length (L)
Coral Reefs
Kenyan Coastline –From J. Tychsen

20. The influence of Wave Reflection from Maldive Islands
Reflection of waves
Sri Lanka
Maldive Islands

21. Investigating the impact
of coral reefs and the
influence of gaps

22. Coral Reefs

23. Gaps created by fishermen
to manoeuvre boats

24. Theoretical Considerations
Wave Parameters (U0 , λ, a) U0
Reef Parameters (M, P, L, H)
Reef Gap (ω)
Depth of water (H0)
Location (x, y, z) UC UG
UC
UG
Gap
M
z
P
x
y
H
ω L
Wave Parameters Uo , λ, a

25. The velocity in the gap at a location (x, y, z) is given by
U G = f {[ P, M , L, H ], [ω ], [ H 0 ], [ x, y, z ], [U 0 , a, λ ]}
reef parameters gap location wave
parameters
Reef Parameters: Height H, Length L, Porosity P and other hydrodynamic and
physical characteristics of the ridge represented by the dimensionless parameter M.
Gap in the reef ω
Depth of water at the location is H0
Oncoming wave parameters: Velocity amplitude U0, the Wave length λ and
Wave amplitude a.

26. On dimensional grounds equation (1) can be written as
 L H x y z ω a λ 
U G = f 1  P, M , , , , , , , , 
 ×
H H0 L H H H H0 H0 
(2)
Since the interest here is the velocity just at the lee edge of the reef or
x
= 1 and the velocity distribution at the centerline ( y = 0)
L
 L H z ω a λ 
U G = f 2 P, M , , , , , ,  (3)
 H H0 H H H0 H0 

27. ω
When the gap is absent =0
H
and this is equivalent to the condition far away from the gap where the effects of the
gap are hardly felt. Therefore the lee-side velocity far away from the gap (or that in
the absence of the gap) can be written as
 L H z a λ 
U C = f 3  P, M , , , , ,  (4)
 H H0 H H0 H0 
Since the present experiments are designed to demonstrate the possibility of large
velocity amplifications and their dependence on the porosity of the barrier
L H λ
M, , and were kept Thus (3) and (4) yields:
H H0 H0 constant.
∆U U G − U C  a z
= = π  P, ,  (5)
UC UC  H0 H 

28. a z
In the experiments the dependence on P, and are investigated
H0 H
Porosity P = 20% and 50%
Amplitude a = 20, 30 and 40 cm for H0=30 cm
Measurements at z = 5, 10, 15, 20 cm for H=20 cm
∆U U G − U C  a z
= = π  P, , 
UC UC  H0 H 
U0 UC UG

29. Schematic diagram of the experimental set-up
(all values are in cm s)
86 cm
W
60 cm
30 cm
20 cm
ADV Positions
26 cm
(a) Plan view (b) Elevation

30. Simulation of Coral Reefs in 2D Physical Modelling
Representation of high dense (20% porosity) and
low dense (50% porosity) structures

31. Collaborative Research
Arizona State University / University of Moratuwa
(May/June 2005 and Nov/Dec 2006)
PIV method
Large flume studies
ADV method
Simulated reefs

32. Representation of high dense (20% porosity) and
low dense (50% porosity) structures
Gap

33. University of Arizona

34. 1 1
0.75 0.75
0.5
Z/H 0
0.5
Z/H 0
0.25 0.25
0 0
0 0.5 1 1.5 2 0 0.5 1 1.5 2
U/U0,surface U/Uo,surface
(a) 50% porosity (b) 20% porosity
Normalized Velocity as a function of normalized height 2a = 10cm
U0 Velocity without the reef
= 0.4Hz
UC Velocity behind the reef
UG Velocity in the reef gap

35. 1 1
0.75 0.75
0.5 0.5
Z/H 0
Z/H 0
0.25 0.25
0 0
0 0.5 1 1.5 2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
U/U0,surface porosity
(a) 50% (b) 20% porosity
U/U0,surface
Figure 11: Normalized Velocity as a function of normalized height
2€ = 20cm,
(a) 50% porosity (b) 20% porosity
Normalized Velocity as a function of= normalized height 2a = 20cm
0.4Hz
U0 Velocity without the reef
UC Velocity behind the reef
UG Velocity in the reef gap

36. 1 1
0.75 0.75
0.5 0.5
Z/H 0
Z/H 0
0.25 0.25
0 0
0 0.5 1 1.5 2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
U/U0,surface
U/U0,surface
(a) 50% porosity (b) 20% porosity
Normalized Velocity as a function of normalized height 2a = 30cm
U0 Velocity without the reef
UC Velocity behind the reef
UG Velocity in the reef gap

37. 1
0.75
0.5
Z/H 0
0.25
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
U/U 0,surface
20% porosity
U0 Velocity without the reef
UC Velocity behind the reef
UG Velocity in the reef gap

38. 1 1
0.75 0.75
0.5 0.5
Z/H 0
Z/H 0
0.25 0.25
0 0
0 0.5 1 1.5 2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
U/U0,surface
U/U0,surface
(a) 50% porosity (b) 20% porosity
Normalized Velocity as a function of normalized height 2a = 30cm
U0 Velocity without the reef
UC Velocity behind the reef
UG Velocity in the reef gap

39. Case Study------ Great Train Tragedy at Hikkaduwa

40. Hikkaduwa Case Study
Key areas of investgation
(i) Discontinuity in the Continental Shelf-
Edge waves
(ii) Coral Mining
(iii) Negative Slope

41. (ii) Coral Mining
Illegal coral mining has created a
defenseless “low resistance path”
Ilegal Coral mining over the
last few decades
Presence of
Corals and Rocks

42. Heavy Coral Mining
Mild negative slope

43. KERRY
SHIEH©2005
From Dr.D Subandano

44. Is it role of vegetation ?
Tsunami Heights =3-4m
From Dr. Danny Hilman Natawijaya West NIAS - SIRAMBU