Dr. Junior Darsan Department of Oceanography and Coastal Processes Institute of Marine Affairs

Transcription

1 Dr. Junior Darsan Department of Oceanography and Coastal Processes Institute of Marine Affairs 1

2 Outline Introduction Objectives Methodology Results Erosion Mitigation Implications Conclusions Recommendations 2

3 Introduction Under the Coastal Conservation Project of the Environmental Research Programme, coastal geomorphological data have been systematically collected from the 198 s. This data provides information to stakeholders on matters pertaining to management and sustainability of Trinidad and Tobago s beaches. This paper focuses on the 25 beaches and bays monitored, comprising 64 beach profiling stations around Trinidad. Cocos Bay Guayaguayare Bay 3

4 Introduction Beaches and coastal dunes are depositional features found along coasts. Beaches are dynamic and respond to coastal forcing mechanisms. Beach sediment is critical in the protection of the coastline by dissipating wave energy; thereby acting as a buffer to erosion. Beaches however are damaged by: 1. sand removal (erosion or mining) 2. badly designed coastal protection structures 3. any activity that prevents sediment replenishment 4

5 How do we analyze the status of beaches? Beaches can either be classified as being in a state of Erosion, Accretion or Dynamic Equilibrium (D.E.). Erosion can occur either horizontally (where the coastline recedes) or vertically (where the sand elevation decreases). Accretion occurs when there is an increase in sediment along the beach profile; both horizontally (increasing beach width) and vertically (building the berm). When these two cycles of erosion and accretion occur without any long term deleterious effects on the beach, a state of Dynamic Equilibrium (DE) is said to exist. 5

6 Research Objectives To investigate the littoral, morphological and sedimentological characteristics of beaches and bays. To identify the effects of tidal cycles and seasons. To identify the effects of extreme events (Storms, Hurricanes and Anthropogenic Changes) To evaluate and monitor erosion and accretion rates. To monitor the stability status and trends of beaches over a timeseries. 6

7 Methodology 7

8 Data Collected 1. Littoral Processes 2. Beach Morphology 3. Sediments 8

9 1. Littoral Processes: (i) Wind Speed (ii) Wind Direction (iii) Wave Period (iv) Wave and Breaker Height (v) Wave and Breaker Angle (vi) Breaker Type (vii) Longshore Currents: (a) Current Speed (b) Current Direction 9

10 2. Beach Morphology: (i) Beach Profiles (ii) Beach Plan (iii) Beach Orientation (iv) Coastline Retreat Beach profiling done using standard surveying methods as described by Goudie (199). 3. Sediments: (i) Upper Foreshore (MHWM) (ii) Mid-Beach (iii) Lower Foreshore (MLWM) Sediments are dry sieved to obtain Folk & Ward (1957) parameters. (mean, sorting, skewness & kurtosis) 1

11 Map of Trinidad Showing IMA Stations 11

12 Coastal Classification Map of Trinidad 12

13 Selected Results 13

14 Sediment Analysis Beaches are an accumulation of sediments ranging in size from silt to pebble and cobble sizes. Generally, coarser sediments are associated with steeper beachface gradients and vice-versa. Beach sediment around Trinidad generally consists of fine sand that are well to moderately sorted. Beaches that contain a matrix of both sand and pebble size sediments have bi-modal distributions that are poorly sorted e.g. Dhein s Bay and Chagville Bay. 14

15 Maracas Bay Sediment Histogram for Maracas Station Percentage (%) Unimodal Well Sorted UB MB LB FineSand Sediment Size in (Phi ) Dhein s Bay Sediment Histogram for Dhein's Bay 1 9 Sand Pebbles Percentage (%) Bi-modal Poorly Sorted UB MB LB Sediment Size in (Phi ) Chagville Bay Sediment Histogram for Chagville Station Matrix of Sand and Pebbles Percentage (%) Sediment Size in (Phi ) Bi-modal Poorly Sorted 15 UB MB LB

16 Sediment Analysis Mean Grain Size (mm) Maracas Bay 1 Maracas Bay 2 Maracas Bay 3 Maracas Bay 4 North Coast Mean Grain Size Distributions Tyrico Bay 1 Tyrico Bay 2 Las Cuevas Bay 3 Las Cuevas Bay 2 Las Cuevas Bay 1 Blanchisseuse Bay 2 Grande Riviere 1 (Average Grain Size =.45mm) Grande Riviere 2 Grande Riviere 3 Grande Riviere 4 Salybia Bay East Coast Mean Grain Size Distributions (Average Grain Size =.3mm) Mean Grain Size (mm) Guayamara Bay Saline Bay Cocos Bay 1 Cocos Bay 2 Cocos Bay 3 Cocos Bay 4 Cocos Bay 5 Mayaro Bay 2 Mayaro Bay 3 16

17 Mean Grain Size (mm) South Coast Mean Grain Size Distributions (Average Grain Size =.28mm) Mean Grain Size (mm) Columbus Bay Granville Bay Irois Bay 9a Irois Bay 1b Guayaguayare Bay 3 Guayaguayare Bay 2 Guayaguayare Bay 1 Quinam Bay Los Iros Bay Erin Bay Punta del Arenal West Coast Mean Grain Size Distribution (Average Grain Size =.3mm) La Brea 2 La Brea 3 Guapo Bay 1 Guapo Bay 4a Guapo Bay ch1 Guapo Bay 4b Guapo Bay 5 Dhein's Bay Chagville Beach 1 Chagville Beach 3 17

18 Stability Status of Beaches North Coast The north coast is open to the Caribbean Sea and exposed to the Northeast Trade Winds and Mid-Atlantic storm swells. These bays are exposed to moderate to high wave energy where breaker heights can exceed.8 m. All monitored beaches along the north coast were in dynamic equilibrium (DE) except the eastern and western regions of Las Cuevas Bay and the western section of Blanchisseuse Bay. 18

19 North Coast At Maracas and Tyrico Bays, there were variation in the sand elevations, but backshore cliff recession was not observed. Macqueripe Bay is backed by a seawall that protects the backshore from being eroded. Grande Riviere is backed in the central and western regions by metamorphic rocks and therefore fairly resistant to wave erosion. At the eastern section of the bay, the berm is wide and buffers wave energy. The reef at Salybia is fairly efficient in reducing wave energy that would otherwise erode the low backshore cliff. 19

20 Shoreline Stability Status of North Coast Beaches Beach/Bay IMA s Beach Monitoring Station Location Shoreline Stability Status (+Net Annual Accretion (m); -Net Annual Erosion (m); DE Dynamic Equilibrium) Las Cuevas East BM not yet established DE DE DE Central DE -3.5 DE DE DE DE DE DE DE DE West BM not yet established -4. DE Blanchisseuse West DE DE DE DE Salybia West DE -.8 DE DE DE DE DE DE DE revetment 2

21 Las Cuevas Bay Station Elevation (m) BM Beach profiles indicates erosion from Long-term analysis also shows negative changes to beach width and volume. Erosion rate: 1. m/yr (25 213) Distance from Benchmark (m) Beach Width (m) Mean Sea Level 5 1 LAS CUEVAS BAY - Station 1 April 25 - March Number of Days (Units) 1 2 Change in Beach Width Change in Beach Volume Linear (Change in Beach Width) Linear (Change in Beach Volume) Beach Volume (m 3 )

22 Elevation(m) Beach profiles indicate D.E. from Long-term analysis shows positive changes to beach width and volume. Las Cuevas Bay Station BM Mean Sea Level Distant from Benchmark (m) Beach Width (m) Change in Beach Width Change in Beach Volume Linear (Change in Beach Width) Linear (Change in Beach Volume) 2 LAS CUEVAS BAY - Station 2 Changes in Beach Widths and Volumes March March Number of Days (Units) y =.4x R² =.131 y =.3x R² = Beach Volume (m 3 )

23 Elevation (m) BM Blanchisseuse Bay Distance from Benchmark (m) Beach profiles indicate erosion from Beach was in dynamic equilibrium from Long-term analysis shows that the beach is stable. Beach Width (m) Mean Sea Level Change in Beach Width Change in Beach Volume Linear (Change in Beach Width) Linear (Change in Beach Volume) BLANCHISSEUSE BAY - Station 2 Changes in Beach Widths and Volumes March July 213 y =.5x R² =.149 y =.4x R² = Beach Volume (m 3 ) Number of Days

24 East Coast The east coast is a high energy environment and is exposed to the Atlantic Ocean and the Northeast Trade Winds. Several rows of spilling breakers are characteristic of the east coast bays. Coastal erosion in some areas has prompted the construction of coastal defense structures such as seawalls and rip rap revetments. Most stations on the east coast are generally in a state of dynamic equilibrium. 24

25 East Coast Guayamara Bay s beach face gradient is in dynamic equilibrium with the oncoming wave conditions. Both ends of Cocos Bay are experiencing erosion, while the central section is stable. Mayaro north did not experience any erosion, although cliff recession was observed further north of this station. Mayaro south experienced undercutting at the base of the cliffs and subsequent slumping, resulting in erosion. 25

26 Shoreline Stability Status of East Coast Beaches Beach/Bay IMAs Beach Monitoring Station Location Shoreline Stability Status (+Net Annual Accretion (m); -Net Annual Erosion (m); DE Dynamic Equilibrium) Cocos North DE DE DE DE DE DE DE Central DE DE DE DE DE DE South (82km mark) DE Mayaro Central DE DE DE DE DE DE DE DE DE DE 26

27 Cocos Bay Station BM Elevation (m) Mean Sea Level Beach profiles indicate D.E. from However, long-term analysis reveals negative changes to beach width and volume. Erosion rate: 1.4 m/yr (24 213) Distance from Benchmark (m) Beach Width (m) y = -.13x R² =.3723 COCOS BAY - Station 1 February July 213 Change in Beach Width Change in Beach Volume Linear (Change in Beach Width) Linear (Change in Beach Volume) Beach Volume (m 3 ) y = -.15x R² = Number of Days (Units)

28 3. Cocos Bay Station Elevation (m) BM Mean Sea Level Distance from Benchmark (m) 3 COCOS BAY - Station 3 February May Beach profiles indicate D.E. from Long-term analysis shows no net change to beach width although sand volumes are decreasing. Beach Width (m) y = -.9x R² =.123 Change in Beach Width Change in Beach Volume Linear (Change in Beach Width) Linear (Change in Beach Volume) y =.1x R² = Beach Volume (m 3 ) Number of Days (Units)

29 Elevation (m) BM Cocos Bay Station Coastline Retreat Mean Sea Level Distance from Benchmark (m) Beach profiles indicate Erosion from Long-term analysis shows negative changes to beach width and volume. Erosion rate: 1.45 m/yr (24 213) Beach Width (m) y = -.4x R² =.8165 Change in Beach Width Change in Beach Volume Linear (Change in Beach Width) Linear (Change in Beach Volume) COCOS BAY - Station 5 January July 213 y = -.13x R² = Beach Volume (m 3 ) Number of Days (Units)

30 Elevation (m) BM Mayaro Bay Station Mean Sea Level Distance from Benchmark (m) Although the two profiles conducted in 28 show lower sand elevations, the profile recovered. Long-term analysis shows that the beach is stable. Beach Width (m) y = -.1x R² = MAYARO BAY - Station 2 March May Number of Days (Units) y =.9x R² = Change in Beach Width Change in Beach Volume Beach Volume (m 3 )

31 South Coast The southern coast consists of an alternating series of low and high cliffs, coastal plains and small emergences of wetlands at Morne Diablo, Erin and Icacos. The geology of this area consists of unconsolidated silts and clays which provide little resistance to wave attack. The five bays monitored along this coastline include Guayaguayare, Quinam, Los Iros, Erin and Punta del Arenal. 31

32 South Coast These beaches are in dynamic equilibrium with the exception of the western section of Guayaguayare Bay which is eroding. This area of Guayaguayare Bay was the site chosen for the laying of the gas pipelines where trenching occurred both in the backshore and offshore regions. Punta del Arenal is the only bay on the south coast that experienced accretion. This bay has been experiencing accretion for a number of years. 32

33 Shoreline Stability Status of South Coast Beaches Beach/Bay IMAs Beach Monitoring Station Location Shoreline Stability Status (+Net Annual Accretion (m); -Net Annual Erosion (m); DE Dynamic Equilibrium) Guayaguayare West DE Punta del Arenal 1km South of Corral Pt. N/A.16 DE DE DE

34 BM Coastline Retreat Mean Sea Level Distance from Benchmark (m) Elevation (m) Guayaguyare Bay (West) Station During , landward recession was observed each year. Most severe erosion in 26, when 2.2 m of the coastline was eroded Negative changes to the beach width and volumes reflect the eroding beach. Erosion rate:.8 m/yr (24 213) Beach Width (m) Atlantic LNG pipeline 24 1 GUAYAGUAYARE BAY - Station 1 October May 213 Change in Beach Width Change in Beach Volume Linear (Change in Beach Width) Linear (Change in Beach Volume) 2 3 y = -.5x R² =.5223 y = -.97x R² = Number of Days (Units) Beach Volume (m3/m)

35 Elevation (m) Punta del Arenal BM Mean Sea Level Distance from Benchmark (m) At 12 m from the benchmark, there has been approximately.5 m increase in sediment levels from , building the berm. The rest of the beach profile remains in dynamic equilibrium. Long-term analysis confirms the accretionary state. Beach Width (m) Change in Beach Width Change in Beach Volume Linear (Change in Beach Width) Linear (Change in Beach Volume) 1 y =.9x R² = Punta Del Arenal March 1985 July y =.34x R² =.2941 Number of Days (Units) Beach Volume (m 3 )

36 West Coast The west coast faces the sheltered Gulf of Paria where wave energy is moderate. This coast consists of rugged outcrops in the northern section, mangroves and mud flats in the central section and low to moderate high cliffs in the southern section. There is extensive urban and industrial development, and the existence of mangroves and mud flats limits beach environments. 36

37 West Coast Erosion is occurring at a phenomenal rate at Corral Point and the north-eastern section of Columbus Bay. All other beaches along this coastline are in dynamic equilibrium with the exception of North Chatham, which is eroding. 37

38 Shoreline Stability Status of West Coast Beaches Beach/Bay IMAs Beach Monitoring Station Location Shoreline Stability Status (+Net Annual Accretion (m); -Net Annual Erosion (m); DE Dynamic Equilibrium) Irois North Chatham Guapo 5 DE DE DE DE DE 5A DE DE DE Construction DE Columbus Bay Central DE DE DE DE DE

39 Elevation (m) Distance from Benchmark (m) Erosion since October 1996 At IMA s benchmark cliff recession occurred every year during at an average rate of.96 m/yr. Forest reserve under threat Erosion rate: 1.5 m/yr (24 213) Irois Bay, North Chatham BM Coastline Retreat Beach Width (m) Mean Sea Level y = -.8x R² = IROS BAY STATION 1 January 2 - May Number of Days (Units) y = -.38x R² =.2573 Change in Beach Width Change in Beach Volume Linear (Change in Beach Width) Linear (Change in Beach Volume) Beach Volume (m 3 ) -4-5

40 Elevation (m) BM Guapo Bay Station 5a Accretion Shoreward of Breakwater Mean Sea Level Breakwater Beach widths and volume between October 1999 and April 28 shows a decrease in both parameters until 26, when a sharp increase was observed. Due to construction works in the vicinity of this station, monitoring was not performed again until August 27. During that time, re-shaping of the backshore, construction of the seawall to the west and the breakwater resulted in a significant increase in the beach width and volume. Distance from Benchmark (m) Beach Width (m) Change in Beach Width Change in Beach Volume 1 2 GUAPO BAY - Station 5A October July Number of Days (Units) y =.51x R² = Beach volume (m 3 )

41 Columbus Bay Coastline Retreat Elevation (m) Mean Sea Level Distance (m) Beach profiles indicate that the beach is eroding. Erosion rate:.8 m/yr (28-213).

42 Coastal Mitigation In Trinidad 42

43 Hard Engineering Revetments Seawalls Groynes Breakwaters 43

44 Hard Engineering Cocos Bay Revetment: - Low topography backshore - Exposed to Atlantic Ocean - Revetment constructed in 25 Rip-rap revetment: Southern End, Cocos Bay, 212) 44

45 Hard Engineering Macqueripe Sea Wall: New wall constructed due to failure of previous wall Macqueripe seawall,

46 Hard Engineering Guapo Bay Breakwater: Constructed to arrest coastal erosion in the immediate vicinity. Breakwater, Guapo Bay 46

47 1b Bad Applications 1 Chatham Rubble Revetment: Rubble material used to protect residential property Rubble revetment, Chatham,

48 Bad Applications Cocos Bay Tyres: Tyres used to protect beachfront property Northern Cocos Bay, 28 48

49 Soft Engineering Columbus Bay: Sandbag Groynes - Groynes installed (28) - Worked temporarily - Failed eventually due to poor design considerations Groyne, Columbus Bay, 21 49

50 Soft Engineering Beach Nourishment, Chagville Beach, 27 Chagville Beach Nourishment: Chagville beach, IKONOS (27) Chagville Beach Nourishment Nourishment, Jan 21 - A man made beach - Low natural sediment supply - Beach reverted to its natural sand levels by 212. Profile, Aug 212 5

51 Implications of Coastal Erosion Loss of revenue for Coconut Estates: Annual losses= $31,68/yr Losses over last 3yrs= $95,4 (at Columbus Bay) Loss of valuable land: 6.5 acres (value unknown) up to 21 at Columbus Bay. Threat to Infrastructure: Beachfront property Roads Affects tourism development Threat to Ecologically Sensitive Areas Nariva Swamp Mangrove Forests 51

52 Conclusion 52

53 Conclusion Coastal erosion is directly linked to wave energy, geology and morphology of the coastline. Coastal erosion can also be induced by anthropogenic activities. Most of the beaches and bays monitored between were in a state of dynamic equilibrium. The north coast beaches are predominantly stable and in dynamic equilibrium. Although this coastline is backed by more resistant metamorphic rocks, erosion was sometimes observed as a lowering of sand levels and not as a result of cliff recession. 53

54 Conclusion On the east coast, significant erosion is occurring at the northern and southern sections (stations 1 and 5) at Cocos Bay, while the central region (spit) of the bay is dynamic and stable. All south coast beaches monitored exhibited dynamic equilibrium with the exception of the western region of Guayaguayare Bay and Punta del Arenal. Guayaguayare Bay (west) has been eroding, while Punta del Arenal is the only monitored bay on the south coast that has been accreting. The west coast beaches were generally in dynamic equilibrium with the exception of North Chatham in Irois Bay and at the western section of Guapo Bay which have been eroding. 54

55 Coastline Stability Map of Trinidad 55

56 Erosion Hotspot: East Coast Cocos Bay, Northern view,

57 Erosion Hotspot: South Coast Guayaguayare Bay, Eastern view,

58 Erosion Hotspot: West Coast Columbus Bay,

59 Erosion Hotspot: West Coast Irois Bay, North Chatham,

60 Recommendations 6

61 Recommendations Additional monitoring stations needed at erosion hot-spot areas. For bays that have been showing preliminary signs of erosion, longer monitoring is needed to ascertain their stability status. Appropriate mitigation strategies should be deployed at bays that have been identified as eroding. This research allows for informing management decisions, as well as for monitoring the performance of erosion mitigation devices. 61

62 Thank You! 62