The Use of Low Cost Breakwaters for Mangrove Restoration in Muddy Foreshores

Transcription

1 The Use of Low Cost Breakwaters for Mangrove Restoration in Muddy Foreshores Douglas A. Gaffney, P.E. Ranata Robertson October 23, 2012

2 The Use of Low Cost Breakwaters for Mangrove Restoration in Muddy Foreshores Geological Setting Mangroves in Guyana Breakwater Design Design at Victoria Guyana Construction Integrated Approach to Mangrove Restoration

3 Geological Setting Muddy Foreshores Specific Gravity Atterberg s Limit Shear Strength Allison and Lee, 2004

4 Mudbanks

5 Allison and Lee, 2004

6 Leading Edge of the Mud Bank

7 Trailing Edge of the Mud Bank

8 Mangroves in Guyana Species Erosion Seawalls

9 Mangroves in Guyana Guyana s coastline is 430 Km. Portions of the upland lie 0.5m to 1.0m below sea level, and are protected by a system of sea defences amounting to 340km in length. Out of this 340 Km, sixty five percent (65%) is protected by Natural Sea Defence (Mangroves).

10 Mangroves in Guyana Guyana currently has 22,632 hectares of mangrove forest. Avicennia germainas (black mangrove) and Laguncularia (white mangrove) is found on the coast while Rhizophora mangle (red mangrove) is found in the riverine areas.

11 Black and White Mangrove

12 Typical Mangrove Cross Section

13 Crane, WCD

14 Crane, WCD

15 Crane, WCD

16 Standard Breakwater Design Sand (Tombolos and Salients) Galveston Mordecai Island

17 Types of Breakwaters In relation to the water level Emergent Submerged (also called sill) Attached or Detached Crest Width Narrow Wide Segmented By shape or orientation Generally shore parallel Chevron (for bi-directional waves) Angled

18 Detached Segmented Breakwater Note the salients and tombolos

19 Nearshore Curved Breakwater Note the change in wave energy from one side to the other Mordecai Island, NJ

20 Example Cross Section

21 NEARSHORE BREAKWATERS Galveston Island State Park, TX

22 Scour Apron Grand Isle, Louisiana

23 Scour Apron (reinforces the foundation) Fly Ash Lagoon, PA

24 Design at Victoria, GY Geotextile Tubes and scour apron

25 Modelling the Tube

26 Plan at Victoria, ECD Plan View

27 Design Objectives Two (2) 50 meter sand filled geotextile tubes 20 ft circumference (8 feet wide by 4 feet high) sand-filled (260 cubic meters plus losses and anchor tubes) Two (2) 52 meter X 5.4 m scour aprons Double layer due to soft foundation Oriented to be perpendicular to incident waves at 60 degrees from magnetic north Designed to encourage tombolo growth which will capture more sediment and mud to raise the forseshore: proper elevation will promote revegetation Breakwaters will reduce future storm energy Breakwaters will reduce focusing of reflected wave energy off the seawall

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30 Construction Geotextile Tubes and scour apron Filling Equipment

31 Fill Material Fill Gradation Tube Foundation Tube Alignment Tube Anchorage Tube Overlaps Tube Filling

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33 Sand on Site

34 Integrated Approach Contrast to no engineering Don t constrain mangroves behind seawalls

35 Lewis, Get the hydrology right first. 2. Do not build a nursery, grow mangroves and just plant some area currently devoid of mangroves (like) a convenient mudflat). There is a reason why mangroves are not already there or were not there in the recent past or have disappeared recently. 3. Once you find out why, see if you can correct the conditions that currently prevent natural colonization of the selected mangrove restoration site. If you cannot correct those conditions, pick another site..

36 Lewis, Use a reference mangrove site for examining normal hydrology for mangroves in your particular area. Either install tide gauges and measure the tidal hydrology of a reference mangrove forest or use the surveyed elevation of a reference mangrove forest floor as a surrogate for hydrology, and establish those same range of elevations at your restoration site or restore the same hydrology to an impounded mangrove by breaching the dikes in the right places.

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38 The Use of Low Cost Breakwaters for Mangrove Restoration in Muddy Foreshores Conclusions Breakwaters, specifically ones constructed of geotextile tubes, may offer a relatively low cost approach to restoring and protecting mangroves Typical Breakwater Design has to be modified to suit the environment Site data and an understanding of the environment is crucial