oce presentation slides

Coastal Structures:Dunes (Soft Structure) and Bulkhead (Hard Structure)

Cyrus, Dexter, Qi Wen & Janet

Content

Introduction to Coastal Protection Measures

Introduction to Dunes

Formation of Dunes

Stages of Succession along a Coastal Sand Dune

Introduction to Bulkhead

Types of Bulkhead & Advantages & Disadvantages

Case Studies (Bulkhead)

Conclusion

Introduction to Coastal Protection Measures

Introduction to Coastal Protection Measures

Use to protect coastline from Coastal Erosion.

There are two types of Coastal Protection Measures- Soft Engineering-Hard Engineering

1. Dunes (Soft Engineering) 2. Bulkhead ( Hard Engineering)

Introduction to Sand Dunes

Introduction to Sand Dunes

In a plant free landscape (e.g. after a landslide or lava flow, a fresh sandy coastline or derelict industrial waterland) plants become established over a number of years and move from the pioneer stage through succession to mature “climax” community of stable vegetation.

Formation of Sand Dunes

1. When speed larger than 15kph, the large supply of sand will be picked up and blown by the wind.

2. When the wind is slowed sufficiently by the plants growing at the shoreline, a pile of sand will begin to accumulate providing an ever increasing wind break which eventually grow into a dune.

3. With high wind speeds, the sand is continually pushed over the crest of the dune falling down the steeper windward slope.

Stages in Succession along A Coastal Sand Dune

There are 5 main stages in succession across a coastal sand dune:

Stage 1: Embryo Dune

Stage 2:Yellow (Main) Dune

Stage 3: Semi Fixed Dune

Stage 4: Grey/fixed Dune

Stage 5: Climax Vegetation

Stage 1: Embryo Dune

Strandline: a line of seaweed litter and debris along the high tide mark which provides nutrients for sea rocket and couch

Stage 2: Yellow (Main) Dune

As pioneer plants grow they provide shelter for the thin humus layer which is being formed by the decomposition of the pioneer biota

The environment here is still very hash though and only very hardy plants can survive such as Marram Grass.

Stage 3: Semi fixed Dune

At around 10m above sea level, the marram grass dies out as the sand supply is reduced, meaning a thin humus layer is now permanent and soil can form.

Improved conditions and increased moisture content leads to a greater variety of vegetation type.

A greater variety of plants can now be supported, including dandelion, ragwort and rest harrow.

Stage 4: Grey/ fixed Dune

With the increasing variety of plants, the soil becomes deeper and more fertile and can support an almost complete covering of vegetation. Typical plants: Mosses, Lichens, Clover and herbs such as Red Fescue.

Stage 5: Climax Vegetation

When a dune succession reaches its climax stage, the vegetation found largely depends on the mineral composition of the sand.

Calcareous (alkaline, shell based) – rich grassland

Silica (acidic rock fragments) – heath land then deciduous trees

Slacks

Low lying areas in between dunes (found at all stages in the succession). These are often marshy due to rise in water table. March species develop, along with peat in the waterlogged soil

Introduction to Bulkhead

Introduction to Bulkheads

1. Structure built parallel (or nearly parallel) to the shore along a bank or shoreline.

2. A bulkhead may also called as a retaining wall.

3. A bulkhead is fairly effective at deflecting wave run up. Bulkhead are often used in situations where there is little or no fronting beach to protect the shore.

Types of Bulkhead

Timber Wall Bulkhead

Wire Fence Bulkhead

Advantages & Disadvantages

1. Provide good protection to property.

2. May provide direct boat access to the shore

3. Fair and easy to repair

Advantages

Disadvantages1. Structure integrity depends upon toe protection, bracing, or

anchoring.

2. May increase erosion immediately at downdrift.

3. Pile driving requires special equipment/ skill

4. Relatively expensive to build

Socioeconomic Effects

• limit recreational uses of the shoreline

• Floating trash will trap in any sharp angle turns on the bulkhead.

Effects on Coastal Process

1. Cause some depletion of the down drift shore

2. Waves reflected by the bulkhead may increase beach depletion, or beach steepening immediately in front of or adjacent to the structure.

Case Studies (Bulkhead)

Sandy Hook Spit, New Jersey

(a) Sandy Hook Spit, New Jersey

16km

Groins

FACTORS OF EROSION:i. Diffraction and refraction of wavesii. Negative sediment budget due to groins

PROBLEM = EROSION

The Use of Bulkheads

The use of Bulkheads:i. Protect infrastructure – road access

Bulkhead (1976) Bulkhead (1988) Bulkhead (Now- from Google earth)

Effects on Coastal Process• Other protection measures:

i. Beach Nourishmentii. Artificial Dunes

Other purposes of protection:i. For natural

environmentii. Provide habitat for

endangered piping plover

iii. Provide recreational platform

Effects on Coastal Process(b) Leonardo, Sandy Hook Bay, New JerseyPROBLEMS:1. LOW ELEVATION• Water level can rise 7.5 ft.

above MSL (during Hurricane Sandy)

• Subjected to storm-induced flooding

• wave damage

EFFECT OF USING BULKHEADS1. EROSION• Minimal dry sand at the toe of bulkhead2. GROYNE AND BULKHEAD• Act as groin trap to trap sand/sediments3. REGULAR REPAIR/REPLACEMENT BULKHEAD• Easily damaged especially during

storm/hurricane

Conclusion

Conclusion

SOFT STRUCTURE - DUNES HARD STRUCTURE - BULKHEAD

Provide habitat for coastal native vegetation

• Trap windblown sand• Minimize sand losses

from wave attack

Impacts: -Inappropriate vegetation- Human activities

Dune management

• Prevent erosion• Retain/Hold back land from moving

water ward

Advantages & Disadvantages of bulkhead

Effects of using bulkhead

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Q & A

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