Coastal structures

Prepared by

Ahmmed Zulfiqar Rahaman

Coastal structures

Coastal structures can be anything human-made in the coastal area. They are constructed principally to protect the coast line.

Coastal structures

Water resources engineers build these structures to:

slow down or prevent coastal erosion (for example, groins and sea walls)

increase access or mooring sites (for example, marinas, bridges, causeways and boat ramps)

support coastal subdivisions (for example, power poles and storm water pipes)

What will happen if erosion occurs in this structure?

Lets see……

cliff

crack

crack

cave

arch

pillar

stack

stump

reef

coastal retreat

To escape from such kind of erosion by wave action coastal structures are needed indeed……

More than 70 percent of all coastal structures are on the shoreline for protection from probable damage by storm surge or coastal erosion called Shoreline protection

Coastal structures

On the basis of purposes

there are different types of Coastal structures.

They are-

Revetments

Bulkheads & sea walls

Breakwaters

Groins & jetties

Coastal bridges & piers

Dikes & levees.

Coastal structures Revetments:

Revetments are shoreline structures constructed parallel to the shoreline.

Revetments are structures placed on banks or cliffs in such a way as to absorb the energy of incoming water or explosives caused by water waves & current.

Fourth Cliff -Scituate, MA

Coastal structures

Revetments: A cheaper alternative to sea walls is the revetment Usually built for shore protection

Point Allerton -Hull, MA

Revetments: Modern revetments have concrete or shaped blocks of rip rap stone laid on top of a layer of finer material. Riprap has the advantage of good permeability plus it looks more natural.

Fourth Cliff -Scituate

Coastal structures

Revetments components:

Armor layer: using armor stone(high wave energy environments) or rip-rap stone(lower wave energy environments)

Filter layer: using smaller stone & geo textile fabrics to act as a filter & under layer, which support the armor layer

Toe protection prevents under cutting & provides support for all about layer materials.

Coastal structures Bulkheads: Bulkheads are vertical retaining walls to hold or prevent the soil from sliding seaward

Timber Bulkhead -Green Harbor Entrance Channel Marshfield

Coastal structures

Timber Bulkhead -Green Harbor Entrance Channel Marshfield

Bulkheads reduce land erosion…..

Coastal structures

Timber Bulkhead -Green Harbor Entrance Channel Marshfield

secondary purpose is to protect upland areas against damage from wave action.

Bulkheads……

Coastal structures

Sea walls: form of hard and strong coastal defense constructed on the inland part of a coast to reduce the effects of strong waves separate the land & water.

Maumeebay (seawall)

Coastal structures

Great Brewster Island Boston Harbor Islands

Granite Block Seawall

Coastal structures

Curved or stepped sea walls are designed to enable wave to break and to dissipate wave energy and repel waves back to the sea.

Concrete Seawall Lynn Shore Reservation Lynn

Coastal structures

- seawalls are vertical structures constructed parallel along the shore line.

- should last 20-30 years.

Seawall near Siwash rock in stanely park

Sea walls:

Coastal structures

Breakwaters: breakwaters are large scale structures constructed seaward of, & usually to the shore line they attempt to break incoming waves before they reach the shoreline or the facility (e.g. marina) they are protecting .

Coastal structures

Single breakwaters :

Single breakwaters may be attached or detached , depending on what they are being designed to protect

Coastal structures

Single breakwaters :

a single detached breakwaters may protect a small section of shoreline

a single attached breakwater may a long structure to shelter marines & harbors from wave action.

Coastal structures

Breakwaters…….

Coastal structures

Head land breakwaters(HB): -a series of breakwaters constructed in an “attached” fashion to the shoreline & angled in the direction of predominant waves

-the shoreline behind the structures evolves into a natural “crenulate” or log spiral embayment.

Coastal structures

Detached breakwaters:

detached breakwaters are constructed away from the shoreline, usually a slight distance offshore

they are designed to promote beach deposition on their leeside.

Groins: groins are the oldest and most common shore-connected, beach stabilization structure they help to create or widen beaches by capturing sand moving along the shoreline.

Mortared Stone Groin “The Five Sisters”Broad Sound

Winthrop

A groin is an active structure extending from shore into sea, most often perpendicularly or slightly obliquely to the shoreline

Coastal structures

Types of Groins:

normal or straight

inclined or angled

submerged

Permeable

single

groin system or field

T L or Y shaped

Rubble-mound and sand-filled bag

attracting, deflecting or repelling

Coastal structures

Types of groins

Coastal structures

Characteristic of Groins:

they are usually made of tropical hardwoods which are more resistant to marine borers and erosion.

a few are made of concrete, steel or in more recent times large rocks.

they are spaced about 50-100 metres apart.

may have a life of 15-20 years

Coastal structures

Concrete groins…….

Ukraine ( the black sea)

Coastal structures

Shoreline response to groins

Coastal structures

Jetties and piers a structure extended into a sea, lake, or river to influence the current or tide or to protect a harbor a protecting frame of a pier a landing wharf

Timber Pier -George’s Island Boston Harbor Islands

Jetties & piers……..

Tybee

Jetties are shore-normal stone structures commonly used for training navigation channels & stabilizing inlets

Coastal structures

Characteristics of jetties: they prevent intrusion of long-shore sediment transport they cause higher flow velocities that scour the channel to a depth required for safe navigation.

Coastal structures

Coastal bridges and piers

Biloxi bridge

Coastal structures

Coastal bridges and piers

Cuba bridge

Coastal structures

Dikes & levees

dikes are typically earth structures

they keep elevated water levels from flooding interior lowlands.

sometimes they refer to as dam

Coastal structures

Dikes & levees

Levees…..

Coastal structures

Gabion

Emergency shore protection:

for emergency shore protection gabions are used. gabion is a metal cage filled with rocks, about 1 metre by 1 metre square

Coastal structures

Gabion:

Coastal structures

Gabion:

they are stacked to form a simple wall.

used to protect a cliff or area in the short term only

gabions have the advantage of ease of use and are

relatively cheap but their life span is short.

Coastal structures

Now-a-days X-block & Tripodal blocks are vastly used for building such coastal structures.

Coastal structures

Tripodal block……

Coastal structures

Coastal structures in Bangladesh

Bangladesh has 710 km long coastline

Coastal structures

Natural calamities or storm surge visit our country every year So to protect our coastline from wave action or coastal erosion, coastal structures are obviously needed

Sonadia dip

Coastal structures

Coastal structures in Bangladesh

The coastal region of Bangladesh has 123 embanked polders, constructed in late sixties to protect the land

Coastal structures

St. Martin island jetty…..

Kutubdia

Coastal structures Importance of coastal structures

Coastal structures are important assets for the economic health of many coastal communities to: Protect harbors and inlets that are important commercial and military navigation links Protect shore-based infrastructure Provide beach and shoreline stability control Stabilize navigation channels Protect navigation, coastal communities, roadways, bridges, etc. Provide flood protection Provide recreational activities

Why should we let such coastal area disappear…….???

Coastal structures

Summary

Coastal structures influence waves currents sediment transport in the near shore environment

A systematic approach is needed in order to minimize negative effects