Design of Rubble Mound Seawall

Harbour Structure Analysis Project

By :Shailesh ShuklaM.Tech – D&H

Under the Guidance of :Mrs. K. Muthuchelvi Thangam,Scientist B, IMU

OBJECTIVE :

The main objective of constructing a seawall is to protect the structures or properties along the shore from being hit by the waves coming from the sea.

Depending upon the type of sea conditions different types of seawall are constructed.

It also acts as a barrier for soil erosion.

INTRODUCTION :

Seawall is a protective structure, made of stone or concrete; extends from shore into the sea to prevent a beach or coastal boundary from washing away.

It is designed to prevent coastal erosion and other damages due to wave action and storm surge, such as flooding.

Seawalls are normally very massive structures because they are designed to resist the full force of waves and storm surge.

A structure separating land and water areas.

PURPOSE OF SEA WALLS :

protect areas of human habitation

impede the exchange of sediment between land and sea.

Figure: Seawall at Bandra in Mumbai

Coastal erosion is wearing of land and removal of beach or dune sediments by wave action, wave currents and tidal currents.

Erosion of the coast depends on many factors like nature of the beachbeach material the shape of the coasttidal level changeshuman interference

Coastal Structures :Seawalls, revetments, anti-sea erosion bundsSystem of groynes or jetties – shore connectedSystem of offshore breakwaters - away from the shore

Types of sea walls : A seawall is typically a sloping concrete structure; it can be

smooth, stepped-faced or curved-faced.

A seawall can also be built as a rubble-mound structure, as a block seawall, steel or wooden structure.

There are three types of sea walls- Vertical sea walls Curved sea walls Mounted sea wall

1. Vertical sea walls:

The first implemented, most easily designed and constructed type of seawall.

Vertical sea walls deflect wave energy away from the coast.

Loose rubble can absorb wave energy

2. Curved sea walls: Concave structure introduces a dissipative element.

The curve can prevent waves from overtopping the wall and provides extra protection for the toe of the wall.

Curved seawalls aim to re-direct most of the incident energy, resulting in low reflected waves and much reduced turbulence.

3. Mounted sea walls :

Current designs use porous designs of rock, concrete armour.

Slope and loose material ensure maximum dissipation of wave energy.

Lower cost option.

Advantages of sea walls:

Long term solution in comparison to soft beach nourishment.

Effectively minimizes loss of life in extreme events and damage to

property caused by erosion.

Can exist longer in high energy environments in comparison to

‘soft’ engineering methods.

Can be used for recreation and sightseeing.

Forms a hard and strong coastal defense.

Disadvantages of sea walls:

Very expensive to construct.

Can cause beaches to dissipate rendering them useless for beach goers.

Scars the very landscape that they are trying to save and provides an

‘eyesore.’

Reflected energy of waves leading to scour at base.

Can disrupt natural shoreline processes and destroy shoreline habitats

such as wetlands and intertidal beaches.

Altered sediment transport processes can disrupt sand movement that

can lead to increased erosion down drift from the structure.

DESIGN CONSIDERATIONS :The function of the seawall is to dissipate the wave energy and allow formation of beach in front of it. As such, the sloping rubble mound seawall is the most suitable type of seawall.

Figure : Wave Action on curved seawall

Rubble mound Seawall : The rubble mound seawall is generally designed to consist of three

layers i.e. core, secondary layer and an armour layer.

A minimum of two layers of stones (units) in the armour and secondary layer is always necessary.

While the thicknesses of these layers are determined by the size of stones used, the levels including that of the core are determined based on maximum water level, design wave height, wave run-up, permissible overtopping and method of construction.

steps to design an adequate and efficient rubble mound seawall :Determine the water level range for the site.

Determine the wave heights .

Determine the beach profile after the storm condition / monsoon.

Select the suitable location and configuration of the seawall .

Select suitable armour to resist the design wave .

Select size of the armour unit .

Determine potential run-up to set the crest elevation.

Determine amount of overtopping expected for low structures.

Design under-drainage features if they are required.

Provide for local surface runoff and overtopping runoff and

make any required provisions for other drainage facilities such as

culverts and ditches.

Consider end condition to avoid failure due to flanking.

Design toe protection .

Design filter and under layers.

Provide for firm compaction of all fill and back-fill materials.

This requirement should be included on the plans and in the

specifications. Also, due allowance for compaction must be made in

the cost estimate .

Develop cost estimate for each alternative.

Provision for regular maintenance and repairs of the structure.

Position of the seawall :Determination of the beach profile and the water levels are

important.

The highest and the lowest water levels at the site must be known. The highest water level helps in deciding the exact crest level while the lowest water level guides the location of the toe.

With steeper slopes, damage to armour stones is more as compared to flat bed slope.

The seawall should be located in such a position that the maximum wave attack is taken by the armour slope and the toe.

If located above the high water level contour, the waves will break in front of the seawall causing scouring and subsequent failure.

Under estimation of maximum water level, incorrect information of beach slope considered, steeping of foreshore.

Presence of a large number of smaller stones than design size are a few of them. large percentage of undersized armour.

Stones having excessively rounded corners attribute to repetitive displacements and consequent attrition and abrasion.

The displacement of the armours has resulted in the exposure of secondary layer, which is completely exposed to the fury of waves

Under design of Armour :

Figure 3: Under Design of Armour layer Leads to Failure of Seawall

TOE PROTECTION : Toe stability is essential because failure of the toe will generally lead to

failure throughout the entire structure. Toe is generally governed by hydraulic criteria.

Factors that affect the severity of toe scour: wave breaking (near the toe),wave run-up and backwash, wave

reflection and grain size. Toe protection must consider geo-technical as well as hydraulic factors. The toe apron should be at least twice the incident wave height for sheet-pile

walls and equal to the incident wave height for gravity walls.

Figure 4: Seawall with toe protection

provision of filters :

In seawall there is removal of fill material by overtopped water, there is no proper filter between the sloping fill and the seawall.

Failure of toe leads to dislodging of armour, makes seawall ineffective.

Reformation of profile is to be done and it is necessary to provide a proper filter before reforming the section.

This can be done by dumping additional stones or retrieving some of the displaced stones.

Rounded Stones :

In order to achieve efficient interlocking, the rock should be sound and the individual units should have sharp edges.

Blunt or round edges result in poor interlocking and hence poor stability (lower stability factor KD)

Rounded stones result in lower porosity and are less efficient in dissipation of wave energy.

Figure 7: Rounded Stones in Armour Layer of a Seawall

WEAK POCKETS :Several weak spots are often present in rubble mound structures,

which may be attributable to reasons such as lack of supervision, quarry yielding smaller stones or deliberate attempts to dispose of undersized stones etc.

Concentration of stones much smaller than the required armour should therefore be avoided at any cost

Concentration of stones much smaller than the required armour should therefore be avoided at any cost

Design Wave Estimation, Wave Height and Stability Considerations :

Wave heights and periods should be chosen to produce the most critical combination of forces on a structure .

Wave characteristics may be based on – Analysis of wave gauge records Visual observations of wave action Published wave hind casts Wave forecasts or the maximum breaking wave at the site Using refraction and diffraction techniques

When selecting the height of protection, one must consider- the maximum water level any anticipated structure settlement freeboard wave run-up and overtopping.

Elevation of the structure is perhaps the single most important controlling design factor and is also critical to the performance of the structure.

Height of Protection :

Wave Run-up :Run-up is the vertical height above the still-water level (SWL) to

which the uprush from a wave will rise on a structure.

It is not the distance measured along the inclined surface.

Overtopping:Overtopping is generally preferable to design shore protection

structures to be high enough to preclude overtopping.

In some cases, however, prohibitive costs or other considerations may dictate lower structures than ideally needed.

In those cases it may be necessary to estimate the volume of water per unit time that may overtop the structure.

Overtopping :

Figure 6: Overtopping of Waves Over Seawall

The usual steps needed to develop an adequate seawall design

follow. Determine the water level range for the site Determine the wave heights  Significant wave height Hs = mean of 1/3 of the maximum waves

Depth of water = H Design pile foundations using EM 1110-2-2906.

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DESIGN PROCEDURE :

Location = India / Kerala / Alappuzha  / KamalapuramLength of Seawall = 1550 mLatitude and Longitude = 9E 24’ 13.93” N 76E 20’33.66” E

LENGTH AND LOCATION OF SEAWALL : 

The above site has been selected due to the erosion of land along the coast as compared in the pictures above.

2003 2013

Reason for Site Selection :

Design Calculation:

DETERMINING SIGNIFICANT WAVE HEIGHT :

DETERMINING WAVE PERIOD :

Select a suitable armor unit type and size

Weight of armour unit,

Wa = 𝛒a H3 / KD∆3cotø

where 𝛒a = unit wt. of armour unit

H = significant wave height

KD = stability coefficient

∆ = relative mass density

∆ = ( 𝛒a / 𝛒w ) – 1

𝛒w = density of sea water = 1.025 T/m3

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DESIGN PROCEDURE :

Crest width of armour layer

B = n K∆(Wa / 𝛒a)1/3

where n = number of stones

K∆ = layer coefficient

Thickness of armour layer

t = n K∆ (Wa / 𝛒a)1/3

where n = number of stones

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DESIGN PROCEDURE :

Under layer thickness is same as armour layer

So Weight of under layer = Wa /10 to Wa /15

Where Wa = Wt. of armour unit

Weight of core layer = Wa /100 to Wa /400

Width of toe berm = 2 x Hs

Depth of toe berm = 0.4 x d

Where Hs = design wave height

d = depth of water

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DESIGN PROCEDURE :

Significant wave height = 1.524 + 1 = 2.524 m

Depth of water = 2.524 m Time Period of Approaching waves = 7.468 sec

Weight of armour unit,

Wa = 𝛒a H3 / KD∆3cotø = 2.65 x (2.524) 3 / 2 x {(2.65/1.025) -1} 3 x 1.5 = 3.56 T

Crest width of armour layer :

B = n K∆(Wa / 𝛒a)1/3

where n = number of stones = 3 K∆ = layer coefficient

B = n x 1 x (3.56/2.65) 1/3

= 3 x 1.1034 m = 3.31 m

Thickness of armour layer :

t = n K∆ (Wa / 𝛒a)1/3

where n = number of stones = 2 = 2 x 1 x (3.56/2.65) 1/3

= 2 x 1.1034 m = 2.2068 m

Weight of under layer = Wa /10 to Wa /15 Where Wa = Wt. of armour unit

= 3.56/10 to 3.56/15 = 0.356 T to 0.237T

Weight of core layer = Wa /100 to Wa /400 = 3.56/100 to 3.56/400 = 0.0356 T to 0.0089 T

Weight of toe berm = Wa /10 to Wa /15 Where Wa = Wt. of armour unit

= 3.56/10 to 3.56/15 = 0.356 T to 0.237T

Width of toe berm = 2 x Hs = 2x 1.524= 3.05 m

Depth of toe berm = 0.4 x d = 0.4 x 3.224 = 1.29 m

Structure height = Thickness of armour layer + Thickness of under layer +Depth of toe berm + Thickness of bedding layer = 2.2068 +2.2068 +1.2896 + 1 = 6.7m

Weight of Structure = Weight of armour unit + Weight of under layer + Weight of core layer + Weight of toe berm = 3.56 + 0.356 + 0.0356 + 0.356 = 4.31T

Length of Seawall = 1550m

Weight of Structure = 4.31 x A *1550 = 1472.27T

Thanks