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Design of Rubble Mound Seawall


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The project involves design of 1550 m long Rubble Mound Seawall at the coastline near Alappuzha (Kamalapuram), Kerala.

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Design of Rubble Mound Seawall

  1. 1. Design of Rubble Mound Seawall Harbour Structure Analysis Project By : Shailesh Shukla M.Tech – D&H Under the Guidance of : Mrs. K. Muthuchelvi Thangam, Scientist B, IMU
  2. 2. 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.
  3. 3. 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.
  4. 4. PURPOSE OF SEA WALLS : protect areas of human habitation impede the exchange of sediment between land and sea. Figure: Seawall at Bandra in Mumbai
  5. 5.  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 beach  beach material  the shape of the coast  tidal level changes  human interference  Coastal Structures :  Seawalls, revetments, anti-sea erosion bunds  System of groynes or jetties – shore connected  System of offshore breakwaters - away from the shore
  6. 6. 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
  7. 7. 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
  8. 8. 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.
  9. 9. 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.
  10. 10. 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.
  11. 11. 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.
  12. 12. 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
  13. 13. 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.
  14. 14. 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.
  15. 15. 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.
  16. 16. 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.
  17. 17. 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
  18. 18. 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
  19. 19. 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.
  20. 20. 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
  21. 21. 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
  22. 22. 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
  23. 23.  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 :
  24. 24. 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.
  25. 25. Overtopping : Figure 6: Overtopping of Waves Over Seawall
  26. 26.  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. 26 DESIGN PROCEDURE :
  27. 27. Location = India / Kerala / Alappuzha / Kamalapuram Length of Seawall = 1550 m Latitude and Longitude = 9̊ 24’ 13.93” N 76̊ 20’33.66” E LENGTH AND LOCATION OF SEAWALL :
  28. 28. 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 :
  31. 31. 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 31 DESIGN PROCEDURE :
  32. 32. 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 32 DESIGN PROCEDURE :
  33. 33. 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 33 DESIGN PROCEDURE :
  34. 34. 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
  35. 35. 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
  36. 36. 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
  37. 37. 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
  38. 38. 23 May 2015