Concrete Tidal Wall PEMBAHASANTembok laut beton adalah struktur bangunan yang keras dan kuat yang dibangun di bagian dalam pantai untuk mengurangi kekuatan gelombang (wikipedia).

Gambar 1. Tampang melintang concrete tidal wall

Tembok laut beton pada umumnya berukuran standar, hanya tinggi puncak saja yang dihitung berdasarkan kondisi hidrolis setempat.

Dalam penentuan desain awal sebuah tembok laut beton harus sudah ditentukan mengenai fungsi dan strukturnya. Dengan tujuan utama sebagai pengurang atau penghambat air laut masuk ke wilayah daratan, maka sebaiknya bangunan ini ditempatkan di daerah agak jauh dari pantai dan disarankan untuk membuatnya di daerah perumahan penduduk. Jika ombak atau gelombang yang terjadi masih sangat besar, maka diperlukan juga beberapa bangunan pelindung pantai lainnya di depan tembok laut ini. Pondasi tiang (sheet pile) juga menjadi parameter yang harus di perhatikan. Pondasi ini sangat bergantung kepada parameter tanah seperti tegangan geser (shear capacity) dan daya dukung tanah (bearing capacity), tetapi pada kesempatan ini saya hanya akan membahasa mengenai bangunan tembok laut tanpa menggunakan pondasi tiang.

Pembuatan tembok laut beton ini didasarkan pada besarnya serangan gelombang. Di sini ada dua jenis beton penahan yang akan dibahas, yakni :

- Tembok laut tanpa pelindung (tidak memperhitungkan gelombang yang terjadi).- Tembok laut dengan lapisan pelindung tumpukan batu (memperhitungkan serangan gelombang).

berikut adalah hal-hal yang pelu diperhatikan dalam penyusunan desain utama tembok laut :1. Tinggi puncak, meliputi desain permukaan air dan tinggi jagaan (kalkulasi tinggi puncak).Untuk menentukan tinggi puncak gelombang pada bangunan beton, digunakan perumusan :

h = HAT + SLR + LSdengan ; h : Desain permukaan air, sesuai MSL (m).HAT : High Astronomical Tide, mengacu terhadap MSL (m).SLR : Estimasi kenaikan permukaan air laut untuk 25 tahun (m).LS : Estimasi penurunan muka tanah untuk 25 tahun (m).

Catatan : untuk perhitungan serangan gelombang selalu diperhitungkan terbatas dan relatif karena penempatan tembok laut beton ini berada di wilayah pemukiman. Pembuatan pelindung dasar rubble mound hanya untuk mengurangi gerusan yang berada di depan bangunan. Hempasan dan runup gelombang tidak diperhitungkan dalam skala signifikan. Dan tinggi jagaan Rc, nilai minimal yang dipakai adalah 0,5 m.

2. Tanggul tanah, meliputi keseimbangan vertikal dan panjang piping.Tujuan dari pembuatan tanggul tanah ini adalah untuk membuat tembok laut beton ini menjadi kedap air. Untuk ukurannya ditentukan oleh kriteria sebagai berikut :- Tekanan air tanah (uplifting).Tekanan air tanah yang berada di belakang bangunan tembok laut harus memiliki tinggi minimal untuk mencegah terjadinya timbulan (heave). Timbulan adalah suatu proses dimana air yang berada di bawah tanggul tanah timbul ke atas permukaan. Tekanan bawah yang didesak oleh berat tanah harus lebih besar dari tekanan atas yang disebabkan oleh permukaan air melewati bangunan.

- Piping.Panjang timbunan yang berada di belakang bangunan tembok laut ditentukan oleh kebutuhan piping. Panjang pola aliran di bawah bangunan harus cukup besar untuk mencegah terbentuknya saluran-saluran kecil karena aliran air ini dapat merusak bangunan. Akan tetapi, jika panjang timbunan terlalu pendek maka air akan merembes dan mengakibatkan banjir di daratan. Berikut adalah parameter piping untuk material tanah yang berbeda.

3. Stabilitas geoteknikal, meliputi prosedur dan kesalahan mekanisme (kontruksi cutoff-wall).Stabilitas geoteknikal ditentukan oleh gaya horizontal dan vertikal dari tanah dan air. Perhitungan tentang masalah ini dilakukan oleh ahli geoteknik.

Dengan adanya perhitungan mekanika tanah maka kita bisa mengetahui apakah suatu bangunan sesuai dengan stablitas yang kita inginkan. Hal ini ditentukan oleh ketahanan struktur tembok beton terhadap kesalahan mekanisme sebagai berikut :

- Terangkat (uplifting)Tekanan air di bawah bangunan menginduksi tekanan di atasnya. Tekanan di atas adalah gaya gerak untuk mengankat bangunan ke atas. Jika tanpa pondasi tiang, maka gaya tekan hanya terdiri dari berat bangunan saja.

- Bergeser (sliding)

- Tidak stabil (overtuning)Tekanan pada setiap sisi menyebabkan bangunan cenderung berotasi pada bangunan kaki. Perubahan momen gerak disebabkan oleh tekanan tanah pada satu sisi, sementara tekanan vertikal pada bangunan dan tekanan yanah pada sisi lain menginduksi momen.

- Terbalik (ecccentricity)Tanah yang berada di bawah bangunan dapat turun karena ketidakstabilan gaya residual, seperti kapasitas tanah yang terjadi lebih rendah dari nol pada kaki.Read more "Concrete Tidal Wall..." Diposkan oleh Putu Aditya Setiawan di 18:59 0 komentar Label: kompetisi website kompas muda-KFC Seawall types

TypeIllustrationAdvantagesDisadvantagesExample

VerticalVertical seawalls are built in particularly exposed situations. These reflect wave energy. Under storm conditions a non-breaking standing wave pattern can form, resulting in a stationary clapotic wave which moves up and down but does not travel horizontally.[11][12] These waves promote erosion at the toe of the wall and can cause severe damage to the sea wall.[13] In some cases piles are placed in front of the wall to lessen wave energy slightly.

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. These are partial to a lot of expensive damage in a short period of time. Vertical design can be undercut by high-wave energy environments over a long period of time.

CurvedCurved or stepped seawalls are designed to enable waves to break to dissipate wave energy and to repel waves back to the sea. The curve can also prevent the wave overtopping the wall and provides additional protection for the toe of the wall.

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. More complex engineering and design process. The deflected waves can scour material at the base of the wall causing them to become undermined.

MoundMound type seawalls, using revetments or riprap, are used in less demanding settings where lower energy erosional processes operate. The least exposed sites involve the lowest-cost bulkheads and revetments of sand bags or geotextiles. These serve to armour the shore and minimise erosion and may be either watertight or porous, which allows water to filter through after the wave energy has been dissipated.[14]

Current designs use porous designs of rock, concrete armour. Slope and loose material ensure maximum dissipation of wave energy. Lower cost option. Less durable. Shorter life expectancy. Cannot withstand or protect from high-energy conditions effectively.

Trade-offsA cost benefit approach is an effective way to determine whether a seawall is appropriate and whether the benefits are worth the expense. Besides controlling erosion, consideration must be given to the effects of hardening a shoreline on natural coastal ecosystems and human property or activities. A seawall is a static feature which can conflict with the dynamic nature of the coast and impede the exchange of sediment between land and sea. The table below summarises some positive and negative effects of seawalls which can be used when comparing their effectiveness with other coastal management options, such as beach nourishment.Advantages and disadvantages of seawalls according to Short (1999)[15]

AdvantagesDisadvantages

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. 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.

Generally seawalls can be a successful way to control coastal erosion, but only if they are constructed well and out of materials which can withstand the force of ongoing wave energy. Some understanding is needed of the coastal processes and morphodynamics specific to the seawall location. Seawalls can be very helpful; they can offer a more long-term solution than soft engineering options, additionally providing recreation opportunities and protection from extreme events as well as everyday erosion. Extreme natural events expose weaknesses in the performance of seawalls, and analyses of these can lead to future improvements and reassessment.

SeawallFrom Wikipedia, the free encyclopediaIt has been suggested that Tsunami barrier be merged into this article. (Discuss) Proposed since July 2014.

An example of a modern seawall in Ventnor on the Isle of Wight (tetrapods not part of wall)

People socializing and walking at the Malecn, Havana

Seawall at Urangan, QueenslandA seawall (or sea wall) is a form of coastal defense constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a seawall is to protect areas of human habitation, conservation and leisure activities from the action of tides and waves.[1] As a seawall is a static feature it will conflict with the dynamic nature of the coast and impede the exchange of sediment between land and sea.[2]The coast is generally a high-energy, dynamic environment with spatial variations occurring over a wide range of temporal scales.[3] The shoreline is part of the coastal interface which is exposed to a wide range of erosional processes arising from fluvial, aeolian and terrestrial sources, meaning that a combination of denudational processes will work against a seawall.[4] Given the natural forces to which seawalls are constantly subjected, maintenance (and eventually replacement) is an ongoing requirement if they are to provide an effective long-term solution.The many types in use today reflect both the varying physical forces they are designed to withstand, and location specific aspects, such as: local climate, coastal position, wave regime, and value of landform. Seawalls are classified as a hard engineering shore based structure used to provide protection and to lessen coastal erosion. However, a range of environmental problems and issues may arise from the construction of a seawall, including disrupting sediment movement and transport patterns, which are discussed in more detail below.[5] Combined with a high construction cost, this has led to an increasing use of other soft engineering coastal management options such as beach replenishment.Seawalls may be constructed from a variety of materials, most commonly: reinforced concrete, boulders, steel, or gabions. Additional seawall construction materials may include: vinyl, wood, aluminium, fibreglass composite, and with large biodegrable sandbags made of jute and coir.[6] In the UK, sea wall also refers to an earthen bank used to create a polder, or a dike construction.