Efek Tegangan Geser Dasar yang Terjadi pada Lapisan Pelindung Terhadap Karakteristik Kemiringan Dasar Saluran

Cahyono Ikhsan, Adam Pamudji Raharjo, Djoko Legono, Bambang Agus Kironoto

Abstract


Abstrak. Armouring adalah suatu proses terbentuknya lapisan pelindung (armour layer) di dasar sungai yang diawali dari pergerakan sedimen dasar (bedload) sampai terjadinya erosi permukaan dasar (eroded surface) sehingga tercapai lapisan dasar yang stabil atau kondisi equilibrium yaitu tidak ada lagi sedimen dasar (bedload) yang terangkut. Armour layer merupakan jenis gravel dengan susunan gradasi butir yang hampir seragam terletak di permukaan dasar dan menghambat gerak bedload yang melintas di atasnya. Penelitian dilakukan di laboratorium Hidraulika PS-IT UGM, menggunakan perangkat utama sediment recirculacy flume pada dinding terbuat dari plexiglass dengan dimensi panjang 10,00 m, lebar 0,60 m, tinggi 0,45 m. Kemiringan dasar saluran yaitu 1%, 1.4%, 1,8%,
2,2%, dan 2,6%. Debit aliran konstan pada kapasitas 25 l/s, 30 l/s, 40 l/s dan 45 l/s. Material berupa pasir dan gravel dicampur merata, dengan 5 variasi grain size yang berbeda. Pada tiap running terdapat 2 fase kejadian yaitu fase eroded surface dan fase equilibrium. Instrumen yang digunakan pada saat running adalah digital currentmeter, point gauge, sediment traps. Pada saat proses eroded surface, struktur armour layer nampak semakin kokoh di permukaan karena tegangan geser yang dimiliki butir armour lebih besar dari tegangan geser dasar, jadi butir armour merupakan struktur butir sedimen yang memiliki critical shear stress lebih besar dari bed shear stress.

Abstract. Armouring is one of the development processes of armour layer on the base of a river, started from the movement of the transported base sediment of eroded surface reaching to stabile base layer condition or equilibrium, where there is no more transported base sediment. Armour layer is a type of gravel with almost uniform composition of granule gradation in surface base position which hampers the sediment moving above it. This research was carried out at the Hydraulic Laboratory of PS-IT UGM, using the main infrastructure of sediment recirculacy flume on the wall made from plexiglass in 10.00 m length, 0.60 m width, and 0.45 m height. The base slopes were 1%, 1.4%, 1.8%, 2.2%, and 2.6%. The flow debit was constant at 25 l/s, 30 l/s, 40 l/s and 45 l/s capacities. The material consisted of sand and gravel which was evenly mixed within 5 different variations of grainsize. At each running, there were two phases of eroded surface and equilibrium. The instruments used during the running were digital current meter, point gauge, sediment traps. Within the eroded surface process, the armour layer structure looked standing out in the base surface due to the granule critical shear stress of armour that was larger than the base shear stress, thus the armour layer was the structure of sediment granule with larger critical shear stress than bed shear stress.


Keywords


Shear stress, kemiringan dasar, armour layer, grainsize, flume

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References


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DOI: http://dx.doi.org/10.5614%2Fjts.2016.23.3.4

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