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SEDIMENTOLOGI dan STRATIGRAFI(7)

HIDRODINAMIK dan BENTUKDASAR

Hadi NugrohoPS Teknik GeologiUniversitas Diponegoro2009

1Hidrodinamik dan Bentukdasar Bedforms and Structures in Granular Sediments The transport of sediment grains is often accompanied by the development and organisation into morphological features termed bedforms.

Much of the loose sediment on the Earths surface is moulded into these features which range in size from the humble (simple) and ubiquitous ripples to gigantic mountains of desert sand.

2Hidrodinamik ..... KonsepTranspor Sedimen Pengendapan dan Rezim Aliran

Pada variasi perbedaan kekuatan arus (stream power) transpor sedimen, pengendapan dan bentukdasar (bedforms) berubah-ubah serta memperlihatkan suatu karakteristik.(Simons, Richardson dan Nordin, 1965)

3HidrodinamikV = mean velocity = shear stress = viscosity = specific weightD = depthS = slope of the energy gradientt0Stream Power = V x t0 = . . D t0 = . . D. S t04

HUBUNGAN STREAM POWER, UKURAN BUTIR DAN BENTUKDASAR (Simons dkk, 1965 & Allen,1968a ). Note : a. straight-crested ripples b. undulatory ripples c. linguoid ripples d. lunate ripples5Hidrodinamik

Hubungan antara kecepatan aliran, ukuran butir dan bentukdasar ; hasil studi pada flume, kedalaman aliran 20 cm dan maksimum ukuran butir 1.5 mm, pasir kasar.(Sumber: Harms dkk, 1975) 6

DIAGRAM HJULSTROMS 7Hidrodinamik

Relationship between Current Velocity, Water Depth and to Various Bedforms.Source: Reineck and Singh, 19758HidrodinamikFlow regimesBedformsBed material concentrations (ppm)Mode of sediment transportType of roughnessLower flow regimeSmall ripples10 - 200 Discrete stepsForm roughness predominatesSmall ripple on megaripples100 - 1200 Megaripples200 - 2000TransitionWashed out megaripples1000 - 3000Variables

Upper flow regimePlane beds2000 - 6000ContinuousGrain roughness predominatesAntidunes2000Chute and pools2000Classification of Flow Regime (After Simons et al., 1965)9Hidrodinamik Rezim Aliran Rendah(Lower Flow Regime)

- Stream power < 0.5 J, tranquil flow ; and F < 1.- Resistance to flow is large and sediment transport is relatively small.The usual mode of bed material transport is for the individual grains (< 0.7 mm) to move up the back of the small ripples or megaripples, and avalanche down the lee face.

10Hidrodinamik- The bedform is either small ripples or megaripples or combinatian for grain diameter < 0.7 mm; and plane beds for grain diameter > 0.7 mm.

11Hidrodinamik- Terbentuk onggokan dan scour hasil erosi.- Transpor sedimen dengan cara diseret dan jatuh bebas ke dalam scour.- Pengaruh utama gravitasi/gayaberat.- Bentukdasar, untuk ukuran butir < 0.6 mm, terdiri dari: (1) Small ripples, termasuk di dalamnya (a) straight-crested ripples, (b) undulatory ripples, dan (c) linguoid ripples; ukuran butir > 0.6 mm: plane beds

12Hidrodinamik(2) Mega ripples: (a) straight-crested ripples, (b) undulatory ripples dan (d) lunate ripples. - Struktur sedimen, antara lain: cross-lamination tipe small trough set dan tabular set.- Sudut kemiringan cross-lamination searah dengan arah arus.13Hidrodinamik Rezim Aliran Transisi(Transition Flow Regime)- Stream power (t0 ) 0.5 2.5 J; and Froude number more or less 1 (F 1). - The bed configuration from the megaripples of the lower flow regime to the plane bed and antidunes of the upper flow regime is erratic.- The bedform may range from the one typical of the flow regime to that typical of the upper flow regime.

14Hidrodinamik Rezim Aliran Tinggi(Upper Flow Regime)

- Stream power (t0 ) more then 2.5 J; the pattern rapid folw, and Froude number > 1. - In the upper flow regime resistance of flow is small and sediment transport is large.

15Hidrodinamik- The main mode of sediment transport is for the individual grains to roll almost continuously downstream in sheets a few grain diameter thick.- The usual bedforms are plane beds and antidunes, or chute and pools (Simons et al., 1965).

16Hidrodinamik- Terbentuk onggokan-onggokan yang disebabkan karena penumpukan pada endapan yang terbentuk lebih awal.- Cara transportasi menerus, karena momentum air dan secara massal.- Pengaruh utama gaya inertial/momentum.- Struktur sedimen acretion akan terbentuk pada punggung onggokan: bentuk stratifikasi mendatar, sudut-rendah < 100; bila ukuran butir kasar, terbentuk imbricated pebble.

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