Lecture 8 - Sedimentary notes PDF

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Lecture 8: Debris Flows- Flows of high concentration suspensions- Mudflows and debris flows are gravity-driven high concentration suspensions of rock and rockdebris in water- The high concentration of rock debris gives them the internal strength necessary to transporthuge boulders as well as buildin...

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Lecture 8: Debris Flows

- Flows of high concentration suspensions - Mudflows and debris flows are gravity-driven high concentration suspensions of rock and rock debris in water

- The high concentration of rock debris gives them the internal strength necessary to transport -

huge boulders as well as buildings and bridges and to exert extremely high impact forces against objects in their paths Debris flows are coarser and less cohesive than mudflows — cohesion decreases as particles increase Lahars are mudflows or debris flows composed of volcanic source As lahars become dilute in downstream direction they become hyper concentrated streamflows

Debris flow: - Very poorly sorted — rise from nature of flow and development of the yield strength and cant settle through the flow - Huge range of material sizes - Has a high concentration of particles - Has random orientation of clasts - Matrix supported - No sedimentary structures present - Energetic, low concentration flow e.g. Sheet flood are very different - Reverse gradding, where there is a coarsening upwards — small material goes to the bottom through gaps — initially dry material with wetter material as you move upwards — Grain flow deposits (dry) - Tested on large scale experiments Case Study: Lahars Generated from Snowmelt Triggered by Pyroclastic Flows – Nevada Del Ruiz, Colombia, 1985 - Nevado del Ruiz is the northernmost and highest Colombian volcano with historical activity - Beginning in November 1984, the volcano began showing clear signs of unrest, including earthquakes, increased fumarolic activity from the summit crater, and small phreatic explosions - Volcanic eruption on an ice cap caused this to take place - Snow was melted, then the mixture was a fluid, went down slope via gravity - Deposits were what happened at the end of the flow - A lot of erosion from high energy flow and then moved down hill along with the flow, volume increased nearly 10X from the source - Become depositional in flatter regions and erosional in steeper regions Dynamic Viscosity - RECAP - For the flows we’ve just looked at there are high concentrations of particles, controls the dynamics of the flow - High concentration suspensions - The dynamic viscosity (usually given the symbol u) is defined as

- For simple fluids, the dynamic viscosity is constant during deformation — Newtonian fluids - Fluids whose viscosity is not constant during deformation — Non-Newtonian fluids

- Most fluids are Newtonian fluids — pass through (0,0), gradient is constant - Plastic and Bingham plastic fluids need a certain amount of stress for deformation to occur, don't pass (0,0), y intercept = yield stress

- Most debris flows are considered as plastic flows

- Interaction of particles in a fluid, use the empirical law (Einstein-Roscoe eqn)

- Above 40% have a yield stress — different deformation behaviour, react with each other - Above 60-70% cant have a flow - Natural debris flow — relate to the Bingham plastic - A Bingham plastic does not deform until a critical shear stress is reached, sometimes called a yield stress — after this yield stress will deforms as a Newtonian fluid

- Viscous forces tend to resist motion - du/dy = rate of shear - Mass/Stress of fluid given as a force =

- Stress of the slope against the fluid balances stress of fluid on the slope - These materials have a field stress, if below = no deformation, if above = deformation occurs - Umax = maximum velocity of the flow...