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When good rivers turn bad: Extreme flooding
Streams and mass wasting


Gravity- Click to enlargeGravity is a force that acts everywhere on the Earth's surface, pulling everything in a direction toward the center of the Earth. On a flat surface, parallel to the Earth's surface the force of gravity acts downward. So long as the material remains on the flat surface it will not move under the force of gravity. On a slope, the force of gravity can be resolved into two components: a component acting perpendicular to the slope, and a component acting tangential to the slope.

Gravity- Click to enlargeThe perpendicular component of gravity, gp, helps to hold the object in place on the slope.

The tangential component of gravity, gt, causes a shear stress parallel to the slope and helps to move the object in the down-slope direction.

Gravity- Click to enlargeOn a steeper slope, the shear stress or tangential component of gravity, gt, increases, and the perpendicular component of gravity, gp, decreases.

Another force resisting movement down the slope is grouped under the term shear strength and includes frictional resistance and cohesion among the particles that make up the object.

When the sheer stress becomes greater than the combination of forces holding the object on the slope, the object will move down-slope.

Thus, down-slope movement is favored by steeper slope angles (increasing the shear stress) and anything that reduces the shear strength (such as lowering the cohesion among the particles or lowering the frictional resistance.

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1. Origins of extreme weather | 2. Finding hidden treasure | 3. Streams and mass wasting
4. The Johnstone River, FNQ

The resources contained in this unit are courtesy of Earth Science Australia http://earthsci.org/