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

Debris flows

Debris flows - Click to enlarge
An ancient debris flow showing just how big they can be.

Debris flows are one of the most dangerous of all mass wasting events. They can occur suddenly and inundate entire towns in a matter of minutes. Debris flows are made of exactly what the name suggests: debris. This debris can include anything from the smallest mud particles to boulders, trees, cars, and parts of buildings. Debris flows occur when rain water begins to wash material from a slope or when water sheets off of a freshly burned stretch of land. Chapparral land is especially susceptible to debris flows after a fire. The rapidly moving water cascades down the slopes, and into the canyons and valleys below. It picks up speed and some debris as it descends the valley walls. In the valley itself, months of dry ravel, loose soil and rocks that have rolled or slid off the slope, begins to move with the water. As the system gradually picks up speed, the flow takes on the characteristics of a basic river system. The faster the water flow, the more the water can pick up. As the water picks up more mud and rocks, it begins to resemble a fast flowing river of concrete. This wall of debris can move so rapidly that it can pluck boulders the size of cars from the floors of the canyons and hurl them along the path of the flow. It's the speed and enormity of carried particles that makes a debris flow so dangerous. Boulders crash through homes and the mud-water mix fills in the rooms sometime totally overtaking the house.

Catch basin
Catch basin in British Columbia, Canada
Debris chute
Debris Chute in British Columbia, Canada

People have tried many methods for stopping or diverting debris flows. In California, catch basins have been constructed to "catch" the debris. Some basins have special overflow ducts with screens to remove the water from the flow and allow more room for the bigger items that may be washed in and take up needed space.

Debris flows happen so rapidly that there is really no way to monitor one until it is on top of you. Instruments in catch basins and flow channels can measure the rate or discharge of the flow by calculating the amount (volume) of material per unit of time (usually seconds).

Mud flows and Lahars

Slope Mud flows and Lahars - Click to enlargeMud flows and Lahars are special forms of debris flows that are mainly made of the smallest mud and silt particles. Extremely heavy rain, or a sudden thaw can trigger these types of flows. In the case of lahars, a sudden thaw of mountain snow due to a volcanic eruption can send a torrent of mud, ash, and hot water down the slope of the volcano and over neighboring towns.As can be seen from this photograph of the unfortunate town of Armero, a lahar can overtake a town far from a volcano. This lahar rushed down stream and river valleys into the town and killed over 23,000 people. They had no warning. The town was quickly buried by mud that later, as rescuers attempted to find victims, dried and hardened like cement.

Slope Mud flows and Lahars - Click to enlargeThis map shows how the lahar found it's way to Armero and the extent of the flow.

In parts of Canada and Scandinavia, a special type of mudflow can occur. Marine mud at the margins of a receding glacier can have a property known as quick clay. There is a high water content in these marine muds due to their relatively low compaction. These clays can change into a viscous fluid with only the slightest disturbance. They become flows that can move very rapidly even on a slight grade. Since these flows also give no warning, they can be very destructive.

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