Debris Flow

A combination of boulders, wood and water

Changing climate has caused increase in debris flow - increases in rainfall seen = such increased frequencies make it hard to protect against all flows

Flows can be.....

• ...on slopes - material becomes unstable and flows downhill and along gullies
• ....or in channels -  mainly water charged triggered by heavy rainfall and flow down a pre-existing channel

Can also be referred to as a mudslide -

• triggered by intense rainfall or snowmelt and travels as a viscous fluid
• Can flow fast or slow depending on water capacity

Process can be divided into three stages:

• INITIATION - needs an angle/ slope gradient of more 25 degrees
• TRANSPORTATION - needs a gradient of more than 15 degrees
• DEPOSITION - needs a gradient of less than 15 degrees

Can form fans of material or levees on the side of flow channels - seds are usually coarse grained therefore deposits are poorly to very poorly stratified

May occur rapidly with little to no warning - usually occurs in areas such as Japan, USA and China (mountainous areas) = have a similar distribution to landslides

Mitigation

Can be wither passive or active:

• PASSIVE - no direct engineering as not trying to control the hazard. When one hazard has been identified the area is cleared and avoided
• ACTIVE - engineering required once the hazard has been identified e.g. ring nets

Preventative structures need to consider:

• Frequency
• Max discharge and flow depth
• Size of debris
• Likely flow paths
• Elevation of the slope
• Magnitude of the event
• Impact 

Not all…