Page "Landslide" Paragraph 23

Muddy-debris flows can start as a result of slope-related factors and shallow landslides can dam stream beds, resulting in temporary water blockage.

As the impoundments fail, a " domino effect " may be created, with a remarkable growth in the volume of the flowing mass, which takes up the debris in the stream channel.

The solid-liquid mixture can reach densities of up to 2 tons / m³ and velocities of up to 14 m / s ( Chiarle and Luino, 1998 ; Arattano, 2003 ).

These processes normally cause the first severe road interruptions, due not only to deposits accumulated on the road ( from several cubic metres to hundreds of cubic metres ), but in some cases to the complete removal of bridges or roadways or railways crossing the stream channel.

Damage usually derives from a common underestimation of mud-debris flows: in the alpine valleys, for example, bridges are frequently destroyed by the impact force of the flow because their span is usually calculated only for a water discharge.

For a small basin in the Italian Alps ( area = 1. 76 km² ) affected by a debris flow, Chiarle and Luino ( 1998 ) estimated a peak discharge of 750 m < sup > 3 </ sup >/ s for a section located in the middle stretch of the main channel.

At the same cross section, the maximum foreseeable water discharge ( by HEC-1 ), was 19 m³ / s, a value about 40 times lower than that calculated for the debris flow that occurred.