FLOWS   

A.  INTRODUCTION

        Flows include any slope movement in which the materials mix
        together as they move down slope (as opposed to slides, which
        involve the movements of coherent masses along slip surfaces).
       
  

B.  TYPES OF FLOWS

        1.  CREEP  

             Creep is the imperceptibly slow down slope movement of
             earth materials, typically soil and unconsolidated materials.
             Because creep is a net motion that results from alternating 
             freezing and thawing or wetting and drying, it occurs even
             on gentle slopes

        2.  DEBRIS FLOWS

             Debris flows are dense mixtures of water and "debris" (i.e.,
             rock and/or sediments).  The percentage of water included
             in a debris flow controls its behavior, especially the velocity
             at which it moves down slope (compare this moderately wet
             flow, which slowly "oozed" down slope, to these highly fluid
             flows, which behave like streams of mud and debris).

        3.  DEBRIS AVALANCHES

             Debris avalanches are flows that occur on steep slopes and
             move in distinct channels known as tracks or chutes.  Once
             set in motion, avalanches move very quickly down slope.  

             Note: snow avalanches are not included in this presentation
             because they have different causes than is described below
             for debris avalanches.

C.  CAUSES OF FLOWS AND AVALANCHES

        Debris flows and avalanches occur when water mixes with loose
        debris on hillslopes.  There are two primary sources of the water
        that triggers these slope movements: 

        1.  RAINFALL

             Unusually heavy and/or long-duration rainfall is often a cause  
             of debris flows and debris avalanches.  A dramatic example is
             Hurricane Mitch and Casita Volcano in Nicaragua.

         2.  SNOWMELT

              Melting snow can also trigger debris flows and/or avalanches. 
              This often happens in mountainous regions during the Spring
              thaw (see, for example, Wolf Mountain, Jackson, Wyoming, or
              the Slumgullion earthflow in Colorado).
                
       
   

D.  THRESHOLDS

        How much water is needed to initiate a debris flow or avalanche?

        Rainfall intensity/duration threshold curves have been developed
        for various regions based on historic events (California example).
        The question is, why do these debris flow threshold relationships
         vary from place to place even within the same state?  

        To help answer this question, examine this linked threshold curve.

        What environmental factors influence threshold relationships?    

        2.  VEGETATIVE COVER 

             Vegetation intercepts rainfall and helps to anchor the soil.

             Vegetation can be lost through wildfires and volcanic eruptions.

        3.  ANTECEDENT MOISTURE

             The amount of rainfall that occurs prior to heavy rain storms is
             also important (see example from Seattle, Washington).

        4.  SOIL PROPERTIES

              The thickness and water absorption capacity of the soil helps
              to determine how much rain is required to initiate a flow.