U.S. GEOLOGICAL SURVEY — REDUCING RISK FROM VOLCANO HAZARDS
Distinguishing between Debris Flows and Floods from Field Evidence in Small Watersheds
ost-ﬂood indirect measurement techniques to back-calculate ﬂood magnitude are not valid for debris ﬂows, which commonly occur in small steep watersheds during intense rainstorms. This is because debris ﬂows can move much faster than ﬂoods in steep channel reaches and much slower than ﬂoods in low-gradient reaches. In addition, debris-ﬂow deposition may drastically alter channel geometry in reaches where slope-area surveys are applied. Because high-discharge ﬂows are seldom witnessed and automated samplers are commonly plugged or destroyed, determination of ﬂow type often must be made on the basis of ﬁeld evidence preserved at the site.
High-discharge ﬂows (often indiscriminately referred to as “ﬂash ﬂoods”) in streams draining small (<20 km2), steep watersheds can involve mixtures of water and sediment in varying proportions. The relative concentration of suspended sediment plays an important role in the behavior and hazards of ﬂows. Three basic ﬂow processes are generally recognized in streams, although (1) they represent a continuum, and boundaries between ﬂow types are not sharp, and (2) any one ﬂow event may exhibit different ﬂow types at different points along the ﬂow path and at different times during the same event. The three types of ﬂow include:
Water Flow (here ~ 5 percent sediment by volume)
(here ~40 percent sediment by volume)
Debris Flow (here ~65 percent sediment by volume)
Water Flow – The amount of suspended sediment is insufﬁcient to substantially affect how ﬂowing water behaves. Newtonian ﬂuid properties are preserved. Water may appear very muddy; but most of the suspended sediment is transported near the bed. Bedload may include material up to boulder-size.
Hyperconcentrated Flow – The amount of suspended sediment is sufﬁcient to signiﬁcantly change ﬂuid properties and sedimenttransport mechanisms. Large volumes of sand are transported in dynamic suspension throughout the water column, although maintenance of high sediment loads depends on ﬂow velocity and turbulence. Flows can be highly erosive.
Debris Flow – Sediment and water mixture becomes a slurry, similar to wet concrete, capable of holding gravel-sized particles in suspension when ﬂowing slowly or stopped. In steep canyons ﬂow can achieve high velocities, transport large boulders in suspension, and cause catastrophic damage from impact or burial.
In low-gradient channels and on alluvial fans ﬂow can be slow, impeded by drier, coarse sediment at the ﬂow margins, but it can nevertheless rapidly inﬁll channels, divert streams, and destroy automobiles, buildings, and infrastructure.
U.S. Department of the Interior
U.S. Geological Survey
Arrows in above ﬁgures indicate ﬂow direction
What’s a “ﬂood?”
For the purpose of this discussion, a ﬂood is deﬁned as a high discharge, overbank ﬂow involving either water ﬂow at “normal” suspended-sediment concentrations
(generally less than 5 – 10 percent sediment by volume), or hyperconcentrated ﬂow (having from 5 – 10 percent to anywhere between 20 – 60 percent sediment by volume, depending on the relative amount of silt and clay in the ﬂuid mixture). In both cases, ﬂow behavior is controlled by the water. Flow behavior of debris ﬂows, in contrast, is signiﬁcantly controlled by the entrained sediment.
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USGS Fact Sheet 2004-3142
DEBRIS FLOW OR FLOOD?
Deposit Margins/Surfaces l No dunes or ripples on surface
l l Accumulations of coarse clasts at margins
(typically openwork); otherwise coarse clast distribution on surface is fairly random Positive relief (convex surface morphology where ﬂow “freezing” occurs); surfaces commonly studded with boulders
Flow levees common but not always formed
Consolidated sediments packed into “nooks and…