USGS
U.S. GEOLOGICAL SURVEY — REDUCING RISK FROM VOLCANO HAZARDS

Distinguishing between Debris Flows and Floods from Field Evidence in Small Watersheds

P

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)

~1 m

Hyperconcentrated Flow
(here ~40 percent sediment by volume)

Debris Flow (here ~65 percent sediment by volume)
~1 m

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
January 2005

Debris Flow

DEBRIS FLOW OR FLOOD?

Deposit Margins/Surfaces l No dunes or ripples on surface

l

Lobate margins

l

l

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