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

Distinguishing between Debris Flows and Floods 

from Field Evidence in Small Watersheds

U.S. Department of the Interior USGS Fact Sheet 2004-3142

U.S. Geological Survey January 2005

USGS

Post-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.

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.

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 – 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.

Printed on recycled paper

~1 m

Water Flow (here ~ 5 percent sediment by volume)

Debris Flow (here ~65 percent sediment by volume)

~1 m

Arrows in above figures indicate flow direction

Hyperconcentrated Flow

(here ~40 percent

sediment by volume)
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Deposit Margins/Surfaces

l No dunes or ripples on surface

l Lobate margins

l Accumulations of coarse clasts at margins 

(typically openwork); otherwise coarse 

clast distribution on surface is fairly 

random

l Positive relief (convex surface morphology 

where flow “freezing” occurs); surfaces 

commonly studded with boulders

l Flow levees common but not always formed

l Consolidated sediments packed into “nooks and crannies” – e.g., between roots in root 

wads, in cavities in trees, buildings, stream banks, etc.

l Deposits frequently dammed locally by small log jams or boulder clusters

l Fragile clasts may be present on surface (e.g., soil clasts, glass bottles)

l Sandy mud coatings on boulders, logs, banks

l No gravel imbrication

Debris Flow

DEBRIS FLOW OR FLOOD?

Damage to Vegetation 

l Severe in thalweg

l Proportional to flow velocity

l Mud coatings diagnostic

Eroded

splintered

wood

Gravel 

may be 

embedded 

in wood

Splintered

stump

Mud coating

on back side

Bark remaining

only on 

downstream

side of trunk

Maximum

stage

Branches tapered by erosion

Key Points to Remember

in Field Evaluations

l Consider all the evidence available – stratigraphic, 

sedimentologic, geomorphic, and effects on vegetation and 

structures; not all features may be present.

l A high-discharge event may involve different flow types; later 

part of flow may modify evidence left by earlier part of flow. For 

example, a debris flow peak may be followed (and typically is ) 

by recessional hyperconcentrated flow or water flow that may 

veneer the debris-flow deposit with fluvial sediments or erode 

and rework part of the debris-flow deposit.

l Examine channel over wide area (not just at gage site)

Channel Cross Section
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High velocity debris flow in Swiss Alps

Damage to Vegetation

l Normally moderate to light; can be 

severe in steep canyons

l No mud coatings

l Erosion of tree bark usually light and 

irregular; concentrated near bed 

(from saltating bed material) and 

near (sometimes above) maximum 

stage (from floating debris)

l Finer branches commonly bent but 

not broken or stripped

Flood

— FIELD EVIDENCE

© Photo courtesy of Toni Venzin, 1987, Trun, Switzerland

Deposit Margins/Surfaces

l Dunes, ripples common 

on water flow deposits 

(WFDs); no ripples/dunes 

on hyperconcentrated flow 

deposits (HCFDs)

l Longitudinal bars 

frequent on WFDs 

(typically armored with 

coarse clasts); coarse 

clasts concentrated in bars

l Surfaces cut by channels and scour depressions; 

HCFD surfaces usually studded more randomly 

with cobbles and boulders than WFDs

l Gravel levees do not form, though flotsam may 

accumulate at flow margins

l No mud coatings on boulders, logs, or banks

l Gravel clasts may be imbricated

Maximum

stage

Backpack for scale

Slope-Area Measurements for Debris Flow

Although peak discharge is the standard indicator of flood magnitude and the 

desired datum for USGS flood data bases, seldom can it be determined reliably 

for debris flows by slope-area measurements. This is because debris-flow 

velocity cannot be predicted by the Manning equation, owing to complex factors 

that affect flow resistance in dense sediment slurries and preclude accurate 

determination of Manning’s “n.” For example, boulder fronts can add friction 

at the head of flow, while incorporation of sediment into a flow or deposition of 

sediment from a flow will alter the bulk rheology and grain-fluid interactions, and 

flow velocities can be profoundly affected. Furthermore, the usefulness of a peak 

discharge measurement for a debris flow (if obtained at a critical-depth site, for 

example) is limited because orders-of-magnitude changes in peak discharge can 

occur over very short distances (hundreds of meters) downstream. Estimates 

or measurements of total planimetric area inundated or total volume delivered 

beyond a canyon mouth are more useful data for some hazard assessments. 

Looking Downstream
Debris-Flow Deposits

l Sand and fine gravel grains typically angular to subangular 

(hillslope source)

l Nonstratified, extremely poorly sorted (= massive diamictons)

l Normal and/or inverse grading common in vertical sections

l Matrix filling all voids except at margins or where washed out 

l Coherent, semi-indurated consistency; difficult to dig—outcrops 

break off in small chunks when struck or kicked

l Multiple flow units commonly indistinguishable stratigraphically

l Coarse-clast distribution fairly random in centers of deposit 

surfaces but more concentrated at deposit margins;deposit 

surfaces commonly convex upward

l Clasts oriented randomly except at flow margins

For more information, contact:

U.S. Geological Survey, Cascades Volcano Observatory

1300 SE Cardinal Court, Suite 100, Vancouver, WA 98683

Tel: (360) 993-8900; Fax: (360) 993-8980

http://vulcan.wr.usgs.gov/

or

USGS Volcano Hazards Program

http://volcanoes.usgs.gov/

Vertical Sections of Coarse Water-Flow Deposits

Vertical Sections of 

HyperconcentratedFlow Deposits

References for more information:

Costa, J.E., 1988, Rheologic, geomorphic, and sedimentologic 

differentiation of water floods, hyperconcentrated flows, and debris 

flows, in Baker, V.R., Kochel, R.C., and Patten, P.C. (eds) Flood 

Geomorphology: Wiley-Intersciences, New York, p. 113-122

 Jakob, M., and Hungr, O., 2005, Debris Flow Hazards and Related 

Phenomena: Springer-Praxis, Heidelberg

Thomas C. Pierson

Graphics and design by Christine Janda and Bobbie Myers

Photographs by T.C. Pierson, except as labeled

Flood Deposits l Most grains (all sizes) are rounded to subrounded (stream bed 

source)

l WFD sediments usually stratified – distinct laminae and beds, 

commonly with cross-bedding; HCFDs show faint horizontal 

to massive bedding with outsized individual gravel clasts and 

lenses sometimes appear as massive but poorly consolidated 

diamictons 

l In WFDs, abrupt changes in mean grain size in vertical sections; 

sorting moderate to good within individual bedding units; 

coarser clasts may be imbricated

l In HCFDs, sorting usually poor to very poor; textural changes 

usually not abrupt

l Voids common between larger clasts (openwork texture) in 

water-flow deposits, not in HCFDs

l WFD consistency is loose and friable when dry (easy to dig), 

although HCFDs slightly more consolidated

l WFD surfaces commonly have longitudinal bars (usually 

armored with coarse clasts), dunes, and/or ripples; abrupt 

changes in mean grain size between bars and channel axes are 

typical

Vertical Sections of Debris-Flow Deposits

Sediment Bedding and Textures