Debris flows are a common hazard in many hillside areas of Colorado. Heavy rainfalls commonly trigger flash floods on steep slopes. These torrents pick up anything in their paths and may contain more solid material than liquid. They tear into the hillsides and deposit accumulated material when they reach flatter ground, creating an alluvial fan.
A debris flow (commonly called a mud slide) is a moving mass of loose mud, sand, soil, rock, water and air that travels down a slope under the influence of gravity. To be considered a debris flow, the moving material must be loose and capable of “flow”, and more than half of the solids in the mass must be larger than sand grains, including gravel-, pebble-, cobble- and boulder-sized material. The speed of a debris flow may reach 100 miles per hour, although most commonly they are slow and move only a few feet per year downslope.
A mud slide or mud flow is a mass of water and fine-grained earth materials that flows down a stream, ravine, canyon, arroyo, or gulch. To be considered a mud flow, more than half of the particles must be sand sized or smaller that can flow very rapidly. A mud flow is the sandy, more water-saturated analog of a debris flow.
The media often use the term mud slide for events that are technically debris or mud flows.
Noting that we were getting hundreds of search hits on a previous version of an info-brochure that we originally published in 2010, we decided to issue a new, updated version: HAZ-2021-01 Post-wildfire Hazards: Mud Slides :: Debris Flows. The subject is unfortunately very relevant given the exceptional drought conditions in the US West and elsewhere in the world exacerbating the threat of major wildfires.
Debris and mud flows are a combination of fast moving water and a great volume of sediment and debris that surges down a slope with tremendous force. The consistency is similar to pancake batter. They are similar to flash floods and may occur suddenly without time for adequate warning. Where the drainage channel trends less steep, the liquid mass spreads out and slows down to form a part of a debris fan or a mud flow deposit. In the steep channel itself, erosion is the dominant process as the flow picks up additional solid material. A drainage may have several mud flows a year, or none for several years or decades. They are common events in the steep terrain of Colorado and vary widely in size and destructiveness. Cloudbursts provide the usual source of water for a mud flow in Colorado.
The likelihood of mud flows and mud flow damage is increased by actions that increase the amount of water or soils involved. Removal of vegetation on steep slopes, dumping debris and fill in a mud flow path and improper road building or earth moving can contribute to a mud flow. The failure of a dam, irrigation ditch or other water management structure can initiate mud/debris flow if the escaping water can swiftly accumulate a large volume of soil materials. Similarly, a landslide that temporarily blocks a stream may cause or contribute to a debris flow.
H.B. 1041, Part 1, 106-7-103 (12) defines a mud flow as follows:
Mud flow means the downward movement of mud in a mountain watershed because of peculiar characteristics of extremely high sediment yield and occasional high runoff.
H.B. 1041, Part 1, 106-7-103 (4) defines a debris-fan floodplain as follows:
Debris-fan floodplain means a floodplain that is located at the mouth of a mountain valley tributary stream as such stream enters the valley floor.
A mud flow is a geologic phenomenon whereby a wet, viscous fluid mass of fine-to-coarse-grained material flows rapidly and turbulently downslope, usually in a drainageway. Typically a torrential rainfall or very rapid snowmelt runoff is the initiating factor. The result is rapid erosion and transport of poorly consolidated surficial materials that have accumulated in the upper reaches of the drainage area. Included in this complex process are such strict terms as earthflow, mud flow, and debris flow. Very high material viscosities usually result in slow earthflow movement or a combination of slow movement and internal fracturing of landslides.
Fluvial (water) transport of materials is characterized by flow of very low viscosity water and fine-grained sediments in suspension.
Mud is composed predominantly of silt and clay, whereas the term debris is commonly applied to material that consist mostly of boulders and cobbles mixed with displaced soil and vegetation.
Mud flows are typically recurrent event in certain drainage basins. The combination of climatic and geologic conditions that produce mud flows is a characteristic of mud flow-prone drainages. The moving mixture of water, soil, rock and vegetation most commonly has the consistency of freshly mixed concrete. As it moves down a drainageway, a mud flow may incorporate nearly anything in its path—trees, rocks, and debris left by previous flows—that in turn increase the erosive power and destruction energy of the moving mass. In the lower reaches of the drainageway, the stream channel may be deeply eroded, overrun and flooded by the flow, or filled, and the location and configuration altered.
A debris fan is a triangular-shaped landform that forms by deposition of material at the intersection of a tributary valley with a larger valley. The material consists of stream-flood sediments and/or mud flow material and is deposited where the stream changes gradient as it enters the larger valley.
Like the mud flows to which they are related and sometimes associated, flooding and deposition of material on debris fans are recurrent events. The cause of flooding is a cloudburst, extended rain or rapid snowmelt followed by rapid runoff into the drainage-way. As the water and associated debris move downstream, they pick up and carry large amounts of material—rocks, vegetation, soil, and at times man-made works. Farther downstream, where the drainage course is less confined by valley walls and where the stream gradient is lower, the water spreads out into multiple channels. It is this area, typically near or at the mountain front, that is called a debris-fan floodplain. At this point stream and debris velocities are lower, and there is insufficient energy to move the debris. The debris load is deposited as a mixed mass forming the debris fan, and the water progressively changes from multiple-channel flow to sheet flow.
Most mud flows in Colorado originate in drainage basins that head in high barren mountainous areas. Such areas are more susceptible to erosion by rapid runoff than are gentler, vegetated slopes. Associated debris fans and their floodplains occur mostly along mountain fronts and steep valley sides.
Severity of problem
Mud flows become a serious threat to human-made works and human life when people inadvertently choose to live in active mud flow areas. Mud flows can occur with no more advance warning than a rising storm cloud or rapid increase in springtime temperature. Most mud flows in Colorado occur in the spring and summer, the months with the greatest snowmelt runoff and rainfall.
Many scenic mountain valley areas in Colorado are under intense development pressure. The uncertain periodicity of mud and debris flows and floods, combined with the short memories of people can result in very dangerous circumstances if these mud flow prone areas are developed.
Because debris fans and mud flows are genetically related, problems associated with them are similar. The location of debris fans at mountain fronts makes them more accessible to people and development pressure.
Criteria for recognition
Nearly all mud flow areas in Colorado are located in the lower parts of tributary streams of major streams as they enter the major valley. They are most easily recognized by occurrence of recent mud flow deposits and by the distinctive undulating topography of the fan areas. The maximum extent of these deposits and the associated fan represents the probable maximum extent of mud flows and danger. This is true even though some parts of the fan may be covered by vegetation, indicating temporary inactivity. Mud flow material is a heterogeneous mixture of mud, angular pebble- to boulder-sized or larger rocks, soil, vegetation, and coarse debris of trees. The top of a mud flow or debris fan is usually rough to undulatory when larger sized material predominates and relatively smooth if most of the material in the flow is fine grained. The color and composition of the flow material is commonly similar to the predominant bedrock near the upper reaches of the drainage basin from which it was derived. At the edge of the flow area, there is a pronounced transition from disturbed vegetation and undulatory ground surface to normal vegetation and slope conditions. The most recent mud flows are nearly devoid of vegetation. The gross appearance of the mud flow area is most commonly a mud and debris-laden streambed terminating down valley as a fan in the depositional area. In the case of certain drainages that carry a large volume of water as well as occasional mud flows, the stream may cut its channel deeply into the fan rather than shifting channels constantly. In such cases the typical debris-fan topography is absent or not easily recognized and the mud and debris may be deposited in or near the stream occupying the major valley.
Preliminary recognition of debris fans is aided by their location near mountain fronts, their irregular surface, the multiplicity of small stream channels on their surface, their triangular (fan) shape, poorly sorted deposits typical of debris flows. Other criteria for recognition include bruised and/or partially buried standing trees. Careful inquiries may provide documentation of historic occurrences.
Consequences of improper land use
The consequences of improper utilization of mud flow and debris-fan areas range from occasional inconvenience to human inhabitants to loss of life and total destruction of all works of man in the area affected. Few mud flow-prone areas are suitable sites for construction of permanent structures. The unpredictable nature and often rapid movement of mud flows makes even the location of semi-permanent structures, such as mobile homes, extremely hazardous. Even in cases where either frequency or magnitude of mud or debris flows is such that some development is acceptable, the nature of old mud flow deposits is uncertain, and normal human activities such as excavations and lawn irrigation could upset and possibly reactivate movement of the deposits. In addition many fan areas have very high seasonal water tables that can adversely affect on-site sewage disposal and other planning considerations.
In general, the more hazardous mud flow and debris flow areas should be avoided. In less severe cases, careful mitigation measures and compatible kinds of development are recommended.