The rushing water carries alluvium to a flat plain , where the stream leaves its channel to spread out. Alluvium is deposited as the stream fans out, creating the familiar triangle-shaped feature. The narrow point of the alluvial fan is called its apex , while the wide triangle is the fan's apron.
Alluvial fans can be tiny, with an apron of just a few centimeters spreading out from the trickle of a drainpipe. They can also be enormous. Over time, water flowing down the Koshi River in Nepal, for example, has built up an alluvial fan more than 15, square kilometers almost 5, square miles wide.
This "megafan" carries alluvium from the Himalaya Mountains. A bajada is the convergence, or blending, of many alluvial fans. Bajadas are common in dry climate s, such as the canyons of the American Southwest. Bajadas can be narrow, from the flow of two or three streams of water, or they can be wide, where dozens of alluvial fans converge. Alluvial fans and bajadas are often found in desert s, where flash flood s wash alluvium down from nearby hills.
They can also be found in wetter climates, where streams are more common. Alluvial fans are even found underwater. A subaqueous fan is created as an underwater current deposits alluvium from a submarine hill or glacier. Sometimes, fans are formed without the aid of water. These are called colluvial fan s. Colluvial fans are created by mass wasting. Mass wasting is simply the downward movement of rock, soil, or other material. Alluvium is material transported by water, while colluvium is material transported by mass wasting.
Landslide s are an instance of mass wasting that often create colluvial fans. A debris cone is a type of alluvial fan with a steep slope , closer to the shape of a half-cone than a flat fan.
Debris cones can be created by the slow accumulation of alluvium over many centuries. They can also form as boulder s and other large materials gather during landslides, floods, or other instances of mass wasting. Alluvial fans can be very diverse habitat s. Shrub s such as rabbitbrush and greasewood, or even trees such as ash or willow, are common in the area of alluvial fans. These plants have very deep root s, which can access the water that helped create the alluvial fan, but has now sunken far below it.
Courtesy of H. At the toe of most fans, sheetflood that has a relatively low sediment concentration because of deposition on vegetated surfaces can once again gain an erosive capacity if it is concentrated into a number of swales and small channels before entering trunk streams that drain the entire mountain front.
On streamflow fans where the sediment balance has turned negative, either at present or for some period in the recent past, the channels are deeper because sedimentation on their floors and margins is replaced by incision. The flow and sediment conveyance capacity increase because form roughness is less in the absence of aggressive bar growth. Thus, many of these channels are incised below the surrounding fan surface, and avulsion occurs less frequently or not at all in the current climatic and hydrologic regime.
The separation of the flow into diverging, smaller channels is reversed, and one or a few trunk streams convey the floodflow to the toe of the fan. Because these major conduits are incised they are not so frequently diverted by mid-channel bar deposits and they do not shift across the fan as quickly as those on actively accumulating fans.
Instead, they tend to gather local runoff generated on the fan surface because the rills and small channels produced by such runoff repeatedly erode toward the stable trunk channel. The channel network is slightly convergent downfan, and mapped contours show upfan re-entrants that reflect. Some channel banks may be colonized by trees and bushes, adding another stabilizing influence.
Because their surfaces are no longer accumulating sediment, such fans or parts of fans are said to be inactive. Incised streamflow fans are particularly well-developed in regions where a major climatic change has altered the conditions that favored sediment accumulation e.
They are also well-developed in areas where fans have been steepened tectonically as in parts of southern California. In these cases, there is a strong isolation of deeply incised channels from the surrounding ''fossil" fan surface. Thus the problem of recognition is complicated because all degrees of isolation occur, ranging from aggressive accumulation to deep incision. Chapter 3 describes field methods for identifying and mapping degrees of activity and for dating the time of latest activity on various parts of fans.
Intermediate cases of channel stability and confinement are particularly widespread and important to recognize and evaluate. They occur, for example, in diverging channel systems distributaries where the sediment balance of a reach is near-steady state. Such channels may gradually become shallower downfan until their floodwaters simply disperse as sheetflood, repeatedly spreading thin layers of sediment and water and building an apron of relatively well watered and fine sediment that supports thick vegetation.
In some years, there is accumulation of sediment, and in others there is net removal, so the bed may rise or fall by a few decimeters, but neither the scour nor the filling trend persists for long enough to raise or lower the channel bed significantly in relation to the fan surface. Alternatively, there may be a persistent but very gradual trend that causes the channel to rise, lower, or shift laterally at a rate that is difficult to detect with commonly available information e.
In other cases, a reach that has stabilized may be perturbed by runoff or sediment that enters it from an unstable reach upstream.
Thus analysis of the stability of a reach requires taking a broad view of the potential for change in channels upfan. Spatial context is important in any analysis of flooding and sedimentation hazards on a streamflow fan. Hjalmarson provides an illustrated account of various distributary-flow channels with a range of flow path stability and intensity of flood hazard. Debris flow fans occur where strongly episodic sediment transport is triggered by collapse of an accumulation of weathered rock, soil, or sediment in a steep source region or by concentration of flow onto a steep accumulation of sediment that is then trenched rapidly in such a way that a high sediment concentration is developed with a mixture of sizes, including a significant proportion of fine sediment.
The sediment-water ratio of the mixture must be so high that the flowing debris has a low permeability and water cannot drain out upward quickly enough to allow the water to separate from the sediment and the sediment to settle onto the bed. The resulting poorly sorted slurry is dense and highly viscous and travels as a laminar flow except where agitated by waterfalls and cascades, by larger rocks in the bed, or by engineering structures.
Observers often describe such flows as looking like wet concrete. Flows with. Debris flows consist of the full range of sediment sizes supplied from the source area, and flows generated from rocks of different types within the source basin may contain different proportions of clay.
The greater the proportion of fines, the greater is the internal strength of the flow because of "cohesive" bonding caused by electrical charges shared between clays and water films. Some flows are sufficiently dense and viscous to transport boulders; others leave the largest boulders behind. As the sediment-water ratio decreases i.
The flow properties of the slurries determine the fate of the debris flows when they emerge onto the fan, the nature of sediment deposition, and the resulting morphology of the deposit. These properties depend on the magnitude of the discharge and the rheological properties of the debris, which in turn are controlled by its sediment-water ratio and clay content. Discharge rate, clay content, and sediment-water ratio of each debris flow are set by the generating mechanism and the particular combination of circumstances that trigger the flow.
For example, a large rainstorm or snowmelt may generate landslides that fall into stream channels containing significant discharge, and the resulting mixture may produce a dilute debris flow. Collapse of wet debris into a steep channel network that already contains a large volume of fallen debris from centuries of slow mass-wasting on adjacent hillslopes may result in scour of that accumulation into a particularly dense and viscous, boulder-charged debris flow.
The volumes and peak flow rates of debris flows depend on 1 the magnitude of the water supplied from a rainstorm, snowmelt, lake outburst, or volcanic eruption, and 2 the volume of loose debris that is available to be liquefied by this water during the initial collapse, undermining and assimilation, or scour from the valley floor along the steep portion of the debris flow track. Thus, the debris flows that supply and mold any one fan have a probability distribution of discharges and rheological properties, which determine the nature and magnitude of flood risk.
Fortunately, these aspects of flood risk can be read from the morphology of the fan and its source basin. The range of rheological properties among debris flows emanating from the source valley usually accounts for differences in morphology on different parts of a single debris flow fan. Flows with the highest sediment-water ratios and therefore the greatest strength come to rest on relatively steep gradients typically 6 to 8 degrees on the upper parts of the fan in the form of bouldery snouts and levees.
These deposits block channels scoured by water floods between debris flow episodes and divert later flows of water or debris into new channels.
The result is a topographically rough surface of berms, lobes, and bouldery channel blockages on the upper parts of debris flow fans Figure Somewhat more dilute and weaker flows travel through the steepest channel reaches, but deposit bouldery levees as their margins are slowed. If the peak discharge rate of a debris flow exceeds the conveyance capacity of the channel, its upper part is partially decanted overbank and it travels some distance across the fan surface until it becomes slow enough and thin enough to stop as a bouldery or gravelly sheet with a sharp edge.
Stranding of boulders in levees and overbank sheets causes a progressive downfan reduction in the boulder content of flow deposits. The most dilute and weakest debris flows remain channelized as far as the lower parts of the fans, where gradients may be as low as 2 to 3 degrees.
Some of these flows halt within the channel, raising its bed and lowering its depth, while others spread over the banks onto the surface of the.
Courtesy of T. The result is a smooth surface with only an occasional boulder on the lower parts of a debris flow fan. On debris flow fans, streams are often confined to nondiverging, boulder-lined channels left by the debris flows, and therefore they neither shift across the fan nor overtop the banks in most cases, except on the lower parts of the fan where shallow channels were originally formed by the dilute, low-viscosity flows described above.
Of course, if the debris lining the channels is gravelly rather than bouldery, the capacity for channel shifting and eventual realignment by water floods is greater. Many channels on debris flow fans are single-thread depressions blocked at their upper ends by bouldery accumulations, so they are never invaded by stream floods or debris flows. Like alluvial fans, debris flow fans are subject to varying amounts of deposition and parts or even much of the fan may be inactive under the present climate.
For example, the debris flow fans emanating from the east side of the Sierra Nevada in the northwestern part of Owens Valley have more or less ceased to accumulate since the end of the last glaciation in the mountains, and the oldest parts of the fans date from previous glaciations.
Parts of fans debouching from the unglaciated southern Sierra and from the White Mountains on the western side of Owens Valley continue to receive. Descriptions of large debris flow fans in Owens Valley, California, are provided by Whipple and Dunne , and smaller debris flow fans in a wetter environment are described by Kellerhals and Church One approach to flood risk on debris flow fans concludes that even on active fans the probability of a debris flow is less than 1 percent in any one year, and therefore the "year flood" is not a debris flow but a runoff event.
This is a generalization that fails to appreciate an important aspect of debris flow initiation, namely, that it is not an independent, random event in the same way that runoff floods are assumed to be. Debris accumulates in source localities and along stream channels over timescales from decades to centuries between failures that evacuate the debris Benda and Dunne, ; Dunne, ; Reneau and Dietrich, Thus, a frequency count of dated debris flows in a region might indicate that the average frequency of occurrence is, say, years per fan with a probability of occurrence in any one year of 0.
However, if a geologist were to walk up any one of the source basins, he or she might find many potential failure sites and the channels below them to be occupied by thick layers of sediment that have accumulated since the previous debris flow occurred centuries earlier.
In a neighboring valley, recent debris flows may have stripped such sediment from the valley and reset the clock so that the probability of debris flow is virtually zero for the foreseeable future. Thus flood risk estimates can be refined by first recognizing from field evidence that debris flows are the dominant sediment transporting agent on a particular fan and then examining the source basin to determine whether debris availability favors an enhanced risk of a debris flow in the event of a large rainstorm or snowmelt.
Many fans are fed by both water floods and debris flows. Others were formed by debris flows under a different climatic regime and are now the sites of stream sedimentation and flooding only. Thus, both streamflow sediments and debris flow sediments and their associated morphologies attest to the nature of the flood risk on different parts of the same fan.
The debris flow sediments are usually concentrated on the upper, steeper parts of the fans, producing a surface laced with berms, lobes, and channel plugs. The lower, streamflow part of the fan has the characteristics of an alluvial fan described above, although there may also be a contribution of dilute debris flow deposition on these distal areas.
An indication of the relative contributions of debris flows and water floods can be obtained through systematic identification and mapping of the distribution of the two types of sediments on the fan surface and in vertical sections along the sides of channels. At the heads of some alluvial fans, channels are strongly incised in a fanhead trench, from which they emerge at some distance downfan to take on the character of a diverging braided channel network or a linear, boulder-leveed channel characteristic of debris flow fans, as described above.
This report calls this point the hydrographic apex. Several reasons for the transition are identifiable in the field. The simplest case arises on a composite fan where episodic debris flow. In this case, the water floods will scour away some of the debris flow sediment, establishing a lower-gradient channel incised within the debris flow deposits. At some distance down the fan, where the gradient of the debris flow sediment surface has diminished, the required stream gradient intersects the fan surface and a single-thread or braided channel or a swath of sheetflooding emerges from the fanhead trench at what Hooke called an intersection point , that is, a transition between flow and sediment transport process regimes.
In other cases, trenching at the fanhead or even over the entire fan may occur as a result of channel incision of older fan deposits, either because the sediment supply has diminished or because the transport capacity has increased owing to climate or vegetation changes within the source basin, or to tectonism. The roles of climatic change and tectonism in trenching the heads of fans are reviewed thoroughly by Bull Since streamflow alluvial fans typically occur in arid and mountainous environments, one of the first difficulties encountered in the quantification of alluvial fan flooding processes is the magnitude-frequency relationship for flows supplied to the apex.
The sparseness of hydrologic monitoring stations in such regions and the shortness of most records render most estimates of probable flood discharges highly uncertain. Fans receive high water discharges from hurricanes or typhoons on the subtropical eastern sides of continents, and from more localized rainstorms or from intense, persistent snowmelt in mountainous western North America.
In southern Europe, particularly in southeastern Spain, the most destructive discharges are again generated by rainstorms. In each of these regions, the history of hydrologic analysis and prediction has been one of surprises. Stream flooding on alluvial fans differs from most riverine flooding in that the hazard not only derives from the inundation itself, but also is intimately connected with sedimentation processes.
These latter have immediate impact during the flood itself, and they have long-term geomorphic influence through the rearrangement of sediment on the fan. High flood stages in channels are accompanied by high flow velocities, and by heavy loads of floating wood, and other debris in some environments.
High velocities are promoted by the relatively steep, hydraulically simple nature of the channels. The flood hazard is markedly increased, however, by the potential for channel change during the flood itself. The loose bed material may be scoured several meters deep.
On some fans the loose, unconsolidated nature of the sediments allows rapid channel widening by bank collapse if the flood persists for several hours or days. On others, the deposition of bars along a channel margin causes the channel to shift against the opposite, concave bank at rates of up to tens of meters per flood.
Thus, rapid scour and filling of the channel cause changes in the channel conveyance capacity between and during floods. The largest and most widespread threat arises, however, through the process of avulsion "tearing away" in which water escapes from a channel by scouring a new path through the bank. This process may begin by sudden bank collapse or by gradual overflow of water from the rising flood. As the relatively dilute surface water flows overbank, it often travels down a gradient that is steeper than the channel because of the convexity of the fan cross section and the perched nature of some channels above the general fan surface and so is able to pick up sediment and scour a new path.
The water may also take advantage of a former channel, or a series of abandoned channel segments. In the short-term, this process may be easily predicted if there are obvious low sections of bank or narrow levees separating the channel from much lower parts of the fan. However, it is difficult to anticipate all such weak sections of the banks and to predict the exact flow path that the diversion is likely to follow across the irregular fan surface, especially since the diverted flow has the transport capacity to modify that surface.
On the timescale of decades, it is virtually impossible, with either field inspection or mathematical modeling of sediment transport, to anticipate the locations of in channel deposition and bank erosion that might provoke avulsion. The problem is aggravated by the fact that a diversion on the upper part of the fan may alter flow paths on the lower part of the fan in ways that are independent of local fan morphology. Topographic changes in the channel network far upstream of a channel reach have the greatest potential for radically altering the risk of inundation, overtopping channel banks, or undermining a site downfan, but are probably the most difficult threat to anticipate and quantify.
Sheetfloods spread extensively on low relief lower parts of fan, and as they decelerate they often deposit sheets and low bars of sand or gravel. Even though velocities and depths are low, inundation by turbid water can be very destructive. Debris flows are dense approximately 1. They can transport boulders up to several meters in diameter, either as individuals supported in the matrix of the flow or as dams of boulders pushed along at the front.
Some debris flows consist of waves of slurry behind bouldery dams Sharp and Nobles, ; Suwa and Okuda, Large woody debris, engineering artifacts, and vehicles are also transported by debris flows and can because blockage, flow diversion, and extra damage to houses and other structures downfan.
As they approach their final deposition point, debris flow sediments acquire a finite yield strength that prevents them from draining away like water. They remain as permanent covers on fan surfaces, and are expensive to remove from urban areas or channels. On the other hand, however, they are less likely than water flows to undermine and destroy a road, so once cleared the road is generally still useable.
In extreme cases, such as after particularly large debris flows on fans near active volcanoes, deposits may be so thick and extensive that they permanently bury settlements. The deposits also block drainage in valley floors and at tributary junctions. Some aspects of the prediction of debris flow-frequency and magnitude at the fan apex are more difficult than is the case for water floods, but other characteristics of debris flow occurrence simplify the problem.
Within the United States, there are no monitoring stations with records long enough to provide a representative sample of debris flow occurrence on which a probability analysis might be based. A procedure commonly used by flood control agencies involves using records of runoff for prediction of a water flood peak with a 1 percent probability of occurrence,.
Although such a procedure might give a reasonable answer for those hyperconcentrated flows generated by runoff processes, it is wrong to mix in a probability analysis the results of runoff processes gauging station records of floods with debris flows, which are the usually triggered by some form of mass failure.
A particularly misleading situation arises when the assumption of interannual independence that has been found to be a useful approximation for rain-generated and snowmelt floods is applied to debris flow occurrence. This is because the occurrence of a debris flow in one year substantially reduces the probability of future debris flows by removing the sedimentary accumulations required for their generation and growth Benda and Dunne, ; Keaton, ; Keaton et al.
Fortunately, it is often possible to identify through field observations those conditions that favor the generation and growth of debris flows. For example, deep accumulations of colluvium on bedrock indicate a relatively high probability of debris flow occurrence in comparison to that in a basin in which most of the colluvium was evacuated in a relatively recent meteorologic event, after a forest fire, or after a climatic change.
Thick accumulations of sediment along channels upstream of a fan indicate that no debris flow has passed for a considerable amount of time and therefore that the conditions are evolving toward a failure that could convey large quantities of sediment from canyon floors to the fan. Such observations combined with a probability analysis of rainfall or snowmelt required to trigger a mass failure are required for estimating the debris flow risk at the apex.
Estimating the probable magnitude is more time-consuming, since it requires documenting volumes of sediment in old debris flow deposits or in the valleys above the fan. Avulsions of debris flows occur on boulder-rich fans and are particularly difficult to forecast because of the uncertainty about the magnitude and rheology of the next debris flow.
However, some clues to the likelihood of an avulsion occurring can be obtained from field inspection of the morphology and sedimentology of the fan itself. In particular, useful indications of the avulsion potential might be provided by 1 the volumes of sediment susceptible to liquefaction in the source area Keaton, ; Keaton et al. Calculations of the channel conveyance capacity for debris flows with a range of rheology can be made for various channel cross sections down the fan to judge the potential for overbank flow and spreading Whipple, ; Whipple and Dunne, At the distal margins of debris flow fans, low-strength flows often spread widely in a manner similar to sheetflooding on streamflow alluvial fans Figure A particularly hazardous situation arises on debris flow fans around active volcanoes because of the huge volumes of sediment that can be liquefied and the persistence of the liquefaction.
For example, the October lahar volcanic debris flow generated by a typhoon from the slopes of Mt. Pinatubo in the Philippines deposited approximately 50 million cubic meters 1.
Helens eruption of May 18, , deposited approximately million cubic meters 3. Colluvial fans are created by mass wasting. Mass wasting is simply the downward movement of rock, soil, or other material. Alluvium is material transported by water, while colluvium is material transported by mass wasting. Landslide s are an instance of mass wasting that often create colluvial fans. A debris cone is a type of alluvial fan with a steep slope , closer to the shape of a half-cone than a flat fan.
Debris cones can be created by the slow accumulation of alluvium over many centuries. They can also form as boulder s and other large materials gather during landslides, floods, or other instances of mass wasting. Alluvial fans can be very diverse habitat s. Shrub s such as rabbitbrush and greasewood, or even trees such as ash or willow, are common in the area of alluvial fans. These plants have very deep root s, which can access the water that helped create the alluvial fan, but has now sunken far below it.
Creating a settlement on an alluvial fan can be dangerous.
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