Why are alluvial fans different sizes

Introduction When it rains in arid environments, such as deserts, it often floods because there is little vegetation to trap water in soils and slow the runoff. Note the conical shape and surficial flows actively depositing sediment. Three types of flows are common: 1 debris flows, 2 sheet flows, and 3 channelized flows.

Sediment Transport Processes Debris flows are slurries of mud, rock debris, and just enough water to make the sediment into a viscous flow. Note that most of the flow is restricted to the channel and the lateral variability in grain size due to energy loss on the flow boundary. This example is not on an alluvial fan; however, it does demonstrate the idea that sheet flows are not restricted to channels and typically flow over large surface areas. Larger channels may form episodically, resulting in fluctuating grain size distributions in a typical vertical facies.

Note the episodic gravel deposits between thicker, more consistent sand deposits. As mentioned before, the finer sediments are deposited most distally away from the inlet of the alluvial fan. Characteristics of Deposited Sediment Characteristics of Alluvial Fan Deposits Poorly sorted beds diamictites that are of an approximately uniform thickness but of limited lateral extent, deposited by debris flows; Moderately to well sorted sandstone beds, often normally graded with pebbles at the base deposited in ephemeral channels; these show some cross stratification due to turbulent flow dynamics; Normally graded sandstone beds that are laterally extensive deposited by sheet flows; Average grain size decreases down slope and the abundance of debris flow deposits decreases down slope.

Types of Deposits Typical of Arid Environments Wind blown sand well sorted, texturally mature medium or finer sand Flash flood deposits poorly sorted breccia, including debris flows Playa lake deposits silt, mud and evaporites. These environments have glacial deposits left by glaciers that flow in from areas with higher precipitation e.

Typical Vertical Sequence of Facies Representing this Environment A is a debris flow dominated alluvial fan B is a sheet flood dominated alluvial fan. Key Words Aridity - Aridity defines a desert, not temperature. Our average in Davis is about mm.

Arid environments are characterized by little vegetation. In this example, only areas of channelized debris flows and younger debris flows could be considered to be active. Desert pavements are surfaces of tightly packed gravel that armor, as well as rest on, a thin layer of silt, presumably formed by weathering of the gravel. They have not experienced fluvial sedimentation for a long time, as shown by the thick varnish coating the pebbles, the pronounced weathering beneath the silt layer, and the striking smoothness of the surface, caused by obliteration of the original relief by downwasting into depressions Ritter et al.

Vegetation types often differ from an alluvial surface of one age to that of another. The reasons seem to be related to the texture and composition of the sediment, as well as to the abundance and availability of moisture in the sediments. For example, on a fresh alluvial deposit, incipient soils contain little organic carbon or clay. As a result, the soils are low in nutrients and have little water-holding capacity.

Older deposits are more enriched in carbon and clay content and have higher water-holding capacities. Use and interpretation of diagnostic vegetation, just like the use and interpretation of desert pavement, varnish, or soil properties e.

For example, some mesquite species are riparian, but others can live anywhere in diminutive form. Palo Verde are more lush along waterways, but also can live well away from streams.

None to weakly developed most varnished clasts reworked from older surfaces or bedrock. None, carbonate rubble on surface surface destroyed to strongly developed petrocalcic horizon surface preserved. Plant type, as well as vegetation density and diversity, is associated with surface age. Some plant species are riparian ironwood , others are xerophitic cacti , and others are completely intolerant of moist soil e. Vegetation density and diversity are low but not nonexistent in streambeds, become most dense and diverse for intermediate-age surfaces middle to late Holocene , and become less dense and less diverse for old ridge-and-ravine surfaces.

Streamflow limits vegetation in the channels by scour and removal of plants and their root support systems, but promotes vegetation on low terraces by watering them with overbank flow and water infiltrated into the bed. Vegetation is limited on old surfaces because they receive only direct rain, are often erosional, and can be less fertile carbonate soil cropping out at the surface, for example.

On the Tortolita piedmont, in Arizona, surface age and vegetation are related in the following manner, with the most dominant plant listed first Pearthree et al. Limestone-clast pitting a. Measurements made on the surfaces of and 96 intermediate-age and older clasts. Because recent deposits are likely to be within a zone of frequent flooding, it is unlikely that mature vegetation will occur on historical deposits.

It sometimes is possible to find evidence of flood damage on vegetation, thus providing a clear means of identifying parts of fans that recently have been active. Alluvial fan flooding, as described in the committee's definition, is characterized by flow path uncertainty below the hydrographic apex and caused by abrupt deposition of sediment, proximity to a sediment or a debris flow source area, sufficient energy to carry coarse sediment at shallow flow depths, and the absence of topographic confinement which may allow higher flows to initiate a new, distinct flow path of uncertain direction.

Although such flooding occurs on the active part of an alluvial fan, the fact that an area is defined as active does not mandate that it also. Riverine flooding also occurs along the channels of many alluvial fans, especially those that are deeply incised. Even though flood hazards happen to be on alluvial fan landforms, they should be dealt with by FEMA under the guidelines established for river floodplains.

Identifying those parts of the active part of alluvial fans that are susceptible to alluvial fan flooding also requires examination of the types of flooding as recorded by flood deposits. This map is not the same as a Flood Insurance Rate Map, which is a map of different flood hazard zones. The map recommended here is one that delineates the boundaries of areas of all types of flood hazards that occur on the active parts of the alluvial fan. Such a map requires the identification of locations where flow paths are uncertain, where erosion and deposition are likely to occur, where channels with confined flow exist, where channel avulsions have occurred or might occur, where sheet flow occurs, where debris flows occur, and where channelized streamflow with overbank flooding occurs.

Mapping similar to that recommended here has been done, for example, for the Alberta Creek fan in Canada Kellerhals and Church, Such maps can be included in flood insurance studies. It is not uncommon for active parts of fans to contain stable channels that will not be susceptible to alluvial fan flooding.

These channels might become unstable in the downstream direction, as in the case of entrenched alluvial fans. On the other hand, unstable channels can become stable in the downstream direction, as in the case of the dissected toe of the Cedar Creek alluvial fan shown earlier Figure a.

FEMA maps of alluvial fans should strive to indicate those channels susceptible to riverine flooding as well as those areas prone to alluvial fan flooding. Some parts of alluvial fans are characterized by sheetflow, which is the flow of water as broad sheets that are completely unconfined by any channel boundaries.

Sheetflow might occur where flow departs from a confined channel and no new channel is formed. It might also occur where several shallow, distributary channels join together near the toe of a fan and the gradient of the fan is so low that the flows merge into a broad sheet.

Because such sheetflows can carry high concentrations of sediment in shallow water and follow unpredictable flow paths, they can be classified as alluvial fan flooding processes if they occur on alluvial fans. Sheetflow generally occurs on downslope parts of fans, where channel depths are low and the boundaries of channels become indiscernible.

They are also more common at distal locations because of the likelihood of fine-grained sediments and shallow ground water; during prolonged rainfall, the ground can become saturated, resulting in extensive sheetflooding as runoff arrives from unslope. Fine-grained sediments can aggravate the likelihood of sheetflow because some clay minerals swell when wet, forming an impermeable surface at the beginning of a rainstorm.

Some parts of alluvial fans are characterized by debris flows. Debris flows pose hazards that are very different from those of sheetflows or water flows in channels see Chapter 2. Identifying those parts of alluvial fans where debris flow deposition might occur requires the examination of deposits from past flows.

Debris flow deposits can be distinguished from fluvial. Exposures in channel banks can be examined and can be supplemented with shallow trenches in different deposits.

In an example of a channel bank exposure described by Hereford et al. For FEMA to carry out the mandates of the National Flood Insurance Program NFIP , areas that are subject to flooding during a year flood—that is, areas subject to a 1 percent chance of flooding in any year—must be identified.

The two previous sections described methods of identifying landforms subject to alluvial fan flooding and the active portions of the fan that are subject to flooding. But identification of possible hazard areas is only the first step.

The third step, one that is critical for floodplain managers and regulators, is to determine the severity and to delineate the extent of the year flood, that is, the area exposed to a 1 percent risk of flooding in any given year.

Although field work and study of aerial photographs and topographic maps are essential for carrying out the three stages necessary to identify alluvial fans and stable and unstable components of fans, the three-stage analysis can be quantified by the use of hydrologic methods.

Although it is beyond this committee's scope and resources to explore in detail the numerous methods that have been developed to evaluate flood hazards, it is appropriate to give a general overview of the methods available to delineate the actual flood hazards on a fan. Thus this section briefly addresses the techniques, the types of analysis, and the appropriate perspectives that may be of assistance in the delineation of the hazards on alluvial fans and explores their potential for assisting FEMA in its mapping responsibilities.

This discussion is not intended to be a complete exploration of all the methodologies that have been developed for hydraulic analyses, but rather it is a general introduction to several methods currently in use. In the future FEMA might consider conducting a detailed review of these methods and how they are applied. The mapping of flood risks for purposes of the NFIP is based on the flooding that is likely to result from an event that has the probability of occurrence of 1 percent in any given year, an event more commonly known as the year flood.

Within relatively stable river systems, it has been a standard practice to delineate the year floodplain using a modeling technique based on the assumption that the flow is clear water and the hydraulic conditions are such that flow is gradually varied. In many instances, this technique also is used to model more dynamic systems with some acknowledgment of its limitations, because the areas of hazard within a river valley are usually apparent and confined to a geologic floodplain.

Areas subject to alluvial fan flooding often are not as readily apparent as those subject to riverine-type flooding. The physical characteristics of the fan-shape also make the use of simplifying assumptions seem less logical and therefore less acceptable. Active alluvial fans are changeable, and erosion and deposition occur to some degree with most events. Inactive fans may also have flow paths that are unconfined and subject to uncertainty largely because of the numerous channel forks and joins.

When floodwater contains a significant amount of sediment or the flooded area is subject to scour and deposition, the flow behavior becomes less predictable. High concentrations of sediment and debris in flowing water can cause it to behave differently than clear water flows. Some of these differences, such as the unit weight, are quantitative in nature. Other differences, such as the vertical velocity distribution for a debris flow, display qualitative differences when compared to clear water.

Lobate to digitate; narrow; steep front and flanks; flat tops with low relief pressure ridges. Matrix-rich muddy ; matrix-supported clasts; poorly sorted; b max range 80— mm; stratification absent.

Matrix-rich; matrix-supported clasts; poorly sorted; b max range 60— mm; stratification absent. To investigate flood hazards, there are three general categories of interest: clear water flows that can be analyzed with traditional hydraulic methods, hyperconcentrated sediment flows that can be analyzed to a great extent by sediment transport theory, and debris flows that can be assessed by various empirical methods such as the bulking factor, the Bingham model, and other methods.

Appendix 5 of FEMA 37, Guidelines and Specifications for Study Contractors , describes a method for delineating the boundaries of flood hazards on a fan-shaped surface. This method, however, is the cause of some confusion. The method considers the conditional probability of the occurrence of a flood with a given magnitude, taking a certain path through the spatial domain, and inundating a point of interest.

The equation that allows one to apply this method is called the total probability equation. Its purpose is to compute, for example, when the combined probability of two events is equal to 0.

The events can be the occurrence of a flood, the failure of a levee, the coincidence with a different flood, the chance that floodwaters take a certain flow path, and so on. The purpose of using this method is to account for uncertainty when it cannot be easily set aside.

The use of the total probability equation is not limited to alluvial fans, and it is used by other federal agencies in addition to FEMA NRC, The method of solving the total probability equation proposed by Dawdy has been used in the preparation of several FIRMs in the western United States.

This method assumes that all areas of the fan are subject to flooding and that there is a fixed relationship between flooding depth and discharge. These assumptions apply when there is absolute uncertainty regarding how floods will occur.

The advantages of these assumptions are that they are reproducible, they lend themselves to uncomplicated regulatory implementation, and, in certain situations, they are the easiest assumptions to defend. When it comes to mitigation and the implementation of floodplain management regulations, however, it may be appropriate to review the assumption of complete uncertainty. There may be historical flow paths that are preferred during small floods. From a mitigation perspective, it would make sense to reinforce these paths rather than ignore them.

These FIRMs then are not necessarily useful to floodplain mangers and regulators who are often unaware of the procedures followed to identify the hazard to assist them in determining the hazards on a particular fan area. Since the decision on how or whether to solve the total probability equation is usually made by the flood insurance study contractor, the safe, default assumption of complete uncertainty is typically embraced to save, among other things, time. Most of the alluvial fan areas examined by the committee, however, show obvious, preferred flow directions.

Alternative solutions to the total probability equation can be applied to these areas, but the guidelines in FEMA 37 are not clear about this and suggest only that the default assumption should be a starting point. Furthermore, permission to deviate from this assumption must be obtained in writing from FEMA. All flooding sources have uncertainty.

There is an apparent contradiction between the existing definition of alluvial fan flooding, which is very inclusive, and the actual method being used to delineate the hazard, which is limited to fan-shaped landforms. Flood behavior is predictable within the expected range of uncertainty.

When the uncertainty can no longer be set aside but must be dealt with directly to achieve a reasonable result, then the total probability equation becomes a useful method for delineating flood hazards.

The applicability of the method, however, does not mean that an area is subject to alluvial fan flooding. It is merely a way of expressing uncertainty.

FEMA has not developed guidelines on the general solution of the total probability equation. The principles of risk-based analysis USACE, provide a framework for a more general and realistic way to identify areas subject to flooding with an annual probability of 1 percent.

The degree of uncertainty associated with a prediction of a given flood scenario is assessed by bringing to bear evidence derived from geomorphologic and other studies for example, an alluvial fan with a series of branching channels. Figure shows a flow diagram for conducting an analysis of diverging channels by considering various scenarios. Figure The broad spectrum of types of flooding that can occur and that have been observed on alluvial fans illustrates the futility of developing a ''cookbook" method to apply to all fans in all geographic areas.

Reviews of current research and discussions with local officials charged with regulating development in alluvial fan flooding areas indicate a prevailing preference for analysis of the flood hazards based on site-specific evaluations. The types of information that should be gathered include both geomorphic and process considerations.

For example, some flow associated with channels meets the criteria for alluvial fan flooding, even though it occurs in channels in much the same way that riverine flow occurs in channels. The difference between the two is that in alluvial fan flooding the channels are likely to shift position with time and flows often abandon one channel to form another, resulting in much unpredictability regarding the locations of future flow paths.

The following questions can provide guidelines to identify areas where flow paths are uncertain and flow is likely to leave confined channels to move in unpredictable directions:.

Because alluvial fan flooding is associated with high rates of erosion, sediment transport, and deposition, it is common for such flows to shift position as sediment is dropped and forms obstructions to the flow. In some events, previous channels are completely blocked by deposits, and a new channel is formed. This process is known as avulsion and can be identified from aerial photos or field mapping by the presence of topographic lows abandoned channels , the upstream parts of which filled with sediment.

Areas of potential channel avulsion sometimes can be identified from construction of longitudinal and cross-fan profiles, because avulsion is likely to occur in places where sedimentation has raised the channel floor surface to a level that is nearly as high as the surrounding surface of the alluvial fan. In addition, human modification of alluvial fan surfaces and urban development on alluvial fans have resulted in cases where human-made obstructions themselves have been the cause of alluvial fan flooding.

For example, construction of culverts to divert water from one part of a fan to another sometimes results in rapid sedimentation downstream from the mouth of the culvert.

The result can be that alluvial fan flooding then occurs in an area that might not have been mapped as susceptible to this type of flooding before human alteration of the landscape. Special attention is needed to identify areas where engineered works might aggravate or cause alluvial fan flooding during the time period designated as active by the investigator. Specific steps that should be followed before undertaking any final delineation of alluvial fan flooding hazards include detailed office and field reviews of historical information and the evaluation of the present landform.

Initial office procedures include the review of topographic maps and aerial photographs to determine the location and the morphology of the landform to determine whether it is a true alluvial fan. Other data that should be gathered early include historical maps and old photographs to document channel changes, changes in channel morphology, and the areas of the fan that may be classified as either active or inactive.

Soil and geologic mappings should be examined to confirm the relative geologic age of fan deposits. Climatologic data and appropriate hydrologic analyses will be needed to determine the magnitude. Aerial photographs and geologic information of the catchment area will provide indications of the amount of sediment and debris that can be delivered to the fan. Field investigations by a trained observer should include gathering information on elevation differences across the fan and in a transverse direction if detailed topographic maps are not available.

Vegetation types, soil characteristics, and the presence of desert varnish should be added to the office maps to confirm the active or inactive portions of the fan. Observations and measurements of channel conditions should be made to determine areas of possible avulsion. Detailed inspection of diffluences or abandoned channels should indicate the most likely flow paths. The results of the initial office and field investigations should provide sufficient information to direct the final analysis.

The previous three stages demonstrate that flood risk on alluvial fans is not unpredictable, but rather that it is predictable with varying degrees of uncertainty.

The assumption of a uniform risk FEMA, or complete uncertainty across an alluvial fan can be used as a formalized guess that allows one to delineate risk on the FIRM using a straightforward technique.

This technique may be reasonable for the delineation of hazards on certain alluvial fans. The method proposed by Dawdy is an insightful application of the total probability equation. Although the assumptions used to solve the equation may vary for each situation, the method itself is sound and quite general.

A FIRM showing alluvial fan flooding hazards mapped considering complete uncertainty is of little use for floodplain management. By making a conservative trade-off in favor of all possibilities, this type of FIRM ignores the importance and the more threatening hazard of flow in existing channels and historical flow paths and conversely penalizes safer areas.

The FEMA Guidelines and Specification for Study Contractors asserts that flow paths for alluvial fan flooding are unpredictable and the assumption of uniform uncertainty must be used in the hazard delineation unless written approval is sought. Approaching the wide range of alluvial fan flooding conditions from the inflexible perspective of this special case is part of the reason for the conflict surrounding this matter. The committee recommends that all efforts at mapping start with the existing channel.

For situations where there is an entrenched channel on an alluvial fan, the uncertainty may be set aside. However, elsewhere the uncertainty associated with flow path direction might cause one to select FEMA's uniform risk method. For the majority of the cases, however, consideration of specific, foreseeable scenarios based on stages 1 and 2 make the most sense.

For some undissected fans, the assumption of uniform flow path uncertainty may apply. Alluvial fan deposits in western Fresno County, California.

Journal of Geology — Alluvial fans and near surface subsidence in western Fresno County, California. Geological Survey Professional Paper A. Progress in Physical Geography 1 2 — Geomorphic Response to Climatic Change.

New York: Oxford University Press. Christenson, G. Correlation and age of Quaternary alluvial fan sequences, Basin and Range province, southwestern United States. GSA Special Paper Cooke, R. Warren, and A. Desert Geomorphology. Dawdy, D. Flood frequency estimates on alluvial fans.

Dorn, R. The role of climatic change in alluvial fan development. Abrahams and A. Parsons, eds. London, England: Chapman and Hall. Washington, D. Guidelines and specifications for study contractors. Document no. Hereford, R. Thompson, K. Burke, and H. Geological Survey Open-File Report Davis, Calif.

Keaton, J. This service is more advanced with JavaScript available. Advertisement Hide. Authors Authors and affiliations William E. Galloway David K. This process is experimental and the keywords may be updated as the learning algorithm improves. This is a preview of subscription content, log in to check access. William E.