Fluvial seismology

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Fluvial seismology is the application of seismological methods to understand river processes, such as discharge, erosion, and streambed evolution. Flowing water and the movement of sediments along the streambed generate elastic (seismic) waves that propagate into the surrounding Earth materials. [1] [2] Seismometers can record these signals, which can be analyzed to illuminate different fluvial processes such as turbulent water flow and bedload transport. [1] Seismic methods have been used to observe discharge values that range from single-digits [3] up through tens of thousands of cubic feet per second (cfs). [1]

Contents

An experiment in 1990 in the Italian Alps was one of the earliest to demonstrate that seismometers could detect discernible fluvial signals within the seismic noise generated by flow. [3] Six seismometers recorded average velocity of ground oscillations along an alpine river that was also monitored for discharge and bedload with a sediment trap. [3] They determined the lowest flow values require to initiate and maintain bedload transport. [3] Since then, fluvial seismology has become a rapidly growing area of research.

Fluvial seismology is a sub-discipline of environmental seismology, a relatively young field in which unconventional seismic signals can be detected within what was previously considered ‘noise’. [4] [5] Seismic noise is found across the full spectrum of frequencies studied in seismology (0.001–100 Hz). [6] While traditional seismology is concerned with tectonic earthquakes and the structure of the solid earth, [5] environmental seismology is concerned with waves that originate from outside the solid earth or whose signal is affected by environmental conditions (temperature, hydrology). [4] The principles of fluvial and environmental seismology can be applied to all sorts of surficial processes, including debris flows, landslides, lahars, glacial movement and icequakes, etc.

Applications

Bedload transport is among the most efficient means of erosion [6] and plays a dominant role in river evolution and morphology. [7] Understanding the forces that a river and the sediment it transports exert on the streambed are a key component of river morphological evolution. [8] High-flow, high-energy storm and flooding events in particular have an outsize effect on stream morphology and development. [7] Some applications of fluvial seismology include:

Signals

Fluvial seismology is generally confined to high-frequency seismic noise with a frequency > 1 Hz (period < 1 s). [5] [10] Observations concern the 1–100 Hz range, [11] which a theoretical forward model of seismic wave generation shows that turbulent water flow across a riverbed generates. [11]

Observations are generally made less than 100 m from the shore of the river but one study shows distinct river signals from 2 km away. [6] Deploying seismometers at different distances from the river can be helpful in distinguishing signal sources. [11]

The two main signals that have thus far been extracted from seismic noise generated by rivers are 1) the turbulent flow of water and 2) bedload transport of sediments.  Other proposed signals include interaction of the water surface with air. [1] Others suggest that further analysis may be able to discriminate between types of bedload transport – saltation vs shearing. [6]

Generally speaking, studies have found that the signal due to turbulent water flow is lower-frequency than that of bedload transport. [11] [10] For example, one study found that while discharge and water level were correlated with a signal from 1–80 Hz, the relationship was particularly strong in the 2–5 Hz and 10–15 Hz windows. [10] Meanwhile, the 30–50 Hz signal was attributed to bedload transport. [10]

Hysteresis

Hysteresis is a well-documented phenomenon observed in the seismic observation of rivers in which the same discharge does not always produce the same seismic signal. [6] If water turbulence were only source of seismic signals, the same discharge would always produce the same amplitude of the seismic response.  

Hysteresis has been observed over timescales of hours (single storms) to full years. [2] [7] [8] Hysteresis has been observed in fluvial systems in both clockwise [1] [2] [8] and counterclockwise forms, though clockwise is much more common. [12] Clockwise hysteresis is often attributed to changes in bedload transport, with a larger seismic signal observed on the rising arm of the discharge curve than on the falling arm. [3] [6]

Hysteresis is most often attributed to changing amount of sediment transported by the river. [3] [12] But while hysteresis is characteristic of the effect of bedload transport in gravel-bed rivers, [7] it is not necessarily caused by bedload transport alone. [12] Furthermore, not all bedload transport necessarily produces hysteresis. [13] Hysteresis may also be caused by changing turbulent flow as the result of changing river morphology, [13] [12] such as changing surface roughness of the streambed. [7] [12]

Improvements

Methods of fluvial seismology provide a means for continuous indirect observations of phenomena that are 1) difficult and dangerous to measure, 2) infrequent, and 3) estimated or poorly constrained. For example, bedload transport is difficult to measure directly while also dangerous during high-flow conditions. [7] [1] As a result, observations may be infrequent and limited to low-discharge conditions only, when high-flow conditions are of particular importance to stream evolution. Estimations may be limited to lab-conducted, empirically derived flume experiments. [2]

The use of seismology to understand fluvial processes is an improvement on several existing methods (such as sediment traps, direct sampling, impact plates or geophones buried in streambed) because  

  1. recordings can be made completely outside the channel, which makes observations
    • non-invasive and the observation methods do not affect flow or natural conditions [7] [1]
    • easier and more time-efficient [1]
    • safer, particularly during high-volume flood events, which are of particular interest and have an outsized effect on morphology [7]
    • cost effective by avoiding the heightened risk of losing in-stream instruments during collection [7]
  2. recordings are continuous and allow for monitoring across timescales of a single storm/flood [2] event to multi-year.
  3. can be deployed and monitored remotely. For example, in areas at high flood risk telemetered seismic data can be used to forewarn down-stream communities of potentially dangerous and catastrophic floods (in a way akin to earthquake detection and warning). [6] [9]

Related Research Articles

<span class="mw-page-title-main">Seismology</span> Scientific study of earthquakes and propagation of elastic waves through a planet

Seismology is the scientific study of earthquakes and the generation and propagation of elastic waves through the Earth or other planetary bodies. It also includes studies of earthquake environmental effects such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, glacial, fluvial, oceanic microseism, atmospheric, and artificial processes such as explosions and human activities. A related field that uses geology to infer information regarding past earthquakes is paleoseismology. A recording of Earth motion as a function of time, created by a seismograph is called a seismogram. A seismologist is a scientist working in basic or applied seismology.

<span class="mw-page-title-main">Braided river</span> Network of river channels separated by small, and often temporary, islands

A braided river consists of a network of river channels separated by small, often temporary, islands called braid bars or, in British English usage, aits or eyots.

<span class="mw-page-title-main">Alluvial fan</span> Fan-shaped deposit of sediment

An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but are also found in more humid environments subject to intense rainfall and in areas of modern glaciation. They range in area from less than 1 square kilometer (0.4 sq mi) to almost 20,000 square kilometers (7,700 sq mi).

<span class="mw-page-title-main">Fluvial sediment processes</span> Sediment processes associated with rivers and streams

In geography and geology, fluvial sediment processes or fluvial sediment transport are associated with rivers and streams and the deposits and landforms created by sediments. It can result in the formation of ripples and dunes, in fractal-shaped patterns of erosion, in complex patterns of natural river systems, and in the development of floodplains and the occurrence of flash floods. Sediment moved by water can be larger than sediment moved by air because water has both a higher density and viscosity. In typical rivers the largest carried sediment is of sand and gravel size, but larger floods can carry cobbles and even boulders. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used, as in periglacial flows and glacial lake outburst floods. Fluvial sediment processes include the motion of sediment and erosion or deposition on the river bed.

In hydrology, discharge is the volumetric flow rate of a stream. It equals the product of average flow velocity and the cross-sectional area. It includes any suspended solids, dissolved chemicals, or biologic material in addition to the water itself. Terms may vary between disciplines. For example, a fluvial hydrologist studying natural river systems may define discharge as streamflow, whereas an engineer operating a reservoir system may equate it with outflow, contrasted with inflow.

<span class="mw-page-title-main">Abrasion (geology)</span> Process of erosion

Abrasion is a process of erosion which occurs when material being transported wears away at a surface over time. It is the process of friction caused by scuffing, scratching, wearing down, marring, and rubbing away of materials. The intensity of abrasion depends on the hardness, concentration, velocity and mass of the moving particles. Abrasion generally occurs in four ways: glaciation slowly grinds rocks picked up by ice against rock surfaces; solid objects transported in river channels make abrasive surface contact with the bed and walls; objects transported in waves breaking on coastlines; and by wind transporting sand or small stones against surface rocks.

<span class="mw-page-title-main">Bar (river morphology)</span> Elevated region of sediment in a river that has been deposited by the flow

A bar in a river is an elevated region of sediment that has been deposited by the flow. Types of bars include mid-channel bars, point bars, and mouth bars. The locations of bars are determined by the geometry of the river and the flow through it. Bars reflect sediment supply conditions, and can show where sediment supply rate is greater than the transport capacity.

Three components that are included in the load of a river system are the following: dissolved load, wash load and bed material load. The bed material load is the portion of the sediment that is transported by a stream that contains material derived from the bed. Bed material load typically consists of all of the bed load, and the proportion of the suspended load that is represented in the bed sediments. It generally consists of grains coarser than 0.062 mm with the principal source being the channel bed. Its importance lies in that its composition is that of the bed, and the material in transport can therefore be actively interchanged with the bed. For this reason, bed material load exerts a control on river channel morphology. Bed load and wash load together constitute the total load of sediment in a stream. The order in which the three components of load have been considered – dissolved, wash, bed material – can be thought of as progression: of increasingly slower transport velocities, so that the load peak lags further and further behind the flow peak during any event.

A mouth bar is an element of a deltaic system, which refers to the typically mid-channel deposition of the sediment transported by the river channel at the river mouth.

<span class="mw-page-title-main">Alluvial river</span> Type of river

An alluvial river is one in which the bed and banks are made up of mobile sediment and/or soil. Alluvial rivers are self-formed, meaning that their channels are shaped by the magnitude and frequency of the floods that they experience, and the ability of these floods to erode, deposit, and transport sediment. For this reason, alluvial rivers can assume a number of forms based on the properties of their banks; the flows they experience; the local riparian ecology; and the amount, size, and type of sediment that they carry.

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Lunar seismology is the study of ground motions of the Moon and the events, typically impacts or moonquakes, that excite them.

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<span class="mw-page-title-main">Subhasish Dey</span>

Subhasish Dey is a hydraulician and educator. He is known for his research on the hydrodynamics and acclaimed for his contributions in developing theories and solution methodologies of various problems on hydrodynamics, turbulence, boundary layer, sediment transport and open channel flow. He is currently a distinguished professor of Indian Institute of Technology Jodhpur (2023–). Before, he worked as a professor of the department of civil engineering, Indian Institute of Technology Kharagpur (1998–2023), where he served as the head of the department during 2013–15 and held the position of Brahmaputra Chair Professor during 2009–14 and 2015. He also held the adjunct professor position in the Physics and Applied Mathematics Unit at Indian Statistical Institute Kolkata during 2014–19. Besides he has been named a distinguished visiting professor at the Tsinghua University in Beijing, China.

<span class="mw-page-title-main">River incision</span>

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<span class="mw-page-title-main">Seismic Experiment for Interior Structure</span> Scientific instrument aboard the InSight Mars lander

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Legacy sediment (LS) is depositional bodies of sediment inherited from the increase of human activities since the Neolithic. These include a broad range of land use and land cover changes, such as agricultural clearance, lumbering and clearance of native vegetation, mining, road building, urbanization, as well as alterations brought to river systems in the form of dams and other engineering structures meant to control and regulate natural fluvial processes (erosion, deposition, lateral migration, meandering). The concept of LS is used in geomorphology, ecology, as well as in water quality and toxicological studies.

<span class="mw-page-title-main">MERMAID</span>

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Subsurface mapping by ambient noise tomography is the mapping underground geological structures under the assistance of seismic signals. Ambient noise, which is not associated with the earthquake, is the background seismic signals. Given that the ambient noises have low frequencies in general, the further classification of ambient noise include secondary microseisms, primary microseisms, and seismic hum, based on different range of frequencies. We can utilize the ambient noise data collected by seismometers to create images for the subsurface under the following processes. The filtered ambient noise raw data Since the ambient noise is considered as diffuse wavefield, we can correlate the filtered ambient noise data from a pair of seismic stations to find the velocities of seismic wavefields. A 2-dimensional or 3-dimensional velocity map, showing the spatial velocity difference of the subsurface, can thus be created for observing the geological structures. Subsurface mapping by ambient noise tomography can be applied in different fields, such as detecting the underground void space, monitoring landslides, and mapping the crustal and upper mantle structure.

References

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