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Seismologically determined bedload flux during the typhoon season.

Chao WA, Wu YM, Zhao L, Tsai VC, Chen CH - Sci Rep (2015)

Bottom Line: We observe hysteresis in the high-frequency (5-15 Hz) seismic noise level relative to the associated hydrological parameters.Based on spectral characteristics of the seismic records, we also detected 20 landslide/debris flow events, which we use to estimate the sediment supply.Our study demonstrates the possibility of seismologically monitoring river bedload transport, thus providing valuable additional information for studying fluvial bedrock erosion and mountain landscape evolution.

View Article: PubMed Central - PubMed

Affiliation: Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan.

ABSTRACT
Continuous seismic records near river channels can be used to quantify the energy induced by river sediment transport. During the 2011 typhoon season, we deployed a seismic array along the Chishan River in the mountain area of southern Taiwan, where there is strong variability in water discharge and high sedimentation rates. We observe hysteresis in the high-frequency (5-15 Hz) seismic noise level relative to the associated hydrological parameters. In addition, our seismic noise analysis reveals an asymmetry and a high coherence in noise cross-correlation functions for several station pairs during the typhoon passage, which corresponds to sediment particles and turbulent flows impacting along the riverbed where the river bends sharply. Based on spectral characteristics of the seismic records, we also detected 20 landslide/debris flow events, which we use to estimate the sediment supply. Comparison of sediment flux between seismologically determined bedload and derived suspended load indicates temporal changes in the sediment flux ratio, which imply a complex transition process from the bedload regime to the suspension regime between typhoon passage and off-typhoon periods. Our study demonstrates the possibility of seismologically monitoring river bedload transport, thus providing valuable additional information for studying fluvial bedrock erosion and mountain landscape evolution.

No MeSH data available.


Related in: MedlinePlus

NPCCFs results and source locations of river seismic noise.(a) Stacked NPCCFs for three station pairs NZ03-NZ07 (top), NZ01-NZ03 (middle) and NZ01-NZ07 (bottom) on the day before (red) and during (black) the typhoon period. Gray shaded areas indicate the time windows with highly coherent signals. The dashed lines show the predicted arrival times for the best location (black star in right panel) obtained from back-projection analysis. (b) The shaded colors show a probability map for the locations of noise sources. Red colors denote areas of higher probability which are most probable sources of noise. The Chishan River is shown by a cyan line. The inverted triangles indicate the short-period seismic stations.
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f3: NPCCFs results and source locations of river seismic noise.(a) Stacked NPCCFs for three station pairs NZ03-NZ07 (top), NZ01-NZ03 (middle) and NZ01-NZ07 (bottom) on the day before (red) and during (black) the typhoon period. Gray shaded areas indicate the time windows with highly coherent signals. The dashed lines show the predicted arrival times for the best location (black star in right panel) obtained from back-projection analysis. (b) The shaded colors show a probability map for the locations of noise sources. Red colors denote areas of higher probability which are most probable sources of noise. The Chishan River is shown by a cyan line. The inverted triangles indicate the short-period seismic stations.

Mentions: To understand the spatial distribution of river seismic noise between off-typhoon periods and during typhoon passage, we use the phase cross-correlation (PCC; ref. 16) function to compute the daily noise cross-correlation function for each pair of receivers in our seismic array (further details in Methods). In Figure S3, we show the vertical-component one-day noise phase cross-correlation function (NPCCF) for the station pair NZ01-NZ03 (9.78 km apart) and observe the emergence of a signal at positive lags between 4 and 8 sec in the NPCCF. This asymmetry of the NPCCF between the acausal and causal parts indicates an inhomogeneous source distribution with a preferential azimuth for the incoming seismic waves. Next, the daily NPCCFs are stacked non-linearly using time-frequency domain phase-weighted stacks17, which attenuates signals if they do not appear with a certain regularity and coherence on individual NPCCFs in the pre-typhoon period (August 20 to August 27) and during the typhoon passage (August 28 to September 3). The stacked NPCCFs for each station pair are shown in Figure 3a. The stacked NPCCFs for pre-typhoon period are clearly different from those during typhoon passage, which indicates that strong coherent signals are generated only during the typhoon passage when the Chishan River is under extreme flow conditions (i.e., high transport capacity). Here, we use the back-projection method18 (see Methods) to locate the sources of river seismic noise which best explain the stacked NPCCFs for all pairs of stations. In contrast to recent studies which showed that the sources of river seismic noise are concentrated along the steepest portions of rivers1219, here we find the source of these highly coherent phases is localized to downstream reaches of the river (white square in Figure 1b and Figure 3b) that have gentler slopes relative to upstream20, and instead correspond to high-curvature parts of the river. We propose that the high curvature might cause sediment particle and turbulent flow impacts to be enhanced, in which case we would expect to see a similar asymmetry in the noise cross-correlation functions computed during the off-typhoon period, even though the stream power is not strong. Indeed, the stacked NPCCFs during the pre-typhoon period show strong correlation at positive lags but with less coherency than the results during typhoon passage, especially for the NZ01-NZ03 and NZ01-NZ07 station pairs (red traces in Figure 3a).


Seismologically determined bedload flux during the typhoon season.

Chao WA, Wu YM, Zhao L, Tsai VC, Chen CH - Sci Rep (2015)

NPCCFs results and source locations of river seismic noise.(a) Stacked NPCCFs for three station pairs NZ03-NZ07 (top), NZ01-NZ03 (middle) and NZ01-NZ07 (bottom) on the day before (red) and during (black) the typhoon period. Gray shaded areas indicate the time windows with highly coherent signals. The dashed lines show the predicted arrival times for the best location (black star in right panel) obtained from back-projection analysis. (b) The shaded colors show a probability map for the locations of noise sources. Red colors denote areas of higher probability which are most probable sources of noise. The Chishan River is shown by a cyan line. The inverted triangles indicate the short-period seismic stations.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4317699&req=5

f3: NPCCFs results and source locations of river seismic noise.(a) Stacked NPCCFs for three station pairs NZ03-NZ07 (top), NZ01-NZ03 (middle) and NZ01-NZ07 (bottom) on the day before (red) and during (black) the typhoon period. Gray shaded areas indicate the time windows with highly coherent signals. The dashed lines show the predicted arrival times for the best location (black star in right panel) obtained from back-projection analysis. (b) The shaded colors show a probability map for the locations of noise sources. Red colors denote areas of higher probability which are most probable sources of noise. The Chishan River is shown by a cyan line. The inverted triangles indicate the short-period seismic stations.
Mentions: To understand the spatial distribution of river seismic noise between off-typhoon periods and during typhoon passage, we use the phase cross-correlation (PCC; ref. 16) function to compute the daily noise cross-correlation function for each pair of receivers in our seismic array (further details in Methods). In Figure S3, we show the vertical-component one-day noise phase cross-correlation function (NPCCF) for the station pair NZ01-NZ03 (9.78 km apart) and observe the emergence of a signal at positive lags between 4 and 8 sec in the NPCCF. This asymmetry of the NPCCF between the acausal and causal parts indicates an inhomogeneous source distribution with a preferential azimuth for the incoming seismic waves. Next, the daily NPCCFs are stacked non-linearly using time-frequency domain phase-weighted stacks17, which attenuates signals if they do not appear with a certain regularity and coherence on individual NPCCFs in the pre-typhoon period (August 20 to August 27) and during the typhoon passage (August 28 to September 3). The stacked NPCCFs for each station pair are shown in Figure 3a. The stacked NPCCFs for pre-typhoon period are clearly different from those during typhoon passage, which indicates that strong coherent signals are generated only during the typhoon passage when the Chishan River is under extreme flow conditions (i.e., high transport capacity). Here, we use the back-projection method18 (see Methods) to locate the sources of river seismic noise which best explain the stacked NPCCFs for all pairs of stations. In contrast to recent studies which showed that the sources of river seismic noise are concentrated along the steepest portions of rivers1219, here we find the source of these highly coherent phases is localized to downstream reaches of the river (white square in Figure 1b and Figure 3b) that have gentler slopes relative to upstream20, and instead correspond to high-curvature parts of the river. We propose that the high curvature might cause sediment particle and turbulent flow impacts to be enhanced, in which case we would expect to see a similar asymmetry in the noise cross-correlation functions computed during the off-typhoon period, even though the stream power is not strong. Indeed, the stacked NPCCFs during the pre-typhoon period show strong correlation at positive lags but with less coherency than the results during typhoon passage, especially for the NZ01-NZ03 and NZ01-NZ07 station pairs (red traces in Figure 3a).

Bottom Line: We observe hysteresis in the high-frequency (5-15 Hz) seismic noise level relative to the associated hydrological parameters.Based on spectral characteristics of the seismic records, we also detected 20 landslide/debris flow events, which we use to estimate the sediment supply.Our study demonstrates the possibility of seismologically monitoring river bedload transport, thus providing valuable additional information for studying fluvial bedrock erosion and mountain landscape evolution.

View Article: PubMed Central - PubMed

Affiliation: Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan.

ABSTRACT
Continuous seismic records near river channels can be used to quantify the energy induced by river sediment transport. During the 2011 typhoon season, we deployed a seismic array along the Chishan River in the mountain area of southern Taiwan, where there is strong variability in water discharge and high sedimentation rates. We observe hysteresis in the high-frequency (5-15 Hz) seismic noise level relative to the associated hydrological parameters. In addition, our seismic noise analysis reveals an asymmetry and a high coherence in noise cross-correlation functions for several station pairs during the typhoon passage, which corresponds to sediment particles and turbulent flows impacting along the riverbed where the river bends sharply. Based on spectral characteristics of the seismic records, we also detected 20 landslide/debris flow events, which we use to estimate the sediment supply. Comparison of sediment flux between seismologically determined bedload and derived suspended load indicates temporal changes in the sediment flux ratio, which imply a complex transition process from the bedload regime to the suspension regime between typhoon passage and off-typhoon periods. Our study demonstrates the possibility of seismologically monitoring river bedload transport, thus providing valuable additional information for studying fluvial bedrock erosion and mountain landscape evolution.

No MeSH data available.


Related in: MedlinePlus