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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

Time-series of precipitation and water discharge, and relationships between river seismic noise and water level.Comparisons of (a) cumulative precipitation (black dashed line), time-series of precipitation rate (black line), water discharge (gray line), and the occurrence of seismologically-detected landquake (hillslope failure) events (solid stars). Solid and dashed rectangles indicate time periods of typhoon passage and off-typhoon period, respectively, which are shown in Figure 5b. The average accumulated rainfall and precipitation rate are calculated from records at two rain gauge stations C0V150 and C0V250 (Figure 1b). The vertical line indicates the time point with maximum qb; and (b) vertical-component seismic noise PSDs at Station NZ03 and water level in meters above sea level (m.a.s.l.). The hourly PSDs are computed over the frequency band 5–15 Hz after removing contributions from anthropogenic sources (by removing data points during local time 4:00–20:00; gray dots in Figure S2). Water level data in the time interval indicated by the thick gray line is not useable due to recording problems. (c) Comparison of PSD amplitudes with water level at the water gauge station 1730H058 (Shan-Lin Bridge 2). The dashed line is the approximate PSD as a function of water level used in the inversion of bedload flux in this study. The color scale represents the time progression from 28 August to 5 September 2011.
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f4: Time-series of precipitation and water discharge, and relationships between river seismic noise and water level.Comparisons of (a) cumulative precipitation (black dashed line), time-series of precipitation rate (black line), water discharge (gray line), and the occurrence of seismologically-detected landquake (hillslope failure) events (solid stars). Solid and dashed rectangles indicate time periods of typhoon passage and off-typhoon period, respectively, which are shown in Figure 5b. The average accumulated rainfall and precipitation rate are calculated from records at two rain gauge stations C0V150 and C0V250 (Figure 1b). The vertical line indicates the time point with maximum qb; and (b) vertical-component seismic noise PSDs at Station NZ03 and water level in meters above sea level (m.a.s.l.). The hourly PSDs are computed over the frequency band 5–15 Hz after removing contributions from anthropogenic sources (by removing data points during local time 4:00–20:00; gray dots in Figure S2). Water level data in the time interval indicated by the thick gray line is not useable due to recording problems. (c) Comparison of PSD amplitudes with water level at the water gauge station 1730H058 (Shan-Lin Bridge 2). The dashed line is the approximate PSD as a function of water level used in the inversion of bedload flux in this study. The color scale represents the time progression from 28 August to 5 September 2011.

Mentions: At the river gauge 1730H058 (Shan-Lin Bridge 2; Figure 1b), water level was continuously measured every hour by a stage recorder. However, the suspended sediment concentration, average flow depth, average flood flow velocity, and the channel-bed width were only measured fortnightly. In particular, between 1 July and 30 September 2011, these fluvial measurements were made at an average frequency of four samples per month20. As expected, the derived water discharge positively correlates with the average cumulative precipitation (correlation coefficient of 0.95), especially during typhoon passage (Figure 4a). However, we observed strong hysteresis in the HF seismic noise levels relative to the associated hydrological parameters (Figure 4b,c and see section Methods). If turbulent dissipation were the only source of river seismic noise, we would expect a linear scaling between the observed PSD and water level12. Although river-induced seismic noise is partly generated by flow turbulence21, this mechanism fails to explain the well-developed hysteresis of the observed noise level PSD versus water level. The spatial offset between the seismic and hydrological stations provides another possible explanation for hysteresis. To check whether the spatial offset (of ~25 km in this study) is responsible for the observed hysteresis, we computed the expected temporal lag between the seismic and hydrological data using a field-measured flood flow velocity of 0.5–1.5 m/s (ref. 20). The resulting value of between 4.6 and 13.9 hours is well below the difference between the peak PSD and peak water level (24 hours; Figure 4b), and indicates that most of the hysteresis is not due to the spatial offset between the seismic and river gauge stations. Based on the time difference of the peak PSDs between Stations NZ01 and NZ03 (4 hours; Figure S2) with a spatial offset of 11.4 km, an apparent flow velocity (~0.8 m/s) can be estimated. This seismically estimated value is consistent with field measurements20. Thus, in order to invert for the sediment load flux by fitting the observed seismic noise PSDs, we apply a time correction (~6.94 hours) for the spatial offset (~25 km) between the seismic and river gauge stations using the average field-measured flood flow velocity (1.0 m/s). The resulting hysteresis trend (approximated as a dashed line in Figure 4c) is then used in the inversion.


Seismologically determined bedload flux during the typhoon season.

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

Time-series of precipitation and water discharge, and relationships between river seismic noise and water level.Comparisons of (a) cumulative precipitation (black dashed line), time-series of precipitation rate (black line), water discharge (gray line), and the occurrence of seismologically-detected landquake (hillslope failure) events (solid stars). Solid and dashed rectangles indicate time periods of typhoon passage and off-typhoon period, respectively, which are shown in Figure 5b. The average accumulated rainfall and precipitation rate are calculated from records at two rain gauge stations C0V150 and C0V250 (Figure 1b). The vertical line indicates the time point with maximum qb; and (b) vertical-component seismic noise PSDs at Station NZ03 and water level in meters above sea level (m.a.s.l.). The hourly PSDs are computed over the frequency band 5–15 Hz after removing contributions from anthropogenic sources (by removing data points during local time 4:00–20:00; gray dots in Figure S2). Water level data in the time interval indicated by the thick gray line is not useable due to recording problems. (c) Comparison of PSD amplitudes with water level at the water gauge station 1730H058 (Shan-Lin Bridge 2). The dashed line is the approximate PSD as a function of water level used in the inversion of bedload flux in this study. The color scale represents the time progression from 28 August to 5 September 2011.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Time-series of precipitation and water discharge, and relationships between river seismic noise and water level.Comparisons of (a) cumulative precipitation (black dashed line), time-series of precipitation rate (black line), water discharge (gray line), and the occurrence of seismologically-detected landquake (hillslope failure) events (solid stars). Solid and dashed rectangles indicate time periods of typhoon passage and off-typhoon period, respectively, which are shown in Figure 5b. The average accumulated rainfall and precipitation rate are calculated from records at two rain gauge stations C0V150 and C0V250 (Figure 1b). The vertical line indicates the time point with maximum qb; and (b) vertical-component seismic noise PSDs at Station NZ03 and water level in meters above sea level (m.a.s.l.). The hourly PSDs are computed over the frequency band 5–15 Hz after removing contributions from anthropogenic sources (by removing data points during local time 4:00–20:00; gray dots in Figure S2). Water level data in the time interval indicated by the thick gray line is not useable due to recording problems. (c) Comparison of PSD amplitudes with water level at the water gauge station 1730H058 (Shan-Lin Bridge 2). The dashed line is the approximate PSD as a function of water level used in the inversion of bedload flux in this study. The color scale represents the time progression from 28 August to 5 September 2011.
Mentions: At the river gauge 1730H058 (Shan-Lin Bridge 2; Figure 1b), water level was continuously measured every hour by a stage recorder. However, the suspended sediment concentration, average flow depth, average flood flow velocity, and the channel-bed width were only measured fortnightly. In particular, between 1 July and 30 September 2011, these fluvial measurements were made at an average frequency of four samples per month20. As expected, the derived water discharge positively correlates with the average cumulative precipitation (correlation coefficient of 0.95), especially during typhoon passage (Figure 4a). However, we observed strong hysteresis in the HF seismic noise levels relative to the associated hydrological parameters (Figure 4b,c and see section Methods). If turbulent dissipation were the only source of river seismic noise, we would expect a linear scaling between the observed PSD and water level12. Although river-induced seismic noise is partly generated by flow turbulence21, this mechanism fails to explain the well-developed hysteresis of the observed noise level PSD versus water level. The spatial offset between the seismic and hydrological stations provides another possible explanation for hysteresis. To check whether the spatial offset (of ~25 km in this study) is responsible for the observed hysteresis, we computed the expected temporal lag between the seismic and hydrological data using a field-measured flood flow velocity of 0.5–1.5 m/s (ref. 20). The resulting value of between 4.6 and 13.9 hours is well below the difference between the peak PSD and peak water level (24 hours; Figure 4b), and indicates that most of the hysteresis is not due to the spatial offset between the seismic and river gauge stations. Based on the time difference of the peak PSDs between Stations NZ01 and NZ03 (4 hours; Figure S2) with a spatial offset of 11.4 km, an apparent flow velocity (~0.8 m/s) can be estimated. This seismically estimated value is consistent with field measurements20. Thus, in order to invert for the sediment load flux by fitting the observed seismic noise PSDs, we apply a time correction (~6.94 hours) for the spatial offset (~25 km) between the seismic and river gauge stations using the average field-measured flood flow velocity (1.0 m/s). The resulting hysteresis trend (approximated as a dashed line in Figure 4c) is then used in the inversion.

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