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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

Study area maps.(a) Map of southern Taiwan, with the thick gray arrow depicting the path of Typhoon Nanmadol during 28–29 August 2011. (b) Distribution of the water gauge station (white circle), short-period seismic stations (inverted triangles), and rain gauge stations (rectangles) used in this study. The black inverted triangle shows a co-located seismic station and rain gauge station. The main river is shown by the blue line. Maps are created using GMT (Generic Mapping Tools, http://gmt.soest.hawaii.edu/) software.
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f1: Study area maps.(a) Map of southern Taiwan, with the thick gray arrow depicting the path of Typhoon Nanmadol during 28–29 August 2011. (b) Distribution of the water gauge station (white circle), short-period seismic stations (inverted triangles), and rain gauge stations (rectangles) used in this study. The black inverted triangle shows a co-located seismic station and rain gauge station. The main river is shown by the blue line. Maps are created using GMT (Generic Mapping Tools, http://gmt.soest.hawaii.edu/) software.

Mentions: To study the characteristics (e.g., frequency range and spatial distribution) of river-generated seismic noise, especially during the passage of Typhoon Nanmadol on 28–29 August 2011, we use records from a short-period seismic array deployed along the Chishan River in the mountain area of southern Taiwan (Figure 1 and see Methods). The Chishan River is a gravel-rich mountain stream, and the drainage area of the Chishan River and the length of its river-channel are 842 km2 and 117 km, respectively. Figure S1 shows two examples of one-day continuous records from Station NZ03, located in the vicinity of the river, which reveal large seismic noise amplitudes during the typhoon passage. Moreover, daily variations in the seismic noise are also clear, with larger noise amplitudes during the day (local time 08:00–18:00), reflecting anthropogenic activities (e.g. traffic, excavation, and construction work) in this area. A spectrogram of the vertical-component continuous seismic signal recorded at Station NZ03, with the shortest river-station distance (r0 = 600 m) of any station, is shown in Figure 2a. The short-period (≤1 sec) seismic signal is particularly well observed during Typhoon Nanmadol (August 29 to September 1). Figure 2b presents one-day average power spectral density (PSD; for further details, see Methods) amplitudes of the three-components during the typhoon passage for the same station NZ03, which are larger by ~5–12 dB relative to those during the pre-typhoon period (dashed line in Figure 2b). These temporal and spectral analyses reveal the existence of high-frequency (5–15 Hz, HF) seismic noise near the river during the typhoon passage, and this HF noise is consistent with previous studies1213. Figure S2 shows that there is an increase in the hourly seismic noise level at Station NZ03 from −173 to −152 dB during typhoon passage. In contrast, at Station NZ07, located far from the river (r0 = 1700 m), the HF signals generated by river processes are not as dominant (Figure S2). These spatial variations in the seismic noise level demonstrate that the river seismic noise has a limited propagation distance for small-magnitude typhoon events due to the rapid decay of the HF signals generated by sediment transport and hydrodynamics (Supplementary S1; Figure S2). These observations provide evidence that the HF noise signals are clearly linked to river processes, so we choose to consider this HF band of 5–15 Hz in our study.


Seismologically determined bedload flux during the typhoon season.

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

Study area maps.(a) Map of southern Taiwan, with the thick gray arrow depicting the path of Typhoon Nanmadol during 28–29 August 2011. (b) Distribution of the water gauge station (white circle), short-period seismic stations (inverted triangles), and rain gauge stations (rectangles) used in this study. The black inverted triangle shows a co-located seismic station and rain gauge station. The main river is shown by the blue line. Maps are created using GMT (Generic Mapping Tools, http://gmt.soest.hawaii.edu/) software.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Study area maps.(a) Map of southern Taiwan, with the thick gray arrow depicting the path of Typhoon Nanmadol during 28–29 August 2011. (b) Distribution of the water gauge station (white circle), short-period seismic stations (inverted triangles), and rain gauge stations (rectangles) used in this study. The black inverted triangle shows a co-located seismic station and rain gauge station. The main river is shown by the blue line. Maps are created using GMT (Generic Mapping Tools, http://gmt.soest.hawaii.edu/) software.
Mentions: To study the characteristics (e.g., frequency range and spatial distribution) of river-generated seismic noise, especially during the passage of Typhoon Nanmadol on 28–29 August 2011, we use records from a short-period seismic array deployed along the Chishan River in the mountain area of southern Taiwan (Figure 1 and see Methods). The Chishan River is a gravel-rich mountain stream, and the drainage area of the Chishan River and the length of its river-channel are 842 km2 and 117 km, respectively. Figure S1 shows two examples of one-day continuous records from Station NZ03, located in the vicinity of the river, which reveal large seismic noise amplitudes during the typhoon passage. Moreover, daily variations in the seismic noise are also clear, with larger noise amplitudes during the day (local time 08:00–18:00), reflecting anthropogenic activities (e.g. traffic, excavation, and construction work) in this area. A spectrogram of the vertical-component continuous seismic signal recorded at Station NZ03, with the shortest river-station distance (r0 = 600 m) of any station, is shown in Figure 2a. The short-period (≤1 sec) seismic signal is particularly well observed during Typhoon Nanmadol (August 29 to September 1). Figure 2b presents one-day average power spectral density (PSD; for further details, see Methods) amplitudes of the three-components during the typhoon passage for the same station NZ03, which are larger by ~5–12 dB relative to those during the pre-typhoon period (dashed line in Figure 2b). These temporal and spectral analyses reveal the existence of high-frequency (5–15 Hz, HF) seismic noise near the river during the typhoon passage, and this HF noise is consistent with previous studies1213. Figure S2 shows that there is an increase in the hourly seismic noise level at Station NZ03 from −173 to −152 dB during typhoon passage. In contrast, at Station NZ07, located far from the river (r0 = 1700 m), the HF signals generated by river processes are not as dominant (Figure S2). These spatial variations in the seismic noise level demonstrate that the river seismic noise has a limited propagation distance for small-magnitude typhoon events due to the rapid decay of the HF signals generated by sediment transport and hydrodynamics (Supplementary S1; Figure S2). These observations provide evidence that the HF noise signals are clearly linked to river processes, so we choose to consider this HF band of 5–15 Hz in our study.

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