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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

Temporal changes of sediment flux ratio.(a) Results from the inversion of PSD data for bedload flux qb. The dashed line is the observed PSD for Station NZ03 averaged over 5–15 Hz. The data points are the predicted PSDs as a function of the estimated flow depth. The observed hysteresis is indicated by the black-arrowed curves. (b) The sediment suspended load flux (qs) derived from the discharge rating curve as a function of the seismologically-determined bedload flux (qb) obtained by fitting the observed PSD data. Three black dashed lines are shown to indicate given values of the sediment flux ratios of the bedload to suspended load, and are used to aid the discussion of the temporal changes of sediment flux ratio.
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f5: Temporal changes of sediment flux ratio.(a) Results from the inversion of PSD data for bedload flux qb. The dashed line is the observed PSD for Station NZ03 averaged over 5–15 Hz. The data points are the predicted PSDs as a function of the estimated flow depth. The observed hysteresis is indicated by the black-arrowed curves. (b) The sediment suspended load flux (qs) derived from the discharge rating curve as a function of the seismologically-determined bedload flux (qb) obtained by fitting the observed PSD data. Three black dashed lines are shown to indicate given values of the sediment flux ratios of the bedload to suspended load, and are used to aid the discussion of the temporal changes of sediment flux ratio.

Mentions: Clear hysteresis at Station NZ03 (r0 = 600 m) in our study, which is pronounced in the HF band with ~9 dB difference amplitude in the rising versus falling limb (Figure 4c), is in rough agreement with previous observations and predictions that show the peak signal due to water noise at lower frequencies2129. Thus, we suggest that a large portion of the HF seismic signal is due to bedload transport. However, we note that the excited frequency bands from turbulent flow and bedload sources are significantly influenced by model parameters21 (e.g., r0 and Q0). If we assume that the full river seismic noise is due to bedload transport, and that the grain size distribution (Supplementary S4.1; Figure S6a) does not change, given the estimates of water flow depth (Supplementary S4.2; Figure S6b), the average channel-bed width (W = 60 m; ref. 20), the average channel-bed slope (θ = 0.6°; ref. 26), shallow shear-wave speed, and the seismic quality factor (Q0 = 12; ref. 30), then we can predict the average bedload-induced seismic noise PSD amplitude in the 5–15 Hz range with a given bedload flux. This estimated river seismic noise level can then be used to invert for the total bedload flux. If transport is at capacity, our inversion scheme has enough flexibility in the bedload flux to fit a wide range of PSD observations. Indeed, predicted hysteresis (open circles in Figure 5a) in the seismic noise level PSDs as a function of the estimated water flow depth is in good agreement with the observations. The maximum value of qb = 4 × 10−3 m2/s occurs during the typhoon passage, coinciding with the occurrence of frequent landquake events (Figure 4a). A comparison of the sediment flux of bedload and the derived suspended load is shown in Figure 5b and is discussed in the Discussion.


Seismologically determined bedload flux during the typhoon season.

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

Temporal changes of sediment flux ratio.(a) Results from the inversion of PSD data for bedload flux qb. The dashed line is the observed PSD for Station NZ03 averaged over 5–15 Hz. The data points are the predicted PSDs as a function of the estimated flow depth. The observed hysteresis is indicated by the black-arrowed curves. (b) The sediment suspended load flux (qs) derived from the discharge rating curve as a function of the seismologically-determined bedload flux (qb) obtained by fitting the observed PSD data. Three black dashed lines are shown to indicate given values of the sediment flux ratios of the bedload to suspended load, and are used to aid the discussion of the temporal changes of sediment flux ratio.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f5: Temporal changes of sediment flux ratio.(a) Results from the inversion of PSD data for bedload flux qb. The dashed line is the observed PSD for Station NZ03 averaged over 5–15 Hz. The data points are the predicted PSDs as a function of the estimated flow depth. The observed hysteresis is indicated by the black-arrowed curves. (b) The sediment suspended load flux (qs) derived from the discharge rating curve as a function of the seismologically-determined bedload flux (qb) obtained by fitting the observed PSD data. Three black dashed lines are shown to indicate given values of the sediment flux ratios of the bedload to suspended load, and are used to aid the discussion of the temporal changes of sediment flux ratio.
Mentions: Clear hysteresis at Station NZ03 (r0 = 600 m) in our study, which is pronounced in the HF band with ~9 dB difference amplitude in the rising versus falling limb (Figure 4c), is in rough agreement with previous observations and predictions that show the peak signal due to water noise at lower frequencies2129. Thus, we suggest that a large portion of the HF seismic signal is due to bedload transport. However, we note that the excited frequency bands from turbulent flow and bedload sources are significantly influenced by model parameters21 (e.g., r0 and Q0). If we assume that the full river seismic noise is due to bedload transport, and that the grain size distribution (Supplementary S4.1; Figure S6a) does not change, given the estimates of water flow depth (Supplementary S4.2; Figure S6b), the average channel-bed width (W = 60 m; ref. 20), the average channel-bed slope (θ = 0.6°; ref. 26), shallow shear-wave speed, and the seismic quality factor (Q0 = 12; ref. 30), then we can predict the average bedload-induced seismic noise PSD amplitude in the 5–15 Hz range with a given bedload flux. This estimated river seismic noise level can then be used to invert for the total bedload flux. If transport is at capacity, our inversion scheme has enough flexibility in the bedload flux to fit a wide range of PSD observations. Indeed, predicted hysteresis (open circles in Figure 5a) in the seismic noise level PSDs as a function of the estimated water flow depth is in good agreement with the observations. The maximum value of qb = 4 × 10−3 m2/s occurs during the typhoon passage, coinciding with the occurrence of frequent landquake events (Figure 4a). A comparison of the sediment flux of bedload and the derived suspended load is shown in Figure 5b and is discussed in the Discussion.

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