HR: 1330h
AN: H42C-1099    [PDF]
TI: Threshold conditions for channel initiation based on hydro-geomorphic observation in the eastern Ashio Mountains, Japan
AU: * Hattanji, T
EM: hattan@atm.geo.tsukuba.ac.jp
AF: Graduate school of Geoscience, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8571 Japan
AU: Onda, Y
EM: onda@atm.geo.tsukuba.ac.jp
AF: Graduate school of Geoscience, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8571 Japan
AU: Matsukura, Y
EM: matukura@atm.geo.tsukuba.ac.jp
AF: Graduate school of Geoscience, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8571 Japan
AB: Threshold conditions for channel initiation in a humid forested mountain in the eastern Ashio Mountains, Japan, were examined from the monitored data of hydro-geomorphic processes coupled with the concept of channel initiation model. The study area is underlain by Triassic bedded chert, and has an annual rainfall of 1476 mm. Field investigation at 22 channel heads showed an inverse correlation between A (source area m$^{2}$) and S (local channel gradient below a channel head): A = 768 S$^{-2.5}$ (r$^{2}$ = 0.56). Observation of discharge at 12 springs revealed scattered area-discharge relations at the base-flow regimes, but a better area-discharge correlation (p $<$ 0.001, r$^{2}$ = 0.79) was observed at the recession stage of the largest storm flow. Stream discharge and rainfall were automatically monitored at two channel-head basins (C1 and C3U) during summer to autumn rainy season (June to October) for 3 years (2000-2002). Relation among peak discharge Q$_{p}$, source area A, and rainfall properties can be empirically expressed by a linear function here: Q$_{p}$ = k$_{p}$r$_{e}$A, where, k$_{p}$ is dimensionless coefficient on runoff peak generation, r$_{e}$ is effective rainfall rate for runoff peak generation. Maximum 4-hour rainfall (R$_{4}$) is suitable for the effective rainfall rate. Based on the regression analysis (p $<$ 0.001, r$^{2}$ = 0.85), rainfall-runoff relationship is expressed as Q$_{p}$/A = 68.66 $\times$ 10$^{-6}$ (R$_{4}$ - 0.014) m s$^{-1}$. Bed-load discharge was also observed at the channel-head sites (C1 and C3U) for 3 years (2000-2002). The result indicates that once the peak-stream discharge exceeds a critical value, bed-load discharge abruptly increases. Estimated critical discharge Q$_{cr}$ is 0.035 m$^{3}$ s$^{-1}$ at C1, and is 0.007 m$^{3}$ s$^{-1}$ at C3U. The critical discharge can be approximately expressed by a power function of Q$_{cr}$ = $\gamma$ / S$^{2}$, where $\gamma$ = 6.4 $\times$ 10$^{-3}$ m$^{3}$ s$^{-1}$. Considering the condition of Q$_{p}$ = Q$_{cr}$, the threshold condition for bed-load transport is AS$^{2}$ = $\gamma$ / k$_{p}$ r$_{e}$ = 93.4/(R$_{4}$ - 0.014) m$^{2}$. Comparing the observed channel-head location with the threshold condition, R$_{4}$ = 120 mm (0.12 m), which corresponds to the estimated return period of 20 years, is the most suitable rainfall condition for channel initiation. The channel head, which may be initiated by a rare intensive storm with quite long return period ($>>$ 20 years), cannot be maintained probably by the sediment supply from slopes.
DE: 1719 Hydrology
DE: 1824 Geomorphology (1625)
DE: 4558 Sediment transport
DE: 5415 Erosion and weathering
SC: Hydrology [H]
MN: 2003 Fall Meeting