Determination of characteristic water discharges causing most intensive influence on the river channel-forming process (by example of rivers of Tomsk region)

The term of river channel-forming water discharge is widely used for characterizationof the influence of a stream on its channel experiencing deformations. Many different interpretations of this term exist and manymeasures of channel-forming discharge are most commonly applied: channel-forming discharge determined using a technique proposedby N.I. Makkaveev, bankfull discharge, and discharges of different frequencies/durations and probabilities of exceedance. Authors considerall these interpretations as characteristic discharges. The study purpose is determination of characteristic discharges causing themost intensive influence on the channel-forming processes in lowland rivers of the southeast of the West Siberian plain. As initial informationwe use long-term discharge observations at nine reference gauge stations on six rivers of Tomsk region: the Ob at Kolpashevo,the Tom at Tomsk, the Chulym at Teguldet, Ziryanskoe, Sergeevo and Kommunarka, the Vasyugan at Sredniy Vasyugan,Chaya at Podgornoe, and the Iksa at Plotnikovo. The applied methods include the Makkaveevs technique for estimations of Qf, techniquesfor determination of bankfull discharge Qbf , and statistical techniques for discharge frequency and probability of discharge exceedancecalculation. For all stations the values of channel-forming discharges Qf corresponding to some of three specific intervals havebeen determined and their dependences on the period of time shown. In order to simplify the estimation of Qf for ungauged rivers bankfulldischarges Qf for nine reference stations were determined. Comparison of bankfull Qbf and channel-forming discharges Qf of themiddle interval allows concluding that the former provide means for precise estimations of the latter. Our study has also shown that theduration (or frequency) of the middle interval Qf varies between 2.33 and 5.54% while the duration of the lower interval Qf changesbetween 17.1 and 35.8 %, so it is possible to make estimations of the middle and lower interval Qf using these values of duration. On thecontrary, the probabilities of exceedance of annual maximum and annual mean discharges related to values of the upper interval andmiddle interval discharges Qf, respectively, are different for different river stations and their use for determination of Qf did not prove tobe so successful. Nevertheless, the values of frequencies/durations and probabilities of exceedance could provide alternative means forthe determination of channel-forming discharges for ungauged rivers.

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