1990 Fiscal Year Final Research Report Summary
Flow and Suspended Sediment in Compound Channels with Trees in the Floodplain
| Project/Area Number |
01460181
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| Research Category |
Grant-in-Aid for General Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Research Field |
Hydraulic engineering
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| Research Institution | University of Tokyo |
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Principal Investigator |
TAMAI Nobuyuki Univ. of Tokyo, Civil Eng., Prof., 工学部, 教授 (90010818)
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| Co-Investigator(Kenkyū-buntansha) |
KAN Kazutoshi Sibaura Inst. of Tech., Assoc. Prof., 助教授 (70052884)
NAKAI Masanori Univ. of Tokyo, Civil Eng., Assistant, 工学部, 助手 (10217648)
KAWAHARA Yoshihisa Univ. of Tokyo, Civil Eng., Assist. Prof., 工学部, 講師 (70143823)
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| Project Period (FY) |
1989 – 1990
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| Keywords | Compound Channel / Trees / Suspended Sediment / Coherent Structure / Momentum Transfer / Resistance |
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| Research Abstract |
Flow in compound channels experiences additional shear and three-dimensional flow features when the water surface level exceeds the level of floodplain. Deposition of suspended sediment imposes crucial role in the planning of river courses and trees in the floodplain add further complexity.
Deposition of suspended sediment was experimentally studied in a compound channel with circular cylinders in the floodplain. The arrangement of cylinders was done in three ways. When the array of cylinders is low, the advection plays influential role in the transport of suspended sediment over the floodplain. As the ratio of the coverage of cylinders in the floodplain increases, the velocity over the floodplain becomes low and the diffusive action becomes substantial. The maximum effective deposition rate was determined and the relation with the velocity field and coherent structure of turbulence was obtained.
Resistance of overbank flow is decomposed into surface resistance and form resistance. Form resistance, caused by trees in the floodplain, is explicitly formulated by the form drag of trunks and momentum loss due to macroturbulence generated by a branch-leaf complex. Hydrodynamic parameters in a formula are determined by the critical review of existing knowledge on coherent structures in open channel flows. Without any tuning of coefficients the proposed formula gives good estimation of discharge through floodways.
An algebraic stress turbulence model was developed for a compound channel flow. The calibration of coefficient showed the the normal component of turbulence is under-estimated when the best value for a duct flow. With a new coefficient the model succeeded in reproducing the primary and secondary velocity fields, and major component of the Reynolds stress. It is elucidated the the major role is played by the advection due to secondary flow for the transverse momentum transport over the turbulent diffusion.
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[Publications] Sano, Y., Nozaki, I., Hamaya, T., Nakai, M. and Kan, K.: "Flows and sediment transport in compound channels with trees on flood plains" Proc. of Hydraulic Engineering, JSCE,. Vol. 35,. 483-488 (1991)
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