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Browsing by Author "Johannesson, Helgi"

Now showing 1 - 4 of 4
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    Computer Simulated Migration of Meandering Rivers in Minnesota
    (1985-09) Johannesson, Helgi; Parker, Gary
    In the present work, a computer model for simulation of the migration of meandering rivers, developed by Beck [1985a], was applied to four rivers in the State of Minnesota. The computer model is based on a theoretical approach developed by Ikeda, Parker, and Sawai [1981]. The major objective was to evaluate the overall performance of the computer model and to estimate the rates of river migration in Minnesota. By using the model to analysis the past history of the four rivers, it was found that the model is able to reproduce the past river migration but needs considerable calibration. In all the case studies it was necessary to increase the dimensionless friction factor, Cf considerably. In all the study areas the rivers flow alternately through forested and non-forested areas. It was found that the rivers typically eroded about two times faster thtough non-forested areas than through forested areas. This result is of some interest, especially in agricultural areas. It indicates the importance of having a grove of trees lining the river instead of farming all the way to the river bank.
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    Diagnostic Study of the Siltation Problem at the Wilmarth Power Plant Cooling Water Intake on the Minnesota River
    (St. Anthony Falls Hydraulic Laboratory, 1988-08) Johannesson, Helgi; Parker, Gary; Garcia, Marcelo; Okabe, Kazunori
    The Northern States Power Company's Wilmarth Power Plant is located at the east bank of the Minnesota River just downstream of Mankato. The plant receives its cooling water from an adjacent intake on the river. A permanent sand bar is located on the east bank Of the river just upstream of the intake. The presence of this bar and its imminent downstream migration threatens the supply of cooling water from the river for the operation of the plant.
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    Model Study of the Minnesota River Near Wilmarth Power Plant, Minnesota
    (St. Anthony Falls Hydraulic Laboratory, 1988-12) Parker, Gary; Garcia, Marcelo; Johannesson, Helgi; Okabe, Kazunori
    The Wilmarth Power Plant, owned and operated by Northern States Power Company, is located on the east bank of the Minnesota River, just downstream of the City of Mankato. The plant receives its cooling water from an intake on the river. The plant has been in operation for over 3.5 years. In 1981, a tendency for sand to accumulate in the intake screenhouse was observed. The plant. superintendent in 1983, J. M. Pappenfus, commented as follows in a 1l1cmorandum; "We are suspicious that the river has recently changed" [1]*. Since then, dredging has been required to prevent further siltation at the intake.
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    Theory of River Meanders
    (St. Anthony Falls Hydraulic Laboratory, 1988-11) Johannesson, Helgi; Parker, Gary
    A mathematical model is developed for the calculation of flow field and bed topography in curved channels with an erodible bed. A small perturbation approach is used to linearize the governing equations. The downstream convective acceleration of the secondary flow is shown to give rise to a phase lag between secondary flow and channel centerline curvature, and also to suppress the magnitude of the secondary flow. The model further accounts for the convective transport of primary flow momentum by the secondary flow. This oft-neglected influence of the secondary flow is shown to be an important cause of the redistribution of the primary flow velocity. The governing equations retain the full coupling between the flow field, the bedload transport, and the bed topography. This coupling is shown to increase significantly the lateral bed slope in the upstream part of a channel bend, even beyond the value for fully developed bend flow which is approached in the downstream part of a channel bend. This coupling is also shown to give rise to resonant behavior for certain combinations of input variables; the common origin of the two phenomena is explained. The predicted flow field and bed topography compare very well with both laboratory and field data. Further, assuming the banks to be erodible, the model is used to predict wavelengths of river meanders. The results compare favorably with both laboratory and field data.