The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.

Flow Resistance in Environmental Channels: Focus on Vegetation

Juha Järvelä

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Civil and Environmental Engineering for public examination and debate in Auditorium R1 at Helsinki University of Technology (Espoo, Finland) on the 11th of June, 2004, at 12 o'clock noon.

Overview in PDF format (ISBN 951-22-7074-9)   [943 KB]
Dissertation is also available in print (ISBN 951-22-7073-0)


This thesis aims to improve the reliability of the determination of flow resistance in environmentally acceptable channels and floodplains. Special emphasis was placed on addressing the hydraulic effects of vegetation. For this reason, laboratory flume studies with living vegetation were employed. The most notable finding was that, when compared to leafless conditions, the presence of leaves increased the friction factor up to seven-fold. This was strongly dependent on the flow velocity. In addition, the linkage between flow resistance, channel properties, and physical habitat was investigated. For this purpose, field studies were conducted in degraded, restored, and natural channel reaches.

To determine friction factor f or Manning's n for non-submerged woody vegetation, a new procedure based on the measurable characteristics of vegetation and flow was developed. A major advantage of this procedure over the old methods was its ability to estimate the flow resistance of woody vegetation in both leafless and leafy conditions. In determining the velocity profile and flow resistance caused by submerged flexible vegetation, the approach developed by Stephan (2002) was found to be suitable. However, a new formulation was proposed for the shear velocity based on deflected plant height. This modification offered better practical applicability than the original formulation, which requires complicated turbulence measurements.

In the field studies, the experimental results for friction factors were, excluding those for low flows, in agreement with the values presented in the literature. Overall, the gathered field data from degraded, restored, and natural channel reaches formed a reference data set, which could be useful in other similar restoration or engineering projects. The field studies showed that both flow resistance and cross-sectional geometry were vital factors in determining local hydraulic conditions. The parameters defining these two factors were found to be simple but nonetheless valuable in evaluating the success of a project which aims to restore local hydraulics. A new procedure for applying the success criteria in the post-project evaluation of local hydraulics was developed.

This thesis consists of an overview and of the following 6 publications:

  1. Järvelä J., 2002. Flow resistance of flexible and stiff vegetation: a flume study with natural plants. Journal of Hydrology 269, numbers 1-2, pages 44-54. © 2002 Elsevier Science. By permission.
  2. Järvelä J., 2002. Determination of flow resistance of vegetated channel banks and floodplains. In: Bousmar D. and Zech Y. (editors), River Flow 2002. Lisse, Swets & Zeitlinger, pages 311-318. © 2002 Swets & Zeitlinger. By permission.
  3. Järvelä J., 2003. Influence of vegetation on flow structure in floodplains and wetlands. In: Sánchez-Arcilla A. and Bateman A. (editors), Proceedings of the 3rd IAHR Symposium on River, Coastal and Estuarine Morphodynamics (RCEM 2003). Madrid, IAHR, pages 845-856. © 2003 International Association of Hydraulic Engineering and Research (IAHR). By permission.
  4. Järvelä J., 2004. Determination of flow resistance caused by non-submerged woody vegetation. International Journal of River Basin Management 2, number 1, pages 61-70. © 2004 International Association of Hydraulic Engineering and Research (IAHR). By permission.
  5. Järvelä J. and Helmiö T., 2004. Hydraulic considerations in restoring boreal streams. Nordic Hydrology 35, number 3, in press. © 2004 by authors and © 2004 IWA Publishing. By permission.
  6. Helmiö T. and Järvelä J., 2004. Hydraulic aspects of environmental flood management in boreal conditions. Boreal Environment Research 9, in press. © 2004 Boreal Environment Research. By permission.

Keywords: hydraulics, flow resistance, roughness, vegetation, rivers, floodplains

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© 2004 Helsinki University of Technology

Last update 2011-05-26