A study on the role of hydraulic retention time in eutrophication of the Asahi River Dam reservoir

Osami Kawara, Eisaku Yura, Shinobu Fujii, Toyohisa Matsumoto

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60 Citations (Scopus)


The Asahi River Dam reservoir is one of the important freshwater resources for Okayama City, Japan. The eutrophication of the reservoir has been warned of since the 1980s. In this study, we discuss the relationship between hydraulic retention time and increase of phytoplankton, and the influence of wind-driven currents on the spatial distribution of phytoplankton based on the observations and numerical simulations. Observations were carried out from 1993 to 1995. The numerical simulation of hydraulics in the reservoir was carried out by applying an orthogonal curvilinear finite difference method. Judging from phosphorus and nitrogen concentrations, the water quality of the Asahi River Dam reservoir is usually eutrophic. However, high concentrations of chlorophyll-a are not always observed. The observed dependence of chlorophyll-a concentration on hydraulic retention time is reproduced fairly well by a simple relationship derived from the balance of phytoplankton based on the assumption of complete mixing. The hydraulic retention time is a limiting factor of an increase in phytoplankton in the Asahi River Dam reservoir. It takes a retention time of 2 weeks for the sufficient increase of phytoplankton. The results obtained by the simulations show that the wind-driven currents play an important role in the spatial distribution of phytoplankton.

Original languageEnglish
Pages (from-to)245-252
Number of pages8
JournalWater Science and Technology
Issue number2
Publication statusPublished - 1998
EventProceedings of the 1997 1st IAWQ-IWSA Joint Specialist Conference on Reservoir Management and Water Supply - An Integrated System - Prague, Czech Repub
Duration: May 19 1997May 23 1997


  • Asahi River Dam reservoir
  • Eutrophication
  • Growth rate of phytoplankton
  • Hydraulic retention time
  • Numerical simulation

ASJC Scopus subject areas

  • Environmental Engineering
  • Water Science and Technology


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