Author(s): Marina Kishida; Yui Shinozaki; Kaito Enomoto; Naoki Shirakawa
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Keywords: Normal flow; Ammonium nitrogen; Water quality; Class-A river; Environmental flow
Abstract: Normal flow in Japanese Class-A rivers is established with an environmental quality standard that does not consider ammonium nitrogen (NH4-N). In this study, distributions of NH4-N with normal flow were calculated in six representative rivers through an integrated model to estimate NH4-N concentration in rivers. After the calculation, some hydraulic conditions that may cause high NH4-N concentration were extracted. As a result, longitudinal distributions were categorized into five types with hydraulic conditions Especially flow velocity of higher than 0.64m/s and water depth below 0.3m were considered as thresholds. Then, normal flows with corresponding to those conditions were extracted from decision points of normal flow in all Japanese class-A rivers. Almost half of subjective points showed water depth lower than 0.3m, while eight river systems were proved to have a risk of high NH4-N concentration due to flow velocity. Such rivers were Naka river system in Ibaraki, Naka river system in Tokushima, Kushiro river system, Arakawa river system, Tenryu river system, Kurobe river system, Asahikawa river system, and Yodo river systems. Therefore, it is concluded that the state of high NH4-N concentration may occur under the present normal flow standard.
Year: 2020