Evaluation of ecohydraulic effectiveness of an artificial fish nest in an agricultural drainage canal

Shigeya Maeda, Hibiki Tanigawa, Koshi Yoshida, Hisao Kuroda

Monday 29 june 2015

16:15 - 16:30h at Europe 1 & 2 (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 3G. Environment - Ecohydraulic

Effectiveness of an artificial fish nest installed on the sidewall of an agricultural drainage canal made by concrete is evaluated in terms of energy expenditure (swimming cost) by a fish in a turbulent current in a rural area in Ibaraki Prefecture, Japan. The size of cavity of the fish nest is 1.14m width, 1.00m height, and 0.90m length. The three-dimensional instantaneous water velocity components are collected at 80Hz in and around the fish nest and in a canal section without fish nests. The observations were carried out three times, on 10 October 2013, 24 April 2014, and 16 June 2014. The first observation date is in the non-irrigation period, but the second and third ones are in the irrigation period. Fifteen observational stations are categorized into two groups. Nine stations are positioned in and around the fish nest (Group 1). Remained six stations (Group 2) are located away from the fish nest in the drainage canal. The water depth, the time-averaged flow velocity, the turbulent flow characteristics and the associated energy expenditure by a fish in Groups 1 and 2 are compared in order to analyze the ecohydraulic effectiveness of the artificial fish nest. The water depth and the time-averaged flow velocity in the three observation days at all the stations range from 0.124 to 0.535m, and from 0.00805 to 0.429m/s, respectively. In order to quantify the flow turbulence, the turbulent kinetic energy, the pulsation standard, and the degree of turbulence (turbulence intensity) are computed. Then the energy spent by a fish to overcome the water resistance in a turbulent current is estimated using a theoretical equation that includes the time-averaged flow velocity and the degree of turbulence. It is found that the turbulent kinetic energy in Group 1 is significantly small, but the degree of turbulence in the group is significantly high. The energy expenditure by a fish in 30 seconds in Group 1 is estimated as significantly small, which indicates the fish nest contributes to create a preferable fish habitat in the agricultural drainage canal.