# Development of Driftwood Capture Trellis for Capturing Driftwood in Agricultural Drainage Ditches
## 3 Results
### Hydraulic Performance Evaluation of the Driftwood Capture Trellis  
Design standards for hydraulic structures are created depending on the design discharge and the characteristics of the region in Korea. In addition, the size of an agricultural drainage ditch is set so that a stable flow occurs at the design discharge. In order to consider various discharge and flow conditions, an equation for calculating discharge and Froude number was proposed. Given the capture efficiency obtained for the velocity and depth of the flow, discharge capture efficiency curves and Froude number were estimated using the continuity and Manning equations (Figure 8).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Velocity (m/s)**} & \multicolumn{5}{c}{**Unit (\%)**} \\ \cline{2-7}  & **80** & **120** & **160** & **200** & **240** & **280** \\ \hline
0.3 & 100.00 & 100.00 & 99.44 & 94.69 & 95.44 & 97.31 \\
0.6 & 96.81 & 95.94 & 90.44 & 86.13 & 87.63 & 94.31 \\
0.9 & 94.69 & 93.13 & 81.00 & 86.50 & 89.81 & 93.94 \\
1.2 & 89.50 & 87.75 & 82.19 & 75.31 & 84.81 & 81.70 \\
1.5 & 86.13 & 84.38 & 75.88 & 71.69 & 67.94 & overflow \\
1.8 & 80.50 & 62.00 & 50.88 & 52.25 & overflow & overflow \\ \hline \hline \end{tabular}
\end{table}
Table 7: Capture efficiency of the driftwood capture trellis by experimental conditions.  
Figure 7: Capture efficiency of the driftwood capture trellis according to depth: (a) H = 80 mm; (b) H = 120 mm; (c) H = 160 mm; (d) H = 200 mm; (e) H = 240 mm; (f) H = 280 mm.  
The size of agricultural drainage ditch systems is planned considering various discharge rates. Therefore, in order to install a driftwood capture trellis for agricultural drainage ditches, a regression equation was calculated using the estimated curve to consider the effect of capture efficiency according to flow rate and flow state. The regression equation used to estimate the capture efficiency of the driftwood capture trellis was \(\text{CE}=101.6-243.8\times\text{Q}\) for flow and \(\text{CE}=98.4-15.6\times Fr\) for \(Fr\). Here, CE is capture efficiency, Q is discharge, and \(Fr\) is Froude Number. According to the estimation curve, within the discharge range of 0.01 to 0.144 m\({}^{3}\)/s, the capture efficiency of the driftwood capture trellis was \(\approx\) 60% or greater. The less the rate of discharge, the higher the capture efficiency: 80% capture efficiency was maintained at 0.10 m\({}^{3}\)/s or lower. As for the Froude number, the capture efficiency was estimated at ~80% or higher when \(Fr<1\), which represents a subcritical flow, and \(\approx\) 60% or higher if \(1<Fr<2.1\), which represents a supercritical flow.  
The driftwood capture trellis was found to capture approximately 60% of the driftwood in agricultural drainage ditches according to discharge rate and Froude number. However, when the flow rate in the drainage was 0.144 m\({}^{3}\)/s or greater, overflow occurred due to a reduction in the opening area caused by the captured driftwood. Based on these findings, the driftwood capture trellis in the agricultural drainage ditch system developed in this study would guarantee over 60% capture efficiency at a flow rate of 0.144 m\({}^{3}\)/s or less. Moreover, using such hydraulic characteristics as flow rate and Froude number, it is possible to establish installation standards for driftwood capture trellis according to the target capture efficiency. The capture efficiency of the regression equation proposed in this study is based on data obtained in various experiments. Although qualitative results were calculated using the regression equation, as discharge and increase, the volatility of the capture efficiency greatly increases. It is necessary to install driftwood capture trellises in consideration of this uncertainty.