# Development of a Two-Dimensional Hybrid Sediment-Transport Model
## 4 Examples and Application
### Jiji Reservoir  
The sediment-transport model was used to simulate the deposition process in the Jiji Reservoir (Figure 11) in Taiwan, which was previously simulated by the CCHE2D sediment-transport model using structured meshes [1]. The Jiji Weir (Figure 11) was built across a mountain river, Chuoshui Creek, in Taiwan. The flow pattern of Chuoshui Creek was strongly affected by the precipitation pattern, with a very small discharge in dry seasons and extremely large flows in typhoon seasons. Due to the steep slopes and exposed soils, the flood water could transport high sediment loads and deposit them in the reservoir, resulting in rapid reductions in reservoir storage capacity. Once the reservoir was full, the sediment would be flushed through the spillway of the weir to the downstream channel. Figure 11 shows the study domain from JiLu Bridge to Jiji Weir, which was about 2 km long. As can be seen, the reservoir suffered from serious sedimentation problems. In 2003, the total amount of sediment yield was about 14.77 million m\({}^{3}\), though only about 13% was transported downstream. According to the bed samples measured in July of 2004 (Figure 12), coarse sediments were deposited mainly upstream of the reservoir, close to the JiLu bridge, while fine sediments were deposited within about 1000 m of the weir. Figure 12: Bed sample locations with \(d_{50}\) in mm. Table 4 lists six sediment-size classes used in the simulation, varying from medium salts to small boulders. The bed samples of 2004 (Figure 12) were used as the initial bed composition within the reservoir. Figure 13 shows the initial bed elevation and the quadrilateral mesh generated for this reservoir based on the measured topography data in 2004. The simulation period was from July 2004 to October 2007. Due to the flow pattern of Chuoshui Creek, small discharges were assumed to have little effect on the sediment transport in the reservoir, which were therefore ignored in the simulation. The actual hydrograph and sedigraph consisted of all ten typhoon events during the study period with peak flow discharges varying from 1020 m\({}^{3}\)/s to 8343 m\({}^{3}\)/s, as shown in Figure 14a. Figure 14b shows the corresponding stage hydrograph at Jiji Weir. Such treatment shortened the simulation time from 3 years to 33 days. Both suspended-load and bed-load transport models were applied to this reservoir with the adaptation coefficient of the suspended load, \(\alpha\), set as 0.01, and the adaptation length of bed load \(L_{b}\) = 4000 m.  
Figure 15a shows the measured bed change pattern from 2004 to 2007. At the downstream end, sediment deposits approached the weir and formed a large hump across the reservoir, though a large scour hole also developed about 100 m from the weir. In the middle, sediment transport followed a general pattern of erosion on the left and deposition on the right, while erosion dominated upstream close to the Jilu Bridge. Note that the erosion on the left of the deposition delta (Figure 11) was partially caused by dredging activities with unknown amounts ([1,2]). Comparing to the measured values, the simulated bed changes (Figure 15b) captured the humps near the weir and the general deposition pattern on the right but missed the hole near the weir and predicted slight deposition on the left, as expected. \begin{table}
\begin{tabular}{l c c c c c c} \hline
**Size (mm)** & **0.0272** & **0.297** & **2.38** & **9.52** & **152** & **457** \\ \hline Lower Bound & 0.001 & 0.074 & 0.59 & 4.76 & 19.1 & 305 \\ Upper Bound & 0.074 & 0.59 & 4.76 & 19.1 & 305 & 610 \\ \hline \end{tabular}
\end{table} 
Table 4: Size classes in the Jiji Reservoir. Figure 13: Initial bed elevations and computational mesh for the Jiji Reservoir with 6440 nodes and 6240 cells. Figure 14: Boundary conditions (**a**) Hydrograph and sedigraph at JiLu Bridge. (**b**) Stage hydrograph at Jji Weir. Figure 15: Cont. Twenty measured cross sections were available within the Jiji Reservoir, and their locations are indicated in Figure 15a.