The function of a stilling basin downstream of a dam spillway or gates is to dissipate energy from the high-speed flow from the gates, and thereby prevent erosion further downstream of the dam. These structures are generally lined with concrete or riprap, and can include velocity reducing components and dissipating elements like chute blocks, baffle blocks, wing walls and an end sill. Funagira Dam was built in 1977. The data do not explain the type of stilling basin implemented downstream of Funagira gates. However, the challenges downstream of Funagira dam can be explained as follows:
- The riverbed suffers from erosion possibly due to the partial gate opening and irregular bed level. This leads to formation of eddies which increase erosion.
- The left bank is currently protected using concrete tetra blocks. However, the soil between and under the tetra blocks is eroding, leading to movement of the blocks.
As an example, the bed changes during the flood season of 2015, downstream of the dam. A study with a physical scale model was conducted by J-power to check the problems and how they can be tackled in detail. In this part we did perform stilling basin modelling (using Delft3D software) to investigate the impact of bed protection design to the expected hydraulic behavior of the basin.
Part of the analysis can be seen here. The modelling results and the calculations show that:
- The protection of the stilling basin would increase the bed level at the first 50 m downstream of the dam by around 4 m. However, it is not clear whether the tetra blocks are placed with maintaining the design bed level of the basin bed or just distributed on top of the bed without bed excavation or treatment.
- The bed topography of 2015, without excavation, will raise around 4 m. This may push the hydraulic jump a bit further downstream (raising water level). The energy is expected to be dissipated within the protected area. However, due to the fact that the stilling basin has a different bed level in front of the gates, the flow pattern would correspond differently. Use of the tetra blocks will increase the roughness in that area but at the same time increase the bed level. It is advisable to investigate whether the roughness increase is sufficient to compensate the expected loss of water depth there and improve the dissipation or not.
- The flow passing through gate 1 is unstable, probably due to the high bed level of the stilling basin there. Deepening of the area in front of gate 1 may reduce the current at the left bank. Lowering of the bed at this location might be checked by the scale model to verify its positive effect.
- The protection could be erected to the original design, as we assume that the bed suffers from erosion. However, it is obvious that if some locations do not suffer from erosion, this may be because of the bed characteristics there. It is advisable to implement the protection to provide an almost equal water depth along the transverse section of the stilling basin. This may provide similar flow patterns in the stilling basin as the equal-gate operation prevails.
- Implementing an end sill may be useful to reduce the velocity downstream of the dam. However, this may increase the tail water level which may have its consequences to the power generation, morphology, gate efficiency, etc. These consequences may need to be investigated.
Figure 1 The effect of bed protection on the depth average velocity for a discharge of 11130 m3/s passing through the dam (using Delft3D4 software).
Figure 2 The scale model protection prototypes of tetra blocks downstream of Funagira Dam (Source: J-power)
