Assume there is a small stream with a maximum discharge of 4 cums (m3/s). We have to collect the flood runoff and also reduce the peak flow during the flood season. According to the river morphology there is a narrow section with an alluvial foundation. The topographical condition is such that limited us to have a structure with 10 m height. Which option is better on your opinion a) Earthen dike, b) Gabion weir to reduce the flow peak and collecting runoff to recharge the groundwater and etc.
Thanks
Dear Saeed,
in general, I would recommend earth dam (dyke) instead of gabion structure in case of 10 m height. However, it is possible that you won't need such a high structure. To decide it, you will need the shape of design flood wave which is necessary to assess the transformation potential in the reservoir defined by your dam. Another crucial input is the elevation volume curve. Then, you can design the bottom outlet allowing only the safe discharge flowing downstream of your dam. To assess the transformation of flood wave, the calculation is in principle as follows for each time step:
- calculating the inflow volume to the reservoir (from the flood hydrograph)
- reading the corresponding rise of water level (based on stored volume = volume from previous step + the volume coming in within the respective time step)
- calculating the outflow from the reservoir (based on the water level and the outlet rating curve)
- calculating new value of stored volume involving the outflow
- continuing to the next time step
The procedure can be slightly modified but the principle is very simple in each case. Usually, it is considered that the reservoir volume (which is free for flood storage = flood control volume) should be at least one third of the flood wave volume but it of course depends on many factors.
Hope, this will help.
Regards,
Vaclav
Dear Vaclav, thank you for your answer. As I mentioned the purpose of the construction of such structure is to first reduce the peak flow by storing flood and recharge the groundwater, there is a good location at the right abutment to consider a spillway during the high flood to avoid over-topping. The main concept of using a dike is the construction cost of the project. If we consider an outlet structure the cost will be increased.
Thanks again for your opinions.
Dear Saeed,
this is a difficult situation. The problem is that water will most likely stay for a long time in your reservoir without any bottom outlet. I do not expect that it will infiltrate very fast as you are speaking about 10 m high structure. It of course depends on the properties of soils. Threrefore, you should build your dam permeable enough. Then, you can expect that your reservoir will be able to drain stored water and be ready for the next flood. In such case, you should of course forget the general recommendation about reservoir volume. You will need the volume large enough to store almost whole flood wave. To be accurate, you will need the volume equal to the volume of flood wave until the safe discharge is reached on the falling limb.