a. Retaining weir (check dams)
Retaining dams built upstream which serves to slow down and gradually reduce the volume of lava flood. To deal with the forces that are at the required weir lava flood barrier that is strong enough. In addition to accomodating clash large rocks, the lighthouse and the wings of the weir should be made of concrete or couples who are quite thick and recommended the same as the maximum diameter of the stones that are expected to traverse.
Very often the collapse of the retaining dam is due to weakness in the construction connection, therefore these connections must be done with the best. Although there is little difference in the behavior of sediment movement, but the method of making a design for control almost the same, except for the difference in the construction of the lighthouse wing and body size of the spillway and weir material. For motion control weir fluvial sediments are fine-grained material, mercunya can be made thinner.
Materials for concrete body other than concrete and masonry may also of wood, wire gabion, or piles of stones. As for the mass movement of the retaining dam is usually used concrete and masonry. Type weir that is used is the type of gravity is lower than 15 m.
b. Regulatory weir (sabo dam)
In addition can also withhold a portion of sediment movement, the main function of regulating weir is to regulate the amount of fluvial sediments that move in high viscosity, so that the amount of sediment that overflowed into the downstream is not excessive. Thus the amount of sediment entering will be balanced with the ability haulage river water flow, so that the sediment in the fan area deposition can be avoided.
In rivers are not expected flood of lava, but many swept fluvial sediments in the form of movement, then the regulator dams are built in rows next to the upper reaches of the fan deposition. For rivers potentially lava flood, then the dams built in the location of lava control system and fan depositional areas. If the foundation soil composed of soft rock, then scours can be prevented by making the weir saplings (sub dam).
Sometimes a dam requires several pieces of sub-dam, so that it can achieve stable slope at the bottom of the river flow downstream. The basic stability of the river channel can be seen from the size of grains of sediment, river discharge and sediment transport capacity, then did the number of sub-dam can be set. Further, they should also note the depth of scour in the event of major floods and set the number of sub-dam is needed, in order to avoid the collapse of dams in a row.
Determining the seat of the weir, usually based on their development objectives as set out below:
– For the purpose of prevention of sedimentation suddenly with jurnlah very large that can arise as a result of landslides, sediment shed, lava floods and others then the seat of the weir should be arranged at a location downstream from the source of sediments that this unstable, ie on the flow of the river, so the river rises in the presence of the dam
– For the purpose of prevention of decrease in the riverbed, place downstream of the weir should be sought placement in the segment of the river. If sections of the river long enough, then take a few pieces of the weir constructed sequentially forming arranging so-arranging, so that more upstream weir foundation can be buried by sediment retained by the weir in the downstream.
– For the purpose of obtaining a large carrying capacity, then the seat of the weir so arranged at a location downstream segment wide stream so as to form a kind of pouch. Sometimes weir is placed on the main river downstream of the mouth of the tributaries that are usually in the form of river rapids (torrent) can serve as a sediment retention dam for both the main river and of its tributaries.
Figure 39. Building Sabo dam