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Remodeling for the purpose of increasing the storage of the reservoir involves raising of the reservoir level above the normal operating level and this would result in additional submergence of land in the upstream. In the case of ungated spillways, the normal operating level is at the crest level and there always is a temporary submergence caused by the overflow depth during the passage of floods. Thus, the full reservoir level (FRL) can be raised up to the maximum depth of overflow corresponding to the allowable limit of the upstream submergence. However, with the gated spillways, the FRL is required to be raised, which brings additional area under the submergence, beyond that corresponding to the original FRL. Thus, the remodeling is to be accomplished under the above mentioned pre-requisites.
When the FRL is raised in order to increase the storage in the reservoir, it also reduces the spillway outflow discharge because of the additional flood absorption rendered by the increased space in the reservoir. Thus, if additional submergence on upstream is permissible, an alternative of installing larger gates on the original crest level may be advantageous as compared to the alternative of lowering the crest and installing large gates.
Flash boards on ungated crests have been used traditionally to retain additional storage. These are suitable for small spillways and to impound small water depths. The boards are so designed that they fall down as soon as the water level reaches the top of the boards. Their disadvantage is that once they fall down, the additional storage created by them is lost and they have to be put in to the condition again. An improved design has been developed by Francois Lemperiere, with which the boards can be designed to retain water depth up to a predetermined level, can be allowed to be overtopped up to a designated discharge and would fall down for higher discharge. Thereafter, the boards have to be replaced. This design has been patented under US Patent 5061118.
Installing crest gates
Sometimes ungated spillways have piers on the crest for supporting a road bridge. In such cases, the temporary storage created by the depth of overflow during floods can be retained permanently by installing crest gates, yet retaining the Maximum Water Level (MWL) the same. It is advantageous to install inflatable rubber weirs on the crests to achieve ease and flexibility in the operation of gates.
A labyrinth weir on the crest of the spillway can convert the temporary storage (corresponding to the depth of overflow) into permanent storage in such a manner that the existing MWL remains the same. The original depth of overflow is divided into the height of the labyrinth weir and the depth of overflow over the labyrinth weir to pass the design discharge. This is shown schematically in figure 1.
Fuse gates can increase both spillway capacity and storage. If only the spillway capacity is to be increased, the crest of the fuse gate is set near the original crest level, thus increasing the depth of overflow substantially just before the fuse gate tilting. For increasing the storage only, the crest of the fuse gate is set higher than the original crest level. At the Terminus dam (Lake Kaweah), USA, world’s largest fuse gates, 6.4m (21 ft) in height have been installed, to increase the storage from 183,300 acre-ft to 225,300 acre-ft, about 25%.
The standard fuse gates are usually not reusable because of the damage due to falling over a considerable height. Recently, a design of what is termed as Recoverable fuse gate has been evolved, which is basically an improved version of the classical flash board.
Concrete fuse plugs
Hydrocoop, France have developed simple arrangement employing concrete fuse plugs that serve the purpose of creating additional storage or increased spilling capacity in the same manner as the fuse gates described above. The functioning of the plugs is depicted in figure 2. There are two types of the plugs; those that tilt before overtopping and those tilting after they overtop. Plugs that tilt before overtop have their heights about double the length whereas the plugs allowing considerable overflow before tilting have their heights about 3 to 10 times the length. Figure 2 shows general arrangement of fuse plug for increasing storage capacity.
Piano Keys Weir
The Piano Keys weir, popularly known as PK weir can be considered as an improved version of labyrinth weir. Its development is due to the commendable efforts of F. Lempérière under the famous Hydrocoop, France. PK weirs may be favored when the unit discharge to be handled is more than 20 m2/s, and where the construction of labyrinth weirs would be expensive. Another advantage of the PK weir is that it can be conveniently installed on a truncated spillway sill or on a non overflow dam unlike the labyrinth weir, which occupy larger space. Its functioning can be explained with reference to figure 3.
It is found that PK weir installation can affect about 20% increase in the storage capacity or the outflow capacity can be almost doubled with the same reservoir level.
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