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Though, all the aspects concerning hydrology, geology, topography and operational needs are taken into account while designing a spillway, eventually some spillways of existing dams require remodeling. This may be due to a variety of reasons. The most prominent are:
· Safety of the dam- This may be hydrologic safety or structural safety
· Need for increasing the storage capacity of the reservoir
· Environmental consideration, i.e. reducing fish mortality during overflow
· Replacement of aging components such as gates
The most compelling case is concerning the hydrologic safety of the dam. In the last decade, existing dams were specifically surveyed in various countries to assess their safety. One of the most comprehensive surveys was conducted by the Corps of Engineers of the US Army (USACE), which included more than 80,000 dams. It was revealed that about 36% of the dams were unsafe due to various reasons. However, the most alarming finding was that about 80% of these were unsafe because of inadequate spillway.
Significant advances in the science of hydrology during the past three decades have taken place. Serious shortcomings in the spillway inflow floods of many dams that were designed prior to the beginning of the nineteenth century have been revealed. Actual floods that passed down the spillways often equalled or exceeded the design values, just when these dams were believed to be operating as anticipated. History is replete with the cases of inadequate spillway capacities and failures due to overtopping of dams. The example of the spillway of Machhu II dam, Gujarat, India would serve to amply illustrate this aspect. The inflow flood for this spillway, at the design stage was presumably arrived at with calculations on a simple formula involving only the catchment area and nothing else. Accordingly, for a catchment area of 1928 sq.km, the design flood was estimated as 5663m3/s. After seven years of operation of the spillway, the catastrophic floods of August 1979 resulted in an actual discharge of 16307 m3/s passing down the spillway. The earthen dam failed due to the overtopping of about two meters, while the masonry spillway remained intact. The hydrology of the project was revised and the modified inflow flood was 20925 m3/s. Since the existing spillway was not capable of handling this discharge, an additional spillway was added to take care of the increased flood.
While addition of a spillway resolved the problem at Machhu II dam,it is clear that such a solution would not be possible for every spillway that is found to be inadequate. Conditions existing at a particular site would call for a solution unique to that site. In many cases, modifications to the existing spillway may be the only alternative available. Besides, requirements arising out of the need for additional storage, structural strengthening of the spillway or some appurtenances, etc. would call for a modification. This is generally known as remodeling.
As mentioned above, the scope and the extent of remodeling would be governed by the local conditions at the site. It would be the designer’s imagination and experience that would shape the remodeling to cater to the needs. Case studies of the projects underwent the remodeling serve as the guide lines. These are discussed to illustrate the approaches and methodologies.
CATERING TO THE HIGHER INFLOW FLOOD
If an additional spillway to handle the increased flood is not feasible, other alternative may be to extend the spillway laterally to replace a portion of non overflow or earth dam. This may be possible only if there is additional space available.
Several other alternatives are available depending upon whether the spillway is gated or ungated.
· Increasing the depth of overflow is an easier means if the resulting increase in the Maximum Water Level (MWL) is acceptable. Sometimes, increased area under the reservoir submergence may not be allowed and under this circumstance another solution is to be found out. Also, the ratio of the actual increased depth of overflow to the design head should not be larger than say 1.3 or so. This is to ensure that the sub-atmospheric pressures developed for the increased head are within acceptable magnitude to avoid cavitation. The encroachment in the available free board for the non- overflow dam has to be made good by increasing the dam height. If the requirement is small, usually a parapet is added to recoup the margin lost.
· If the increased area under the submergence or encroachment in the free board is not acceptable or if raising the height of the non- overflow or earth dam is not feasible, the only means available is to lower the crest level of the spillway so that the actual depth of overflow is more than the design head and MWL is retained the same. This is shown schematically in figure 1.
· If the increased discharge is too large to be handled by the available increase in the depth of overflow, the alternative of installing a labyrinth weir on the crest will be suitable. A fuse gate can also be installed instead of a labyrinth weir. These alternatives are shown in figures 2 and 3.
· 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 4. 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. Installing PK weirs at the Goulours dam in France indicated that a discharge capacity of 69 m2/s could be obtained on a 11.5 m long PK weir with a head of 1 m. A conventional spillway would have required a head of 2 m on a 25 m length.
It must, however, be noted that higher discharge is passed down the spillway through the same width as before, resulting in the increased discharge intensity. Its implications on the performance of the energy dissipator should be studied for the condition of higher discharge. This is particularly so for the hydraulic jump stilling basin or roller buckets, which if subjected to discharges higher than the design discharge, could result in a sweep out condition leading to excessive erosion. If the increase in discharge is small, a slight deficiency could be tolerated as passing of the higher discharge would be rare and the damage would not be of the magnitude to endangering the safety of the dam.
Increasing the depth of overflow along with replacing the existing gates with a larger size gate and increasing the height of the non overflow or earth dam (if the available free board is not adequate) is the easiest means for passing the increased discharge. This is, however, subject to the conditions that additional submergence is acceptable, the crest piers can accommodate larger gates and the non overflow or earth dam can be raised as necessary. In case, the additional submergence cannot be accepted or the non overflow or earth dam cannot be raised as required, the only alternative will be lowering the crest level of the spillway and installing larger gate so that the MWL remains unchanged. This is indicated in figure 5.
Some examples of increasing the discharge capacity with remodeling involving lowering the crest level /installation of crest gates:
Riolunato dam in Italy : Lowering the crest level and increasing its hydraulic efficiency effected a 55% increase in spillway outflow
Peublo Viejo dam spillway in Guatemala : Loweing the crest level and installing gates effected 12% increase in spillway outflow
Nacimiento dam in USA : Loweing the crest level and installing rubber weir on crest ensured 42% increase in spillway outflow
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