Design, safety and management of the reservoir

I found very good online article on the dam management on the view of hydrology and hydraulic. The article was achieved from Dr. Sobri’s academic website (Link: http://civil.utm.my/sobriharun/drama/). Very interesting. The article as follows:

A Dam Management team requires knowledge of civil engineering (structural, hydraulics, hydrology, geology and geotechnical), electrical and mechanical engineering. The main objective of dam safety management is to ensure that the dams able to sustain on two aspects; strength and operation. The standard operation procedures (SOP) determine the successful of dam safety management.The strength normally related to structural, geology and geotechnical assessments. The knowledge in hydraulics, hydrology, electrical and mechanical engineering are required in the reservoir operation.Construction of a large dam ( > 15 meter high) or a major large dam ( > 30 meter) would impose the risk to nearby inhabitants. Dam risk management program would help the operator and manager on minimising the risk and maximizing the safety.

To ensure the safety of the inhabitants along downstream of dam during the heavy rainfall, the flow modelling through spillway adopting minimum of 100 years design (ARI or return period) rainfall shall be performed by the dam managers. Several critical storm patterns could be inspected during the simulation of outflow hydrograph and potential flood inundation map along the downstream river.

Fig. 1: Reservoir system

Sediments flow into reservoir system due to erosion from upstream catchment could reduce the reservoir storage capacity in long-term. The erosion control at upstream catchment is less costly compared to removal of sediments in the reservoir. The nature of inflow pattern into the reservoir system also changes with time and this variation would affect the spillway capacity of reservoir system. The computed probable maximum flood (PMF) value shall be below the designed value to indicate that the dam is safely design. It is the PMF condition with adequate capacity of the spillway to discharge the PMF to the downstream in the event of extreme rainfall. PMF provides an upper limit of the interval within the engineer must operate and design.

The spillway capacity of a dam is regularly inspected for at least 10,000 years return period of PMF. The PMF is derived from probable maximum precipitation (PMP) using rainfall-runoff model. For several stages of spillway operation (gradual releases) of a major large dam, the capacity could be inspected from 10-yr, 100-yr, 1000-yr, 10,000-yr and 100,000-yr. If the spillway capacity below the design level, the possibility of upgrading the system should be adopted by dam manager. Gradual water release has advantages that would alter the shape of rising limb for the outflow hydrograph through spillway due to extremely huge flood magnitude.

Dam failure is possible due to lack of management on two attributes; the structural strength and optimal operation. Most of the dams in the world failed due to the overtopping and piping. In general, the overtopping failure occurs after unexpected extremely heavy rainfall and the piping failure occurs due to unsatisfactory maintenance of seepage in the earth dam.

The dam break modelling also necessary to reveal illustration of potential flood inundation due to the greatest possible heavy rainfall (probable maximum precipitation, PMP). Normally, the hydrologists adopt within 10,000 years to 100,000 years design rainfall that equivalent to probability of occurrence by 0.01 percent and 0.001 percent respectively. The results of dam break modelling are the timing, depth and extent of flood the inundation along downstream that would likely possibly happen due to dam failure.

The international organizations related to dam building and safety management are the International Commission on Large Dams (ICOLD) and the World Commission on Dams (WCD).

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