Reservori Filling

1.

Introduction

A reservoir filling study is normally undertaken to investigate the duration required of filling the reservoir with river flow to various critical elevations of the works. The results will be used for planning purposes in the construction. 2

Critical Elevation

The durations that reach the critical elevations after reservoir impoundment are listed as follows: (a)

Minimum Operating Level The minimum operating level is required for the commissioning of the hydromechanical equipment and turbines in the power station commences only if the reservoir level reaches this level. For planning purposes this time will assist with assessing the period before operation of the power station can commence.

(b)

Spillway Crest Level The crest level of the spillway is important for planning purposes, if the reservoir impoundment is to be carried out prior to the completion of spillway.

3

Assumption and Methodology

3.1

Methodology

The methodology in estimating the rate of reservoir filling is briefly discussed. •

The rate of reservoir filling is estimated by statistical approaches. The statistical approach estimates the probability of a given discharge, based on available historical data, which could occur if the statistical characteristics of the data are unaffected by the construction of the dam.

•

Two scenarios have been considered in the analysis, one with while the other without riparian release of a specified discharge in m3/s.

•

The percentiles of the historical stream flow records are first estimated. Here, the kth percentile is defined as the value of the data such that k percent of the value are less than that value. Here, the italic number “k” any real number ranging from 0 to 100%. Therefore, the probability of occurrence greater than that value is equal to 100 – k Percentile. Two approaches are attempted to estimate the discharges at various percentiles and then probabilities: Interpolation and Normal distribution. The results are used for comparison purposes only; only to indicate the results are of similar order of magnitude. They can be estimated by using the functions available in the Microsoft Excel spread sheet.

Interpolation:

Various k percentiles are interpolated from the range of stream flow data. They are estimated from the cumulative frequency graph.

Normal Distribution: Various k percentiles are estimated based on the mean and standard deviation of the historical monthly stream flow records. They are estimated from cumulative area under the standard normal curve. Explanations on the two methods are discussed in the built-in functions of Microsoft Excel. Details may be referred to standard statistical textbooks and the followings are recommended for references: (a)

Knode. D., and Bohrnstedt, G. W. (1994), “Statistical For Social Data Analysis”, F.E. Peacork Publishers, Inc., USA.

(b)

Ang, A. H. S. and Tang, W. H. (1975), “Probability Concepts In Engineering Planning And Design”, Vol. 1: Basic Principles & Vol. 2: Decision, Risk, and Reliability, John Wiley and Sons , New York, USA.

3.2

Assumptions

•

The occurrence of stream flow discharge at each month is independent of preceding and subsequent monthly.

•

Evaporation, seepage losses and rainfall on the reservoir surface are ignored in the analysis. The reason is that the stream flow data at the gauging station are obtained before the reservoir is built. They represent the net river inflow, after deducting the losses due to evaporation, seepage and adding rainfall on the catchment.

•

Additional analysis will be carried out by estimating the rate of reservoir filling with the normal distribution. The probability of occurrence of an event equals to or above a prescribed value is determined by assuming the data fitted well with the normally distribution.

4

Procedures

The procedures for computing the reservoir filling rate are given as follows: •

Determine the long-term monthly stream flow series available near the dam site from the long-term monthly stream flow series.

•

Determine the probability greater than a specific discharge from the relatinship: 100-% k percentile estimated by Interpolation and Normal Distribution.

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The impounding and closure works are assumed to commence at the beginning of different months, such as January, April, July and October.

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Two cases will be considered: with and without riparian release.

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The cumulative days required for filling the reservoir to the specific water level at different probabilities will be calculated. Note that the probability greater than a specific discharge is used in the calculation. Therefore, it estimates the minimum water level that the stream flow is able to fill the reservoir under similar catchments and statistical characteristics for a given probability.