Statistics

INTRODUCTION
MAXIMUM WATER LEVEL IN BAGMATI
OBSERVATION IN BURHI GANDAK
CORRELATIONS IN BURHI GANDAK
LAG TIME ANALYSIS IN BURHI GANDAK
OBSERVATION IN BAGMATI
FREQUENCY ANALYSIS IN BAGMATI
CORRELATION ANALYSIS IN BAGMATI
LAG TIME ANALYSIS IN BAGMATI

Introduction:

Floods are complicated natural events. It depends on several parameters, so it is very difficult to model analytically. The floods in a catchment depends on the characteristics of the catchment, rainfall and antecedent conditions. So the estimation of the flood peak is a very complex problem. There is several alternative methods to estimate the magnitude of a flood peak:

• Rational method
• Empirical methods
• Unit –hydrograph technique and
• Flood- frequency method

Flood frequency method is a statistical method .The value of the annual maximum flood from a catchment area for large number of successive years is called the annual series.These data are then arranged in decreasing order of magnitude.
Then the probability of each event being equalled to or exceeded is calculated as:-

                          P=  m/N+1          
             Where m=order number of the event and
                          N=total number of events
Then the recurrence interval also called return period or frequency is calculated as

              T= 1/P

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Maximum Water Level in Bagmati

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OBSERVATION IN BURHI GANDAK-

From year 1991 to 2006  i .e(16 years) monthly observation has been done during flood season. There is no flood in the month of June during past 16 years. Years of flood has been shown in the table below-  

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CORRELATIONS IN BURHI GANDAK :-

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LAG TIME ANALYSIS IN BURHI GANDAK : -

Water level curves shows that the lagging time between different sites are as follows:

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OBSERVATION IN BAGMATI: -

The study shows that during past 14years water level has exceeded the danger level in year 1998,1999, 2000, 2003 and 2006 only in the month of June at Benibad. But at Hayaghat it remains below the danger level in the month of june from 1993 to 2006. Water level has exceeded the danger level in most of the year from 1993 to 2006 in the month of July.
Past 14 years analysis shows that at Benibad water level always exceeded the danger level in the month of August. But at Hayaghat water level remains  below the danger level in August 2006, However it always crossed the Danger Level in past 10 years in the month of August.
During past 14 years the Water Level crossed the danger level in the month of September at Benibad for most of the years.
The maximum Water Level trend in the month of October during past 14 years shows that in this year there is record Water Level as 49.54m at Bendibad and 46.34m at Hayaghat. During past 14 years water Level had never crossed the Danger Level at Hayaghat, but in this year it crossed the Danger Level in October.

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Frequency Analysis in Bagmati :-

June.

The maximum Water Level at Benibad in the month of june-2006 is 48.76m which has return period of 3 years and  whose probability was 0.33.
Frequency analysis gives the idea that the Water Level in the month of June will cross the Danger Level (48.68 m) having return period of 2.5 years with probability of 0.4.
July.
The maximum Water Level at Benibad in the month of July 2006 is 49.39m which has return period of 3.4 years and whose probability is 0.29.
Frequency analysis gives an idea that the Water Level in the month of July at Benibad will cross the danger level (48.68m) having return period of 1.25 years whose probability is 0.80.
August
The maximum Water Level at Benibad is the month of August-2006 is 49.17m which has return period of 1.7 years and whose probability is 0.59. According to frequency analysis the Water Level in the month of August of Benibad will cross the danger level having return period of  1.2 years with probability 0.82.

September
The maximum Water Level at Benibad in the month of September-2006 is 49.66m, which has return period of 17 years with probability 0.06.
According to frequency analysis the Water Level in the month of September at Benibad will cross the danger level having return period of 1.20 years with probability 0.80

October.

The maximum water level at Benibad in the month of October-2006 is 49.54m  which has return period of 15 years with probability 0.07.
According to frequency analysis the Water Level in the month of October at Benibad will cross the danger level having return period of 1.5 years with probability 0.67.

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Correlation Analysis in Bagmati :

Correlation analysis has been done by arranging all data in the column. The correlation coefficients has been obtained as follows:-

The above table shows that there is good relationship between the water level at Dubbadhar and water level at Kansar in Nepal and the second good relationship is between the water level at Benibad and water level at Hayaghat in India. But the water level at Kansar in Nepal in not so correlated with the water level at Benibad. This may be due to the different tributaries of Bagmati between Kansar and Benibad. This shows that the trend of Bagmati River at Dubbadhar and Kansar is same and also the trend at Benibad and  Hayaghat is more or less same.

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LAG TIME ANALYSIS IN BAGMATI: -

Water level  curves shows that  the lagging time between different sites are as follows:

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