Short Notes: Floods, Flood Routing and Flood Control | Short Notes for Civil Engineering - Civil Engineering (CE) PDF Download

 Page 1


Floods, Flood Routing and Flood Control 
A flood an unusually high stage in a river, normally the level at which the river 
overflow its banks and inundates the adjoining area .The design of bridges, culvert 
waterways and spillways for dams and estimation of score at a hydraulic structure 
are some examples wherein flood-peak values are required. To estimate the 
magnitude of a flood peak the following alternative methods are available: 
1. Rational method 
2. Empirical method 
3. unit-hydrograph technique 
4. Flood- frequency studies 
Rational Method 
If tp = tc 
 
Where, Qp = Peak discharge in m
3
/sec 
PC = Critical design rainfall in cm/hr 
A = Area catchment in hectares 
K = Coefficient of runoff. 
tD = Duration of rainfall 
tC = Time of concentration 
Empirical Formulae 
(a) Dickens Formula (1865) 
 
Where, Qp = Flood peak discharge in m
3
/sec 
A = Catchment area in km
2
. 
CD = Dickens constant, 6 = CD = 30. 
(b) Ryve’s formula (1884) 
 
Where, 
Page 2


Floods, Flood Routing and Flood Control 
A flood an unusually high stage in a river, normally the level at which the river 
overflow its banks and inundates the adjoining area .The design of bridges, culvert 
waterways and spillways for dams and estimation of score at a hydraulic structure 
are some examples wherein flood-peak values are required. To estimate the 
magnitude of a flood peak the following alternative methods are available: 
1. Rational method 
2. Empirical method 
3. unit-hydrograph technique 
4. Flood- frequency studies 
Rational Method 
If tp = tc 
 
Where, Qp = Peak discharge in m
3
/sec 
PC = Critical design rainfall in cm/hr 
A = Area catchment in hectares 
K = Coefficient of runoff. 
tD = Duration of rainfall 
tC = Time of concentration 
Empirical Formulae 
(a) Dickens Formula (1865) 
 
Where, Qp = Flood peak discharge in m
3
/sec 
A = Catchment area in km
2
. 
CD = Dickens constant, 6 = CD = 30. 
(b) Ryve’s formula (1884) 
 
Where, 
CH = Ryve’s constant 
= 8.8 for constant area within 80 km from the cost. 
= 8.5 if distance of area is 80 km to 160 km from the cost. 
= 10.2 if area is Hilley and away from the cost. 
(c) inglis Formula (1930) 
 
Where, A = Catchment area in Km
2
. 
QP = Peak discharge in m
3
/sec. 
Flood Frequency Studies 
(i) Recurrence interval or return Period: 
 where, P = Probability of occurrence 
(ii) Probability if non-occurrence: q = 1-P 
(iii) Probability of an event occurring r times in ‘n’ successive years: 
 
(iv) Reliability: (probability of non-occurrence /Assurance) = q
n
 
(v) Risk = 1-q
n 
? Risk = 1(1-P)
n
 
(vi) Safety Factory =   
(vii) Safety Margin = design value of hydrological parameter – Estimated value of 
hydrological parameter 
Gumbel’s Method 
The extreme value distribution was introduction by Gumbel (1941) and is commonly 
known as Gumbel’s distribution. it is one of the most widely used probability 
distribution functions for extreme values in hydrologic and meteorologic studies for 
prediction of flood peaks, maximum rainfall, maximum wind speed. 
Page 3


Floods, Flood Routing and Flood Control 
A flood an unusually high stage in a river, normally the level at which the river 
overflow its banks and inundates the adjoining area .The design of bridges, culvert 
waterways and spillways for dams and estimation of score at a hydraulic structure 
are some examples wherein flood-peak values are required. To estimate the 
magnitude of a flood peak the following alternative methods are available: 
1. Rational method 
2. Empirical method 
3. unit-hydrograph technique 
4. Flood- frequency studies 
Rational Method 
If tp = tc 
 
Where, Qp = Peak discharge in m
3
/sec 
PC = Critical design rainfall in cm/hr 
A = Area catchment in hectares 
K = Coefficient of runoff. 
tD = Duration of rainfall 
tC = Time of concentration 
Empirical Formulae 
(a) Dickens Formula (1865) 
 
Where, Qp = Flood peak discharge in m
3
/sec 
A = Catchment area in km
2
. 
CD = Dickens constant, 6 = CD = 30. 
(b) Ryve’s formula (1884) 
 
Where, 
CH = Ryve’s constant 
= 8.8 for constant area within 80 km from the cost. 
= 8.5 if distance of area is 80 km to 160 km from the cost. 
= 10.2 if area is Hilley and away from the cost. 
(c) inglis Formula (1930) 
 
Where, A = Catchment area in Km
2
. 
QP = Peak discharge in m
3
/sec. 
Flood Frequency Studies 
(i) Recurrence interval or return Period: 
 where, P = Probability of occurrence 
(ii) Probability if non-occurrence: q = 1-P 
(iii) Probability of an event occurring r times in ‘n’ successive years: 
 
(iv) Reliability: (probability of non-occurrence /Assurance) = q
n
 
(v) Risk = 1-q
n 
? Risk = 1(1-P)
n
 
(vi) Safety Factory =   
(vii) Safety Margin = design value of hydrological parameter – Estimated value of 
hydrological parameter 
Gumbel’s Method 
The extreme value distribution was introduction by Gumbel (1941) and is commonly 
known as Gumbel’s distribution. it is one of the most widely used probability 
distribution functions for extreme values in hydrologic and meteorologic studies for 
prediction of flood peaks, maximum rainfall, maximum wind speed. 
Gunbel defined a flood as the largest of the 365 daily flows and the annual series of 
flood flows constitute a series of largest values of flows. 
Based on probability distribution. 
 
 Where, XT = Peak value of hydrologic data 
K = Frequency factor 
 yT = Reduced variate 
 
T = Recurrence interval in year 
yn = Reduced mean = 0.577 
Sn = Reduced standard deviation. 
Sn = 1.2825 for N ? 8 
 
Confidence Limit 
Since the value of the variate for a given return period, xT determined by Gumbel’s 
method can have errors due to the limited sample data used. An estimate of the 
confidence limits of the estimates is desirable the confidence interval indicates the 
limits about the calculated value between which the true value can be said to lie with 
specific probability based on sampling errors only. 
For a confidence probability c, the confidence interval of the variate xT is bounded by 
value x1 and x2 given by 
 
Where, f(c) is a function of confidence probability ‘C’. 
Page 4


Floods, Flood Routing and Flood Control 
A flood an unusually high stage in a river, normally the level at which the river 
overflow its banks and inundates the adjoining area .The design of bridges, culvert 
waterways and spillways for dams and estimation of score at a hydraulic structure 
are some examples wherein flood-peak values are required. To estimate the 
magnitude of a flood peak the following alternative methods are available: 
1. Rational method 
2. Empirical method 
3. unit-hydrograph technique 
4. Flood- frequency studies 
Rational Method 
If tp = tc 
 
Where, Qp = Peak discharge in m
3
/sec 
PC = Critical design rainfall in cm/hr 
A = Area catchment in hectares 
K = Coefficient of runoff. 
tD = Duration of rainfall 
tC = Time of concentration 
Empirical Formulae 
(a) Dickens Formula (1865) 
 
Where, Qp = Flood peak discharge in m
3
/sec 
A = Catchment area in km
2
. 
CD = Dickens constant, 6 = CD = 30. 
(b) Ryve’s formula (1884) 
 
Where, 
CH = Ryve’s constant 
= 8.8 for constant area within 80 km from the cost. 
= 8.5 if distance of area is 80 km to 160 km from the cost. 
= 10.2 if area is Hilley and away from the cost. 
(c) inglis Formula (1930) 
 
Where, A = Catchment area in Km
2
. 
QP = Peak discharge in m
3
/sec. 
Flood Frequency Studies 
(i) Recurrence interval or return Period: 
 where, P = Probability of occurrence 
(ii) Probability if non-occurrence: q = 1-P 
(iii) Probability of an event occurring r times in ‘n’ successive years: 
 
(iv) Reliability: (probability of non-occurrence /Assurance) = q
n
 
(v) Risk = 1-q
n 
? Risk = 1(1-P)
n
 
(vi) Safety Factory =   
(vii) Safety Margin = design value of hydrological parameter – Estimated value of 
hydrological parameter 
Gumbel’s Method 
The extreme value distribution was introduction by Gumbel (1941) and is commonly 
known as Gumbel’s distribution. it is one of the most widely used probability 
distribution functions for extreme values in hydrologic and meteorologic studies for 
prediction of flood peaks, maximum rainfall, maximum wind speed. 
Gunbel defined a flood as the largest of the 365 daily flows and the annual series of 
flood flows constitute a series of largest values of flows. 
Based on probability distribution. 
 
 Where, XT = Peak value of hydrologic data 
K = Frequency factor 
 yT = Reduced variate 
 
T = Recurrence interval in year 
yn = Reduced mean = 0.577 
Sn = Reduced standard deviation. 
Sn = 1.2825 for N ? 8 
 
Confidence Limit 
Since the value of the variate for a given return period, xT determined by Gumbel’s 
method can have errors due to the limited sample data used. An estimate of the 
confidence limits of the estimates is desirable the confidence interval indicates the 
limits about the calculated value between which the true value can be said to lie with 
specific probability based on sampling errors only. 
For a confidence probability c, the confidence interval of the variate xT is bounded by 
value x1 and x2 given by 
 
Where, f(c) is a function of confidence probability ‘C’. 
 
Se = Probability error 
Where, N = Sample size 
B = factor 
s = Standard deviation 
 
Flood Routing 
Flood routing is the technique of determining the flood hydrograph at a section of a 
river by utilizing the data of flood flow at one or more upstream sections. The 
hydrologic analysis of problems such a flood forecasting. Flood protection Reservoir 
design and spillway design invariable includes flood routing. 
Prism Storage: it is the volume that would exist if the uniform flow occurred at the 
downstream depth. i.e., the volume formed by an imaginary plane parallel to the 
channel bottom drawn at the outflow section water surface. 
Wedge Storage: it is the wedge like volume formed between the actual water 
surface profile and the top surface of the prism storage. 
Flood Routing 
 
Page 5


Floods, Flood Routing and Flood Control 
A flood an unusually high stage in a river, normally the level at which the river 
overflow its banks and inundates the adjoining area .The design of bridges, culvert 
waterways and spillways for dams and estimation of score at a hydraulic structure 
are some examples wherein flood-peak values are required. To estimate the 
magnitude of a flood peak the following alternative methods are available: 
1. Rational method 
2. Empirical method 
3. unit-hydrograph technique 
4. Flood- frequency studies 
Rational Method 
If tp = tc 
 
Where, Qp = Peak discharge in m
3
/sec 
PC = Critical design rainfall in cm/hr 
A = Area catchment in hectares 
K = Coefficient of runoff. 
tD = Duration of rainfall 
tC = Time of concentration 
Empirical Formulae 
(a) Dickens Formula (1865) 
 
Where, Qp = Flood peak discharge in m
3
/sec 
A = Catchment area in km
2
. 
CD = Dickens constant, 6 = CD = 30. 
(b) Ryve’s formula (1884) 
 
Where, 
CH = Ryve’s constant 
= 8.8 for constant area within 80 km from the cost. 
= 8.5 if distance of area is 80 km to 160 km from the cost. 
= 10.2 if area is Hilley and away from the cost. 
(c) inglis Formula (1930) 
 
Where, A = Catchment area in Km
2
. 
QP = Peak discharge in m
3
/sec. 
Flood Frequency Studies 
(i) Recurrence interval or return Period: 
 where, P = Probability of occurrence 
(ii) Probability if non-occurrence: q = 1-P 
(iii) Probability of an event occurring r times in ‘n’ successive years: 
 
(iv) Reliability: (probability of non-occurrence /Assurance) = q
n
 
(v) Risk = 1-q
n 
? Risk = 1(1-P)
n
 
(vi) Safety Factory =   
(vii) Safety Margin = design value of hydrological parameter – Estimated value of 
hydrological parameter 
Gumbel’s Method 
The extreme value distribution was introduction by Gumbel (1941) and is commonly 
known as Gumbel’s distribution. it is one of the most widely used probability 
distribution functions for extreme values in hydrologic and meteorologic studies for 
prediction of flood peaks, maximum rainfall, maximum wind speed. 
Gunbel defined a flood as the largest of the 365 daily flows and the annual series of 
flood flows constitute a series of largest values of flows. 
Based on probability distribution. 
 
 Where, XT = Peak value of hydrologic data 
K = Frequency factor 
 yT = Reduced variate 
 
T = Recurrence interval in year 
yn = Reduced mean = 0.577 
Sn = Reduced standard deviation. 
Sn = 1.2825 for N ? 8 
 
Confidence Limit 
Since the value of the variate for a given return period, xT determined by Gumbel’s 
method can have errors due to the limited sample data used. An estimate of the 
confidence limits of the estimates is desirable the confidence interval indicates the 
limits about the calculated value between which the true value can be said to lie with 
specific probability based on sampling errors only. 
For a confidence probability c, the confidence interval of the variate xT is bounded by 
value x1 and x2 given by 
 
Where, f(c) is a function of confidence probability ‘C’. 
 
Se = Probability error 
Where, N = Sample size 
B = factor 
s = Standard deviation 
 
Flood Routing 
Flood routing is the technique of determining the flood hydrograph at a section of a 
river by utilizing the data of flood flow at one or more upstream sections. The 
hydrologic analysis of problems such a flood forecasting. Flood protection Reservoir 
design and spillway design invariable includes flood routing. 
Prism Storage: it is the volume that would exist if the uniform flow occurred at the 
downstream depth. i.e., the volume formed by an imaginary plane parallel to the 
channel bottom drawn at the outflow section water surface. 
Wedge Storage: it is the wedge like volume formed between the actual water 
surface profile and the top surface of the prism storage. 
Flood Routing 
 
 
Muskingum Method 
 
 
S = Sp + Sw 
Where, S = Total storage in the channel. 
Sp = Prism storage 
= if (Q) = function of outflow discharge. 
Sw = Wedge storage 
= f(I) = function of inflow discharge. 
 
Where, X = Weighting factor