Flood Routing and Flood Control MCQ Quiz in বাংলা - Objective Question with Answer for Flood Routing and Flood Control - বিনামূল্যে ডাউনলোড করুন [PDF]

Some rivers show a flow of water all the time, even long after the last rainstorm in the watershed is called a perennial river.

Rivers that flow for only a short time after a rainstorm, and for the rest of time, usually most of the time, their beds are dry is called an ephemeral river.

Some rivers lie between these two extremes i.e. during the wetter part of the year they flow as a perennial stream, whereas during the drier part of the year they flow as an ephemeral stream is called an intermittent river.

If the water surface in the river lies above the local groundwater table in the river banks, then the river loses water to its banks is said to be an influent river or losing river.

Concept:

Hydrologic risk

\(Risk\,(R)=1-(1-{1\over T})^n\)

Or Risk (R) = 1 - (1 - P)ⁿ

Where T = Flood return period

P = Probability of occurring flood in a given year

n = Number of years of risk calculate

Calculation:

Given

Flood return period (T) = 100 years

Service life of the project (n) = 2 years

Risk (R) = \(1-(1-{1\over 100})^2\)

Risk (R) = 1 - (1 - 0.01)²

Risk (R) = 1 - (0.99)²

Risk (R) = 1 - 0.9801 = 0.0199

Risk (R) = 0.0199 × 100 = 1.99%

Risk (R) = 1.99%

Explanation

A flood is an unusually high stage in a river, normally the level at which the river overflows its banks and inundates the adjoining area.

Estimation of Flood Peak is done by 

1. Empirical Method

• Applicable only in regions where they are developed.
• Peak flood discharge expressed as, \({Q_p} = C{A^n}\)

2. Rational Method

• Peak flood discharge \({Q_p} = \frac{1}{{36}}K{P_c}A\) 
• where, K = coefficient of runoff, Pc = mean rainfall intensity (cm/hr). A = Catchment area (km2)

​So, from the above formula, we can say Option 4 is not true.

3. Unit - Hydrograph Method

• Used to predict the flood discharge if rainfall, infiltration characteristics are known.

4. Flood - frequency analysis

• Prediction for future floods is made on the basis of the available records of the past floods.
• Hazen Formula, Probability \(p = \frac{m}{{N + 1}}\)
• where, m = order no. of the event, N = Total no. of events in the data
• The recurrence interval, T = 1/p

The Modified Puls routing method and Goodrich routing method:

The Modified Puls routing method and Goodrich routing method is a reservoir routing (Lumped routing) which is used in

• the design of the capacity of spillways and other reservoir outlet structures
• the location and sizing of the capacity of reservoirs to meet specific requirements

Explanation:

• Hydrologic processes such as floods are very complicated processes and depend on characteristics of catchment, rainfall, etc.
• This makes the estimation of flood peak a very complex process. The statistical method of frequency analysis is one such method.

• For calculating the return period from Weibull’s formula, the data is required to be ranked with the highest ordinate having the least (first) rank.

For Weibull Formula:

The values of annual maximum flood from a given catchment area for a large number of successive years are arranged in decreasing order of magnitude and the probability P of each event being equaled or exceeded is given by

\(P = \frac{m}{{N + 1}}\)

Where N = Total number of observations, and m = rank of the value.

The recurrence interval, \(T = \frac{1}{P} = \frac{{N + 1}}{m}\)

Concept:

Risk = The probability of a flood to occur at least once in n successive years

Risk = 1 – qn = 1 – (1 - p)n

where,

p is the probability of occurrence of an event and it is given by 

p = 1/T

where,

T is the return period, q is the probability of not occurrence of an event

Calculation

Given, n = 25 years,  Risk = ??

Return Period (T) = 100. 

⇒ p = 1/100, q = 1 - 1/100 = 99/100

∴ Risk (R) = 1 – qn = 1 - (99/100)²⁵

Concept:

1. Probable maximum flood (PMF):

This is the flood resulting from the most severe combination of critical meteorological and hydrological conditions that are reasonably possible in the region. It is computed by using the probable maximum storm (PMS) which is an estimate of the physical upper limit to storm rainfall over the catchment. 

The design flood commonly adopted in India for spillways of major projects is PMF or a time period of 1000 yr flood whichever is higher.

2. Standard Project Flood (SPF):

This is the flood resulting from the most severe combination of meteorological and hydrological conditions considered reasonably characteristic of the region. It is computed from the Standard Project Storm (SPS) over the watershed considered and may be taken as the largest storm observed in the region of the watershed.

It is not maximized for the most critical atmospheric conditions but it may be transposed from an adjacent region to the watershed under consideration.

The design flood commonly adopted in India for barrages of major projects is SPF or a time period of 100 yr flood whichever is higher.

3. Flood of a specific return period:

It is estimated by frequency analysis of the annual flood values of adequate length. Sometimes when the flood data is inadequate, frequency analysis recorded storm data is made and the storm of a particular frequency applied to the unit hydrograph to derive the design flood.

Note:

The design flood commonly adopted in India for spillways of major projects is the Flood with a return period of 10,000 years.

Important Point:

Design Flood for Various type of dams are as follows:

For large dams (defined as those with gross storage greater than 60 million m3 or hydraulic head greater than 30 m), IDF should be based on PMF.

For intermediate dams (gross storage between 10 and 60 million m3 or hydraulic head between 12 m and 30 m), IDF should be based on SPF.

For small dams (gross storage between 0.5 to 10 million m3 or hydraulic head between 7.5 m to 12 m), IDF may be taken as 100 years return period flood.

Muskingum method:

A common method of Hydrologic channel routing is Muskingum method, in which the channel storage in a reach is expressed as a function of both inflow and outflow discharge.

S = k [x Im + (1 - x)Qm]

Where, s = Storage in a channel reach, I = Inflow Discharge, Q = Outflow Discharge, k = Storage time coefficient, x = Weightage factor (0-0.5)

For naturally occurring channels m = 1, Hence the equation becomes

S = k [x I + (1 - x)Q]

For x = 0, storage is only a function of outflow discharge. Such reservoirs are also known as Linear Reservoir.

Probable Maximum Flood (PMF):

This is the flood resulting from the most severe combination of critical meteorological and hydrological conditions that rare reasonably possible in the region.

It is computed by using the Probable Maximum Storm (PMS) which is an estimate of the physical upper limit to storm rainfall over the catchment.

Standard Project Flood (SPF):

This is the flood resulting from the most sever combination of meteorological and hydrological conditions considered reasonably characteristic of the region.

The SPF is computed from the Standard Project Storm (SPS) over the watershed considered and may be taken as the largest storm observed in the region of the watershed.

It is not maximized for the most critical atmospheric conditions but it may be transposed from an adjacent region to the watershed under consideration.

Design Flood:

For large dams: IDF should be based on PMF.

For intermediate dams: IDF should be based on SPF.

For small dams: IDF may be taken as 100 years return period flood.

Explanation:

• A common method of Hydrologic channel routing is Muskingum method, in which the channel storage in a reach is expressed as a function of both inflow and outflow discharge.

S = k [x I^m + (1 - x)Q^m]

Where, s = Storage in a channel reach

I = Inflow Discharge

Q = Outflow Discharge

k → Coefficient is known as the storage time coefficient. It was unit of time, it is approx. equal to time of travel of flood wave through the channel reach.
x → Weightage factor. Its range series from 0 to 0.5

• For naturally occurring channels m = 1, Hence the equation becomes

S = k [x I + (1 - x)Q]

• For x = 0, storage is only a function of outflow discharge. Such reservoirs are also known as Linear Reservoir.

The Modified Puls routing method and Goodrich routing method:

The Modified Puls routing method and Goodrich routing method is a reservoir routing (Lumped routing) which is used in

• the design of the capacity of spillways and other reservoir outlet structures
• the location and sizing of the capacity of reservoirs to meet specific requirements