Flood Routing and Flood Control MCQ Quiz - Objective Question with Answer for Flood Routing and Flood Control - Download Free PDF

The Muskingum method is primarily a hydrological channel routing technique used to manage water flow through river channels. It considers the balance between inflow and outflow, simplifying the storage and discharge characteristics of the river reach to predict changes in water levels. While it incorporates some hydraulic principles, it does not provide a complete numerical solution of the St. Venant equations and is distinct from methods used specifically for reservoir routing.

Dicken's formula for estimating flood discharge is given by: Q=CA(3/4)

Explanation:

Geotextile Reinforcement for Embankments

• Geotextile reinforcement involves the use of synthetic fabrics to improve soil stability and strength in embankment construction.
• These materials provide excellent drainage properties and prevent soil erosion, making them ideal for flood-prone areas.
• Geotextiles help distribute loads, reduce settlement, and increase the overall stability of embankments.

Additional Information

• Soil nailing: Used to stabilize slopes and retaining walls.
• Shotcrete application: Applied to stabilize surfaces, particularly in tunnels and retaining walls.
• Jet grouting: Used for soil stabilization by injecting grout into the ground.

Explanation:

Dicken’s Method

It was adopted for Northern India.

\({Q_P} = {C_D}{A^{\frac{3}{4}}}\)

where

QP = Peak discharge in m3/s.

A = Area in km2.

CD = coefficient applicable in the region.

Additional Information

Different empirical method for determining flood discharge are as follows:

Ryve’s Method

\({Q_P} = {C_R}{A^{\frac{2}{3}}}\)

This formula is used only in Southern India.

Jarvi’s Method

This formula is applicable for Eastern India.

\({Q_P} = C\sqrt A\)

Inglis Method

This formula is used only in Maharashtra. Here three different cases are taken into consideration.

\({Q_P} = \frac{{124A}}{{\sqrt {A + 10.4} }} \cong 123\sqrt A \)

Concept:

• Return Period (T): It is the average time interval between the occurrence of rainfall of magnitude equal to or in excess of a specific magnitude.
• Probability (P): The probability of rainfall whose magnitude is equal to or in excess of specific magnitude having a return period of T is given as, \(\text{P} = \frac{1}{T}\)
• q = probability of rainfall not occurring in a given year = 1 - P
• Probability of rainfall not occurring at all in 'n' successive years = qⁿ
• Probability of rainfall occurring at least once in 'n' successive years = 1 - qⁿ

​
Calculation:

Given,

n = 20 years, T = 40 years

∵ We know that, \(\text{P} = \frac{1}{T}\)

⇒ \(\text{P} = \frac{1}{40}\) = 0.025

∴ q = 1 - P = 0.975

∵ We know that the Probability of rainfall occurring at least once in 'n' successive years = 1 - qn

⇒ Probability of rainfall occurring at least once in '20' successive years = 1 - 0.975²⁰ = 0.397

Explanation

Inglis Method

This formula is used only in Maharashtra. Here three different cases are taken into consideration.

\({Q_P} = \frac{{124A}}{{\sqrt {A + 10.4} }} \cong 123\sqrt A\)

where

QP = Peak discharge in m3/s.

A = Area in km2.

Important Points

 

Ryve’s Method

This formula is used only in Southern India.

\({Q_P} = {C_R}{A^{\frac{2}{3}}}\)

Rational Method

This method is based on the principle of the relationship between rainfall and runoff. Hence, it can be considered to be similar to empirical method.

\({Q_P} = \frac{1}{{36}}k{P_c}{A^1}\)

Dicken’s Method

It was earlier adopted only in Northern India but now it can be used in most of the States in India after proper modification of the constant.

\({Q_P} = {C_D}{A^{\frac{3}{4}}}\)

Concept:

Flood Routing:

• 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 is called flood routing.
• There are basically two types of flood routing methods:

1. Reservoir routing
2. Channel Routing

Attenuation:

• The following figure shows the result of reservoir routing.

• Owing to the storage effect, the peak of the outflow hydrograph will be smaller than that of the inflow hydrograph. This reduction in the peak value is called attenuation.

Lag:

• The peak of the outflow occurs after the peak of the inflow; the time difference between the two peaks is known as lag.

Additional Information

• Total volume in storage can be considered under two categories as:

1. Prism storage - It is the volume that would exist if the uniform flow occurred at the downstream depth.
2. Wedge Storage - It is the wedgelike volume formed between the actual water surface profile and the top surface of the prism storage.

Important Points

• Linear reservoirs are those reservoirs in which the storage is the function of outflow.

Concept:

The probability for the annual maximum flood magnitude to occur = \(\frac{1}{{Return\;period\;}} \)

Calculation:

\(\begin{array}{l} p = \frac{1}{T} = 0.1\\ q = 1 - p = 0.9 \end{array}\)

Probability of flood at least once in next 5 years

= 1 – no flood in next 5 years

= 1 – q⁵

= 1 – (0.9)⁵ = 0.409

Concept

Probable Maximum Precipitation (PMP) is termed as “theoretically the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographic location at a given time of the year.

∴ Option 2 is correct.

(PMP) is a theoretical concept that is widely used by hydrologists to arrive at estimates for probable maximum flood (PMF) that find use in planning, design, and risk assessment of high-hazard hydrological structures such as flood control dams upstream of populated areas.

As it is maximum precipitation for a given duration, not maximum precipitation for all recorded storms.

∴ Option 4 is wrong.

Concept:

The probability for the annual maximum flood magnitude to occur = \(\frac{1}{{Return\;period\;}} \)

Calculation:

Given,

Return period T = 10 years

 n = 5 years

p is the probability of occurring an event 

q is the probability of not occurring an invent.

\(\begin{array}{l} p = \frac{1}{T} = 0.1\\ q = 1 - p = 0.9 \end{array}\)

Probability of flood at least once in next 5 years

= 1 – no flood in next 5 years

= 1 – q⁵

= 1 – (0.9)⁵ = 0.40

Concept:

The probability for the annual maximum flood magnitude to occur = \(\frac{1}{{Return\;period\;}} \)

Calculation:

Given,

Return period T = 10 years

 n = 2 years

p is the probability of occurring an event 

q is the probability of not occurring an invent.

\(\begin{array}{l} p = \frac{1}{T} = 0.1\\ q = 1 - p = 0.9 \end{array}\)

Probability of flood at least once in next 2 years

= 1 – no flood in next 2 years

= 1 – q²

= 1 – (0.9)2 = 0.19

in Terms of % = 19%

Explanation:

Dicken’s Method

It was adopted for Northern India.

\({Q_P} = {C_D}{A^{\frac{3}{4}}}\)

where

QP = Peak discharge in m3/s.

A = Area in km2.

CD = coefficient applicable in the region.

Additional Information

Different empirical method for determining flood discharge are as follows:

Ryve’s Method

\({Q_P} = {C_R}{A^{\frac{2}{3}}}\)

This formula is used only in Southern India.

Jarvi’s Method

This formula is applicable for Eastern India.

\({Q_P} = C\sqrt A\)

Inglis Method

This formula is used only in Maharashtra. Here three different cases are taken into consideration.

\({Q_P} = \frac{{124A}}{{\sqrt {A + 10.4} }} \cong 123\sqrt A \)

Explanation

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.

Concept:

Return period (T):- It is defined as the average time interval after which a flood of given discharge is equalled or exceeded.

\({\bf{T}} = \frac{1}{{\bf{p}}}\)

Where,

p = probability of occurrence or exceedance of an event with rank ‘m’.

The probability of occurrence or exceedance is given by three methods:-

Method	Probability(p)
Weibull Method	\(\frac{{\rm{m}}}{{{\rm{N}} + 1}}\)
California Method	\(\frac{{\rm{m}}}{{\rm{N}}}\)
Hazen Method	\(\frac{{\left( {{\rm{m}} - 0.5} \right)}}{{\rm{N}}}\)

 

N = Total number of events

Thus as per Weibul method,

\({\rm{p}} = \frac{{\rm{m}}}{{{\rm{N}} + 1}}\)

\(\therefore {\rm{T}} = \frac{1}{{\rm{p}}} = \frac{{\left( {{\rm{N}} + 1} \right)}}{{\rm{m}}}\)

Hence the required return period is (N+1)/m