Runoff MCQ Quiz in मल्याळम - Objective Question with Answer for Runoff - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

Concept:

There are two types of Infiltration index

1) ϕ - Index

Average rate of infiltration during the period of storm producing runoff

\(\phi - {\rm{Index}} = \frac{{{{\rm{P}}_{\rm{e}}} - {\rm{R}}}}{{{{\rm{t}}_{\rm{e}}}}}\)

Where,

P_e = Rainfall during storm producing runoff

t_e = Duration of storm producing runoff

R = Runoff

2) W - Index

Average rate of infiltration during entire storm duration

\({\rm{W}} - {\rm{Index}} = \frac{{{{\rm{P}}} - {\rm{R-Losses}}}}{{{{\rm{t}}}}}\)

Where,

P = Total rainfall during storm

R = Runoff

t = Duration of total storm

Calculation:

Given,

Runoff volume = 27.45 million m³ = 27.45 × 10⁶ m³

Basin area = 1830 km² = 1830 × 10⁶ m²

Rainfall intensities at an half and hour interval: 5, 9, 20, 13, 6, 8, 16 and 3 mm/hr 

\({\rm{Runoff\;in\;mm}} = \frac{{27.45 × {{10}^6}}}{{1830 × {{10}^6}}} = 0.015{\rm{\;m}}=15mm\)

P = (5 + 9 + 20 + 13 + 6 + 8 + 16 + 3)/2 = 80/2 = 40 mm

\({\rm{W}} - {\rm{Index}} = \frac{{{\rm{40}} - {\rm{15}}}}{{\rm{4}}} = 6.25\;{\rm{mm/hr}}\)

Elimintating rainfall intensities less than 6.25 mm/hr, i.e 5, 6, 3 mm/hr

∴ P = (9 + 20 + 13 + 8 + 16)/2 = 66/2 = 33 mm

As three intensities of half and hour interval is eliminated

∴ t = 4 - (0.5 × 3) = 2.5 hr

\(\phi - {\rm{Index}} = \frac{{{\rm{33}} - {\rm{15}}}}{{{\rm{2}}.{{\rm{5}}_{}}}} = 7.2\;{\rm{mm/hr}}\)

Concept:

The ϕ – index is a rate of infiltration in which, the rate of infiltration exceeds the value at which volume of runoff become equals to the volume of rainfall.

\( {\phi_{\left( {index} \right)}} = \frac{{{Total\; Infiltration}}}{{Total\;time\;of\;the\;storm}}\)

\( {\phi_{\left( {index} \right)}} = \frac{{{P_{total}} - Q}}{{Total\;time}}\)

where

P_Total = Total precipitation

Q = Runoff

Given:

First set of data:

t = 6 hr

P = 6 cm

Q = 3 cm

Second set of data:

t = 9 hr

P = 12 cm

Q = ? cm

Calculations:

\( {\phi_{\left( {index} \right)}} = \frac{{{P_{total}} - Q}}{{Total\;time}}\)

\({\phi _{index_1}} = \frac{{6 - 3}}{6} = 0.5\) cm/hr

\({\phi _{index_1}} = {\phi _{index_2}} = 0.5\)

\({\phi _{index_2}} = 0.5= \frac{{12 - Q}}{9} \)

\(\Rightarrow 9\times 0.5= {{12 - Q}}{} \)

Actual runoff (Q)= 12 – 9 × 0.5 = 7.5 cm

Based on the time delay between the precipitation and the runoff, the runoff is classified as

a) Direct Runoff → It is that part of runoff which enters the stream immediately after rainfall. It includes surface runoff, prompt interflow and rainfall on the surface of system. In case of snow melt, the resulting flow entering the stream is also a direct runoff.

Concept:

The surface run-off is the quantity of water that reaches the stream channels.

When rainfall occurs, it flows over the ground and reaches to the nearby stream and the amount of rainfall that flows in stream is known as surface runoff.

During the flow over the ground, some of the water is trapped by buildings and vegetation. Some of the water fills the depressions present in the ground. This amount of water is calculated as water lost.

The remaining of the water is absorbed by the soil.

Concept:

Runoff and Streamflow:

• Surface runoff is a term used to describe the flow of water, from rain, snowmelt, or other sources, over the land surface.
• Runoff is that part of the rainfall that is not absorbed by the soil by infiltration.
• The term streamflow is used to describe the drainage after runoff reaches a defined channel.
• The total quantity of surface water that can be expected in the given period from a stream at the outlet of its catchment is known as the yield of the catchment in that period.

Concept:

The shape of the basin governs the rate at which the water enters the stream and it is generally expressed by ‘Form Factor’ and ‘Compactness Coefficient’ as defined below:

\({\rm{Form\;factor}} = \frac{{{\rm{Aevrage\;width\;of\;the\;basin}}}}{{{\rm{Axial\;length\;of\;the\;basin}}}} = \frac{{\rm{B}}}{{\rm{l}}} = \frac{{\frac{{\rm{A}}}{l}}}{l} = \frac{{\rm{A}}}{{{l^2}}}\)

\({\rm{Compactness\;coefficient}} = \frac{{{\rm{Perimeter\;of\;the\;basin}}}}{{{\rm{Circumference\;of\;a\;circle\;whose\;area\;in\;equal\;to\;the\;area\;of\;the\;basin}}}}\)

\(∴ {\rm{Compactness\;coefficient}} = \frac{{\rm{p}}}{{2 \times \sqrt {{\rm{\pi }} \times {\rm{A}}} }}\)

Calculation:

\(∴ {\rm{Form\;factor\;}}\left( {\rm{F}} \right) = \frac{{\rm{A}}}{{{{\rm{l}}^2}}} = \frac{{50}}{{{8^2}}} = \frac{{50}}{{64}} = 0.78125 \approx 0.78\)

\(∴ {\rm{Compactness\;coefficient\;}}\left( {\rm{C}} \right) = \frac{{\rm{p}}}{{2 \times \sqrt {{\rm{\pi }} \times {\rm{A}}} }} = \frac{{25}}{{2 \times \sqrt {{\rm{\pi }} \times 50} }} = 0.997 \approx 1\)

∴ The form factor is approximately 0.78.

Concept:

Infiltration \(= \frac{{P - R}}{t}\)

Where, P = Precipitation

R = Run-off

t = Time Period

Calculation:

Area = 3 km2 = 300 x 10⁴ m²

Intensity = 2 cm/hr

t = 8 hours

Runoff = 0.3 Mm3 = 0.3 x 10⁶ m³

Total runoff in cm \(= \frac{{0.3 \times {{10}^6}{m^3}}}{{300 \times {{10}^4}{m^2}}} = 10cm\)

1) Rational Method

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

2) Dicken’s Method

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

3) Ryve’s Method

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

4) Inglis Method

\({Q_P} = \frac{{124A}}{{\sqrt {A + 10.4} }} \cong 123\sqrt A \)         => 1/2

Where, Q_P = Peak discharge in m³/s.

A = Area in km².

C_R = C_D = coefficient applicable in certain region.

6 < C_D < 30

6 < C_R < 11

Concept:

The Rational Method is used to calculate runoff or stormwater flow, and is given by:

Q = K × A × I × R

Given Units:

• Q = Discharge in liters per second (L/s)
• A = Area in hectares (ha)
• R = Rainfall intensity in cm/hour
• I = Runoff coefficient (dimensionless)

Conversion Details:

• 1 hectare = 10,000 m²
• 1 cm/hour = 0.01 m/hour
• Volume/hour from 1 ha and 1 cm/hr = 10,000 × 0.01 = 100 m³/hour
• 100 m³/hour = 100 × 1000 = 100,000 liters/hour
• Q in L/s = 100,000 ÷ 3600 ≈ 27.78 L/s

Final Answer:

K = 28

Concept:

ϕ - index:

• The ϕ index is the average rainfall above which the rainfall volume is equal to the runoff volume.
• The ϕ -index is derived from the rainfall hyetograph with the edge of the resulting run-off volume.

ϕ = \(P - R \over t_e\)

where 

P = total rainfall depth

R = runoff depth

t_e = duration of rainfall

Calculation:

P = Rainfall depth = 15× 6 = 90 mm

R = runoff depth = \(runoff \: volume\over\: basin\: area\)=  \(21.6 \times 10^6 \times 1000\over 300\times 10^6\) = 72 mm

P - R = 90 - 72 = 18mm

ϕ = \(P - R \over t_e\) = \(18\over 6\) = 3mm/hr.

⇒ ϕ = 3mm/hr.