(SEM VII) THEORY EXAMINATION 2018-19 ENGINEERING HYDROLOGY

SECTION A

(Attempt all questions in brief – 2 × 10 = 20 marks)

(a) Depression Storage

Depression storage is the part of rainfall that gets temporarily stored in small surface depressions such as puddles, ditches, and low-lying areas. This water does not immediately contribute to runoff. It either evaporates or infiltrates into the soil. Depression storage reduces direct runoff, especially during light rainfall events.

(b) Methods to Assess Recharge of Groundwater

Groundwater recharge in an area can be assessed using methods such as water-table fluctuation method, soil moisture balance method, and rainfall infiltration studies. These methods estimate how much rainfall percolates into the ground and increases groundwater storage.

(c) Frequency Analysis

Frequency analysis is a statistical method used in hydrology to predict the probability of occurrence of hydrological events such as floods or rainfall. It helps estimate return periods, like 10-year or 100-year floods, which are essential for designing hydraulic structures.

(d) Global Water Budget

The global water budget explains the distribution and movement of water on Earth through processes like evaporation, precipitation, runoff, and infiltration. It shows that oceans contain the largest share of Earth’s water, while freshwater availability is limited, making water resource management important.

SECTION B

(Attempt any one – 10 marks)

(a) Unit Hydrograph: Definition, Assumptions, and Uses

A unit hydrograph is defined as the direct runoff hydrograph resulting from one unit depth of effective rainfall occurring uniformly over a catchment for a specified duration.

The main assumptions are that rainfall is uniformly distributed, the catchment characteristics remain constant, and the runoff response is linear.

Unit hydrographs are used to estimate flood hydrographs, design drainage structures, and predict runoff from storms of different durations.

(b) Differences Explained Briefly

Actual and Potential Evapotranspiration
Actual evapotranspiration is the real amount of water lost due to evaporation and transpiration under existing moisture conditions, while potential evapotranspiration is the maximum possible loss assuming unlimited water supply.

Field Capacity and Permanent Wilting Point
Field capacity is the moisture content after excess water drains away from soil, whereas permanent wilting point is the moisture level below which plants cannot recover.

Depression Storage and Interception
Depression storage refers to water stored in surface depressions, while interception is rainfall caught by vegetation before reaching the ground.

Infiltration Capacity and Infiltration Rate
Infiltration capacity is the maximum rate at which soil can absorb water, while infiltration rate is the actual rate at which water enters the soil at a given time.

SECTION C

(Attempt any one – 10 marks)

(a) Unit Hydrograph Numerical Concept

In this question, a 12-hour unit hydrograph is given as triangular in shape with a base width of 144 hours and a peak discharge of 23 m³/s.

The area under a unit hydrograph represents the total runoff volume produced by one unit depth of rainfall. The area of the triangular hydrograph is calculated using the formula for the area of a triangle. This runoff volume is then equated to rainfall excess over the catchment to determine the catchment area.

Such numerical problems test understanding of the relationship between rainfall, runoff, and basin characteristics.

(b) Hydrograph of a Catchment (Conceptual Explanation)

A hydrograph represents variation of discharge with time at a particular point in a river. It consists of rising limb, peak flow, and recession limb.

The shape of a hydrograph depends on rainfall intensity, duration, basin area, slope, soil type, and land use. Hydrographs are used for flood forecasting, reservoir operation, and water resources planning.