All questions of Engineering Hydrology for Civil Engineering (CE) Exam

Explanation:

The world's total quantity of water is about 1.386 billion cubic kilometers. Out of this, only about 2.5% is freshwater, and the remaining 97.5% is saline water.

The percentage of total quantity of water in the world that is saline can be calculated as follows:

Saline water quantity = 97.5% of 1.386 billion cubic kilometers
= 1.3515 billion cubic kilometers

Total water quantity = 1.386 billion cubic kilometers

Percentage of saline water = (Saline water quantity / Total water quantity) x 100
= (1.3515 / 1.386) x 100
= 97.48%

Therefore, the percentage of total quantity of water in the world that is saline is about 97%.

Conclusion:

The world's water resources are mainly composed of saline water, which is not suitable for human consumption or agricultural purposes. The limited freshwater resources need to be conserved and managed sustainably to meet the growing demand for water worldwide.

B

The mass curve of rainfall of a storm is a plot of accumulated precipitation against time in chronological order. It shows how the total rainfall changes over time during a storm. It is useful for understanding how much and how quickly rainfall will accumulate during a storm, which can help with flood forecasting and stormwater management. The mass curve can also help to identify how much rainfall is concentrated in a short period of time ( peak rainfall rate) and how much rainfall is spread over the duration of the storm.

C

Thermohygrograph is a device that records continuous data of temperature and humidity. It is commonly used in industries and laboratories where temperature and humidity control is crucial. Here is a detailed explanation of the answer:

Temperature and Humidity Recording:
Thermohygrograph gives a continuous recording of temperature and humidity. It has two sensors, one for temperature and the other for humidity. These sensors record the respective values and plot them on a chart over a period of time. The chart is usually a circular disc that rotates once a day or a week, depending on the model.

Working Principle:
The working principle of a thermohygrograph is based on the expansion and contraction of a special type of paper that is sensitive to temperature and humidity. The paper is coated with a chemical that changes color with the change in temperature and humidity. The paper is wrapped around the rotating disc, and the sensors record the values and plot them on the paper.

Applications:
Thermohygrograph is widely used in industries that require controlled temperature and humidity, such as pharmaceuticals, food processing, and storage facilities. It is also used in laboratories to monitor the temperature and humidity during experiments. It is an essential tool for climate studies and weather forecasting.

Advantages:
Thermohygrograph provides a continuous recording of temperature and humidity, which is useful for identifying any fluctuations or variations in the environment. It helps in maintaining the quality and safety of products that are sensitive to temperature and humidity. It also helps in predicting weather patterns and climate changes.

Conclusion:
In conclusion, Thermohygrograph is a device that records continuous data of temperature and humidity. It is widely used in industries and laboratories for monitoring and controlling the environment. It provides accurate and reliable data that helps in maintaining the quality and safety of products.

Explanation:

The correct answer is option 'D' - Slope-area method. Let's understand why the slope-area method is not a direct stream flow determination technique.

Slope-Area Method:
The slope-area method is a commonly used indirect method for determining stream flow in open channels. It involves measuring the cross-sectional area of the channel and the slope of the water surface. By applying the principles of conservation of mass and energy, the flow rate can be calculated. However, it is not considered a direct stream flow determination technique because it relies on measurements and calculations rather than direct measurement of the flow.

Direct Stream Flow Determination Techniques:
Direct stream flow determination techniques involve directly measuring the flow rate of water in a stream or channel. These techniques provide more accurate and precise measurements compared to indirect methods. Some of the commonly used direct stream flow determination techniques include:

1. Dilution Method:
The dilution method involves adding a known quantity of a tracer (such as a dye or salt) to the stream and measuring the change in concentration downstream. By knowing the initial concentration of the tracer and the rate of dilution, the flow rate can be calculated.

2. Ultrasonic Method:
The ultrasonic method utilizes ultrasonic sensors to measure the velocity of the water flow. By combining the velocity measurements with cross-sectional area measurements, the flow rate can be determined.

3. Area-Velocity Method:
The area-velocity method involves measuring the cross-sectional area of the channel and the velocity of the water flow. By multiplying the area and velocity, the flow rate can be calculated.

These direct stream flow determination techniques provide accurate and reliable measurements, making them suitable for various applications in water resources engineering, hydrology, and environmental studies.

Conclusion:
The slope-area method is not considered a direct stream flow determination technique because it relies on measurements and calculations rather than direct measurement of the flow. The dilution method, ultrasonic method, and area-velocity method are examples of direct stream flow determination techniques that provide more accurate and precise measurements.

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Explanation:
The hydrological cycle is the continuous movement of water on, above, and below the surface of the Earth. It involves the processes of evaporation, transpiration, precipitation, infiltration, and runoff. Water moves through different reservoirs such as the atmosphere, oceans, rivers, lakes, groundwater, and glaciers. The residence time of water refers to the average time that a water molecule spends in a particular reservoir.

Residence time of water in oceans:
- The oceans are the largest reservoir of water on the planet, covering about 70% of the Earth's surface.
- The average residence time of water in the oceans is about 3,000 years.
- This means that a water molecule in the ocean will stay there for an average of 3,000 years before it evaporates into the atmosphere or is transported to another reservoir.

Residence time of water in groundwater:
- Groundwater is the water that is stored in the pores and fractures of rocks beneath the Earth's surface.
- The average residence time of water in groundwater is about 1,400 years.
- This means that a water molecule in groundwater will stay there for an average of 1,400 years before it is discharged into a stream or river, or extracted by a well.

Residence time of water in rivers:
- Rivers are the channels through which water flows from one place to another, eventually reaching the oceans.
- The average residence time of water in rivers is about 16 days.
- This means that a water molecule in a river will stay there for an average of 16 days before it is discharged into the ocean.

Residence time of water in atmospheric moisture:
- Atmospheric moisture refers to the water vapor in the Earth's atmosphere.
- The average residence time of water in atmospheric moisture is about 9 days.
- This means that a water molecule in atmospheric moisture will stay there for an average of 9 days before it either condenses into clouds and precipitates, or is transported to another location by winds.

Conclusion:
From the above explanation, it is clear that the average residence time of water in oceans is larger than that of the global groundwater. Hence, option D is the correct answer.

Collecting Area of Standard Symons Type Raingauge

The standard Symons type raingauge is an instrument used to measure rainfall. It has a cylindrical container with a funnel-shaped top that collects rainwater. The collected water is then measured to determine the amount of rainfall. The collecting area of the standard Symons type raingauge is given as:

a) 12.7 cm

Explanation:

The collecting area of a raingauge is the surface area of the funnel-shaped top that collects rainwater. The standard Symons type raingauge has a diameter of 10 inches (25.4 cm) and a height of 10 inches (25.4 cm). The funnel-shaped top of the raingauge has a diameter of 8 inches (20.32 cm) at the top and a diameter of 5 inches (12.7 cm) at the bottom. Therefore, the collecting area of the raingauge is the area of the circular base with a diameter of 5 inches (12.7 cm).

Hence, the correct answer is option A, i.e., 12.7 cm.

Solution:

Given: Probability of design capacity flood in 50 years = 1/50

We need to find the probability of exactly one flood of design capacity in the 75-year life of the structure.

To solve the problem, we can use the Poisson distribution formula, which gives the probability of a certain number of events occurring in a fixed interval of time, given the average rate of occurrence.

The formula is:

P(X=k) = (e^(-λ) * λ^k) / k!

where X is the random variable representing the number of events, k is the number of events we want to find the probability for, λ is the average rate of occurrence, and e is the base of the natural logarithm.

Let's calculate the value of λ first:

The expected number of design capacity floods in 75 years is:

λ = (75 / 50) * 1 = 1.5

Now, let's use the Poisson distribution formula to find the probability of exactly one flood of design capacity in 75 years:

P(X=1) = (e^(-1.5) * 1.5^1) / 1! = 0.336

Therefore, the answer is (c) 0.336, which is option (c).

The % of earth covered by oceans is about 71%.

Explanation:

Coverage of the Earth's Surface

The Earth's surface is covered by different types of landmasses such as mountains, valleys, deserts, forests, and water bodies such as oceans, seas, lakes, and rivers.

Percentage of the Earth's Surface Covered by Oceans

The oceans are the largest water bodies on Earth and cover about 71% of the Earth's surface. The remaining 29% of the Earth's surface is covered by landmasses such as continents and islands.

Importance of Oceans

Oceans are important for various reasons such as:

- They regulate the Earth's climate and weather patterns
- They provide a habitat for diverse marine life
- They serve as a source of food and livelihood for millions of people
- They facilitate trade and transportation
- They offer recreational activities such as swimming, surfing, and diving.

Conclusion

In summary, the % of Earth covered by oceans is about 71%, making them a crucial part of the Earth's ecosystem and human activity.

Under identical conditions, evaporation from seawater is typically about 2-3% less than evaporation from fresh water. This is because the salt in the seawater raises the boiling point of the water, making it more difficult for the water to turn into vapor. Additionally, the surface tension of seawater is higher than that of freshwater, which also reduces evaporation.

Explanation:

An instantaneous unit hydrograph is a hydrograph that represents the direct runoff response of a catchment to a unit rainfall excess that occurs instantaneously over the catchment. It is a fundamental concept in hydrology and is widely used in the analysis and design of stormwater management systems.

Key Points:

- Direct Runoff: An instantaneous unit hydrograph represents the direct runoff component of the total streamflow. It does not include the baseflow component, which is the groundwater contribution to streamflow.

- Unit Rainfall Excess: The unit rainfall excess refers to a rainfall excess of a unit magnitude, such as 1 cm or 1 inch. It is the portion of rainfall that exceeds the infiltration capacity of the catchment and directly contributes to streamflow.

- Instantaneous Precipitation: The unit rainfall excess is assumed to occur instantaneously over the catchment. This means that the entire unit excess rainfall is assumed to be applied to the catchment at a single point in time.

- Catchment Response: The instantaneous unit hydrograph represents the response of the catchment to the unit rainfall excess. It shows how the streamflow hydrograph changes over time in response to the input of rainfall.

- Duration: The instantaneous unit hydrograph does not have a specific duration. It is a theoretical concept that represents the response of a catchment to a unit rainfall excess regardless of the duration of the excess rainfall.

Conclusion:

In summary, an instantaneous unit hydrograph is a hydrograph that represents the direct runoff response of a catchment to a unit rainfall excess that occurs instantaneously over the catchment. It is a theoretical concept that helps in understanding the relationship between rainfall and streamflow in a catchment.

Hydrographs and Flood Events
A hydrograph is a graphical representation of the flow rate or discharge of a river or channel over a specific period of time. It depicts the rise and fall of water levels in response to precipitation, and is commonly used in hydrology and hydraulic engineering to analyze flood events.

Isolated Storm and Flood Hydrograph
An isolated storm refers to a singular rainfall event that occurs over a relatively short period of time, typically within a few hours. When this storm occurs, it can produce a flood hydrograph that illustrates the changes in river discharge over time. The flood hydrograph resulting from an isolated storm typically consists of three distinct phases: surface runoff, interflow, and base flow.

Surface Runoff
Surface runoff is the portion of precipitation that flows over the ground surface and eventually enters streams or rivers. It occurs when the rate of rainfall exceeds the infiltration rate of the soil, causing excess water to flow overland. Surface runoff is a major contributor to flood events, as it can rapidly increase river discharge. In the flood hydrograph, the surface runoff phase is characterized by a steep rising limb, indicating a rapid increase in river discharge.

Interflow
Interflow refers to the lateral movement of water within the soil, just below the ground surface. It occurs when the soil becomes saturated and excess water starts to move horizontally through the soil layers. Interflow contributes to the overall flow of water towards streams or rivers, but its contribution to flood events is relatively smaller compared to surface runoff. In the flood hydrograph, the interflow phase is represented by a slower rising limb, indicating a gradual increase in river discharge.

Base Flow
Base flow is the portion of streamflow that is sustained by groundwater seepage into a river or channel. It represents the long-term average flow of a river under normal weather conditions, and it is typically lower than the peak flow associated with flood events. However, during a flood event, the base flow can contribute to the overall river discharge. In the flood hydrograph, the base flow phase is depicted by a relatively flat portion after the peak flow, indicating a gradual recession of river discharge.

Conclusion
In summary, the flood hydrograph resulting from an isolated storm consists of three phases: surface runoff, interflow, and base flow. Surface runoff is the primary contributor to flood events, as it represents the rapid increase in river discharge during heavy rainfall. Interflow and base flow play smaller roles in flood events but contribute to the overall hydrograph shape. Understanding the different phases of a flood hydrograph is crucial for analyzing and managing flood risk in hydraulic engineering and hydrology.

Muskingum Method for Flood Routing

The Muskingum method is a hydrological technique used for flood routing in rivers and streams. It is a linear reservoir model that predicts the outflow hydrograph from an upstream river reach to a downstream river reach. The method is based on the principle of conservation of mass and the assumption that the flow in a river can be represented by a single channel with uniform characteristics.

Equation for Flood Routing

The Muskingum method of flood routing gives the following equation:

Q2 = C0I2 + C1I1 + C2Q1

where Q2 is the outflow at the downstream reach, Q1 is the inflow at the upstream reach, I1 and I2 are the inflows at the current and previous time steps, and C0, C1, and C2 are the coefficients that determine the routing characteristics.

Coefficients in the Equation

The coefficients in the Muskingum equation are determined by the channel characteristics and the routing parameters. The values of the coefficients depend on the channel geometry, roughness, and slope, as well as the time step and the length of the reach.

The coefficients have the following properties:

- C0 + C1 + C2 = 1, which ensures that the outflow is a linear combination of the inflows.
- C0, C1, and C2 are all positive and less than or equal to 0.5, which ensures stability and accuracy of the method.
- The values of the coefficients depend on the ratio of the time step to the reach length, which affects the storage and attenuation characteristics of the reach.

Conclusion

In summary, the Muskingum method is a widely used technique for flood routing in rivers and streams. The method is based on a linear reservoir model and uses the Muskingum equation to predict the outflow hydrograph at the downstream reach. The coefficients in the equation are determined by the channel characteristics and the routing parameters, and have specific properties that ensure stability and accuracy of the method.

The Hydrologic Cycle

The hydrologic cycle is the continuous circulation of water between the earth's surface and the atmosphere. It involves various processes such as evaporation, precipitation, infiltration, and runoff. The balance between the inflow and outflow of water is crucial for the sustainability of water resources.

Mass Inflow and Outflow

The mass inflow and outflow refer to the amount of water that enters and leaves a particular system. The mass inflow includes the amount of water that enters a basin through precipitation, surface runoff, and subsurface flow. The mass outflow includes the amount of water that leaves the basin through evaporation, transpiration, surface runoff, and subsurface flow.

Change in Mass Storage

The change in mass storage refers to the difference between the inflow and outflow of water in a particular system. It is the amount of water that is stored in the basin at a particular time. The change in mass storage can be positive or negative depending on the inflow and outflow of water.

The Correct Answer

The hydrologic cycle states that the mass inflow and outflow are equal, and the change in mass storage is the difference between the inflow and outflow of water. Therefore, the correct answer is option D, "mass inflow - mass outflow = change in mass storage." This equation is important in hydrology as it helps to understand the water balance of a particular system and to manage water resources effectively.

Explanation:

When a floodwave passes through a given section of a river, the maximum stage and maximum discharge will occur at some point during the flood event. The question asks about the relationship between the occurrence of these two events.

Option A: The maximum discharge passes down before the maximum stage is attained

This option suggests that the peak flow of water passes downstream before the water level reaches its highest point. However, in a flood event, the water level usually rises before the peak flow occurs. This option is not correct.

Option B: The maximum stage is attained before the maximum discharge passes down

This option suggests that the water level reaches its highest point before the peak flow passes downstream. In many cases, this is indeed the situation during a flood event. As the water level rises, it eventually reaches its maximum, and then the peak flow of water follows. This option is plausible.

Option C: The two events occur simultaneously

This option suggests that the maximum stage and maximum discharge occur at the same time. This is a possibility during a flood event, especially if there are no significant delays in the propagation of the floodwave. The water level and flow rate could both reach their peaks simultaneously. This option is plausible.

Option D: No specific sequence would be universally assignable

This option suggests that there is no consistent relationship between the occurrence of the maximum stage and maximum discharge. However, in reality, there is often a relationship between the rise in water level and the subsequent peak flow. This option is not correct.

Conclusion:

Based on the explanations above, the correct answer is option C: the maximum stage and maximum discharge occur simultaneously.