Runoff and Hydrograph

I. Introduction

Runoff and hydrograph are important concepts in engineering hydrology. Runoff refers to the movement of water over the land surface, while a hydrograph is a graphical representation of the flow rate of a river or stream over a period of time. Understanding runoff and hydrograph is crucial for various engineering applications such as flood forecasting, water resource management, and design of hydraulic structures.

II. Runoff and its Components

Runoff is composed of three main components: surface runoff, subsurface runoff, and baseflow.

1. Surface Runoff

Surface runoff is the portion of rainfall that flows over the land surface and eventually reaches streams, rivers, and other water bodies. It occurs when the rainfall intensity exceeds the infiltration capacity of the soil or when the soil is saturated.

1. Subsurface Runoff

Subsurface runoff, also known as interflow, is the movement of water through the soil layers below the land surface. It occurs when the soil becomes saturated and cannot absorb any more water.

1. Baseflow

Baseflow is the portion of streamflow that comes from groundwater seepage into streams and rivers. It represents the long-term average flow of a river or stream and is sustained even during dry periods.

III. Factors Affecting Runoff

Several factors influence the amount and timing of runoff:

1. Climate factors

• Precipitation: The amount, intensity, and duration of rainfall directly affect runoff. Higher rainfall intensity and longer duration result in increased runoff.
• Temperature: High temperatures can increase evaporation rates, reducing the amount of water available for runoff.
• Evaporation: The process by which water changes from a liquid to a gas and returns to the atmosphere.
• Transpiration: The process by which plants release water vapor into the atmosphere.

1. Physical factors

• Soil type and properties: Different soil types have varying infiltration capacities, which affect the amount of runoff.
• Vegetation cover: Vegetation intercepts rainfall, reducing the amount of water that reaches the ground and potentially increasing infiltration.
• Topography: Steep slopes promote faster runoff, while flat areas allow for more infiltration.
• Land use: Urban areas with impervious surfaces generate more runoff compared to natural or agricultural areas.

IV. Basin Yield

Basin yield refers to the total amount of runoff generated by a watershed or drainage basin. It is influenced by various factors, including precipitation, soil characteristics, vegetation cover, and land use. Calculating basin yield is important for water resource management, flood control, and designing hydraulic structures.

V. Rainfall-Runoff Relationships

Rainfall-runoff relationships describe the relationship between the amount of rainfall and the resulting runoff. Various methods can be used to estimate runoff from rainfall:

1. Rational Method: The rational method is a simple approach that estimates peak runoff based on the catchment area, rainfall intensity, and a runoff coefficient.

2. Soil Conservation Service Curve Number Method: This method uses a curve number to estimate runoff based on soil type, land use, and antecedent moisture conditions.

3. Time-Area Method: The time-area method estimates runoff by dividing the catchment area into different time intervals and calculating the runoff volume for each interval.

4. Unit Hydrograph Method: The unit hydrograph method uses a hypothetical unit hydrograph to estimate runoff based on the rainfall excess.

VI. Flow-Duration and Flow Mass Curve

Flow-duration and flow mass curves are graphical representations of streamflow characteristics over a specified period of time.

1. Flow-Duration Curve

A flow-duration curve shows the percentage of time that a given flow rate is exceeded. It helps in understanding the frequency and duration of different flow rates.

1. Flow Mass Curve

A flow mass curve represents the cumulative volume of water passing through a stream or river over time. It helps in analyzing the total flow volume and the rate of flow accumulation.

VII. Hydrograph and its Separation

A hydrograph is a graphical representation of streamflow over time. It consists of two main components: direct runoff and baseflow.

1. Direct Runoff

Direct runoff is the portion of streamflow that directly results from rainfall or snowmelt. It is influenced by factors such as rainfall intensity, duration, and antecedent moisture conditions.

1. Baseflow

Baseflow is the portion of streamflow that comes from groundwater seepage into streams and rivers. It represents the long-term average flow of a river or stream and is sustained even during dry periods.

VIII. Unit Hydrograph Theory and Application

A unit hydrograph is a hydrograph resulting from a unit amount of effective rainfall uniformly distributed over a watershed. It is a useful tool for estimating the runoff hydrograph for a given rainfall event.

The theory and application of unit hydrograph include:

• Definition and explanation of unit hydrograph
• Derivation of the unit hydrograph convolution equation
• Unit hydrograph from complex storms
• Unit hydrograph for various durations
• Synthetic unit hydrograph
• S-curve hydrograph
• Distribution graph
• Use and applications of unit hydrograph

IX. Dimensionless Unit Hydrograph

A dimensionless unit hydrograph is a unit hydrograph that has been normalized to remove the effects of basin size and rainfall duration. It allows for the comparison of hydrographs from different watersheds and rainfall events.

The calculation and interpretation of a dimensionless unit hydrograph are important for understanding the hydrologic response of different watersheds.

X. Real-World Applications and Examples

Runoff and hydrograph analysis have various real-world applications in engineering projects. Some examples include:

• Design of stormwater management systems
• Flood forecasting and management
• Reservoir operation and water supply planning
• Erosion control and sedimentation management

Case studies of runoff and hydrograph analysis in these applications can provide valuable insights into their practical implementation.

XI. Advantages and Disadvantages of Runoff and Hydrograph Analysis

There are several advantages to conducting runoff and hydrograph analysis:

• Improved understanding of the hydrologic response of a watershed
• Better prediction of flood events and their impacts
• Enhanced water resource management

However, there are also some disadvantages to consider:

• The accuracy of the analysis depends on the availability and quality of data
• Simplifying assumptions and limitations of the analysis methods
• Uncertainty in predicting future rainfall patterns and climate change impacts

XII. Conclusion

In conclusion, understanding runoff and hydrograph is essential for engineering hydrology. Runoff is composed of surface runoff, subsurface runoff, and baseflow, and it is influenced by climate and physical factors. Basin yield and rainfall-runoff relationships help in estimating runoff from a watershed. Hydrographs represent streamflow over time and can be separated into direct runoff and baseflow. The unit hydrograph theory and dimensionless unit hydrograph provide tools for estimating and comparing hydrographs. Real-world applications of runoff and hydrograph analysis are diverse, but there are also limitations and uncertainties to consider. Overall, a thorough understanding of runoff and hydrograph is crucial for effective water resource management and flood control.

Summary

Runoff and hydrograph are important concepts in engineering hydrology. Runoff refers to the movement of water over the land surface, while a hydrograph is a graphical representation of the flow rate of a river or stream over a period of time. Understanding runoff and hydrograph is crucial for various engineering applications such as flood forecasting, water resource management, and design of hydraulic structures. Runoff is composed of three main components: surface runoff, subsurface runoff, and baseflow. Factors affecting runoff include climate factors (precipitation, temperature, evaporation, and transpiration) and physical factors (soil type and properties, vegetation cover, topography, and land use). Basin yield refers to the total amount of runoff generated by a watershed and is influenced by various factors. Rainfall-runoff relationships describe the relationship between rainfall and runoff, and different methods can be used to estimate runoff from rainfall. Flow-duration and flow mass curves are graphical representations of streamflow characteristics over time. A hydrograph represents streamflow over time and can be separated into direct runoff and baseflow. Unit hydrograph theory and application involve the use of a hypothetical unit hydrograph to estimate runoff. A dimensionless unit hydrograph is a normalized unit hydrograph that allows for the comparison of hydrographs from different watersheds. Runoff and hydrograph analysis have various real-world applications in engineering projects, but there are also limitations and uncertainties to consider. Overall, understanding runoff and hydrograph is essential for effective water resource management and flood control.

Analogy

Understanding runoff and hydrograph is like understanding the flow of traffic on a road. Runoff is similar to the cars moving on the road, while a hydrograph is like a graph showing the number of cars passing by at different times. Factors affecting runoff, such as climate and physical factors, can be compared to factors affecting traffic flow, such as weather conditions and road characteristics. Just as analyzing traffic flow helps in managing road congestion and planning transportation systems, understanding runoff and hydrograph is crucial for managing water resources and designing hydraulic structures.