ANALYSIS OF RAINFALL RUNOFF IN HYDROLOGY
- 2. RUNOFF • Runoff is the draining or flowing off of precipitation from a catchment area through a surface channel. • It thus represents the output from the catchment in a given unit of time. Consider a catchment area receiving precipitation. • For a given precipitation, the evapotranspiration, initial loss, infiltration and detention storage requirements will have to be first satisfied before the commencement of runoff. • When these are satisfied, the excess precipitation moves over the land surfaces to reach smaller channels. • This portion of the runoff is called overland flow and involves building up of a storage over the surface and draining off of the same.
- 3. • Usually the lengths and depths of overland flow are small and the flow is in the laminar regime. • Flows from several small channels join bigger channels and flows from these in turn combine to form a larger stream, and so on, till the flow reaches the catchment outlet. • The flow in this mode, where it travels all the time over the surface as overland flow and through the channels as open-channel flow and reaches the catchment outlet is called surface runoff .
- 4. Different routes of runoff
- 5. • A part of the precipitation that infiltrates, moves laterally through upper crusts of the soil and returns to the surface at some location away from the point of entry into the soil. • This component of runoff is known variously as interflow, through flow, storm seepage, subsurface storm flow or quick return flow • The amount of interflow depends on the geological conditions of the catchment. • A fairly pervious soil overlying a hard impermeable surface is conducive to large interflows. • Depending upon the time delay between the infiltration and the outflow, the interflow is sometimes classified into prompt interflow, i.e. the interflow with the least time lag and delayed interflow. • Another route for the in filtered water is to undergo deep percolation and reach the groundwater storage in the soil. • The groundwater follows a complicated and long path of travel and ultimately reaches the surface. • The time lag, i.e. the difference in time between the entry into the soil and outflows from it is very large, being of the order of months and years. • This part of runoff is called groundwater runoff or groundwater flow. • Groundwater flow provides the dry-weather flow in perennial streams.
- 6. • Based on the time delay between the precipitation and the runoff, the runoff is classified into two categories; as 1. Direct runoff, 2. Base flow. 1. Direct Runoff • It is that part of the runoff which enters the stream immediately after the rainfall. • It includes surface runoff, prompt interflow and rainfall on the surface of the stream. • In the case of snow-melt, the resulting flow entering the stream is also a direct runoff. • Sometimes terms such as direct storm runoff and storm runoff are used to designate direct runoff. 2. Base Flow • The delayed flow that reaches a stream essentially as groundwater flow is called base flow. • Many times delayed interflow is also included under this category. • In the annual hydrograph of a perennial stream the base flow is easily recognized as the slowly decreasing flow of the stream in rainless periods.
- 7. HYDROGRAPH A hydrograph is a graphical representation that shows the variation in discharge (flow rate) of a river, stream, or other water body over time. It is an essential tool in hydrology and water resource management, used to analyze how water flow responds to various factors such as precipitation, snowmelt, and upstream water releases. Components of a Hydrograph • Discharge Axis (Y-axis):Typically represents the discharge or flow rate, measured in cubic meters per second (m³/s), cubic feet per second (cfs), or liters per second (L/s). • Time Axis (X-axis):Represents time, which can range from minutes to days, weeks, or even years, depending on the specific study or application. • Rising Limb: The part of the hydrograph where discharge is increasing. This usually occurs after a precipitation event or during snowmelt. • Peak Discharge: The maximum discharge reached during a particular event. It indicates the highest flow rate at the given time. • Falling Limb (Recession Limb):The part of the hydrograph where discharge is decreasing after the peak. This represents the return to normal or base flow conditions. • Base Flow: The portion of the flow that is sustained by groundwater seeping into the river or stream, not directly influenced by immediate surface runoff.
- 8. Types of Hydrographs 1. Single-Peak Hydrograph: • Characterized by one distinct peak, typically resulting from a single precipitation event.
- 10. Example
- 11. 2. Multi-Peak Hydrograph: • Shows multiple peaks, which can result from a series of precipitation events or varying sources of runoff.
- 12. Uses of Hydrographs 1. Flood Analysis and Forecasting: • Hydrographs are used to predict the timing and magnitude of floods, helping in the design of flood control measures and early warning systems. 2. Water Resource Management: • Helps in planning and managing water resources, including reservoir operations, irrigation scheduling, and urban drainage systems. 3. Environmental Studies: • Used to study the ecological impacts of varying flow conditions on riverine habitats and aquatic life. 4. Hydrological Modeling: • Provides data for calibrating and validating hydrological models that simulate river and watershed behavior.
- 13. DETERMINATION OF PEAKFLOW • Determining the peak flow in a river during a rainstorm event involves several steps and the use of various hydrological and hydraulic methods. • Here's a common approach: 1. Data Collection • Rainfall Data: Collect data from rain gauges in the watershed area. This data includes rainfall intensity, duration, and distribution. • Watershed Characteristics: Gather information about the watershed, including area, land use, soil type, slope, and vegetation cover. • Streamflow Data: Obtain historical streamflow records if available. These can help in understanding the typical response of the river to rainfall events.
- 16. 3. Hydrograph Development Unit Hydrograph: • Develop a unit hydrograph for the watershed. • This is a time-distributed runoff resulting from a unit input of rainfall excess (e.g., 1 inch or 1 cm) over a specified duration. Convolution: • Convolve the effective rainfall hyetograph with the unit hydrograph to generate the storm event hydrograph. (Read and make notes on convolution. Include appropriate illustrations) 4. Peak Flow Determination Direct Calculation: • For simple methods like the Rational Method, the peak flow can be directly calculated. Hydrograph Analysis: • For methods involving hydrographs, identify the peak of the generated hydrograph to determine the peak flow.
- 17. 5. Verification and Adjustment Historical Data Comparison: • Compare the results with historical peak flow data (if available) to ensure the estimates are reasonable. Model Calibration: • If detailed models are used, calibrate them using historical rainfall and runoff events to improve accuracy. Examples of helpful Tools and Software • HEC-HMS (Hydrologic Engineering Center's Hydrologic Modeling System): For detailed hydrologic modeling. • SWMM (Storm Water Management Model): For urban drainage systems. • ArcGIS: For spatial data analysis and watershed delineation.
- 18. Example Consider a small watershed with the following characteristics: • Runoff coefficient (C) = 0.7 • Rainfall intensity (I) = 2 inches per hour • Watershed area (A) = 1 square mile (640 acres) Determine the peak discharge using the rational method. Solution Using the Rational Method:
- 19. FACTORS AFFECTING RUNOFF 1. Climate Factors • Precipitation Intensity and Duration: Heavy, short-duration storms generate more runoff than light, long-duration rainfall because the ground's infiltration capacity is exceeded more quickly. • Precipitation Type: Rainfall typically produces more immediate runoff than snowfall. However, snowmelt can lead to significant runoff when temperatures rise. • Temperature: Affects the rate of evaporation and transpiration, which can reduce the amount of water available for runoff. 2. Soil Characteristics • Infiltration Capacity: The rate at which soil can absorb water. Soils with high infiltration rates (e.g., sandy soils) produce less runoff compared to soils with low infiltration rates (e.g., clay soils). • Soil Moisture Content: Saturated soils cannot absorb additional water, leading to increased runoff. • Soil Structure and Texture: Fine-textured soils (e.g., clay) tend to have lower infiltration rates and higher runoff compared to coarse-textured soils (e.g., sand).
- 20. 3. Land Use and Land Cover • Vegetation: Dense vegetation increases infiltration and reduces runoff by intercepting rainfall and promoting water uptake by plant roots. • Urbanization: Impervious surfaces (e.g., roads, buildings) prevent infiltration, leading to increased runoff. • Agricultural Practices: Tillage, crop type, and soil conservation practices can significantly influence runoff. 4. Topography • Slope: Steeper slopes lead to faster runoff and less infiltration, whereas gentle slopes allow more water to infiltrate into the soil. • Slope Length and Shape: Long, continuous slopes can increase runoff, while terracing or other landforms that break the slope can reduce it.
- 21. 5. Hydrological and Geological Factors • Watershed Size and Shape: Larger watersheds typically generate more runoff. The shape of the watershed affects how quickly water reaches the main channel. • Drainage Density: The length of streams per unit area of the watershed. Higher drainage density usually leads to quicker runoff. • Geology: The presence of permeable rock formations can enhance infiltration and reduce runoff, while impermeable formations increase runoff. 6. Human Activities • Deforestation: Reduces the interception and infiltration capacity of the land, increasing runoff. • Construction and Development: Creates impervious surfaces and often includes drainage systems that quickly channel water to streams, bypassing natural infiltration. • Water Management Practices: Dams, levees, and stormwater management systems can alter natural runoff patterns. 7. Antecedent Moisture Conditions • Previous Rainfall Events: Recent rainfall can saturate the soil, leading to higher runoff from subsequent storms. • Seasonal Variations: Different seasons can affect soil moisture levels, vegetation cover, and temperature, all of which influence runoff.
- 22. Example Scenario Consider a small watershed with varying land use: • Urban Area: High runoff due to impervious surfaces. • Forested Area: Low runoff due to high infiltration and interception by trees. • Agricultural Land: Moderate runoff depending on soil type, crop cover, and farming practices. • A rainstorm occurs, and the urban area quickly generates runoff, while the forested area absorbs much of the water, leading to delayed and reduced runoff. • The agricultural land produces moderate runoff, which may increase if the soil is already saturated or if crops are not actively growing.
- 23. • Consequently, hydrographs can take different shapes dependent upon the characteristics of the drainage basin. • The various flows and stores of the drainage basin are affected by these characteristics, and these in turn will affect the shape of the hydrograph and the volume of water in a river. • Some of these are as shown below.
- 28. Example 2 Consider a catchment area of 500 acres with the following characteristics, and determine its peak flow. Land Use: 50% residential area (lawn), 25% forest, 25% agricultural land (row crops). Soil Type: Predominantly soil group B (moderate infiltration rate). Rainfall Event: 4 inches of rainfall over 24 hours. Initial Abstraction (Ia): 0.2S (where S is the potential maximum retention). Solution : • Determine Curve Numbers (CN) for each land use and soil type. • Calculate the weighted Curve Number (CN) for the catchment. • Compute the runoff using the SCS Curve Number Method. 1. Determine Curve Numbers (CN): Using standard CN tables, we find the following values for soil group B: Residential area (lawn): CN = 55 Forest: CN = 55 Agricultural land (row crops): CN = 78