Hydrograph Concepts Notes

Detailed Notes on Hydrograph Concepts with Examples

1. Hydrograph Definition

A hydrograph is a graphical representation of stream discharge (flow rate) over time at a specific point in a river or stream. It typically illustrates how a river's flow changes in response to a precipitation event, showing the relationship between rainfall and runoff.

Discharge Units:

Cubic meters per second (m³/s) in the metric system

Cubic feet per second (cfs) in the imperial system

Example:

After a heavy rainfall, a hydrograph for a river might show a sharp rise in discharge, peaking after a few hours, and then gradually falling as the water drains from the watershed.

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2. Hyetograph Definition

A hyetograph is a bar graph representing rainfall intensity over time, commonly used in hydrological analysis to understand how rainfall contributes to runoff.

Graph Representation:

X-axis: Time (in hours or minutes)

Y-axis: Rainfall intensity (in mm/h or inches/h)

Example:

For a 6-hour rainfall event, a hyetograph may show higher intensity in the first hour (e.g., 10 mm/h) followed by a steady decline to 2 mm/h. This pattern can influence how the hydrograph responds, typically leading to a quick rise in discharge.

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3. Unit Hydrograph (UH) Definition

A Unit Hydrograph (UH) represents the direct runoff hydrograph (DRH) resulting from one unit of effective rainfall (e.g., 1 cm or 1 inch) uniformly distributed over a watershed over a specific time duration. It is a fundamental tool in hydrology used for predicting river discharge.

Key Assumptions:

Linearity: The response of the watershed is directly proportional to the input rainfall.

Time Invariance: The hydrograph response for a given rainfall input remains consistent over time.

Example:

A 1-hour unit hydrograph for a watershed may show a peak flow of 50 m³/s after 3 hours, representing the response to 1 cm of effective rainfall. If 2 cm of rainfall occurs, the hydrograph values can be scaled proportionally to 100 m³/s.

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4. Direct Runoff Hydrograph (DRH) Definition

A Direct Runoff Hydrograph (DRH) represents the streamflow generated from direct runoff, excluding baseflow (the groundwater contribution). It is used to estimate the volume of runoff caused by a specific rainfall event.

Components of Streamflow:

Direct Runoff: Water from rainfall that flows quickly to streams.

Baseflow: Groundwater flow that sustains river discharge during dry periods.

Example:

After a storm, the streamflow rises sharply, forming the DRH portion of the hydrograph. Once the rain stops, baseflow becomes dominant, causing a gradual decline in streamflow.

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5. DRH Determination from a Flood Hydrograph

A flood hydrograph consists of both direct runoff and baseflow. To isolate the DRH, baseflow separation is necessary.

Formula:

Baseflow Separation Methods:

1. Straight-line Method: Draw a straight line from the beginning to the end of the stormflow to estimate baseflow.

2. Fixed Base Length Method: Assumes baseflow ends after a set time, typically related to watershed characteristics.

3. Variable Slope Method: Draws a curve to represent the natural decay of baseflow.

Example:

If a flood hydrograph shows a peak flow of 300 m³/s, and baseflow is estimated to be 50 m³/s, the direct runoff component would be:

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6. UH Computation from DRH

A Unit Hydrograph can be derived from the Direct Runoff Hydrograph using the following steps:

1. Calculate the Effective Rainfall Depth:

The total volume of runoff can be determined from the area under the DRH curve.

Convert the volume into a uniform rainfall depth using the watershed area.

2. Normalize the Hydrograph:

Divide each ordinate (discharge value) of the DRH by the effective rainfall depth to obtain the unit hydrograph.

Example:

Watershed Area: 100 km²

Total Runoff Volume: 10 million m³

Effective Rainfall Depth = 

Normalize the DRH ordinates using this rainfall depth.

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7. UH Duration Transformation

Duration transformation allows generating a Unit Hydrograph for a longer duration by combining multiple unit hydrographs of shorter duration.

Concept:

Two 1-hour unit hydrographs can be added with a lag of 1 hour to obtain a 2-hour Unit Hydrograph.

Similarly, adding ‘T’ hour UHs lagged by ‘T’ hours results in a 2T-hour hydrograph representing two units of runoff.

Normalization:

To convert the 2T-hour hydrograph into a unit hydrograph, divide each ordinate by 2.

Example:

Suppose two 1-hour unit hydrographs have peak values of 30 m³/s. After applying the lag and summing, the peak reaches 60 m³/s.

To create the 2-hour unit hydro

graph, divide the peak by 2:

This transformation is particularly useful when modeling rainfall events of different durations using limited unit hydrographs.