Runoff and Hydrographs

Why it matters

Runoff and hydrographs are essential in civil engineering for designing drainage systems, flood control, and water resource management. Understanding these concepts helps engineers predict water flow patterns and manage water resources effectively, especially in regions prone to heavy rainfall and flooding.

Key ideas

• Runoff: The portion of precipitation that flows over the land surface towards streams and rivers. It is influenced by factors such as soil type, land use, and topography.
• Hydrograph: A graphical representation of the flow rate of water over time at a specific point in a river or stream. It shows how a watershed responds to precipitation.
• Components of a Hydrograph:
  • Rising limb: Indicates increasing discharge due to rainfall.
  • Peak discharge: The maximum flow rate.
  • Recession limb: Decreasing discharge as the watershed returns to base flow.
• Factors affecting runoff and hydrographs:
  • Rainfall intensity and duration
  • Soil saturation and permeability
  • Land cover and vegetation
  • Topography and slope

Formulas

• Q = C·i·A
  • Q: Estimated peak runoff discharge (m³/s), not runoff volume
  • C: Runoff coefficient (dimensionless)
  • i: Rainfall intensity (m/s)
  • A: Catchment area (m²)

The rational method is a peak-flow method for suitable small catchments, assuming approximately uniform rainfall intensity over the contributing area for at least the time of concentration. Use intensity appropriate to the design frequency and duration. It does not supply a full hydrograph or a universal time-to-peak equation.

Worked example

Given:

• Rainfall intensity, i = 72 mm/h = 0.072/3600 = 0.000020 m/s
• Catchment area, A = 5000 m²
• Runoff coefficient, C = 0.75

Find: Runoff, Q

1. Use the formula Q = C·i·A
2. Substitute the given values: Q = 0.75 × 0.000020 × 5000
3. Calculate: Q = 0.075 m³/s

Final Answer: 0.075 m³/s

Common mistakes

• Confusing the units of rainfall intensity and area.
• Incorrectly estimating the runoff coefficient, which varies with land use and surface type.
• Ignoring the impact of soil saturation on runoff.

For GATE CE

Questions often involve calculating runoff using given rainfall data and catchment characteristics. Practice problems on interpreting hydrographs and understanding the impact of different factors on runoff.

Quick check

1. What is a hydrograph used for?
2. Name two factors that affect runoff.
3. What does the rising limb of a hydrograph indicate?

Answers: 1. To represent flow rate over time. 2. Soil type, rainfall intensity. 3. Increasing discharge due to rainfall.

Reference

FHWA: Introduction to Highway Hydraulics.