JNTU B.Tech II HYDROLOGY Previous Question Paper (March - 2022 - R13)

The Depth-Area Relationship describes how the average depth of precipitation over a storm area decreases as the size of the area increases. For a given storm duration, the average rainfall depth is highest at the storm center and tapers off towards the periphery. This relationship is typically expressed mathematically as:

where P is the average depth over area A, P₀ is the maximum depth at the storm center, and k and n are coefficients dependent on the storm characteristics and region. This relationship is critical for hydrologic design, as it allows engineers to estimate the total volume of runoff generated over a specific catchment area based on point rainfall data.

The Depth-Area-Duration (DAD) analysis is a systematic method used to determine the maximum volume of precipitation that can occur over a range of areas and durations for a specific region. The resulting DAD curves represent the upper limit of rainfall intensity for various storm sizes.

• Data Preparation: Rainfall data from a network of gauges is analyzed to identify major storm events.
• Isohyetal Mapping: For a selected storm, isohyets (lines of equal rainfall) are drawn to visualize the spatial distribution of precipitation.
• Area Calculation: The area enclosed by each isohyet is measured using a planimeter or GIS tools.
• Average Depth Calculation: The average rainfall depth for each incremental area is calculated by integrating the volumes between isohyets.
• Curve Plotting: The calculated average depths are plotted against the corresponding areas for different storm durations (e.g., 6, 12, 24 hours).

These curves are essential for estimating the Probable Maximum Precipitation (PMP), which is a fundamental parameter in the design of large hydraulic structures like dams and spillways, ensuring they can safely handle extreme flood events.