1. What is Time of Concentration?

The time of concentration (t_c) is the time required for water to flow from the most remote point in a watershed to the outlet. It's a fundamental parameter in hydrologic analysis for determining peak discharge rates.

2. How Does the Calculator Work?

The calculator uses the Kirpich formula:

Where:

• \( t_c \) — Time of concentration (minutes)
• \( L \) — Length of flow path (feet)
• \( S \) — Slope (dimensionless, ft/ft)

Explanation: The formula empirically relates watershed characteristics to the time it takes for runoff to reach the outlet.

3. Importance of Time of Concentration

Details: Time of concentration is crucial for designing drainage systems, calculating peak flows, and developing stormwater management plans. It affects the intensity-duration-frequency curves used in hydrologic design.

4. Using the Calculator

Tips: Enter the length of the longest flow path in feet and the average slope (rise/run) as a dimensionless value. Both values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is the range of applicability for Kirpich formula?
A: Best suited for small agricultural watersheds with areas less than 200 acres and slopes between 3-10%.

Q2: How does land use affect time of concentration?
A: Urbanized areas typically have shorter times due to impervious surfaces, while forested areas have longer times due to higher roughness.

Q3: What are typical values for time of concentration?
A: Small watersheds may range from 5-30 minutes, while larger watersheds can have times of several hours.

Q4: Are there alternative formulas to Kirpich?
A: Yes, other methods include the NRCS Velocity Method, FAA formula, and Kerby formula, each with different applications.

Q5: How accurate is the Kirpich formula?
A: It provides reasonable estimates for small watersheds but may need adjustment factors for different surface conditions.