Lecture 7 NR

Hydrograph Points in TR-55

• Focus on significant points in hydrology from TR-55.

Time of Concentration and Travel Time

• Travel Time:

  • Defined as the time required for water to travel from one location to another.

• Time of Concentration:

  • More specific term.

  • Time it takes for the hydrologically furthest drop in a watershed to reach the outlet.

  • Corrected definition: it is the time for the furthest point in a watershed to reach the outlet.

Calculating Time of Concentration

• The time of concentration is computed by calculating various travel times across different segments of the watershed and summing them up.

• Conceptual Understanding:

  • Time of concentration = total travel time

  • Travel time = individual segments of time summed up.

• General Guidance on Travel Time:

  • Often presented as the ratio of flow length (L) to flow velocity (V).

  • Equation:
    $Travel Time = \frac{L}{V}$

• Units:

  • Velocity is typically measured in meters per second or feet per second.

  • Travel time often expressed in hours; hence the need to convert seconds to hours by multiplying by 3,600 seconds/hour.

Factors Influencing Time of Concentration

• Factors affecting time of concentration include:

  • Watershed Size:

  • Larger watersheds may increase time due to greater flow lengths.

  • Watershed Shape:

  • Different shapes can affect how quickly water flows toward an outlet.

  • Surface Roughness:

  • The higher the friction (more vegetation, rough surfaces), the slower the water will flow.

  • Channel Slope:

  • Steeper slopes result in faster water movement.

  • Flow Patterns:

  • The existing flow dynamics can influence travel and concentration times.

Importance of Standards in Calculation

• Calculation methodologies are influenced by engineering standards in the U.S., which help standardize practices across the profession.

• The presence of data influences the depth and detail of time of concentration calculations.

• Methods of estimation can vary based on standards or the available data.

Methods for Estimating Time of Concentration

1. Shortcut Method (Kerpich Method):

   • Computes a total time of concentration without segmenting into individual travel times.

   • Originated from data collected from seven rural watersheds, primarily in Tennessee, with slopes of 3% to 10%.

   • Equation:

     • $T_{c} = 0.0078 \cdot L^{0.77} \cdot \frac{L}{H^{0.385}}$

   • Units: time of concentration in minutes.

   • Variables:

     • L = hydrologic flow length (longest distance from the outlet),

     • H = elevation difference between the outlet and the furthest hydrologically distant point.

   • Limited applicability, particularly in urban-settings due to its assumptions.

2. Detailed Method (TR-55 Travel Time Method):

   • Involves calculating travel times for three distinct flow categories:

   1. Sheet Flow:

      • Initial flow where water spreads over land (maximum length for sheet flow is typically 300 feet).

      • Influences on travel time: surface roughness and slope.

      • Uses Manning's kinematic solution: $T = \frac{L}{N \cdot P \cdot S}$ where:

        • T = travel time,

        • N = Manning's roughness coefficient,

        • P = two-year, twenty-four hour rainfall,

        • S = slope (as derived from topographic maps).

   2. Shallow Concentrated Flow:

      • Transition phase between sheet flow and channel flow.

      • Calculated easily with available tables and has a straightforward equation.

   3. Open Channel Flow:

      • Water that has been funneled into a defined channel (e.g., stream or river).

Summary of Calculated Travel Time Methods

• For TR-55 overview, three flows yield three calculations that contribute to overall time of concentration.

• Next Steps:

  • Explore travel time method in detail, visualizing how each phase contributes to the total travel time of water from the furthest watershed point to the outlet.