Hydrograph Method for Runoff Estimation
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What do isochrones represent in the context of runoff estimation?

  • Different catchment areas based on size
  • Historical rainfall data for a region
  • Equal travel times for rainfall runoff to the outlet (correct)
  • Variations in rainfall distribution across a catchment

    • In the time-area method, how is the catchment area organized?

  • Based on soil type and vegetation density
  • Segmented by elevation and slope characteristics
  • Arranged chronologically with respect to annual rainfall
  • Divided into sub-catchments according to runoff travel time (correct)

    • Which term derives from Greek, meaning 'equal time'?

  • Sub-catchment
  • Isochrone (correct)
  • Runoff hydrograph
  • Discharge estimation

    • What is the main purpose of using the Time-Area method in discharge estimation?

    <p>To estimate discharge based on catchment characteristics</p> Signup and view all the answers

    What does Figure 2.6 illustrate in the context of hydrology?

    <p>The Time-Area Hydrograph Method</p> Signup and view all the answers

    What is the initial loss for pervious catchment conditions with sandy soil?

    <p>10 mm</p> Signup and view all the answers

    Which of the following factors affects continuous loss in pervious conditions?

    <p>Soil type</p> Signup and view all the answers

    In a mixed urban area with impervious surfaces, what would be the continuous loss value?

    <p>0 mm/hr</p> Signup and view all the answers

    When using the time-area hydrograph method for runoff estimation, what does ARI stand for?

    <p>Annual Return Interval</p> Signup and view all the answers

    For a pervious catchment with clay soil, what is the expected continuous loss rate?

    <p>0.5 - 3 mm/hr</p> Signup and view all the answers

    What is the average recurrence interval (ARI) for rainfall design in this scenario?

    <p>20 years</p> Signup and view all the answers

    What is the total design rainfall for a duration of 30 minutes?

    <p>70.54 mm</p> Signup and view all the answers

    Which time period shows the highest rainfall amount in the temporal rainfall pattern?

    <p>10-15 min</p> Signup and view all the answers

    What is the rainfall excess between 10 and 15 minutes?

    <p>27.92 mm</p> Signup and view all the answers

    What is the continuous loss for the pervious area during the rainfall event?

    <p>9.50 mm/hr</p> Signup and view all the answers

    What area (m2) does the isochrone time period 5-10 minutes represent?

    <p>79,304 m2</p> Signup and view all the answers

    At what time was the inflow (I) equal to 4.690 m³/s?

    <p>0.08 hr</p> Signup and view all the answers

    What is the initial loss for the impervious area specified in the calculation?

    <p>1.5 mm</p> Signup and view all the answers

    During the time period 0-5 minutes, what is the estimated runoff excess?

    <p>1.94 mm</p> Signup and view all the answers

    What is the correct formula to calculate the outflow O2 at 0.04 hr?

    <p>O2 = 1.173 m³/s</p> Signup and view all the answers

    How does the storage volume change over the time slots provided?

    <p>Increases initially, then decreases</p> Signup and view all the answers

    In the hydrograph at the outlet, what is the maximum discharge recorded?

    <p>37.3495 m3/s</p> Signup and view all the answers

    What pattern is observed in outflow O1 at different time intervals?

    <p>Fluctuates with inflow rate</p> Signup and view all the answers

    What is the rainfall excess for the period 20-25 minutes?

    <p>7.21 mm</p> Signup and view all the answers

    What reference provides the normalized-rainfall temporal pattern?

    <p>Appendix 2.C5</p> Signup and view all the answers

    At time 0.33 hr, what is the value for inflow (I)?

    <p>5.440 m³/s</p> Signup and view all the answers

    Which time segment has an outflow O2 of 0.078 m³/s?

    <p>0.04 hr</p> Signup and view all the answers

    What is the cumulative rainfall excess by the end of the 30-minute period?

    <p>70.54 mm</p> Signup and view all the answers

    What does S2/ ∆t represent in the outflow calculations?

    <p>Change in storage volume over time</p> Signup and view all the answers

    What is the drainage area for the isochrone time period 25-30 minutes?

    <p>45,306 m2</p> Signup and view all the answers

    What is the initial loss for a pervious area with loam soil?

    <p>10 mm</p> Signup and view all the answers

    What happens to outflow O1 from 0.17 hr to 0.21 hr?

    <p>Increases significantly</p> Signup and view all the answers

    What is the total inflow at 0.54 hr?

    <p>0.000 m³/s</p> Signup and view all the answers

    How is the outflow O2 affected by incremental changes in inflow?

    <p>It increases with increasing inflow</p> Signup and view all the answers

    Which outflow value is calculated at a time of 0.42 hr?

    <p>0.375 m³/s</p> Signup and view all the answers

    What concept does 'Linear Interpolation' refer to in the context of the example?

    <p>Estimating values between known data points</p> Signup and view all the answers

    Which statement best describes the purpose of measuring outflow O1?

    <p>To assess the efficiency of the system</p> Signup and view all the answers

    What is the significance of the time intervals used in the calculations?

    <p>They impact the accuracy of calculations</p> Signup and view all the answers

    What is the significance of the values located in row 2 of the matrix?

    <p>They are related to the average recurrence interval (ARI).</p> Signup and view all the answers

    Which of the following rows is likely to show the peak flow values?

    <p>Row 10</p> Signup and view all the answers

    From the data provided, which rainfall design duration can be inferred?

    <p>An average of 24 hours</p> Signup and view all the answers

    In the context of the matrix, what does the legend primarily describe?

    <p>The identification of different types of drainage systems.</p> Signup and view all the answers

    What do the numbers along the axis of the matrix likely represent?

    <p>Length and area in meters and square meters.</p> Signup and view all the answers

    Which row is most likely to outline a design storm event?

    <p>Row 8</p> Signup and view all the answers

    The term 'Drain' in the legend most directly relates to which of the following?

    <p>Water management and extraction.</p> Signup and view all the answers

    What is the purpose of the Time-Area Hydrograph Method referenced?

    <p>To analyze peak flow and drainage time.</p> Signup and view all the answers

    How are the values grouped in the matrix relevant to flood management?

    <p>They provide data for flood risk calculations.</p> Signup and view all the answers

    Which of the following values is most likely to correspond to low rainfall intensity?

    <p>$18.4 mm$</p> Signup and view all the answers

    Which value from the identified rows likely relates to peak runoff volume?

    <p>$31.9 mm$</p> Signup and view all the answers

    What parameter does the value $23.6 mm$ in the matrix likely represent?

    <p>The rainfall intensity during a storm.</p> Signup and view all the answers

    What do the numbers $71$ in the rows potentially signify?

    <p>A designated area for measurement.</p> Signup and view all the answers

    Which parameter is essential for determining the contour drainage line?

    <p>Slope of the terrain.</p> Signup and view all the answers

    Study Notes

    Time-Area Hydrograph Method

    • Used to estimate runoff hydrographs
    • Divides catchment area into sub-catchments based on runoff travel time to the outlet
    • Isochrones are lines of equal travel time for rainfall runoff to the outlet

    Isochrones

    • Named from the Greek words "iso" (equal) and "chrone" (time)
    • Represent equal travel time for rainfall runoff to reach the outlet

    Rainfall Losses

    • Can be assumed constant or decaying
    • Constant loss is simpler to calculate, while decaying loss is more practical
    • Recommended loss values for rainfall excess estimation vary based on catchment condition (impervious or pervious):
      • Impervious:
        • Initial Loss: 1.5 mm
        • Continuous Loss: 0 mm/hr
      • Pervious:
        • Initial Loss: 10 mm
        • Continuous Loss:
          • Sandy Soil: 10 - 25 mm/hr
          • Loam Soil: 3 - 10 mm/hr
          • Clay Soil: 0.5 - 3 mm/hr

    Worked Example

    • Calculates a 20-year ARI (Annual Recurrence Interval) runoff hydrograph for a 97 ha mixed urban area in Wangsa Maju, Kuala Lumpur

    Worked Example

    • The worked example demonstrates how to calculate a design rainfall for a drainage system
    • Location of the worked example is Ibu Pejabat JPS, Kuala Lumpur
    • Average recurrence interval (ARI) is 20 years
    • Duration of the rainfall is 30 minutes
    • Design rainfall intensity for the location is 141.1 mm/hr
    • Total design rainfall for 30 minutes is 70.54 mm

    Rainfall Temporal Pattern

    • Rainfall temporal pattern for 30 minutes duration is taken from Appendix 2.C5
    • Rainfall temporal patterns are normalized to a total rainfall of 100%
    • The total rainfall excess for the worked example is 70.54 mm

    Rainfall Excess (Runoff)

    • Rainfall excess is calculated by subtracting losses from rainfall
    • Losses include initial losses and continuous losses
    • Infiltration losses are categorized by catchment condition and soil type
    • Pervious areas are 40% of the total catchment area
    • Impervious areas are 60% of the total catchment area

    Hydrograph

    • The hydrograph is calculated using the time-area method
    • The hydrograph is established by superposition of the rainfall excess over the various isochrones
    • Total discharge is calculated by multiplying the rainfall excess with the area between each isochrone

    Hydrologic Pond Routing

    • Hydrologic pond routing is used to modify the flow as it travels through the drainage system
    • It accounts for changes in the hydrograph due to the flow system

    Runoff Routing

    • The process of calculating how quickly water flows through a system.
    • The example calculates the amount of water flowing into a hypothetical lake by using inflows and outflows, including the time component.
    • The lake has two inflows (I1 and I2), and two outflows (O1 and O2).
    • The storage volume and outflow are calculated based on the stage which is the water level.
    • The average inflow over the time step is calculated as the mean of the previous and current time step.
    • Inflow, Outflow and Storage Volume are calculated in m³/s.
    • Time is calculated in hours.
    • Stage is calculated in meters.
    • The outflow is calculated using the stage (water level) where there is a direct relationship between stage and outflow.
    • The amount of water stored in the lake is determined by the stage.
    • The relationship between stage and outflow is linear.
    • For this example, the relationship between storage volume and discharge is defined as S = 0.54 * O^2.

    Calculation Examples

    • The example calculates how much water is flowing into the lake for each time step.
    • The calculation includes inflow (I) and outflow (O), which represent the amount of water entering or leaving the lake respectively.
    • The outflows are calculated using a combination of the storage volume and the inflow.
    • The storage volume is calculated by combining the stage, which is the height of the water, with a constant value for the lake.
    • The outflow is calculated by subtracting the storage volume from the inflow.

    ### Key Figures

    • I1, I2: Inflow rates [m³/s] at the first and second point of entry
    • O1, O2: Outflow rates [m³/s] at the first and second point of exit
    • S1, S2: Storage volumes [m³] at the first and second time step
    • ∆t: Time difference between the current and the previous time step

    Tables

    • Table 1: The water flowing into the lake is given in m³/s.
    • Table 2: The storage volume (S), discharge (O), and water flowing through the lake is given in m³/s.
    • The first row of data is for time = 0.
    • The next row of data is for time = 0.04.
    • The last row of data is for time = 0.58.

    Graph

    • There is a graph comparing the inflow and outflow over time, with the discharge on the y-axis and time on the x-axis.
    • The inflow appears to have a single peak at t= 0.21 hr.
    • The outflow appears to peak at t= 0.38 hr.
    • The outflow appears to be stable over the remaining data at 35 m³/s.

    Key Points

    • The example has been completed after the time step when the water level is constant, resulting in the inflow being zero after a certain point.
    • This example demonstrates how to use the concept of inflow and outflow to calculate the amount of water flowing through a lake.
    • The amount of water that flows out of the lake is calculated by subtracting the amount of water stored in the lake from the amount of water that flows into the lake.
    • The outflow is controlled by stage.
    • This technique could be applied to a wide variety of situations where calculating the amount of water flowing through a system is relevant.

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    Description

    This quiz explores the Time-Area Hydrograph Method, focusing on how to estimate runoff hydrographs by dividing catchment areas into sub-catchments based on runoff travel time. It covers the concept of isochrones, rainfall losses, and includes a worked example for calculation. Test your understanding of these critical hydrological concepts!

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