Engineering Hydrology Class 3 Topics and Goals: I.Develop s (estimate precipitation) II.Develop simple constant intensity design storm III.Develop SCS design storm Ocean
s Why do we want to derive the? Convolve the Design with the unit hydrograph (transfer function) to predict a runoff hydrograph (output function). Design storms are convolved with a Unit Hydrograph (think of it as a transfer function ) to make real-time predictions of ultimate discharge. Such predictions are essential for effective engineering design for detention basins, drainage systems, etc.
s Peak Discharge Predicting peak discharge is of particular interest!!
s The simplest design storm is a constant intensity design storm: The time of concentration, or t c, is generally defined as the time required for a drop of water to travel from the most hydrologically remote point in the subcatchment to the point of collection. We ll study this later (week 8)!
s The simplest design storm is a constant intensity design storm: The time of concentration, or t c, is generally defined as the time required for a drop of water to travel from the most hydrologically remote point in the subcatchment to the point of collection. We ll study this later (week 8)!
s The simplest design storm is a constant intensity design storm, composed as follows: (1)Read the IDF to determine rain intensity (2) Assume that the time of concentration of the watershed is the duration of peak intensity of the storm. (3)Plot the resulting hyetograph.
s : Check out NOAA Atlas 14 (IDF Data Server) Intensity Duration - Frequency
s
s The average intensity for 24 hr and 5-yr return period is ~0.3 in/hr
s : Example Example: For Tallahassee watershed, (1) The design standard specifies a 10-year return period for design, (2) The watershed time of concentration is 22 minutes. From the Bedient et al. text, Chapter 6 (Tallahassee IDF):
s : Example The average intensity for a 22 minute duration and 10-yr return period is ~5.5 in/hr
s : Example Example: For Tallahassee watershed, (1) The design standard specifies a 10-year return period for design, (2) The watershed time of concentration is 22 minutes. From Figure 6-5 in your reading packet (Tallahassee IDF): The average intensity for a 22 minute duration and 10-yr return period is ~5.5 in/hr; which yields a storm volume of 2.01 inches
s : Example Example: For Tallahassee watershed, (1) The design standard specifies a 10-year return period for design, (2) The watershed time of concentration is 22 minutes. The average intensity for a 22 minute duration and 10-yr return period is ~5.5 in/hr; which yields a storm volume of 2.01 inches [5.5 in/hr X (22 / 60)]
s SCS 4 dimensionless rainfall distributions (as determined by the SCS); valid for areas < 400 mi 2, durations up to 24 hr, and frequencies of 1 yr to 100 yr
s SCS 4 dimensionless rainfall distributions (as determined by the SCS); valid for areas < 400 mi 2, durations up to 24 hr, and frequencies of 1 yr to 100 yr Fraction of 24-hr Rainfall 1.0 0.8 0.5 0.3 0 IA I III II 0 4.0 8.0 12.0 16.0 20.0 24.0 Time (hr)
s SCS 4 dimensionless rainfall distributions (as determined by the SCS); valid for areas < 400 mi 2, durations up to 24 hr, and frequencies of 1 yr to 100 yr Incremental Fraction Rainfall 0.40 0.30 0.20 0.10 0 II I III IA 0 6 12 18 24 Time (hr)
s : In Sum Intensity Duration - Frequency Constant-Intensity Check out NOAA Atlas 14 (IDF Data Server) http://dipper.nws.noaa.gov/hdsc/pfds/