Hyetographs - Hydrology - Lecture Slides, Slides of Water and Wastewater Engineering

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Hyetographs & Hydrographs 1 Docsity.com Objectives • Understand what a hyetograph represents • Understand what a hydrograph represents • Understand what a unit hydrograph represents • Know how unit hydrographs can be used to develop various hydrographs 2 Docsity.com 5 Hyetograph-Depth 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1 2 3 4 5 6 Time Interval (15-minutes) De pt h (in ch es ) Runoff Abstraction Docsity.com 6 Hyetograph-Intensity 0 1 2 3 4 5 6 1 2 3 4 5 6 Time Interval (15-minutes) In te ns ity (i nc he s pe r h r) Docsity.com Rainfall Intensity • Usually varies over storm duration Hyetograph-Intensity 0 1 2 3 4 5 6 1 2 3 4 5 6 Time Interval (15-minutes) In te ns ity (i nc he s pe r h r) 7 Docsity.com Mass Hyetograph • Another way to represent a hyetograph is a mass curve (accumulated time versus accumulated rainfall) – Rainfall ratio (inches/total inches) – Time ratio (time/total time) 10 Docsity.com 11 Mass Hyetograph 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Time Ratio R ai n fa ll R at io Docsity.com Deriving the Mass Hyetograph Time Time Increment Sum time Precip. Intensity Sum Precip. Time Ratio Rainfall Ratio (minutes) (minutes) (in) (in/hr) 0 0 1:00-1:15 15 15 0.4 1.60 0.4 0.17 0.10 1:15-1:30 15 30 0.7 2.80 1.1 0.33 0.26 1:30-1:45 15 45 1.3 5.20 2.4 0.50 0.57 1:45-2:00 15 60 1 4.00 3.4 0.67 0.81 2:00-2:15 15 75 0.5 2.00 3.9 0.83 0.93 2:15-2:30 15 90 0.3 1.20 4.2 1.00 1.00 90 4.2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.2 0.4 0.6 0.8 1 Ra in fa ll Ra tio Time Ratio Mass Hyetograph 12 Docsity.com Hydrograph • Graphical representation of time (hours) versus discharge (cfs or cms) at a particular point on a stream or channel which drains the watershed area 15 Docsity.com Hydrograph • Shape represents basin characteristics – Area – Shape – Stream pattern – Land/channel properties/slopes – Infiltration capacity 16 Docsity.com Hydrograph • Represents rainfall that runs off into the stream (rainfall-excess or direct runoff) • Base flow (from groundwater) is not part of the hydrograph 17 Docsity.com Hydrograph-Definitions • Lag Time – Time interval from the center of mass of the rainfall-excess to the peak of the resultant hydrograph • Time to Peak – Time interval from the start of the rainfall-excess to the peak of the resultant hydrograph • Rainfall Duration – Time interval from the start to the end of rainfall-excess • Time of Concentration – Time interval from the end of the rainfall-excess to the point of inflection on the hydrograph recession curve or – Time required for runoff to travel from the hydraulically most distant point on the watershed to the point of interest 20 Docsity.com Hydrograph • Associated with the time duration and frequency of the rainfall-excess producing it • If the duration D of the rainfall-excess is 30 minutes and the frequency is 2 years, the hydrograph is referred to as a 30-minute, 2- year frequency hydrograph 21 Docsity.com Hydrograph • Area under the hydrograph represents a volume of water (total volume of rainfall that fell on the basin and appeared as runoff) • If the area under the hydrograph represents 1 inch (or 1 cm) of rainfall-excess over the entire drainage basin then the hydrograph is called a UNIT hydrograph 22 Docsity.com Example • After a 2-hour storm, a station downstream from a 45 square mile drainage basin measures 9400 cfs as a peak discharge and 3300 acre-feet as total runoff. a) Find the 2-hour unit hydrograph peak discharge. b) What would be the peak runoff and design flood volume if a 2-hour storm dropped 2.5 inches of net precipitation? 25 Docsity.com a) Find Unit Hydrograph Peak Flow • Find volume of runoff which represents 1” of excess runoff over the 45 square mile drainage area • 45 square miles * 1” = 2400 acre-ft • Since the actual measured 3,300 acre-ft is greater than the 2400 acre-ft for 1” of runoff, the storm had more than 1” of excess runoff 26 Docsity.com a) Find Unit Hydrograph Peak Flow • The ratio of 3300/2400=1.375; therefore the storm had 1.375” of excess runoff • If the peak discharge is 9400 cfs for a runoff of 1.375” then the ratio of the peak discharge of the unit hydrograph must be: • 2400/3300 (or 1/1.375) = 0.727 • Peak discharge=0.727*9400 = 6,800 cfs 27 Docsity.com Hydrographs • From rainfall data (not typical) • Synthetic – TR-55 – Snyder – Simple Triangular – Modified Rational Method 30 Docsity.com Error in Book: Example 10-3 31 Flow (cfs) Time (hrs) Unit Hydrograph 0 0 1 0.086 2 0.19 3 0.38 4 0.71 5 2.3 6 5.5 7 6.4 8 5.8 9 4.4 10 3.2 11 2.5 12 2.1 13 1.7 14 1.4 15 1.2 16 1 17 0.84 18 0.69 19 0.54 20 0.45 21 0.36 22 0.25 23 0.19 24 0.15 sum 42.336 cfs volume 152409.6 cubic ft Watershed Area 1828915.2 square ft Watershed Area 42 acres 0 1 2 3 4 5 6 7 0 5 10 15 20 25 30 Di sc ha rg e (c fs ) Time (hrs) Hydrograph Area calculated is apparently for the unit hydrograph (not the 5.5” hydrograph). Therefore they should have divided by 1” (instead of 5.5”). Answer should be approx. 42 acres. Docsity.com Next Lecture • More hydrographs • Subtract groundwater • Synthesize unit hydrographs for different storm durations using the: – Lagging storm method – S-Curve method 32 Docsity.com