cee5734_week2_2011

# cee5734_week2_2011 - CEE 5734 Urban Hydrology and...

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Glenn E. Moglen Department of Civil & Environmental Engineering Virginia Tech 1. Complete Rational Method Design 2. Introduction to SCS / NRCS Methods CEE 5734 – Urban Hydrology and Stormwater Management Week 2

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Questions Review of Week 1 Finish Rational Method Design Start – SCS Methods Introduction Definitions/Concepts Major quantities Curve Number Time of Concentration SCS Graphical Method Today’s Agenda
If submitting homework electronically: Submit via “Digital Dropbox” (please do NOT use the “Assignment” link) Naming scheme: Please use something like: “xxxx_Homework_#.pdf” when submitting Submit a single document preferably a .pdf or .doc file. Multiple file submissions are unwieldy. If sending an email, please include “5734” in the subject line. Some Administrative Stuff

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Need to get velocity in 27 inch pipe. Use circular geometry table (see website): From table, y/d = 0.77. (also note that A/d 2 = 0.6489), therefore: Travel time in pipe is 600 ft / 1.2 ft/s = 500 s (8.3 min) Sample Rational Method Calculation – Pipe 1-2 (cont.) ( 29 2930 . 0 ) 001 . 0 ( 49 . 1 030 . 0 ) 0 . 4 ( 25 . 2 1 49 . 1 1 2 / 1 3 / 8 2 / 1 3 / 8 3 / 8 3 / 2 = = = S n Q d d AR ft/s 2 . 1 ) 25 . 2 ( 6489 . 0 0 . 4 2 2 2 = = = = d d A Q A Q v
Sample Rational Method Calculation – Area 2 Runoff to Inlet 2: From Blacksburg IDF curve, precipitation intensity is: 4.72 in/hr (for 10-year, 9 minute duration- interpolated) A 1 = 5.3 C 1 = 0.20 T c,1 = 15 min A 2 = 7.2 C 2 = 0.40 T c,2 = 9 min A 3 = 6.4 C 3 = 0.60 T c,3 = 7 min L 1-2 = 600 ft T t,1-2 =8.3 min L 2-3 = 500 ft T t,2-3 = ??? Pipe 1-2 Pipe 2-3 Pipe 3… s ft 14 s ft 5936 . 13 ) 2 . 7 ( ) 72 . 4 ( ) 40 . 0 ( 3 3 = = = = ciA Q p

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You might think Pipe 2-3 carries 4 + 14 = 18 ft 3 /s. But longest T c controls. T c coming from Area 1 is 15 + 8.3 = 23 minutes T c coming from Area 2 is 9 minutes. T c = 23 minutes controls. Sample Rational Method Calculation – Pipe 2-3 A 1 = 5.3 C 1 = 0.20 T c,1 = 15 min A 2 = 7.2 C 2 = 0.40 T c,2 = 9 min A 3 = 6.4 C 3 = 0.60 T c,3 = 7 min L 1-2 = 600 ft T t,1-2 =8.3 min L 2-3 = 500 ft T t,2-3 = ??? Pipe 1-2 Pipe 2-3 Pipe 3…
Calculated weighted c value: For T c = 23 minutes, i = 3.23 in/hr Since 14 ft 3 /s from Area 2 alone is greater than the combined area, 14 ft 3 /s is the design discharge for pipe. Sample Rational Method Calculation

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cee5734_week2_2011 - CEE 5734 Urban Hydrology and...

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