Lecture 6-Watershed Concepts & Simple Rainfall Runoff PDF

WATERSHED CONCEPTS AND SIMPLE RAINFALL RUNOFF ANALYSIS 9 8 Before Development...

WATERSHED CONCEPTS AND SIMPLE RAINFALL RUNOFF ANALYSIS 9 8 Before Development 7 After Development 6 5 4 3 2 1 0 10:48:00 PM 12:00:00 AM 1:12:00 AM 2:24:00 AM 3:36:00 AM 4:48:00 AM 6:00:00 AM 7:12:00 AM Engr. Adones Caduyac, MSc Master of Science in Civil and Infrastructure Engineering/Major in Water Engineering, Part time Faculty, Full time Senior Water Resources Engineer,  TOPICS WATERSHED CONCEPTS RATIONAL METHOD MODIFIED RATIONAL METHOD SCS CURVE NUMBER METHOD WATERSHED CONCEPTS  RAINFALL-RUNOFF ANALYSIS 1. RATIONAL METHOD Design Rainfall Intensity (I) Ratio of total amount of rainfall to time in mm/hr. is determined from intensity-duration-frequency (IDF) charts developed by the Philippine Atmospheric, Geoph ysical and Astronomical Services Administration Rainfall Intensity (mm/hr) based on 52 years of rainfall records: Return Duration Period (year) 10 min 20 min 30 min 1 hr 2 hr 3 hr 6 hr 12 hr 24 hr 2 136.2 106.5 72.6 50.2 34.1 26.7 17.4 10.5 6.3 5 167.4 136.5 107.6 74.2 51.7 40.8 26.6 16.1 9.4 10 205.3 156.4 130.8 90.1 63.3 50.2 32.8 19.8 11.5 15 226.7 172.1 143.8 99.0 69.9 55.5 36.2 21.9 12.7 20 241.7 183.1 153.0 105.3 74.5 59.2 38.6 23.3 13.5 25 253.3 191.6 160.0 110.1 78.1 62.0 40.5 24.5 14.2 50 288.8 217.7 181.7 125.0 89.0 70.8 46.3 27.9 16.1 100 324.1 243.6 203.3 139.8 99.8 79.5 52.0 31.4 18.1 Time of Concentration (Tc) Travelled time of stormwater from the farthest po int to the outlet. Tc = To + Tch (To) Overland Flow (Sheet) Time 107 𝑥 𝑛 𝑥 𝐿1/3 𝑇𝑜 = (DPWH-DGCS, 2015) 𝑆 1/2 (Tch) Travel Time thru Channels and Pipes 𝑇𝑐ℎ = 𝐿/𝑉 V = (1/n ) R2/3 S1/2 (Manning’s Equation) Or Vmin = 0.8m/s, Vmax = 5m/s (as per DPWH – DGCS) Can be validated by the actual concentration of runoff in the flooded area. Example: Rolling hills along A.S fortuna is flooded after 15mins of rainfall according to residents in the area or based on observation. Coefficient of Runoff (c) FROM DPWH-DGCS Sample Computation of Coefficient of Runoff (c) Catch Runoff Coefficie Areal Avera Land Cover Area ment nt ge "c" Residential 0.63 2.121 Industrial 0.65 3.84 28.78 S3 Open Spaces 0.25 1 0.40 Water/Pond 0.75 2.99 Gravel Surface Road 0.45 2.95 Open Spaces 0.25 2.066 S2 0.55 Water/Pond 0.75 3.144 10.97 Open Spaces 0.25 7 S1 Water/Pond 0.75 1.22 0.36 Inudstrial 0.65 0.78 Roofed Area 0.8 0.153 Open Space/Gr assed 0.25 7.883 S4 Roofed Area 0.8 0.217 0.61 Water/Pond 0.75 16.66 Road 0.9 1.31 ASSIGNMENT 3. Modified Rational Method 2. SCS CURVE NUMBER METHOD