Download Rational Method - Hydrology - Lecture Slides and more Slides Water and Wastewater Engineering in PDF only on Docsity! Determining Peak Flows 1 Docsity.com Several Methods • Statistical Analysis of Streamflow Records • Transfer Methods (using gaged data to estimate an ungaged location) • Regional Methods (region-specific equations developed based on areas, slopes, etc) • Models (Rational method/TR-55) 2 Docsity.com Objectives • Know the rational method equation • Know the assumptions used in developing the rational method • Know how to calculate peak flows using the rational method 5 Docsity.com Assumptions • Rainfall intensity is uniform over the drainage basin during the duration of the rainfall • Maximum runoff rate occurs when the rainfall lasts as long or longer than the time of concentration • The frequency for rainfall and runoff are equal 6 Docsity.com General Use • Rational method generally used for small drainage basins • Do not use the rational method for drainage areas larger than 100-200 acres • Rational method not applicable if there is significant ponding (ponds, wetlands) within the drainage area 7 Docsity.com Derivision of Rational Method • Volume of rain = Volume observed as Runoff • I*A*Tc=Tc*Qp • Qp=IA • To account for infiltration/evaporation/storage add a coefficient C (C<1) • Qp=CIA 10 Docsity.com Equation: Q=CIA • Q-peak flow (cfs) • C-runoff coefficient (dimensionless) • I-average rainfall intensity (in/hr) • A-drainage area basin (acres) 11 Docsity.com Do units make sense? acre-in/hr = cfs? Do the calculation: 1 acre-in/hr = 1.008 cfs 12 Docsity.com C-Typical Values (by use) Farmland or Pasture 0.05-0.3 Unimproved 0.1-0.3 Parks or Cemeteries 0.1-0.25 Railroad Yard 0.2-0.4 Playgrounds (not asph or conc) 0.2-0.35 Business Districts neighborhood 0.5-0.7 city (downtown) 0.7-0.95 Residential single family 0.3-0.5 multi-plexes, detached 0.4-0.6 multi-plexes attached 0.6-0.75 suburban 0.25-0.40 apartments, condominiums 0.50-0.70 Industrial light 0.5-0.8 heavy 0.6-0.9 Ref: Civil Engr. Ref. Manual, 6th ed., Michael Lindeburg, PE, Professional Publications, Inc. ISBN: 0- 921045-45-0 15 Docsity.com Rational Coefficient C Must be weighted if you have different area types within the drainage area Drainage area = 8 acres: 2 acres; C=0.35 (residential suburban) 6 acres; C=0.2 (undeveloped-unimproved) Weighted C=[(2)(.35)+(6)(.2)]/8 = 0.24 16 Docsity.com Example: Determine the peak flow for a 100-acre watershed in central NY (mostly forested). Assume the intensity is 3” per hour. What is the peak flow if the watershed is developed into light industrial usage? Assume the intensity is 3” per hour. 17 Docsity.com Detention Structures • Store runoff temporarily and then release it in a controlled manner to limit the peak flow leaving a site – Mitigates destructive effects of increased runoff – May improve water quality – May increase recharge 20 Docsity.com Detention Structures • Basically a tank or pond • Complex because water coming in (inflow hydrograph) is not constant • Water going out is not constant (single or multistaged outlet structure where Qout varies based on water elevation in the structure) • Water elevation in the structure is not constant 21 Docsity.com
