engn.4010 watershed analysis - umass lowellfaculty.uml.edu/.../engn.4010watershedanalysis.pdf ·...
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Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
CiAQ RATIONAL METHOD
Where:Q = Maximum Rate of Runoff (cfs)C = Runoff Coefficienti = Average Rainfall Intensity (in/hr)A = Drainage Area (in acres)
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
RATIONAL METHODAssumptions and Limitations:• Watershed area < 200 acres• The method is applicable if time of
concentration (tc) for the drainage area is less than the duration of peak rainfall intensity.
• The time of concentration (tc) is the time required for water to travel from the hydraulically most remote point of the basin to the point of interest.
From Texas DOT Hydraulics Design Manual
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
RATIONAL METHODAssumptions and Limitations:• The calculated runoff is directly
proportional to the rainfall intensity.• Rainfall intensity is uniform
throughout the duration of the storm.• The frequency of occurrence for the
peak discharge is the same as the frequency of the rainfall producing that event.
From Texas DOT Hydraulics Design Manual
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
RATIONAL METHODAssumptions and Limitations:• Rainfall is distributed uniformly over
the drainage area.• The minimum duration to be used for
computation of rainfall intensity is 10 minutes.
From Texas DOT Hydraulics Design Manual
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
RATIONAL METHODAssumptions and Limitations:• The rational method does not
account for storage in the drainage area. Available storage is assumed to be filled.
From Texas DOT Hydraulics Design Manual
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
CiAQ RATIONAL METHOD
Where:Q = Maximum Rate of Runoff (cfs)C = Runoff Coefficienti = Average Rainfall Intensity (in/hr)A = Drainage Area (in acres)
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
RUNOFF COEFFICIENT (C)Definition:Dimensionless ratio intended to indicate the amount of runoff generated by a watershed given a average intensity of precipitation for a storm.
From Texas DOT Hydraulics Design Manual
PRC
Where:R = Total depth of runoffP = Total depth of precipitation
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
From Ohio DOT Hydraulics Manual
RUNOFFCOEFFICIENT
(C)
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
RAINFALL INTENSITY (i)The determination of rainfall intensity (i) for use in the Rational Formula involves consideration of three factors:
• Average frequency of occurrence.• Intensity-duration characteristics for a
selected rainfall frequency.• The time of concentration (tc).
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
TIME OF CONCENTRATION (tc)Definition:The time required for a parcel of runoff to travel from the most hydraulically distant part of a watershed to the outlet.
• tc represents the time at which all areas of the watershed that will contribute runoff are just contributing runoff to the outlet.
From Texas DOT Hydraulics Design Manual
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
TIME OF CONCENTRATION (tc)• If the chosen storm duration > tc,
then the rainfall intensity will be less than that at tc (Peak discharge < optimal value).
• If the chosen storm duration < tc, then the watershed is not fully contributing runoff to the outlet for that storm length (i.e. optimal value will not be realized).
• Therefore, choose storm duration = tc for peak discharge.
From Texas DOT Hydraulics Design Manual
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
TIME OF CONCENTRATION (tc)Morgali and Linsley Method (1965)
Assumptions and Limitations:• For use with sheet flow (rarely more than 400 ft).• Cannot be solved for tc without i. Therefore,
iteration is required.
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
TIME OF CONCENTRATION (tc)Morgali and Linsley Method (1965)
From FHWA-NHI-01-021
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
TIME OF CONCENTRATION (tc)Kirpich Method (1940)
Assumptions and Limitations:• For small drainage basins dominated by
channel flow.
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
TIME OF CONCENTRATION (tc)Kerby-Hatheway Method (1959)
Assumptions and Limitations:• Primarily used for
overland flow.
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
TIME OF CONCENTRATION (tc)Shallow Concentrated Flow
VLtc
2/1)(28.3 skV
From FHWA-NHI-01-021
Where:tc = Time of ConcentrationL = Length of FlowV = Velocity (ft/s)k = Intercept Coefficients = Slope (%)
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
TIME OF CONCENTRATION (tc)Open Channels and Pipe Flow
VLtc 2/13/248.1 sr
nV
From FHWA-NHI-01-021
Where:tc = Time of ConcentrationL = Length of FlowV = Velocity (ft/s)n = Roughness Coefficientr = Hydraulic Radius (flow area/wetted
perimeter) (ft)s = Slope (ft/ft)
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
TIME OF CONCENTRATION (tc)Numerous Flow Segments
Where:tc = Time of Concentrationtt1 = Travel time of Segment 1n = Number of segments
tnttc tttt ...21
Revised 10/2015
ENGN.4010 ENGINEERING CAPSTONE DESIGNWatershed Analysis
RAINFALL INTENSITY (i)Intensity-Duration-Frequency (IDF) Curves
From FHWA-NHI-01-021 IDF Curve for Boston, MA(Hydrology Handbook for Conservation Commissioners, 2002)