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The Hydrologic Cycle
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Hydrology
Watersheds
Meteorology Study of the atmosphere including
weather and climate
Surface water hydrology Flow and occurrence of
water on the surfaceof the earth
Hydrogeology Flow and occurrence
of ground water
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Intersection ofHydrology and Hydraulics
Water supplies Drinking water
Industry
Irrigation
Power generation Hydropower
Cooling water Dams
Reservoirs
Levees
Flood protection
Flood plainconstruction
Water intakes
Discharge anddilution Wastewater
Cooling water
Outfalls4
Engineering Uses ofSurface Water Hydrology
Average events (average annual rainfall,evaporation, infiltration...) Expected average performance of a system Potential water supply using reservoirs
Frequent extreme events (10 year flood,10 year low flow)
Levees Wastewater dilution
Rare extreme events (100 to PMF) Dam failure Power plant flooding
Probable maximum flood
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Flood Design Techniques
Use stream flow records Limited data
Can be used for high probability events
Use precipitation records Use rain gauges rather than stream gauges
Determine flood magnitude based onprecipitation, runoff, streamflow
Create a synthetic storm Based on record of storms
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10
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9/30 12/31 4/1 7/2date
Streamf
low(m3/s)
Forecasting Stream Flows
Natural processes -not easily predicted ina deterministic way We cannot predict the
monthly stream flow
We will use probabilitydistributions insteadof predictions
Seasonal trend with large variation
10 year daily average
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Surface DrainageSurface DrainageSurface Drainage
Design FlowsDesign Flows
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Objectives
Identify drainagerequirements and design
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Surface Drainage surface water is removed from
pavement Redirects water into appropriately
designed drainage systems (channelsor pipes)
Eventually, discharges into naturalwater systems
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Surface Drainage
Two types of water Surface water rain and snow (?)
Ground water can be a problem when awater table is near surface
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Surface DrainageSystem Design
Three phases1. Estimate of the quantity of water toreach the system
2. Hydraulic design of system elements3. Comparison of different materials that
serve same purpose
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Calculating Peak Runoff
Rainfall Runoff Analysis /Rational Method
Qp = CiA
C = constant runoff coefficienti = rainfall intensityA = drainage area
(tc
= time of concentration < rainfall duration)
0
0.2
0.4
0.6
0.8
1
0 1 2 3 4 5 6
t / Tp
Q / Qp
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Rational Formula - Methodto Choose Rainfall Intensity
Intensity = f(storm duration)
Expectation of stream flow vs. Time duringstorm of constant intensity
Watershed
Outflow
Q
t
Qp
tcClassic Watershed
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Rational Formula - Timeof Concentration (Tc)
Time required (after start of rainfallevent) for most distant point in basinto begin contributing runoff to basinoutlet
Tc affects the shape of the outflow
hydrograph (flow record as afunction of time)
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Hydrologic Analysis:Rational Method
Useful for small, usually urban,watersheds (
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Runoff Coefficient rural area
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Runoff Coefficient urban area
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Runoff Coefficient ForHigh Intensity Event
(i.e. 100-year storm)
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Runoff Coefficient ForHigh Intensity Event
(i.e. 100-year storm)
C = 0.16 forlow intensityevent forcultivatedfields
C = 0.42 forhigh intensity
event
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Runoff Coefficient
When a drainage area has distinctparts with different C values
Use the weighted average
C = C1A1 + C2A2 + .. + CnAnAi
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Watershed Area
measured in hectares Combined area of all surfaces thatdrain to a given intake or culvertinlet
Determine boundaries of area thatdrain to same location i.e high points mark boundary Natural or human-made barriers
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Watershed Area
Topographic maps
Aerial photos
Digital elevation models
Drainage maps
Field reviews
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Intensity Average intensity for a selected frequency and
duration over drainage area for duration of storm Based on design event (i.e. 5-year storm)
Overdesign is costly Underdesign may be inadequate
Duration is important Based on values of Tc and T
Tc
= time of concentration T = recurrence interval or design frequency
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Time of Concentration (tc)
Time for water to flow from hydraulically mostdistant point on the watershed to the point ofinterest
Rational method assumes peak run-off rate occurswhen rainfall intensity (I) lasts (duration) >= Tc
Used as storm duration dont use Tc
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Infiltration Measurement:Double Ring Infiltrometer
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Double RingInfiltrometer
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Measuring Infiltrationwith Rainfall Simulator
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Rainfall IntensityIncreased until Surface
Runoff Occurs
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Streamflow Measurements
Flood forecasting
Flood analysis Reservoir operations
Low flows water quality concerns
Design structures culverts, bridges,stormwater systems
Evaluate changes in land use on
watersheds and/or
changes in climatic regimes
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Streamflow & Fluvial Geomorphology
(Adapted from Dunne & Leopold, 1978; Leopold, 1994, 1997)
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Flow velocity varies with depth andchannel width
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Measuring StreamflowDischarge
Current meter method: measureflow & stage (elevation) over timeto establish a discharge ratingcurve: Continuously measurestage (stilling well) and derive Qfrom stage.
Pre-calibrated Structures forsmall streams, ditches &research applications
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Current Meter Method
Q = [Velocity x Area] Area is channel cross-sectional area
Need to know width of channel (w), Depth ofchannel (d), and Velocity of flow (V) (ft/s orm/s)
Procedure Depth varies across a channel
Velocity varies
Therefore need to divide the channel intomanageable segments (slices); Typically use10-20 segments
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Discharge (Q) Measurement
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Discharge (Q) Measurement Compute the Q for each segment (slice)
=
n
iiVAQ1
Sum the Q for each segment to compute the
total Q for the stream
Where on a stream do you collect Q data?
Need a quasi stable section (Control Section)
Look for a relatively straight reach w/uniform
flow such as a riffle section
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Discharge (Q) Measurement
Each segment has a fixed width
Identify the midpoint for each segment & Measure:
Channel depth from water surface Velocity
Depth of velocity measurement depends upon channeldepth
IF Depth > 0.5m (1.6 ft) take 2 measurements andcompute the average One @ 20% depth
One @80 % depth
Average the two readings
IF Depth < 0.5m, take one reading @ 60% depth
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Discharge (Q) Measurement
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Measuring Streamflowwith a Current Meter
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Discharge (Q) Measurement
Pygmy meter
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Discharge (Q) Measurement
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Precalibrated Structures
Weirs
Flumes
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Weirs
Obstruct flow andforce it through a
notch
Stage-Q relationshipestablished
for each type
of notch
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Weirs
Generally used in small streams
Various types V-notch for accurate low flow
Rectangular Handles higher flows
Less accurate at low flows
Trapezoidal -- an intermediate weir
Concerns Sediment & debris are trapped
Leakage
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Cipolletti (Trapezoidal) Weir
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Trapezoidal Weir
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Rectangular Weir
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90 degree V-notch Weir
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90 V-notch Weir
Q = 2.5H2/3
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Flumes
An artificial open channel built to contain flow
within a designed cross-section and length
No impoundment
Water height in flume measured with a stilling
well
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Flumes Used to measure flow in:
water and wastewater treatment plants
irrigation channels
agricultural runoff
runoff plots research applications
small watersheds
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Large Crest Flumes
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Long-throated Flume
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Short-throated Flume
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Parshall Flume
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H Flume
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Floods
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stimating Disc arge Qwhere flow was not directly
measured Large flood events that can not be
measured with conventional methods
Peak discharge in streams wherethere is no gauge
Stage will give us X-sectional area Its the velocity (v) or (u) thats
problematic
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Discharge (Q) MeasurementMannings Equation
2 / 3 1 / 2R S
vn
=
u or v = average velocity (m/s)
R = hydraulic radius
= [Area/wetted perimeter]
S= Energy gradient, Approximated by water surfaceslope
n = Mannings roughness coefficient
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Manning Roughness Coefficients
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Simplified Methods:predicting peak flow
discharge
Simplified Methods:Simplified Methods:predicting peak flowpredicting peak flow
dischargedischarge
Rational MethodRational Method
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Rational MethodEmpirically based method
Qp = CIA (cfs)
Qp = [0.00278]CIA (cms, m3/s)
Most commonly used formula for estimating Qpfrom rainfall in small urban watersheds
Widely accepted method for design of storm
sewer capacity
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Rational Method
Qp = [1/360]CIA (cms)
C = dimensionless runoff coefficient
Vegetation type
Soil type
Amount of impervious area
High C values = high RunOff rates
I = rainfall intensity for the storm of interest
(mm/hr)
A = watershed area (acres)
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Rational Method
Only valid for storms that last as long as the
watersheds Tc
If Tc is < Tc for watershed then Qpeak will be
overestimated
Assumes precipitation is uniformly distributed over
entire watershed
Assumes the RI of the flood peak is the same as the RI
of rainfall
1 in/hr of runoff from 1 acre will yield 1 cfs Designed to be used on watersheds < 200 acres (81
hectares) NOT FOR LARGE WATERSHEDS
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