CE 590 Hydrology Hydrology Lecture # 3 Paul P. Mathisen Worcester Polytechnic Institute Civil and...
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Transcript of CE 590 Hydrology Hydrology Lecture # 3 Paul P. Mathisen Worcester Polytechnic Institute Civil and...
CE 590CE 590
Hydrology Hydrology
Lecture # 3Lecture # 3
Paul P. MathisenWorcester Polytechnic Institute
Civil and Environmental Engineering
CE 590 HydrologyCE 590 Hydrology
Hydrologic principles
Rainfall and runoff Surface water & ground water flow Watersheds and terrestrial inputs to
surface water bodies
Examples developed from Wachusett Reservoir
Instructor: Paul Mathisen (with some assistance from others)
http://cee.wpi.edu/hydro
Web site:
Last time ...Last time ...
Brief reviewBrief review Some clarifications on definitionsSome clarifications on definitions Quantitative analysis of rainfallQuantitative analysis of rainfall Rainfall measurementRainfall measurement
Today ...Today ...
ReviewReview AbstractionsAbstractions Catchments Catchments RunoffRunoff Intro to infiltrationIntro to infiltration
Water BudgetWater Budget
T
G
R
EPI
Rainfall / runoffRainfall / runoff
rainfallrainfall
depression storage
overland flow
streamflow
AbstractionsAbstractions
Processes acting to reduce total Processes acting to reduce total ppt into effective ppt., which ppt into effective ppt., which ultimately produces runoffultimately produces runoff– InterceptionInterception– surface or depression storagesurface or depression storage– infiltration infiltration – evaporationevaporation– evapotranspirationevapotranspiration
InterceptionInterception
Abstraction by vegitation or other surface Abstraction by vegitation or other surface covercover
throughfall - part of ppt that reaches the throughfall - part of ppt that reaches the groundground
fx of storm, vegatative cover, seasonfx of storm, vegatative cover, season amountsamounts
– light storms - 25 percentlight storms - 25 percent– moderate storms - 7 to 36 % in growing seas.moderate storms - 7 to 36 % in growing seas.– heavy & longer storms - smallheavy & longer storms - small
InterceptionInterception
ComponentsComponents– Interception storage - retained by Interception storage - retained by
foliagefoliage– evaporation lossevaporation loss
L= S + K E t
Where ..L=interception loss (mm)S = interception storage depth (mm)K= evap.foliage surf/its horiz projectionE= evap. Rate (mm/hr)t= storm duration
Surface/depression Surface/depression storagestorage
Abstracted ppt is retained in puddles, Abstracted ppt is retained in puddles, ditches , and other depressions in surfaceditches , and other depressions in surface
milder the relief, greater the depression milder the relief, greater the depression storagestorage
ex - ex - – sand - 5mmsand - 5mm - pervious urban - 6.25 - pervious urban - 6.25
mmmm– loam - 3.75 mmloam - 3.75 mm - paved areas - 1.5 mm- paved areas - 1.5 mm– clay - 2.5 mmclay - 2.5 mm
Depression storageDepression storage
QuantificationQuantification– lump in with other components such as lump in with other components such as
infiltrationinfiltration– Peak flow correction factor (SCS TR55)Peak flow correction factor (SCS TR55)
– VVss = S = Sd d (1 - e(1 - e-kPe-kPe) where Vs is the equiv ) where Vs is the equiv depth of depress storage (mm), Pe is depth of depress storage (mm), Pe is precip excess, Sd is depression storage precip excess, Sd is depression storage capacity (mm) [typically 10 to 50 mm], capacity (mm) [typically 10 to 50 mm], and k is a constand k is a const
Infiltration Infiltration
R
(in)
t (hrs)
Infiltration - seepage of rainfall into the ground(contribution to groundwater)
Antecedent moistureAntecedent moisture Infiltration has an important effect on Infiltration has an important effect on
abstracted ppt. abstracted ppt. Infiltration is dependent on initial level of Infiltration is dependent on initial level of
soil moisture, or antecedent moisturesoil moisture, or antecedent moisture Antecedent ppt index (API) Antecedent ppt index (API) typical depletion ratetypical depletion rate
– IIi i = K I= K I I-1 I-1
– where Iwhere Iii is index for day, I is index for day, Ii-1i-1 is index for is index for preceding day, and K= recession factor (.85-.98)preceding day, and K= recession factor (.85-.98)
Antecedent Precipitation Antecedent Precipitation IndexIndex
High API, greater runoffHigh API, greater runoff Alternative definitionsAlternative definitions
– Antecedent moisture condition (AMC)Antecedent moisture condition (AMC)– by SCS .. I(dry), II(avg), or III(wet)by SCS .. I(dry), II(avg), or III(wet)– SSARR - soil moisture index (SMI) - SSARR - soil moisture index (SMI) -
relates runoff to SMI and ppt. relates runoff to SMI and ppt. intensityintensity
Runoff percent (R/P)*100Runoff percent (R/P)*100
Surface runoffSurface runoff
runoff depends on the antecedent runoff depends on the antecedent ppt indexppt index
water flowing on earth’s surfacewater flowing on earth’s surface overland flowoverland flow flow in rills, gullies and streamsflow in rills, gullies and streams
3 components contribute 3 components contribute to runoffto runoff
surface flowsurface flow interflowinterflow groundwater flowgroundwater flow
Surface runoff in Surface runoff in catchmentscatchments
Overland flow - sheet flow over land Overland flow - sheet flow over land surf.surf.
rill flow - small rivulets (conc of rill flow - small rivulets (conc of overland)overland)
gully flow - runoff with erosive gully flow - runoff with erosive cababilitycabability
streamflow - concentrated runoffstreamflow - concentrated runoff river flow - confluence of streamsriver flow - confluence of streams
Catchment characteristicsCatchment characteristics
areaarea SlopeSlope ShapeShape Flow lengthFlow length streams (location, density, nature)streams (location, density, nature)
Catchment areaCatchment area
Drainage areaDrainage area– leads to potential runoff volumeleads to potential runoff volume
catchment dividecatchment divide– may differ from groundwater dividemay differ from groundwater divide
Estimate: Q Estimate: Q peakpeak=Ca=Cann
A
divide
Catchment shapeCatchment shape FormForm
– KKff=A/L=A/L22 where Kf is form ratio, and L is where Kf is form ratio, and L is catchment lengthcatchment length
CompactnessCompactness– KKcc=0.282P/A=0.282P/A1/21/2 where K where Kcc is a compactness is a compactness
ratio, and P is the perimeter ratio, and P is the perimeter Catchment response - conc. & timing of runoffCatchment response - conc. & timing of runoff If Kf high, or Kc~1, then rapid runoffIf Kf high, or Kc~1, then rapid runoff Factors: relieve, veg. cover, drainage density, ..Factors: relieve, veg. cover, drainage density, ..
Flow length - Flow length - approximate equationsapproximate equations
Approximate estimateApproximate estimate– LLoo=1/(2D)=1/(2D)
– where D=drainage density since overland where D=drainage density since overland flow length is approx 1/2 of mean distance flow length is approx 1/2 of mean distance between channelsbetween channels
More precise estimateMore precise estimate– LLoo=1/[2D{1-(S=1/[2D{1-(Scc/S/Sss)})}1/21/2]]
– where Swhere Scc is the mean channel slope and S is the mean channel slope and Sss is the mean surface slopeis the mean surface slope
Linear measuresLinear measures
G
LcL
Catchment length -length Catchment length -length along principle watercoursealong principle watercourse
length to centroid (often est. length to centroid (often est. as point to 2 or more as point to 2 or more bisecting straight linesbisecting straight lines
OrderOrder– 0 = overland flow0 = overland flow– 1 = gets flow from 0 orders1 = gets flow from 0 orders– 2 = gets flow when 2 1st order 2 = gets flow when 2 1st order
streams combinestreams combine– etc.etc.
Estimating the overland Estimating the overland flow lengthflow length
collector collectorcollector
Lo
Lo Lo
Slope/catchment reliefSlope/catchment relief
Relief - an elevation differenceRelief - an elevation difference max relief = max elev diff between max relief = max elev diff between
highest & lowest pointshighest & lowest points Relief ratio=max relief/longest Relief ratio=max relief/longest
straight lgthstraight lgth
Land surface slopesLand surface slopes
Often use grid Often use grid methods to get methods to get slopes slopes
hypsometric hypsometric analysis = curve analysis = curve showning showning elevation of elevation of catchment above catchment above this elevationthis elevation
Ei-Emin
Emax-Emin
Ai/Ac
Stream channelsStream channels S1 - S1 - from max and min elev’sfrom max and min elev’s S2 - S2 - const slope that makes const slope that makes
shaded area above equal to shaded area above equal to shaded area belowshaded area below
S3 - S3 - equivalent slope - equivalent slope - – break channel into break channel into
subreaches & obtain slopesubreaches & obtain slope E
Distanceup Down
Slope 0.10 - mountains 0.000006 - some tidal rivers
S3=Li
(Li/Si 1/2)
2
Stream-types and Stream-types and baseflowbaseflow
perennial perennial – (always flowing)(always flowing)– flow maintained by “base flow” during flow maintained by “base flow” during
dry weatherdry weather ephemeralephemeral
– (only in response to ppt)(only in response to ppt) intermittentintermittent
– (only in certain times of the year)(only in certain times of the year)
HydrographsHydrographs
• Characteristics of the hydrograph
• Distribution of uniform rainfall
RunoffRunoff
Volume or flow rateVolume or flow rate varies with timevaries with time may express in flow per unit may express in flow per unit
drainage area, per unit runoff drainage area, per unit runoff depth, or per bothdepth, or per both
surface flow - direct runoffsurface flow - direct runoff also get indirect runoffalso get indirect runoff
Q(cfs)
T (hr)
Runoff coefficientsRunoff coefficients
R=k PR=k P
surfacesurface k k urban residential: singleurban residential: single 0.30.3 aptsapts 0.50.5 commerial and industrialcommerial and industrial 0.90.9 forestsforests 0.05-0.200.05-0.20 parks; farmsparks; farms 0.05-0.300.05-0.30 asphalt and pavementasphalt and pavement 0.85-1.000.85-1.00
First part of class ...First part of class ...
ReviewReview AbstractionsAbstractions Catchments Catchments RunoffRunoff
Next ...Next ...
InfiltrationInfiltration– DefinitionsDefinitions– Physical aspectsPhysical aspects– Simple modelsSimple models– Physically based modelsPhysically based models– measurementmeasurement
3 components contribute 3 components contribute to runoffto runoff
surface flowsurface flow interflowinterflow groundwater flowgroundwater flow
InfiltrationInfiltration Infiltration - process by which ppt is abstracted by seeping Infiltration - process by which ppt is abstracted by seeping
into soil below ground surfaceinto soil below ground surface define it by define it by
– an instantaneous infiltration rate (mm/hr)an instantaneous infiltration rate (mm/hr)– an average infiltration rate (mm/hr)an average infiltration rate (mm/hr)
Function of rainfall intensity, soil properties and soil type, Function of rainfall intensity, soil properties and soil type, surface conditions, vegitative cover, and water qualitysurface conditions, vegitative cover, and water quality
Physical problemPhysical problem
unsaturatedflow
saturatedflow
Close-upview onnext slide
Moisture in the Moisture in the unsaturated zoneunsaturated zone
unsaturated saturated
air water water
moisture content=Vwater/Vtot
porosityn=Vw /Vt=Vvoid/Vtot
Capillarity & capillary Capillarity & capillary fringefringe
capillary forces result in rise of fluid
rwgR
(2 cos )hc =
hc
2R
In subsurface, we get a capillary fringe
capillary fringe
Vadose/unsaturated zone
Conditions in unsaturated Conditions in unsaturated zonezone
total potential or headtotal potential or head h = z + h = z + where z is the where z is the
elevation head and elevation head and is the is the pressure head (or moisture pressure head (or moisture potential)potential)
is a function of is a function of Darcy’s law applies:Darcy’s law applies:
– q=K(q=K( dh/dz dh/dz
Controls on the Range of Controls on the Range of moisture contentmoisture content
Field capacity Field capacity – maximum amount of moisture the soil maximum amount of moisture the soil
structure can hold agains the force of structure can hold agains the force of gravitygravity
– upper level of moisture before rapid upper level of moisture before rapid drainagedrainage
Wilting pointWilting point– soil mosture at which permanent soil mosture at which permanent
wilting of plants starts to occurwilting of plants starts to occur
[NEXT TIME] [NEXT TIME] Infiltration - quantitative Infiltration - quantitative
approachesapproaches
Estimation from water balancesEstimation from water balances Horton EquationHorton Equation Philip’s EquationPhilip’s Equation IndexIndex Green-Ampt modelGreen-Ampt model MeasurementMeasurement