CE 590

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• CE 590
• Hydrology
• Lecture 3

Paul P. Mathisen Worcester Polytechnic
InstituteCivil and Environmental Engineering
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CE 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

Web site
http//cee.wpi.edu/hydro
Instructor Paul Mathisen (with some assistance
from others)
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Last time ...

• Brief review
• Some clarifications on definitions
• Quantitative analysis of rainfall
• Rainfall measurement

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Today ...

• Review
• Abstractions
• Catchments
• Runoff
• Intro to infiltration

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Water Budget
E
P
T
I
R
G
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Rainfall / runoff

• rainfall

depression storage
overland flow
streamflow
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Abstractions

• Processes acting to reduce total ppt into
  effective ppt., which ultimately produces runoff
• Interception
• surface or depression storage
• infiltration
• evaporation
• evapotranspiration

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Interception

• Abstraction by vegitation or other surface cover
• throughfall - part of ppt that reaches the ground
• fx of storm, vegatative cover, season
• amounts
• light storms - 25 percent
• moderate storms - 7 to 36 in growing seas.
• heavy longer storms - small

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Interception

• Components
• Interception storage - retained by foliage
• evaporation loss

Where .. Linterception loss (mm) S
interception storage depth (mm) K evap.foliage
surf/its horiz projection E evap. Rate
(mm/hr) t storm duration
L S K E t
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Surface/depression storage

• Abstracted ppt is retained in puddles, ditches ,
  and other depressions in surface
• milder the relief, greater the depression storage
• ex -
• sand - 5mm - pervious urban - 6.25 mm
• loam - 3.75 mm - paved areas - 1.5 mm
• clay - 2.5 mm

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Depression storage

• Quantification
• lump in with other components such as
  infiltration
• Peak flow correction factor (SCS TR55)
• Vs Sd (1 - e-kPe) where Vs is the equiv depth
  of depress storage (mm), Pe is precip excess, Sd
  is depression storage capacity (mm) typically 10
  to 50 mm, and k is a const

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Infiltration
Infiltration - seepage of rainfall into the
ground (contribution to groundwater)
R
(in)
t (hrs)
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Antecedent moisture

• Infiltration has an important effect on
  abstracted ppt.
• Infiltration is dependent on initial level of
  soil moisture, or antecedent moisture
• Antecedent ppt index (API)
• typical depletion rate
• Ii K I I-1
• where Ii is index for day, Ii-1 is index for
  preceding day, and K recession factor (.85-.98)

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Antecedent Precipitation Index

• High API, greater runoff
• Alternative definitions
• Antecedent moisture condition (AMC)
• by SCS .. I(dry), II(avg), or III(wet)
• SSARR - soil moisture index (SMI) - relates
  runoff to SMI and ppt. intensity
• Runoff percent (R/P)100

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Surface runoff

• runoff depends on the antecedent ppt index
• water flowing on earths surface
• overland flow
• flow in rills, gullies and streams

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3 components contribute to runoff

• surface flow
• interflow
• groundwater flow

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Surface runoff in catchments

• Overland flow - sheet flow over land surf.
• rill flow - small rivulets (conc of overland)
• gully flow - runoff with erosive cabability
• streamflow - concentrated runoff
• river flow - confluence of streams

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Catchment characteristics

• area
• Slope
• Shape
• Flow length
• streams (location, density, nature)

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Catchment area

• Drainage area
• leads to potential runoff volume
• catchment divide
• may differ from groundwater divide
• Estimate Q peakCan

divide
A
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Catchment shape

• Form
• KfA/L2 where Kf is form ratio, and L is
  catchment length
• Compactness
• Kc0.282P/A1/2 where Kc is a compactness ratio,
  and P is the perimeter
• Catchment response - conc. timing of runoff
• If Kf high, or Kc1, then rapid runoff
• Factors relieve, veg. cover, drainage density, ..

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Flow length - approximate equations

• Approximate estimate
• Lo1/(2D)
• where Ddrainage density since overland flow
  length is approx 1/2 of mean distance between
  channels
• More precise estimate
• Lo1/2D1-(Sc/Ss)1/2
• where Sc is the mean channel slope and Ss is the
  mean surface slope

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Linear measures

• Catchment length -length along principle
  watercourse
• length to centroid (often est. as point to 2 or
  more bisecting straight lines
• Order
• 0 overland flow
• 1 gets flow from 0 orders
• 2 gets flow when 2 1st order streams combine
• etc.

G
L
Lc
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Estimating the overland flow length
Lo
Lo
Lo
collector
collector
collector
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Slope/catchment relief

• Relief - an elevation difference
• max relief max elev diff between highest
  lowest points
• Relief ratiomax relief/longest straight lgth

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Land surface slopes

• Often use grid methods to get slopes
• hypsometric analysis curve showning elevation
  of catchment above this elevation

Ei-Emin
Emax-Emin
Ai/Ac
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Stream channels
Slope 0.10 - mountains 0.000006 - some tidal
rivers

• S1 - from max and min elevs
• S2 - const slope that makes shaded area above
  equal to shaded area below
• S3 - equivalent slope -
• break channel into subreaches obtain slope

E
2
?Li
up
Down
S3
Distance
? (Li/Si 1/2)
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Stream-types and baseflow

• perennial
• (always flowing)
• flow maintained by base flow during dry weather
• ephemeral
• (only in response to ppt)
• intermittent
• (only in certain times of the year)

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Hydrographs

• Characteristics of the hydrograph
• Distribution of uniform rainfall

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Runoff
Q (cfs)

• Volume or flow rate
• varies with time
• may express in flow per unit drainage area, per
  unit runoff depth, or per both
• surface flow - direct runoff
• also get indirect runoff

T (hr)
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Runoff coefficients

• Rk P
• surface k
• urban residential single 0.3
• apts 0.5
• commerial and industrial 0.9
• forests 0.05-0.20
• parks farms 0.05-0.30
• asphalt and pavement 0.85-1.00

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First part of class ...

• Review
• Abstractions
• Catchments
• Runoff

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Next ...

• Infiltration
• Definitions
• Physical aspects
• Simple models
• Physically based models
• measurement

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3 components contribute to runoff

• surface flow
• interflow
• groundwater flow

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Infiltration

• Infiltration - process by which ppt is abstracted
  by seeping into soil below ground surface
• define it by
• an instantaneous infiltration rate (mm/hr)
• an average infiltration rate (mm/hr)
• Function of rainfall intensity, soil properties
  and soil type, surface conditions, vegitative
  cover, and water quality

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Physical problem
Close-up view on next slide
unsaturated flow
saturated flow
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Moisture in the unsaturated zone
unsaturated
saturated
air
water
water
porosity nVw /VtVvoid/Vtot
moisture content ?Vwater/Vtot
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Capillarity capillary fringe
capillary forces result in rise of fluid
2R
hc
?
(2? cos ?)
hc
rwgR
In subsurface, we get a capillary fringe
Vadose/unsaturated zone
capillary fringe
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Conditions in unsaturated zone

• total potential or head
• h z ?? where z is the elevation head and ? is
  the pressure head (or moisture potential)
• ??is a function of ?
• Darcys law applies
• qK(?? dh/dz

?
?
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Controls on the Range of moisture content

• Field capacity
• maximum amount of moisture the soil structure can
  hold agains the force of gravity
• upper level of moisture before rapid drainage
• Wilting point
• soil mosture at which permanent wilting of plants
  starts to occur

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NEXT TIME Infiltration - quantitative
approaches

• Estimation from water balances
• Horton Equation
• Philips Equation
• ? Index
• Green-Ampt model
• Measurement