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The Urban Hydrosphere
Elie Bou-‐Zeid Princeton University
Civil & Environmental Engineering Lecture 6
The Hydrologic Cycle Slide 2
Quick recap
Over a watershed: Precipita=on = Evapotranspira=on + Runoff + Infiltra=on + Storage (at the surface) + Transfer from other watersheds
P = ET + R + I + S + T
Hydrograph: Plot of discharge/flow rate (y-‐axis) in a river, ouOall, etc. versus =me (x-‐axis)
Hyetograph: Plot of Rainfall (y-‐axis), as a cumula=ve volume or a rate, versus =me (x-‐axis)
Slide 3
Main water problems related to urbaniza=on
Water supply: large use of water in small area, water has to be imported from other watersheds, some=mes far away
Water quality: Agricultural produc=on around urban areas to feed the large popula=on, runoff from dirty and hot streets with heavy metals and other pollutants, and sewers pollute local water bodies
Hydrology: large changes in water cycle that are very difficult to control
Slide 4
Urban hydrology Vs. natural hydrology Impervious surfaces AND engineered drainage systems combine to Drain water rapidly causing more “flashy” streamflows and
floods Reduce infiltra=on and ground water recharge and evapora=on Increase runoff since most precipita=on is intercepted and
cannot infiltrate or evaporate later Reduce baseflow of local stream that usually drain the areas,
also drained water to these streams will be ho^er and more polluted
Generally, reduce water quality
Same data is needed for studies: precipita=on, catchment characteris=cs, drainage system, etc
Slide 5
Engineering Challenge
How to control peak flows and the water levels in drainage system to reduce flood damage at all points? (very demanding)
How to predict peak flow and/or runoff volumes? (less demanding)
How to design a system that will work well when the popula=on dras=cally increases or when climate changes?
Slide 6
Meted: Weather and the built environment
h^p://www.meted.ucar.edu/ Meted: Great resource for interac=ve educa=onal modules in meteorology from the US Na=onal Center for Atmospheric Research
Free, but you have to register We will now see the “impacts on the watershed” part of one module on “Weather an the built environment”
Interac=ve modules at: h^p://www.meted.ucar.edu/broadcastmet/wxbuiltenv/
Slide 7
Changes in P
Urban areas are rough, almost act like one building reduce wind speed and deflect flow upward
They are hot produce buoyant upflows Both effects tend to produce a rise in the air mass, <w> > 0, i.e. convergence and liiing
Slide 8
Changes in P Slide 9
Changes in P
Urban areas are rough, almost act like one building reduce wind speed and deflect flow upward
They are hot produce buoyant upflows Both effects tend to produce a rise in the air mass, <w> > 0, i.e. convergence and liiing
When a passing storm experiences this addi=onal liiing precipita=on will increase, mainly downwind of the city … it seems
Historic storm data and hyetographs might not be valid anymore for design, also a problem with climate change
Slide 10
Water import/export Water brought from other watersheds or drained to other
watershed can be very significant.
Imported water can recharge groundwater though pipe leakage and irriga=on
Otherwise imported water for domes=c use goes to wastewater treatment plants, then its fate depends on where the treatment plant usually send it.
Slide 11
Water import in Tel Aviv > P (Hoang Duong et al. 2011)
Slide 12
Changes in Storage
Natural terrain stores water in surface ponds due to the topography: li^le depressions in the surface, etc.
Urbaniza=on fla^ens the terrain, removing any natural storage sites
In many places in the US and many countries, all new development or construc=ons must have a reten=on basin or pond : which is an ar=ficial lower area/hole that can hold water during high rain events
Slide 13
Reten=on/Deten=on basin Slide 14
Change in Infiltra=on
Change in infiltra=on is almost propor=onal (a bit simplis=c) to the impervious frac=on: a 50% impervious surface frac=on means 50% of the surface cannot allow infiltra=on
But it could be more if a lot of ar=ficial soils are used: they tend to be compacted and have lower hydraulic conduc=vity
In addi=on, water wells are dug in many ci=es to extract groundwater, this can lead to a lowering of the water table
In coastal ci=es, lowering of the water table salt water intrusion leading to the groundwater becoming brackish (salty)
Sea level rise can exacerbate the problem
Slide 15
Simple models: Richards equa=ons (diffusivity form) for soil water content θnat
D is the soil water diffusivity (needed in unsaturated soil only, since the gradient of soil moisture would be zero in saturated soils)
K is the hydraulic conduc=vity Fθ represents source and sink terms, at the surface
Fθ = P + QF – R – ET, anthropogenic water QF is the transferred water applied at that loca=on
Underground Fθ can represent leaking pipes D and K are altered in urban areas, usually reduced, and Fθ
has mainly anthropogenic sources
∂θnat∂t
= ∂∂z
D∂θnat∂z
+ K + Fθ⎛⎝⎜
⎞⎠⎟
Slide 16
Changes in Runoff: more intercep=on Slide 17
Frac=on of impervious terrain Slide 18
Changes in runoff: faster surface drainage over smooth asphalt streets
Slide 19
… and compacted soils
Changes in runoff: faster subsurface drainage in storm drainage system
Slide 20
Changes to the Hydrograph Leopold, 1968 Slide 21
Runoff Hydrograph construc=on requires high =me resolu=on (whether with models or measurements) since runoff in urban terrain happens fast
Spa=al variability of P is higher than in natural terrain more rain gages needed, also to catch storm direc=on
Flow rou=ng now is in drainage network (pipes and channels) and overland many models are available, see Bedient and Hubert chapter 6 for a list or see h^p://www.hydrocad.net/tr-‐55.htm
Slide 22
Runoff
Completely impervious : P=R+S+T E=I=0
During storms T<<P, aier some=me in the storm S≈0 P=R
Par=ally impervious, need to take into account E and I
Slide 23
Runoff, as in natural terrain We need to characterize: drainage area, roughness, slope,
land-‐use, soil types, frac=on of impervious surfaces, storage characteris=cs.
Connec=vity of impervious areas is also important. A roof that stores water does not contribute to flood peak.
Simplest ra=onal methods to predict peak flow at last outlet:
Q=C I A (only if flow is at equilibrium) Q = peak flow (m3/s)
C=runoff coefficient ≈ runoff/rainfall (calibra=on parameter, but variable; variability for urban surface is actually lower)
I is rainfall intensity for the design storm (has to be in m/s)
A is catchment area m2
Slide 24
Runoff Coefficient (Bedient and Huber, 1990) Slide 25
Measures to control urban runoff (Bedient and Huber, 1990)
Slide 26
Measures to control urban runoff (Bedient and Huber, 1990)
Slide 27
Porous asphalt Slide 28
Inlet restric=on
Blocking the inlet of storm water drainage pipes to get water to flow on the street instead of in the stormwater drainage network
Useful in combined wastewater/rainwater network because it prevents water from backing up and going out in residences (basement flooding)
Slide 29
Changes in Evapora=on
Impervious surfaces have a small water reten=on capacity, but all water lei there evaporates quickly usually
Evapora=on then occurs from the vegetated/soil/waterbody frac=on of the surface
Ho^er and drier city air enhances evapora=on
Increased turbulence increases evapora=on
Building shading reduces evapora=on
Slide 30
Aerodynamic evapora=on models
Eddy covariance E w qρ ′ ′=
MOST model E =ku*! qs ! q( )
ln z ! dz0v
"
#$%
&'!! v
z ! dL
"#$
%&'
z0v (z010
Bulk model E = Ce!u (qs " qa ) H = Ch!cpu (Ts " Ta ) Which can also be formulated as
8
( )( ) where ( ) ( ) is called the wind function
when u is in m/s, a good function is ( ) 1.25.10e s a eE f u e e f u a bu
f u u−
= − = +
=
Slide 31
Aerodynamic formula=ons
Purely fluid mechanical No considera=on of energy or water budgets
Problema=c if we are relying on mean measurement and just using some turbulence transfer func=on
Great if we have fast sensors that can measure turbulence (eddy covariance)
But what if we are in a model or with slow sensors that cannot capture turbulence?
Slide 32
Energy Budget evapora=on models, more widely used in environmental models
Based on: Rn = H+LE+G Penman for wet surfaces: poten=al E
Ep =Δ
Δ + γQnLv
+ γΔ + γ
EA OR Ep =ΔγHLe
+ EA
Qn = Rn −G
EA = f (u)(ea* − ea ) (e : water vapor pressure, * for saturation)
γ ≈cp p
0.622Le= 67 hPa K−1 is called the psychrometric constant
Δ = de* / dT
Slide 33
Energy Budget evapora=on models
Reduced Penman for unsaturated soils E = βeEp
Slide 34
Brutsaert et al., Hydrology: An Introduc=on, 2005
Energy Budget evapora=on models
Penman-‐Mon=eth for vegeta=on
In urban terrain, Qn, EA, <u> and Ce and rs are all changed
Also historic data of evapora=on parameters runoff, and even rainfall, should be treated with care since urbaniza=on changes the catchment
E = ΔΔ + γ 1+ rsCeu( )
QnLv
⎛
⎝⎜⎞
⎠⎟+ γ
Δ + γ 1+rsρ
⎛⎝⎜
⎞⎠⎟
EA
Slide 35
h^p://www.toolkit.net.au/tools/Aquacycle Slide 36
Urbaniza=on of models, more in the next lecture
Slide 37
Slide 37