The Urban Hydrosphere
Elie Bou-Zeid Princeton University Civil & Environmental Engineering Lecture 6
Slide 2 The Hydrologic Cycle Slide 3 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, ou all, etc. versus me (x-axis) Hyetograph: Plot of Rainfall (y-axis), as a cumula ve volume or a rate, versus me (x-axis) Main water problems related to Slide 4 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 5 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 6 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 7 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 8 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,
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,
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. Water import in Tel Aviv > P (Hoang Slide 12 Duong et al. 2011) Slide 13 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 14 Reten on/Deten on basin Slide 15 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 Simple models: Richards equa ons Slide 16
(diffusivity form) for soil water content θnat ∂θ ∂ ⎛ ∂θ ⎞ nat = D nat + K + F ∂t ∂z ⎝⎜ ∂z θ ⎠⎟ 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 Slide 17 Changes in Runoff: more intercep on Slide 18 Frac on of impervious terrain Changes in runoff: faster surface Slide 19 drainage over smooth asphalt streets … and compacted soils Changes in runoff: faster subsurface Slide 20 drainage in storm drainage system Slide 21 Changes to the Hydrograph Leopold, 1968 Slide 22 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 23 Runoff
Completely impervious : P=R+S+T E=I=0 During storms T< 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 25 Runoff Coefficient (Bedient and Huber, 1990) Measures to control urban runoff Slide 26 (Bedient and Huber, 1990) Measures to control urban runoff Slide 27 (Bedient and Huber, 1990) Slide 28 Porous asphalt Slide 29 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 30 Changes in Evapora on Impervious surfaces have a small water reten on capacity, but all water le 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 31 Aerodynamic evapora on models Eddy covariance Ewq= ρ ′′ ku q q *!( s ! ) z0 MOST model E = z0v ( " z ! d % " z ! d % 10 ln $ ' !! v $ ' # z0v & # L & Bulk model E = C !u(q " q ) H = C !c u(T " T ) e s a h p s a Which can also be formulated as Efuee= esa()(− ) where fue()=+ ( abu )is called the wind function when u is in m/s, a good function is fu ( )= 1.25.10−8 u Slide 32 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? Energy Budget evapora on models, more Slide 33 widely used in environmental models Based on: Rn = H+LE+G Penman for wet surfaces: poten al E Δ Qn γ Δ H Ep = + EA OR Ep = + EA Δ +γ Lv Δ +γ γ Le Qn = Rn − G * EA = f (u)(ea − ea ) (e : water vapor pressure, * for saturation) c p γ ≈ p = 67 hPa K−1 is called the psychrometric constant 0.622 Le Δ = de* / dT Slide 34 Energy Budget evapora on models Reduced Penman for unsaturated soils E = βe Ep Brutsaert et al., Hydrology: An Introduc on, 2005 Slide 35 Energy Budget evapora on models Penman-Mon eth for vegeta on Δ ⎛ Q ⎞ γ E = n + E ⎜ L ⎟ A Δ +γ 1+ rC u ⎝ v ⎠ ⎛ rs ⎞ ( s e ) Δ +γ 1+ ⎝⎜ ρ ⎠⎟ In urban terrain, Qn, EA, 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 Slide 36 h p://www.toolkit.net.au/tools/Aquacycle Urbaniza on of models, more in the Slide 37 next lecture Slide 37