veolia eau : urban hydrology and meteorological radars - by michel desbordes

Urban Hydrology and Urban Hydrology and Meteorological RadarsMeteorological Radars

Prof. Michel DesbordesProf. Michel Desbordes

Polytech’MontpellierPolytech’Montpellier

Montpellier UniversityMontpellier University

Urban HydrologyUrban Hydrology

Urban Hydrology is a scientific and technical Urban Hydrology is a scientific and technical discipline devoted to the study of discipline devoted to the study of relationships between urbanization and the relationships between urbanization and the natural water cycle. It leads:natural water cycle. It leads:

- to scientific studies related to the effects of - to scientific studies related to the effects of urbanization on weather and storms, on urbanization on weather and storms, on atmospheric exchanges, on environmetal and atmospheric exchanges, on environmetal and sanitary impacts, etc…sanitary impacts, etc…

- to technical studies in order to control - to technical studies in order to control urban runoff drainage, flooding risk, impacts urban runoff drainage, flooding risk, impacts on receiving waters bodies, etc…on receiving waters bodies, etc…

Urban Hydrology (2)Urban Hydrology (2)

At the end of 19At the end of 19thth century, in major european century, in major european and american cities, engineers have tried to and american cities, engineers have tried to estimate the runoff carried to sewer systems. estimate the runoff carried to sewer systems. A model has been widely chosen: the « A model has been widely chosen: the « rational formula »:rational formula »:

Q = k . iQ = k . im m (t(tcc) . C. A) . C. AQ being the peak runoff created by an upstream Q being the peak runoff created by an upstream

area A, with a « concentration time » tarea A, with a « concentration time » tcc and a and a runoff coefficient C, irunoff coefficient C, imm being the being the meanmean rainfall rainfall over A and during tover A and during tcc (Kuilching, Chicago, (Kuilching, Chicago, 1889; Llyod-Davies, London 1906; in France 1889; Llyod-Davies, London 1906; in France Belgrand for Paris gave 40 l/(s.ha))Belgrand for Paris gave 40 l/(s.ha))


Two main questions when applying the rational formula:Two main questions when applying the rational formula:

- how can t- how can tcc be estimated? be estimated?

- what is i- what is imm??

For tFor tcc : in anglosaxon countries: formulae based on flow : in anglosaxon countries: formulae based on flow velocity in sewers; in France: formulae based on sewers velocity in sewers; in France: formulae based on sewers structure analysis (Caquot, 1941; Desbordes, 1974, structure analysis (Caquot, 1941; Desbordes, 1974, 1984)1984)

For iFor imm : very few raingauge networks. General formula: : very few raingauge networks. General formula:

iimm = = αα i imaxmax with with αα = A = A--εε and iand imaxmax the maximum rainfall over the maximum rainfall over A. Later A. Later εε = f(t = f(tc c , T) with T return period of the design , T) with T return period of the design storm.storm.


Now, considering iNow, considering imm means that the rainfall- means that the rainfall-runoff relationship is runoff relationship is linearlinear. So appears the . So appears the so called generalized rational formula or « so called generalized rational formula or « isochrone curves » method which is a linear isochrone curves » method which is a linear distributed model (Larrieu, 1954):distributed model (Larrieu, 1954):

Q(t) = Q(t) = ∫∫tt∫∫x,yx,y i(x,y,t) . C(x,y) . dA(x,y,t)/dt . i(x,y,t) . C(x,y) . dA(x,y,t)/dt . dx.dy.dtdx.dy.dt

with dA/dt the time-area concentration curve. with dA/dt the time-area concentration curve. So, So, i (x,y,t) must be known.i (x,y,t) must be known.


Research at the end of the 60’ s has shown that the Research at the end of the 60’ s has shown that the rational formula was related to the kinematic wave rational formula was related to the kinematic wave hydrodynamic model (Eagleson, MIT, 1969) which hydrodynamic model (Eagleson, MIT, 1969) which means that the rational formula means that the rational formula is not linearis not linear and and that that the spatial distribution of rainfallthe spatial distribution of rainfall must be must be knownknown when applying the formula. when applying the formula.

1970: Developement of raingauge networks in urban 1970: Developement of raingauge networks in urban areas. METROMEX (Métropolitan Meteorological areas. METROMEX (Métropolitan Meteorological Experiment): 225 rain gauges over Saint-Louis (USA) Experiment): 225 rain gauges over Saint-Louis (USA) with Radar measurements in 1980 (Braham, 1981). with Radar measurements in 1980 (Braham, 1981). In France Seine-Saint-Denis Department at the In France Seine-Saint-Denis Department at the beginning of the 70’s, then Bordeaux, Lyon, Nancy, beginning of the 70’s, then Bordeaux, Lyon, Nancy, Marseille, etc…(Blanchet, 1993)Marseille, etc…(Blanchet, 1993)

Meteorological RadarsMeteorological Radars

Dynamic sampling of rain fields, rainfall intensities Dynamic sampling of rain fields, rainfall intensities estimates through calibrations using rain gauges estimates through calibrations using rain gauges networks. Aims in Urban Hydology:networks. Aims in Urban Hydology:

- testing of distributed rainfall-runoff models - testing of distributed rainfall-runoff models performances, of non linearity of these models, of performances, of non linearity of these models, of the adequacy of design linear models (such as the adequacy of design linear models (such as linear linear

« reservoir » model (Desbordes, 1974))« reservoir » model (Desbordes, 1974))

- looking for real time control of sewerage - looking for real time control of sewerage systems (Frerot, 1987) in order to prevent systems (Frerot, 1987) in order to prevent environmental impacts or local floodings (CSO, environmental impacts or local floodings (CSO, runoff storage, etc…)runoff storage, etc…)

Meteorological Radars (2)Meteorological Radars (2)

- predict urban runoff flooding risk, improve - predict urban runoff flooding risk, improve flooding crisis management, and post crisis flooding crisis management, and post crisis analysis (ESPADA concept in Nîmes, Marseille, analysis (ESPADA concept in Nîmes, Marseille, etc…)etc…)

Theoretically the radar reflectivity Z is related to Theoretically the radar reflectivity Z is related to rainfall R by: Z = a Rrainfall R by: Z = a Rbb with a ≈ 200 and b ≈ 1,6. with a ≈ 200 and b ≈ 1,6.But, under intense rainfall, attenuation can But, under intense rainfall, attenuation can reach up to 90% so that rainfall intensity reach up to 90% so that rainfall intensity estimate is less than 50% that measured by estimate is less than 50% that measured by gauges. This implies the radar calibration with gauges. This implies the radar calibration with another measure of rainfall intensity.another measure of rainfall intensity.


An approach to correct for such an attenuation An approach to correct for such an attenuation has been first proposed in 2001 by F. Fabry has been first proposed in 2001 by F. Fabry (McGill univ., Canada): rainfall drops of heavy (McGill univ., Canada): rainfall drops of heavy storms under radar signal must, in turn, emit storms under radar signal must, in turn, emit radiation at same frequency. These microwaves radiation at same frequency. These microwaves emissions look like an increase in measured « emissions look like an increase in measured « noise » at far range, i.e. behind the storm. As noise » at far range, i.e. behind the storm. As noise levels seem to be relatively stable at each noise levels seem to be relatively stable at each radar site during dry weather, one can think radar site during dry weather, one can think that it may be possible to estimate noise that it may be possible to estimate noise changes during heavy storms and to relate changes during heavy storms and to relate them to attenuation in order to correct the them to attenuation in order to correct the radar data in term of rainfall intensity.radar data in term of rainfall intensity.


Use of multiparameters radars (such as dual Use of multiparameters radars (such as dual polarity) (1990) in order to discriminate polarity) (1990) in order to discriminate between rain, hail, snow, ice, birds, etc…between rain, hail, snow, ice, birds, etc…

Use of Doppler radars in order to explore Use of Doppler radars in order to explore wind fields and improve numerical wind fields and improve numerical weather prediction (NWP) models (G.B. weather prediction (NWP) models (G.B. 2007)2007)


Today, after 30 years:Today, after 30 years:

- a 10 to 20 % uncertainty on radar - a 10 to 20 % uncertainty on radar measurement implies a 25 to 50 % measurement implies a 25 to 50 % uncertainty on flow estimates (Bristol uncertainty on flow estimates (Bristol University, SHE/MIKE 11 model, fully University, SHE/MIKE 11 model, fully « physically » based and distributed) « physically » based and distributed)

- lowering the spatial resolution from 5 to - lowering the spatial resolution from 5 to 1 km introduces a stroboscopic effect 1 km introduces a stroboscopic effect under strong windsunder strong winds


Research needs for Urban Hydrology Research needs for Urban Hydrology (according to Inter Agency Committee on (according to Inter Agency Committee on Hydrological Uses of Weather Radar, UK, Hydrological Uses of Weather Radar, UK, 2010; more than 600 referenced 2010; more than 600 referenced publications covering the past 30 years) publications covering the past 30 years) http://www.iac.rl.ac.uk/http://www.iac.rl.ac.uk/

- is a 5’ resolution time sufficient or should - is a 5’ resolution time sufficient or should it be lowered to 2,5’ for urban hydrology?it be lowered to 2,5’ for urban hydrology?- should the spatial resolution be lowered - should the spatial resolution be lowered from 1 km to 500 m?from 1 km to 500 m?


- is the dual polarity radar able to improve the - is the dual polarity radar able to improve the estimates of rainfal intensity of heavy storms?estimates of rainfal intensity of heavy storms?

- are the radar data able to improve the spatial - are the radar data able to improve the spatial interpolation techniques of rainfalls as interpolation techniques of rainfalls as measured by rain gauge networks?measured by rain gauge networks?

- is it possible to give better estimates of - is it possible to give better estimates of uncertainties related to rainfall data as given uncertainties related to rainfall data as given by radar measurements?by radar measurements?


- can historical heavy rainfall data given - can historical heavy rainfall data given by radars be used to study the behaviour by radars be used to study the behaviour of storm drainage systems of major cities?of storm drainage systems of major cities?

- can radar data related to extreme - can radar data related to extreme rainfall events, which have lead to severe rainfall events, which have lead to severe damages in areas with very few or no rain damages in areas with very few or no rain gauges, be used for post crisis analysis or gauges, be used for post crisis analysis or evidence of damages origin?evidence of damages origin?

veolia eau : urban hydrology and meteorological radars - by michel desbordes

Technology