Urban Hydrology and Meteorological Radars

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Urban Hydrology and Meteorological Radars

• Prof. Michel Desbordes
• PolytechMontpellier
• Montpellier University

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Urban Hydrology

• Urban Hydrology is a scientific and technical
  discipline devoted to the study of relationships
  between urbanization and the natural water cycle.
  It leads
• - to scientific studies related to the effects
  of urbanization on weather and storms, on
  atmospheric exchanges, on environmetal and
  sanitary impacts, etc
• - to technical studies in order to control urban
  runoff drainage, flooding risk, impacts on
  receiving waters bodies, etc

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Urban Hydrology (2)

• At the end of 19th century, in major european and
  american cities, engineers have tried to estimate
  the runoff carried to sewer systems. A model has
  been widely chosen the rational formula
• Q k . im (tc) . C. A
• Q being the peak runoff created by an upstream
  area A, with a concentration time tc and a
  runoff coefficient C, im being the mean rainfall
  over A and during tc (Kuilching, Chicago, 1889
  Llyod-Davies, London 1906 in France Belgrand for
  Paris gave 40 l/(s.ha))

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Urban Hydrology (3)

• Two main questions when applying the rational
  formula
• - how can tc be estimated?
• - what is im?
• For tc in anglosaxon countries formulae based
  on flow velocity in sewers in France formulae
  based on sewers structure analysis (Caquot, 1941
  Desbordes, 1974, 1984)
• For im very few raingauge networks. General
  formula
• im a imax with a A-e and imax the maximum
  rainfall over A. Later e f(tc , T) with T
  return period of the design storm.

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Urban Hydrology (4)

• Now, considering im means that the
  rainfall-runoff relationship is linear. So
  appears the so called generalized rational
  formula or isochrone curves method which is a
  linear distributed model (Larrieu, 1954)
• Q(t) ?t?x,y i(x,y,t) . C(x,y) . dA(x,y,t)/dt .
  dx.dy.dt
• with dA/dt the time-area concentration curve. So,
  i (x,y,t) must be known.

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Urban Hydrology (5)

• Research at the end of the 60 s has shown that
  the rational formula was related to the kinematic
  wave hydrodynamic model (Eagleson, MIT, 1969)
  which means that the rational formula is not
  linear and that the spatial distribution of
  rainfall must be known when applying the formula.
• 1970 Developement of raingauge networks in urban
  areas. METROMEX (Métropolitan Meteorological
  Experiment) 225 rain gauges over Saint-Louis
  (USA) with Radar measurements in 1980 (Braham,
  1981). In France Seine-Saint-Denis Department at
  the beginning of the 70s, then Bordeaux, Lyon,
  Nancy, Marseille, etc(Blanchet, 1993)

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Meteorological Radars

• Dynamic sampling of rain fields, rainfall
  intensities estimates through calibrations using
  rain gauges networks. Aims in Urban Hydology
• - testing of distributed rainfall-runoff models
  performances, of non linearity of these models,
  of the adequacy of design linear models (such as
  linear
• reservoir model (Desbordes, 1974))
• - looking for real time control of sewerage
  systems (Frerot, 1987) in order to prevent
  environmental impacts or local floodings (CSO,
  runoff storage, etc)

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Meteorological Radars (2)

• - predict urban runoff flooding risk, improve
  flooding crisis management, and post crisis
  analysis (ESPADA concept in Nîmes, Marseille,
  etc)
• Theoretically the radar reflectivity Z is
  related to rainfall R by Z a Rb with a 200
  and b 1,6.
• But, under intense rainfall, attenuation can
  reach up to 90 so that rainfall intensity
  estimate is less than 50 that measured by
  gauges. This implies the radar calibration with
  another measure of rainfall intensity.

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Meteorological Radars (3)

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

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Meteorological Radars (4)

• Use of multiparameters radars (such as dual
  polarity) (1990) in order to discriminate between
  rain, hail, snow, ice, birds, etc
• Use of Doppler radars in order to explore wind
  fields and improve numerical weather prediction
  (NWP) models (G.B. 2007)

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Meteorological Radars (5)

• Today, after 30 years
• - a 10 to 20 uncertainty on radar measurement
  implies a 25 to 50 uncertainty on flow
  estimates (Bristol University, SHE/MIKE 11 model,
  fully physically based and
  distributed)
• - lowering the spatial resolution from 5 to 1 km
  introduces a stroboscopic effect under strong
  winds

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Meteorological Radars (6)

• Research needs for Urban Hydrology (according to
  Inter Agency Committee on Hydrological Uses of
  Weather Radar, UK, 2010 more than 600 referenced
  publications covering the past 30 years)
• http//www.iac.rl.ac.uk/
• - is a 5 resolution time sufficient or should
  it be lowered to 2,5 for urban hydrology?
• - should the spatial resolution be lowered from
  1 km to 500 m?

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Meteorological Radars (7)

• - is the dual polarity radar able to improve the
  estimates of rainfal intensity of heavy storms?
• - are the radar data able to improve the spatial
  interpolation techniques of rainfalls as measured
  by rain gauge networks?
• - is it possible to give better estimates of
  uncertainties related to rainfall data as given
  by radar measurements?

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Meteorological Radars (8)

• - can historical heavy rainfall data given by
  radars be used to study the behaviour of storm
  drainage systems of major cities?
• - can radar data related to extreme rainfall
  events, which have lead to severe damages in
  areas with very few or no rain gauges, be used
  for post crisis analysis or evidence of damages
  origin?