Urban Hydrology and
Meteorological Radars
Prof. Michel Desbordes
Polytech’Montpellier
Montpellier University
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…
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))
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 = α imax with α = A-ε and imax the maximum rainfall over A. Later
ε = f(tc , T) with T return period of the design storm.
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.
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 70’s, then Bordeaux, Lyon, Nancy,
Marseille, etc…(Blanchet, 1993)
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…)
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.
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.
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)
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
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?
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?
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?