Claeys, P.

De Bondt, K.

Petrucci, G.

I-Revues

NOVATECH 2016 
1 
The role of natural ponds in controlling the urban 
runoff regime 
Le rôle des étangs dans le contrôle du ruissellement 
urbain 
 
Guido Petrucci, Kevin De Bondt, Philippe Claeys  
 
Analytical, environmental and geo-chemistry (AMGC), Vrije Universiteit Brussel 
(VUB), Pleinlaan 2, 1050 Brussels, Belgium 
guido.petrucci@vub.ac.be 
 
RÉSUMÉ 
L’urbanisation, au-delà de l’augmentation des surfaces imperméables, entraine souvent une réduction 
des étangs et des zones humides naturellement présents le long des cours d’eau. Ce processus, peu 
étudié, contribue à déterminer les modifications du régime hydrologique des bassins versants en 
réduisant la capacité de stockage et de rétention du ruissellement. Sur la base de données issues de 
sept points de mesures dans un bassin versant (50 km²) de la Région de Bruxelles, cette 
communication présente une comparaison entre le régime hydrologique de zones boisées et 
urbaines. Ensuite, par la simulation d’une série de scénarios représentant la croissance de 
l’imperméabilisation et la disparition des étangs dans un sous-bassin versant actuellement boisé, le 
rôle respectif de ces deux processus associés à l’urbanisation est évalué. Le résultat principal est que, 
si les deux processus ont des effets comparables, seule leur combinaison permet d’expliquer le 
passage du régime naturel au régime urbain observé.  
 
ABSTRACT 
Urbanization, beside an increase in impervious cover, is often associated with a reduction of natural 
ponds and wetlands along streams. This process is poorly studied in urban hydrology literature, but 
contributes to modify the hydrological regime of urbanizing catchments by reducing the storage 
capacity available to retain runoff. This communication presents a research using data from seven 
gauging stations in a catchment (50 km²) of the Brussels Region. First, these data are used to 
compare the hydrological regime of urbanized and forested catchments. Then, a set of scenarios are 
simulated over a forested subcatchment. Scenarios include an increase in the impervious cover of the 
catchment and the removal of existing ponds, in order to test the respective role of each of these 
processes. The main result is that, even if the two processes have similar effects, only the combination 
of both allows explaining the shift from the observed natural regime to the urban one. 
 
KEYWORDS 
Flow-duration curves, Hydrologic regime, Ponds, Urbanization, Urban runoff 
SESSION 
2 
1 INTRODUCTION 
The impact of urbanization on runoff regime is extensively studied since several decades (Leopold, 
1968; Fletcher et al., 2013). Urbanization affects both runoff peaks, responsible of flooding, and the 
whole regime of low flows, with consequences on the stability and ecology of downstream water 
bodies (Walsh et al., 2005). Several processes associated with urbanization modify the runoff regime. 
Among them, the most studied and known is the sealing of the natural landscape with impervious 
surfaces (Jacobson, 2011), reducing infiltration and increasing surface runoff. Another aspect 
sometimes mentioned is the increased smoothness of urban surfaces and artificial drainage, 
accelerating runoff (Salvadore et al., 2015). A third process, poorly considered by urban hydrology 
literature, is the removal of natural ponds and wetlands (Fig. 1). Natural ponds play a role of “buffer” of 
the runoff regime, thanks to their storage capacity and the exchanges with groundwater. These ponds 
are often filled and covered during the urbanization processes, reducing this buffer effect. Artificial 
ponds and reservoirs are considered as a flood-reduction measure, but their functioning is not always 
the same as natural ones: they are often built on a by-pass of the normal water flow in order to fill only 
during heavy storms and they can be isolated from groundwater. 
 
Figure 1 - Example of ponds removal with urbanization in Brussels 
This research studies the role of natural ponds on the runoff regime of urban catchments. The case-
study used, the Woluwe catchment, is a part of the Brussels Capital Region, including several gauged 
subcatchments, both natural and urbanized. The methodology consists in two steps. The first, based 
on data-analysis, aims at the identification of the runoff regime of urban and natural subcatchments. 
The second, based on modelling and scenario analysis, is used to distinguish the role of 
imperviousness and natural pond filling in modifying the runoff regime of the natural catchments. 
2 METHODOLOGY 
2.1 Case study: the Woluwe catchment 
The Woluwe is a small river flowing in the East of the Brussel Capital Region (Fig. 2). Its catchment 
covers roughly 50 km²: 30 km² of urban areas (average imperviousness of about 20%) and the 
remaining 20 km² covered by a forest. The drainage system is constituted by the river, collecting water 
from the forested area and from some parks in the valley, and by a combined sewer draining water 
from the urban parts of the catchment. About 40 combined sewer overflows (CSO) link the two 
systems. The natural drainage system includes several large ponds (although less than in the pristine 
state), both in the valley bottom and in the upstream subcatchments. The gauging network includes 
two stations on the river, two on its tributaries and three on the sewer. Discharge data used, available 
on the Flowbru website, covers different periods ranging between 2 and 12 years. 
2.2 Data analysis and modelling 
Data from the seven gauging stations are used to produce flow-duration curves (FDC, Vogel and 
Fennessey, 1994; Petrucci et al., 2014). For gauges where only 2 years of data are available, the FDC 
is computed over the complete duration. For the others, a median annual FDC was computed. 
To assess the respective role of imperviousness and pond filling, a detailed model of one of the 
NOVATECH 2016 
3 
forested subcatchments was realized (RKB). The model used is SWMM, calibrated over one year of 
5’ time-step data using a genetic algorithm (Petrucci and Bonhomme, 2014). Once the model 
calibrated on the actual conditions (forested, with ponds), three scenarios were simulated and 
compared to the case of WMB, a subcatchment with similar characteristics except for urbanization: 
A) increase in imperviousness of 30% (value of the WMB catchment); B) filling of the existing ponds; 
C) combination of 30% imperviousness and filling of the ponds. 
 
Figure 2 – Map and scheme of the Woluwe catchment in the Brussels region. In the map: grey lines are the limits 
of the urban gauged subcatchments; black dots are the gauging stations. Green lines are the limits of the natural 
gauged subcatchments, with gauging stations marked by green triangles. Labels are the codes of the gauging 
stations, as in the scheme and in Fig. 3. 
3 RESULTS AND KEY FINDINGS 
Results of data analysis for the seven stations are synthesized in Fig. 3. The distinction between the 
natural and urban catchments in two groups of curves is evident. Natural catchments have extremely 
stable discharges, always ranging between 0.02 and 0.3 L/s/ha (note: BOI station is downstream of a 
sewer overflow, so its relatively high discharges at low frequencies can be due to interactions with the 
sewer). Conversely, urban catchments discharge ranges between 0.03 and 8 L/s/ha, spanning over 
two orders of magnitude. The highest variability occurs during wet periods (frequency < 5%), while in 
dry periods low flows are represented by waste water. 
 
Figure 3 – Flow duration curves for the seven gauging stations 
SESSION 
4 
Results of scenario simulation are presented in Fig. 4. Two “extreme” references are provided: the 
green FDC, extremely flat over the year, represents the natural conditions with forest and ponds; the 
black FDC represents, conversely, data from the WMB catchment, close and geographically similar to 
RKB, but urbanized (30% imperviousness, no ponds but a stormwater reservoir for flood prevention). 
In terms of low flows, the three scenarios present a strong reduction in discharge, which in the real 
catchment is compensated by the presence of waste water. With 30% imperviousness, discharge at 
the outlet is present for 70% of the year (100% in the pristine state), without ponds for 55%; the 
combined effect is that water flows at the outlet only 16% of the time.  
In terms of high flows the filling of ponds has a smaller effect than the increase of imperviousness: for 
a 1 ‰ frequency, discharge increases from 0.15 to 0.6 L/s/ha without ponds and to 1.1 L/s/ha for 30% 
imperviousness. But the interesting result is that the combination of the two modifications produces a 
curve for wet periods that is extremely close to that of the real urban catchment (1 ‰ discharge is 4 
L/s/ha). Apparently, neither the ponds nor imperviousness can, alone, explain the runoff regime of the 
urban catchment, but the two together do it extremely well. 
Moreover, considering both low and high flows, the combined effect of ponds removal and 
imperviousness is larger than the sum of the two effects considered separately. The reduction of the 
buffer effects of ponds enhances the impact of increased urbanization, and vice versa. 
10
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RKB 2008-2014 - Data
RKB 2008-2014 - No ponds
RKB 2008-2014 - +30% impervious
RKB 2008-2014 - +30% imp. & no ponds
WMB 2013-2014 - Data / urban
 
Figure 4 – Flow duration curves for the urbanization scenarios of the RKB catchment 
 
LIST OF REFERENCES 
Fletcher T. D., Andrieu H., Hamel P. (2013). Understanding, management and modelling of urban hydrology and 
its consequences for receiving waters: A state of the art. Advances in Water Resources, Volume 51, 261-279. 
Jacobson C.R. (2011). Identification and quantification of the hydrological impacts of imperviousness in urban 
catchments: A review. Journal of Environmental Management 92(6), 1438-1448. 
Leopold L. B. (1968). Hydrology for urban land planning: A guidebook on the hydrologic effects of urban land use. 
US Government Printing Office. 
Petrucci G., Bonhomme C. (2014). The dilemma of spatial representation for urban hydrology semi-distributed 
modelling: trade-offs among complexity, calibration and geographical data. Journal of Hydrology 517, p. 997-
1007. 
Petrucci G., Rodriguez F., Deroubaix J.-F., Tassin B. (2014). Linking the management of urban watersheds with 
the impacts on the receiving water bodies: the use of flow duration curves. Water Science and Technology, 
70.1, 127-135. 
Salvadore E., Bronders J., Batelaan O. (2015). Hydrological modelling of urbanized catchments: A review and 
future directions. Journal of Hydrology, 529, 62-81. 
Vogel R. M. and Fennessey N. M. 1994 Flow-Duration Curves. I: new interpretation and confidence intervals. 
Journal of the American Water Resources Association 120(4), 485-504. 
Walsh C. J., Roy A. H., Feminella J. W., et al. (2005). The urban stream syndrome: current knowledge and the 
search for a cure. Journal of the North American Benthological Society, 24(3), 706-723. 


Le rôle des étangs dans le contrôle du ruissellement urbain

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Le rôle des étangs dans le contrôle du ruissellement urbain

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