Pezzaniti D.

I-Revues

SESSION 2.1 
NOVATECH 2007  279 
Towards sustainability of waterways in 
urbanising catchments  
Deux stratégies pour améliorer la durabilité des systèmes urbains 
d'assainissement pluvial 
John R Argue and David Pezzaniti 
Urban Water Resources Centre, School of Natural and Built Environments, 
Mawson Lakes Campus, University of South Australia,  
Mawson Lakes, South Australia 5095 
John.argue@unisa.edu.au ; david.pezzaniti@unisa.edu.au 
RESUME 
La rétention est le processus dominant qui caractérise le comportement des 
précipitations/ruissellements d’un bassin versant forestier. Toute tentative de création 
de paysages urbains durables doit inclure des pratiques de rétention aussi larges que 
possible au moyen de jardins sur toiture, de réservoirs d’eau de pluie, de dispositifs 
de goutte à goutte dans le sol, d’accès aux aquifères, etc. Ceci induit un transfert de 
l’égalité de avant-et-après Qpeak (pratique de rétention de développement de sous-
zone) à l’égalité de Runoff Volume à partir de l’élément de sous-zone comme critère 
pour le développement/redéveloppement urbain durable. Cette approche est illustrée 
pour le cas d’une sous-division résidentielle de 60 unités. Ce résultat peut être 
généralisée aux cas de développement de bassins versants forestiers. L’article 
conclut par une revue et un rejet des objections avancées contre l’utilisation des 
pratiques de rétention pour la collecte des eaux pluviales au prétexte de l’impact des 
flux environnementaux et des conséquences des successions d’orages.  
ABSTRACT 
The dominant process that characterises the rainfall/runoff behaviour of a forest 
catchment is retention. Any attempt to create sustainable urban landscapes must 
incorporate retention practices as widely as possible including roof gardens, rainwater 
tanks, in-ground ‘leaky’ devices, access to aquifers, etc. A consequence of this is the 
shift from equality of before-and-after Qpeak (sub-area development detention practice) 
to equality of Runoff Volume from the sub-area element as the criterion for 
sustainable urban development/re-development. This approach is illustrated for the 
case of a 60-unit residential sub-division. The result can be generalised to forest 
catchment development cases. The paper concludes with a review and rebuttal of 
objections raised against retention practice use in storm drainage on the grounds of 
impact on environmental flows and the consequences of storm successions.     
KEYWORDS 
Forest catchment, retention, sustainability, urbanising, waterways
SESSION 2.1 
280 NOVATECH 2007  
INTRODUCTION  
 
When storm rainfall of sufficient magnitude to produce runoff is generated in a forest 
catchment, it passes into creeks and waterways within time period measured, 
typically, in hours: the annual total of such runoff – in temperate zone catchments – is 
likely to be less than 20% of rainfall input. There is a second stream of flow which is  
discharged from the natural catchment, that delivered to waterways as ‘base flow’ 
from aquifers: considered on an annual basis, this stream is comparable in magnitude 
to the surface runoff flow but its delivery period is measured in weeks, months or 
years. These elements of the hydrological performance of a (temperate zone) forest 
catchment are illustrated in Figure 1. 
  
Figure 1: Destiny of 10 units of annual rainfall input in a temperate zone forest catchment 
(notional values) 
 
Primary elements (circled) : 10 units of (annual) rainfall - 2 units are intercepted and 
evaporate; 2 units become surface runoff; 6 units infiltrate. 
 
Secondary elements (boxed) : 1 unit of surface runoff evaporates. 4 units of  
infiltrated water are taken up by tree roots leading to transpiration while the other 2 
units are divided between seepage to streams, lakes and deep aquifers. ½ unit of 
stream/lake water percolates to the deep aquifer.  
 
Outflows to the ocean : surface runoff - 1½ units; aquifer discharge - 1½ units. 
  
The goal of sustainability in the waterways of urban catchments needs to be 
informed by these assessments, in particular, that the dominant hydrological process 
operating in a natural catchment is retention (see Chandler, 2001). Traditional 
stormwater drainage practice has followed an opposite path by seeking to collect and 
dispose of storm runoff  “…as completely and as quickly as possible”.  
 
SESSION 2.1 
NOVATECH 2007  281 
1 DETENTION OR RETENTION ?  
 
The first, significant move away from the ‘rapid removal’ approach to urban storm 
drainage design was the introduction of detention technology (Wright-McLaughlin 
Engineers, 1969). Detention refers to the holding of surface runoff for relatively short 
periods to reduce peak flow rates before releasing it into, typically, constructed 
watercourses which convey the flow downstream to the ocean. The volume of surface 
runoff involved in the temporary (detention) ponding process is relatively unchanged 
by it. The conventional way in which detention technology has been applied in an 
urban catchment is to ensure that -  
 
                    Qpeak flows  from a site before and after development (or re- 
                    development) are equal, in the design storm of critical duration. 
 
Retention of storm runoff  in the context of urban drainage includes procedures and 
schemes whereby stormwater is held for relatively long periods causing it to continue 
in the urban water cycle via domestic use (in-house and outdoors), industrial uses 
and the natural processes of infiltration, percolation, evaporation, transpiration and 
aquifer recharge. It is embodied in the regime-in-balance strategy which requires 
that (Argue, 2004)  - 
 
           Runoff volumes from a site  before and after development (or re- 
          development) are equal, in the design storm of critical duration. 
2 ILLUSTRATION 
 
Consider a hypothetical, 60 lot sub-division in Parramatta, NSW, where each 
developed allotment (1,000 m2), illustrated in Figure 2, is re-developed for dual-
occupancy. Modelling using the catchment-wide critical design storm  (1-hour, ARI, Y 
= 100-years) for which intensity, i = 68.8 mm/h, is carried out on the sub-division.  
Equivalent impervious areas (EIAs) for the two cases are listed in Table 1. 
 
              Figure 2 : Layouts of residential original and re-developed allotments 
 
 
Verge and paved 
carriageway 
EIA = 150 m2 
House 2 
250 m2 
House 1 
250 m2 
P
av
in
g 
30
0 
m
2 
P
er
vi
ou
s 
20
0 
m
2 
Verge and paved 
carriageway 
EIA = 150 m2 
P
er
vi
ou
s 
(T
ot
al
) 5
50
 m
2 
House 
300 m2 
P
er
vi
ou
s 
22
0 
m
2  
P
av
in
g 
15
0 
m
2 
Pervious 
330 m2 
SESSION 2.1 
282 NOVATECH 2007  
Component ORIGINAL SITE RE-DEVELOPED SITE 
Roof area 300 m2 500 m2 
Paved area 150 m2 300 m2 
Pervious area 132 m2 48 m2 
Share of road reserve 150 m2 150 m2 
TOTAL EIA 732 m2 998 m2 
Table 1 : Components of original and re-developed sites (EIAs) 
Runoff volumes -   Original allotment: 732 m2 × 0.069  =  50.5 m3 
Re-developed allotment: 998 m2 × 0.069  =  68.9 m3 
Hence, on-site retention storage volume required per allotment for regime-in-
balance strategy equals (68.9 – 50.5) m3 = 18.4 m3.  Figure 3 shows design storm 
hydrographs for the original and re-developed cases – 
• Single allotments with runoff uncontrolled: Figure 3a ; 
• 60 allotments each ‘lagged’ by one minute: Figure 3b ; 
• Re-developed allotment with 18.4 m3 retention storage: Figure 3c ; 
• 60 re-developed allotments  - 18.4 m3 retention storage each: Figure 3d. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2 : Regime-in-balance strategy applied to 60 residential allotments 
Volume of OSR device 
= 18 4 m3 per site
SESSION 2.1 
NOVATECH 2007  283 
3 DISCUSSION 
 
The prospect of using retention technology as a major element of stormwater 
management planning raises a number of issues. First among these is the  
regime-in-balance strategy itself: does it lead to true sustainability ? Then there is 
the need to know what retention ‘tools’ are available to implement the technology. 
This leads to the question: what are the consequences for environmental flows of 
using retention systems widely in an urban catchment. Finally, there is the issue of 
storm successions: if retention storages are employed in an urbanising catchment, 
how confident can the designer be that they will empty between successions of 
significant storms ? These matters are addressed in the following sub-sections. 
 
3.1 The regime-in-balance strategy 
Comparison of Figures 3b and 3d reveals great similarity in the two surface runoff 
hydrographs for – 
• 60 original allotments with flow uncontrolled (present system); and, 
• 60 re-developed allotments with 18.4 m3 on-site retention storage. 
 
This shows that application of the regime-in-balance strategy leads to almost 
identical surface runoff hydrographs – for the original (developed) and re-developed 
catchment states - under conditions of the design storm of critical duration. 
Corresponding modelling of the transformation from forest or ‘greenfields’ to urban 
development under the same design storm condition produces the same outcome. 
The implications of this are – 
 
1. The main waterways and associated aquatic and terrestrial ecosystems of a 
forest or rural catchment can be substantially sustained in their natural 
states provided all development (outside the defined floodplain) is ordered 
in that catchment under the regime-in-balance strategy; and,  
  
2. The stormwater infrastructure (formal waterways) of a developed catchment 
can continue to function without the need for augmentation provided all 
development (outside the defined floodplain) is ordered in that catchment 
under the regime-in-balance strategy.  
 
Of course, these claims must be qualified: the strategy reviewed above employs one, 
only, design storm condition in its implementation, whereas true sustainability 
demands correspondence being observed over the full range of storm inputs. This 
ideal is sought in some contemporary practices, in particular UK (CIRIA, 2001) and 
the Pacific north-west of North America (Stephens et al, 2002).  
 
“Substantial sustainability” in (1), above, sees the ‘before and after’ correspondence 
delivered by the regime-in-balance strategy as applied to a mid-range design storm 
condition arrived at by consensus among stakeholders in the urbanising catchment. 
This may be the 5-year, 10-year or 20-year event. In relation to the developed 
catchment (formal) stormwater infrastructure – case (2) above – the ‘consensus’ 
frequency is likely to be in the range 20-years to 100-years. 
SESSION 2.1 
284 NOVATECH 2007  
3.2 Retention practices 
There is an extensive range of devices and systems available to the practitioner for 
designing and reaping the benefits of stormwater retention. These include roof 
gardens, green roofs, rainwater tanks; porous and permeable paving; in-ground 
‘leaky’ devices; above-ground and underground devices with aquifer access; and 
above-ground or underground storages with slow-drainage provision, termed 
‘extended detention’.  
 
Numerous papers presented at the ICUD series of conferences (Eriksson et al, 2005)  
and previous Novatech conferences (Chocat et al, 2005)) should be consulted to 
access the literature of the subject as well as national practice/guideline manuals 
such as ATV (1990) – Germany; CIRIA (2001) – UK; Ontario Ministry of the 
Environment (2003) – Canada; and Argue (2004) – Australia.    
 
3.3 Environmental flows 
It is sometimes claimed that widespread inclusion of retention practices in a ‘mixed’ 
catchment with significant urban and remnant (forest) components, will adversely 
affect environmental flows in the natural waterways. In a typical application of the 
regime-in-balance strategy, some stormwater is retained – typically, part of roof-
originating runoff – and the remainder (ground-level runoff) passes downstream. 
Figure 3c, above, illustrates this clearly. While the destiny of the surface runoff 
component is well understood, that of its subterranean counterpart held, temporarily, 
in, say, a ‘leaky’ device, is less certain. What is known is that this component along 
with other products of surface infiltration enters the soil moisture store, is taken up by 
tree roots (leading to transpiration) and provides ‘base flow’ to local streams via 
shallow aquifer routes, as illustrated in Figure 1. 
  
Certainly, the paths taken by the two streams – surface runoff and ‘base flow’ 
reaching local waterways - are very different, as are their journey time scales, but this 
should not disguise the fact that their quantities, considered on a catchment-wide 
basis over the course of an average year, are similar. Retention techniques and 
processes can therefore be employed to ‘mimic’ natural catchment hydrological 
behaviours and, therefore, provide a basis for claiming waterway sustainability.  
 
Environmental flow considerations also affect the scale at which the  
regime-in-balance strategy should be applied in an urbanising catchment. Simple 
application might see ‘before and after’ (surface runoff) volume equality approved on 
a site-by-site basis by a regulatory agency. But this ignores the consequences of 
‘over-supply’ of surface runoff delivered from catchment components such as paved 
carriageways and multi-storey carparks which present little or no opportunity for ‘site’ 
application of the strategy.  
 
It would seem prudent to embrace this aspect of catchment performance at a 
neighbourhood or district rather than site scale in such circumstances. 
Neighbourhood-scale (or district-scale) planning to compensate for ‘over-supply’ 
would see on-site storages on residential allotments or other land use elements 
therefore set at greater magnitudes than required, individually, by the strategy. 
SESSION 2.1 
NOVATECH 2007  285 
3.4  Storm successions 
Another objection often brought against on-site retention of stormwater relates to the 
situation which may occur when a storage, partly drained after a significant storm 
event, is subject to additional input from a new storm following the first in close 
succession. This issue raises two questions of prime importance – 
• How long will a typical, in-ground, ‘leaky’ retention device take to empty 
from full under ‘natural’ (percolation) drainage conditions ? and, 
• What time delay can be expected between successive storms of significant 
magnitude ? 
The first of these questions has been answered for the most commonly used devices 
– ‘leaky’ wells, gravel-filled (and similar) trenches and “soakaway” (mattress-shaped) 
installations - in the following formulae  (Argue, 2004) -   
‘leaky’ wells:         
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
+
−=
4
D
4
D
10
h H
log
k4
D6.4T , s                         (1) 
gravel-filled (or similar) trench :  
                         ⎥
⎦
⎤
⎢
⎣
⎡
+++
−=
)bL(H2Lb
Lblog
)bL(k2
Lbe6.4
T 10
h
s , s             (2) 
 “soakaways” and ‘dry’ ponds:       
h
s
k
e.H2
T ≈ , s                                 (3) 
Where T  =  emptying time in seconds;  D  =  ‘leaky’ well diameter (m); 
kh  =  soil hydraulic conductivity (m/s); L  =  length of trench (m); 
b = width of trench (m);  
HbL
available space void
e s = ; 
H  =  depth of ‘leaky’ well, trench, “soakaway” or ‘dry’ pond. 
The second question requires a set of criteria against which to compare the emptying 
times determined for devices. These are also offered as ‘interim’ values (Table 2) 
which display a semi-log relationship between frequency (ARI) and emptying time 
(Argue, 2004). The values are considered generally conservative by practitioners 
using them in Australia. However, they enable the well-known design storm method to 
be used with confidence to dimension stormwater retention (storage) installations 
required under the regime-in-balance strategy.   
Ave Recurr Interval 
(ARI),  years 
1-year 
or less  
2-
years 
5-
years 
10-
years 
20-
years 
50-
years 
100-
yrs 
Emptying time, T in days 0.5 1.0 1.5 2.0 2.5 3.0 3.5 
TABLE 2 : INTERIM RELATIONSHIP BETWEEN ARI AND ‘EMPTYING TIME’ 
 
SESSION 2.1 
286 NOVATECH 2007  
 
4 CONCLUSION 
The dominant hydrological process which affects the movement of water within a 
forest or natural catchment – from the first point of rainfall contact with its canopy to 
the discharge of ‘base flow’ into streams and the ocean - is retention. Traditional 
urban storm drainage, on the other hand, has sought to collect and dispose of 
stormwater “…as completely and as quickly as possible”. 
Any attempt to create sustainable stormwater systems in urbanising landscapes 
should therefore be directed towards mimicking the processes of nature and  – in the 
case of urban waterways – to adopt retention as the practice of first priority. The 
regime-in-balance strategy reviewed in the paper provides a systematic and 
practical way to implement this approach.  
However, its use must be accompanied by an understanding on the part of 
practitioners that it does not reproduce complete natural catchment waterway 
matching across the full range of flood circumstances as urbanisation takes its 
course.  It is therefore recommended that the strategy be applied to a ‘median’ level 
of flooding appropriate to the catchment and its place in the life of the basin 
community. The adopted level of (flood) risk should result from consensus reached 
among catchment  stakeholders. 
The paper concludes with discussion and resolution of some common ‘misgivings’ 
about retention technology applied in urbanising catchments, in particular, the issues 
of environmental flows and storm successions.   
LIST OF REFERENCES  
 
Argue, J R (2004), WSUD: basic procedures for ‘source control’ of stormwater – a Handbook for 
Australian practice. J R Argue, Editor, Urban Water Resources Centre Univ of South 
Australia, Adelaide, November. ISBN 1 920927 18 2 
ATV (Abwassertechnische Vereingung E V, 1990), Construction and dimensioning of facilities for 
decentralised percolation of non-harmful polluted surface water. Standard ATV – A 138E, 
Hennef, Germany. 
Chandler, R (2001), Turbid relations: managing creeks in an ultra-urban city.  Proc. 4th Novatech 
Conf. on Innovative Technologies in Urban Storm Drainage, Graie, Lyon June. 
Chocat, B, Desbordes, M and Brelot, E (2005),  Novatech Conferences: CD-Rom 1992 – 2004. 
International conferences on sustainable techniques and strategies in urban water 
management, Lyon, France. 
CIRIA (2001), Sustainable urban drainage systems; design manual for England and Wales. 
Construction Industry Research and Information Assoc., Report C522, London.. 
Eriksson, E,  Genc-Fuhrman, H,  Vollertsen, J,  Ledin, A,  Hvitved-Jacobsen, T and Mikkelsen, P 
S (2005), Proceedings 10th International Conference on Urban Drainage: CD-Rom, 
Technical Univ of Denmark and Aalborg University, Copenhagen, Denmark, August. 
Ontario Ministry of the Environment (2003), Stormwater management and design manual. 
Available www.ene.gov.on.ca/envision/gp/4329eindex.htm ISBN 0-7794-2969-9. 
Stephens, K,  Reid, D and Graham, P (2002), Stormwater planning: a guide for British Columbia. 
Greater Vancouver Regional District, Vancouver, BC.   
Wright-McLaughlin Engineers (1969), Urban storm drainage. Criteria manual, Volumes 1 and 2, 
Denver Regional Council of Governments, Denver, USA, March. 
 
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Deux stratégies pour améliorer la durabilité des systèmes urbains d'assainissement pluvial

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Deux stratégies pour améliorer la durabilité des systèmes urbains d'assainissement pluvial

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