Going with the flow

PROFESSOR JOHN R. ARGUE

ABSTRACT:

The dominant hydrological process which takes place in a forested catchment is that of retention.  This is evident from the point of contact which incident rainfall makes with the forest canopy, through the mechanism of infiltration that takes place within the catchment, to the slow-release of water from aquifers to provide base flow for creeks and waterways.  Similar mechanisms can be provided in the developed catchment to mimic the processes of Nature to yield drainage lines that support forest corridors and waterway/terrestrial habitats.  The ‘tools’ available to do this include roof gardens, rainwater tanks and “leaky” in-ground installations.  The linked problem of containment of pollution generated in the urban landscape can also be addressed through use of appropriate devices and systems.  The application of these concepts to two scenarios is explored: firstly, urban development of a natural catchment and, secondly, progressive re-development of an initially over-developed (urban) catchment.  The paper concludes with an overview of the parameters, guidelines, qualifications and criteria needed to ‘engineer’ such systems and produce developed catchment outcomes that show harmony between urbanisation and the maintenance or restoration (cases of over-development) of environmental values in streams and waterway floodplains.

INTRODUCTION

The process of establishing any of our major population concentrations in Australia, commenced with the founding of a settlement centred on a dependable surface water source typically with ready access to or in close proximity to the sea.  An inevitable casualty of this process was the local native vegetation, cleared to create space for the colony and its associated infrastructure of residential accommodation, roads, a market place, public buildings, barracks, etc.  This clearing was extended to fringe areas close to the settlements where a variety of vital agricultural activities – market gardens, livestock paddocks, etc. – was established to provide food for the colonists.

At a later stage of development, government-directed clearing of native vegetation to create space for expanded agricultural production gave rural settlers essential income enabling them to survive and establish their first dwellings and farms.

Whether it has been this process of vegetation clearing in the name of ‘settlement’ and the creation of commercial agriculture that has caused indigenous vegetation to be perceived as “the enemy” in the Australian psyche can only be a matter of speculation.  However, it is certainly true that development of the great bulk of suburban landscapes of our major cities has taken place at the expense of native forests.  To add insult to injury, this sequence has been followed in the cities more often than not by recognition of the need for trees and other vegetation, and satisfying that need by the introduction of exotic lawn, shrub and tree species that are foreign to our climate and to the available water resources of our land.

A parallel scenario of environmental damage can be described in relation to the small streams and minor waterways that in pre-settlement times meandered through valleys and across flood plains on their way to the “bottom lands” and, ultimately, the sea.  In the course of development, these have been converted into concrete drains or networks of stormwater pipes, and the vegetated corridors they once supported – as well as their associated water-based and terrestrial ecosystems – have been lost.

This paper sets out to show how an ‘enlightened’ approach to development can create new urban domains with minimum impact on the waterway-associated elements of any natural landscape – its floodplains, trees and habitats.  But the paper also contributes towards solving the far greater problem of transforming already over-developed urban landscapes into domains where pre-development (waterway) environmental values can become re-established.  The initiative that provides the philosophical base and principles for achieving acceptable outcomes in both of these areas is termed (in Australia) water-sensitive urban design or “WSUD” (stormwater ‘source control’).

THE NATURAL CATCHMENT

In its wild state, the natural catchment is a “…system of drainage paths, creeks and streams well matched to the rainfall/runoff processes operating in that basin.  Generally, less than 20% of storm rainfall on a natural catchment is discharged from it as surface runoff.  This discharge causes a minimum of scour and erosion and, consequently, carries a relatively low sediment load” (Argue, 1986).  The discharge referred to here cannot be described as ‘pollution free’, however it does represent a level of chemical/biological content which is acceptable as a benchmark for evaluating the impacts of later development (Millis, 2002).

There is a second stream of outflow from the natural catchment, that delivered by aquifers into the creeks and waterways as ‘base flow’ long after – usually weeks or months – the causative rainfall event.  The quantity of discharge involved in this outflow is comparable to surface runoff, but its exit from the catchment takes place over a much longer time scale.

The scenario described here is represented in Figure 1 with a main stream providing capacity through its flood plain for the conveyance of all (surface) flood flows and base flows, as well as support for stream-associated indigenous fauna and flora in a plethora of biotic communities and aquatic environments.  The (natural) catchment exhibits a number of processes, apart from surface runoff and aquifer outflow recognised by the hydrologist – interception, detention, evaporation, infiltration, percolation – all of which can be subsumed under the broad heading of storage within the catchment or retention (of incident rainfall).

A further, important characteristic of natural catchments and one intimately linked with retention, is that of response (or ‘lag’) time which is the delay evident between the period of rainfall input and the appearance of a portion of it as a surface runoff flood wave at the point of catchment discharge.  This is a direct consequence of the surface-retardation processes listed above – interception and detention in particular – which results in delayed passage of water flow through the catchment media of trees, understorey, forest floor humus, natural obstacles, surface depressions, etc.

A high proportion of incident rainfall is retained in the catchment initially as soil moisture: the ultimate destiny of this component (of incident rainfall) can be take-up by forest vegetation roots leading to evapotranspiration, or alternatively, deep percolation into aquifer strata and, hence, base flow supply to local streams.

TWO WSUD DEVELOPMENT SCENARIOS

‘Enlightened’ development in a natural or substantially natural catchment – Scenario 1

The most obvious consequence of development in a natural catchment – the conversion of natural space into ‘hard surface’ generating significantly greater surface runoff – is universally recognised.  What is less appreciated and its consequences often misunderstood is the effect that the presence of connected paved area has on catchment ‘lag’ time (see above): reductions to one-third, one-fifth, etc. in this parameter are not uncommon in ‘traditional’ developed catchments.  These two impacts – paved surfacing and reduced ‘lag’ time – account for, between them, the significant increases noted in flood runoff peaks compared to pre-development levels of (surface) outflow.

Where the opportunity presents itself to plan the course of urbanisation in an undeveloped or partly-developed catchment, perhaps on the outskirts of a metropolis, then WSUD techniques (Argue, 2004) can be employed to manage storm runoff in a truly water-sensitive manner.  The primary principle upon which such planning must be based is for :stormwater generated on each site in the developed catchment – above that occurring on the same site in the natural catchment – to be fully retained for, at least, the duration of the ‘lag’ time determined for the natural catchment.

The presence of pollutants of various types in the urban landscape – sediment, phosphorous, nitrogen, hydrocarbon – of course, must also be taken into account in the planning process: but the requirements of this aspect can be integrated into on-site stormwater management practices and/or into ‘final filter’ installations discharging to the urban streams.

The outcome of application of this approach is illustrated in Figure 2 and manifests itself in a developed catchment whose hydrological behaviour, in terms of surface runoff and underground seepage/aquifer processes, resembles that of the original basin or sub-catchment in its undeveloped state.  It follows that the central drainage paths – where these processes have been applied – can exhibit not only hydraulic and hydrological similarity to the original waterways, but also, similarity of ecosystem behaviour – trees and habitats.

Some overseas practices, notably those of North America (Stephens et al, 2002; Gilliard, 2004) and UK (CIRIA, 2001), attempt to reproduce in their sensitively designed urban catchments the same runoff hydrographs as observed in or attributed to the original (natural) catchments, in all incident storms – small, medium and large.  The quest for this goal introduces yet another parameter of importance, that of storm magnitude linked to a flood wave frequency indicator – “once in 2 years”, “once in 10 years”, “once in 100 years”, etc., called ‘average recurrence interval’ or ARI.  Success in achieving true hydrograph correspondence between the natural and developed catchments through the full range of ARIs is certainly an ideal and worthy objective, but one whose attainment in real world circumstances must be viewed with considerable skepticism.

A practical compromise on this issue which delivers an ‘acceptable’ rather than ideal outcome is to base similarity of natural and developed catchment surface runoff performance on the selection of a ‘representative’ or design storm which combines the ‘lag’ time derived for the natural catchment with an adopted ARI of ‘median’ proportions – for example, “once in 5 years”, “once in 10 years”, “once in 20 years”, etc.

It follows that ‘enlightened’, water-sensitive development can be achieved in a natural catchment provided there is :

• retention of all (surface) runoff above that occurring in the natural catchment;
• maintenance of the same ‘lag’ time as occurs in the natural catchment;
• recognition of the need for pollution control installations ‘at-source’ and/or at waterway entry points, and,
• an appropriate ARI adopted leading to ‘acceptable’ runoff similarity (natural/developed catchments).

Recognition of these characteristics in the engineering of developed catchment infrastructure has the potential to deliver urbanisation outcomes that preserve natural waterway drainage paths along with their associated aquatic and terrestrial environments and habitats in a truly sustainable manner.

‘Enlightened’ re-development of an over-developed catchment – Scenario 2

Where ‘over-development’ has occurred in a drainage basin, typically between 40% and 70% or more of the entire catchment is covered by roofs/paving or other relatively impervious surfaces, and runoff is conveyed rapidly by storm­water pipes and channels from points of origin in the catchment to its main stream(s).  The natural creek branches are modified – straightened – in the process of urbanisation and their original alignments used as the basis for a string of concrete pipes and channels, often contained within narrow municipal drainage easements.  Development has been allowed to encroach, frequently, right to the boundaries of these easements.  This process is illustrated in Figure 3 : it has two well-recognised consequences :

• much greater surface runoff generated than occurred in the original, natural catchment; and,
• greatly reduced response (or ‘lag’) time, compared with that of the original catchment.

In the case of over-developed catchments, riparian property owners in the valley “bottom lands” are likely to experience frequent inundation by floodwaters carrying high silt loads and high concentrations of coliform, phosphorous, nitrogen, heavy metals and petroleum hydrocarbons.  Furthermore, because of rapid removal of stormwater from the urban landscape and consequent limited infiltration occurring during runoff events, supplementary watering is commonly undertaken to compensate for low soil moisture levels (after Argue, 1986; see King, 2003).

The task of remediating an over-developed catchment calls for a broad, two-step approach :

STEP 1 : Apply practices throughout the landscape aimed at retaining as much surface runoff as possible from every element of development or re-development taking place in the catchment; this process should be integrated with a strategy to incorporate pollution control installations within all modified sites and/or at points of discharge to urban waterways; and,

STEP 2 : Restore the natural drainage path, as closely as is practicable, to its pre-developed state.

The process of retention (STEP 1) in the present case differs significantly from its counterpart in the natural catchment ‘enlightened’ development process reviewed previously.  Here, it is not sufficient to retain only the storm runoff excess (above that of the natural catchment) on a site-by-site basis (as above), because development/re-development of sites occurs only on an opportunistic rather than ‘planned’ basis.  It is therefore imperative that as much surface runoff as practicable be retained (for, at least, the duration of the ‘lag’ time) in every development/re-development situation that arises in order to ‘make up’ for the numerous sites where no remedial action in the short term is likely.

The greatest difficulty encountered by the practitioner attempting to follow this procedure relates to established roadways.  Techniques exist which can, given total community commitment, lead to retention measures being retrofitted into other urban classifications – residential (individual dwellings, housing clusters, high-rise apartment blocks), industrial sites, commercial/sporting complexes, etc. – as required by ‘enlightened’ re-development.

However, retrofitting retention technology into an established roadway – short of complete or substantial reconstruction using permeable paving – presents, usually, insurmountable difficulties (including cost).

There are other components of the urban form, in addition to roadways, which may present similar difficulties, for example, sealed car park areas, multi-storey car parks, commercial/industrial complexes, etc., where opportunities to modify existing building layouts to incorporate required stormwater retaining installations are severely limited.  Or the cost of installations, were they to be constructed, may be prohibitive.

In circumstances such as these, it is possible to achieve the overall objectives of WSUD (stormwater ‘source control’), by a combination of full retention in those components where this is possible, and limited retention in the others.  The process by which STEP 1 is implemented in an over-developed catchment has much the same basic profile as its earlier counterpart, and is :

• retain as much (surface) runoff as possible at each development/re-development site*;
• maintain the same (catchment-wide) ‘lag’ time determined for the over-developed catchment;
• recognise the need for pollution control installations ‘at-source’ and/or at waterway entry points, and,
• adopt an appropriate flood ARI for the catchment reflecting the consensus interests of its stakeholders.

The case for remediating an over-developed catchment through application of these actions as a first step, together with continuation of the process to embrace STEP 2 (restoration of the natural drainage path), requires high levels of commitment, patience and funding which should not be underestimated by any community setting out to “correct the errors of the past”.  The ultimate outcome of such commitment is transformation of the catchment – illustrated in Figure 3 – towards the situation depicted in Figure 2.  How engineering can be employed to bring about this transformation is explored in the following section.

‘ENGINEERING’ OF WSUD IN DEVELOPING AND OVER-DEVELOPED CATCHMENTS – AN OVERVIEW

It is quite impossible to include in this article a comprehensive explanation of the detailed procedures, strategies, guidelines, criteria and data bases needed to ‘engineer’ practical solutions to the problems posed by developing and over-developed catchments throughout Australia.  Suffice it to say that these procedures, strategies, etc. can be found among over 200 pages of the document “WSUD : basic procedures for ‘source control’ of stormwater – a Handbook for Australian practice” (Argue, 2004).  This section of the paper presents an overview of this material related directly to the concepts discussed in the previous section.

Retention

The earlier content (above) proposed retention of storm surface runoff as the main mechanism for achieving the goal of developed catchment hydrological similarity to natural catchment behaviour.  Further, it also proposed retention as the primary technology for moving an over-developed catchment progressively towards a state where storm runoff could be managed in a truly water-sensitive and sustainable manner.  What ‘tools’ are available to the engineer to achieve these objectives and how can they be used in ‘enlightened’ design?

Before answering these questions, it is necessary to clarify how retention practices differ from the detention techniques currently used throughout Australia to solve problems of flooding.

“Detention refers to the holding of runoff for relatively short periods to reduce peak flow rates and later releasing it into natural or artificial watercourses to continue in the hydrological cycle as channel flow, evaporation, groundwater recharge, input to lakes and the ocean, etc.  The volume of surface runoff involved in the temporary ponding process is relatively unchanged by it.

Retention refers to 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, evapotranspiration, but not, usually, via direct discharge to natural or artificial watercourses” (Argue, 2004).

While detention practices undoubtedly lead to flood (peak) flow decreases in urban waterways, the fact that “…The volume of surface runoff involved in the temporary ponding process is relatively unchanged by it.” renders this technology quite inappropriate as a basis for ‘simulating’ the hydraulic and hydrological behaviour of natural catchments  in an urbanised landscape.  So what are the stormwater retention ‘tools’ available to the engineer?  They fall into two broad classes :

Class 1 :  ‘Simple’ systems receiving runoff from relatively clean surfaces; and,

Class 2 :  Complex systems which include elements designed to filter pollutants of various types – sediment, phosphorous, nitrates, hydrocarbons, etc. – prior to entering in-ground storage.

Class 1 installations include :

• Roof gardens, ‘green’ roofs and roof water storages (rainwater tanks).
• Surface infiltration systems including porous/permeable paving.
• “Leaky” underground devices providing storage and emptying by seepage.
• Above-ground or underground storages with aquifer access and no recovery of water entering aquifers.
• ASR (aquifer storage and recovery) schemes providing recovered water available for various surface uses.

Some basic devices, appropriate to the space limitations of urban residential circumstances, are illustrated in Figure 4.

Class 2 installations are predominantly associated with systems and installations accepting runoff from paved or other ground-level surfaces, for example, courtyards, walkways, driveways, carriageways, car parks, etc. Storm runoff generated on these surfaces must not be passed directly to ‘simple’ devices of the types illustrated in Figure 4. All Class 2 installations should involve two components – some form of filter (upstream), followed by a component capable of storing (and providing additional treatment of) the filtered stormwater prior to discharge by percolation into the surrounding media. This arrangement is illustrated, schematically, in Figure 5.

The range of devices, systems and products available for ‘filter’ duty is massive and growing, ranging from the simplest – grassed swales – through bioretention/biofiltration systems and gross pollution traps, to sand and peat-based cleansing systems. Adequate coverage of this field is beyond the scope of the present article, but the reader is referred to the following for more information – Schueler et al, 1987; Ferguson, 1994; Debo and Reese, 2003; NSW EPA, 1999; CSIRO, 1999; CIRIA, 2001; Ontario Ministry of the Environment, 2003; France, 2003; Argue, 2004; Engineers Australia, 2005.

‘Lag’ time, ARI and (on-site) device storage volume for Class 1 installations

Each of the stormwater retention devices or systems listed or illustrated above involves some element of storage.  The task of computing the magnitude of this storage is determined by its place in Class 1 (‘simple’ systems) or Class 2 (‘complex’ systems), above.  The most obvious choice of device for on-site retention of stormwater is, of course, the rainwater tank: the ‘size’ of rain tank needed for a particular set of (site) circumstances involves knowledge of roof area, demand – daily, seasonal, etc. – and location rainfall (continuous record of rainfall for at least 10 years).  There are a number of available programs under the classification of “stormwater harvesting” to undertake this task.

Design of Class 1 systems for, primarily, surface runoff management – for example, from a domestic roof – where the (storage) device is subject to in-ground percolation, employs the ‘design storm’ approach and requires knowledge of five parameters :

1. Magnitude of the on-site (equivalent) impervious area, A, draining directly to the device;
2. Duration of the ‘critical storm’, T_C, that applies in the entire developed catchment;
3. Average recurrence interval, ARI = Y years, adopted for stormwater management in the catchment;
4. Average rainfall intensity corresponding to storm duration T_C and ARI = Y years, i_Y ; and,
5. Hydraulic conductivity (or permeability) of the host soil.

Application of these values into the appropriate equation(s) yields the dimensions required for the selected device (Argue, 2004).  Item (1) in the above list is readily recognised in the process of site-by-site stormwater management planning.  Item (2) corresponds to the catchment response or ‘lag’ time discussed in earlier sections, and is determined in a particular catchment by a well-recognised engineering modelling procedure (IEAust, 1987) : it is equal, approximately, to the catchment “longest travel time”.  Item (3) arises from consultation with all stakeholders and reflects the level of flooding which the catchment community considers ‘acceptable’: it may take values of Y = 10-years, 20-years, 50-years, etc. : economic considerations may play a major role in the selection of “Y”.  Item (4) is readily available from many sources, in particular, Commonwealth Bureau of Meteorology.  Item (5) requires information obtained from the site (soil test) where the device is to be located. The design process which involves input from these five parameters is called the design storm method, well known to storm drainage engineers, both nationally and internationally.  A vital element of this method is identification of the critical storm [duration, T_C, intensity, i_Y, where Y = adopted average recurrence interval (ARI) in years] brings together information from Items 2, 3 and 4, above.  These data for the particular catchment and site circumstances under review lead to the dimensions of an in-ground device of appropriate size.

It should be noted that ‘appropriate size’ depends on whether development of a natural catchment is being addressed (Scenario 1, above), or whether re-development of an existing catchment (Scenario 2, above) is the primary design concern.  In the case of the former, storage volume is determined as the difference in runoff between the ‘natural’ and developed conditions, as explained earlier.

However, the latter case – re-development in an over-developed catchment – calls for on-site storage to be “…as much…as possible…”  An important qualification (see earlier footnote) must be recognised here.  There is a logical maximum to the available storage volume which should be associated with any element of catchment re-development.  This maximum equals the total volume of runoff delivered to the on-site retention device from that element in the course of the catchment’s critical storm (see above).  Any retained volume greater than this quantity has no influence on the peak flow discharged from the catchment’s (downstream) discharge point.

The size of storage required to retain cleansed effluent from a filter in a Class 2 system is a complex issue which should be solved using ‘continuous simulation’ modelling techniques, similar to those needed to design rainwater tanks (Argue, 2004; Engineers Australia, 2005).

The problem of ‘storm successions’: emptying time in runoff management installations

The design storm method referred to above provides the dimensions of an installation whose primary role is in flood management.  It yields, in a given case, a storage volume which – in keeping with the ‘primary principle’ stated above – must be “… fully retained for, at least, the duration of the ‘lag’ time determined for the natural catchment.”  With the storage thus filled following a ‘design storm’, the designer then faces a new problem : ”how long will emptying (of the storage) take and will there be sufficient time between succeeding storm events to provide space to store the next burst of rainfall?”

These important matters are also addressed in the WSUD (stormwater ‘source control’) literature (Argue, 2004) which includes emptying time formulae for a variety of commonly used installations – gravel-filled trenches, “soakaways” and ‘dry’ ponds – as well as a table of emptying time criteria which relate permissible emptying time to average recurrence interval (ARI).  This table is presented in Table 1:

TABLE 1 INTERIM RELATIONSHIP BETWEEN ARI AND ‘EMPTYING TIME’

CONCLUSION

The paper has briefly reviewed the hydrological processes taking place in a forested catchment and demonstrated that the principal mechanism which explains its response to incident rainfall is that of retention – retention (and evaporation) of rainfall caught in the forest canopy, retention of surface runoff on the forest floor, and storage of infiltrated water within the soil mass.  Retention is also demonstrated in the slow-release of seepage water to catchment streams and minor waterways from aquifers.

It is also demonstrated that similar mechanisms can be provided in the developed catchment through application of the principles and technology of water-sensitive urban design (WSUD stormwater ‘source control’).  This can be achieved through widespread use of devices such as roof gardens, rainwater tanks and “leaky” in-ground installations designed to ‘mimic’ the overall behaviour of natural catchment elements not only in the surface domain but also at depth – aquifer/waterway interaction.  The linked problem of pollution containment can also be addressed in the developed catchment through judicious use and placement of appropriate devices and systems.

The consequence of these undertakings is the opportunity to support and sustain the principal features of the natural catchment – its minor streams and their associated forest corridors and waterway/terrestrial habitats – in harmony with typical urban development.

The more difficult problem of moving an over-developed catchment towards a state of harmony between development and high environmental values is also addressed.  The same principles of retention of surface runoff are invoked to achieve this goal but, it is stressed, the time-scale is likely to be very long and the requirements of commitment and finance on the part of the catchment community, likely to be very demanding.

The paper concludes with an overview of the ‘engineering’ that is required to achieve the outcomes reviewed above, in particular :

• Retention storages – what devices and systems are available?
• How can the size of a selected device be determined?
• How can the problem of (close) ‘storm successions’ be solved?

The paper reflects – overall – a positive outlook for future development in urbanising or already developed catchments, provided the planning of change is undertaken with water-sensitive design principles such as those of WSUD (stormwater ‘source control’) firmly in mind.  The ultimate consequence of this approach, if executed with administrative commitment and widespread community support, can be urban catchments which display balance and harmony between the demands of residential and commercial/industrial development and sustainable high quality urban stream environments and their associated corridor forests and waterway/terrestrial habitats.

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