September 2016 - Sustainable Urban Drainage Systems (SuDS)


Today I am writing about the integration of water and nature in the urban context and in particular, about the bioretention systems in urban environments.

This topic is so broad and interesting from the point of view of landscape architecture that I have found it difficult to summarise this entry. So, in order to make it more readable, there will be a second entry on this topic following this one.

In this first entry I would like to do a general introduction about the sustainable urban drainage systems (SuDS) and in the next entry I will discuss some examples of their use in New Zealand.

I hope you enjoy the reading!


With the climate changing, it seems evidently that extreme meteorological events are more frequent. Flooding and flash floods go hand in hand with periods of drought.

Children swimming in the improvised pool this school playground turned into in the 2011 floods in Nelson and Tasman. I thought they are a great representation of the Kiwi spirit to keep a good sense of humour despite the adversities.
Photo from

When urban environments are hit by one of these events, the enormous amount of run off produced can cause physical, economical and social losses to a great scale. In many occasions, the current drainage systems get saturated, become overloaded and incapable of fronting up to the enormity of these precipitations.

In coastal cities like Nelson, the coincidence of high intensity of rainfall with high tides put a stress on the current drainage network which becomes a threat due to its incapacity to discharge the excess run off.

These events have promoted the popularity of an increasing global movement towards a more ecological and sustainable way to understand our cities. From this, the SuDS (Sustainable Urban Drainage Systems) were born. They are part of an urbanism with a deeper connection between nature and city.

In many instances, they incorporate nature by means of increasing the amount of green areas, with wetlands, green roofs, green walls, rain gardens, bioretention systems, vegetated swales, etc. For a good performance, the SuDS need to be designed specifically for each situation and at the same time, they need to work at a greater scale to achieve a better efficiency of the drainage system (they should not be aimed to provide isolated solutions. The designer needs to keep the big picture in mind.)

The introduction of a greater amount of vegetation in urban areas has additional benefits, such as the beautification of streets, the improvement of the environmental quality and the rapprochement of the population to nature, sometimes it even has managed to increase the biodiversity of some areas.

SuDS: Bioretention systems, rain gardens and vegetated swales

There seems to be some confusion around these terms and many times they are used indistinctively. In some countries, such as United States there is a clear distinction made between these three systems. However, in most of the documentation I have found in New Zealand, the term rain gardens is used in a general manner for these three systems.

I have found interesting the definitions used in The United States. The three systems are aimed at the intake of water with the objective to reduce the runoff flow and improve its quality but there are some slight differences. I have summarised them below for your information:

Rain Gardens
These gardens tend to be used in residential areas and they are usually smaller than the bioswales. They are planted on the existing soil, which if needed, can be improved with sand or compost.
Bioretention systems
These are rain gardens with underground drainage. They usually have a gravel layer to help with infiltration and a buried perforated pipe.
These are swales designed to cope with a specific amount of runoff from an impervious area. As they usually need to accommodate big quantities of water, their ground needs to be designed in a similar manner to the bioretention systems. They need to be deeper than the rain gardens. They are also longer and wider than these. The main difference with the bioretention systems is that the bioswales are usually constructed along roads or parking lots.

These systems treat the water runoff by infiltration, evaporation and transpiration with the objective of reducing, collecting and filtering the water. They are an efficient and economical solution and, as mentioned above, they also have environmental and social benefits. Moreover, they are capable of absorbing and retaining pollutants, heavy metals and pathogens present in the runoff. As time passes and their vegetation matures, they become more effective.

There are various studies that have evaluated the effects of the vegetation in the elimination of pollutants. For your information, I would like to mention that according to EPA 1999, the pollutants are reduced as follows:

• 81% Total suspended solids
• 67% Oxygen demanding substances
• 38% Nitrate
• 9% Phosphorus
• 62% Hydrocarbons
• 42% Cadmium
• 51% Copper
• 67% Lead
• 71% Zinc

But I would like to reiterate that this data is only indicative because the capability to eliminate pollutants is dependant on various factors such as the environment, the design of the system and the context.

As this blog entry is written from a New Zealand point of view, from now and onwards, I will use the term rain gardens in general for these three systems.

Design Factors

The extract below is from Christchurch City Council’s Guidelines for the Design, Construction and Maintenance of rain gardens. It shows that is important to take into account various factor in the design.

Other factors to keep in mind are related to the size and the level of the water table. The gardens become unviable when they are too big or the level of the water table is high as they are difficult to construct and maintain, hard to obtain uniform infiltration ratios and a high water table could mean that the gardens become flooded for long periods of time drowning their vegetation.

Other Councils in New Zealand such as Auckland City Council have also produced guidelines to aid in the design. For more information please check the following links:

Auckland City Council Guidelines

Christchurch City Council Guidelines

Trees in rain gardens - The study of Morton’s arboretum in Chicago

Another interesting aspect of these gardens is the dilemma of whether to incorporate trees in the design. Planting trees in urban environments is always a controversial topic due to the restrictions in space for their proper development and the preoccupation of how the roots affect the multitude of underground services nearby.

The American Society of Agronomy and the Crops Science Society and the Soil Science Society of America carried out a study in 2015 through which they evaluated how different tree species worked in rain gardens.

The study utilized as a reference the arboretum located in Morton’s car park located on the outskirts of Chicago, near Meadow Lake. This study concluded that the use of trees was beneficial for reducing the amount of water runoff. It proved that trees returned a great quantity of water to the atmosphere (46 to 72% in the trees observed). This was mainly done through the stomata in their leaves. Moreover, they found that their roots kept the soil healthy due to the channels they create as these help water infiltration and the activity of beneficial organisms that clean and store water. They noted that between half and three quarters of the car park runoff was eliminated through the rain gardens.

There were various tree species studied at different maturity levels. When they observed the conductivity of the stomata, the growth diameter and the health condition of the species, they noticed that not all species were suitable for the rain gardens and not all them produced an equivalent growth and transpiration curve. This meant that different trees contributed in a very different manner to the functional capabilities of the rain gardens. They concluded that the species with greater stomata conductivity from mature trees were the ones that contributed better.

I have not seen whether the study looked into how the roots of these trees affected any underground public services or whether they had enough space to grow and keep the trees healthy.

For more details on this study, you can visit the following link: Tree Species Suitability to Bioswales and Impact on the Urban Water Budget

Maintenance of rain gardens:

These systems require more maintenance that the traditional drainage systems and this means some councils feel reticent to their implementation. I think when evaluating the construction of these systems, not only the maintenance cost should be taken into account, but also the long and short term benefits these systems provide, versus the traditional ones should be added to the equation.

A regular maintenance would help these gardens working to optimal conditions. Tasks such as controlling erosion, sedimentation and accumulation of debris especially in filters and grates, plant development and weed control without pesticides that could seep into the water are all part of a good maintenance programme.

To finish this first entry on this topic, I would like to add a touch of humour with these funny pictures from They show an urban area with a traditional drainage system and then the same area is drawn with a SuDS. On the next entry I will also publish a list of all the references I have used for this research for your information if you want to read further.