Catchment News

What happens when we urbanise a catchment?

John Stack from Dublin City Council tells us about the changes that happen as a catchment urbanises, how an urban catchment can be managed, and looks at the Santry River as an urban case study.

A catchment may be defined as any area of land where precipitation collects and drains to a common outlet, such as river, lake or coastal waterbody. A catchment includes all the surface water from rainfall, snow melt, and nearby streams that run downslope towards the common outlet, as well as the groundwater underneath. Catchments are complex 3-D systems influenced not only by their topography, but also by the geology underneath.

One cannot discuss a catchment without considering the water (aka hydrological) cycle, and a discussion of the impact of urbanisation on catchments is effectively a discussion of the impact of urbanisation on the water cycle. The water cycle describes the natural processes by which water moves from one location to another and the pathways which facilitate this movement. The pathways of the water cycle are interrupted by urbanisation. Figure 1 illustrates (a) the natural water cycle, (b) the urban water cycle, and (c) a sustainable option for the urban water cycle.

In a natural catchment, channel form is governed largely by topography and geology. In an urban catchment, channels are engineered to facilitate development. Channels are typically straightened, diverted and culverted. Furthermore, the area draining to the channel becomes artificial, constrained by drainage infrastructure.

What is Urbanisation?

Urbanisation is the process by which a natural, pervious landscape is converted into one that is hard, sealed and impervious. It may also be described as the process by which towns and cities are formed and become larger as more and more people begin living and working in central areas. There are good social and economic reasons for urbanisation. This article is not intended to demonise urbanisation; however, it is important to sustainably manage the consequences of urbanisation.

Prior to urbanisation, precipitation would have soaked into the ground, recharging groundwater, and feeding the river naturally (and slowly). As the water passed through the soil layer, it was purified, resulting in clean water supplying the baseflow of the river. In the urban context, precipitation is rapidly moved from the point at which it lands, to the drainage infrastructure, and into the nearest water course. This is known as stormwater runoff or urban runoff. The typical movement pattern of storm water in this instance is sheet flow, where rainwater flows rapidly across hard surfaces. In this scenario, water is seen as a waste product to be disposed of quickly, when it should be valued as an asset. When treated as a waste, it exacerbates flooding and becomes contaminated with pollutants.

Urban Runoff

Urban runoff is precipitation that has collected on roofs, roads, footpaths and other sealed surfaces. It flows directly to the surface water system and into the nearest waterbody. As the surface water system has no treatment processes, pollutants are washed into our surface waters. As the surface water system is designed to move water rapidly away, it can lead to flooding further downstream.

Impact of Urbanisation on a Catchment

Urbanisation dramatically changes the landscape of a catchment and disrupts the natural water cycle. Rainfall and other precipitation that would traditionally have been managed by the natural water cycle now becomes stormwater and urban runoff, resulting in flooding and pollution of watercourses. Figure 2 illustrates the differences between natural cover and impervious cover.

Case Study: The River Santry, Dublin

The River Santry rises to the west of Dublin Airport and flows roughly eastwards through Ballymun, Santry, Edenmore, and Raheny, before eventually discharging to Dublin Bay at the rear of the North Bull Island, which is a RAMSAR site, and is contained within the Dublin Bay Biosphere.

The river is approximately 13 km in length and has a reasonably sized catchment of approximately 14 km2. The district of Santry itself has changed from one centred around a large estate and, in later years, a small village to a modern, dispersed, and mixed-use suburb. Where there were once fields full of crops, and a variety of natural woodlands, there are now housing estates, a sports stadium, a shopping centre, industrial estates and a dense road network.

The change is reflected throughout the whole catchment, having changed from a principally agricultural catchment to an urban/suburban one. There have been tremendous consequences for the river as a result of this urbanisation.

Figures 3 and 4 clearly illustrate how the landscape has changed since the period 1837 – 1842, and the present day.

The pressures on the River Santry are summarised in Table 2. The table illustrates the difference in the pressures faced in an urban catchment compared to a rural catchment, where agriculture and diffuse pollution predominate.

For Water Framework Directive classification purposes, the River
Santry is divided into 2 sections. The upper section is classified as Poor, while the lower section is unassigned. Figures 5 and 6 show water quality parameters downstream of Clonshaugh on the River Santry over time (2008 to 2018).

Pressure	Description
Agriculture	Farms at the headwaters
Field drainage network	• Network capacity issues • Effects of sewer overflow spillages – spillages from foul sewer network to waters • Sewer chokes • Exfiltration due to infrastructural deficiencies
Misconnections	Discharges from household appliances and toilets to the surface water network
Urban Runoff	• Road runoff • Runoff from other hardstanding areas • Car Parks • Cobblelocked driveways • Roofs • Illegal disposal of polluting matter to gullies (e.g. old engine oil, etc.) • Cross connections from foul to surface water network (both public and private) • Animal/bird faeces • Private drain blockages • Car washing on kerbside/in driveways • Discharges not subject to licence (e.g. business/people washing shop/house fronts and floors and throwing thewashwater into the gullies • Unlicensed discharges
Historical changes to morphology	River channel altered, culverted, canalised, diverted
Historical landfills	A historical landfill may be impacting on the river
SW Network Management	Contaminated river silts, dredging and disposal
Disruption to the hydrological cycle	• Urbanisation prevents rainwater from infiltrating to ground and recharging groundwater and the river naturally. • Urbanisation creates sheet flow from hardstanding surfaces during rainfall, which leads to flooding • Flow from hardstanding surfaces increases sediment impact on the river

Table 2: pressures within the Santry River catchment

What can we do?

Traditional stormwater management has treated precipitation as something to be disposed of rather than something to value. The consequences are loss of groundwater recharge, reduced base flows in rivers, increased flooding and poorer water quality.

Stormwater should be managed more naturally and in a way that generates increased environmental and economic benefits by restoring the natural water cycle as much as possible. Many cities are achieving this through the use of Green Infrastructure (GI). Everybody can, and should, be involved in the management of stormwater, including the private householder, business owners, estate managers, developers, and the State Agencies.

There are two focus areas to be addressed when mitigating urbanisation:

• New developments – design for stormwater management in a way that mimics the natural water cycle as much as possible, recharge it into the ground, retain it on site, and slow down its rate of flow. Preserve natural features, such as floodplains with a natural vegetation buffer along streams that can slow, filter and store polluted runoff.
• Retrofitting of Green Infrastructure in existing developments/ properties and within the public realm Property owners can use green roofs, water butts, rain gardens and planters to slow down and reuse water
• Local Authorities and developers can use Green Infrastructure in an integrated and coordinated way to manage stormwater on a bigger scale

Green infrastructure incorporates both the natural environment and engineered systems to provide clean water, conserve ecosystem values and functions, and provide a wide range of benefits to people and wildlife. Green Infrastructure boosts the economy, enhances community health and safety, and provides recreation, wildlife and other benefits. Table 3 outlines the outcomes of implementing Green Infrastructure throughout a catchment.

Outcomes from the effective use of Green Infrastructure in a catchment

Reduces runoff volume, peak flow and flow duration

Slows down flow to increase the time of concentration and encourage infiltration and evapotranspiration

Restore groundwater recharge

Protect downstream water resources, including wetlands

Reduce downstream flooding and property damage

Reduce the frequency of operation of combined sewer overflows (CSOs)

Improve water quality by preventing pollution from entering into the watercourse

Reduce thermal pollution

Improve habitats, biodiversity and ecology

Table 3: Outcomes form the effective use of Green Infrastructure in a catchment

John Stack, Senior Executive Engineer, Water Framework Directive Office, Dublin City Council