Testing Grounds
Collaborations between Daniel Jan Martin, Geoffrey London, Nigel Bertram, Tessa Ansell and James Rietveld as part of projects supported by the CRC for Water Sensitive Cities.
The Swan Coastal Plain can be divided into two distinct hydrogeological types: sandy-low-groundwater and clayey-high-groundwater. In the sandy-low-groundwater condition, the ground behaves like a sponge. Water that falls on the ground and discharges to the ground through soakwells will often move vertically, downward, and recharge of the superficial aquifer is likely. This water then gets drawn back up through bores which tap the aquifer largely for the irrigation of lawns, gardens, public spaces and horticulture. A lack of nutrients in the sandy soils means that irrigation demands are higher in these areas.
In the clayey-high-groundwater condition, clays form layers and pockets beneath the ground that tend to hold and transport water in unpredictable ways. Here, soakwells can transport water onto the clays, which saturate the subsurface. With increasing urbanisation and density, there may be greater risks of flooding. These clayey areas are often in close proximity to Perth’s hydrological grid, to surface water systems, rivers and wetlands rich in ecology. This scenario is harmful to river and wetland ecology, as the quantity and rate of water can disrupt the hydrochemistry, and the nutrients carried from urban areas can affect the water quality. Much of Perth’s future urban development will occur in these clayey-high-groundwater areas.
Dozens of medium-density typologies have been tested in terms of their building footprints (red), infiltration (amber) and retention (blue) potentials, landscape qualities (green) and connection to internal spaces (light blue). Priorities of infiltration and retention can be broadly defined for these sandy-low-groundwater and clayey-high-groundwater conditions, respectively. These typologies were developed alongside site and landscape concepts for design propositions on sandy and clayey sites.
DESIGN FOR CLAY
Present development on clayey-high-groundwater areas in Perth is predicated on hydrological erasure, which involves sandy fill on clayey sites, and the avoidance of design response to the hydrogeological conditions. This involves stripping remnant trees and vegetation and covering over existing water and fertile soils. These have become incredibly costly prerequisites to building, both financially and environmentally. Rather than allowing sandy fill to negate the need to design with the clay conditions, we propose that design could instead respond to them, in ways that acknowledge the presence of water and deal with the necessary functional issues. These design explorations challenge the heavyweight construction norms in Western Australia.
Urban and building design for clayey-high-groundwater sites should explore the opportunities to collect and retain water for reuse and integration in urban life. Collectively harvesting roof runoff, stormwater and groundwater through subsoil drainage are some of the strategies that can be employed to localise water inputs and outputs. In medium-density scenarios, there are many opportunities to locally harvest, store and manage water which are not possible in single residential settings. Much larger tanks can be employed to store abundant winter rains further into Perth’s dry Mediterranean summer. The stored water can then be recycled and reticulated to dwellings and urban spaces through ‘third pipes’, forming closed loop systems. These systems may be maintained, monitored and charged for through strata and peer-to-peer water sharing schemes. Lightweight construction methods are critical, allowing existing surface hydrology and vegetation to remain. There are emerging efficiencies of timber construction in Perth, joists on piers, that tread lightly, retain existing topography, trees and vegetation, while raising structure above the seasonal groundwater highs. Lightweight boardwalks in the urban realm can enable a connected landscape through raised communal spaces, and native retention gardens and courtyards, that connect with the clayey terrain. These schemes retain the water that falls on them and make its presence visible. Rather than routing it away through hidden infrastructure, residents are engaged and gathered. In times of large rainfall events, these schemes overflow to retention areas in adjacent public spaces.
DESIGN FOR SAND
These infill housing schemes for sandy-low-groundwater sites considered how to densify while increasing infiltration into the ground and aquifers below. There is potential to recover the water-sensitive endemic vegetation types and tree species. The open woodlands which once covered these areas can provide inspiration for treed landscapes can provide shade and communal space over a ground plane designed to allow water to infiltrate the sandy soils. Open space was designed to be permeable, through permeable paving and driveway surfaces. Any impermeable surfaces are directed to infiltration garden beds and swales across the site. These infiltration gardens became key drivers in the architecture, forming courtyards and urban spaces between dwellings. Water from roof runoff is directed to rainwater tanks, decreasing scheme water consumption while overflowing in winter months into the ground. Endemic sandy planting requires little irrigation and strategies such as the community bore enable groundwater use to be monitored and balanced against infiltration within these schemes.