Turtle-top Cities

Forest and Bird, Issue 298, 1 November 2000, Page 34

Turtle-top Cities

PETER NAGELS

Modern cities reject the rain at their peril.

Story and photographs by

lash floods in our cities could become more frequent as the style of our housing changes. Suddenly, stormwater is becoming a problem in cities undergoing redevelopment. Nature’s rains which formerly drained away into the land via soak pits and quar-ter-acre sections now have nowhere to go as runoff is increasingly spilled overground by the growing expanses of concrete and other hard sealing. As a result, stormwater systems overload, there is erosion, property gets damaged, and sewerage systems spill. Downstream, beaches and other waterways are polluted by runoff from roads and industry, and sewage. The tendency to cover the earth with concrete and other seals is the product of many people overlooking the collective result of their individual environmental impacts. Rainwater should be valued and collected. A Danish consultant, Karl Iver DahlMadsen, hired by North Shore City to ‘peer review its handling of sewage likened the modern city to a turtle; when nature rains on it, the water runs off. In some areas of the fast-developing city, more than 70 percent of the land has been made impervious. There are precious few natural places left to absorb the rain.

‘In a certain sense, the city is protecting itself against rainwater as if the rain is toxic, he says. "Every time a drop of rain hits the city surface, it is transported downhill and out into the sea.’ There are possible alternatives; leaving grass islands in driveways is one; slowing the run-off naturally, by planting roadsides with appropriate species, is another. Developments could be planned to include natural areas, incidentally providing somewhere for the run-off to go. The fringe of North Shore City is rapidly developing in the Albany basin where town-planning restrictions have been varied to produce a new pattern of intensive development. In many places this has produced rows of townhouses, or warehouses with studio flats over; most surrounded by concrete lanes and yards. In the older suburbs too, land is also being redeveloped with two or three townhouses in place of each of old family home. This creates predictable problems for already strained services, including sewerage; but it also accelerates run-off as more earth vanishes under buildings and impervious surfaces. These pictures illustrate a problem common in developing towns and cities, and suggest some possible solutions. -PETER NAGELS is a committee member of North Shore Forest and Bird, and works as a stormwater engineer.

e When it rains, water rapidly runs from the impervious areas, swelling streams, flooding low-lying areas, and damaging structures, before polluting the sea. The cumulative effect of many hard surfaces can be devastating. Incremental paving occurs in all urban areas. Earlier styles incorporated grass with relatively low impacts and minor stormater run-off. Even small storms can transfer considerable amounts of land-based materials to the coast. The effects are often underestimated. Many storms occur at night and the true impacts are not seen. Below, opposite, is the effect of a short-duration storm (approximately 10 millimetres of rain) on Takapuna Beach. In contrast, water transfers to the coast via groundwater infiltration where there is less paving, exiting slowly through beach sands on Cheltenham Beach (below).

Sewage caught up in stormwater, frequently pollutes North Shore beaches after rainstorms. Stormwater discharges from the central city onto Takapuna Beach are concentrated, polluted, voluminous and sometimes violent (previous page, far left).

e Motor vehicles are the catalyst for the formation of most impervious surfaces. Roads, carparks, driveways and off-street parking all contribute. In the Auckland region, 40 percent of land is dedicated to the demands of vehicles and transportation, but the effects are not taken seriously enough. Less damage is done where natural vegetation is allowed to slow run-off.

e Edge maintenance of motorways often involves regular spraying with herbicides. But spraying results in unnecessary erosion and environmental damage. Fortunately may road edges, particularly in rural areas, contain good remnants of native plants, such as kanuka, manuka, bracken, cabbage trees and flax. These slow stormwater run-off and help retain the natural character of the countryside. Weed pests, such as blackberry, pampas and honeysuckle, should not be allowed to establish on road edges, but roadside sterilisation (below) should be avoided.

Some Solutions he impact of rainstorms can be alleviated, and the environment improved, by valuing rain and planning to cope with it. Methods include: e A good degree of healthy landscaping, minimising imprevious surfaces. e Raintanks — collection of rainwater (a natural resource) for re-use. e Enhance natural systems: swales, raingardens (soak areas), wetlands.