The Landscape Guide

10.10 Hydrology and Context

Contents list

The developed world's policy for surface water management has been one of Difference. In future, the policy should normally be one of Similarity or Identity. In urban areas, the old method of urbanization was to encourage surface water to run off hard surfaces into underground pipes, which discharged into rivers. In rural areas, the ancient policy was to install agricultural drainage. Such policies increased both the volume and the rate of surface water discharge, causing rivers to flood and necessitating their canalization. A formula, known as the Rational Method, was used to calculate peak discharge flow from a development area:

Q = CiA

where Q = peak rate of flow; C = coefficient of runoff (based on impermeability); i = intensity of rainfall; and A = area of catchment.

The coefficient of runoff was worked out using figures of the type shown in Table 1. Rainfall intensity was assumed to be constant throughout a storm. Once peak runoff had been estimated, underground pipes were sized to accommodate maximum flows. Meekly, it was accepted that "Development of an area of land for residential or industrial purposes increases the amount of runoff from that area' ( Bartlett, 1981). Due allowance was made, by oversizing pipes, for projected future developments within the catchment.

Cultural historians can delight in the use of "rational' as an adjective to describe the above method. The purportedly rational method was used for calculated environmental destruction. As the figures in Table 1 show, the process of urbanization tended towards 100% runoff into underground pipes and culverted rivers. Land was made impervious. Vegetation was killed. Underground aquifers were depleted. All this was done by men in white coats and dark suits, waving the flag of reason.

A more enlightened policy is to plan for a better future instead of following trends. Zero Runoff Increase (ZRI) should be adopted as a planning goal. Table 1 should become an exhibit in the Chamber of Urban Horrors. Table 2 should take its place.

Table 2 has question marks instead of figures for percentage impermeabilities. They need to be estimated for each development project, taking evaporation, transpiration, infiltration, detention and runoff into account. This will require data on rainfall, soil conditions, mean temperatures, mean wind speed and surface roughness.

In existing urban areas, policy decisions need to be set for decreasing surface water discharge (Table 3). A zoning policy is desirable (Figure 9e). It should be based on studies of building types, land use types and habitat conditions.

The proposed techniques use bioengineering in addition to ground engineering, pavement engineering and roof design. They are conservation policies. They will help to conserve the natural environment, but they may often be anti-conservation policies with regard to the traditional character of urban areas.