The effect of land-use changes on the extent of flooding in the Porirua Basin

Abstract

The deforestation and subsequent urbanisation of a catchment will have a dramatic impact on its hydrological response to storm events. Theoretically deforestation would cause an increase in the magnitude of storm events while also increasing annual discharge volumes. Storms would become more frequent while the response times for each storm would decrease. Urbanisation would serve to further enhance the effects of deforestation. In the Porirua Catchment deforestation was complete by the 1900's. In 1956 a State Housing Scheme initiated concentrated urban development of this catchment, and this has continued until present with approximately 30 percent of the catchment now under urban land-use. It is imperative that we can understand, and quantify, the effects that such urban growth has had on the catchment's hydrology. It is only in this way that we can predict the effects of continued urbanisation, and assess those areas in greatest need of planned mitigation. Porirua's existing hydrometric record, which was installed in the late 1960's, only covers the latter stages of urban development. Although analysis of this dataset shows real increases in storm frequency, coupled with a significant drop in lag times, these changes cannot be quantified. For this reason design catchments of homogeneous land-use were analysed, and the corresponding 'large event' land-use coefficients were then used to model predicted urban growth. This model was then tested against a previously recorded large event, and was shown to accurately predict flood extents. Predicted urban growth scenarios could then be modelled, with the most extreme of these modelling an urban increase of 42 percent. Under this urban growth scenario flood extents were doubled despite an increase of only 15 percent in flood peaks. When current mitigation measures are taken into account, these urban scenarios can be used to isolate those areas that are under risk of flooding should urban growth continue to this level.