Integrated Catchment Management Approach
Abstract
An integrated catchment management study involving numerical modelling with both hydrologic/hydraulic and groundwater models was undertaken to assess engineering solutions to mitigate flooding within a "land-locked" area at St. Leonards Road, Meola Catchment, Three Kings. The work was carried out as a result of the Meola Catchment Integrated Catchment Management Plan Study (ICMPS) for Metrowater and Auckland City Council (Sinclair Knight Merz, 2000a), which had identified the St. Leonards Road area as flood prone.
The preliminary modelling used the XP-SWMM hydrologic/hydraulic model to simulate stormwater flows for design storms and extreme historical events. This provided information to assess the location of hydraulic restrictions within the combined wastewater/stormwater network. The preferred stormwater disposal method in most urban volcanic catchments in Auckland is soakage to ground through drainage bores or pits. A drilling investigation was subsequently conducted to determine the hydraulic characteristics of the underlying geological materials with respect to the ground drainage potential for stormwater disposal. The geology was confirmed as low permeability volcanic tuff deposits, hence the drainage potential was limited.
The practicality and cost of various other water-engineering options to mitigate flooding were assessed using XP-SWMM. Options evaluated included in-line and off-line storage mechanisms and reticulation of separated stormwater to the Peary Road area (approximately 200 m north). The Peary Road area is underlain by high permeability basalt and is known to have numerous private soakage bores and public soakage pits. Given the magnitude of stormwater flows for the design storms simulated, the most feasible option was reticulation to Peary Road with disposal via a number of new soakage facilities.
The focus of the study shifted to gaining an understanding of the hydrogeological aspects associated with stormwater disposal at Peary Road (Sinclair Knight Merz, 2001). The work included GPR surveying to identify the interface between the tuff and the basalt; exploratory drilling and hydraulic testing of bores to determine the nature of the basalt aquifer; and development of a MODFLOW model. The purpose of the modelling was to assess the ability of the aquifer to accept the proposed peak stormwater loads (as determined by XP-SWMM), to assess the groundwater impacts from stormwater disposal, and to optimise the soakage disposal network and infrastructure requirements. These aspects of this study are described in this paper.