South East Queensland catchment modelling for stormwater harvesting research: instrumentation and hydrological model calibration and validation

South East Queensland catchment modelling for stormwater harvesting research: instrumentation and hydrological model
calibration and validation

Stormwater is one of the last major untapped sources of water in the urban landscape. South East Queensland (SEQ) urban runoff varies between 240 and 750 GL/year, of which about half is required to maintain environmental flow requirements in the lower reaches of the SEQ river systems. The challenge for using stormwater includes its capture, storage, appropriate treatment, and supply to end users at cost effective prices. Major potential end uses include
dual reticulation in greenfield urban developments (in lieu of rainwater tanks to achieve the mandated mains water saving of 70 kL/household/year) and irrigation of high value public open spaces such as playing fields.

There is a consensus view amongst freshwater ecologists that the increased frequency and peak discharge of runoff has seriously degraded the ecosystem health of urban creeks. Hence, stormwater harvesting is one method to reduce adverse ecosystem impacts and achieve the runoff objectives (contaminants, frequency, amount, peak discharge) defined in the SEQ Regional Plan (2009) Implementation Guideline #7. However, the science linking
hydrological response to creek ecosystem response is poorly understood. Until it can be demonstrated that stormwater harvesting does not adversely affect environmental flows, state regulators are disinclined to promote (or even approve) the practice in the Resource Operating
Plan/Resource Allocation Plan (ROP/RAP) environment of water regulation in SEQ.

An inevitable hydrological consequence of urbanisation is an increase in the fraction of impervious areas (roads, roofs, paving etc.) and consequent increases in runoff, frequency of runoff events, and peak discharges at various return intervals. The consequences of this changed hydrology are elevated concentrations of nutrients and contaminants, degraded channel morphology, reduced biota richness, and increased dominance of tolerant species (Walsh et al., 2005). Land uses in an urban catchment which reduce the frequency of runoff events, (and hence runoff %) and the peak discharges are considered to be beneficial to the restoration of stream ecosystem function. It follows that stormwater harvesting practices that can reduce the frequency of small events and take the top off peak discharge rates should be beneficial to the creek ecosystem. However, the other view is that abstraction of water from streams and rivers is likely to cause environmental harm, and until the safe environmental flows (based on the natural flow regime) are defined, extraction for beneficial uses should not be allowed. This stance has been adopted by the water regulator (DERM) as required by the Queensland Water Act (2000) and supported by numerous studies that many of our (inland) river systems are degraded due to over-extraction of water (both in its timing and amount) for human purposes.

Therefore, catchment hydrology modelling is an essential part of this project. Calibrated hydrologic models are widely used for stream flow simulation and to define hydrologic characteristics of streams. A reliable flow simulation depends on availability of reliable stream flow data and a reliable rainfall runoff model. Therefore, considerable effort has been invested in instrumentation of 12 catchments located in SEQ across a land use gradient from nil to significant urbanisation, in order to obtain continuous rainfall and creek flow data. A US EPA Stormwater Management Model (SWMM) has been calibrated and validated for each catchment at an hourly resolution, using two years of continuous hourly rainfall and runoff data. This technical report documents the work involved in doing so from instrumentation to catchment calibration and validation.

This report consists of a detailed description of catchments, hydrologic instrumentation techniques, rating curve development, estimation of catchment impervious fraction, description of the US EPA Stormwater Management Model, calibration and SWMM parameter estimations, and sensitivity analysis of SWMM parameters. The results of running the SWMM catchment models under baseline (current) land use conditions, and a series of stormwater harvesting and urbanisation scenarios will be reported separately.

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