1. Introduction
1.1 How Is Stormwater Infrastructure Designed?
Design criteria for a stormwater system usually has a frequency or probability attached to it - for example, a stormwater network (pipes, channels, overland flow paths) is sometimes designed to carry a rainfall event with a 1% chance of occurring in any given year.
Stormwater infrastructure upgrades are designed by simulating a rainfall event through a computer - generated model of the stormwater network. The results of such a modelling exercise help to identify areas where the stormwater infrastructure is under-capacity, and surrounding properties are at risk of flooding during large rainstorms.
The simulated rainfall event is often called a 'design storm'. A 1% AEP (annual exceedence probability) design storm is a short-duration (often 24 hour) rainfall event that has been developed to simulate a large storm that would most likely occur only once in 100 years.
An alternative method is to run a model with a time-series of real rainfall. Because of a lack of long-term rain gauges in New Zealand, however, it is rare to find a complete 100-year record.
1.2 How Could Climate Change Affect Stormwater Infrastructure?
Climate change has the potential to affect both the amount of rain falling on a catchment, and also the frequency with which heavy rainfall events occur. Because of this, stormwater infrastructure might need to be designed differently to take account of climate change.
1.3 The Study
An investigation was undertaken into the effects of different possible future climate scenarios, and different modelling approaches on stormwater infrastructure design. The project was instigated and managed by North Shore City Council, and partial funding was received from the NSCCO.
This report summarises the results of the project, with particular emphasis on the impact of climate change and modelling approaches on:
- Infrastructure Performance; and
- Infrastructure design
1.4 Project Background
North Shore City Council (NSCC) have historically undertaken catchment modelling and stormwater infrastructure design based on a 'design storm' approach as specified by the Auckland Regional Council in their 'Guidelines for Stormwater Runoff Modelling in the Auckland Region' (TP108: ARC, 1999). Following the successful use of time-series modelling in the planning of NSCC's wastewater network upgrades, NSCC sought to investigate the implications of using a dynamic time-series modelling approach to stormwater planning and infrastructure design.
The Wairau catchment in North Shore City was chosen for this case study, due to the availability of almost 20 years' continuous rainfall and flow data for its central drainage channel, the Wairau Creek. Implications of infrastructure upgrade in this area are also potentially significantly expensive due to the high flows, encroaching development, and highly engineered infrastructure.
During the preparatory stages of this investigation, consideration was also given to the potential climate change impacts on infrastructure design. At this stage, NSCC approached the New Zealand Climate Change Office (NZCCO) of the Ministry for the Environment (MfE), offering to include the Wairau catchment as a Case Study of how a council could consider the implications of climate change in its activities.
As part of its portfolio of climate change work, NZCCO has begun a programme to assist regional councils and territorial authorities to better understand and take into account climate change effects when carrying out their day-to-day operations. In particular, the programme aims to develop guidance materials for local authorities to assist them in assessing and managing the risks of climate change in their planning process. The climate change component of this report was, therefore, able to be partly funded by NZCCO.
This paper summarises some of the results of this study, in particular, the results of the time-series and design storm modelling, discussing how various different rainfall scenarios would affect the Wairau Catchment and its drainage infrastructure, and discussing these effects and how they might also apply to other catchments.
1.5 Project Objectives
The main objectives of this project were to:
- Compare the use of a "Design Storm" to the use of a "Dynamic Time Series" modelling approach for the design of stormwater infrastructure in the Wairau Catchment and determine the potential catchment management and infrastructure design implications of each approach.
- Determine the potential impact of climate change on the design requirements for stormwater infrastructure in the Wairau Catchment and the stormwater planning implications.
1.6 Study Approach
This study consisted of three distinct phases; stochastic rainfall generation, design storm development, and hydrological and hydraulic modelling of the stormwater network.
Stochastic Rainfall Generation - Current and Future Climate Scenarios
The existing 30-year rainfall record from the Wairau rain gauge was used to generate a number of stochastic rainfall series 'blocks' with the same rainfall characteristics, using a Neyman Scott Rectangular Pulses (NRSP) model. Five such 30-year series were created, providing a total of 150 years of rainfall time series data referred to as the "Present" scenario.
Assumptions regarding possible future climate change were used to generate similar stochastic rainfall series for three potential future scenarios.
This work was carried out by Dr Paul Cowpertwait of Massey University (Cowpertwait, 2003) and resulted in four synthetic 150-year continuous rainfall series with 5-minute increments.
Design Storm Development
In order that the "Dynamic modelling approach" could be properly compared to a "Design storm modelling approach", it was essential that the two approaches were based on the same rainfall data. This required the development of design storms using the same methodology as was used to develop the design storm in the ARC's stormwater runoff modelling guidelines (ARC, 1999). Intensity duration frequency curves and design storms were developed for two of the stochastically generated rainfall series- "Present" and "Future 3".
This work was carried out by Beca Carter Hollings & Ferner Ltd who were also responsible for the preparation of the ARC's TP108 (ARC, 1999).
Hydrological and Hydraulic Modelling
URS were commissioned to carry out hydrological and hydraulic modelling of the Wairau stormwater catchment and network in order to compare the two different approaches to modelling. The modelling was carried out using the latest version of DHI's MOUSE software, and the model was calibrated for both the dynamic and design storm modelling using actual recorded rainfall events in the catchment and corresponding flows recorded at a site on the main channel of the Wairau Creek.
1.7 The Wairau Catchment
The Wairau catchment is in North Shore City, and has a stormwater drainage area of approximately 1,300 hectares. The catchment has relatively steep terrain in the upper catchment areas, draining to a central valley and creek, and comprises a mixture of residential and industrial land use. The overall imperviousness of the Wairau catchment was estimated at 47% (Harrison Grierson, 2002). The stormwater system in the upper parts of the catchment is predominantly piped, and there are no longer any significant natural stream channels in the catchment. The catchment drains to the Wairau Creek, a highly modified stream channel with a number of in-line stormwater detention ponds, and then to the Wairau Estuary and the Hauraki Gulf. Rainfall and stormwater flows in the Wairau catchment have been monitored continuously since 1981. The Wairau catchment has a fast response to rainfall, and the drainage infrastructure is significant. Because of the large, highly engineered infrastructure, and built-up catchment, stormwater infrastructure upgrades can be costly. There are also a large number (1,280) of houses affected by the '100 year flood hazard' area, and there is a need to define this area as accurately as possible, in order to correctly ascertain the public health and safety risk, potential property damage and consequent NSCC liability.
