The Process for Designing for Shallow Groundwater. and Small Rainfall Event Management in Urban. Developments

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1 The Process for Designing for Shallow Groundwater and Small Rainfall Event Management in Urban Developments Bill Till* and Emma Monk *Presenting Author The Decision process for stormwater management in WA is a component of the Stormwater management manual for Western Australia (DoW ). It was first published in 2005 and an update was published in The Department is currently progressing a major update of the Decision Process to strengthen its guidance in relation to surface water and groundwater interactions. The update amends the criterion for small rainfall event management to achieve water quality outcomes, aligns the criteria with the proposed intensity-frequency-duration (IFD) terminologies in the Australian rainfall and runoff (Engineers Australia) review, includes the provision of urban amenity in the stormwater management design process and provides more transparent guidance on the water sensitive approach to stormwater management. This paper documents the revised criteria for rainfall event management proposed in the Decision process for stormwater management in WA: Draft for comment. Introduction 1

2 The Decision process for stormwater management in WA is a component of the Stormwater management manual for Western Australia (DoW ) and outlines various water related issues or components that may need to be managed in urban development such as: stormwater, groundwater, water quality and protection of receiving waters. Since its original publication in 2005, there has been considerable progress made in the application of water sensitive urban design principles and design criteria. To address industry concerns regarding the interpretation of the Department s criteria, particularly by local governments, the Department has been undertaking reviews of its groundwater and surface water management criteria. Groundwater Management in Urban Developments Urban development on the Swan Coastal Plain, which is underlain by a superficial aquifer, presents unique design issues due to the significant surface and groundwater interactions. Most of the areas where the superficial aquifer is at depth have already been developed and land developers now have to import considerable volumes of sand or provide subsoil drainage to provide adequate clearance to seasonal groundwater levels. To address the issue of whether to fill or drain land prior to urban development, the Department published a discussion paper in March 2012, Guidelines for assessing the need for and setting controlled groundwater levels. This document provided guidance on how to set the controlled groundwater level (CGL), which is defined as the invert level of open channels or piped drains that intercepted and control the seasonal peak groundwater level. The document presented a process to determine whether the risk of groundwater rise was sufficient to require a design response by increasing the development ground levels and/or installing groundwater control drainage. It then outlined the design considerations required to design a functional groundwater control system, whilst protecting the water resource attributes of the catchment and delivering a functional and liveable urban form. Following extensive stakeholder consultation, the Water resource considerations when 2

3 Water resource considerations when controlling groundwater levels in urban development (Department of Water 2013) guideline was published. Surface Water Management in Urban Developments Current Small Rainfall Event Management Criteria The criteria for surface water management in urban developments to achieve water sensitive urban design outcomes in Western Australia were first published in the 2005 edition of the Decision process for stormwater management in WA. One criterion recommended the retention or detention of the 1-year ARI event as high in the catchment and as close to the source as possible, subject to adequate site conditions. Another criterion recommended that detention systems preserve the pre-development 1-year ARI peak discharge rate. The 2005 edition also provided criteria for the management of minor and major rainfall events, water quality management, protection of waterways and wetlands, and groundwater level management. The 1-year ARI event criteria were modified in the 2009 edition of the Decision Process and later further clarified in the Water sensitive urban design brochure series (Department of Water 2011) as: 1-year, 1-hour ARI criterion: Retain or detain stormwater runoff from constructed impervious surfaces generated by up to the 1-year, 1-hour ARI event at its source, preferably in lots and road reserves; and 1-year ARI peak flow and volume criterion: Maintain pre-development peak flow rates and total volumes runoff from the whole sub-catchment at outlets from the site at the critical 1-year ARI event. The key objectives of these criteria are: 1-year, 1-hour ARI criterion: Minimise the effective imperviousness of a development area and improve the water quality of stormwater runoff before it is mobilised into the 3

4 downstream stormwater systems and receiving water bodies. 1-year ARI peak flow and volume criterion: Protect the ecological values of receiving environments by maintaining the pre-development hydrological regime and maintaining waterway channel form and stability. These criteria are currently applied to the design and retrofit of stormwater management systems in developments across Western Australia, and the department uses these criteria when assessing planning applications. The Need for a Review of the Small Rainfall Event Management Criteria Western Australia is one of the largest government jurisdictions in the world, stretching from latitude 14 degrees south to 34 degrees south, a north/south distance of 2,500 kilometres. It crosses three climate zones from the tropical climate of the Kimberley, across the arid climate of the Pilbara, to the Mediterranean climate of the south west. Due to the significant variability in rainfall intensities, frequencies and patterns across the state, the 1-year, 1-hour ARI design rainfall derived from the Bureau of Meteorology s (BOM) intensity frequency duration (IFD) data system varies from 11 to 37 mm/hr across the state. The Department of Water s existing criteria has resulted in different rainfall depths, peak discharges and total volumes of small events to be managed in urban developments. For example, as per the 1-year, 1-hour ARI criterion, a typical development in Kununurra (located in the Kimberley) would need to manage runoff generated from 37 mm rainfall at-source, compared to 16 and 24 mm for a similar development in Perth (located in the south west) and Karratha (located in the Pilbara) (Fig. 1) (Till and Bhandari 2014). 4

5 Figure 1. Rainfall to be managed at-source for various towns and cities in Western Australia under current criterion (Till and Bhandari 2014). The key objective of this criterion is to improve the quality of stormwater generated from constructed impervious surfaces. To achieve water quality outcomes, it is not necessary to have significantly variable requirements (such as rainfall depths) for different cities and towns in Western Australia. Therefore, this criterion was reviewed from the perspective of the desired outcome of improved water quality protection, with the need for consistency and equitability for developments across the State. An analysis of spatial variability of small rainfall events across Western Australia was conducted using the number of daily rainfall events recorded over the entire record period of 15 5

6 rainfall stations. The stations represented the major towns and cities in Western Australia. Initially, the size of daily rainfall events was limited to 10, 15 and 20 mm and the number of events that were less than or equal to these amounts were counted for the entire recorded period. Mean counts calculated for a typical year were expressed as a percentage of total daily rainfall events per year and plotted. Figure 2 shows the results for a rainfall depth of 15mm/day. Figure 2. Percentage of rainfall events that can be captured per year if a rainfall event of 15 mm/day is retained (Till and Bhandari 2014). A review of the rainfall depth used by other Australian and international jurisdictions for water quality management found that management of approximately 15 mm rainfall depth was adequate to achieve water quality management in urban areas. Figures across jurisdictions 6

7 varied between 10 and 25 mm, with 15 mm identified as a requirement to minimise the transmission of pollutants. Additionally, the majority of rainfall events can be managed within lots and road reserves if the 15 mm rainfall depth is retained and/or detained at-source. More than 80% of annual rainfall events can be managed if 15 mm per day rainfall is captured (Fig. 2) for all climatic regions across Western Australia and more than 90% can be captured in the south-west climatic region. The conclusion from this review identified that the water sensitive urban design (WSUD) principles of ecological protection, urban amenity and water quality maintenance or improvement could be achieved by adopting a criterion for the at-source management of the first 15mm of rainfall events for all climatic regions of Western Australia. A discussion paper titled Review of the design criteria for managing stormwater runoff generated by small events was distributed for selected stakeholder comment in October No significant issues were raised by stakeholders. The detailed assessment of this analysis was delivered in a presentation at the Hydrology and Water Resources Symposium in Perth in February Proposed Rainfall Event Management Criteria Small rainfall events ecological protection Manage water quality The proposed criterion for managing water quality is: Stormwater runoff from constructed impervious surfaces generated by up to a 15 mm rainfall depth should be managed (retained and/or detained) as close to the runoff source as possible. Lot runoff should be managed within lots and road runoff should be managed within road reserves. Where site conditions do not allow for the full lot runoff volume to be retained and/or detained on-lot, retain and/or detain as much as possible on-lot, then retain and/or detain the remaining volume within the road reserve, in preference to within 7

8 parks. The 15 mm volume should be available within 24 hours of the end of the rainfall event. The management of 15 mm rainfall depth is appropriate for achieving the objectives of managing small rainfall events within residential, commercial and industrial lots and road reserves because it: captures and reduces the mobilisation of pollutants commonly produced in stormwater runoff on residential, commercial and industrial lots and streets, such as soluble materials, fine dusts, silts, oil, grease and other non-volatile hydrocarbons helps to maintain the pre-development hydrologic condition of the catchment accounts for managing more than 80% of annual rainfall events for all climate regions in Western Australia and more than 90% for the south-west climate region. 8

9 Maintain receiving environment form and hydrology This criterion is being amended to ensure consistency with the Australian rainfall and runoff (Engineers Australia undated) updates and to provide clarity on the area at which it is to be applied: Maintain pre-development peak flow rates and total volume runoff from the outlets of the development area for the critical 1 exceedance per year (EY) event. The key objective of the 1-year ARI peak flow and volume criterion is to protect the ecological values of receiving environments by maintaining the pre-development hydrological regime (e.g. maintaining peak flow rates, volumes, frequency, duration and velocity of stormwater runoff) from the outlet of the development area. These pre-development hydrological aspects assist in maintaining waterway channel form and stability. It is therefore appropriate to align these aspects with the local climatic and hydrological conditions. Major rainfall events flood protection Manage catchment flooding This criterion is being amended to ensure consistency with the Australian rainfall and runoff (Engineers Australia undated) updates. The proposed criterion for managing catchment flooding is: Maintain pre-development flood height for the critical 1% annual exceedance probability (AEP) event, unless otherwise established in an approved water management strategy or plan and the asset manager(s) / owner(s) have provided written permission. This criterion assists with protecting people and property from flooding and preventing additional inundation of water bodies by preventing increased flood heights within and downstream of the development area. Prevent building inundation This criterion is being amended to ensure consistency with the Australian rainfall and runoff (Engineers Australia undated) updates. The proposed criterion for preventing building 9

10 inundation is: Protect people and property from flooding by constructing residential, commercial and industrial building habitable floor levels at least 0.3 m above the 1% annual exceedance probability (AEP) flood height of the urban drainage system and at least 0.5 m above the 1% annual exceedance probability (AEP) flood height of waterways (or as published in a current plan or based on advice from the Department of Water). This criterion assists with preventing building inundation by constructing habitable floor levels to minimum distances above 1% AEP. Minor rainfall events serviceability, amenity and road safety Maintain serviceability, amenity and road safety Once the small and major rainfall event systems have been designed, the effects of minor rainfall events on transport networks, drainage networks and parks should be assessed and designs altered to maintain serviceability, amenity and road safety. Providers of transport networks, drainage networks and parks should define the relevant design exceedance per year (EY) or the annual exceedance probability (AEP). The proposed minor rainfall events criterion is: Design stormwater management systems in transport networks, drainage networks and parks to maintain serviceability, amenity and road safety during minor rainfall events. Updating the Decision Process Document Although there has now been almost universal acceptance and commitment to WSUD principles, application of WSUD criteria has been impeded by a business as usual approach of drainage designers following the traditional Australian rainfall and runoff minor/major design approach. A traditional approach leads to the principal design focus and effort being on the design of minor (flood) event infrastructure. The traditional objective was for the quick conveyance and disposal of all runoff up to the designated minor event, usually 5 or 10 year ARI. Applying WSUD criteria was addressed as a modification to the minor event design, to 10

11 manage water quality issues, without compromising the flood protection functionality of the infrastructure. A traditional drainage design approach on the Swan Coastal Plain of WA has led to some less than optimal outcomes over recent years as the urban sprawl has spread into the difficult to develop seasonally inundated areas. When concentrating on the minor event, drainage designers overlooked the detail required for the management of seasonal groundwater variability and the benefits of managing the small rainfall events at the runoff source. Managing shallow groundwater levels is a significant limiting factor in drainage design because the controls for these controlled groundwater level outlets are more critical than the controls for minor and major drainage systems, which can operate even when back flooded. Addressing the more frequent and lowest hydraulic flows before progressing to the less frequent and higher hydraulic flows enables the designer to optimise the proposed design for the most frequent functionality of the proposed urban infrastructure. It also informs designs for vegetation to suit the frequency and quantum of small rainfall events. In addition, the designer can maximise losses at the commencement of all rainfall events, no matter how much rainfall depth ultimately occurs. By taking this approach, the ultimate coefficient of runoff of a flood event is minimised and assists with managing major rainfall events. The draft Decision Process revises the order in which the hydrologic and hydraulic issues of urban development should be addressed by designers. The revised order of addressing design issues is: Address groundwater issues and risks for the development area Address the small rainfall event design criteria Determine the flood height, peak flow rates, volumes and flow paths for the 1% AEP event, and downstream impacts, including potential increased flows as a result of the development and strategies to mitigate these impacts. 11

12 Address serviceability, amenity and road safety issues, which are the objectives of minor event designs following the Australian Rainfall and Runoff minor/major design approach. Manage water quality. Protect and manage water bodies. Provide urban amenity. Approaching urban stormwater management design in the above order should result in: protection of public health and safety protection of public and private infrastructure and buildings minimisation of adverse effects on the water regime, water quality, habitat diversity and biodiversity in sensitive receiving environments provision of urban amenity. Conclusion Although the uptake of WSUD principles in urban development is increasing across Australia, this paper proposes that a fundamental change in the approach to the urban drainage design process is required to embed these principles and outcomes into business as usual. Managing small rainfall events is proposed as one of the first design steps to fully address WSUD principles in the urban water management design process. The Department of Water s Decision process for stormwater management in WA: Draft for comment is a tool to guide urban water management design, with the ultimate aim to transition to a new business as usual. Acknowledgements The authors would like to acknowledge the Department of Water for permission to publish this paper. 12

13 References Department of Water ( ) Stormwater management manual for Western Australia, Department of Water, Perth, Western Australia. Available via < Department of Water (2009) Decision process for stormwater management in WA, Department of Water, Perth, Western Australia. Available via < Department of Water (2011) Water sensitive urban design brochure series, Department of Water, Perth, Western Australia. Available via < Department of Water (2013) Water resource considerations when controlling groundwater levels in urban development, Department of Water, Perth, Western Australia. Available via < Engineers Australia (undated) Terminology draft discussion paper, Australian rainfall and runoff, Engineers Australia. Available via < Till, B. and Bhandari, A. (2014) Review of the design criteria for managing stormwater runoff generated by small rainfall events at source in Western Australia, Proceedings of the 35th Hydrology and Water Resources Symposium, Engineers Australia, Perth, Western Australia. 13