Benchmarking Erosion and Sediment Control Performance in South East Queensland Healthy Waterways Initiative

Transcription

1 Benchmarking Erosion and Sediment Control Performance in South East Queensland Healthy Waterways Initiative

2 This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Requests and enquiries concerning use or reproduction should be forwarded to info@waterbydesign.com.au July 2014 Healthy Waterways Ltd This publication should be cited as: Water by Design (2014). Benchmarking Erosion and Sediment Control Performance in South East Queensland Healthy Waterways Ltd. Brisbane. This document is available for download from Disclaimer The material contained in this publication is produced for general information only. It is not intended as a substitute for professional advice and does not take into account specific applications. Before the use of, or reliance on, any material in this publication, it is the responsibility of the user to determine the suitability and applicability, and where appropriate consult a qualified professional advisor. To the extent permitted by law, Healthy Waterways Limited expressly disclaim any and all liability to any person in respect of anything done by any person in reliance on any information in this publication. The information contained in this publication does not necessarily represent the views of Healthy Waterways Limited. Acknowledgements The Benchmarking Erosion and Sediment Control Peformance in South East Queensland document was developed by the Water by Design program of Healthy Waterways Limited with financial assistance from the Queensland Government acting through the Department of Environment and Heritage Protection. Site audits and collection of compliance data undertaken by Stephen Tracey of Healthy Waterways. Further input and final production by Scott Paten of Healthy Waterways. Healthy Waterways would like to thank those South East Queensland Councils that agreed to participate in this benchmarking exercise and the local government officers involved for their assistance. Water by Design Water by Design is a program of Healthy Waterways Ltd. It helps individuals and organisations to sustainably manage urban water. For more information, visit Healthy Waterways Healthy Waterways is an independent, not-for-profit organisation working to protect and improve South East Queensland s waterways. Together with our members from government, industry and the community, we have one clear vision to achieve healthy waterways for a healthy economy. Healthy Waterways works to understand and communicate the condition of our waterways to drive and influence future targets, policy and actions. We pride ourselves in providing best practice scientific advice that informs effective and efficient waterway management. For more information, please visit Benchmarking Erosion and Sediment Control Performance in South East Queensland

3 Benchmarking Erosion and Sediment Control Performance in South East Queensland

4 1. Introduction Healthy waterways and rapid urbanisation need not be mutually exclusive. Simple, effective, affordable erosion and sediment control (ESC) measures have been routinely used for decades in other States and internationally, to minimise off-site impacts from sediment whilst supporting sustainable urban land development. The high rainfall intensity of the coastal subtropics is capable of eroding more than 400 tonnes of exposed soil per hectare each year from an urban construction site (BMTWBM 2009). Coupled with the extensive urban land development currently underway in South East Queensland (SEQ) this situation presents a real challenge in reducing the massive volumes of sediment currently being transported from land development sites to our waterways and coastal environments. The release of sediment from land development sites can also cause a wide range of economic, social and environmental costs, for which the broader community subsequently bears the financial burden. See Photo 1 and 2. Furthermore, the current level of industry ESC knowledge and often inadequate implementation of even basic ESC principles and practices means that the value of existing industry investment in ESC is often being under-utilised both in terms of cost-effectiveness and environmental outcomes. See Photo 3. During 2013 Healthy Waterways Limited (Healthy Waterways) set about benchmarking the standard of erosion and sediment control on SEQ land development sites within 6 local government authority (LGA) areas undergoing significant urban development. The intention of the benchmarking was to assess whether current ESC implementation by the land development industry was consistent with doing all that s reasonable and practicable to minimise the impact on receiving waterways and also to establish a baseline of ESC performance to measure future improvements against. It is clear from the benchmarking results (presented in this report) that the standard of ESC implementation on land development sites in SEQ is very poor. Photo 1: Impact on local community 4 Benchmarking Erosion and Sediment Control Performance in South East Queensland

5 Photo 2: Impact on waterway health Photo 3: Poor ESC installation. An example of a poorly installed sediment fence on a land development site. Benchmarking Erosion and Sediment Control Performance in South East Queensland

6 2. Methodology Land development sites under construction were assessed by a Healthy Waterways officer experienced in ESC, often accompanied by an LGA officer. The sites were usually selected by the LGA officer. Each site was assessed against 4 broad criteria: potential for water contamination minimising erosion managing site hydrology minimising sediment releases These criteria were derived from the Queensland Department of Environment and Heritage Protection s (EHP) Procedural Guide for assessing the lawfulness of releases from construction sites (EHP, undated - a; b) and simplified somewhat because the assessment was intended to provide a representative snap shot rather than a detailed assessment of regulatory compliance with ESC requirements Water contamination. The purpose of assessing the potential for water contamination is that irrespective of the standard of ESC applied onsite, if there is no potential for water contamination then the site would be considered as compliant. For example, a flat construction site with a one metre (1m) high impervious perimeter bund, would need to receive more than 1m of rainfall to cause a release of contaminated stormwater. So even if there were no ESC measures on the site, providing that the bund is intact, stable and effective, the site would be considered compliant at that time. Assessing the potential for water contamination therefore centres around the question Is there actual water contamination occurring or potential for water contamination to occur? Photo 4 and 5 shows some typical examples of water contamination. Each site was inspected and for each of the 4 assessment criteria was given a semi-quantitative compliance rating based on actual site observations that day. These compliance ratings are defined as follows: Substantially compliant the site demonstrated a high degree of onsite ESC installation, monitoring, maintenance and expected performance consistent with current best practice ESC principles and practices; Partially compliant - the site demonstrated a moderate degree of onsite ESC installation, monitoring, maintenance and performance with respect to current best practice ESC principles and practices; Non-compliant - the site demonstrated a low degree of onsite ESC installation, monitoring, maintenance and performance with respect to current best practice ESC principles and practices. The rationale and assessment methodology for each assessment criteria is discussed further here. 6 Benchmarking Erosion and Sediment Control Performance in South East Queensland

7 Photo 4: Water contamination. An example of actual water contamination and the release of untreated, highly turbid stormwater from the outlet of a sediment basin. Photo 5: Water contamination. Another example of water contamination caused by unmitigated stormwater runoff on a construction site. Benchmarking Erosion and Sediment Control Performance in South East Queensland

8 2.2. Minimise erosion The Revised Universal Soil Loss Equation (RUSLE) is the standard used by ESC professionals to predict the volume of soil lost from a site, over a given time period and considering site specific attributes such as surface cover, slope, rainfall intensity and soil type. The attribute with the greatest effect on soil loss is surface cover. Put simply, well vegetated or heavily mulched surfaces, or other techniques that cover the soil surface are much less prone to erosion than exposed soils. Estimated soil loss in SEQ (using RUSLE) for say a pasture or natural forested area would typically be less than 20 tonnes per hectare per year, whereas the estimated unmitigated soil loss for a construction site in coastal areas of South East Queensland is likely to be around 300 to 400 tonnes per hectare per year (BMTWBM 2009). It is acknowledged that staging is not always possible or warranted, such as on small sites. However, on large sites, where it is deemed more cost-effective to clear the whole site at the same time, the areas not being actively worked should be temporarily stabilised such as by grassing or mulching. See Photo 7. Photo 7: Minimising erosion - Substantially compliant. An example of good erosion minimisation. A large site where several stages have been cleared at one time, and the stages not being actively worked have been mulched with the cleared vegetation. For a 10 hectare site this equates to 3000 to 4000 tonnes of soil per year eroded from the site (or between 300 and 400 truck loads of soil). Whereas if the land development site was planned such that clearing and construction works were staged in, for example, 2 hectare sections, the predicted annual soil loss would drop by 80%. Importantly, this approach does not necessarily result in a net increase in overall construction costs provided it is planned for and integrated into the development proposal. Assessing the degree to which erosion has been minimised centres around the question Have all reasonable and practicable measures been implemented to minimise erosion, such as by staging clearing, temporary stabilisation or progressive revegetation? See Photo 6. Photo 6: Minimising erosion Non-compliant. An example of a large, steep construction site on which limited attempts have been made to minimise erosion through staging, temporary stabilisation or progressive revegetation. Likewise the duration of soil exposure can be minimised by progressively stabilising completed areas, rather than waiting until the engineering works are completed. See Photo 8. This approach has proven not only to reduce soil erosion, but has real benefits to the developer in terms of enhanced resale opportunities because grassy house blocks are more appealing to buyers than bare earth. See Photo 9. 8 Benchmarking Erosion and Sediment Control Performance in South East Queensland

9 Photo 8: Minimising erosion - Substantially compliant. An excellent example of advanced revegetation of the areas at finished level being stabilised prior to the completion of the surrounding engineering works. Photo 9: Minimising erosion - Substantially compliant. This site is approaching completion. The road verges and allotments are progressing towards being completely vegetated. Developers find that grassed allotments are more saleable than bare earth. Benchmarking Erosion and Sediment Control Performance in South East Queensland

10 2.3. Manage site hydrology The importance of managing site drainage on construction sites cannot be overstated. Stabilised stormwater flow paths are needed to convey dirty stormwater to sediment controls and to convey clean stormwater around or through the site without causing erosion or contaminating the clean stormwater. If stormwater flow paths are not stable they are likely to erode (see Photo 10), which increases the concentration of sediment in the stormwater flows, and in overdesign storm events produces a higher concentration of sediment released from the site. Unstabilised flow paths are also more likely to fail in major storm events due to erosion. Failure can mean that contaminated stormwater is not conveyed to a sediment basin for effective treatment prior to release to the environment, or is released at an uncontrolled location with other consequences, such as damage to infrastructure and costly repair bills. See Photo 11. Stormwater flow paths should be designed and constructed to convey the design flow without failing or causing erosion (see Photos 12 and 13). Assessing how well site hydrology is being managed is determined by considering the question Have all reasonable and practicable measures been implemented to minimise erosion caused by concentrated stormwater flows, such as by stabilising flow paths, diverting clean runoff around or through the site, and diverting contaminated runoff to an effective control device such as a sediment basin? Photo 11: Managing site hydrology Non-compliant. An example of the damage and cost implications of not adequately managing stormwater flows from a construction site. Photo 10: Managing site hydrology Non-compliant. No attempt to manage stormwater surface flows. 10 Benchmarking Erosion and Sediment Control Performance in South East Queensland

11 Photo 12: Managing site hydrology - Substantially compliant. Effective stormwater flow management, stabilised flow path conveys clean stormwater through the construction site without contaminating the clean stormwater or causing erosion. Photo 13: Managing site hydrology - Substantially compliant. An example of a flow path stabilised with geotextile fabric, conveying turbid site stormwater runoff to a sediment basin for treatment. Benchmarking Erosion and Sediment Control Performance in South East Queensland

12 2.4. Minimise sediment release Sediment basins for construction sites are designed to capture (for subsequent treatment) all the runoff from a typical design storm event. In SEQ this usually equates to around 30 to 50mm of rainfall depth over a 5 day period (not uncommon in the wet season). Any rainfall in excess of the design volume will exceed the capacity of the basin and be released without treatment, so it is vitally important that sediment basins are designed, constructed and maintained to ensure that the design volume of the basin is ready to receive each rain event. Photo 15: Minimising sediment release - Substantially compliant. An example of a sediment basin with stabilised inlet and spillway, designed to capture the 80th percentile 5 day storm event. Sediment basins and traps are assessed on their efficacy including location, construction, adequacy and maintenance. See Photos 14 and 15. Photo 14: Minimising sediment release - Substantially compliant. An example of a sediment basin with stabilised inlet and spillway, designed to capture the 80th percentile 5 day storm event. 12 Benchmarking Erosion and Sediment Control Performance in South East Queensland

13 It is also critical that inlets and especially outlets/ spillways are designed, constructed and maintained to appropriate standards, because failures can have a catastrophic impact and cost on public infrastructure, private property and local waterways. See Photos 16, 17 and 18. Photo 18: Minimising sediment release Non-compliant. An example of the impact of sediment release from a construction site on an adjacent waterway after a rain event. Other sediment traps such as bunds and sediment fences are intended to trap sediment before it gets to the sediment basin. Failure of sediment traps often leads to downslope gully erosion, because the failure usually causes a concentration of flow over surfaces which are not stabilised. See Photo 19 and 20. Assessing how well a construction site minimises sediment release is determined by considering the question Have all reasonable and practicable measures been implemented to minimise sediment release from the site, such as by capturing sediment in traps and basins and maintaining them in effective working order? Photo 16: Minimising sediment release Non-compliant. An example of the impact of sediment release on public infrastructure after a rain event. Photo 17: Minimising sediment release Non-compliant. An example of the impact of sediment release from a construction site on an adjacent residence after a rain event. Photo 19: Minimising sediment release Non-compliant. This sediment fence has been constructed poorly, failed during a rain event, causing more erosion and higher turbidity in the stormwater released from the site. Photo 20: Minimising sediment release Non-compliant. A poorly constructed sediment fence and grass filter strip are inadequate and do not provide an effective substitute for a sediment basin at the release point of this construction site. Benchmarking Erosion and Sediment Control Performance in South East Queensland

14 3. Benchmarking ResuIts The results of the ESC benchmarking exercise are presented in the following charts using a simple traffic light colour coding scheme to graphically illustrate compliance levels. See Figure 1. Figure 1. Compliance Colour codes Substantially compliant Partially compliant Non compliant When reviewing these results the reader should note the following information: The assessments are not attempting to determine the lawfulness of any release or potential release. Rather, the objective is to assess to what extent all reasonable and practicable measures to prevent or minimise water contamination have been implemented. This approach has been based on the EHP Procedural Guide. The assessments are not based on development consents or ESC Plans but observations of actual onsite conditions at the time of the inspection. The benchmarking exercise is not intended to be critical of any LGA s performance with respect to regulating the performance of the land development industry, nor is it intended to be critical of any individual site. It is a data gathering exercise and as such, individual LGAs and individual construction sites have not been identified in this report. Figure 2.Overall ESC compliance rating for 57 land development sites in 6 local government areas within South East Queensland (January to June 2013) 5% 21% Non compliant Partially compliant 74% Substantially compliant 14 Benchmarking Erosion and Sediment Control Performance in South East Queensland

15 Figure 3: Compliance levels for erosion and sediment control on 57 land development sites in 6 local government areas within South East Queensland (January to June 2013). 100% 90% 86% 80% 70% 70% 60% 50% 40% 30% 23% 2% 0% Minimise erosion Manage sediment export Minimise sediment export Overall ESC rating As a percentage of each category 20% 12% 10% 7% 58% 35% 7% 74% 21% 5% Benchmarking Erosion and Sediment Control Performance in South East Queensland

16 4. Conclusions This report highlights the very low levels of average ESC compliance observed on land development sites within South East Queensland during The results indicate the disproportionate under-performance of these sites in minimising erosion and managing stormwater runoff. Typically the consequence of sites demonstrating poor erosion and drainage control is that an increased pressure, and risk, of poor sediment control outcomes occurs, leading to increased levels of sediment pollution within our waterways. Similarly the report shows the significant opportunity to improve ESC compliance. Erosion and sediment control on land development sites is founded on well-established and industry tested principles and practices. The view of most leading ESC practitioners is that improving ESC performance on land development sites is not a technical challenge, but rather an institutional challenge. This challenge centres upon recognising the significant financial, social and environmental costs of sediment pollution to current and future generations of South East Queensland communities, and taking proactive steps to change current attitudes and behaviours towards ESC compliance on land development sites. 16 Benchmarking Erosion and Sediment Control Performance in South East Queensland

17 5. References BMTWBM (2009) Indicative Land Use Loads For The Sunshine Coast Region Using WaterCAST. Sunshine Coast Regional Council August Queensland Government (undated: a), Procedural Guide Summary sheet: Standard work method for the assessment of the lawfulness of releases to waters from construction sites in South East Queensland EM1137, Version 1, Department of Environment and Heritage Protection. Queensland Government (undated: b), Procedural Guide Compliance notes: Standard work method for the assessment of the lawfulness of releases to waters from construction sites - South East Queensland EM1135, Version 1, Department of Environment and Heritage Protection. Benchmarking Erosion and Sediment Control Performance in South East Queensland

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