PUBLIC POLICY RESEARCH FUNDING SCHEME

公共政策研究資助計劃

Project Number : 項目編號：

2014.A8.016.14C

Project Title : 項目名稱：

Developing Sustainable Hong Kong through Low Impact Development: from Science to Innovation Policy 從科學到創新政策探討以低影響開發打造可持續香港

Principal Investigator : 首席研究員：

Dr CHUI Ting Fong May 徐婷芳博士

Institution/Think Tank : 院校 ／智庫：

The University of Hong Kong 香港大學

Project Duration (Month): 推行期 (月) ：

15

Funding (HK$) : 總金額 (HK$)：

455,975.00

This research report is uploaded onto the Central Policy Unit’s (CPU’s) website for public reference. The views expressed in this report are those of the Research Team of this project and do not represent the views of the CPU and/or the Assessment Panel. The CPU and/or the Assessment Panel do not guarantee the accuracy of the data included in this report.

Please observe the "Intellectual Property Rights & Use of Project Data” as stipulated in

the Guidance Notes of the Public Policy Research Funding Scheme. A suitable acknowledgement of the funding from the CPU should be included in any

publication/publicity arising from the work done on a research project funded in whole or in part by the CPU.

The English version shall prevail whenever there is any discrepancy between the

English and Chinese versions. 此研究報告已上載至中央政策組(中策組)網站，供公眾查閱。報告內所表達的意見純屬本

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PUBLIC POLICY RESEARCH FUNDING SCHEME

FINAL REPORT

Project No.: 2014.A8.016.14C

Developing Sustainable Hong Kong through Low Impact Development:

from Science to Innovation Policy

從科學到創新政策 探討以低影響開發打造可持續香港

Submitted by:

Dr. Ting Fong May Chui (Principal Investigator)

Assistant Professor

Department of Civil Engineering

The University of Hong Kong

December 7, 2015

1

Abstract

Conventional stormwater drainage system primarily focuses on flood control in urban regions and does not address the need of water quality management and ecosystem service design in fast urban sprawl that significantly alters the hydrological cycle. Since the last decade, there have been rising interests worldwide in implementing component-based Best Management Practices throughout urban areas to promote sustainable stormwater management and to reduce environmental impact on receiving water body. This new approach in stormwater management, referred to as low impact development (LID) or sustainable drainage systems (SuDS), also provides many other benefits such as increasing freshwater supply and mitigating heat island effect in urban areas. Though gaining popularity worldwide, its application in Hong Kong (HK) is still limited. To strive for sustainable development, HK should further explore and promote the use of LID in varying landscape environments. The implementation of LID and the formulation of its public policy should be supported by technical, social and economic understandings of LID in HK. Therefore, the goal of this project is to facilitate the planning and implementation of LID in HK, promoting science towards public policy. It first compiles relevant data and information to develop numerical models to evaluate the hydrologic performance and cost effectiveness of LID in HK. LID can alleviate some flooding risks but are not as effective when compared to many overseas locations. Technical recommendations, regarding suitable applications, optimal dimensions, required areal coverage, etc. are also made. Social and economic analysis is then performed. Understanding of HK people towards stormwater management is not strong though they still in general support LID implementation. The willingness to pay (WTP) of HK people, however, is low in terms of the amount per person for public places, and property price percentage for private properties when compared to some studies performed overseas. However, the life-cycle net benefit estimation of city-scale LID implementation is still positive, suggesting that it is socially and economically beneficial to implement LID given the assumptions of this study. Finally, based on the results of this project together with overseas experience, possible approaches and policy for LID implementation are proposed for HK, including making reference to existing framework for greening, forming steering committee, carrying out pilot projects, establishing incentives, educating and promoting to both professionals and general public, and setting regulations. LID implementation and its public policy formulation is a long and complicated process. This project is a first and important step, providing the necessary fundamental data and analysis for HK. Its results could lead to more in-depth evaluations of the technical, social, economic, political considerations for the successful implementation of LID in HK in the future.

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摘要 城市的急速發展嚴重改變了城市的水文循環，傳統的雨水排放系統側重于對洪澇災害

的控制，忽略了對水質的管理和生態系統的評估。過去十幾年間，雨水最佳管理與實

踐（Best Management Practice, BMP）的觀念受到世界各地關注，被廣泛應用於現代化

城市設計中。這不僅有利於減少水體受到負面影響，還能促進雨水管理的可持續發

展。作為創新的可持續雨水管理方法，低影響開發（Low Impact Development, LID）

或可持續排水系統（Sustainable Drainage Systems, SuDS）的策略不但能緩解洪澇災

害，減少水體污染，還有很多其他有利的影響，比如增加淡水供給，以及減輕城市熱

島效應。該方法目前被許多發達國家採用，但香港對其認知和應用仍然很少。爲了改

善這一狀況，實現可持續發展，香港應將 LID 融入到各景觀發展方案中。而LID的施

行以及相關公共政策的制定需要科技、社會、經濟各方面的分析。該項目有利於LID技術導則和處理設施的策劃和實施，為公共政策的制定提供科學依據。首先，該項目

收集相關的數據和信息來建立數字模型，藉以來評估LID在香港的水文性能和成本效

益。LID在香港可以緩解一些洪澇風險，但效益不及很多海外的應用。該項目也作出

技術建議，如LID適當的應用、最佳尺寸、需要面積覆蓋等。其次，進行社會和經濟

分析。香港人對雨水管理的理解不深入，但仍普遍支持LID的實施。香港人願意支付

的金額，以公共場所人均量和私人房地產價格的百分比來計算，比國外相對地低。然

而，全市規模LID的淨效益估計仍然是正數，這表明基於該項目的假設下LID是有利於

社會和經濟。最後，結合該項目結果與其他國家的經驗,提出LID可能的辦法和政策，

包括參照現有綠化的框架、建立指導委員會、開展試點項目、建立激勵機制、教育和

推廣、制定條例。 將LID融入香港的城市規劃以及制定相關公共政策是一個長期並且

複雜的過程，該項目是第一步且是非常重要的一步。它將引伸出更多科技、社會、經

濟、政治等各方面深層的分析。

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1 Policy Implications and Recommendation

This project is divided into three parts and there are policy implications and recommendation arising from each part of the project.

The first part of the project evaluates the hydrologic performance of LID in HK and develops some technical guidelines for HK. Due to the large storms in HK, the hydrologic performance and cost effectiveness of an individual LID element are not as good as many other locations worldwide. However, it can still alleviate some flooding risks. Comparing the performance of three potential LID elements for HK (i.e., green roof, bioretention system and porous pavement), green roof is the least cost-effective for peak flow reduction. Thus, it is recommended only if its other benefits, such as building energy savings, aesthetics enhancement, are important. Bioretention system is more cost-effective for peak flow reduction than green roof. However, the operation and maintenance activities of bioretention system are more involved and costly. Porous pavement is the most cost-effective for peak flow reduction among the three LID elements. It is therefore recommended for locations in which stormwater management is the main objective, because it lacks the potential benefits from the vegetation and the soil layers in other LID elements.

The second part of the project performs social and economic analysis to determine whether LID is socially and economically beneficial. Understanding of HK people towards stormwater management is not strong though they still in general support the implementation of LID. This is encouraging relevant government agencies and private developers to consider adopting LID. However, the results also suggest the importance of education and promotion if LID were to be implemented in HK. The willingness to pay (WTP) of HK people, however, is low in terms of the amount per person for public places, and property price percentage for private properties when compared to some studies performed overseas. These results suggest that government intervention (e.g., incentives, policy and regulation) is required for LID implementation in HK. However, HK is densely populated and the property price is very high, the aggregate WTP per unit area might therefore be sufficient to support LID implementation. Another finding is that the WTP differs between public places and private properties, and also varies with age, income, educational level and previous experience with flooding, etc. These results might facilitate strategic incentives and policy formulation for LID implementation. A framework is further proposed in an attempt to estimate the life-cycle net benefit of city scale LID implementation, accounting for economic, environmental and social benefits. Given the assumptions in this study, preliminary estimations show that the life-cycle net benefit is positive suggesting that it is socially and economically beneficial to implement LID. However, there are many uncertainties and assumptions involved in the calculations and further detailed analysis is required. The net benefit is also highly sensitive to the WTP of HK people, in particularly the private WTP, and thus should be examined in more details.

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The last part of the project proposes plausible approaches and public policy for HK LID. Six main areas are identified:

a) Referencing existing framework for greening

Since LID in some ways is similar to greening, LID can possibly make reference and even be integrated into the existing framework for greening. For example, Greening, Landscape and Tree Management (GLTM) Section was established under the Works Branch of Development Bureau in March 2010 to develop strategic policy on greening, landscaping and tree management. Steering Committee on Greening, Landscape and Tree Management (SCGLTM), chaired by Permanent Secretary for Development (Works), was formed with various department representatives to formulate relevant policies, oversee the development and implementation of standards, guidelines, etc. A similar approach can be considered for LID, and a steering committee with a particular focus on sustainable rainwater/stormwater management can possibly be established under the GLTM Section. Promotion and education of LID might also be performed within or in parallel with GLTM Section.

b) Formation of steering committee

A steering committee, with representatives from various parties, can facilitate LID implementation in HK. In short term, the steering committee could assist and supervise the implementation of pilot projects, as well as education and promotion activities. In longer term, the committee could facilitate the drafting of technical circulars, joint practice notes, and ordinance revisions, etc. Based on the results from surveying and interviewing professionals in HK, together with reference to SCGLTM, Table 1 lists the potential parties that could be considered as members of the LID steering committee and their proposed main areas of involvement.

c) Pilot projects

Pilot projects are effective in demonstrating, testing and promoting the implementation of LID in HK. Feasibility and cost effectiveness of LID can also be furthered examined through pilot projects. There are a number of existing projects of roof greening such as those in IFC2, The University of Hong Kong, Headquarters Building of Electrical and Mechanical Services Department, etc. but future applications can consider focusing more on stormwater mitigation. Other departments, such as Civil Engineering and Development Department and Drainage Services Department, have also been planning and have implemented some pilot projects. Future pilot projects can strive for a wider variety of LID elements including porous pavement and bioretention system, etc., and also in different potential locations such as public housing, parks, roadsides, etc.

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Table 1 Potential parties of LID Steering Committee

Proposed parties Proposed main areas of involvement

Development Bureau Leading the committee Development Bureau technical circulars

Agriculture, Fisheries and Conservation Department Environmental and ecological considerations

Architectural Services Department Design considerations in roof greening

Building Department Buildings Ordinance Design considerations in roof greening

Civil Engineering and Development Department

Pilot projects Incorporate LID in New Development Areas

Consultant representative(s) Comment on policy making and LID design guidelines Contractor representative(s) Comment on policy making and LID design guidelines

Drainage Services Department Pilot projects Incorporate LID in urban drainage systems Land Drainage Ordinance

Highways Department Design considerations in, and incorporation of porous pavement and bio-retention system along roadside

Home Affairs Department Community education and promotion

Hong Kong Housing Authority Pilot projects Incorporate LID in public housing

Lands Department Comment on policy making Land management

Leisure and Cultural Services Department

Pilot projects, especially those in public parks and LCSD structures

Local University LID technical development and research

Planning Department Comment on policy making HK Planning Standards and Guidelines

Private developer representative(s) Comment on policy making and LID design guidelines

The Hong Kong Green Building Council Comment on policy making and LID design guidelines

Urban Renewal Authority Retrofit existing sites with LID

Water Services Department Design considerations in, and incorporation of rainwater harvesting system

d) Incentives

Three main forms of incentives can be considered for LID, namely development incentives, financial incentives, awards and recognition programs. In HK, there are existing development incentives for green roofs and podium gardens as they can potentially be exempted from Gross Floor Area and/or Site Coverage calculations in new building development. Similar development incentives can possibly be adopted for other LID elements such as porous pavement and bioretention system. In addition, LID elements can potentially be regarded as greenery measures for meeting greenery requirements. Financial incentives have been successfully adopted overseas to promote LID implementation in which HK can take reference. For example, there are reimbursement programs and/or low-interest loans for private developers to handle the

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potential increase in construction costs if the LID elements can provide certain levels of benefits. Finally, awards and recognition programs are commonly adopted to encourage green participation in HK, such as BEAM Plus from The Hong Kong Green Building Council, Hong Kong Green Label Scheme and Hong Kong Green Awards from Green Council, etc. Similarly recognitions and/or awards can be provided to LID projects and/or companies and/or individuals that have contributed to the innovative design and effective application of LID in HK.

e) Education and Promotion

Professional trainings and guidelines are required to cover the design, construction and maintenance of LID. In conjunction with professional organizations such as the Hong Kong Institution of Engineers, the Hong Kong Institute of Architects and local universities, seminars and workshops could be held and be regarded as continuing professional development programs. Design guidelines could potentially be promulgated as practice notes of Development Bureau or joint departments.

For education and promotion to general public, a sub-topic about LID could be introduced under greening to primary or secondary school students. Ambassador or volunteer schemes could be established for general public to contribute to LID implementation. Guided visits to pilot projects can be made available. Brochures can be distributed in promotional activities such as school seminars and exhibitions. Signage could also be installed next to pilot projects. Finally, discussion forums with general public, both online and real-life, could be held to facilitate communication.

f) Regulations

Regulation could be gradually implemented in the long term. LID can potentially be incorporated to the existing target-based green performance framework for new and existing government buildings, for stormwater mitigation and other environmental benefits. For private development, HK currently does not limit the amount of stormwater runoff as long as proper drainage is provided. Different stormwater runoff discharge limits could possibly be applied to old and new buildings or development areas.

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2 Research

a) Methodology

This project first evaluates the hydrologic performance of LID in HK and develops some technical guidelines and recommendations for HK (Part I). It then performs social and economic analysis (Part II), finally proposes plausible implementation approach and policy for HK LID (Part III).

I Hydrologic Performance in and Technical Guidelines for Hong Kong

The first part of this project creates usable technical knowledge, extracting applicable and relevant technical metrics and analytical tools from existing guidelines and literatures. Based on these information together with existing relevant datasets, numerical models are developed and data analysis are performed to understand the hydrologic performance of LID in HK, and also to derive technical guidelines and recommendations for HK LID.

To estimate the hydrologic performance of LID under the specific geophysical conditions of HK, one particular LID element, green roof, is selected to be examined in details using COMSOL Multiphysics (COMSOL AB, 2010). There is a high potential of adopting green roofs in HK because of the high density of buildings. This numerical modeling study consistently compares the hydrological performance of green roof across different locations. This study also particularly examines the hydrologic responses to large and extreme storm events, providing results that can support the design of green roof for stormwater management. The numerical model is calibrated and verified using experimental results from Singapore (van Spengen, 2010) in which rainfall-runoff processes were monitored on 1 x 1 m green roof platforms with a 21 cm thick soil media. A number of cities around the world, covering tropical, subtropical and temperate zones are selected to represent various rainfall conditions. For each selected city, at least two design storms of different return periods (i.e., two years and fifty years) are studied. The model is then subjected to the design storms, and the peak reduction and delay are particularly evaluated and compared among the different scenarios. Peak reduction is defined as the reduction in the peak discharge from the green roof (i.e., combining the runoff from the top of the green roof and the bottom of the soil layer) when compared to the peak of the design storm, while peak delay is the time difference between the peak green roof discharge compared with the design storm peak

Other than examining the hydrologic performance of green roof in various locations, the effectiveness of different designs of LID elements are further evaluated. The goal is to identify the optimal design by assessing the hydrological performance and the cost-effectiveness of different designs of LID elements at a household scale, and to analyze the sensitivity of the hydrological performance and the cost of the optimal design to different model and design parameters. First, EPA SWMM (Rossman, 2010), automatically controlled by MATLAB, is used to obtain the peak runoff of different designs of three specific LID elements (i.e., green roof, bioretention system and porous pavement) under different design storms (i.e., 2 yr and 50 yr design storms of HK, China and Seattle, U.S.). Then, life-cycle cost is estimated for the different designs, and the optimal design, defined as the design with

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the lowest cost and at least 20% peak runoff reduction, is identified. Finally, sensitivity of the optimal design to the different design parameters is examined. Using the developed numerical models, the most suitable LID elements (e.g., bioretention system, green roof, and porous pavement) for the different contexts in HK can be identified. Specific guidelines such as those on sizing, representative runoff coefficients for the different LID elements under the specific geophysical environment of HK can be derived.

II Social and Economic Analysis

To gain support from various stakeholders, not only does LID has to be proven technically feasible and effective, it also has to be socially and economically beneficial. Therefore, part II of the project performs social and economic analysis using data collection and surveys. Various representative groups of people (e.g., ages, genders, education levels) are interviewed to analyze their social perceptions on LID, importance of natural environment, etc. and their willingness to pay (WTP). Economic analysis is then performed for both individual LID element and also large-scale LID implementation in HK.

Face-to-face interviews were conducted using a survey questionnaire between April and May 2015 with 600 respondents. Respondents were selected from five different districts in HK covering various levels of flooding experience, infrastructure density and residents’ income (Table 2). They were also selected in a way to replicate the demographic characteristics of the overall population in HK, except that they were all 18 year olds or above.

Table 2 Characteristics of sampling locations

District Flooded previously1 Density of infrastructure Income level of resident 2

Mong Kok Yes High Low to Medium

West Kowloon No Low High

Shum Shui Po No High Low

Sheung Wan Yes Medium to High High

Yuen Long Yes Low Low

1 Information extracted from Drainage Services Department of HKSAR (2015) 2 Information extracted from Census and Statistic Department of HKSAR (2011) Before the actual interviews, a pilot study was conducted in March 2015 with 50 respondents from The University of HK, providing feedback to improve the questionnaire. Some layman terms were purposely used in the questionnaire instead of technical jargons, hoping the respondents can better understand the questions. The finalized questionnaire for the actual interviews consists of four parts:

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1. Questions testing respondents’ understanding towards stormwater management in HK

2. A brief introduction to LID together with colored pictures for illustrations

3. Questions examining respondents’ perception towards LID including their perceived benefits of LID and WTP for LID via different payment vehicles

4. Demographic questions

The respondents were first informed of current HK government spending on stormwater management per HK resident per year. They were then asked the amount they think on average each HK resident should pay each year if HK government is going to adopt LID facilities. The respondents also were asked to state their perception towards LID separately for public places and private properties. To estimate their WTP for private properties, they were asked how much more they would pay for a flat with LID in the neighborhood if they were going to purchase a flat in HK provided that all other factors remained the same.

In order to quantify individual and large-scale benefits of LID implementation, a three-step quantification procedure is proposed.

Step 1: Estimate area of each impervious cover type;

Step 2: Identify potential LID project sites for each LID practice;

Step 3: Quantify life-cycle cost and benefit.

In order to express all the benefits in monetary values, the last step of the three-step process is further broken down into four sub-steps as shown in Figure 1:

(a) Define types of costs and benefits;

(b) Determine evaluation method for each type of costs and benefits;

(c) Calculate life-cycle cost and benefit during a specific service time;

(d) Evaluate life-cycle net benefit.

The method developed by CNT and American Rivers (2010) is used to quantify the economic and environmental benefits (blue circle in Figure 1), and the Stated Preference Method is used to quantify the WTP and thus the social benefits (red circle in Figure 1). For economic benefits, costs reductions in water treatment and grey infrastructure, and energy savings are quantified. Environmental benefits include the quantification of air pollutants reduction and CO2 reduction. To evaluate the monetary value of social benefits, contingent valuation can elicit respondents’ WTP for LID through surveys or structured interviews The results of the face-to-face interviews mentioned above, was used to evaluate the WTP and thus the monetary value of social benefits. For life-cycle calculation, the construction of LID is assumed to take one year and all benefits are assumed to occur from the end of the first year and continue to the end of the service period.

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Figure 1 Life-cycle cost and benefit evaluation procedure

III Formulate plausible implementation approach and policy for HK

Public policy is needed for the smooth, integrated and comprehensive implementations of LID because LID can be considered as public goods and also involves various stakeholders that might have high inertia or conflicting interests. This part of the project will first identify and prioritize the key objectives of LID for HK based on the technical, social and economic analysis of Parts I and II. After deriving the key objectives of HK LID, the LID policies of a few overseas locations are examined for their applicability in HK. To propose a local and practical LID implementation approach and policy framework, surveys and interviews were also conducted to collect comments and suggestions from local professionals. A survey questionnaire was conducted at the one-day workshop (see Section on public dissemination) regarding the application of LID in HK. There were more than 100 attendants from different government departments, consulting companies, universities, contractor firms, etc. and 41 samples were collected. Besides the survey questionnaire at the workshop, interviews were conducted to collect specific views from particular professionals, including those from Drainage Services Department, Civil Engineering and Development Department, Highways Department, The University of Hong Kong, the University of Central Florida and Hong Kong Green Building Council.

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b) Results

Similar to the organization of Methodology, the results are divided into the three corresponding parts of the project.

I Hydrologic Performance in and Technical Guidelines for Hong Kong

The hydrologic performance of the green roof in HK is compared with other six cities: Singapore (Singapore), Nagoya (Japan), Mumbai (India), Shanghai (China), London (UK) and New York (US). Figure 2 compares the design storms of two-year return period of the seven cities, demonstrating the much higher intensity storm in HK.

Figure 2 Design storm of two-year return period of various cities

Table 3 summarizes the peak reductions and delays, together with the characteristics of the various storms. As expected, a green roof can mitigate peak discharge of a smaller storm better, and thus the peak reduction is higher in a two-year storm than in a fifty-year storm for the same location. The peak reductions of the two-year storms of all cities are all over 10%, suggesting good performance. However, many main urban drainage systems are designed for more extreme events, and only Nagoya, London and New York yield peak reductions higher than 10% for fifty-year storms. Most cities, except HK and Singapore, experience some peak delays during a 2 year storm. However, during a 50 year storm, only London, New York and Shanghai experience some peak delays. The delays are overall in the orders of a few minutes and do not seem large. However, some urban catchments in fact are very small with time of concentrations in the order of 10 minutes. Therefore, one may not overlook the benefit of a few minutes of delay. The comparisons provided by this part of the study facilitate the transfer of existing knowledge from overseas in green roof, or other LID elements, for stormwater management in HK.

0

50

100

150

200

0 20 40 60 80 100 120 140 160 180 200

Rain

fall

Inte

nsity

(mm

/hr)

Time (min)

Hong Kong

Singapore

Nagoya

Mumbai

London

New York

Shanghai

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Table 3 Summary of simulation results

Two-Year Return Period

City Hong Kong

Singapore Nagoya Mumbai London New York

Shanghai

Total Rainfall (mm) 127 92 58 58 36 41 23 Peak Rainfall (mm/hr) 165 145 88 77 60 38 26

Peak Discharge (mm/hr) 141 107 69 62 42 32 20 Peak Reduction (%) 15 26 22 20 30 15 24 Peak Delay (min)* 0 0 5 5 5 5 20

Fifty-Year Return Period

City Hong Kong

Singapore Nagoya Mumbai London New York

Shanghai

Total Rainfall (mm) 254 163 133 130 71 97 111 Peak Rainfall (mm/hr) 240 250 145 170 105 102 99

Peak Discharge (mm/hr) 220 241 125 165 83 87 91 Peak Reduction (%) 8 4 14 3 21 14 8 Peak Delay (min) 0 0 0 0 5 5 10

Regarding the effectiveness of different designs of LID elements, the main conclusions are summarized as follows. The relationship between peak flow reduction and cost of green roof is almost linear (Figure 3a). Its optimal design tends to be larger in area and thinner when compared to bioretention system and porous pavement. However, it is more cost-effective for increasing the soil depth than expanding the area to handle larger design storms. For bioretention system, the relationship between peak flow reduction and cost forms an “S” shape, in which peak flow reduction only increases significantly in the middle cost range (Figure 3b). Its optimal design tends to be smaller in area and thicker than that of green roof. However, it is more cost-effective to expand the area than increase the soil depth for larger design storms. Similar to bioretention system, the relationship between peak flow reduction and cost for porous pavement form an “S” shape (Figure 3c). Its optimal design tends to have a smaller area and a thinner pavement surface. However, it is more cost-effective for expanding the area than increasing the depth for larger storms.

As shown in Table 4, the cost-effectiveness, measured as the peak runoff reduction/thousand Dollars of LID practices in HK (e.g., 0.02L/103US$·s, 0.15L/103US$·s and 0.93L/103US$·s for green roof, bioretention system and porous pavement for 2 yr storm) is lower than that in Seattle (e.g., 0.03L/103US$·s, 0.29L/103US$·s and 1.58L/103US$·s for green roof, bioretention system and porous pavement for 2 yr storm). For green roof, the main reason is the high land cost in HK, which accounts for nearly 90% of the total cost. For bioretention system and porous pavement, the main reason is the high operation and maintenance costs due to the very limited experience and data in HK. The results of green roof are sensitive to initial saturation and hydraulic conductivity, while those of bioretention system are sensitive to hydraulic conductivity and berm height. Porous pavement is only sensitive to hydraulic

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conductivity. A 20% change in any of the parameter leads to various levels of peak runoff reduction changes for the original optimal design. When a new optimal design is identified for the same 20% peak runoff reduction, the resulting percentage change in the cost is however smaller than the percentage change in the parameter. Overall, the sensitivity to model and design parameters does not affect the main trends observed in and key insights derived by this part of the study.

Figure 3 Cost-effectiveness curves of 2yr and 50yr design storms in Hong Kong and Seattle for (a) green roof, (b) bioretention and (c) porous pavement.

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Table 4 Peak runoff reduction and cost-effectiveness of optimal designs

City Design

storm LID elements

Peak

runoff

(L/s)

Percentage of peak

runoff reduction/

thousand Dollars

(%/103US$)

Volume of peak

runoff reduction/

thousand Dollars

(L/103US$·s)

Hong

Kong

2yr

Green roof 81.8 0.02 0.02

Bioretention 80.0 0.15 0.15

Porous pavement 81.6 0.91 0.93

50yr

Green roof 149.9 0.02 0.04

Bioretention 149.6 0.09 0.16

Porous pavement 149.9 0.61 1.15

Seattle

2yr

Green roof 17.2 0.12 0.03

Bioretention 17.0 1.34 0.29

Porous pavement 17.0 7.33 1.58

50yr

Green roof 41.9 0.12 0.06

Bioretention 42.0 0.78 0.41

Porous pavement 41.2 4.36 2.28

II Social and Economic Analysis

In general, respondents’ understanding towards stormwater management was not strong as shown in Figure 4. Their estimate of government spending on managing stormwater was only half of the actual amount. However, most respondents (91.0%) supported HK government to build LID in public places with “reducing greenhouse effect” and “improving the environment” as their main motivations. The respondents also indicated that it is their responsibility to contribute to the sustainable development in HK. Regarding private properties, most respondents (respectively 89.4% and 80.8%) would like to see more LID installed and support government to implement regulations to encourage developers to build LID. The main motivations are the same as those for public places, and the respondents also considered LID to be beneficial to both residents and society. However, most respondents (68.1%) considered the additional real estate value brought by LID as the least important factor.

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Figure 4 Understanding towards stormwater management in Hong Kong

Respondents who were relatively younger, with higher income, better educated and from a previously flooded district were willing to pay more for LID in public places. For LID in private properties, most of the respondents were only willing to pay less than 1% of the property price. Interestingly, respondents with highest education and income levels were willing to pay the smallest percentage of the property price. The mean and median WTP are HKD 87.6 and HKD 149.6 for public places, and are 1.8% and 2.5% of the property price for private properties, which are lower than those from previous overseas studies. However, with the high population density and property price in HK, the aggregate WTP per unit area can therefore still high.

Regarding citywide economic analysis, the total maximum potential LID areas in HK is estimated to be about 73 km2, which accounts for 27% of impervious cover and 6.5% of total land area of HK. The proposed procedure considers the service life-cycle of LID to calculate the net benefit and annual unit net benefit with a 30 year life service period. Given the assumptions adopted in this study, preliminary estimations show that the life-cycle net benefit is positive. However, there are many uncertainties and assumptions involved in the calculations and further detailed analysis is required. The net benefit is also highly sensitive to the WTP of HK people, in particularly the private WTP, and thus should be examined in more details. Although city expansion decreases both the mean and medium net benefits of LID, the values still remain positive, supporting LID implementation.

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III Formulate plausible implementation approach and policy for HK

Based on the results from Parts I and II as well as surveys and interviews of this part of the project, some key objectives of LID for HK are proposed. LID elements can be introduced in developed areas as retrofits for stormwater management, but they are more likely to be first introduced in New Development Areas (NDAs), supplementing the conventional drainage system. Similar to Singapore, enhancing community livability and environmental protection is slso important. The main results of the survey questionnaire performed at the one-day workshop are summarized in Table 5.

Table 5 Result of the survey

Questions Results

Most popular LID elements

1st: Green roof and podium garden in non-residential building 2nd: Bioretention system in parks 3rd: Green roof and podium garden in residential building 4th: Porous pavement in pedestrian walkways

Most popular implementation

strategies

1st: Development incentives 2nd: Pilot projects 3rd: Awards and recognition 4th: Financial assistance

Steering Committee

Most respondents agreed to setting up a steering committee Most suggested parties to be involved: Government departments:

Development Bureau (DevB) Water Services Department (WSD) Drainage Services Department (DSD) Lands Department (LandsD) Building Department (BD) Leisure and Cultural Services Department (LCSD) Highways Department (HyD) Agriculture, Fisheries and Conservation Department (AFCD) Civil Engineering and Development Department (CEDD) Hong Kong Housing Authority (HA)

Academic researchers and institutions Consultant representatives Contractor representatives Management and maintenance parties Private developer representatives Green groups General public (e.g. District Council, local residents)

Most suggested incentives

1st: Awards and recognition 2nd: Government policy directives and supports (e.g. case sharing, guidelines, initiation) 3rd: Financial assistance (e.g. subsidies, tax relief) 4th: Development incentives (e.g. Increase allowed GFA, exclude in plot ratio)

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Most suggested projects for LID

New Development Areas (NDAs) Public Housing

Suggested guidelines subjected to revision

1st: Stormwater Drainage Manual 2nd: Building ordinance 3rd: Hong Kong Planning Standards and Guidelines 4th: DevB Technical Circular 5th: HyD guidance notes 6th: Practice Notes for Authorized Persons 7th: Land drainage ordinance 8th: BEAM plus

New guideline of LID

Half of samples agree to have a new guideline of LID. Most suggested parties involved: DSD WSD BD Planning Department LCSD AFCD DevB HyD Environmental Protection Department HA

Valuable comments and suggestions were also obtained from the interviews of various professionals. There are two main approaches of LID implementation in HK. The first is to incorporate LID into NDAs such as Anderson Road Quarry Site, Tung Chung New Town Extension, NDAs of Kwu Tung North, Fanling North, Hung Shui Kiu in the medium term, and other potential development areas such as North New Territory and eastern Lantau Island in the long term. Incorporating LID in NDAs is in general more feasible since there are less physical constraints, and the LID could be designed together with the conventional drainage system. Another approach is to retrofit existing structures with LID when the existing drainage system requires upgrade, replacement, and/or rehabilitation. However, more research studies are required to integrate LID with existing stormwater drainage facilities given the unique conditions of HK. More possible implementation approach and policy are proposed and explained under Section 1 Policy Implications and Recommendation.

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c) Public Dissemination

A one-day workshop on October 6, 2015 was organized by the PI together with Ir Dr. Onyx Wai from The Hong Kong Polytechnic University, who is also an expert in LID. Other than the PI disseminating her project results, representatives from different HK government departments were invited to share their recent initiatives in this area. Academics from both overseas and local universities were also present to share their specific research projects. Information of the speakers and talks are included in the workshop rundown in Appendix A. Over 100 participants, from different government departments, consultants, universities, etc. attended the presentations and engaged in different discussions. Figure 5 are some of the photos taken during the workshop.

Speakers and participants engaging in discussions

Participants listening to speaker’s presentation

Speakers and participants interacting over lunch

Organizers presenting certification of appreciation

to speaker Figure 5 Photos taken at the one day workshop promoting low impact development and disseminating research results

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3 Project Effectiveness

a) Objectives versus Results

Table 6 below compares the project results with the original project objectives:

Table 6 Objectives versus project results

Objectives Results I Develop LID technical metrics and

analytical tools for the dedicate use in HK

- Relevant data and information, both local and overseas, are compiled and numerical models are developed to evaluate the hydrologic performance of LID elements suitable for HK (i.e., green roof, bioretention system and porous pavement)

- Hydrologic and cost effectiveness of different LID elements are compared, and application recommendations are made

- Optimal dimension, as well as required areal coverage of each LID element are determined

II Transform scientific theories and technical knowledge into social and economic outcomes in HK;

- Perception of HK people and their willingness to pay for LID are examined in dilife-cyclefferent areas of HK

- Cost and benefits of each LID element is estimated, and life-cycle net benefit of citywide LID implementation is determined

III Formulate plausible public policy for HK LID with reference to overseas experience.

- Based on overseas experience as well as the results from Parts I and II, implementation approaches and policy for LID are proposed and summarized in Section 1

The exact methods used could be different from those stated in the original proposal because more information and experience become available as the project progresses. However, the three objectives have been achieved and thus the project is overall effective in facilitating LID implementation and sustainable development of HK. The one-day workshop was also organized to promote LID and share the research results with over 100 professionals.

b) List of Publications

Other than meeting the original objectives, this project has also produced a number of publications. As listed below, three journal manuscripts have been submitted. One conference paper has been accepted and an oral presentation was made in August, 2015 in Hague, the Netherlands. Another presentation will be made in December, 2015 in San Francisco, U.S.

Journal manuscripts

1. Chui T.F. M., X. Liu and W. Zhan. Assessing cost-effectiveness of specific LID practice designs in response to large storm events. Submitted.

2. Chui T.F. M. and W.Y., Ngai. Willingness to pay for sustainable drainage system in a highly urbanized city: A contingent valuation study. Submitted.

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3. Zhan, W. and T.F.M. Chui. Evaluating life cycle net benefit of low impact development in a city. Submitted.

Conference presentations

1 Chui T. F. M. and A. C. W. Wong, 2015. Comparison of green roof performance in stormwater mitigation. E-proceedings of 36th IAHR World Congress. Hague, the Netherlands. 2I Flood risk management and adaptation: 81365.

2 Chui T.F. M., X. Liu and W. Zhan, 2015. Assessing cost-effectiveness of green infrastructures in response to large Storm events at household scale. Proceedings of the American Geophysical Union (AGU) Fall Meeting, San Francisco, CA, United States, H21J-1521.

References

Center for Neighborhood Technology and American Rivers (CNT and American Rivers), 2010. The Value of Green Infrastructure: A Guide to Recognizing Its Economic, Environmental and Social Benefits. Available at: http://www.cnt.org/repository/gi-values-guide.pdf. [Accessed 16 September 2015].

COMSOLAB, 2010. COMSOL Multiphysics reference guide (Version 4.1). Stockholm, Sweden; 2010

Rossman, L. A., 2010. Storm water management model user's manual, version 5.0. National Risk Management Research Laboratory, Office of Research and Development, US Environmental Protection Agency.

van Spengen, J., 2010. The effects of large-scale green roof implementation on rainfall-runoff in a tropical urbanized subcatchment. A Singapore case study. Faculty Civil Engneering and Geosciences. Deft, The Netherland: Delft University of Technology (TuDelft).

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Appendix A Workshop Rundown

The University of Hong Kong Department of Civil Engineering

The Hong Kong Polytechnic University

Department of Civil & Environmental Engineering

Workshop on Sustainable Rainwater Management 06 October, 2015, Tuesday, 9:00am -5:00pm

Room P5-03, Graduate House, No. 3 University Drive, The University of Hong Kong

Many urbanized cities including Hong Kong (HK) are striving for sustainable development and management of water resources and environment. This workshop focuses on the sustainable management of rainwater - maximizing freshwater supplies and mitigating flood hazards using environmental friendly approaches. Representatives from different HK government departments would share their recent initiatives in this area. Academics from both overseas and local universities would also present their specific research projects.

Event rundown: 08:30-09:00 Registration 09:00-09:30 Promoting Low Impact Development via

the Land Development Permitting Procedure and the Water Utility District System

Dr. Chang Ni-Bin, Professor at University of Central Florida (UCF), Director of UCF Stormwater Management Academy

09:30-10:00 Green Water Management Initiatives in New Development Areas of Hong Kong

Mr. Lau Chun-kit, Ricky, Deputy Head (Port and Land) of Civil Engineering Office, Civil Engineering and Development Department

10:00-10:30 Revitalising Water Bodies Mr. Ho Yiu Kwong, Chief Engineer/Land Drainage, Drainage Services Department

10:30-11:00 Break 11:00-11:30 Sustainable Use of Rainwater Resources in

Hong Kong Mr. Andrew Y.K. Hui, Chief Engineer /Development (2), Water Supplies Department

11:30-12:00 Developing Sustainable HK through Low Impact Development: from Science to Innovation Policy

Dr. Ting Fong May Chui, Assistant Professor at The University of Hong Kong

12:00-12:30 Discussion (Chairman: Ir PK Chan, former AD of DSD) 12:30-14:00 Lunch 14:00-14:30 Multi-purpose Rainwater Harvesting for

Water Resource Recovery and the Cooling Effect

Ir Dr. Alicia K.J. An, Assistant Professor at City University of Hong Kong

14:30-15:00 Coupling Multi-scale and Multi-attribute Models to Assess the Risk and Resilience of the Sustainable Urban Drainage System

Dr. Chang Ni-Bin, Professor at University of Central Florida (UCF), Director of UCF Stormwater Management Academy

15:00-15:30 Break 15:30-16:00 Laboratory Investigation of Bioretention and

Porous Pavement in Reducing Stormwater Runoff

Dr. Lam Kit Ming, Associate Professor at The University of Hong Kong

16:00-16:30 Extensive Green Roofs for Runoff Mitigation Ir Dr. Wai Wing-hong, Onyx, Professor at The Hong Kong Polytechnic University

16:30-17:00 Discussion and Closing Remarks (Chairman: Dr. May Chui, Assistant Professor at HKU)

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Venue map: http://www.gradhse.hku.hk/GI/Data/201307hkumap.jpg

Language: English Registration fee: Nil Registration: By invitation, or advance online registration on a first come first served basis at https://hkuems1.hku.hk/hkuems/ec_hdetail.aspx?guest=Y&ueid=39246 Certificate: Attendance certificate will be issued Enquiry: Ms. Ruby Kwok by email at rubykwok@hku.hk or by phone at 2219 4986

Workshop funded by Central Policy Unit, HKSAR Government, and supported by International Association for Hydro-Environment Engineering and Research – Hong Kong Chapter (IAHR-HK)