Sustainable Stormwater Management with Low Impact Development (LID)

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1 Sustainable Stormwater Management with Low Impact Development (LID) Chui Ting Fong, May Department of Civil Engineering The University of Hong Kong HKIE Seminar Nov 27,

2 Hydrologic Impact of Urbanization Urbanization Hydrological problems (e.g. Surface runoff, groundwater recharge and river discharge) Pictures from California Water and Land Use Partnership 2

3 Low Impact Development Development approach that works with nature to manage stormwater close to its source. Functions include - restoring infiltration, subsurface storage - Improving water quality, etc. Picture from California Water and Land Use Partnership 3

4 Rain Garden (Bioretention System) Figure from FAWB,

5 Green Roof Green roof 5

6 Porous Pavement and also vegetated swale, constructed wetland, etc. 6

7 Benefit Environmental Economic Social Table extracted from US EPA, 2010 Type - Increase carbon sequestration Benefits of LID - Improve water quality - Additional recreational space and aesthetic value - Efficient land use - Improve human health - Flood protection - Drinking water source protection - Replenish groundwater - Improve watershed health - Protect or restore wildlife habitat - Reduce sewer overflow events - Restore impaired waters - Meet regulatory requirements for receiving waters - Reduce hard infrastructure construction costs - Maintain aging infrastructure - Increase land values - Encourage economic development - Reduce energy consumption and costs - Increase life cycle cost savings - Establish urban greenways - Provide pedestrian and bicycle access - Create attractive streetscapes and rooftops that enhance livability and urban green space - Educate the public about their role in stormwater management - Urban heat island mitigation 7

8 Overseas Experience Started as early as in 1980s and gained popularity in 2000s U.S (LID or best management practices) U.K. (sustainable drainage systems, SuDS) Australia (water sensitive urban design, WSUD) Singapore (Active, Beautiful, Clean (ABC) Waters Programme) Some knowledge is transferable to HK 8

9 Hong Kong Stormwater has been managed relatively independently from water supply Ecological considerations in channel designs 9

10 LID in HK Most LID elements are not commonly adopted Research projects at universities DSD has harvested rainwater at two of its sewage pumping stations in Kowloon City AECOM works with HKHA on rainwater harvesting using bioretention system CEDD plans to implement various LID elements at Anderson Road Quarry Site 10

11 My LID Projects Individual Techniques Bioretention system Green roof Nullah decking Porous pavement Catchment Scale Implementation Singapore and Hong Kong catchments Bioretention location optimization Socio-economics and public policy 11

12 Individual Green Roof Design Function as a microcatchment on the top of a building Include a number of hydrological process Provide potential stormwater management benefits depending on the green roof design Picture from Stovin (2010) 12

13 Research Objectives Evaluate the factors affecting the hydrological behavior of green roof Suggest suitable parameters to maximize its hydrological benefits 13

14 One-dimensional Numerical Model Interception (Trinh and Chui, 2011) Evapotranspiration (Penman-Monteith eq.) Rainfall infiltration Vegetation layer Outlet discharge QQ pppppppp = BBCC dd AA 0 OOOOOO = QQ pppppppp AA 2ggg Free flow Soil layer (Richard s eq.) ρρ CC mm + ρρρρρρρρρρ HH pp tt + ρρρρ HH pp + 1 Storage reservoir h tt = IIII OOOOOO 14

15 Model Calibration Time series Rainfall event Rainfall event Rainfall depth (mm) Runoff depth (mm) Measured Simulated 05/07 17: /07 15: /07 12: /07 15: /07 11: /07 08: /07 11: /07 23: /07 07:

16 Simulation Scenarios Using rainfall condition in July 2009 in Singapore Hydraulic conductivity (K): 1.8x10-6, 10-6, 10-5, and 10-4 m/s (no storage reservoir) Soil thickness: 21, 35 and 50 cm (no storage reservoir) Storage reservoir capacity: 1.5, 3, 4.5 and 6 cm reservoir height (21 cm soil thickness and K = 1.8x10-6 m/s) 16

17 Influences of Hydraulic Conductivity 17

18 Influences of Soil Layer Thickness 18

19 Influences of Storage Reservoir Capacity 19

20 Conclusions from First Project Green roof can have strong hydrological influences reduce peak runoff and mitigate urban flooding varies depending on the design and the rainfall event Higher hydraulic conductivity more infiltration but less peak discharge reduction and delay Thicker soil layer more water retention + more discharge delay but more building load Storage system more rainfall retention + peak discharge delay (when not full) no additional impact (when full) 20

21 A preliminary study to improve our understanding of hydrological behaviour of green roofs for stormwater management Fundamental information for optimizing green roof properties for urbanized areas Requires further physical experiment and numerical model scenarios 21

22 Catchment Planning Many previous studies have examined the effectiveness of LID, particularly as individual elements More understandings should be developed for catchmentscale implementation Figure from Singapore PUB ABC Waters Design Guidelines 22

23 Study Catchments Singapore vs Hong Kong Marina Catchment Catchment of Ma Wat River (~161 km2) (~8 km2) 23

24 Numerical Modeling MIKESHE vs SWMM (Commercial software by DHI) (Available from US EPA) 24

25 Scenarios to be Considered Level of development or restoration Pre-urbanized Urbanized Different LID implementation strategies Only one particular type of green structure Combinations of different types Design rainfall Average performance over a long period 3 month, 2 year, 5 year or higher ARI 25

26 Integrated Distributed Hydrological Modeling MIKESHE allows more accurate depiction of groundwater surface water interactions (e.g., increase of infiltration, restoration of baseflow) 26 Trinh and Chui (2013)

27 Lumped Hydraulic Modeling SWMM performs river routing with relatively less parameters 27

28 Reduction of Peak Flows Either extensive green roofs or bioretention systems are effective in mitigating the peak discharges. 28

29 Effectiveness Decreases with Storm Intensity 29

30 Importance of Catchment Choice Not as effective in the Hong Kong catchment because there is a large undeveloped hillslope which accounts for 77% of the catchment area Very limited space available in the remaining urban area for bioretention systems which are in general more effective than green roofs in retaining stormwater 30

31 Main Study Conclusions Requires substantial level of infrastructures, either converting 5% of the Singapore catchment into bioretention systems, or converting the roofs of all the buildings which account for 14% of the Singapore catchment Effective in the Singapore catchment on an average long term basis in reducing/delaying peak flow and restoring baseflow Not as effective in the Hong Kong catchment during small ARIs (e.g., less than 5 years) 31

32 More Insights LID requires catchment level implementation, which is difficult for developed catchments Numerical modeling facilitates the design, and the choice of software depends on data availability and processes of interests Success of LID depends on Objectives (e.g., reduce peak flow, maintain low flows) Catchment choice (e.g., land uses, existing drainage networks) Overall, catchment scale LID potentially provides more sustainable stormwater management and should be more widely studied and adopted 32

33 My LID Projects Individual Techniques Bioretention system Green roof Nullah decking Porous pavement Catchment Scale Implementation Singapore and Hong Kong catchments Bioretention location optimization Socio-economics and public policy 33