Potential of Low-Impact Development for Stormwater Management in - PowerPoint PPT Presentation

Potential of Low-Impact Development for Stormwater Management in Hong Kong Kit Ming LAM, Ting Fong May CHUI, Ze-ying LI Department of Civil Engineering, University of Hong Kong

Flooding  Urban drainage demand

Culprits for Urban flooding in Hong Kong  Severe (short and intense) rainstorms  Storm drain extension in harbour reclamation  Ageing of storm drains  Steep topography

Urbanization  Need for stormwater drainage reduced permeability of land  Hard engineering solutions Source: http://en.wikipedia.org/wiki/Stormwater Additional Runoff • Erode watercourses Water body pollution • • Flooding

Large infrastructures, e.g., :  Upstream interception and diversion for flood prevention, e.g., Hong Kong Western Drainage Tunnel  Flood storage scheme, e.g., Tai Hang Tung storage tank  Average Annual Rainfall of Hong Kong ~ 2200 mm  Concentrates in May - September

Low impact development (LID)  Environmental friendly solutions Stormwater LIDs: • Devices, practices, or methods used to manage stormwater runoff locally • Mitigate changes to water quality of urban runoff caused by increased impervious surfaces from land development • Can also reduce stormwater volume and peak flows • Work through evapotranspiration, infiltration, detention, and filtration or biological and chemical actions. Also known as: • Stormwater best management practices (BMP) • Treatment train • Sustainable urban drainage systems (SUDS) • Water sensitive urban design (WSUD)

Common LID devices Vegetated swale Permeable/porous pavement Green roof Bioretention (rain garden)

Stormwater LIDs in Urban Drainage Challenges in Hong Kong: Steep topography  low retention rate of runoff × × Very heavy rainfall  Existing drainage design and layouts Retrofit  Lack of spaces  Inadequate awareness of public and engineers

Retrofit stormwater LIDs Experience overseas: Page et al. (2014), Schlea et al. (2014)

Retrofit stormwater LIDs Possibilities for Hong Kong landscape garden  bio-retention Rain Gardens  Effective design for road-side applications

Potential LID applications in Hong Kong Bioretention and Porous pavement: Working principles  Infiltration structures: Retention & Infiltration General benefits • Reduce adverse stormwater impacts due to urban development • Easy to incorporate into urban landscaping • Can be applied on a wide scale Few previous applications in Hong Kong • Rainfall patterns in Hong Kong? • Ease of retrofit? • Installation and maintenance problems?

Bioretention - Introduction Bioretention: • Depressed planter with engineered soil (soil media) • Receive and store stormwater from the vicinity of large area. • Allow stormwater penetrate through it. • Encourage stormwater seeping into the ground. Examples of ponded bioretention cell Overflow Inflow Depression storage Infiltration into (assumed soil media constant) • Absorption by soil media • Percolation into in-situ soil • Underdrain (optional)

Bioretention - Introduction Bioretention usually serves adjoining impervious catchment area Driveway Rooftop Car park Park Source: Curtis Hinman (2007), Rain Garden Handbook for Western Washington Homeowners, Designing your Landscape to protect our streams, lakes, bays, and wetlands, Washington State University Extension Pierce Country

Bioretention - Introduction Performance of rain garden can be determined by  Uncontrollable factors Local rainfall pattern •  Controllable: factors 1. Inflow • Area of the rain garden vs. area of whole catchment, it determines the amount of inflow • Storage depth of the rain garden, or the volume between the soil surface and the controlled 2. Overflow overflow level • Infiltration rate, or the rate of water dissipation through 3. Infiltration infiltration

Bioretention - Experiment Pilot-scale physical model of bioretention cell at HKU • Monitor inflow and overflow under local rainfall events. • Quantify peak reduction and volume retention against rainfall events. • Provide data for verification of numerical model. • Rooftop area = 26 m 2 • Rain garden area = 0.9 m 2 Physical Experiment  A planter-box type rain garden constructed for collecting and treating runoff from a rooftop  Measure inflow, overflow, and underdrain for real rainfall events

Bioretention - Experiment Bioretention cell  Planter box Flow measurement of overflow and underdrain Freeboard = 50 mm Storage depth = 95 mm Soil media (Infiltration rate ∼ 50 mm/hour) = 450 mm Gravel and pipe = 100 mm • Rates of inflow, overflow, and underdrain were monitored in 5-second intervals • Data monitored during (1) August 2 to September 5, 2013; (2) March 20 to April 8, 2014

Bioretention - Results 58 rainfall events identified during the 55 days of monitoring periods Rainfall depths ranged from less than 1 mm to over 90 mm. Observation: Performance of the bioretention cell is affected largely by the rainfall depths.  Typical examples….

Bioretention - Results (1) 04:57-05:16, August 3, 2013. Effective rainfall depth = 4.2 mm Full interception (2) 00:40-01:22 on Aug 14, 2013. Effective rainfall depth = 6.8 mm. Partial interception  Overflow observed but behind the first peak inflow

Bioretention - Results (3) 12:09-14:11 on Aug 13, 2013. Effective rainfall depth = 27.6 mm. Negligible interception  Most inflow bypassed the cell through overflow  Overflow observed and lasted till the end of inflow  Magnitude of underdrain was much smaller than overflow

Bioretention - Results Peak flow reduction ratios for all 58 rainfall events: • 100% reduction for events of depth < 3 mm. • Little reduction for events of depth > 3 mm. Same observation on the Volume retention ratios 

Bioretention - Results Overflow depth vs. inflow depth for events less than 30 mm. Volume retention: • 2.7 mm was retained statistically from the monitored results. Physical set up: • 95 mm storage depth in bioretention cell, serving catchment at 1 / 3.3% • that is 95 × 3.3% = 3.1 mm effective storage depth  The bioretention cell can retain rainfall close to its storage depth only.  (Infiltration rate is not very large)

Bioretention - Results Given a fixed setting (storage depth) of a bioretention cell, its retention performance is governed by: • Rainfall depth (and pattern) • Catchment area ratio (spreading thin the storage depth) • Infiltration rate of soil media With the monitoring results from the physical model, numerical model (e.g., SWMM) is used to estimate the hydrologic performance of a bioretention cell (storage depth = 150 mm) for local rainfall climate and patterns Example: 10-year averaged retention ratio of stormwater in Hong Kong rainfall conditions (2004-2013) 

Porous Pavement - Introduction  Similar infiltration BMP structure as bioretention • commonly used to collect rainfall on its surface and some apron area. Overflow Inflow Storage • can provide storage layer for from stormwater below the surface. pores • allow stormwater to percolate into in-situ soil at the bottom. Infiltration into in-situ soil and underdrain

Porous Pavement - Introduction General configuration: 1) Permeable surface course and bedding layer (permeable enough to allow inflow to penetrate through) 2) Open-graded gravel base course (storage layer to provide additional storage below the surface) (http://www.icpi.org) 3) Underdrain and in-situ soil (to drain the stored water away) 4) Overflow structure (to avoid flooding the pavement)

Porous Pavement - Introduction Major differences from bioretention:  large watershed area ratio, or the catchment is restricted to the area of permeable pavement, thus allowing a considerable storage.  can be designed to capture all the stormwater, leading to a good reduction in peak discharge and high volume retention. Major hydrologic design parameters: • Design storms • Gravel storage • Infiltration capacity of in-situ soil

Porous Pavement – Field Tests Site trials of porous pavements in Hong Kong (2014-15)  A study initiated by DSD (Drainage Services Department, Hong Kong Government)  To explore the hydrologic performance of porous pavements under the local rainfall conditions of Hong Kong  Test site in Western Kowloon 3 test panels, each 4 m × 3 m & 1 control panel (impervious concrete cover)

Porous Pavement – Field Tests Three surface course materials: 1) PICP: permeable interlocking concrete pavers (with fine loose aggregates in between gaps) 2) OCP: open-cell pavers (with fine loose aggregates in open area) 3) PB: permeable paver blocks (with fine sand filling the gaps) (a) PICP (b) OCP (c) PB

Porous Pavement – Field Tests Vertical structures: 1) Surface course: paver blocks (60 to 90 mm thick) 2) Bedding layer: fine aggregates (10 mm), or sand, thickness ∼ 40 mm 3) Base course: washed uniformly graded open aggregates (37.5 mm), Storage layer of thickness 500 mm 4) Sub-soil, with geotextile on top