Effect of revitalizing water Steve H.L. YIM 1,2,3 bodies on thermal environment 1 Department of Geography and Resource Management, CUHK 2 Institute of Environment, Energy and Sustainability (IEES), CUHK DSD R&D Forum 2017 3 Centre for Environmental Policy and Resource Management, CUHK 14-11-2017 香港中文大學 1 The Chinese University of Hong Kong
Thermal environment in Hong Kong Source: USEPA (http://www.epa.gov/hiri/resourc es/pdf/BasicsCompendium.pdf) Monthly mean number of Very Hot days observed at the Hong Kong Observatory (1981-2016) 6 4 2 0 Daily maximum temperature ≥ 33.0 ° C 2
Introduction • This assessment is under the consultancy of Feasibility on Revitalisation of Water Bodies to assess the potential benefits provided by revitalised water bodies. 3
Objectives • To evaluate the effect of various land covers on urban heat island effect and thermal comfort in Hong Kong context • To compare and rank the performance among studied land covers • To provide recommendations to river revitalization 4
Studied rivers Yuen Long Nullah Lam Tsuen River 5
Measurements 1. Surface temperature 3. Short- and long-wave radiations (mean radiant temperature for thermal comfort) FLIR SC660 2. Ambient temperature CNR4 Net Radiometer and relative humidity 4. Wind speed and direction HOBO temperature/RH Davis Anemometer 6
UHI and thermal comfort Semi Grass Water Concrete Full Grass Dry Clay Surface urban heat island Atmospheric urban heat island Thermal comfort Reference 7
Land Cover Types Different land covers (4 types) Yuen Long Nullah A B Pedestrian Level C 1 Concrete 1: Concrete 4: Full Grass 2: Dry Clay 3: Semi Grass 1 Dry Clay 2 Time Period Duration Locations of Measurements Semi-Grass 3 1 15 min A, B, C, 1, 3 2 15 min A, B, C, 2, 4 Full-Grass 4 3 15 min A, B, C, 5, C2 (under tree) 8
Land Cover Types Different land covers (4 types) Lam Tsuen River Pedestrian Level C 5: water Limitation 1: Concrete 4: Full Grass Full-Grass 4 2: small Grass 3: Semi-Grass B small-Grass 2 Water 5 A 3 Semi-Grass 1 Time Period Duration Locations of Measurements A: Wind Speed and Direction Concrete 1 15 min A, B, C, 1, 3 2 15 min A, B, C, 2, 4 9
Measurement schedule Time No. Date Start End 1 June 20* 11:45 21:30 2 June 21* 11:45 21:30 3 June 22 11:45 21:30 4 June 23 11:30 21:30 5 July 8^ 11:30 21:30 6 July 18 14:00 21:30 7 July 23 11:45 21:30 8 July 25 12:00 21:45 9 August 23 12:00 21:45 10 August 24 12:00 21:45 III: (33 o C ≤ T air, 1.2m < IV: (34 o C ≤ T air, 1.2m < Cloud \ T air, 1.2m 34 o C ) 35 o C) Yuen long Nullah CR: Clear Sky 23/6, 23/8 25/7, 24/8 NC: Non-Clear Sky 22/6, 18/7 23/7 III: (33 o C ≤ T air, 1.2m < IV: (34 o C ≤ T air, 1.2m < Cloud \ T air, 1.2m 34 o C) 35 o C) Lam Tsuen River CR: Clear Sky 24/8 22/6, 23/7, 25/7 NC: Non-Clear Sky 23/6, 18/7 23/8 10
Major variables for analyses • Surface temperature • Long-wave radiation flux • Long-wave mean radiant temperature 11
What is Mean Radiant Temperature, MRT? Integral Radiation measurements Sofia Thorsson et al, 2007 4 ( Τ 𝑁𝑆𝑈 = 𝑇 𝑡𝑢𝑠 𝜁 𝑞 𝜏) − 273.15 6 6 𝑇 𝑡𝑢𝑠 = 𝛽 𝑙 𝐿 𝑗 𝐺 𝑗 + 𝜁 𝑞 𝑀 𝑗 𝐺 𝑗 𝑗=1 𝑗=1 Weighted Sum of Short-wave Weighted Sum of Long-wave Solar and Infrared Radiation Measurements Frank Vignola, Joseph Michalsky, and Thomas Stoffel CRC Press 2012 300 nm to 2800 nm short-wave 4.5 to 42 μ m long-wave 12
Hypothetical Study – Part 1 𝑀 𝑗 = 𝑀 𝑘 𝐺 𝑗−𝑘 N 𝑘 % easterly: concrete + water + concrete L_easterly = 2(Fe_3) Lwat + 2(Fe_2 + Fe_4)Lcon + (1-2Fe_234)Lair; A2 A2 LMRT_easterly = (L_easterly /(0.97*sigma))^(0.25) - 273.15; x3: 0 – 100m; A3 % independent variable % increasing A2 x4 = 100 – x3; % in m A2 A4 r3: x3/100; A3 A4 13
Hypothetical Study – Part 1 𝑀 𝑗 = 𝑀 𝑘 𝐺 𝑗−𝑘 N 𝑘 % easterly: concrete + grass + concrete L_easterly = 2(Fe_3) Lgra + 2(Fe_2 + Fe_4)Lcon + (1-2Fe_234)Lair; A2 A2 LMRT_easterly = (L_easterly /(0.97*sigma))^(0.25) - 273.15; x3: 0 – 100m; A3 % independent variable % increasing A2 x4 = 100 – x3; % in m A2 A4 r3: x3/100; A3 A4 14
Hypothetical Study – Part 1 Long-wave radiation :east (W/m 2 ) concrete concrete LMRT: east ( ° C) grass grass water water X3 (m) X3 (m) X1 (5m) X2 X3 (0.5m) Don’t cite; Not yet published 15 X4 10m
Hypothetical Study – Part 2 𝑀 𝑗 = 𝑀 𝑘 𝐺 𝑗−𝑘 N 𝑘 % easterly: concrete + water + concrete L_easterly = 2(Fe_4) Lwat + 2(Fe_2 + Fe_3)Lcon + (1-2Fe_234)Lair; A2 A2 LMRT_easterly = (L_easterly /(0.97*sigma))^(0.25) - 273.15; x3: 0 – 100m; A3 % independent variable % increasing A2 x4 = 100 – x3; % in m A2 A4 r3: x3/100; A3 A4 16
Hypothetical Study – Part 2 𝑀 𝑗 = 𝑀 𝑘 𝐺 𝑗−𝑘 N 𝑘 % easterly: concrete + grass + concrete L_easterly = 2(Fe_4) Lgra + 2(Fe_2 + Fe_3)Lcon + (1-2Fe_234)Lair; A2 A2 LMRT_easterly = (L_easterly /(0.97*sigma))^(0.25) - 273.15; x3: 0 – 100m; A3 % independent variable % increasing A2 x4 = 100 – x3; % in m A2 A4 r3: x3/100; A3 A4 17
Hypothetical Study – Part 2 Long-wave radiation :east (W/m 2 ) concrete concrete LMRT: east ( ° C) grass grass water water X3 (m) X3 (m) X1 (5m) X2 X3 (0.5m) Don’t cite; Not yet published 18 X4 10m
Conclusions • Concrete and dry clay land covers can cause a 20-22°C increase in surface temperature and a 12-13°C increase in LMRT. The concrete and dry clay land covers should be avoided. • Water is effectively to reduce surface temperature (↓7 °C) and thermal discomfort (↓ 4 °C LMRT). Water • Full Grass should be considered to improve the thermal environment including both surface temperature (↓6 °C) and thermal comfort (↓ 3 °C LMRT). Don’t cite; Not yet published 19
Conclusions • The width of belt and the land cover of banks should be carefully designed to mitigate thermal discomfort. • The cooling effect of the width on X1 (5m) revitalisation of water bodies is up to ~30m. X2 X3 (0.5m) 10m X4 • The cooling effect of the width on X1 promenade is up to ~30m. (5m) X2 X3 (0.5m) 10m X4 Don’t cite; Not yet published 20
Conclusions • Effect of land covers on thermal environment should be taken into account in water body designs and management. • Land cover effect information should be integrated into relevant guidance and policy documents. Don’t cite; Not yet published 21
Limitations - General • Yuen Long Nullah is strongly affected by the surrounding concrete bank and the narrower belt; • Lam Tsuen River has more diverse ambient environment, different size of trees and unevenly distributed green features – such as the impact of tree shading effect on measurement over the measurement over concrete land cover type; • Results will be affected by a number of environmental factors, especially in Lam Tsuen River; 22
Limitations – Land covers • A real and fully sunlit bare concrete was missed for better reference at Lam Tsuen River due to renovation of pavement. • Yuen Long Nullah, differences in long-wave radiant fluxes, might be attributed to a larger distance between bare concrete land cover and other types of land covers; • At Yuen Long Nullah, the solar radiation incident on the bare concrete might be blocked by nearest residential building in the early evening. • Length of leaves of any grass land covers (semi-grass, full grass, and small grass) varied due to uncontrolled growth and unexpected mowing work; 23
Limitations – Method • The weather conditions were not significantly different in daytime for each site; • This study was performed for a limited number of time due to the restriction of weather conditions and safety concerns. • Conducting field measurements at a limited number of sites may not allow us to understand the situation in other environments. 24
Acknowledgment • A group of CUHK helpers: Alan Lai, Samuel Tang, Yongli Wang, Ifean Nduka, Marcu Tong, Sammi Luk, Matthew Ma, Gabriel Fan, Kenji Leong, Dennis Leung, Claire Tang, Calvin Chan, Edwin Yeung, Sabrina Hung, Aaran Lam, Andy Tang, Rebecca Liu, Jamie Wong, Henry Wong, Hung Sze Ping, Claudia Ng, Lee Wing Yung • Max Lee from the School of Architecture at CUHK • DSD and AECOM 25
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