In recent years, there are reports and concerns on climate change, green house effect and global warming. Climate change affects a number of meteorological




НазваниеIn recent years, there are reports and concerns on climate change, green house effect and global warming. Climate change affects a number of meteorological
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RD 1036 - An Analysis of Effects of Climate Change on Stormwater Drainage System

1. Introduction


In recent years, there are reports and concerns on climate change, green house effect and global warming. Climate change affects a number of meteorological phenomena and variables, e.g. temperature, visibility, solar radiation, evaporation, snow cover and ice extent, oceanic circulation, sea level, rainfall, El Niño event, monsoons, tropical cyclone, etc. The concerns of climate change could be very different for different professions. For example, in an article of a magazine published by the Association of Chartered Certified Accountants (ACCA), it is mentioned that

the emissions (greenhouse gases) trading sector is on the verge of becoming big business. It now looks as if trading of emissions permits will develop into a significant part of London commodities market, which in turn means that accountants must seriously consider how to account for emissions rights …… It has been apparent for some time that climate change and the introduction of emissions trading regimes are going to have material impacts on some industries and some companies. …… How environmental issues affect corporate profits.” (Accounting & Business, 2004).


For drainage engineers, the impacts of climate changes on the design of stormwater drainage system is particularly an area of concern. As parameters of rainfall and sea level are pivotal in the hydraulic performance of stormwater drainage system, these two parameters will be the focus of this R&D study.


In the current hydraulic design of stormwater drainage system, reference of design values of rainfall and sea level are made to the tables contained in the DSD Stormwater Drainage Manual (SDM) (DSD 2000). These tables are provided by the Hong Kong Observatory (HKO) based on annual maxima of rainfall and sea level up to 1990 using Gumbel distribution analysis. No consideration of climate change is made on the analysis and hence no allowance is made in the design values for climate change.


In this study, the findings on climate change both globally and locally done by the Intergovernmental Panel on Climate Change (IPCC) and the HKO respectively are reviewed to see if their findings can be applied for use. The methods adopted in previous extreme values analyses done for local rainfall and sea levels are reviewed. Then, methodology suitable for this study is searched for and formulated. Updated rainfall and sea level data are obtained from the HKO for extreme value analysis. Hypothesis testings are carried out for the trends of rainfall and sea level data to be estimated from different statistical methods. Some testing scenarios, generating from both stationary and non-stationary extreme value models, as well as the current SDM standard are put into the hydraulic models of northern NT catchment and West Kowloon Catchment CD for a comparison under a 200-year design flood event.


2. Findings of IPCC on Global Climate Change


The IPCC was jointly established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1998 to assess and provide advice on available scientific and socio-economic information on climate change and its impacts, and on the options for mitigating climate change and adopting to it. The report “Climate Change 2001: The Scientific Basis” (IPCC 2001), reviewed by many governments and experts, provides a comprehensive and up-to-date scientific assessment of past, present and future climate change. In this report, the changes of a number of meteorological phenomena and variables are addressed. Complex physically-based climate models were used as the main tool to provide their estimates.


Averaged rainfall is projected to increase globally in the 21st century. However, both increases and decreases in rainfall are projected at regional scale. The world is divided into 23 regions for showing the results of analysis. Hong Kong is included in the eastern Asia defined in the report. Atmosphere-Ocean General Circulation Model (AOGCM) simulation using emissions scenarios A2 and B2 showing that slight increase (5 – 20 %) is projected in summer but with inconsistent sign of changes under different emissions scenarios in winter. It is projected that areas with future increases in mean rainfall will very likely lead to increases in year to year variations. Rainfall extremes are projected to increase more than the mean and the intensity of precipitation events are projected to increases. Besides, the frequency of extreme rainfall events is projected to increases almost everywhere.


Based on tide gauge data, the rate of global average sea level rise during the 20th century is in the range of 1 to 2 mm/year. Projections of global average sea level rise from 1990 to 2100 using a range of AOGCMs and emissions scenario IS92a lie in the range of 0.11 m to 0.77 m. The report points out that this range reflects the systematic uncertainty of modeling. The confidence in the regional distribution of sea level change from AOGCMs is low as there is little similarity between models. Nevertheless, the sea level around Hong Kong is projected by a number of models to increase about 0.2 m to 0.4 m over the 21st century. It is expected that extreme high sea levels will increase in frequency as a result of mean sea level rise. For the changes in extreme sea levels due to storm surges, IPCC cannot ascertain whether there have been changes in the magnitude and frequency in many regions of the world due to lack of adequate data sets. However, a few analyses results done by other researchers were briefly cited in the report. No trend, increasing trend and decreasing trend are reported in different countries / regions based on past decades of data.


Other than the uncertainties in the physical processes of the climate models and the choice of emission scenario, the spatial resolution of the climate models has been criticized. “While global models can give useful information about changes in physical mechanisms that might lead to changes in the distributions of climate variables such as precipitation, the resolution of typical AOGCMs is too coarse to simulate extremes accurately (Senior et al. 2002).”


3. Findings of Local Climate Change


In the Technical Note (TN) No. 107 issued by the HKO (Leung, Y.K., Yeung K.H., Ginn E.W.L. and Leung W.M. 2004) long-term trend analyses are carried out for meteorological observations made in Hong Kong. The TN points out that there are many statistical methods for calculating trends and testing their significance, including parametric methods such as regression and t-test and non-parametric methods such as Mann-Kendall test. The TN adopted parametric least squares linear regression for trend estimation and t-test for its significance. The long-term trends are tested by two tailed t-test at 5% level of significance.


Trends of rainfall are analyzed for its annual total and seasonal total based on data from 1947 to 2002 recorded at the HKO HQ. All trends are estimated to be increasing but not statistically significant at 5% level. The findings are summarized in table 1.


Table 1: Trends of Seasonal and Annual Total Rainfall at HKO HQ (1947-2002)




Spring

Summer

Autumn

Winter

Annual

Trend (mm/year)

2.5

3.4

0.028

0.47

6.5

Percentage increase relative to mean

0.47%

0.31%

0.006%

0.48%

0.3%

Significant at 5% level ?

No

No

No

No

No


In a press release of the HKO (HKO 2005), the results of a study on “Projected Change in Hong Kong’s Rainfall in the 21st Century” was presented. The study was carried out using projections of some global climate models, and rainfall records of the HKO HQ, southern and central China. The annual rainfall at the HKO HQ was projected using regression type statistical downscaling, and was estimated to increase around 1% per 10 years in the 21st century. Depending on the choice of emission scenario and global climate model, the projected annual rainfall in the 21st century may increase or decrease relative to the mean annual rainfall of the 30-year period from 1961-1990. The number of days with hourly rainfall greater than 30 mm was also projected to increase.


The long-term sea level change in Hong Kong is analyzed by staff of the HKO (Wong, Li, and Yeung 2004) using least squares linear fit at North Point / Quarry Bay (NPQB), Tai Po Kau, Lok On Pai and Shek Pik. Settlement correction is applied to the data at NPQB. No settlement correction is applied to other locations as the settlement is found to be insignificant. The findings are summarized in Table 2.

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