Summary of doctor of engineering: Study identified a number of parameters of rain contribute to improve traffic formula designed small drainage structures on roads in Vietnam

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Engineering PhD thesis: Study identified a number of parameters of rain contribute to improve traffic formula designed small drainage structures on roads in Vietnam climatic conditions for theoretical research, analysis essence, the proposed principles, methods, formulas determine the parameters of rain used in the flow calculation formula designed small drainage structures on roads in Vietnam conditions.

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Nội dung Text: Summary of doctor of engineering: Study identified a number of parameters of rain contribute to improve traffic formula designed small drainage structures on roads in Vietnam

MINISTRY OF EDUCATION & TRAINING UNIVERSITY OF TRANSPORT AND COMMUNICATIONS Nguyen Anh Tuan STUDYING THE DETERMINATION OF A NUMBER OF RAIN PARAMETERS TO MAKE CONTRIBUTIONS TO COMPLETING THE FORMULA OF DESIGN FLOW OF SMALL SIZED DRAIN WORKS ON ROAD UNDER THE CLIMATIC CONDITIONS OF VIETNAM Major: Building auto road and city road Code: 62.58.30.01 SUMMARY OF DOCTOR OF ENGINEERING
-2- Hanoi-2014 PREAMBLE 1. Background for the research. Today, the transportation network has been expanding; urban areas, industrial zones have been developed at a rapid pace. We require a formula to calculate the design flow for simple, easy-to-calculate small drainage structures with the acceptable accuracy. In recent decades, Vietnam has been seriously affected by the climate change phenomenon, increasing the extreme climate phenomena, affecting the rainfall regime in Vietnam. Therefore, it affects the rainfall parameters used in calculating the design flow of drainage works on the road Qp. The use of the research results, previous rainfall data in calculating Qp becomes the reduced reliability, with large errors, giving rise to the damages to works, delaying the transport, increasing costs of repairing and maintaining, affecting the living environment… This clearly shows the reality out of the practice, such as the phenomenon of damages to small drainage works on the road due to increasing flood; damages can occur immediately or only a few years after being put into use; phenomenon of water storage at the upstream causes flooding to the fields and villages, affecting the agricultural production often occurs in the Central Region after completing the construction of roads; the phenomena of street inundation after the heavy rains in some urban areas of Vietnam makes difficulties, disturbance for activities and production. . . Obviously, there are outstanding problems, emerged problems of determining the rainfall parameters in formula of calculating Qp of small water drainage works on the roads in Vietnam today.
-3- From the above urgent requirements, the dissertation has been selected for the research. 2. Research objectives. - Theoretical research, analysis of nature, proposing principles and methods, formula for determining the parameters of rain used in the formula of calculating design flow of small drainage works on roads under the conditions of Vietnam. - Applying into the practice: immediately, because there has not been enough conditions, targets of determining specific valuation of parameters on rainfall at 12 typically selected meteorological stations, these values are proposed for reference to the practical use in calculating design flow of drainage works on roads in these areas of Vietnam. 3. Research contents. - The thesis focuses on researching and resolving rain parameters. This is the decisive, important and indefinite parameters in calculating Q p of small drainage works on the roads, totally depending on the particular characteristics of the climate in Vietnam. The rainfall parameters of the research are as follows: +) The study shall determine the calculated daily rainfall H n,p at a design frequency tailored to the characteristics of the weather changes in Vietnam. +) The study shall identify specific factors of the rain shape ΨT and issues of rain zoning in line with the requirements on calculating design flow of small drainage works on the roads in Vietnam. +) The study shall determine parameters on the designed rainfall intensity aT,p, corresponding to the concentration time of designed water and design frequency. Calculated rain intensity parameters a T,p is the general impact parameter in calculating design flow of drainage works on the roads. - The above are the parameters on rain used in the formula of calculating design flow of drainage works on the roads. 4. Research methods. Statistical analysis is the main method used in the thesis. 5. Scientific meanings and pratice of the project.
-4- - Researching characteristics of rainfall changes, contributing to further elucidate the unusual situation of change of rain in the territory of Vietnam in recent decades. Realizing the urgency to recalibrate or gradually replace the new database of rainfall in line with the weather phenomenon under the affects of the climate changes; suggesting solutions to coping with the phenomenon of radical changes in rain in calculating design flow of water drainage works on the roads in Vietnam today. - The study shall determine the parameters of rainfall (calculated daily rainfall Hn, p, calculated rainfall intensity aT,p, logical rain zoning and other characteristics of rain: ΨT, Sp, A, B, m, α) suitable for the climatic conditions in Vietnam used in the formula Q p calculating the small drainage works on the roads. A few researched rain parameters identified in the thesis (such as daily calculated rainfall parameters H n,p, typical characteristic rain shape coefficient ΨT) can also be used in the Sokolopsky formula calculating design flow for medium and large basins; used in calculating showers - flow with NAM - MIKE model with reliable results. - Rain gauge stations in Vietnam are numerous, but most of them are used to measure daily rainfall and few of them are recording rain gauge stations, so that when the method of directly determining the calculated rain intensity parameters are based on recording rain gauge data, then researching the construction of empirical formula in indirectly calculating the rainfall intensity aT,p in the thesis; issues of transferting the calculated daily rainfall Hn,p into the calculated rainfall of each short time HT,p is necessary for calculating the peak flow of the design flood of the basin of drainage works on roads in Vietnam. - To better plan the flood prevention, firstly, it is required to logically zone flood, in accordance with rainfall characteristics of each region. The thesis has researched proposals on criteria, rain zoning methods for mapping the logical rain zoning with a large proportion in accordance with the requirements for calculating flood flow of the small basins of small drainage works on the roads in Vietnam. - The thesis has established the specific parameters of rain, such as calculated daily rainfall H n,p according to the design frequency, typical coefficienmt of the rain shape ΨT , rain strength Sp , factors of the climate
-5- zones A, B, coefficient of the rain shape m, the regression coefficient of the climate zone α, standard rainfall intensity a TO,p used in 07 empirical formula that the dissertation researches and develops to calculate the rainfall intensity aT,p corresponding to the concentration time of calculated water and design frequency, for 12 selected typical meteorological stations, including stations at Muong Lay Town-Dien Bien Province, Tuyen Quang City, Lang Son City, Lang-Hanoi, Ha Dong-Hanoi, Son Tay Town-Hanoi, Vinh City, Dong Hoi City, Da Nang City, Nha Trang City, Buon Ma Thuat City, Can Tho City, with the actual rainfall records from 1960 - 2010, reference recommendations forn the practical use to compute the design flow of small drainage works on the roads today in these regions of Vietnam. 6. New contributions of the dissertation. - Determining the values of calculated daily rainfall H n,p according to the design frequency at 12 selected meteorological stations on the basis of long actual rainfall recording sequence from 1960 to 2010, in which the last time is supposed to correspond to the new context with the impact of climate change phenomena, applied to calculate the design flow of small drainage works on the roads in accordance with current design standards TCVN9845:2013 [5] or using Sokolopsky formula in computing the design flow for medium and large basins (in the regions located with these meteorological stations). - Determining the values of characteristic coefficient of the rain shape ΨT for the regions of 12 studied meteorological stations with the rain gauge period from 1960 to 2010, used to calculate the rainfall intensity corresponding to the water concetrating time of the basins and the design frequency used in the standard TCVN9845:2013 [5] to calculate the design flow of small drainage works on the roads. At the same time, it is an important parameter used to calculate the transfer from the calculated daily rainfall H n,p to the calculated rainfall of each short timeHT,p used in Sokolopsky formula to calculate the design flow for medium and large basins in calculating showers-flow with the NAM- MIKE model with the reliable results. Also, the characteristic rain shape coefficient ΨT can also be used as a criterion for zoning the rain. - Summarizing and researching to improve into 7 types of empirical formula in calculating the calculated rain intensity parameter a T,p
-6- corresponding to the water concentration time of the basin and the design frequency used to calculate the design flow of small drainage works on the roads in Vietnam corresponding to the different conditions on rainfall data available in the design areas, in which there are 3 types of new improved formula of the thesis, the formula remains the coefficients in the formula new building for the thesis area 12 meteorological stations selected for the study period rainfall in 1960-2010. Chapter 1: OVERVIEW ABOUT THE RESEARCH ISSUES 1.1. Domestic and international related researches. 1.1.1. Theoretical basis of the formula for determining the maximum flood current flow due to rain on the catchment [32]. - The formula of the limited intensity determining the design flood peak flow is drawn from the famous flow formula whose basis is the theory of concentrating water from the basin. This theory was firstly analyzed and researched by USSR scientists (N.E. Dongov, M.E. Velikanov and M.M.Protodiakonov) and it is now widely used in the world in the field of hydrological computation. The assumptions are. +) Basins are evenly shaped, the river bed in the middle. +) Rain and losses are evenly distributed over the entire basin and with unchanged intensity during the computation time. +) Considering the rain frequency of flood flow to be equal to the flood flow frequency. - By analyzing rules of water running from slopes of the basin to drainage works, the authors of the theory of concentrating water from the basin have drawn the formula for determining the maximum flow of the design flood flow. Q p = K .ϕ.aτ , p .F (1.1) In which: Qp is the design flow at frequency p; F is the catchment area ϕ is the flow coefficient with regard to the amount of lost rain water aτ,p is calculated rainfall intensity at water concentration time τ of the basin and the frequency p. It is the max average
-7- rainfall intensity during the water concentration time τ and frequency p K is the unit conversion coefficient when elements in the formula are not calculated at the same unit. K = 16.67 when Qp is in m3/s, aτ,p calculated in mm/ph, F in km2. - Formula (1.1) is the basic formula. From the basic formula (1.1), very many authors have studied the application and completed to suit the actual conditions in terms of rainfall intensity distribution law, water running conditions on the natural basins. 1.1.2. Formula defining the design flow for small drainage works on the roads in some countries and in Vietnam. 1.1.2.1. In some countries: countries with the developed science such as the U.K., the U.S., Russia, Japan, China, etc use the limited intensity formular in the form (1.1) to determine the design flow Qp for small water drainage works on the roads. 1.1.2.2. The formula for determining the design flow for small drainage works on the roads in Vietnam - The formula of the limited intensity of the standard TCVN9845:2013: Calculation of flood flow characteristics [5]: applied to the basin with F ≤ 100 km2. Q p = A p .ϕ.H n , p .F .δ 1 (1.9) In which: Ap is the relative flood peak flow module at the frequency p Ap = 16.67Ψτ / τ Hn,p is the calculated daily rainfall at the frequency p δ1 is the factor with regard to effects of ponds, lakes, and swamps. - Limited intensity formula of National University of Civil Engineering (the formula proposed by Prof., Dr.Nguyen Xuan Truc) [3], [32]. The small drainage structures on roads in Vietnam often have the small catchment area (usually less than 1 km 2 to a few km2), so the water concentration time τ is short. Therefore, the calculation of Q p is based on the rain intensity parameters with the short calculation time, but the high rain intensity will be more accurate than using the daily rainfall parameters as in the formula (1.9) of the design standard [5]. With that
-8- viewpoint, since the 1980s, Prof. Dr. Nguyen Xuan Truc has proposed using the following formula (1.10) to calculate the design flow Q p for small drainage structures on roads with the basin area F≤ 30 km2 in Vietnam. Q p = 16.67ϕ.aτ , p .F .φ.δ 1 (1.10) With: φ the flow reduction coefficient depending on the basin area. - Limited intensity formula used in calculating urban road drainage under the design standard TCVN7957:2008 [8]. Q p = C.qτ , p .F (1.11) With: C is the flow coefficient, depending on the nature of the basin surface and p qτ,p is the rain flow module or rain intensity according to the calculated volume corresponding to the water concentration time τ and design frequency p qτ,p = 166.67aτ,p In which: qτ,p in liter/s/ha, aτ,p in mm/ph. 1.1.2.3. Sokolopsky Formula [3], [15], [32]. This is the kind of volume formula used to calculate the flow of water drainage works on the roads with the basin area F>100 km 2. 0.278( H τ , p − H 0 ) Qp = .ϕ. f .F .δ + Q0 (1.12) tl With: Hτ,p is the calculated rain flow at the water concentration flow τ of the basin and the frequency p. It is the max rainfall during the water concentration time τ and the frequency p (mm) H0 is the initial lost rain water (mm) f is the factor of flood shape , tl the duration of rise (h) δ is the coefficient with regard to the influence of reduced flow of ponds, swamps and forests in the basin Q0 the water flow in the river before the flood (m3/s). 1.1.3. The problem of determining the parameters in the formula for calculating the design flow of small drainage works on the roads. The parameters of rain in the formula for calculating the design flow of small drainage works on the roads are: calculated daily rainfall H n,p at the design frequency p; characteristic rain shape coefficient ΨT and the
-9- issue of rain zoning; the calculated rain intensity a τ,p at the water concentration time τ and design frequency p. 1.1.3.1. Calculated daily rainfall Hn,p. - Calculated daily rainfall Hn,p is determined by the statistical analysis method on the basis of sequence of data measuring actual daily rainfall for many continuous years at rain gauge stations within the project design area. - Up to now, the fullest database about Hn,p in Vietnam was founded in 1987 in accordance with the design standard TCVN9845:2013 [5], for 589 points of rainfall gauge across the country with the frequency p=1%, 2%, 4%, 10%, 25%, 50%. Since then now, it has passed over 25 years, the rainfall regime in Vietnam has been changed, influenced by the phenomenon of climate change, so the database has become less accurate so far. 1.1.3.2. Characteristic coefficient of the rain shape ΨΤ and rain zoning. - Up to now in Vietnam, there have been some projects on zoning rain and building the coefficient ΨT ∼ T for each rain zone. The rain partitioning projects are subdivided by the rainfall intensity, i.e. only paying attention to the relation of reducing the rainfall intensity by the calculated interval aT ∼ T. Like the rain partition of the year 1977 of the Irrigation Standard QP.TL.C-6-77 [7], it divided the Northern Region into 10 rain zones; the 1980 rain partition of Hoang Minh Tuyen divided the entire territory of Vietnam into 15 rain zones [60]; the 1991 rain partition of Hoang Niem and Do Dinh Khoi divided the whole nation into 18 rain zones. In 1993, Dr. Trinh Nhan Sam also divided the entire territory into 18 rain zones as above, but the coefficient value ΨT established in rain zones is a bit different [29], [40]. Results of rain zoning in 1993 were included in the design standard TCVN9845:2010 [5], which are now used to calculate Q p for the drainage works on the roads. - Therefore, over the annual periods, the rainfall regime of Vietnam has been changed, leading to the fact that the rain zoning is also adjusted accordingly and characteristic rain shape coefficient value shape ΨΤ in the rain zones are also corrected accordingly.
- 10 - 1.1.3.3. Defining the calculated rain intensity aT,p: with the following instructions. - Direct calculation aT,p: on the basis of sufficient surbey data available for recording rain gauge at the meteorological stations, using the method of statistical analysis to calculate the calculated rainfall intensity value aT,p in calculated rain period T and the design frequency p. When calculating the flow of drainage works, the calculated rain period T is drawn by including the water concentration time τ of the basin. - Determining the calculated rainfall intensity aT,p based on the calculated daily rainfall Hn,p and characteristic rain shape coefficient ΨT ΨT aT , p = .H n , p (1.14) T This method is proposed by Alexayev. It has been used in Vietnam to calculate Qp in the design standard TCVN9845:2013 [5]. - Building the empirical formula to calculate aT, p. The empirical formulas are all built based on the characteristic rain strength S and the rain form factor m to determine the rainfall intensity calculated aT,p. The basic form is: aT,p = Sp / Tm = (A + B.lgN) / Tm. From this basic formulation, it has been developed into a lot of other empirical formulations to suit the climatic conditions in each country, such as the formula of the U.S., India, Russia, China, Japan, Korea China, Indonesia, Malaysia, etc. In these formulas, compared to above basic formulations, they can be added or subtracted a few regression coefficients. In Vietnam, there have been many studies to suit the climatic conditions of the nation, typically: ./ The research of Prof., Dr. Nguyen Xuan Truc in the year 1980 A + B. lg N for 18 meteorological stations: aT , p = Tm ./ The research of Dr.Tran Huu Uyen in the year 1973 for 34 meteorological stations: A + B. lg N aT , p = (T + b.N k ) m ./ The research of Dr.Tran Viet Lien in the year 1979 for 47 A + B. lg N meteorological stations: aT , p = (T + b) m
- 11 - ./ The research of Prof., Dr.Ngo Dinh Tuan in the year 1980 for 10 meteorological stations: −T m a =a T,p .e max, p 1.1.4. Researching, analyzing the factors affecting the design flood flow calculation of the small drainage works on the roads. - Factors affecting the flood flow of small basins of small drainage works on the roads can be divided into 3 categories: climatic conditions, in particular the rain; buffering surface factors; effects of socio-economic activities of human beings. +) Under the climatic conditions of Vietnam, the rain is a factor directly affecting the flood flow of small basins and the rain is the cause of flood flows on small watersheds in Vietnam. +) Buffering surface factors including: geographical location, topographical and geomorphological features, geological and soil conditions, surface features, shape of basin. . . These factors affect the two main stages: decisive to concentrating the flow and loss rate. In addition, geographical location and topographical features affect climate conditions expressed by different climate regions and zones in the country. Topography affects rainfall, flood currents of the basin in both direct and indirect manners. Direct manner: creating heavy rainfall due to the windward, creating the little rainfall due to the leeward. Indirect manner: centralizing water fast or slow due to large or small slope, stream network in the fan shape or feather shape. . . +) Socio-economic activities of the human beings are today increasingly large with negative or positive influence such as effects causing the phenomena of climate change, sea level rise, changing basin buffering surface factors. It is considered through two factorsd of rain and other padding surface. - In addition, the design flow of drainage works on the roads Q p is also influenced by the design frequency p. The greater the frequency value p becomes (p = 10%, 20%, 50%,...), the smaller the calculated Q p is and vice versa, it is designed with smaller value p (p = 3%, 2% , 1%, ...), then the greater the calculated Qp is. - The calculated rain intensity parameter a τ,p is a general affecting quantity in calculating the design flow of drainage works on the roads because it shows the impacts of all factors affecting the calculation of the
- 12 - design flood flow of drainage works on the roads, including the rain factors, factors of padding surface and the design frequency. +) Indeed: aτ,p = Hτ,p / τ ./ Precipitation regime in the design zone is decisive to the largest value the calculated period Hτ,p , so it affects aτ,p . ./ Design frequency p affecting Hτ,p , so it affects aτ,p. ./ Buffering surface factors affecting the water concentration time parameter τ of the basin, so it affects aτ, p . +) Also, unlike the calculated daily rainfall parameter H n,p , then the calculated rain intensity parameter a τ, p also reflects the influence of rain shape. Figure 1.4 shows that Ht,p I II despite the same calculated daily rainfall Hn,p , in the two H τ,p I rain regions I and II, they have H τ,p II Hn,p different shapes, then the highest rainfall in the calculated period Hτ, p in two τ different areas of rain and as a 0 t result, the calculated rain Figure 1.4: Influence of the rain intensity aτ,p = Hτ,p/τ is also shape to the rain intensity aτ, p different. - Thus, the limited intensity formula used to calculate Q p for the small drainage works already has a general affecting parameter a τ,p 1.2. Shortcoming issues at the thesis to be focused on the settlement. - Through the above analysis and evaluation, the most urgent existing problems in computing the design flow of small drainage works on the roads in Vietnam today is the determination of the rain parameters in formulars calculating Qp of small drainage works on the roads. Because the currently used parameters were set in previous, they are not matched with the computational requirements Qp of small drainage works on the roads in the weather and climate conditions in Vietnam today. - Zoning rain (too wide and for long time) as used in the applicable design standard TCVN9845:2013 [5] is not suitable for the computational requirements Qp of small drainage works on roads under
- 13 - the present conditions in Vietnam and it is necessary to propose the researches for correcting. - The calculated rain intensity parameters are of the general and representative characteristics for the rain regime and the shape of the rain zone, for the specific watershed buffering surface, the water concentration time and design frequency, it is characterized by the distinct climates of Vietnam, so it is necessary parameters for calculating the design flow of small drainage works on the roads, reflecting more closely the condition climate and terrain of Vietnam. It is necessary to research and determine the calculated rain intensity parameter a T,p under the climatic conditions of Vietnam used to determine the design flow of small drainage works on the roads Q p in order to ensure the required level of precision. Chapter 2: STUDYING RAIN CHARACTERISTICS UNDER THE EFFECTS OF THE PHENOMENON OF CLIMATE CHANGE IN COMPUTING THE DESIGN FLOW OF SMALL DRAINAGE WORKS ON THE ROADS 2.1. Overview of Vietnam climatic conditions. - Vietnam is within the humid tropical, monsoon climate. The annual rainfall and rain duration are relatively large, averaging from 1100 - 8000 mm/year and from 67-223 days of rain/year, compared with countries in the same latitude, it is about 2.4 times higher [25]. - The rainfall regime in Vietnam is under the effect of three main factors: (i) Weather forms of raining amd raining causes (ii) Wind (iii) Topography. 2.2. Introduction about the network of meteorological stations and rain gauge data sources in Vietnam. - According to sources of the National Centre for Hydrometeorological Forecasting, at present, the whole nation has more than 800 rain gauge points (including meteorological stations, meteorological stations and rain gauge sites are operating) distributed
- 14 - throughout the territory, reaching the national average density of approximately 433 km2/rain gauge point. Self-recording rain gauge equipments have been present at most meteorological stations, accounting for about 20% of the total rainfall stations, each province has ≥ 1 meter meteorological station with self-recording rain gauge machine, some provinces have up to ≥ 2-3 stations, remaining rain gauge points have only daily rainfall measuring equipments. - For the time of observation: in the north, there are uniform monitoring data from around the year 1960 onward; in the south, due to historical circumstances, continuous and reliable monitoring data are from the year 1980 now. - In the thesis, rain gauge data is collected from 1960 - 2010 (stations in the North Vietnam), from 1980 to 2010 (stations in the South Vietnam). The sequence of research data is 30-50 years long, so results ensures the necessary reliability. 2.3. Studying rainfall variability features under the effect of the phenomenon of climate change and its effects over calculating the design flood peak flow of small drainage works on the roads. The study was carried out over 7 norms related to the calculation of the design flow of small drainage works on the roads as follows. 2.3.1. The rainy season, dry season. - To distinguish the rainy season, dry season in the year, the thesis used “excess loss norms” [34]. According to this criterion: the rainy season is the season including continuous months with the rainfall exceeding losses (usually taken as 100mm/month, according to the study [34]) with the exceeding frequency p≥ 50%. That is: p{ (Hmonth)i ≥ 100 mm/month } ≥ 50%. - Research findings about the rainy season in the year in the sequence of actual rainfall records in 1960 - 2010 at 12 selected typical meteorological stations are as in Table 2.2 below. Recognizing that in general, the rainy season in Vietma does not have the shift, the change compared with the results of previous studies, however, there have been fluctuations in recent years.
- 15 - 2.3.2. Rainy months with many days and few days. - To classify the rainy month with many days, use the classification coefficient Krain-for-many-days = the number of rainy days in the month/30 - The month with many rainy days: Krain-for-many-days ≥ 0.5 Table 2.2: Summary of research findings of the month with many rainy days, comparing the research findings of months of the rainy season at 12 meteorological stations selected for studies from 1960-2010 Month with many rainy days in the Rainy months in the year Meteorological stations, year (average) (average) No. places Rainy Months Time of appearance Time of rainy season months 1 Muong Lay Town Station 4 months May → August 6 months April → September 2 Tuyen Quang City Station 6 months March → August 7 months April → October 3 Lang Son City Station 4 months May → August 5 months May → September 4 Lang Station-Hanoi 6 months March → August 6 months May → October 5 Ha Dong Station-Hanoi 6 months March → August 6 months May → October 6 Son Tay Town Station- 6 months March → August 6 months May → October Hanoi 7 Vinh City Station 4 months February → March & 4 months August → November September → October 8 Dong Hoi City Station 4 months September → December 4 months August → November 9 Da Nang City Station 4 months September → December 5 months August → December 10 Nha Trang City Station 4 months September → December 6 months July → December 11 Buon Ma Thuat City Station 6 months May → October 6 months May → October 12 Can Tho City Station 7 months May → November 7 months May → November - Comments: The months of the rainy season is not necessarily the month with many rainy days and vice versa. This shows that the rainfall not spread in the rainy days, but focuses on some heavy rains.
- 16 - 2.3.3. The trend and the variability in the annual rainfall and rainy days in the year - Using the average trend line and double sliding average line of 5 years to study the trend and the variability by the time of the survey precipitation value. Figure 2.1, Figure 2.2 below are examples of the trend of variation of annual rainfall and the number of rainy days in the year at Lang Station-Hanoi City from 1960 to 2010. - Comments: In all 12 research stations, in recent years, the variation of annual rainfall Hyear and the number of rainy days become greater, more different and more unusual, appearing extremely big values in recent years. y y 3 1 3 1 2 2 x x 1-Actual measurements; 2-Average trends; 3-Double sliding average of 5 years Sudden big values appeared in recent years Figure 2.1: Variation trends in Figure 2.2: Variation trends in the annual rainfall Hannual at Lang number of rainy days in the year at Station-Hanoi City from 1960 - Lang Station-Hanoi City from 1960 2010 - 2010 ‘‘ ’’
- 17 - 2.3.4. Trends and variability of the biggest daily rainfall Hdaymax and the biggest calculated rainfall intensity in the year a Tmax. The extreme sudden variation due to effects of the climate change phenomena. y y 1 2 3 1 x 2 3 x 1-Actual measurements; 2-Average trends; 3-Double sliding average of 5 years Sudden big values appeared in recent years Figure 2.3: Variation trends Hdaymax Figure 2.4: Variation trend aTmax at at Lang Station-Hanoi City from T= 60ph at Lang Station-Hanoi 1960 - 2010 City from 1960 - 2010 - Purposes: comparing and clarifying the rules of change in two important rain parameters in calculating design flood flow of small drainage works on the roads Hdaymax and aTmax , assessing the difference between them. This research is especially needed to determine the calculated daily rainfall Hn,p according to the design frequency and calculated rain intensity interval aT,p at the design frequency in chapters 3 and 4 in handling issues of heavy rain. In addition, from this study, it also suggests proactive measures to respond to climate change phenomena in computing the design of small drainage works on the roads today in Vietnam. - Research results: realized at all 12 research stations, parameters Hdaymax and aTmax are constantly changing and not even; in some times, there are sudden significant changes in recent years. At the same station, the trend and level of time-variaton trend of H daymax and aTmax are different, the variability of Hdaymax and the highest 24-hour rainfall in the year H24hmax is not identical, the variability of H24hmax is not identical to that of the highest rainfall in the year HTmax at other short computational interval T. ‘‘ ’’
- 18 - +) The extreme interval suddeness of Hdaymax, aTmax occurs at all research stations. The extreme segment are in both values and time of appearance. ./ Extremeness in value: values (Hdaymax)*, (aTmax)* are suddenly big exceeding values Hn,p , aT,p at the design frequency usually used for small drainage works p = 4%, even exceeding the value at p = 1%. ./ Extremeness on the appearance time: the extreme value (Hdaymax)*, (aTmax)* can occur at the time which is considered to be in the dry season. Even the time when the value (Hdaymax)*, (aTmax)* is extremely big at the same station, they are not coincided. For example, for Lang Station-Hanoi City from 1960 - 2010, the time of appearing the value (Hdaymax)* was on 31 October 2008, but the time of appearing the value (a20phmax)* was on 03 May 2005, (a30phmax)* on 03 May 2005, while in the year 2005, the highest daily rainfall in the year Hdaymax created fell on 27 September 2005. Therefore, with the arrival of rain heavy rain intensity although mutations do not create value in the largest daily rainfall, does not create large daily rainfall values are still causing mutations major flood surge for small drainage structures on roads due to the small basin of small drainage works on the road a short time focus countries. This is of particular concern in the design calculations minor drainage works on the road. +) The change of the highest rainfall in the year Hdaymax, the rain intensity of the highest calculated interval in the year aTmax in recent years make the value Hn,p , aT,p under the frequency set with the sequence of the actual rain measuremens collected up to 2010 tend to increase compared with the calculation result Hn,p , aT,p with the sequence of actual rain measurements up to years backward and results make the calculated value Qp increase. 2.3.5. Average value in many years X and scattering coefficient Cv, hệ bias coefficient Cs of Hdaymax and aTmax. - Purposes: This research serves for computing and defining H n,p , aT,p under the design frequency in chapters 3 and 4 in estimating statistical parameters; in determining the years of necessary observation and sampling nyc to ensure the sampling error because the more statistical sequences Hdaymax, aTmax have the coefficient Cv, the longer the required
- 19 - years of observation nyc are in order to ensure the calculated results Hn,p, aT,p are reliable. The rate Cs / Cv is also a condition for making decision on selecting a suitable probability distribution rule when calculating and determining Hn,p , aT,p to gain the best results. In addition to values X, Cv, Cs, they also relect the nature of change by the time of the parameter Hdaymax, aTmax that we need to research, compare and clarify. - Research results at 12 meteorological stations from 1960 - 2010: the highest daily rainfall has the average value Hdaymax from 89.31 - 237.46 mm, coefficient Cv = 0.24 - 0.55, Cs = 0.34 - 2.99, Cs/Cv = 1.4 - 5.7; the highest rain density in the year aTmax at intervals T = 5mins – 1440mins with the average value aTmax from 3.01 - 0.07 mm/min, Cv = 0.18 - 0.63, Cs = (-0.25) - 3.58, Cs/Cv = 0.05 - 8.0, with the same station coefficients Cv, Cs of aTmax , the shorter intervals T are, the smaller they have values and vice versa. 2.3.6. Cycle of large-small-medium changes of Hdaymax and aTmax. - Purposes: This study serves for testing the representation of the period of sampling of the statistical data sequence Hdaymax, aTmax included in the calculation of the frequency to determine Hn,p , aT,p in chapters 3 and 4. They are deemed to have the representativeness, reflecting the approximate accuracy in line with the general change rule, then the overall sampling period Hdaymax, aTmax must have the number of years in the heavy rain period and some years in the light rain period in succession. Additionally, researching the cycle of changes also reflects the over-time variation of Hdaymax, aTmax that we need to research compare, and clarify. +/ Time of high values: (1983 - 1994), (2007 - 2010). +/ Time of small values: (1956 - 1983), (1999 -2007). +/ Average time: (1994 - 1999). +/ With 2 cycles. Figure 2.7: Cycle of changing the biggest daily rainfall in the year Hdaymax at Lang Station – Hanoi City from 1960 – 2010
- 20 - +/ Time of high values: (1996 - 2010). +/ Time of small values: (1986 - 1996). +/ Average time: (1961 - 1986). +/ With 01 cycle. Figure 2.8: Cycle of changing the biggest rain intensity in the year aTmax at the computational interval T = 30 mins at Lang Station-Hanoi from 1960 – 2010 - In order to determine the change cycle of Hdaymax, aTmax , we use the standard error comulative line St = Σ(Ki – 1) với Ki = (Xi /X) as the variable coefficient. On the standard error comulative line St , if the general trends are considered, we will realize there are groups of 5 cumulative lines always rising up corresponding to the heavy rain periods, or declining corresponding to the light rain periods or always in parallel corresponding to the medium rain periods. A rain cycle includes a period of heavy rain and a little rainy period or a period of heavy rain, a period of little rain and a period of continuous rain average. For example, in above Figure 2.7, Figure 2.8. - Comments: Hdaymax, aTmax change with cycles; the length of a change cycle of them ranges from 20 to 50 years. In the same station, the change cycle of Hdaymax and aTmax are not the same, at the calculation interval, the shorter T is, the more different they are and vice versa. With the length sequence of survey data up to 2010 at the selected meteorological stations, Hdaymax and aTmax at T = 5mins – 1440mins reach at least 01 change cycle, even >= 2 change cycles. 2.3.7. Correlation of changes in value and time of together appearance of Hdaymax and aTmax. - Comparing the change in values: using the graph standards to compare the variation coefficient Ki of Hdaymax and aTmax.