- Theoretical research, analysis of nature, proposing principles and methods, formula for determining theparameters of rain used in the formula of calculating design flow of small draina
Trang 1UNIVERSITY 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
Trang 2SUMMARY OF DOCTOR OF ENGINEERING
Hanoi-2014 PREAMBLE
1 Background for the research.
Today, the transportation network has been expanding; urban areas, industrial zones have been developed at arapid pace We require a formula to calculate the design flow for simple, easy-to-calculate small drainage structureswith the acceptable accuracy
In recent decades, Vietnam has been seriously affected by the climate change phenomenon, increasing theextreme climate phenomena, affecting the rainfall regime in Vietnam Therefore, it affects the rainfall parametersused in calculating the design flow of drainage works on the road Qp The use of the research results, previousrainfall data in calculating Qp becomes the reduced reliability, with large errors, giving rise to the damages toworks, delaying the transport, increasing costs of repairing and maintaining, affecting the living environment…
Trang 3This 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 agriculturalproduction often occurs in the Central Region after completing the construction of roads; the phenomena of streetinundation after the heavy rains in some urban areas of Vietnam makes difficulties, disturbance for activities andproduction 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
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 theparameters of rain used in the formula of calculating design flow of small drainage works on roads under theconditions of Vietnam
- Applying into the practice: immediately, because there has not been enough conditions, targets of determiningspecific valuation of parameters on rainfall at 12 typically selected meteorological stations, these values areproposed for reference to the practical use in calculating design flow of drainage works on roads in these areas ofVietnam
3 Research contents.
- The thesis focuses on researching and resolving rain parameters This is the decisive, important and indefiniteparameters in calculating Qp of small drainage works on the roads, totally depending on the particularcharacteristics of the climate in Vietnam The rainfall parameters of the research are as follows:
+) The study shall determine the calculated daily rainfall Hn,p at a design frequency tailored to thecharacteristics of the weather changes in Vietnam
Trang 4+) The study shall identify specific factors of the rain shape T and issues of rain zoning in line with therequirements 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 theconcentration time of designed water and design frequency Calculated rain intensity parameters aT,p is the generalimpact 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 theroads
4 Research methods.
Statistical analysis is the main method used in the thesis
5 Scientific meanings and pratice of the project.
- Researching characteristics of rainfall changes, contributing to further elucidate the unusual situation ofchange of rain in the territory of Vietnam in recent decades Realizing the urgency to recalibrate or graduallyreplace 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 waterdrainage works on the roads in Vietnam today
- The study shall determine the parameters of rainfall (calculated daily rainfall Hn, p, calculated rainfall intensityaT,p, logical rain zoning and other characteristics of rain: T, Sp, A, B, m, ) suitable for the climatic conditions inVietnam used in the formula Qp calculating the small drainage works on the roads A few researched rainparameters identified in the thesis (such as daily calculated rainfall parameters Hn,p, typical characteristic rain shapecoefficient T) 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
Trang 5- Rain gauge stations in Vietnam are numerous, but most of them are used to measure daily rainfall and few ofthem are recording rain gauge stations, so that when the method of directly determining the calculated rain intensityparameters are based on recording rain gauge data, then researching the construction of empirical formula inindirectly calculating the rainfall intensity aT,p in the thesis; issues of transferting the calculated daily rainfall Hn,pinto the calculated rainfall of each short time HT,p is necessary for calculating the peak flow of the design flood ofthe 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 rainfallcharacteristics of each region The thesis has researched proposals on criteria, rain zoning methods for mapping thelogical rain zoning with a large proportion in accordance with the requirements for calculating flood flow of thesmall basins of small drainage works on the roads in Vietnam
- The thesis has established the specific parameters of rain, such as calculated daily rainfall Hn,p according tothe design frequency, typical coefficienmt of the rain shape T , rain strength Sp , factors of the climate zones A, B,coefficient of the rain shape m, the regression coefficient of the climate zone α, standard rainfall intensity aTO,pused in 07 empirical formula that the dissertation researches and develops to calculate the rainfall intensity aT,pcorresponding to the concentration time of calculated water and design frequency, for 12 selected typicalmeteorological stations, including stations at Muong Lay Town-Dien Bien Province, Tuyen Quang City, Lang SonCity, Lang-Hanoi, Ha Dong-Hanoi, Son Tay Town-Hanoi, Vinh City, Dong Hoi City, Da Nang City, Nha TrangCity, Buon Ma Thuat City, Can Tho City, with the actual rainfall records from 1960 - 2010, referencerecommendations forn the practical use to compute the design flow of small drainage works on the roads today inthese regions of Vietnam
6 New contributions of the dissertation.
Trang 6- Determining the values of calculated daily rainfall Hn,p according to the design frequency at 12 selectedmeteorological stations on the basis of long actual rainfall recording sequence from 1960 to 2010, in which the lasttime is supposed to correspond to the new context with the impact of climate change phenomena, applied tocalculate the design flow of small drainage works on the roads in accordance with current design standardsTCVN9845:2013 [5] or using Sokolopsky formula in computing the design flow for medium and large basins (inthe regions located with these meteorological stations)
- Determining the values of characteristic coefficient of the rain shape T for the regions of 12 studiedmeteorological stations with the rain gauge period from 1960 to 2010, used to calculate the rainfall intensitycorresponding to the water concetrating time of the basins and the design frequency used in the standardTCVN9845:2013 [5] to calculate the design flow of small drainage works on the roads At the same time, it is animportant parameter used to calculate the transfer from the calculated daily rainfall Hn,p to the calculated rainfall ofeach short timeHT,p used in Sokolopsky formula to calculate the design flow for medium and large basins incalculating showers-flow with the NAM-MIKE model with the reliable results Also, the characteristic rain shapecoefficient T 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 rainintensity parameter aT,p corresponding to the water concentration time of the basin and the design frequency used tocalculate 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, theformula remains the coefficients in the formula new building for the thesis area 12 meteorological stations selectedfor the study period rainfall in 1960-2010
Trang 7Chapter 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 flowformula whose basis is the theory of concentrating water from the basin This theory was firstly analyzed andresearched 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 thecomputation 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 ofconcentrating water from the basin have drawn the formula for determining the maximum flow of the design floodflow
F a K
In which: Qp is the design flow at frequency p; F is the catchment area
is the flow coefficient with regard to the amount of lost rain water
a,p is calculated rainfall intensity at water concentration time of the basin and the frequency p It isthe max average rainfall intensity during the water concentration time and frequency p
Trang 8K 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, a,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 theapplication and completed to suit the actual conditions in terms of rainfall intensity distribution law, water runningconditions 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 drainageworks 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
1 ,
1 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].
Trang 9The small drainage structures on roads in Vietnam often have the small catchment area (usually less than 1 km2
to a few km2), so the water concentration time is short Therefore, the calculation of Qp is based on the rainintensity parameters with the short calculation time, but the high rain intensity will be more accurate than using thedaily rainfall parameters as in the formula (1.9) of the design standard [5] With that viewpoint, since the 1980s,Prof Dr Nguyen Xuan Truc has proposed using the following formula (1.10) to calculate the design flow Qp forsmall drainage structures on roads with the basin area F30 km2 in Vietnam
1 , .
67
16 a F
With: 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].
F q C
With: C is the flow coefficient, depending on the nature of the basin surface and p
q,p is the rain flow module or rain intensity according to the calculated volume corresponding to the waterconcentration time and design frequency p
Trang 100 0
,
)(
278.0
Q F f t
H H
H0 is the initial lost rain water (mm)
f is the factor of flood shape , tl the duration of rise (h)
is the coefficient with regard to the influence of reduced flow of ponds, swamps and forests in the basinQ0 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 Hn,p at the design frequency p; characteristic rain shape coefficient T and the issue of rainzoning; the calculated rain intensity a,p at the water concentration time and design frequency p
1.1.3.1 Calculated daily rainfall H n,p
- Calculated daily rainfall Hn,p is determined by the statistical analysis method on the basis of sequence of datameasuring 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 designstandard 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 and rain zoning
Trang 11- Up to now in Vietnam, there have been some projects on zoning rain and building the coefficient TT foreach rain zone The rain partitioning projects are subdivided by the rainfall intensity, i.e only paying attention tothe 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 rainpartition of Hoang Minh Tuyen divided the entire territory of Vietnam into 15 rain zones [60]; the 1991 rainpartition of Hoang Niem and Do Dinh Khoi divided the whole nation into 18 rain zones In 1993, Dr Trinh NhanSam also divided the entire territory into 18 rain zones as above, but the coefficient value T established in rainzones is a bit different [29], [40] Results of rain zoning in 1993 were included in the design standardTCVN9845:2010 [5], which are now used to calculate Qp for the drainage works on the roads
- Therefore, over the annual periods, the rainfall regime of Vietnam has been changed, leading to the fact thatthe rain zoning is also adjusted accordingly and characteristic rain shape coefficient value shape in the rainzones are also corrected accordingly
1.1.3.3 Defining the calculated rain intensity a T,p : with the following instructions.
- Direct calculation a T,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 valueaT,p in calculated rain period T and the design frequency p When calculating the flow of drainage works, thecalculated rain period T is drawn by including the water concentration time of the basin
- Determining the calculated rainfall intensity a T,p based on the calculated daily rainfall H n,p and characteristic rain shape coefficient T
p n
T p
T
Trang 12This method is proposed by Alexayev It has been used in Vietnam to calculate Qp in the design standardTCVN9845:2013 [5].
- Building the empirical formula to calculate a T, p
The empirical formulas are all built based on the characteristic rain strength S and the rain form factor m todetermine 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 climaticconditions 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 fewregression 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 for 18 meteorological stations:
m
p
N B
A
./ The research of Dr.Tran Huu Uyen in the year 1973 for 34 meteorological stations:
m k p
T
N b
T
N B
A
a
)
(
lg
T
b
T
N B
A
a
)(
lg
e a
Trang 131.1.4 Researching, analyzing the factors affecting the design flood flow calculation of the small drainage works
+) Socio-economic activities of the human beings are today increasingly large with negative or positiveinfluence such as effects causing the phenomena of climate change, sea level rise, changing basin buffering surfacefactors It is considered through two factorsd of rain and other padding surface
- In addition, the design flow of drainage works on the roads Qp is also influenced by the design frequency p.The greater the frequency value p becomes (p = 10%, 20%, 50%, ), the smaller the calculated Qp 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 ap is a general affecting quantity in calculating the design flow ofdrainage works on the roads because it shows the impacts of all factors affecting the calculation of the design flood
Trang 14flow of drainage works on the roads, including the rain factors, factors of padding surface and the designfrequency
+) Indeed: a,p = H,p /
./ Precipitation regime in the design zone is decisive to the largest value the calculated period Hp , so itaffects ap
./ Design frequency p affecting H,p , so it affects a,p
./ Buffering surface factors affecting the water concentration time parameter of the basin, so it affects ap
+) Also, unlike the calculated daily rainfall parameter Hn,p , then the calculated rain intensity parameter apalso reflects the influence of rain shape
Figure 1.4 shows that
despite the same calculated
daily rainfall Hn,p , in the two
rain regions I and II, they have
different shapes, then the
highest rainfall in the
calculated period Hp in two
different areas of rain and as a
result, the calculated rain
intensity ap = H,p/ is also
Trang 15- Thus, the limited intensity formula used to calculate Qp for the small drainage works already has a generalaffecting parameter ap
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 formularscalculating 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 weatherand climate conditions in Vietnam today
- Zoning rain (too wide and for long time) as used in the applicable design standard TCVN9845:2013 [5] is notsuitable for the computational requirements Qp of small drainage works on roads under the present conditions inVietnam and it is necessary to propose the researches for correcting
- The calculated rain intensity parameters are of the general and representative characteristics for the rainregime and the shape of the rain zone, for the specific watershed buffering surface, the water concentration timeand design frequency, it is characterized by the distinct climates of Vietnam, so it is necessary parameters forcalculating the design flow of small drainage works on the roads, reflecting more closely the condition climate andterrain of Vietnam It is necessary to research and determine the calculated rain intensity parameter aT,p under theclimatic conditions of Vietnam used to determine the design flow of small drainage works on the roads Qp 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
Trang 162.1 Overview of Vietnam climatic conditions.
- Vietnam is within the humid tropical, monsoon climate The annual rainfall and rain duration are relativelylarge, averaging from 1100 - 8000 mm/year and from 67-223 days of rain/year, compared with countries in thesame 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 wholenation has more than 800 rain gauge points (including meteorological stations, meteorological stations and raingauge sites are operating) distributed throughout the territory, reaching the national average density ofapproximately 433 km2/rain gauge point Self-recording rain gauge equipments have been present at mostmeteorological stations, accounting for about 20% of the total rainfall stations, each province has 1 metermeteorological 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 1960onward; 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 thenecessary reliability
Trang 172.3 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 tothis criterion: the rainy season is the season including continuous months with the rainfall exceeding losses (usuallytaken as 100mm/month, according to the study [34]) with the exceeding frequency p50%
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 rainyseason in Vietma does not have the shift, the change compared with the results of previous studies, however, therehave been fluctuations in recent years
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
No Meteorological stations, Month with many rainy days in the Rainy months in the year
Trang 18Months Time of appearance Rainy
months Time of rainy season
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-Hanoi 6 months March August 6 months May October
7 Vinh City Station 4 months February March &
September October
4 months August November
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
Trang 192.3.3 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 ofannual 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 ofrainy days become greater, more different and more unusual, appearing extremely big values in recent years
Figure 2.1: Variation trends in
annual rainfall H annual at Lang
StationHanoi City from 1960
-2010
Figure 2.2: Variation trends in the number of rainy days in the year at Lang Station-Hanoi City from
1960 - 2010
‘‘ ’’
Trang 202.3.4 Trends and variability of the biggest daily rainfall H day max and the biggest calculated rainfall intensity in the year a T max The extreme sudden variation due to effects of the climate change phenomena.
Figure 2.3: Variation trends H day max
at Lang Station-Hanoi City from
1960 - 2010
Figure 2.4: Variation trend a T max at T= 60ph at Lang Station-Hanoi City from 1960 - 2010
- Purposes: comparing and clarifying the rules of change in two important rain parameters in calculating designflood flow of small drainage works on the roads Hdaymax and aTmax , assessing the difference between them Thisresearch is especially needed to determine the calculated daily rainfall Hn,p according to the design frequency andcalculated rain intensity interval aT,p at the design frequency in chapters 3 and 4 in handling issues of heavy rain Inaddition, from this study, it also suggests proactive measures to respond to climate change phenomena incomputing the design of small drainage works on the roads today in Vietnam
‘‘ ’’
Trang 21- Research results: realized at all 12 research stations, parameters Hdaymax and aTmax are constantly changing andnot even; in some times, there are sudden significant changes in recent years At the same station, the trend andlevel of time-variaton trend of Hdaymax and aTmax are different, the variability of Hdaymax and the highest 24-hourrainfall in the year H24hmax is not identical, the variability of H24hmax is not identical to that of the highest rainfall inthe year HTmax at other short computational interval T.
+) The extreme interval suddeness of Hdaymax, aTmax occurs at all research stations The extreme segment are inboth values and time of appearance
./ Extremeness in value: values (Hdaymax)*, (aTmax)* are suddenly big exceeding values Hn,p , aT,p at the designfrequency 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 isconsidered to be in the dry season Even the time when the value (Hdaymax)*, (aTmax)* is extremely big at the samestation, they are not coincided For example, for Lang Station-Hanoi City from 1960 - 2010, the time of appearingthe 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 27September 2005 Therefore, with the arrival of rain heavy rain intensity although mutations do not create value inthe largest daily rainfall, does not create large daily rainfall values are still causing mutations major flood surge forsmall drainage structures on roads due to the small basin of small drainage works on the road a short time focuscountries 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 inthe year aTmax in recent years make the value Hn,p , aT,p under the frequency set with the sequence of the actual rainmeasuremens 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
Trang 222.3.5 Average value in many years X and scattering coefficient Cv, hệ bias coefficient Cs of H day max and a T max
- Purposes: This research serves for computing and defining Hn,p , aT,p under the design frequency in chapters 3and 4 in estimating statistical parameters; in determining the years of necessary observation and sampling nyc toensure the sampling error because the more statistical sequences Hdaymax, aTmax have the coefficient Cv, the longer therequired years of observation nyc are in order to ensure the calculated results Hn,p, aT,p are reliable The rate Cs / Cv isalso a condition for making decision on selecting a suitable probability distribution rule when calculating anddetermining 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 averagevalue Hdaymax from 89.31 - 237.46 mm, coefficient Cv = 0.24 - 0.55, Cs = 0.34 - 2.99, Cs/Cv = 1.4 - 5.7; thehighest 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 H day max and a T max
- Purposes: This study serves for testing the representation of the period of sampling of the statistical datasequence Hdaymax, aTmax included in the calculation of the frequency to determine Hn,p , aT,p in chapters 3 and 4 Theyare 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 someyears 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
Trang 23+/ Time of highvalues: (1983 - 1994),(2007 - 2010).
+/ Time of smallvalues: (1956 - 1983),(1999 -2007)
+/ Average time:
(1994 - 1999)
+/ With 2 cycles
+/ Time of highvalues: (1996 - 2010)
+/ Time of smallvalues: (1986 - 1996)
Figure 2.7: Cycle of changing the biggest daily rainfall in the year H day max
at Lang Station – Hanoi City from 1960 – 2010
Figure 2.8: Cycle of changing the biggest rain intensity in the year a T max
at the computational interval T = 30 mins at Lang Station-Hanoi from
1960 – 2010