CALCULATION OF MAXIMUM RAINSTORM FLOW RATE IN AREAS OF LATVIA

Transcription

1 CALCULATION OF MAXIMUM RAINSTORM FLOW RATE IN AREAS OF LATVIA ABSTRACT Eriks Tilgalis, Reinis Ziemelnieks Latvia University of Agriculture Department of Architecture and Building Rainstorm water flow rate is highly variable and it is difficult to do accurate calculations of runoff and the flow rate, that result in periodic flooding of streets and squares disturbing traffic and causing material damages. The developed rainwater system calculation method is capable of determining the rainfall maximum flow rates of different surfaces with the probability of % in the entire territory of Latvia. A method for calculating rain flow probability of different recurrence was developed in the 2010 thesis for the city of Riga (R.Ziemelnieks). The aim of this paper is to develop a method for determining the rain flow for other major Latvian cities and populated areas. The improvement of the existing rainwater flow calculation method is given in Latvian Building Normative LBN , which is partly borrowed from the former Soviet Union improved building standards and regulations СНиП (SNIP)(Строительные нормы..,1985), which are based on the promotion work thesis by Soviet scientist Kurganov (Курганов) in 1978(Курганов, 1984). Key words: rainwater, rain flow, surface runoff, intensity INTRODUCTION The effects of global warming each year causes intense unpredictable rain storm water, flooding the streets, rivers, newly built villages, destroying dams and causing financial losses for people. Laws are less able to quickly adapt to today's modern availability of materials and the rapid nature of human life. The situation is becoming worse by the connection of the old rainwater sewerage collectors to the sewerage networks, thus creating an additional load to the sewerage networks of the co-systems and pumping stations during the rain. Media news shows that heavy downpours in Riga and other Latvian cities are becoming more intensive year after year. In Latvia, the Latvian Building Normative LBN is used as the official calculation method which does not give the correct results in practical life. The problem is that there has not been a method developed of correct calculation for the necessary distance in-between rainwater gully traps. MATERIALS AND METHODS This study makes use of the rain observation data from different inhabited locations and towns using the publicly accessible information from the agency of Latvia environment, geology and meteorology (LVGMA) (Tables of Meteorological Observations.., Meteorological and hydrological..). The rain observation data rows were summed up and supplemented in order to improve the existing calculation method of maximum rainwater discharge in towns of Latvia by means of the new k-coefficient. The data of rain intensity during the warm period from April till September were chosen and used when the downpours with the maximum intensity are observed in Latvia causing flooding of territories and streets. Rainwater discharge calculation methods In order to be able to calculate the rain water discharge according to the data obtained with a larger possibility, some improvements were already made of the existing adopted calculations, thus by improving the drawn up formula with k-coefficient of the rainwater maximum discharge by A.Zīverts (Zīverts, 1997). Maximum storm water flow rates are determined according to the following expression Q max = q F. [m 3 s -1 ] (1) Q max - max 20 minutes rainfall flow rate, m3 s-1; q runoff module l (s ha) -1 calculated using the formula by E.Tilgalis; depending on the probability of calculation 300;200;100;50 and 10%, in accordance with LBN , 1 table and Fig.1-4); F the surface run-off area, ha; surface runoff coefficient ( % depending on the surface (Table 2); The rainwater diversion gravity self-flow collector internal diameters were calculated assuming full pipe filling and a minimum allowable internal pipe diameter of mm for the inside section nets to calculate the minimum pipe slope (LBN ), 182

2 which will challenge the self-cleaning process of the pipe systems: Q=ω υ, m 3 s -1 (2) I min=τ(ρgr) -1 (3) The coefficients of rainwater discharge surface for the present surface cover areas have been studied in the world and their values are shown in Table 1. The data in the table show clearly that it is possible to reduce the rainwater discharge several times by using the low density surface covering materials where: ω- cross sectional flow area, m2; υ- flow speed, ms-1; τ flow traction, N m-2; ρ specific gravity of waste water, kg m-3; g - acceleration of free fall m s-2; R- filler of pipe hydraulic radius, m; The kind of material used for the surface cover may be different in the location of use of a conformable area, therefore the rainwater discharges change. In Latvia the coefficients of surface discharge have not been summed up in scientific literature. Runoff coefficients Use of areas or type of Covering material of surface area Table 1 Coefficient, Town office Commercial premises Detached house Flat in a dwelling house Flat(apartments) Inhabited suburb district Light industry Heavy industry Parks, green areas, cemeteries Street, pavement covering by asphalt or concrete Concrete area Concrete cobble stone covering Pedestrian pavements and part for transport House roof covering material (depending on material) Sandy soil with 2% decline or less Sandy soil with 2%-8% decline Sandy soil with 8% decline or more(precipice, slope) Grassland having clayey soil composition Decline 2% or less Decline 2%-8% Decline 8% and more (precipice, slope) Source: Computer applications in hydraulic engineering (basic hydrology-rainfall) (translation report by R.Ziemelnieks) 183 RESULTS AND DISCUSSION By determining the values of the precipitation intensity of maximum minutes and by specifying the coefficients of surface runoff for different kind of covering materials, the calculation method of the existing discharge has been improved. The existing calculations in the Latvian building normative LBN envisage the precipitation discharge having the repetition period once a year (100%), twice (200%) or three times a year (300%). Moreover, the calculation given by the LBN and recommendations partly adopted and improved from the building norms and regulations of the USSR СНиП (SniP) were based on the USSR scientist Kurganov s promotion paper elaborated in 1978 and improved in the course of time and later shown in the handbooks of different kinds and number table materials (Kurganov, 1984). The calculations have been specified and around 1986 Snip projecting materials were officially accessible (Metodiskie norādījumi lietus.., 1983). Sewarage system location conditions of pipe collectors Small streets/main streets Runoff different sewage collector location conditions Table 2 Recurrence intensity (once per given period) of rainwater exceedance period P(years) in living areas, if q20 till 60 over over Over 120 Favorable and environmentally friendly/favorable Unfavorable/ Medium favorable Super unfavorable/ Unfavorable.../Super unfavorable Source: Regulations about Latvian building normative LBN Outer networks and buildings of sewerage The existing method of rainwater calculation amount according to Latvian building normative Cabinet regulations No.214 Regulations about Latvian building normative LBN Outer networks and buildings of sewerage ("LV",198/199 (1658/1659), ) came into force on with the alterations

3 (regulations on LBN , 2010), and it is complicated. Some parts for rainwater calculations in the normative which are given in the table or formulas are useful as coefficients of rainwater discharge surface for street runoff designing Recurrence intensity of rainwater excess period in different sewage collector location has been shown in Table 2. Figure 1. Maximum stormwater runoff module q20 with the probability of 100%; l(s*ha) -1 Figure 2. Maximum stormwater runoff module q20 with the probability of 50%; l(s*ha) -1 The method elaborated provides the possibility to calculate the maximum rainwater discharges in inhabited locations with a different possibility. In the result of the investigation it is possible to conclude that in Latvia it is advisable to use the calculations with the repetition possibility of 50 or 25% (frequency of repetition once in 2 or 4 years) and less. According to the table drawn up by A.Zīverts (Zīverts, 1997) several improvements 184 were carried out, values of k-coefficients were determined and the runoff module was calculated at different provisions for the inhabited locations of Latvia. Data calculations are moved to make practical isoanmale line drawings of the runoff module with the probabilities of 10, 25, 50, 100% in 20min (figures 1, 2, 3, 4). In the result of the investigation it is possible to conclude that in Latvia it is advisable to use the calculations with the

4 repetition possibility 50 or 25% (frequency of repetition once in 2 or 4 years). Figure 3. Maximum stormwater runoff module q20 with the probability of 25%; l(s*ha) -1 Figure 4. Maximum stormwater runoff module q20 with the probability of 10%; l(s*ha) -1 CONCLUSIONS Calculations from given expressions of results criteria probabilities can be practically shown in map of Latvia for each town. The calculation method of maximum rainwater discharge elaborated shows that LBN method is imperfect and gives incorrect results in the rainwater system calculation, it is proved by the fact that during heavy downpours many Latvian streets are flooded. In the future the work must be continued on the improvement of calculation methods of more accurate maximum rain water discharges as well as on different kinds of calculation methods in the territory of Latvia. The newly formed formula and the values given offer an accurate calculation of the rainwater discharge. 185

5 REFERENCES Ziemeļnieks R., Tilgalis E. (2009) Calculation method of rainfall flow rate, LUA Annual 15th International Scientific Conference Proceedings, Research for Rural Development Jelgava, Latvia University of Agriculture, pp Ziemelnieks R., Tilgalis E., Juhna V. (2008) Rainfall effect on sewage co-system activity in Riga, LUA Annual 14th International Scientific Conference Proceedings, Research for Rural Development Jelgava, Latvia University of Agriculture, pp Строительные нормы и правила (1985) СНиП "Канализация. Наружные сети и сооружения" (утв. постановлением Госстроя СССР от 21 мая 1985 г. N 71)(с изменениями от 20 мая 1986 г.). Москва: Госстроем СССР. 129 c. Курганов А.М. (1984) Таблицы паpaметров предельной интенсивность дождя для определения расходов в системах водоотведения: толковый справочник. Москва: Стройиздат. 108 c. Tilgalis Ē. (2004) Notekūdeņu savākšana un attīrīšana: mācību grāmata. Jelgava: LLU, LVAF. 239 lpp. Zīverts A. (1997) Ievads hidroloģijā: mācību palīglīdzeklis, Jelgava: LLU. 111 lpp. Meteoroloģisko un hidroloģisko novērojumu dati [online] [accessed on ]. Available: Noteikumi par LBN Kanalizācijas ārējie tīkli un būves: MK noteikumi Nr.214 [online] [accessed on ]. Available: 186