International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 7, July 2018, pp. 403–411, Article ID: IJCIET_09_07_041 Available online at http://iaeme.com/Home/issue/IJCIET?Volume=9&Issue=7 ISSN Print: 0976-6308 and ISSN Online: 0976-6316 © IAEME Publication Scopus Indexed ANALYSIS OF RAINFALL BY INTENSITY- DURATION- FREQUENCY (IDF) CURVES FOR VAMANAPURAM RIVER BASIN, KERALA Anie John. S Research Scholar, Karunya Institute of Technology and Sciences, Coimbatore, India Dr. Brema. J Professor and Programme Co-ordinator, Karunya Institute of Technology and Sciences, Coimbatore, India ABSTRACT Climate change impact related to water resources can be characterized by changes in temperature and precipitation. The rainfall Intensity Duration Frequency (IDF) relationship is one of the most commonly used tools in water resources engineering particularly to identify design storm event of various magnitude, duration and return period simultaneously. The objective of the present study is to develop IDF curves for the Vamanapuram river basin area, Kerala. Thirty years (1982-2012) of peak rainstorm intensity values with their corresponding durations were extracted and analysed using Microsoft Excel software. The frequency analysis technique used in this study is Gumbel Distribution Method. The linear regression goodness of fit by Microsoft Excel is used to determine the best fit probability distribution. The parameters of the IDF equation and coefficient of correlation for different return periods (2, 5,10,25,50 & 100) are calculated. The IDF curves are recommended for the prediction of rainfall intensities for the selected river basin. Key words: Rainfall Intensity, Rainfall Duration, Intensity-Duration-Frequency (IDF) Curves. Cite this Article: Anie John. S and Dr. Brema. J, Analysis of Rainfall by Intensity- Duration- Frequency (IDF) Curves For Vamanapuram River Basin, Kerala, International Journal of Civil Engineering and Technology, 9(7), 2018, pp. 403–411. http://iaeme.com/Home/issue/IJCIET?Volume=9&Issue=7 1. INTRODUCTION Climate change impact related to water resources can be characterized by changes in temperature and precipitation. The rainfall Intensity Duration Frequency (IDF) relationship is a mathematical relationship between the rainfall intensity, the duration and the return period http://iaeme.com/Home/journal/IJCIET 403 [email protected] Anie John. S and Dr. Brema. J [1]. It is one of the most commonly used tools in water resources engineering particularly to identify design storm event of various magnitude, duration and return period simultaneously [2]. The establishment of such relationship was done as early as 1932 [3]. Analysis of rainfall is usually done by Intensity Duration Frequency (IDF) curves and isopluvial maps [4]. The IDF formulae are the empirical equations representing a relationship among maximum rainfall intensity (as dependant variable) and other parameters of interest such as rainfall duration and frequency (as independent variables). Koutsoyiannis et al. [5] developed a mathematical relationship between the rainfall intensity i, the duration d, and the return period T for IDF curves. There are several commonly used functions found in the literature of hydrology applications [6]. All functions have been widely applied in hydrology. The IDF relation is mathematically as follows: i=f (T, d) (1) Where T is the return period (years) and d is the duration (minutes). The typical generalized IDF relationship for a specific return period is as given in equation (2) a T = e (2) (a v + b) Where a, b, e and v are non-negative coefficients. Thus, the equation that is more general: with v=1 and e=1 it will be Talbot equation; v=1 and b=0 is Sherman; e=1 is Kimijima equation and v=1 is Sherman. This expression is an empirical formula that summarizes the experience from several studies. There has been considerable attention and research on the IDF relationship: Hershfield [7] developed various rainfall contour maps to provide the design rain depths for various return periods and durations. Bell [8] proposed a generalized IDF formula 10 using the one hour, 10 years rainfall depths; P1 , as an index. Chen [9] further developed a generalized IDF formula for any location in the United States using three base rainfall depths: 10 10 100 P1 , P24 , P1 , which describe the geographical variation of rainfall. Kothyari and Garde [10] 2 presented a relationship between rainfall intensity and P24 for India. The engineering application of rainfall intensity is mainly in the estimation of design discharge for flood control structures. The main aim of this work is to understand and quantify uncertainties in series of extreme rainfall at several durations (1, 2, 6, 12, 24 h) collected at three rain gauge stations in Vamanapuram river basin, Kerala from the point of view of climatologically factors, streamflow response and as regards the impact on the dimensions of hydraulic works designed for flood protection. 2. MATERIALS AND METHODS 2.1. Study Area The Vamanapuram river basin with a catchment area of 787 sq. km. is located mainly in Thiruvananthapuram district and a small part falls in Kollam district of Kerala state. Vamanapuram River Basin is bounded by latitudes of8° 35′ 24″ N and 8° 49′ 13″ N and longitudes of 76°44′ 24″ E and 77°12′ 45″ E. The Vamanapuram River Basinis bounded by Nedumangad Taluk of Thiruvananthapuram district in the South, Kottarakkara Taluk of Kollam district in the North, Tamil Nadu in the East and Arabian Sea in the West. The area forms part of the midland terrain of the state, characterized by lateritic uplands with undulating topography and intermittent valleys. The river Vamanapuram is a major river in South Kerala with its network of tributaries. The main stream originates from the foothills of the Ponmudi hills (1074 m above msl) and the tributaries from the surrounding hills like Kallar. Theriver then flow http://iaeme.com/Home/journal/IJCIET 404 [email protected] Analysis of Rainfall by Intensity-Duration- Frequency (IDF) Curves For Vamanapuram River Basin, Kerala through Vamanpuram town and two-branch streams join at Attaramoodu, and at this location the mainstream is called as Kilimanurriver. From there the main stream flows and joins the Kadinamkulam backwater at the northern most extremity. It debouches into the Arabian Sea at Mudalapallipozhi near Perumathura, 25 km north of Thiruvananthapuram city. The two tributaries of this river are the Upper Chittar & Manjaprayar streams. The major portion of the Vamanapuram River flows through midland terrain and the remaining through highlands and lowlands areas. The river basin consists of high to moderate and low flood risk areas. Figure 1 Location Map of the Vamanapuram River Basin For the present study, the relevant maps were created using ArcGIS 9.3 and ERDAS Imagine 9.1 software tools. Satellite imagery was used as primary data source. The study area comprising of Vamanapuram River Basin was delineated from Survey of India Topographic maps of scale 1:50000, numbered 58 D/10, 58 D/13, 58 D/14, 58 H/1, and 58 H/2. The location map and base map of the study area is showed in Figs. 1 and 2. Figure 2 Base map of the Vamanapuram River Basin http://iaeme.com/Home/journal/IJCIET 405 [email protected] Anie John. S and Dr. Brema. J Table 1 Rainfall depth (mm) Vs Discharge for the Vamanapuram River Basin Mean Discharge Sl.No Year Day Rainfall (mm) Guage(m) (cumecs) 1 1992 10/10/1992 10.367 974.8 279.1 2 2010 10/4/2010 8.030 522.0 211.8 3 1996 10/18/1996 7.095 506.3 207.4 4 1998 10/12/1998 8.310 500.3 184.3 5 1993 11/11/1993 6.204 365.1 177.4 6 2001 7/9/2001 5.852 360.1 178.4 7 2000 8/26/2000 5.432 341.4 167.2 8 1994 8/3/1994 6.945 340.7 162.5 9 2009 10/2/2009 5.480 331.4 160.4 10 1997 9/16/1997 5.700 326.3 149.5 11 2007 7/17/2007 5.242 323.3 142.8 12 1990 11/4/1990 6.275 305.4 137.5 13 1984 10/3/1984 5.490 298.5 136.7 14 2005 12/12/2005 5.193 296.2 133.4 15 2006 10/30/2006 4.920 287.2 125.6 16 1999 10/17/1999 5.500 272.5 122.3 17 2011 12/31/2011 4.785 252.3 119.8 18 1991 6/8/1991 5.522 251.3 108.4 19 2008 10/25/2008 4.100 223.1 100.5 20 1989 7/22/1989 4.335 213.6 98.0 21 2003 10/7/2003 3.975 207.7 97.5 22 2002 11/9/2002 3.840 204.0 95.6 23 2004 10/4/2004 3.920 201.2 82.4 24 1982 6/9/1982 4.570 196.3 81.9 25 1986 8/8/1986 4.298 185.3 74.5 26 1988 9/3/1988 4.070 165.5 73.0 27 1985 6/24/1985 3.915 141.1 62.4 28 1987 9/24/1987 3.700 138.5 61.7 29 1995 7/31/1995 3.245 116.5 59.1 30 1983 9/23/1983 2.653 93.5 53.4 SUM= 3844.5 Average 5.467 311.8 AVERAGE=128.15 2.2. Data Collection The major data utilized for the study is rainfall data comprising of rainfall durations and intensities in Vamanapuram River Basin, Kerala. Thirty (30) years of hourly rainfall data included data ranging from 1982 to 2012 from the three rain gauge stations namely Nedumangad, Thiruvananthapuram and Varkala. The data were obtained from Indian Meteorological Department (IMD),Thiruvanathapuram, Kerala.