CHAPTER 3 STUDY AREA

Transcription

1 35 CHAPTER 3 STUDY AREA 3.1 GENERAL There are 34 river basins in Tamil Nadu, and they are grouped into 17 major river basins. Chennai basin is one among them. The Chennai Basin is located between latitudes 12 40' N and 13 40' N and longitudes 79 10' E and 80 25' E at the North East corner of Tamil Nadu. Andhra Pradesh lies on the North side; Palar River basin lies on West and South and the Bay of Bengal on the East which are the main boundaries of Chennai basin. The total area of the basin is 7282 sq km. Out of that 5542 sq km lies in Tamil Nadu, and the rest is in Andhra Pradesh. Araniyar, Kosathalayar, Cooum and Adyar are the four rivers of the Chennai basin group (Figure 3.1). This basin covers partly or fully 26 blocks of Tiruthani, Thiruvallore, Saidapet, Tambaram, Ponneri, Sriperumbudur, Arakonam, and Walajapet taluks (blocks) of Thiruvallore, Kanjeepuram, Chennai and Vellore districts. The major part of the basin area comes under Thiruvallore and entire Chennai districts, and only a few areas fall in Vellore and Kanjeepuram districts. Adyar River collects surplus water from about 200 tanks (artificial ponds) and lakes, small streams and the rainwater drains in the city. The river is almost stagnant except during the rainy season. Rapid industrialization and urbanization directly contribute to waste generation; most of which is discharged into the Adyar River. The Cooum River serves as storm water drainage system within the Chennai city, and has been polluted by untreated sewage and industrial effluents.

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3 37 Population trend and population dynamics are important indicators of future water requirement. Chennai city is trisected by two rivers namely the Adyar and the Cooum. These rivers have their own catchment area to drain the surface runoff. Adyar watershed is a combination of both rural and urban watersheds; 39 % of this watershed falls under Chennai metropolis, and the remaining 61 % falls in Kancheepuram and Tiruvallur districts. It was chosen as one of the study areas. Another study area Nemam watershed is located in Poonamallee taluk (block) of Thiruvallur district, Tamil Nadu. This contributes the surplus to Chembarambakkam tank. 3.2 STUDY AREA India Meteorological Department (IMD) has installed one S-band Doppler Weather Radar (DWR) at Cyclone Detection Radar centre (CDR), Chennai. The DWR derived products such as Surface Rainfall Intensity (SRI) and Precipitation Accumulation (PAC) are available at reliable precision for 100 km radius from the DWR station, Chennai. The Adyar and Nemam watersheds are within 100 km radius from DWR station, hence they had been chosen as the study areas. Satellite data such as Cartosat-1 with 30 m x 30 m resolution, ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) with 30 m x 30 m resolution and SRTM (Shuttle Radar Topography Mission) with 90 m x 90 m resolution provide the Digital Elevation Model (DEM) for the watersheds, and are available on the respective websites. These DEMs are useful in developing drainage network and slope map for the watersheds and could also be utilized for watershed parameter estimation. Watershed delineation and parameter estimation were done using the Indian Remote Sensing (IRS) satellite Cartosat-1 with 30 m DEM and GIS tools for the study areas. The aerial extent of Adyar and Nemam watersheds are sq km and sq km respectively. Figure 3.2 exhibits the study area map with Doppler weather radar station location. Figure 3.3 and Figure 3.4 present the Thiessen polygons for Nemam and Adyar watersheds. The hydrological elements and their connectivity with

4 38 gridded area for Adyar and Nemam watersheds are shown in Figure 3.5 and Figure 3.6 respectively. Figure 3.2 Map of Adyar and Nemam watersheds

5 Figure 3.3 Nemam watershed with Thiessen polygon 39

6 Figure 3.4 Adyar watershed with Thiessen polygon 40

7 Figure 3.5 Adyar gridded watershed with hydrological elements 41

8 42 Table 3.1 Locations of rain gauge stations proximate to the study areas S. No. Rain gauge station Latitude Longitude 1 Nungambakkam 13 04'05" 80 14'46" 2 Chengalpattu 12 42'03" 79 58'41" 3 Sriperumpudur 12 58'01" 79 56'37" 4 Tambaram 12 56'06" 80 07'37" 5 Chembarambakkam 13 00'30" 80 05'05" 6 Meenambakkam 12 59'24" 80 10'53" 7 Thiruvallur 13 08'23" 79 54'24" 8 Korattur Dam 13 05'19" 80 00'39" 9 Anna University Campus 13 00'30'' 80 14'06'' 10 Tharamani 12 59'30'' 80 14'55'' Figure 3.6 Nemam gridded watershed with hydrological elements

9 CLIMATE AND RAINFALL Adyar and Nemam watersheds are relatively at, and slightly undulating terrains. The study areas fall under the semi-arid tropical region. Average monthly minimum and maximum temperatures are about 19º C and 42º C respectively. The mean relative humidity is about %, while the mean wind velocity is 4.84 km/hour. Average sunshine hours/day is 7.25, and the average annual rainfall is about 1200 mm (IWS Report 2007). The locations of the rain gauge stations are given in Table 3.1. Out of the ten rain gauges; nine rain gauges influence the Adyar watershed, and three rain gauges influence the Nemam watershed. The climate of the region is dominated by two monsoons caused by the land-sea thermal contrast. In Chennai, the weather patterns are dominated by the Northeast monsoon, which normally occurs during October-December months. Most of the heavy rainfall in the Northeast monsoon is associated with a clear synoptic system of depression and cyclones, with night time rainfall being most common. Diurnal variations in temperature are moderated by coastal influences. The study areas receive a significant amount of rainfall from cyclonic storms. Cyclonic storm brings heavy rains to the Bay of Bengal and the South-Eastern coast of India, particularly to Chennai during November and December. Three post monsoon cyclonic storm events, namely, Nisha, Phyan and Jal that occurred in the year 2008, 2009 and 2010 respectively were selected for the study. The daily rain gauge rainfall data was collected from the State Ground and Surface Water Resources Data Centre, Tharamani, Chennai. India Meteorological Department (IMD) empirical reduction formula (Rashid et al 2012) was used to estimate the hourly rainfall data from everyday rainfall data for the rain gauge stations of Adyar and Nemam watersheds.

10 SOIL CHARACTERISTICS Hydrologic part is much controlled by different types of soils. Soils are classified by taking their color, texture, fertility and chemical combinations include salts, minerals and the solution effect over them. These soils are generally poor in soil nutrients, medium to high permeability and low water-holding capacity except in patches of clayey soils. These soils do not pose problems for cropping. The Adyar alluvium is m thick and depth of granular zones varies in different places. However, due to indiscriminate withdrawal and use of groundwater along the seacoast, seawater intrusion has been observed (Ramesh 1994). Taxonomies of soils in the study areas are presented in Table 3.2. As far as the agricultural and groundwater point of view, the soil types of the study areas are described based on the thematic maps collected. The hydrological soil maps of Adyar and Nemam watersheds are depicted in Figure 3.7 and Figure 3.8 respectively Alfisols This is a special type of soil, which has a peculiar color shade varying from one area to another area. These red sandy and red loamy soils, deep to very deep, coarse loamy to fine loamy are found near the seashore Entisols These are alluvial soils comprising sand and sandy materials occurring on the beaches and at the confluence of rivers and by the side of the rivers and channels. These are found along the coastal belt in small strips, Easterly part of Ponneri taluk (block), and South of Pulicat lake to Ennore creek, South of Cooum confluence to Adyar Estuary and Thiruvanmiyur - Covelong stretch and throughout the length of beach of the Eastern coast.

11 Vertisols The Vertisols are clayey in nature with high specific water retention capacity but poor in supporting agriculture. These are found as ground mass in extreme Northern portion around Gummidipoondi, Ponneri, Minjur, Madhavaram, and Manali and in the Western portion of the East Coast Road (ECR) around Thiruporur. Hydrogeologically vertisols is grouped under Aquitard. The Valudhavoor patches of clay and the Manali, Mathur clayey soils are the significant members of this type of soil. Vertisols are exclusively used for dwelling and industrial units only. The rate of infiltration varies from 1 to 3 cm/hr for fine red sandy clay, clayey sand, sandy clay, sand fine to medium, sand medium to coarse and very coarse and gravel and for weathered rock, fractured and jointed rock it varies from 0.2 to 0.5 cm/hr, which normally occur in the study areas. Table 3.2 Taxonomy of the soils Sl No Order Sub order Great group Sub group Haplustalfs Udichaplustalfs 1 Alfisols Ustalfs Ultichaplustalfs Paleustalfs 2 Entisols Orthents Ustorthents Oxyaquicpaleustalfs Lithic ustorthents Typicustorthents 3 Vertisols Usterts Chromusterts Enticchromusterts 4 Inceptisols Ochrepts Ustochrepts Udicustochrepts Verticustochrepts

12 Inceptisols This major soil group consists of the red sandy to brownish clayey soil fragments derived from parent rock and is spread all along the Westward side of the ECR. These are suitable for agriculture and hold moderate groundwater reserves. Systematic water bearing rocks are bordering this type and percolate more water into these soil formations for effective agriculture. The agricultural pattern in this type of soil is intensive. Figure 3.7 Hydrological soil map of Adyar watershed

13 47 Figure 3.8 Hydrological soil map of Nemam watershed 3.5 DRAINAGE Adyar River is one of the four main rivers that drain the Chennai basin. It is a small river of 40 km length from its origin to the confluence point with the Bay of Bengal. It usually carries flood water during rainy days. This river finds its source from the Guduvancheri hills and receives the surplus from Manimagalam tank, Nandivanam tank and Chembarambakkam tank (tanks are artificial ponds). This river empties into Bay of Bengal at Adyar estuary in the Eastern part of the Chennai basin area. Due to urbanization this river is polluted by both domestic drains and industrial effluents.

14 LAND USE PATTERN The IRS P6 False Color Composite (FCC) image was used to prepare the land use map of the study areas by Institute for Water Studies (IWS), Tharamani. The land use map was prepared adopting existing classification system developed by National Remote Sensing Centre (NRSC), Hyderabad. This map consists of categories like built-up land, agricultural land, forest land, wasteland and water bodies. A primary occupation in the Nemam watershed is agriculture; and the main agricultural crop is paddy, cultivated on an area of about sq km. Subsidiary crop is groundnut, banana, coconut. The land use of Nemam and Adyar watersheds are portrayed in Figure 3.9 and Figure 3.10 respectively. Figure 3.9 Land use map of Nemam watershed

15 49 Figure 3.10 Land use map of Adyar watershed 3.7 SUMMARY Rainfall-runoff modeling for gauged and ungauged watersheds is essential for water resource projects. The Adyar watershed experienced major flooding in Adyar River, which runs through the city, during the years 1976, 1985, 2005 and Adyar and Nemam watersheds consist of agricultural and urban sector and hence suitable methodology should be opted for runoff estimation. Hence, this research endeavors to simulate rainfall-runoff processes for these two watersheds. The computed runoff values were compared with the observed runoff for the gauged watershed.