CHAPTER 3 STUDY AREA

Size: px

Start display at page:

Download "CHAPTER 3 STUDY AREA"

Abel Craig
5 years ago
Views:

1 48 CHAPTER 3 STUDY AREA 3.1 GENERAL Chennai, the capital city of Tamil Nadu, is located in the North Eastern corner of Tamilnadu. The Chennai Metropolitan Area (CMA) comprises the city of Chennai, 16 municipalities, 20 town panchayats (local governing bodies) and 214 village panchayats. The extent of CMA is 1189 km 2. The CMA falls in three districts of the Tamil Nadu state: Chennai district (area: 176 km 2 ), and parts of Thiruvallur district (area: 637 km 2 out of 3427 km 2 ) and Kancheepuram district (area: 376 km 2 out of 4433 km 2 ). The demographic details of CMA are given in Table 3.1. Table 3.1 Demographic details of CMA S.No Particulars Population in Millions Sex ratio (number of females per 1000 males) Literacy level in % NA 4. Households in lakhs NA 5. Housing units in lakhs NA (Source: Census of India: 1971, 1981, 1991, 2001 and 2011)

2 49 Chennai city is trisected by two rivers namely the Adayar and the Cooum. These rivers have their own catchment area to drain the surface runoff. The Adayar watershed of the Pennar basin was chosen as the study area. This watershed is a combination of both rural and urban watersheds, 39 % of this watershed falls under Chennai Metropolitan area and the remaining 61 % falls in Kancheepuram and Tiruvallur districts. Problems due to flooding in the study area may be attributed to: (i) (ii) (iii) (iv) The change in land use leading to increase in runoff volume with reduction in time of concentration; Encroachment along the banks of the river; Inadequate storm water drains and their poor maintenance; and In urban watershed, inundation occurs even for a low intensityshort duration storm. 3.2 CODIFICATION OF WATERSHED According to All India Soil and Land Use Survey Organization (AISLUS 1990), the codification of Adayar watershed is given as 4C2C2. The letters signify the following: 4 Region Rivers draining in to the Bay of Bengal C Basin Between Cauvery and Krishna (Pennar) 2 Catchment Between Pennaiyar to Pennar C Sub catchment Miscellaneous around Chennai 2 Watershed Adayar a, b, c, d Subwatershed Thirusoolam, Manimangalam, Orathur, Dasarikuppam respectively.

3 LOCATION AND EXTENT The study area extends between the North Latitudes N and N and East Longitudes E and E with an area of about 705 km 2. The area falls in 57 P/131, 57 O/ 161, 66 C/4, 66 C/8, 66 D/1 and D/5 Survey of India Toposheets. This watershed forms part of blocks like Chenglepet, Saidapet, Sriperumbudur and Chennai city. The watershed comprises of about 113 villages and 7 reserve forest. Figure 3.1 shows the index map of the watershed. Figure 3.1 Index map of Adayar watershed

51 Based on Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) and stream flow data, the watershed was delineated into four sub watersheds. Figure 3.

4 CLIMATE AND RAINFALL The Adayar watershed has a tropical climate with high temperature and humidity.

4 51 Based on Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) and stream flow data, the watershed was delineated into four sub watersheds. Figure 3.2 presents the sub and micro watersheds of the Adayar watershed. Figure 3.2 Sub and Micro watersheds of the Adayar watershed 3.4 CLIMATE AND RAINFALL The Adayar watershed has a tropical climate with high temperature and humidity. The watershed falls under the semi-arid tropical area with the hottest climate between April and June, with the maximum temperatures around C and the cold climate between December and February with the minimum temperatures between C. The watershed gets most of its rainfall from the North-East monsoon winds (mid-september to mid-december). The watershed also gets an ample amount of rainfall due to cyclonic storms. The average annual rainfall is about 1200 mm. The highest annual rainfall recorded in the study area is mm in 2005.

5 PHYSIOGRAPHY AND DRAINAGE The study area is a low-lying, flat, slightly undulating terrain with a gentle slope of 3-5 toward the E-ENE direction (Ramesh 1994). The ground slope is gentle towards the coast. The elevation of the area generally ranges between 30 m in the West and sea level in the East. The Adayar is a short river of about 42 km in length and enters the city limit near Nandambakkam Bridge (10 km upstream from the river mouth). The river consists of three arms. The northern arm comes from Chembarambakkam minor basin, the second arm comes from Guduvancheri and the third arm comes from Malaipattu tank near Manimangalam village. The second and third arms join at the weir in Orathur and form the Southern arm. The Southern and Northern arm join at Thiruneermalai as indicated in Figure 3.3. This river is ephemeral in nature and carry flood waters only during monsoon rainfall periods. This river is fed by 109 tanks of Adayar watershed. The flood experiences in Chennai city for the past three decades are given in Table 3.2. The problems associated with Adayar River are: (i) (ii) (iii) Absence of continuous flow in the river; Lack of required width to discharge flood water due to presence of patta (land ownership approved by the Government) lands within the flood plains; Reduction in waterways due to encroachments and solid waste dumping, causing flood hazards; and (iv) Formation of sand bar in the river mouth causing stagnation.

6 53 Northern arm Southern arm Figure 3.3 Drainage map of Adayar watershed

7 54 Table 3.2 Flood experiences in Adayar River for the past thirty years Sl. No. Year Experiences Heavy flood submergence in Adayar-Kotturpuram TNHB Quarters. Flood could not enter into sea due to high tide. Chembarambakkam tank surplused into Adayar River m 3 /s (28,000 cusecs) Floods in Adayar m 3 /s (63,000 cusecs) submergence of encroached flood plains Floods in Adayar, Cooum and Kosasthalaiyar rivers. Poondi reservoir surplused around m 3 /s (80,000 cusecs). Chembarambakkam tank surplused into Adayar River m 3 /s (20,000 cusecs) Three persons marooned in Thanikachalam Nagar - a residential colony in the flood plains of Kodungaiyur drain cm of rainfall recorded in a day corresponding to 100 year return period. Flood in Cooum River m 3 /s (19,000 cusecs), Adayar River m 3 /s (40,000 cusecs), Otteri Nullah, Cooum, Adayar, Buckingham Canal, Virugambakkam- Arumbakkam Drain over flowed, 50,000 people evacuated Chembarambakkam tank surplused into Adayar river m 3 /s (15,000 cusecs) 3.6 GEOLOGY The entire watershed comprises a sequential pile of unconsolidated sediments, semi consolidated sediments, plutonic rocks and residual cappings. Semi consolidated sediments are found in the Western parts of the area and Plutonic rocks are found in the Southern parts of the area, while unconsolidated sediments are found in the Northern parts of the watershed as can be seen in Figure 3.4.

55 Figure 3.4 Geology map of the study area (Source: IWS Tharamani) 3.

sand form potential aquifers. The alluvium deposit is limited in thickness from 10 to 28 m.

8 55 Figure 3.4 Geology map of the study area (Source: IWS Tharamani) 3.7 LITHOLOGY Ground water in Adayar watershed occurs in semi-confined to confined conditions in the porous alluvial formations. Gravel, coarse to fine sand, clay and silty clay constitute the alluvial material and of these, the gravel and sand form potential aquifers. The alluvium deposit is limited in thickness from 10 to 28 m. Alluvium covers a major part of the Adayar watershed followed by crystalline pockets in South Chennai. The alluvium consists of sand, silt and clays and the thickness of alluvium is widely varying. The tube wells constructed in the alluvium vary from depth 10 m to

9 56 30 m. The crystalline rocks underlie the southern part of the watershed and the occurrence of ground water is essentially limited to the weathered mantle and the zones of fracturing of the rocks. These hard rock aquifers are heterogeneous in nature and the water bearing characteristics vary very widely both laterally and vertically. Ground water occurs under the phreatic conditions in the fractured and fissured zones of the rocks (Ramesh 1994) as indicated in Figure GEOMORPHOLOGY The geomorphological set up of the study area is shown in Figure 3.6. Pediplains are found in the major portions of the study area. Alluvial plains are found in the Northern part while the coastal plains are found in the Eastern part of the study area. 3.9 SOIL Different types of soil present in the entire watershed are Alfisols, Inceptisols and Entisols. Due to different stages of weathering of parent material, these soil types are in combination. 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 much problem for cropping. The Adayar alluvium is m thick and depth of granular zones varies at different places. However, due to indiscriminate withdrawal and use of ground water along the seacoast, sea water intrusion is observed (Ramesh 1994). The taxonomy of soils of the study area is derived from the soil map and is presented in Table 3.3. The soil map of the study area is presented in Figure 3.7.

10 57 Figure 3.5 Lithology map of the study area (Source: IWS Tharamani) Figure 3.6 Geomorphology map of the study area (Source: IWS Tharamani)

Lithic ustorthents Typic ustorthents 3 Inceptisols Ochrepts Ustochrepts Udic ustochrepts Vertic ustochrepts 4 Vertisols Usterts Chromusterts Entic chromusterts Note: Alfisol : A fertile leached soil

11 58 Table 3.3 Taxonomy of the soils Sl No Order Sub order Great group Sub group 1 Alfisols Ustalfs Haplustalfs Udic haplustalfs Ultic haplustalfs Paleustalfs Oxyaquic paleustalfs 2 Entisols Orthents Ustorthents Lithic ustorthents Typic ustorthents 3 Inceptisols Ochrepts Ustochrepts Udic ustochrepts Vertic ustochrepts 4 Vertisols Usterts Chromusterts Entic chromusterts Note: Alfisol : A fertile leached soil found in humid areas that is alkaline or basic and contains a clay enriched sub soil. Entisol : The soil that is basically unaltered from their parent material which can be unconsolidated sediment or rock. Inceptisol : The soil that can be formed quickly through alteration of parent material. They have no accumulation of clays. Vertisol : The soil which has a high content of clay. Figure 3.7 Soil map of the study area (Source: IRS Anna University)

12 STUDIES ON ADAYAR RIVER There is a history of flooding in Adayar River flood plain (during the years 1976, 1985, 2005 and 2008) with damages to lives, property and infrastructure. After the disastrous flooding in November 1976, the Government of Tamil Nadu constituted a Task Force Committee report (1976) to study the problem and to suggest concrete measures to prevent the flood damages due to flooding in Adayar River. The study focused on the reach between Nandambakkam Bridge which is 10 km upstream from the mouth of the river and Southern Railway Bridge near Saidapet, which is 5 km upstream from the mouth of the river. Orifice formula was adopted to compute the maximum flood discharge assuming Manning s roughness coefficient as They recommended flood mitigation works for a value of Rs Lakhs (220 Millions) in their report submitted to the Government. This includes formation of flood protection walls, embankments, raising full tank level of reservoirs, excavation of new channels to carry flood water and reconstruction of bridges. The Institute of Hydraulics and Hydrology (IHH report 1981) conducted physical model studies based on the recommendations given by the Task Force Committee Report (1976). A distorted rigid bed model covering the area bounded by Latitudes and longitudes are N, E and N, E to scales 1/500 horizontal and 1/50 vertical was constructed. The model was verified only at low stages and it was assumed that the same relationship is maintained for higher discharges. An Expert Committee (1986) examined (i) the maximum flood that can be expected from the basin other than rainfall; (ii) the effect of high tide on flood levels; (iii) the effect of existing structure across the river including the encroachments along the river; (iv) the problem of siltation of Adayar River

13 60 and methods to clear the same; and (v) the effect of rough sea during cyclonic storms and formation of sand bar in the mouth. After analysis, they concluded that: (i) the river s natural regime like flood plains, natural course etc. should not be tampered with by human intervention; (ii) protective works should be carried out with minimal interference to the river regime; (iii) more number of rain gauges must be installed; and (iv) flood plain zoning and monitoring the future use of the flood plains must be given top priority. However, it is to be noted that the previous studies focused on an area within the city limits and Manning s roughness coefficient adopted was for 1976 and 0.03 for 1985 floods. Thandaveswara et al (1986) established a flood damage-frequency relationship for five regions along the Adayar River. A constant value of 0.03 for Mannings s roughness coefficient and a uniform bed slope of were used in their study. This study is a continuation of the previous works, taking into account the upstream contribution outside the city limits and appropriate Manning s n in a 1D hydraulic model.

CHAPTER 3 STUDY AREA

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