Areas having hard rock surface include Guindy, Nanganallur, Pallikaranai, Alandur, Jaladampet, Velachery, Adambakkam and a part of Saidapet and Perungudi.[91][92] The ground water table in Chennai is at 4-5 m below ground in most of the areas,[92] which was considerably improved and maintained through the mandatory rain water harvesting system.[93] Of the 24.87 km coastline of the city, 3.08 km experiences erosion, with sand accretion along the shoreline can be noticed at the Marina beach and the area between the Ennore Port and Kosasthalaiyar river.[94]