Construction of Subsurface Dams for Sustainable Water Management in the Mandavi and Ganganeru River Basins, YSR District, Andhra Pradesh, India Routu Balaram, S. Ramanaiah and R. Jagadiswara Rao Somasila reservoir constructed across the Pennar River in YSR District, Andhra Pradesh, India stores over 2 billion cubic metres (BCM) of surface water received from the basin proper and that imported from Krishna River to convey drinking water to Chennai and water for irrigation, drinking and industrial use to downstream stakeholders in Nellore and Chittoor districts. This storage became possible because of constructing a subsurface dam beneath the same reservoir to prevent an equivalent amount of groundwater to remain in the upstream. This incidentally boosted up the groundwater resources of several nearby regions such as the Rajampet area bordered by Cheyyer River in YSR District. This paper attempts to make out a case for construction of two small subsurface dams at the confluence of the Mandavi-Ganganeru Rivers to provide sustainable water supply to the chronically drought prone region further west of Rajampet area. As per the latest estimate of the Central Ground Water Board, the annual replenishable groundwater resources of India are 431 BCM, of which the annual groundwater draft is 243 BCM (CGWB, 2012). This implies that the unutilised balance groundwater of 188 BCM remains unutilised because of getting lost to sea as groundwater runoff mostly beneath the multitude of rivers. It is possible to make use of a significant portion of the groundwater so lost by arresting its flow through construction of subsurface dams across rivers close to the coast. Groundwater so arrested in the downstream on saturation emerges as controlled base flow for use in the downstream, while that accumulated in the upstream provides additional water through construction of high-yielding wells in the riverbeds. This strategy appears to be the only economic way to meet the ever-growing fresh water needs of India particularly because of its ever-growing population (Jagadiswara Rao, 2003, 2004). Although no large-sized subsurface dam exists in India close to the coast for the exclusive purpose of preventing groundwater from joining the sea, the foundations of some dams constructed across rivers underlain by permeable formations such as sand, gravel and boulders are actually subsurface dams that disallow downstream flow of groundwater through permeable formations. One such dam is the Somasila reservoir constructed across the Pennar River in YSR (earlier known as Cuddapah and Kadapa) District of southern Andhra Pradesh (Fig. 1). Construction of the Somasila Reservoir With an area of 55,213 sq km, the Pennar Basin has 6.9 BCM of exploitable surface water resources and 5.35 BCM of exploitable groundwater resources (CWC, 1988). In addition, the Pennar River receives imported water from the Krishna basin in varying amounts. Because of the impervious Velikonda hill ranges bordering the entire eastern border of the YSR district, the entire unutilised surface and groundwater runoff generated in the basin was earlier passing through the narrow Somasila gorge. With the construction of the 1
Somasila reservoir together with its subsurface dam across the gorge, most of the unutilised portion of groundwater amounting to around 50%, which was earlier leaving to Nellore district beneath the Pennar Riverbed, now remains within YSR district itself. As a result, there has been enormous boost up of groundwater in the upstream of the Somasila reservoir for a distance of several kilometres both along Pennar and Cheyyer rivers in the mandals of Atlur, Gopavaram. Nandalur, Penagalur, Rajampet, Pullampet and Vontimitta. As a result, large chunks of land get groundwater for irrigation round the year through multitude of shallow tube wells known as filter points and deep bore wells within rocky aquifer without danger of any long-range groundwater depletion. Rajampet area is one such area that benefits from groundwater enhancement to increase irrigated land to grow paddy, sugarcane, groundnut and other crops round the year (Fig. 2). Fig. 1: A Google Earth image showing the Srisailam reservoir constructed across the Pennar River and the location of the confluence of the Mandavi and Ganganeru Rivers. The principal geological formations in this area include Pullampet Shale of low resistance forming pediments and pediplains occupied by cultivated land interspersed by resistant Pullampet quartzite forming hilly tracts covered by forestland. Despite availability of abundant groundwater, the Government has been making additional efforts without much success to develop surface water resources through construction of new reservoirs and construction of rainwater harvesting structures besides importing Krishna River water through Galeru-Nagari Sujala Sravanthi (GNSS) canal. Although useful in recharging groundwater, the Cheyyeru reservoir under construction since 1976 failed to provide adequate irrigation water by gravity flow owing to occurrence of cavernous dolomitic limestones in its foundations. Forestland occurs in the hilly tracts towards west on the Nagari quartzites of Palakonda (or Seshachalam) hill ranges and towards east on the Pullampet shales. 2
Fig. 2: Rajampet area upstream of the Somasila reservoir has abundant fertile agricultural lands (green) irrigated mostly by groundwater. Forestland occurs in hilly tracts towards west on Nagari quartzites and towards east on Pullampet quartzites. Mandavi-Ganganeru Area The hot semiarid tract occupied by the Mandavi and Ganganeru River basins of low fertility comprising of granitic rocks cut across by resistant dolerite dykes and brittle quartz veins occur as an upland area towards west of the Rajampet area (Fig. 3). The Mandavi River, named after the ancient sage Mandavya, rises at an elevation of 1054 m in the Rekkalakonda Reserved Forest in Gurramkonda Mandal of Chittoor district of Andhra Pradesh and enters YSR district after traversing for a distance of 12 km. It then flows for a distance of 82 km in Chinnamandem, Rayachoti, Lakkireddipalle and Veeraballe Mandals to join the Cheyyer River. The Mandavi basin with an area of 1423 sq km also includes Ganganeru basin with an area of 494 sq km towards north. The Ganganeru river flows in a general easterly direction close to the Gulcheru quartzite hill ranges of Palakonda hill ranges towards north with hill streams contributing most of its runoff. The whole area shows a very rugged topography composed mainly of denudational hills, pediments and pediplains of granite and dyke rocks, which are mostly devoid of any vegetation. Dyke rocks cut across granitic rocks mostly in an east-west direction. With a weighted mean annual rainfall of 693 mm, the rainfall in the region is highly erratic with most of it falling in a few days a year. The streams in the basin are ephemeral with no runoff for most part of the year leading to longduration drought damage and flash floods leading to short-duration flood damage. The surface runoff contributes substantial sediment load composed mainly of coarse sand and subordinately suspended particles of silt and clay. 3
Fig. 3: The Mandavi-Ganganeru area west of the Rajampet area occupied by agricultural lands kept mostly fallow (hazy white) but for minor patches (green) of irrigated lands bordering streams. Most cultivated land in the area originally depended on uncertain rainfall for growing crops. In course of time, community-managed spring channels and tanks constructed across streams started providing irrigation water by gravity flow. In course of time, large-diameter dug wells with provision to lift groundwater by animal draught and diesel/electric pumps started providing irrigation water at individual level. The cropping season is mostly in kharif from July to October during the south-west monsoon, with land remaining mostly fallow for the rest of the year. The soils have reduced fertility because of high annual fluctuations in the soil moisture with near water logging conditions in the monsoon and dry barren soil in summer. These fluctuations have led to capillary upward solution movement depositing sodium carbonate in the top soil to make it alkaline with high ph and reduced crop yields. At the time of capturing the satellite image (Fig. 3), most land was fallow exposing alkaline soils that look hazy white, differing from the white shown by the dry sandy riverbeds. Discovery of tapping groundwater round the year in sandy alluvium through indigenous low-cost filter points is a boon for irrigation agriculture in the region. Small chunks of such lands bordering the Mandavi and Ganganeru rivers appear as green patches in the satellite image. Unlike the alkaline lands where crop growth is restricted, the soils where crops grow throughout the year do not produce alkaline soils and continue to remain fertile. The Government of Andhra Pradesh has formulated a project over three decades ago to bring floodwaters of the Krishna River to the region through Handri-Neeva Sujala Sravanthi (HNSS) canal without much success. The aim of this paper is to evolve a strategy for increased enhancement of groundwater in sandy alluvium through construction of two subsurface dams across the Mandavi River to increase land under irrigation in the Mandavi-Ganganeru area. 4
Construction of Subsurface Dams across the Mandavi River The entire surface and ground waters generated in the Mandavi river basin pass through a narrow gorge cut across the Palakonda hill ranges at Vangimalla village in Veeraballe mandal. The riverbed is nearly dry before entering the gorge portion and shows surface water during its journey through the gorge. This is evidently because of a substantial reduction in the width and thickness (that is, cross sectional area) of sand under the riverbed to allow groundwater to emerge as surface water. The Department of Minor Irrigation of the Government of Andhra Pradesh has constructed a subsurface dam at this site in 1990 where vertical walls of impervious Nagari Quartzite abut on either side of the Mandavi River by replacing the entire sand along the dam with impervious puddle clay (Figure 4). Despite constructing the subsurface dam at the best available site, the upstream farmers found no improvement in their well yields. Had the subsurface dam constructed properly, there should be now surface water immediately downstream of it. The water bodies in the downstream used by dhobis for washing clothes before dam construction remained intact even after the dam construction (Fig. 5). The authors studies have indicated that the failure of the subsurface dam in achieving the desired results is because of using puddle clay of doubtful quality as the construction material besides not carrying out adequate scientific investigations in establishing the exact contact surface between the sand and the underlying rocky formation in the riverbed. As there is difficulty locally to obtain puddle clay of high plasticity and masons having expertise to handle puddle for such construction, the authors recommend masonry over puddle clay as the construction material for the construction of the subsurface dam. The authors have carried out detailed exploratory boring at nine sites along the alignment of the proposed subsurface dam at 15-m intervals by using the boring equipment used by the local farmers for construction of filter points in sandy alluvium. This involved use of a 15-cm wide and 30-cm long bucket auger to remove sediment in the zone of aeration (Figs. 6) and a 15-cm wide and 100-cm long shell with non-reversible value to remove sediment in the zone of saturation (Figs. 7). Fig. 8 illustrates the results obtained on the thickness of sand, summer water table, mean water table and bed level along the dam alignment. The mechanical analysis of the grain-size distribution of sand samples collected has been determined using a set of sieves. Standard methods have been used to investigate particle size distribution (grain-size curve) and determine the parameters of d10, d50 and d60, where d10, d50 and d60 are the soil particle diameters in mm that 10%, 50% and 60% of all particles are finer (smaller) by weight. The saturated hydraulic conductivity K s expressed in metres/day has been determined by using the latest formula K s =10.06+118.54(d 10 )- 12.50(d 50 )-7.32(d 60 ) found to be the best by Salarashayeri and Siosemarde (2012) by regression analysis for arriving at reasonably accurate saturated hydraulic conductivity of sand. This formula indicates that the parameter d10 plays a more significant role compared to the parameters d50 and d60, and best called as the effective parameter in the calculation of 5
saturated hydraulic conductivity. Table 1 gives the saturated hydraulic conductivity of 19 test-bore samples as obtained by this method to range from 28 to 45 m/day with a mean of 35 m/day. Fig. 4: A photograph showing the beginning portion of the left flank of the Vangimalla gorge of the Mandavi River best suited for the construction of a subsurface dam. A vertical line marked at the left flank on the rock indicates the alignment of the defunct subsurface dam constructed across the river by the Department of Minor Irrigation, Government of Andhra Pradesh in 1990. 6
Fig. 5: Spring discharge in the Mandavi Riverbed immediately downstream of the subsurface dam constructed by the Minor Irrigation Department, Government of Andhra Pradesh indicates that the subsurface dam constructed in the upstream failed to arrest groundwater flowing through it. Fig. 6: A picture showing exploratory boring by bucket augering in the zone of aeration without inserting the casing pipe. 7
Fig. 7: A picture showing exploratory boring by shelling in the zone of saturation by driving the casing pipe to the junction between sand and the bottom rock. The authors investigations along the proposed dam axis have revealed the width of the Mandavi River to be 140 m, average sand thickness 3.5 m, average water-bearing sand 3.0 m, cross-sectional area 490 sq m and mean saturated hydraulic conductivity 35 m/day. A subsurface dam blocking the entire water-saturated sand arrests over 53 lakh cubic metres of water a year. Construction of such a subsurface dam at the site leads to regulation of flood flow in the Mandavi River, resulting in a more uniform water flow in the downstream for a longer duration. The subsurface dam proposed makes the whole structure watertight to disallow any water to seep downstream through sides and bottom of the dam with the surrounding rock (Fig. 9). Use of cement concrete in the bottom and top of the dam provides further protection. While executing the work, actual construction should be based on the actual depth of rock reached rather than as per the figures obtained during the investigation. Fig. 10 shows the exact location of the subsurface dam proposed in the southwest corner of a satellite image of the Mandavi River (Fig. 10). While boosting up the groundwater availability in the upstream, this subsurface dam reduces the groundwater availability to the downstream Gadikota village. To compensate this reduction, another subsurface dam located in northeast corner of the image downstream of the Gadikota village is necessary. The reduction in the groundwater availability in the Rajampet area, located further downstream of Gadikota village, is more than adequately compensated by the Somasila subsurface dam-cum surface dam reservoir located further downstream. 8
Fig. 8: Profile showing the thickness of sand, summer water table, mean water table and bed level along the alignment of the proposed subsurface dam as obtained by exploratory boring along the Mandavi River at Vangimalla village at Veeraballe mandal. Fig. 9: Cross section of the design of the masonry subsurface dam proposed across the Mandavi River, Vangimalla Village at Veeraballe Mandal. The line marked SS Dam in the southwest corner in Fig. 10 indicates the location of the proposed subsurface dam. 9
Table 1: Saturated Hydraulic Conductivity (K S ) of Test Bore Samples in m/day Sediment Sample No. Effective Size (D10) in mm Median (D50) in mm D60 in mm 1 0.41 1.12 1.44 34 2 0.48 1.22 1.32 42 3 0.49 1.12 1.24 45 4 0.49 1.23 1.30 43 5 0.49 1.23 1.36 43 6 0.41 1.25 1.21 34 7 0.47 1.20 1.80 38 8 0.45 1.18 1.58 37 9 0.40 1.15 1.58 32 10 0.46 1.00 1.89 38 11 0.39 0.99 1.58 32 12 0.41 0.98 1.70 34 13 0.39 0.97 1.66 32 14 0.37 1.15 1.52 28 15 0.36 0.99 0.94 33 16 0.36 0.93 1.00 34 17 0.35 0.99 1.23 30 18 0.35 1.17 1.09 29 19 0.35 1.15 0.97 30 Saturated Hydraulic Conductivity (K S ) in m/day Fig. 40: Location of subsurface dams across the Mandavi River upstream and downstream of Gadikota village. The upstream subsurface dam boosts up the groundwater availability in the upstream. 10
ACKNOWLEDGEMENTS: The authors are grateful to Mr. Jayesh Ranjan, I.A.S., Former District Collector, Kadapa and Dr. G. Muniratnam, General Secretary, Rashtriya Seva Samithi (RASS), Tirupati for encouragement and providing financial assistance for carrying out exploratory boring studies across the Mandavi River in the summer of 2003. Suggested Reading: CGWB (2012) Ground Water Year Book India 2011-12: Central Ground Water Board, Faridabad, 63 p. http://cgwb.gov.in/documents/ground%20water%20year%20book%20- %202011-12.pdf CWC (1988) Water resources of India: Publication No. 30/88. Central Water Commission, New Delhi. Jagadiswara Rao, R. (2003) Ground water development in the riverbeds through subsurface dams: a viable alternative to linking of rivers in India: Journal of Indian Water Works Association, Oct-Dec 2003, p. 307-312; Reprinted in Civil Engineering Construction Review, May 2004, p. 36-40. Jagadiswara Rao, R. (2004) Unseen waters: http://www.indiatogether.org/2004/may/envsubsurf.htm Accessed on 21 Mar 2013. Salarashayeri, A.F. and Siosemarde, M. (2012) Prediction of soil hydraulic conductivity from particle-size distribution: World Academy of Science, Engineering and Technology, v. 61, pp. 454-458. http://www.waset.org/journals/waset/v61/v61-81.pdf About the Authors Mr. Routu Balaram, Junior Geologist, District Water Management Agency (DWMA), Kadapa, A.P., 516001 Email: routubalaram@gmail.com Dr. S. Ramanaiah, Professor, Dept. of Geology, Sri Venkateswara University College of Sciences, Tirupati, A.P., 517502 Email: ramanaiah.svuniversity@gmail.com Dr. R. Jagadiswara Rao, Professor of Geology and Principal Retired, Sri Venkateswara University Colleges of Arts and Sciences, Tirupati, A.P., 517502, Email: rjagadiswara@gmail.com 11