Evaluating the connection between Samanalawewa Reservoir leakage and right bank groundwater distribution: Toward identifying potential karst flow paths

Abstract

The Samanalawewa Reservoir is a major source of hydropower and water supply in Sri Lanka, and continuous water leakage was observed on its right bank. This study evaluated the relationship between reservoir water levels, leakage flow rate, and groundwater distribution to identify potential karst flow paths. The readings of the well-structured groundwater observation wells were used to model the right bank’s groundwater distribution corresponding to changes in reservoir water levels. Groundwater distribution maps were prepared to visualise the groundwater behaviours at different reservoir water levels, and factors influencing fluctuations in the water table were studied. Pearson’s correlation coefficients and related statistical methods were applied to assess the strength of the relationship between leakage flow rates and each groundwater observation well, and a correlation above 0.9 was selected as the highly correlated observation wells. The effect of the spatial proximity on the correlation strength was also studied. Distribution of the highly correlated observation wells leads to the identification of three possible leakage paths. Water entering the right bank through the GW9, GW12 area and sinkholes could be released through the leakage outlet through the area that MS3 and Z6. The second path extends along Adit G, passing through GW2, WB2, GW8, WB8, and GW4. The third path is further upstream (Walawa River) from GW15 through WB3, GW6, GW5, WB8, and GW2. Groundwater level variations along the identified possible leakage paths were examined and showed perfect synchronized with leakage. However, due to the complex karstic nature of the subsurface, confirming the exact leakage path remains challenging. The findings of this study provided valuable insights to understand the leakage mechanism and the importance of using groundwater monitoring, statistical correlation analysis, and hydrogeological assessments as a systematic approach to understanding the underground leakage mechanisms in the Samanalawewa Reservoir.