Preliminary 2017 Forensic Hydrogeologic Evaluation of Elevated Senor Readings Uvas Dam No. 72-12 Santa Clara County

Historic seepage issues and high phreatic excursions have existed since construction of Uvas Dam in 1950. The dam is an earthfill embankment located in San Jose, California. A grouting program in 1995-1996 in the right abutment at Uvas Dam was performed to target and mitigate historic seepage in the right downstream portion of the embankment. The District periodically evaluated the effectiveness of the 1995-1996 grouting program with internal reports in 2000, 2007, and, 2011. These evaluations indicate that during the last ~20 years (1996 to 2016) the grouting program appears to have satisfactorily attenuated the historically high phreatic response in the right ~one-third of Uvas Dam (including the right downstream groin), as evident by lowered piezometric elevation surfaces and weir discharges. New findings from analysis of 2016-2017 piezometer and seepage data is that, even with continued apparent favorable lowering of the phreatic response as a result of the 1995-1996 grouting, there are periods since grouting, such as during the 1996-1999 “four-year” event, and the more recent 2016-2017 “dual-year” event, when sensors approach (or exceed) historic maximum excursions. Our analysis suggests that the two internal downstream embankment drains, known as the inclined Chimney and Blanket Drains, are ineffective and cause high phreatic surfaces to develop in the downstream section of the embankment. A proposed hydogeologic model for historic high phreatic response involves development of an elevated wetting front (“phreatic surface mounding”) in the two internal drains. At some point, the mounded phreatic surface reaches sufficiently high hydraulic head to cause an elevated wetting front to “short circuit” (i.e. develop unexpected seepage paths), along at least two preferred paths that exit the dam face due to relatively short exit distances from the internal downstream face of the drains to the external downstream dam face. The “short circuit path” allows the mounded phreatic surface to intersect the downstream face of Uvas Dam to create historically observed moist to wet seepage “patches” on the dam face.