Wednesday, May 4, 2011

High flow releases from O'Shaughnessy Dam

This has been a very wet winter with recent snow survey results indicating the snowpack in the Upper Tuolumne watershed at 184% of average May 1st snow conditions.  Currently, the snow line is near 5600 feet and has melted on south and west slopes up to approximately 6400 ft; above 6400 feet the snow coverage is 100%.  Warm temperatures over the last week have been melting this low elevation snow and generating streamflows in the watershed. Over all, SFPUC hydrologists believe over 500,000 acre-feet of excess runoff ("spill") is likely to occur over the next 2-3 months at O'Shaughnessy Dam.

As discussed at the April 8 stakeholder meeting, draft UTREP flow recommendations for O'Shaughnessy Dam include high flow releases during the snowmelt season to meet sediment transport and Poopenaut Valley wetland inundation objectives.  This year, the SFPUC and NPS will take those recommendations for a "test drive" to help identify any potential operational issues.  Beginning May 15th, operators at O'Shaughnessy Dam will begin high flow releases, starting with test wetland inundation "benches" at 3300, 3700, 4800, and 5700 cfs.

High flow release from O'Shaughnessy Dam in 2009. (W. Sears photo)

These releases will be monitored by equipment in the Poopenaut Valley and NPS staff will take photos to confirm inundation of wetland surfaces.  A peak release for sediment transport of 8500 cfs is targeted for May 19th and 20th, followed by a 5700 cfs bench or a continued 8500 cfs release if it is necessary to control Hetch Hetchy reservoir elevations. Although variable weather conditions could require changes to this plan, the "test drive" will provide us with a valuable opportunity to evaluate the operational procedures needed to implement a new set of flow recommendations at O'Shaughnessy Dam.