Effect of Snow Covered Area and Delayed Snowmelt on Water Quality and Reservoir Management: 2010 Turbidity Event in Hetch Hetchy Reservoir

The hydrodynamic processes in reservoirs located in snowmelt-dominated watersheds are poorly understood during snowmelt runoff. Very large storm events or unusually large spring inflows can threaten the operational and regulatory water quality goals. Turbidity levels in Hetch Hetchy are typically between 0.1 and 0.3 NTU, but storm events or unusual spring inflow events can produce inflow turbidity of over 15 NTU. This study collected turbidity and temperature data over a two-week period that included the peak inflow during an unusually large inflow event that carried a significant volume of sediment and organic debris-laden water, causing localized reservoir turbidities in excess of 10 NTU. The field data provided the information needed for operational responses and averted exceeding turbidity thresholds. An operator can adjust the elevation of the inlet used to release excess water as well as adjust the inlet elevation of water supplied to the customers. The operator can also manage reservoir storage to maintain a storage volume to dilute turbid inflow, and the timing of the release of excess water can be adjusted to control the location of mixing and control the approach of plumes of turbid water toward the water supply inlet. By combining inflow forecasts with real-time water quality data collection and dam operations, most snowmelt inflow turbidity events can be managed so that interruptions in supply due to water quality issues can be avoided.