A common problem for dams is the loss of storage due to sediment deposition. Historically sediment issues have been addressed by utilizing low level outlets to flush sediment downstream or by direct removal by dredging the material. Unfortunately dam infrastructure is frequently overwhelmed by vast sediment accumulations and low-level gates are plugged or otherwise unable to be operated for sediment removal, which in turn can prevent dam owners from diverting needed flows as sediment blocks diversion intakes. For smaller dams, modification of the structure may allow better handling of large sediment influx and maintain functioning diversions. This is achieved by notching the spillway and installing a bottom hinged or bladder gate that can be lowered to move sediment downstream more efficiently. The size of the notch is typically sized to match the upstream bankfull width and the ponded man-made reservoirs can eventually revert to pre-altered riparian form. One example is being pursued for the South Fork and North Fork Long Canyon Dams, owned and operated by Placer County Water Agency. The two dams were constructed in the mid-1960s and divert flow into the Middle Fork Tunnel used for hydropower generation. Sediment movement in these systems is episodic, but once the incipient motion threshold is exceeded the reservoirs have been known to fill up in a single storm. The low-level outlets were undersized and are not operable. A design effort is underway that will cut a 25-foot wide notch in both dams and construct a headpond in each on the downstream side of the dams, controlled by a weir and Coanda Screen for water water diversion. A bottom hinged maintenance gate is installed at the downstream end of each headpond that can be lowered to move sediment downstream and will allow the upstream channel to re-form and train itself into the notch. While not a complete restoration, the process balances the continued need for water diversions with the ecological needs of the system, including reconnecting the channel to the floodplain, reestablishing sediment connectivity downstream, and allowing downstream fish passage. This presentation will step through the project objectives, constraints, analysis, and proposed design.
