Seite - 1493 - in Book of Full Papers - Symposium Hydro Engineering

Accordingly, a research study was undertaken to develop a high-resolution data set from both field and laboratory settings on the erodibility of 3D (non-cubic) rock blocks [15]. This paper presents first-of-a-kind observations made in real-time leading up to the removal of an instrumented rock block in a prototype unlined spillway channel at a dam site in Northern California. 2. SITE DESCRIPTION AND INSTRUMENTATION Collection of real-time data on 3D rock block erodibility from a prototype setting has several challenges related to selection of a site that is readily accessible, that spills regularly and that has erosion actively occurring. Additionally, instrumentation would need to withstand variable climatic conditions as well as hydraulics loads associated with spill events to allow capture of data. The basic concept for the prototype set-up was to re-create rock blocks within a spillway channel from locations in the rock mass where blocks once existed but had subsequently been eroded. To do so, two instrumented artificial rock blocks were installed in an unlined rock spillway channel at a dam site in Northern California. The dam is located in the Sierra Mountain Range and founded on hard, blocky granodiorite rock. The spillway consists of a concrete ogee control section that discharges onto rock just downstream (Fig. 1). Ten radial gates are present, three of which are larger and have an invert at a slightly lower elevation. The capacity of the spillway with all gates open is 1,161 m3/s and the maximum flow experienced to date was 968 m3/s in January 1997. Typically, the spillway discharges on an annual basis in the winter and spring months associated with rain on snow events or runoff from snowmelt in the Sierra. Scour of the granodiorite rock within the spillway has generally be observed throughout the almost 100-year lifespan of the structure, with the first documented instance in the early 1940’s. Based on aerial photography, approximately 185,000 m3 (the equivalent of 75 Olympic swimming pools) of material has been eroded from the spillway channel (Fig. 2). Despite several large flows through the spillway, catastrophic erosion has not been witnessed, but rather scour appears to occur more gradually. Given the propensity for regular spill events and historic erosion of the spillway, the site provided an excellent opportunity to collect real-time scour data from a prototype setting. As mentioned above, two artificial, instrumented blocks were installed within the spillway channel. The blocks were cast into existing block molds, which are locations were a rock block once existed but has subsequently been eroded (Fig. 3). Block 1 was installed below the upper (small) spillway gates, while Block 2 was installed below the lower (large) spillway gates. 1493