Sediment Management & Blockage

  

Davis Hydro is concerned about the sediment build up and probable eventual blockage of the flow through the conduit at the Rock Creek Dam, and most likely the same effects at Cresta and similar dams at Pit 4. The problem has two elements:

 

  • Sediment build-up in front of the dams, and the resulting problem of
  • blockage in the area of the 30 conduit.

 

Sediment builds up in front of the dams due to lack of adequate turbulence to carry the sediment over the dam. The concern about blockage is more complicated and needs explanation.  Most debris transported in typical rivers does not float on the surface, but is either in the water column or bounces along the bottom. Large debris in particular is carried along the bottom, bouncing on it. This is because most of the large debris is negatively to barely neutrally buoyant. The effect of this is that if a hydropower trash rack intake is located near the bottom it is very likely to get plugged up with large and medium size debris. This will occur most often during high water events when the bottom of most rivers is a marching forest of woody debris from sticks, knots, and stumps to almost deflated tires.

 

This is now a serious concern at Rock Creek and possibly elsewhere because if the bottom comes up to the 30 pipe inlet it will differentially attract debris especially during high flow events - and will plug any trash rack near the bottom.

 

The sediment build up problem was identified as significant in the earliest discussions with PG&E staff (letter to Tom Jereb 2/18/04), and was discussed during our first consultation meeting (meeting notes 2/25/04). The sedimentation built-up report was requested from PG&E at that time, and a dialog was started with Scott Tu of PG&Es geotechnical. See note attached, extracted from e-mail and conversations on potential solutions. Scott recognizes this as a serious problem.

 

The second problem is that the 30" pipe channels water through the same trash rack as water for the drum gates. Discussions with Scott and Bill Zemke on the inadadvisability of using 30 pipe for keeping area clear. The effect of continuous use will be to plug up the trash rack thereby limiting it's use for filling drum gates. This is a design flaw, and will not be easily detectable using existing pressure drop measures until it is too late. A recommended solution is to abandon the 30" as a continuous discharge device because it is guarenteed to plug up, and learn to use the control the release at the spillway or other release mechanisms. Detailed methods for controlling radial gate releases were engineered, and discussed with Cardone, Bolger, and Zemke. See RC Technical Report #3.

 

5' gates to the drum gates must be kept clear using other means, notably by lower gate releases. This is being discussed with Scott and agencies. Informal discussions with Canaday and CFG suggest that this could be a useful avenue to explore for stream gravel recruitment.

 

Summer 2004

 

In Spring 2004 the sedimentation report was made available. The report shows encroachment of sediment up to the 30 pipe, and worse, the 30 pipe is now in a basin - much like a storm drain catchment basin just in front of the dam. This generated a formal letter to Scott, with copies distributed through Bill Zemke and others interested at the ERC meetings. The May 10, 2004 letter had the Draft of a Technical Report that addresses potential solutions. This is a serious problem. In the summer of 2004 Dr. Ely, while making other measurements as a guest of PG&E, noticed heavy gravel being carried through the 30 conduit. This was discussed with Mr. Cardone, and Mr. Tu. Mr. Tu and Mr. Ely agree that this was probably an artifact of rolling horizontal vortex stirred up by the recreation releases going on that day of observation. The implications for future blockage are profound and have been discussed with Tu.

Fall 2004

 

Davis Hydro continues to be concerned about the bottom build up in front of the dam. Updated (9/1/04) Technical Paper RC-2 is released focusing on environmental benefits of gravel passthrough. It contains recommended solution paths for sediment buildup. Mr. Tu does not think they are possible as the gates have been formally abandoned. Discussions with Cordone indicate that the 30 inch bypass is not shut for modest debris events. One solution drawing is at: RC-T-2. The air jet and slow sediment leak solutions have no diagrams. Syphon suction bypass dicussed with Cardone - has maintenance problem, but safe.

 

Summer 2005

 

Davis Hydro has informally cored in the reservoir. Heavy gravel found making gravity coring difficult. Cores discussed briefly with Tu. No new information. No one has heard gravel moving through pipe. Winter flush is seen as "solution" by PG&E.

 

Summer 2006

 

Davis Hydro was guest on PG&E bathymetric survey in RC reservoir. Survey equipment is not capable of defining bottom topography in pit area due to its design. This will to lead to lack of information to PG&E geotechnical people. Specifically, the equipment is designed to only the first acoustic returns. The problem with this is that deep holes will not show up as they are disguised by returns from nearer sediments and dam face artifacts.