October 2009 - South Tahoe Public Utility District

9.5.1 Project Description
Improvements to the Diamond Ditch System proposed in this project will result in increasing the
capacity of the system to transport higher flow rates of recycled water and in stabilizing
segments of the system that are subject to erosion and/or flooding. The increase in system
capacity will allow the District to manage the potential increases in the volume of recycled water
resulting from future flow from the Tahoe Basin. The improvements will permit the District to
provide uninterrupted flows of recycled water while protecting the environment.
It is recommended that the control box overflow at Bar Screen No. 5 be modified if the District
does not implement Project No. 6 (Page 9-70), and the Waterfall Forebay and Pipeline. If
Project No. 6 is not implemented and Bar Screen No. 5 is blocked by debris, recycled water
spillage can potentially overtop a small downstream containment berm and enter the nearby
West Fork of the Carson River. To improve the Diamond Ditch flow rate and to reduce
operational constraints, either of the following is recommended: (1) provide improvements at
Bar Screen No. 5 area, or (2) construct a new bar screen at a location above the Diamond Ditch
Waterfall with a flow-limiting weir and an overflow side outlet that discharges to Wade Valley.
Neither of the recommendations would be necessary if Project No. 6 is constructed.
Modifying the Bar Screen No. 3 flow control structure will also result in increased system
capacity and alleviate the flooding and erosion problems associated with this structure.
The existing recycled water 20-inch diameter steel pipeline crossing the Paynesville Bridge is
not double contained. This project would install a 30-inch diameter casing pipe for the carrier
pipe to reduce the possibility of spillage.
The Diamond Ditch crosses the West Fork of the Carson River a total of six times: there are four
above-ground crossings (including the Paynesville Bridge), and two underground invertedsiphon
type crossings. The District should evaluate all six crossings to determine the need for
installing casing pipe or implementing other upgrades.
The existing Diamond Ditch has eroded downstream of Bar Screen No. 3. Erosion control in the
form of channel riprap lining should be installed. The planning level cost (see Table 8.1, page
8-59) is based on approximately 450 feet for the eroded portion upstream of Bar Screen No. 3
and approximately 300 feet of the eroded section downstream from Bar Screen No. 3.
9.5.2 Project Considerations
The two choke points discussed in Section 4.1.1.2 generally limit the conveyance capacity of
the Diamond Ditch to an estimated 20 cfs. These choke points are at Bar Screen No. 3 and at
Bar Screen No. 5. The Diamond Ditch agreement requires the District to deliver an
instantaneous recycled water flow rate of 25 cfs. This project intends to fix these choke points.
The aspect of providing improvements at Bar Screen No. 5 of this project is not needed with the
implementation of Project No. 6 below. Implementation of Project No. 6 would alleviate
potential operational constraints at the Bar Screen No. 5 area. Otherwise, if the District does
not implement Project No. 6, the District could reduce potential operational restrictions by
providing the improvements at Bar Screen No. 5.
South Tahoe Public Utility District
Recycled Water Facilities Master Plan - October 2009 Page 9-69