South Fork Fire BAER Plan
The South Fork Fire burned in the Wawona area beginning on August 13, 2017. A Burned Area Emergency Response Team investigated the fire impacts to watershed processes such as runoff, sediment transport, and debris flows then assessed potential impacts to identified resources at risk due to these processes.
The purpose of this project is to implement post-fire treatments to protect resources at risk.
These projects are:
1) Sandbags around one NPS-owned house located at the base of a burned slope,
2) Monitoring water quality to assess whether fire retardant has entered the water ways and the public water supply,
3) Install warning signs on Chilnualna Road as it enters the area below burned slopes and on trailheads leading into the burned area,
4) Modify the water supply intake on the South Fork of the Merced River to be less vulnerable to floating debris and allow for emergency withdrawal of water at higher water levels should the intake reservoir fill with sediment during winter storms
5) Remove debris and sediment from the water intake impoundment should the intake be compromised,
6) Monitor and control for invasive plants, and
7) Monitor and assess potential post-fire impacts to cultural resources.
Specific to the public water supply, this project proposes one-time modifications to several components of the water inlet system so that the system can more likely survive and recover from floods/floods carrying debris and debris flows associated with the post-fire watershed. The intake is located adjacent to a small diversion dam in the South Fork of the Merced River. The inlet is at risk because of the potential for high flood flows, flood flows carrying debris, or debris flows. This project will make the screened inlet more protected, protect the pipe from the screened inlet where it is exposed in the river, and provide for drawing the reservoir at a higher elevation if debris clogs the reservoir.
This action will achieve better protection of the screened inlet by extending it about 18-inches toward the north and lowering the inlet about 4 inches. This will put the inlet more behind two large boulders that were previously installed for inlet protection. The work involves removal of cobbles under the pipe, extension of the 2 inch compressed air line, and loosening the coupling. Chipping of the concrete anchors may be required.
A rock/concrete boring tool will be utilized to construct five holes in the east side of the sediment settling basin. The specific locations of the holes will be identified by NPS Wawona water utility staff. Hole locations should consider minimum and maximum water withdrawal elevations required and adequate distance between holes to minimize cracking during drilling. Five plugs shall be installed from inside the settlement basin. The outside east end of the holes shall be covered with a 1/8th inch stainless steel screen. The screen shall be mounted to the masonry wall with steel strap.
A new screen will be built and installed in the sediment settlement basin. The screen shall be removable. The screen will consist of a steel frame with additional vertical and horizontal members to support a stainless steel screen. The screen shall be constructed of stainless steel will small openings (e.g. 1/16th inch). Five to 7 one cubic-yard boulders will be placed to protect the inlet pipe. The large boulders shall be lowered by cables or other means and not allowed to tumble down the rock slope.