Hetch Hetchy Corral Drainage Improvements

Yosemite National Park » Hetch Hetchy Corral Drainage Improvements » Document List

To mitigate the runoff potential of contaminants from the NPS corral at Hetch Hetchy from reaching Hetch Hetchy Reservoir, the San Francisco Public Utility Commission plans to implement drainage modifications.
Components are: 1) There is an existing culvert, with a drop inlet at the upper end of the corral, which goes completely under the southern portion of the corral. The runoff from the road above the corral will be diverted to the ditch on the southeast edge of the road, bypassing the corral entirely. The remaining portion of the runoff that cannot be diverted to the southeast edge of the road will be diverted into the culvert's drop inlet and then pass under the corral.
2) The road is presently paved to just above the corral. The road above the corral is a native surface, and rolling dips and berms will be constructed there to divert the water. The rolling dips will be closely monitored during runoff to assure that they in fact are diverting the water away from the corral. If over time the rolling dips do not stay in place in the native soil surface of the road, additional pavement with permanent dips and berms will be added to approximately 100 feet of the road above the corral.
3) Runoff from a small area on the north side of the corral can presently enter the corral. This runoff can easily be diverted around the site by construction of two small berms that parallel the north fence line.
4) Once these diversions are accomplished, only the precipitation that falls directly on the surface of the corral will have the potential of contributing contaminated runoff. To mitigate this potential, a roof will be constructed over approximately 40 % of the corral, thereby reducing the amount of precipitation reaching the corral's surface. The materials used will match the existing. One pine would need to be removed to implement. A gutter with a downspout that will be connected to the culvert under the corral will be placed on the roof's lower side. After the roof is in place, only approximately 60% of the corral will be exposed to precipitation and runoff. The rolling dips and berms in the native surface of the road and the drainage berm along the north fence line will be constructed by Hetch Hetchy Water and Power's South Fork Maintenance crew in the late summer of 2007. The roof will be constructed by Hetch Hetchy Water and Power's carpenters in the winter of 2007/2008. The NPS will purchase the roof materials. If the paving is determined to be desirable, it will be accomplished during the summer of 2008. There are no cultural resources or archeological concerns associated with this project. The park forester comments, "The small gray pine (Pinus sabiniana) tree within the corral is not remarkable, as there are many similar trees in the area, and along foothills from northern to southern California. This specimen does not have severe outwardly visible defects at this time, but the branch architecture suggests future problems from large branch failures, and the heavy, spiky nature of the cones probably makes it incompatible with the existing land use of small corral. The tree can be removed without significant effect to the environment."