Townwide Drainage Study Update 6-1 SJB037500
April 2015 \\sjs5\project\SJB037500\General\doc\Drainage_Report\2015 Drainage report-revision\2015-04t drainage report.docx TABLE 6-2
TOWN OF ATHERTON DRAINAGE STUDY
BACKGROUND INFORMATION FOR RECOMMENDED PROJECTS
AREA LOCATION PROBLEMS SOLUTIONS
Tier 1
H – SF Waterline/Reservoir
Channel Reline & Erosion Control
Atherton Channel north of I-280 between
San Francisco Water Line and Reservoir
Road.
The major problem is channel erosion. The channel bottom is about a foot lower than the existing channel lining and the side slopes have eroded nearby to the trees. Standing water can cause mosquito breeding problems. Picture was taken during a very dry week. The bottom of the channel has eroded below original elevation .
2001 Photo 2013 Photo 2013 Photo
Re-grade channel bottom to eliminate ponding and repair eroded channel side slope/embankment.
Q – Alameda de las Pulgas/Camino
Al Lago Upgrade Existing Channel
Areas adjacent to Atherton Channel from
Alameda de las Pulgas to Camino Al
Lago.
Existing metal arch box culvert crossing the Las Lomitas School and channel is under capacity for 10-year event.
The school is located in the major flow path.
Because of flow overtopping Alameda de las Pulgas in December of 2012, school classrooms were flooded. Location of overtopping is at the low point of the road. Increases in runoff flow rates due to the change from pervious to impervious surfaces have not been mitigated.
Reduction of demand will be difficult to attain due to the contributing surface area upstream of the existing Alameda de las Pulgas box culvert. Watershed areas are 74 percent off-site of Town limit and 26 percent within the Town limit.
The school is lower than the road, see photo below
Upgrade the existing metal arch box culvert and the channel to increase capacity.
Alternative to upsizing existing box culvert and channel is to install a 3 foot wall along south side of the property and force the higher flows to be diverted through the parking lot and into the road to Camino Al Lago. To reduce the amount of runoff on the diverted flow path, install a bypass inlet and pipe system along the parking lot and downstream channel at Camino Al Lago.
The bypass piping system can be combined with regional detention basin for 10-year flood protection.
This alternative leaves the existing culvert in place. Recommend inspection of existing pipe to determine the stability of the system.
II1 – Reline Channel Slopes,
Phase 1
Atherton Channel The channel side lining has cracks and holes, see 2013 photo. Restore the sides of the channel. Assume total channel length of 1000 lineal feel is in need of repair.
Channel to be repaired excludes sections where other channel improvement projects are identified.
Expand the life expectancy of the channel.
This project would be phase 1 of two projects. Phase 1 would restore channel areas most needing improvement upstream of Alameda de las Pulgas.
To identify specific locations most needing improvements, a condition assessment of the existing channel is recommended prior to design of channel improvements.
Townwide Drainage Study Update 6-2 SJB037500
April 2015 \\sjs5\project\SJB037500\General\doc\Drainage_Report\2015 Drainage report-revision\2015-04t drainage report.docx TABLE 6-2
TOWN OF ATHERTON DRAINAGE STUDY
BACKGROUND INFORMATION FOR RECOMMENDED PROJECTS
AREA LOCATION PROBLEMS SOLUTIONS
JJ1 – Reline Channel Bottom,
Phase 1
Atherton Channel The channel bottom lining has cracks and holes, see 2013 photos. Life expectancy of the channel has been exceeded.
Regrade the bottom of the channel. Assume total channel length of 1000 lineal feel is in need of repair.
Channel to be repaired excludes sections where other channel improvement projects are identified.
This project would be phase 1 of two projects. Phase 1 would restore areas most needing improvement upstream of Alameda de las Pulgas. To identify specific locations most needing improvements, a condition assessment of the existing channel is recommended prior to design of channel improvements.
Tier 2
II2 – Reline Channel Slopes,
Phase 2
Atherton Channel The channel side lining has minor cracks and holes Restore the sides of the channel. Assume total channel length of 1000 lineal feel is in need of repair.
Channel to be repaired excludes sections where other channel improvement projects are identified.
Expand the life expectancy of the channel.
This project would be phase 2 of two projects and would address areas from Alameda de las Pulgas to El Camino Real.
To identify specific locations most needing improvements, a condition assessment of the existing channel is recommended prior to design of channel improvements.
JJ2 – Reline Channel Bottom,
Phase 2
Atherton Channel The channel bottom lining has cracks and holes Exceeded the life expectancy of the channel.
Regrade the bottom of the channel. Assume total channel length of 1000 lineal feel is in need of repair.
Channel to be repaired excludes sections where other channel improvement projects are identified.
This project would be phase 2 of two projects and would address areas from Alameda de las Pulgas to El Camino Real.
To identify specific locations most needing improvements, a condition assessment of the existing channel is recommended prior to design of channel improvements.
U1 – Barry/Elena Upgrade Channel Atherton Channel from Barry Lane to Isabella Avenue
The project was divided into three phases. The first phase would address areas identified as the worst channel condition.
The major problem is overtopping of the channel and flooding basements and properties. The Atherton Channel from upstream of Barry Lane and Elena to Isabella Avenue is undersized for higher flows.
Channel is deteriorating.
This is the first part of the three phased improvement project. Inspect the condition of the channel and determine areas that need significant repair.
Maintain the capacity of the channel by installing a U-shape concrete channel.
E2 – Mandarin Storm Drainage
System
Areas adjacent to Mandarin Way starting
from cul-de-sac to Alameda de las Pulgas
and upstream tributary flow paths that
contribute flows to this area.
The major problem is a large amount of runoff flows through the back property. Flooding on property occurs because the property is a flow path and a low point. Increases in runoff flow rates due to the change from pervious to impervious surfaces have not been mitigated.
There is no release point for this area, such as a storm drainage system, to carry large amounts of flow. Create a release point by installing a storm drainage system along Mandarin Way and connecting to an existing storm drainage system along Alameda de las Pulgas.
Combined storm system with localized detention system.