HomeMy WebLinkAbout02/18/1997, 2 - CONEJO AREA STORM DRAINAGE, SPECIFICATION NO. 97-04 council. z D40-97
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C I TY OF S AN L U IS O B I S P O
FROM: Michael D. McCluskey,Public Works Directo-�/
Prepared by: Barbara Lynch, Civil Engineer`�
SUBJECT: Conejo Area Storm Drainage, Specification No. 97-04
CAO RECOMMENDATION:
Approve concept plans to modify the rock catcher and culvert entrance. CAO to authorize advertising
for bids when necessary permits have been acquired. Authorize staff to proceed with the modifications
to the rock catcher only by July of 1997 if authorization from the property owner of the culvert
entrance can not be obtained to complete the entrance modifications.
Direct staff to cease further efforts to design and construct an overflow protection wall.
DISCUSSION:
In June of 1996, Council heard public comment regarding drainage in the area above Conejo Street
(see Vicinity Map). During the 1995 storm, the rock catcher above Conejo filled with rocks and the
entrance to the culvert behind the homes on Conejo filled with debris. As a result, mud, rocks and
water came through various properties along Conejo Ave, onto Conejo as well as out to Cazadero St.
(see Overflow Map).
During the budget process, $30,000 was identified to make some minor improvements to the existing
drainage system to lessen the likelihood of such flooding. The budget was based on the concept of
expanding the rock catcher on the creek and improving the entrance to the culvert pipe. This would
increase the rock catcher capacity to retain debris, and increase the amount of water able to enter the
culvert pipe. During the same meeting, Council also gave general direction for staff to study the option
of building a two foot wall with a paved overflow along the rear property line of the homes on Conejo
to divert flood water away from the homes and to the end of the block at Cazadero should the advert
plug or stone flow exceed the culvert's capacity.
Improvements per budget:
Staff has completed preliminary plans for the modification to the rock catcher and the culvert entrance.
This project is not ready to advertise because the plans have not been through the permit processes
which are required for work in the creek. Staff will need to obtain environmental, Army Corps, Fish
and Game and Water Quality clearances prior to bidding.
In addition to permits, the City must obtain right-of-entry and construction authorization from a
property owner(1744 Alisal) because the culvert entrance is not in the City's drainage easement. To
date, permission has not been granted by the property owner to do work on that property. The
property owner has expressed concerns as to the appearance of the new entrance, a twenty foot (6
meter)long, concrete structure, as well as to its effectiveness when large quantities of rock come down
the channel. The property owners of the culvert entrance have indicated that the two times they have
seen the Conejo properties flood, it has been because the culvert entrance plugged, not because the
pipe was unable to handle the flow in the channel. If the rock catcher fills up, the rock and debris will
come down to the new entrance and could plug it. Additional rock catching capabilities will reduce the
likelihood of this happening but, if storm crews are unable to reach the catcher in time,the potential for
flooding will still exist. The culvert entrance is not accessible during flooding. Once it becomes
plugged, crews are unable to reach it with equipment until the flows stop.
These improvements will maximize the carrying capabilities of the culvert, but will not upgrade the
system to the 10 year design storm that the City now uses as a design standard. The current capacity
of the system is about 329/6 of the design standard. The improvement will increase the capacity to
about 65%of the full design standard which is also about the capacity of the natural channel.
Two foot wall/overflow study results:
Staff has approached this portion of the work using the conditions experienced during the 1995
flooding as a guideline. The channel was modeled assuming the existing rock catcher was full and the
culvert entrance plugged. The study indicates that a two foot wall would just handle a similar flow,
diverting water out onto Cazadero. Based on the conditions existing during the '95 flooding, these
diverted flows would most likely result in flooding, similar to what occurred on Conejo, to residents on
Cazadero. Homes on the southerly side of Cazadero are below the street elevation. During the storm
which flooded Conejo, water was also running out onto Cazadero from behind the homes on Conejo,
and splashing over the sidewalk in front of these homes. This was occurring even with the bulk of the
flow moving onto Conejo.
Staff feels the construction of such a wall would shift the problem elsewhere. For this reason, staff
does not recommend this option. The City would be causing damage to homes where there has not
traditionally been damage. There is also the potential for the constructed wall to fail during a major
storm and result in damage to homes on Conejo.
The installation of a wall will require the relocation of existing water and sewer connections so that
they are accessible for repairs. The water and sewer are currently in the same area as the drainage
easement.
FISCAL EWPACT:
Wall Project: Budgeted Project:
Engineer's Estimate: Engineer's Estimate:
Wall/Overflow $ 115,000 Catcher/Entrance $20,000
Water service connections $6,000 Contingencies: $ 5,000
Sewer service connections $15,000 Total: $25,000
Contingencies: $ 14,000
Total: $1509000
The recommended project is fully budgeted and requires no additional funding. The studied wall
project is not budgeted, or hilly designed and would require a supplement appropriation of$150,000.
ALTERNATIVES:
1) The City could leave the existing system as is. The channel was designed around 1940 to the
standards in effect at the time. It is unlikely this would satisfy local property owners. There would
be no direct cost for construction of improvements.
2) Build a large storm drain bypass which would pick up the bulk of water, rock and mud near the
rock catcher and carry it to San Luis creek via a pipe in Andrews street. Sufficient water could be
directed into the natural channel to maintain any existing habitat. The preliminary estimates to
construct this project are$250,000.
ATTACffi1M14T:
Vicinity Map
Overflow Map
Plans are available in the Council Office
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