HomeMy WebLinkAboutCT 98-23; La Costa Greens; Tentative Map (CT) (18)CROSBY
MEAD
BENTON
& ASSOCIATES
April 23,1999
Job No. 421-001
Engineers • Planners • Surveyors
LA COSTA GREENS ~ CT. 98-23
PRELIMINARY DRAINAGE EVALUATION OF
PROPOSED DEVELOPMENT AND SAN MARCOS CREEK
Project Description
La Costa Greens is a proposed 5 lot single family subdivision with private street access from the
north side of La Costa Avenue approximately 0.60 miles east of El Camino Real. The site is
made up of 2 existing legal lots; lots 9 and 10 of La Costa Greens Map No. 6708. The boundary
is a rectangle 200 feet wide by 260 feet deep and is bordered by La Costa Avenue on the south
with San Marcos Creek and La Costa Golf Course to the north. An existing apartment site is
adjacent to the east with vacant land on the west side. Import material is proposed for the
project to facilitate access down from La Costa Avenue and will result in elevating the site a
minimum of approximately 2 ¥2 feet above the 100-year flood level.
Existing Condition
The existing topography from south to north can generally be described as sloping away from
La Costa Avenue (average elevation of 45 feet) down an existing 2:1 ± slope to the flatter useable
portion of the site at an average elevation of approximately 20 feet. Continuing north to
approximately 50 feet from the north boundary line, there is a 4:1 ± slope down to an existing
concrete drainage ditch and then continuing north into San Marcos Creek near the north
boundary. Flows from the existing concrete ditch run westerly parallel to the creek on the south
side to pick up local drainage for this site and properties along the north side of La Costa
Avenue to the east. Site runoff currently sheet flows northerly across the site into the concrete
ditch or directly into San Marcos Creek. There is no other drainage coming onto the site. The
total project runoff is from onsite area only. The existing limits of inundation from San Marcos
Creek during a 100-year storm would inundate approximately one third of the site in the
northwest portion of the property.
Proposed Condition
The buildable portion of the site after completion of grading will be a split level pad with a
manufactured 2:1 slope at the north end as an extension of the existing slope that comes up
from the south side of the existing concrete drainage ditch. The upper pad area in the south
half will be occupied by portions of the buildings and the private cul-de-sac driveway
providing access to the 5 lots. The lowest pad elevation will be 20 feet ± with the lowest floor
elevation at 20.5 feet ±.
5650 El Camino Real, Suite 200 • Carlsbad, California 92008 • 760/438-1210 • Fax 760/438-2765
CROSBY
MEAD
BENTON
& ASSOCIATES
Summary and Conclusion
A Hec. II drainage analysis for this reach of San Marcos Creek has been prepared by Rick
Engineering (see attached). The report addresses downstream drainage issues, as well as,
certain improvements to the golf course adjacent to this project. Cross-section No. 6 traverses
this site and therefore provides an accurate water surface elevation (W.S.EL.) for site design
purposes. The 100-year W.S.EL. for cross-section No. 6 is 17.54 feet with a velocity of 2.17 fps.
for the left bank and 2.56 fps. for the right bank per page 12 of the referenced report. Generally
erosive velocities are above 7 fps. Therefore, no slope protection for the proposed pad will be
needed. The horizontal change in the limits of inundation as a result of the proposed grading
would be an approximate 40 foot shift to the north. During a 100-year storm, runoff would
inundate much of the golf course with limits of inundation being approximately 1100 feet wide.
Therefore, this relatively small reduction in width of the overall flood plain would not result in
any appreciable increase in the water surface elevation.
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