HomeMy WebLinkAbout3338B; AGUA HEDIONDA & CALAVERA CREEK DREDGING; IMPACT ANALYSIS REPORT; 2008-11-01IMPACT ANALYSIS REPORT
AGUA HEDIONDA CHANNEL
EMERGENCY DREDGE PROJECT
November, 2008
ca No.41204
EXP. 3. 1.0n ,
Agua Hedionda Channel
Emergency Dredging Project
Impact Analysis
Background
Agua Hedionda Creek Channel through the Rancho Carlsbad community has experienced
significant deposition since it was constructed in the late 1970's. Storms in 2005 created
flows that crested less then a foot below the soffit of the El Camino Real bridge. El
Camino Real is a regionally significant arterial roadway carrying over 50,000 vehicles per
day and containing numerous major utility lines. Therefore on March 2, 2006, the City of
Carlsbad City Manager proclaimed the existence of a local emergency. This declaration
was ratified by the City Council on March 7, 2006.
City staff obtained emergency permits from the resource agencies, based on the
estimated impacts of the proposed emergency dredging. These estimates were based on
biological mapping already performed for the project and the preliminary emergency
dredging plans, since detailed dredging plans were not yet completed. Based on
preliminary plans, the impacts were estimated at 0.8 acres of vegetated waters of the U.S.
and 4.2 acres of unvegetated waters of the U.S./State. Construction work was
accomplished in March, 2006.
During construction, the contractor was directed in such a way as to limit impacts where
possible while providing the benefit of dredging. Original plans were to trim back the
channel banks. In the interest of minimizing impacts and reducing erosion, the actual
dredging operations were limited to the existing toes of the channel side slopes.
Impact Measurement Methodology
On June 28, 2006, GVP Consultants performed field measurement of the emergency
dredging operations: For the area between the El Camino Real and Cannon Road.
bridges, they established a base line and 7 transects. At each of these transects they
measured the limits of disturbance to the neareèt foot with a 100-foot long surveying tape.
The plotted impact areas were measured by a Dietzgen planimeter on a 40-scale working
drawing (see Exhibit 1). Three measurements were made, with a maximum variation of
0.5% from the average. The average measurement was then mathematically converted
from square inches to acres.
For the impacts to non-vegetated waters, the crew walked the entire channel. The
dredging work in the non-vegetated portion of the channel was fairly uniform, since it had
been from the bottom of the existing slopes. The location of the existing slopes was
plotted from topographic mapping generated by photogrammetric methods, based on
aerial data gathered on May 20, 2006 by Towill Surveying Company, in conformance with
National Map Accuracy Standards. The plotting from the as-built topography mapping was
field-verified with spot checks in multiple areas with a 100-foot long surveying tape. The
plotted impact areas were measured by a Dietzgen planimeter on a 100-scale working
drawing (see Exhibit 2). Three measurements were made, with a maximum variation of
1.7% from the average. The average measurement was then mathematically converted
from square inches to acres. -
Agua Hedionda Channel
Emergency Dredging Project
Impact Analysis
Summary of Impacts
Based on the field measurements, the impacts from the emergency dredging was 0.50
acres to vegetated wetlands, and 3.06 acres to non-vegetated waters of the U.S/State.
Certification
This report has been prepared by me or under my direction, and I have exercised
responsible charge of the gathering of the information and the engineering measurements
and calculations, in accordance with Section 6703 of the Business and Professions Code.
Glen K. Van Peski, P.E.
RCE41204 exp. 3/31/09
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