HomeMy WebLinkAboutCT 03-09A; Ocean Bluff; Tentative Map (CT) (27)Investigative Science and Engineering
January 26, 1998
Brian Milich
Catellus Residential Group
5 Park Plaza, Suite 400
Irvine, California 92614
Re: Ocean Bluff, Catellus Site - Acoustical Plan Check (ISE Report #98-001)
Dear Mr. Milich,
At the request of Jack Hentorn & Associates, Investigative Science and
Engineering (ISE) was directed to examine past acoustical work and address concerns
expressed by the City of Carlsbad regarding the effectiveness of proposed acoustical
mitigation of Lots # 78 through 87 for the Ocean Bluff residential development. The
findings regarding the effectiveness of the proposed mitigation design are presented in this
letter report.
Introduction and Definitions
Project Site / Past Work Introduction
The project site is located on the northern side of the eastward extension of
Poinsettia Lane between 'A' Street and Black Rail Road within the City of Carlsbad. A
previous acoustical report (Report on an Acoustical Study: Ocean Bluff, Report No. 9321,
James C. Berry, June 7, 1993) and amended (Addendum to the Report on an Acoustical
Study, Report No. 9321A James C. Berry, July 31, 1993) identifies acoustical mitigation
in the form of a noise attenuating wall which shall be "...in the form of a solid barrier
ranging in height from 7 feet at lot 87 down to 4 feet at lot 78". The proposed design was
shown to mitigate traffic generated noise levels along Poinsettia Lane to compliance levels
adopted by the City.
In a letter from the City of Carlsbad Planning Department dated January 12, 1998,
concern was expressed regarding the effectiveness of the proposed mitigation design since
affected pad elevations had increased while roadway elevations had remained the same.
10478 Rancho Camel Drive • San Diego, CA 92128-3665
Phone: 613-592-7817 • Fax: 619-592-7817 • Cell: 619-987-5185 • Email: rtavares@flash.net
Mr. Brian Milich
Ocean Bluff, Catellus Site - Acoustical Plan Check
ISE Report #98-001
January 26, 1998
Page 2
The City formally requested that the proposed design be checked for compliance with City
acoustical standards.
Definitions
Noise is generally defined as unwanted or annoying sound that interferes with or
disrupts normal activities. Although exposure to high noise levels has been demonstrated
to cause hearing loss, the principal human response to environmental noise is annoyance.
The response of individuals to similar noise events is diverse and influenced by the type of
noise, the perceived importance of the noise and its appropriateness in the setting, the time
of day, the type of activity during which the noise occurs, and the sensitivity of the
individual hearing the sound.
Sound levels are usually measured and expressed in units of decibels (dB). Most
of the sounds we hear in the environment do not consist of a single frequency, but rather a
broad band of frequencies differing in sound level. A method commonly used to quantify
environmental sounds consists of determining all of the frequencies of a sound according
to a weighting system that reflects that human hearing is less sensitive at low and
extremely high frequencies than at the midrange frequencies. This is called "A" weighting,
and the decibel level measured is called the A-weighted sound level (or dBA). In practice,
the level of a noise source is conveniently measured using a sound level meter that
includes a filter corresponding to the dBA curve.
Although the A-weighted sound level may adequately indicate the level of
environmental noise at any instant in time, community noise levels vary continuously.
Most environmental noise includes a conglomeration of sounds from distant sources that
create a relatively steady background noise in which no particular source is identifiable. A
single descriptor called the Leq (equivalent sound level) is used. Leq is the energy-mean
A-weighted sound level during a measured time interval. It is the 'equivalent' constant
sound level that would have to be produced by a given source to equal the fluctuating
level measured.
Finally, the sound measure known as the Community Noise Equivalence Level (or
CNEL) is defined as the "A" weighted average sound level for a 24-hour day. It is
calculated by adding a 5 decibel penalty to sound levels in the evening (7:00 p.m. to 10:00
p.m.), and a 10 decibel penalty to sound levels in the night (10:00 p.m. to 7:00 a.m.) to
compensate for the increased sensitivity to noise during the quieter evening and nighttime
hours.
The City of Carlsbad's Noise Element of the General Plan has identified a
mitigation threshold level of 60 dBA CNEL for residential development. This standard is
Mr. Brian Milich
Ocean Bluff, Catellus Site - Acoustical Plan Check
ISE Report #98-001
January 26, 1998
Page 3
typically applied at usable exterior living areas adjacent to transportation noise sources
such as roadways, rail, and aircraft. This level (60 dBA CNEL) is the design impact
threshold for the Ocean Bluff residential development.
Analysis Methodology
The Fresnel Diffraction Method was used to calculate the acoustical reduction for
suspect lots adjacent to nearby Poinsettia Lane (the identified noise generator for the site).
The method incorporates knowledge of the local topography (distance and elevations) and
proposed barrier placement locations to calculate the amount of acoustical attenuation due
to the proposed design. Engineering data and assumptions applied in the previous
acoustical reports were used where applicable to maintain consistency.
Topographic data (Source: Tentative Map - Ocean Bluff, Hunsaker & Associates,
November 5, 1997) was obtained by taking a series of cross-sectional elevation cuts
perpendicular to the roadway alignment along the area of interest (i.e., between Lots # 78
through 87). This data formed the basis for input to the Fresnel model. Output consisted
of the amount of attenuation (in dBA) due to topographic considerations and the insertion
of the noise wall.
As a final step, traffic noise source levels taken from the original technical report
were added to the calculated attenuation (or Fresnel losses) to determine the final sound
level at the building pads under investigation. If the proposed design was found to yield
inadequate attenuation, the wall height was increased until compliance was achieved.
Findings
The previous acoustical report identifies the future traffic flow along Poinsettia
Lane as 24,800 ADT at a speed of 50 MPH. The report determines the sound level due to
this traffic flow as being 73 dBA CNEL at the edge of the road right-of-way.
The calculated theoretical maximum attenuation due to the combined effects of
topography and barrier insertion loss and the resulting sound level at the pad elevation is
shown below in Table 1.
Mr. Brian Milich
Ocean Bluff, Catellus Site - Acoustical Plan Check
ISE Report #98-001
January 26, 1998
Page 4
Table 1
Calculated Attenuation/Resultant Sound Levels from Proposed Design
Lot#
78
79
80
81
82
83
84
85"
86
87
Source to
Receptor
Distance
115.0
115.0
115.0
115.0
115.0
115.0
105.0
190.0
105.0
175.0
Source to
Barrier
Distance
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
170.0
Source
Height
363.0
362.5
361.5
361.0
359.0
357.0
354.5
352.0
352.0
344.2
Barrier
Height
370.0
370.0
370.0
368.0
367.0
363.0
360.3
358.0
358.0
357.3
Receptor
Height
360.1
359.9
358.9
357.9
356.8
355.8
354.9
353.4
355.0
355.3
Predicted
Fresnel
Loss
13.3
13.4
13.8
13.3
13.4
11.9
12.7
10.8
10.4
9.0
Spherical
Spreading
Loss
4.5
4.5
4.5
4.5
4.5
4.5
4.5
7.1
4.5
8.0
Resultant
Sound
Level
55.2
55.1
54.7
55.2
55.1
56.6
55.8
55.1
58.1
56.0
Notes:
*: Lot #85 contains an additional 3 dBA loss (per FHWA RD-77/108) due to structural attenuation
of Lot #86.
Column Definitions:
1. Parcel Lot No. = Lot number designated per Ocean Bluff Tentative Map (CT 93-09 A).
2. Source to Receptor Distance = Horizontal distance from roadway centerline to pad. Receptor
point setback taken 5 feet from lot edge.
3. Source to Barrier Distance = Horizontal distance from roadway centerline to adjacent barrier
placement location - 5 feet.
4. Source Height = Height of adjacent roadway point in feet MSL.
5. Barrier Height = Height of parcel slope (in feet MSL) adjacent to lot number + height of barrier as
identified in Ocean Bluff Tentative Map (CT 93-09 A).
6. Receptor Height = Height of lot number pad in feet MSL + 5 feet.
7. Predicted Fresnel Loss = Fresnel diffractive loss based upon above topography and wall placement
expressed in dBA. This value is calculated for an incoherent line source at 500 Hz.
8. Spherical Spreading Loss = 15 * LOGio (source to barrier distance / 50 foot reference distance) +
10 * LOGio (barrier to receptor distance / 50 foot reference distance) expressed in dBA. This is
the propagation rule applied in the previous acoustical reports.
9. Resultant Sound Level = 73.0- Column 7 - Column 8 expressed in dBA CNEL.
Mr. Brian Milich
Ocean Bluff, Catellus Site - Acoustical Plan Check
ISE Report #98-001
January 26, 1998
PageS
Conclusions
Based upon the above analysis it can be seen that the project is still in compliance
with the City's 60 dBA CNEL acoustical standards. No additional exterior mitigation for
this project is indicated.
Should you have any questions regarding the above conclusion, please do not
hesitate to contact me at (619) 592-7817.
Sincerely,
Rick Tavares, BIT, REA, INCH
Project Principal
Investigative Science and Engineering
cc: John Conley, ISE
Melissa Allen, Jack Henthorn & Assoc.