From:[email protected]
To:City Council
Cc:Jorge Garcia; Matt Downing; Cory Hanh; Michael Gruver Subject:McNeal Tract 3181 Sea Level Rise Letter Reinforcing 2021 Bluff Retreat Recommendations Date:Sunday, October 1, 2023 5:12:45 PM
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McNEAL TRACT 3181 SL12009-2 Beachcomber Drive Wave Runup Analysis 9-18-23.pdf External Email
Mayor and Councilmembers:
On September 21st we submitted the attached letter from our geologists addressing in detail how Sea Level Rise was included in their 2021 bluff retreat analysis for proposed Tract 3181 (McNeal). This was submitted in light of questions raised by the Coastal Commission staff when this project was presented to the Planning Commission in August, primarily to add clarification for the record.
Last Friday staff requested I submit this directly to you via the City's website portal.
If this raises any Qs I can address, please do not hesitate to contact me. Thank you.
David Watson, AICP
WATSON PLANNING CONSULTANTS
Post Office Box 385
Pismo Beach, California 93448-0385
Tel: 805.704.8728
Em: [email protected]
Website: www.watsonplanning.us
-------- Original Message --------
Subject: Re: McNeal Tract 3181 Sea Level Rise Letter Reinforcing 2021 Bluff Retreat Recommendations
From: Cory Hanh
Date: Fri, September 29, 2023 3:45 pm
To: "[email protected]"
Cc: Michael Gruver
Hi Dave --
Thank you for providing this analysis. To make this part of the publish correspondence, could you please email the analysis to the following email adress: [email protected]? This will ensure that the City Council receives it ahead of the meeting as well. Otherwise, we can certainly included it as part of the case file, but it would not be published as correspondence on the City website.
Have a great weekend!
Note: Please submit applications and plan check
correspondence to [email protected].
Cory HanhPlanning ManagerCommunity DevelopmentDepartment Office: (805) 773-7043Email: [email protected] Website I Facebook I Twitter I YouTube
From: [email protected]
Sent: Friday, September 22, 2023 10:03 AM
To: Michael Gruver
Cc: Fred McNeal ; Tom McNeal ;
Cory Hanh
Subject: RE: McNeal Tract 3181 Sea Level Rise Letter Reinforcing 2021 Bluff Retreat Recommendations
External Email
Perfect Mike, thank you.
David Watson, AICP
WATSON PLANNING CONSULTANTS
Post Office Box 385
Pismo Beach, California 93448-0385
Tel: 805.704.8728
Em: [email protected]
Website: www.watsonplanning.us
-------- Original Message --------
Subject: RE: McNeal Tract 3181 Sea Level Rise Letter Reinforcing 2021 Bluff Retreat Recommendations
From: Michael Gruver
Date: Fri, September 22, 2023 9:59 am
To: "[email protected]"
Cc: Fred McNeal , Tom McNeal
, Cory Hanh
Dave,
Thank you for having this prepared. I am reviewing this now. As this submittal did not meet a deadline I had for staff report preparation, I will not be able to include this as an attachment nor provide a staff response in the staff report itself. We will, however, include this as correspondence for the public record and will note we have received the document during our presentation to City Council. As part of the public record, we will forward the document to Coastal Commission as part of our Notice of Final Action should City Council take action on October 3rd.
Note: Please submit applications and plan check correspondence to [email protected].
Mike Gruver, AICPAssociate PlannerCommunity Development Department Office: (805) 773-7090Email: [email protected]
Website I Facebook I Twitter I YouTube
From: [email protected]
Sent: Thursday, September 21, 2023 10:44 AM
To: Michael Gruver ; Cory Hanh
Cc: Fred McNeal ; Tom McNeal
Subject: McNeal Tract 3181 Sea Level Rise Letter Reinforcing 2021 Bluff Retreat Recommendations
External Email
Mike and Cory:
Following up our discussions after the August PC Hearing regarding the Coastal Commission staff's inquiry on SLR and how it was addressed in the 2021 GeoSolutions Bluff Retreat Study, I asked GS to generate a letter detailing how SLR was incorporated with their recommendations in establishing a 100-year bluff retreat rate.
Attached is that letter reinforcing their analysis and recommendations including SLR considerations.
We would very much appreciate including this letter with the City Council packet and for your Staff Report to acknowledge receipt of this for the record, so it is included in any appeal considerations by CCC.
Thank you, and please don't hesitate to contact me to discuss the letter if you see anything in need of review.
David Watson, AICP
WATSON PLANNING CONSULTANTS
Post Office Box 385
Pismo Beach, California 93448-0385
Tel: 805.704.8728
Em: [email protected]
Website: www.watsonplanning.us
SEA LEVEL RISE AND WAVE RUN-UP ANALYSIS
1.0 INTRODUCTION
As requested by the client, representatives of GeoSolutions, Inc. have generated this letter to detail the manner in which Sea Level Rise was incorporated into our 2021 recommendations concerning bluff retreat recommendations regarding Tract 3181. To do this, we have reviewed potential sea level rise and conducted a sea wave run-up analysis for the proposed residential development to be located at Beachcomber Drive, APN: 010- 144-025, Shell Beach Area, Pismo Beach, California. This review is a supplement to the referenced Coastal Bluff Evaluation (GeoSolutions, 2021).
2.0 SITE DESCRIPTION
Beachcomber Drive is located at
35.1688 degrees north latitude and -
120.6940 degrees east longitude at a
general elevation of 72 feet above
mean sea level. The site is currently
vacant with annual grasses. The
property will hereafter be referred to
as the Site.
3.0 WAVE RUN-UP ANALYSIS
Sea Level Rise
Our 2021 analysis incorporates the
changes in sea level rise anticipated
to occur over the next 100 years, the
State of California Sea-Level Rise
Guidance (2018 update) was utilized.
Table 20 on p.64 of this document (a
portion of this document in contained
within this letter) shows Port San Luis
having a 2-foot sea-level rise by the
year 2100 with a probability of 56%
with a high emission category.
Higher sea-level rise in that year
have a lower probability. A
probabilistic projection on Table 19
shows a low risk aversion sea level rise of 2.4 to 3.7 feet, medium risk of 7.0 and high risk of 8.2 feet by 2120. The high-risk aversion elevation was utilized for the 100-year projection.
Figure 1: Site Photograph
DATE:
September 18, 2023
PROJECT NUMBER:
SL12009-2
CLIENT:
Fred & Tom McNeal
605 Via Cumbres
Fallbrook, California
92028
PROJECT NAME:
Beachcomber Drive
APN: 010-144-025
Shell Beach Area,
Pismo Beach,
California
Beachcomber Drive
September 18, 2023 Project SL12009-2
2 100-Year Design Stillwater Elevation
The design water level in this analysis is the maximum Stillwater level under a typical 100 -year recurrence condition. Water level is dependent upon several factors including tide, storm surge, wind set up, inverse barometer, and climatic events (El Nino). For this study, the maximum highest observed water level in the area was on 1-18-1973 with a highest tide of 7.57 feet. This water level takes into account an El Nino storm surge condition. This data is relative to NAVD88 as published by NOAA datums for 9412110, Port San Luis (see Figure 2).
Figure 2: Port San Luis Elevations
Based upon the highest tide information, the 100-year design Stillwater elevation in NAVD 88 datum would be as follows:
100-year design Stillwater elevation = 7.57 feet (highest water level observed, 1973) + 8.2 feet (sea level change) = 15.77 feet (NAVD 88).
100-Year Design Stillwater Depth (Ds)
To establish the 100-year Design Stillwater Depth (Ds) along the shoreline, the maximum scour depth must first be determined. At the subject site, it is anticipated that the elevation of Beachcomber Drive would control the design Stillwater depth. Beachcomber Drive is approximately 55 feet elevation. Subtracting a design still water elevation of 15.77 (100-year) from the Beachcomber Drive 55-foot elevation, would yield a scour depth of 0 feet for 100-year period. Therefore, the design Stillwater depth (100-year design Stillwater elevation is equal to 0 feet (Ds).
100-Year Maximum Breaking Wave Height (Hb)
Data regarding storms along the Southern California coast during the winter of 1982 -1983 (Denison and Robertson, 1985) were used as a guideline for determining the maximum breaking wave height at the site.
These storm data are considered to be comparable to the 100-year storm events. In January, 1983, wave heights from 6 to 15 feet with 4-6-second periods were recorded; these waves were considered to be the most severe of that winter. For this analysis, the breaking wave height used was 15 feet with a 6 -second
Beachcomber Drive
September 18, 2023 Project SL12009-2
period (Hb = 15 feet). During a storm event similar to those experienced in 1982 -1983, the wave would likely break offshore and run up along the scoured beach surface.
100-Year Wave Run-up Elevation (R)
Wave run-up is the vertical height above the Stillwater level to which a sea wave will rise up on a broad beach, a bluff face, or a bluff protection structure. At the subject site, the wave run-up elevation is estimated at the Site. Determination of wave r un-up height was based upon the Naval Facilities Engineering Command manual (see Figure 3, NAVFAC, 1982), a 15-foot breaking wave height, and a 6-second period (t), a scoured beach surface of -2 feet below the existing beach surface, and a beach slope angle of 2.0 degrees (cot Ɵ = 28.6 degrees).
R = Hb x R/Hb
Ds/Hb = 3.77/15 = 0.25; therefore Figure 72 was used to get R/Hb Hb/gt2 = 15/32.2(6)2 = 0.0129
R/Hb = (from graph) 0.14
R = Hb x 0.14 = 2.1 feet
The addition of the 100-year wave run-up height of 2.1 feet to the 100-year stillwater elevation of 15.77 feet yields a total 100-year wave run-up elevation of 17.87 feet (NAVD88).
Wave Run-Up Discussion
The determined 100-year wave run-up analysis indicated the highest elevation that a sea wave run-up would reach is elevation 17.87 feet (NAVD 88). As the existing property elevation is greater than 45 feet elevation, the potential for sea-level rise and wave run-up to affect the proposed development is low.
Figure 3: NAVFAC Wave Height Chart
Beachcomber Drive
September 18, 2023 Project SL12009-2
Effect of Sea-Level Rise on 2021 Bluff Retreat Analysis and Recommendations
Our 2021 analysis of bluff retreat rate at the Site resulted in a rate of 2.43 inches per year which equates to a 21-foot bluff setback distance for the 100-year design life of the structures. An additional 10-foot buffer was also recommended which serves to allow for uncertainty in aspects of the analysis, allows for future increase in bluff retreat due to sea level rise, and assures that at the end of the design life of the structure that the foundation is not being undermined to produce a total bluff setback for structures of 31 feet. It is our understanding the final plans for the project include a 35-foot setback.
The bluff at the subject property is currently near vertical to a general height of 45-feet. The underlying formational unit of the Edna Member of the Pismo Formation vari es between 18 to 30 feet in height along the bluff face. Marine Terrace Deposits overlies the Edna Member of the Pismo Formation.
Based on the elevation determined in the above sea level rise and wave runup analysis (17.87 feet) and the elevation of formational material along the bluff (18 to 30 feet), there is a low potential for sea level rise to have a significant effect on future retreat rates at the property provided the recommendations from the referenced Coastal Bluff Evaluation (GeoSolutions, 2021) are implemented. The retreat rate, bluff setback and recommendations from the referenced report are still applicable and should be incorporated into the project design.
Should you have any questions regarding content of this report, please do not hesitate to contact us at (805) 543-8539.
Sincerely,
GeoSolutions, Inc.
Jeffrey Pfost, CEG
Principal Engineering Geologist
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Beachcomber Drive
September 18, 2023 Project SL12009-2
REFERENCES
Denison, F.E. and Robertson, H.S., 1985, September. “1982 -1983 Winter Storms Damage, Malibu Coastline,” in California Geology.
GeoSolutions, 2021, Coastal Bluff Evaluation, Beachcomber Drive, APN: 010-144-025, Shell Beach Area, Pismo Beach, California, Project SL12009-1, dated February 12, 2021.
NAVFAC (Naval Facilities Engineering Command), April, 1982. Coastal Protection Design Manual, 26.2, U.S. Navy.
State of California Sea-Level Rise Guidance, 2018 update. California Natural Resources Agency and California Ocean Protection Council. Appendix 3.