1990-658 C
' MICHAEL W. HART, ENGINEERING GEOLOGIST
' GEOLOGIC RECONNAISSANCE
1404 NEPTUNE AVENUE
' ENCINITAB, CALIFORNIA
1
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MICHAEL ANTONORSI
Cl0 ROBERTA KING
r SAN DIEGO, CALIFORNIA
NOVEMBER 20, 1998
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MICHAEL W. HART
r ENGINEERING GEOLOGIST
r
r File No. 363-98
November 20, 1998
r
' Mr. Michael Antonorsi
c/o Roberta King
10995 Vivaracho Way
' San Diego, California 92124
Subject: Residence, 1404 Neptune Ave.
' Encinitas, California
GEOLOGIC RECONNAISSANCE
' Dear Mr. Antonorsi:
In. accordance with an agreement dated November 6, 1998, I have completed a geologic
' reconnaissance of the sea-bluff located at the subject site. It 'is concluded that erosion and bluff
retreat potential at the site can be classified as low to moderate. The sea-bluff can be expected to
retreat at an average long term rate of approximately 0.15 to 0.35 feet per year. The geologic
' conditions of the site and the potential for bluff failure is discussed in the following report.
' Very truly ours;
r 4 n
No. C
Michael W. Hart cp CECL
CEG 706 y~F,ia e ~ rfi. ~
' 2cc addressee OF CA ~
P.O. Box 261227. SAN DIEGO CALIFORNIA 92196. (619) 578-4672
' Residence, 1404 Neptune Avenue
File No. 363-98
' GEOLOGIC RECONNAISSANCE
1404 NEPTUNE AVENUE,
ENCINITAS, CALIFORNIA
Purpose and Scope:
' This report presents the findings of a engineering geologic reconnaissance of the sea-bluff
opposite the residence at 1404 Neptune Avenue in the Leucadia area of Encinitas, California.
' The purpose of this study is to: 1) describe the general geologic characteristics of the site; and 2)
to address the potential for the property to be impacted by erosion and sea-bluff retreat.
' The scope of work included geologic mapping of the sea-bluff along the west edge of the site;
' inspection of the bluff for evidence of past failure; review of published geologic reports and
existing erosion studies; review of historic aerial photographs; and preparation of this written
' report summarizing findings and recommendations.
Site Description•
The site consists of an approximately rectangular 50 feet wide residential lot located between a
sea-bluff on the west and Neptune Avenue on the east
(Figure 1). The existing single-story
residential structure is of wood-frame and stucco construction and has a setback varying from 25
to 35 feet from the edge of bluff. The rear yard is landscaped with a lawn that slopes gently to
the top of bluff. Wood fences exist along the northern and southern property lines. A
stairway
with an observation deck located approximately 15 feet below the elevation of the rear yard grade
has been constructed down the face of the bluff to the beach. The property bounded on
. the
t north and south by similar residential structures.
The sea-bluff west of the site is approximately 60 feet in height and has a compound slope that
I varies from 1:1 (horizontal to vertical) near the top to approximately 2:1 near the base of slope.
The bluff is vertical at the base and is supported by a concrete sea-wall approximately 4 feet in
height. An apparently abandoned storm drain exists near the top of slope just north of the
southern property line. Drainage from this pipe may have contributed to creating the highly
I
MICHAEL W. HART, ENGINEERING GEOLOGIST
Residence, 1404 Neptune Avenue
File No. 363-98
eroded area that occurs below the pipe extending to the base of the bluff.
General Geology and Geologic Structure:
The sea-bluff is comprised of a lower near-vertical section of Eocene-aged bedrock consisting of
claystone and sandstone that is partly supported (and obscured) at the base by a low sea-wall.
This unit is generally correlated with the Delmar Formation. Overlying the Delmar Formation is
a marine terrace deposit consisting of massive sandstones that may be correlatable with the
Lindavista Formation named for the community in the city of San Diego in which it is well-
exposed. Mapping by Tan (1986) also indicates the presence of minor faults trending at nearly
right angles to the bluff in the Delmar Formation nearby or below the site.
The contact between the Pleistocene terrace deposits d e Delmar Formation dips gently to the
west as shown on cross-section A-A' (Figure 3). Bedding within the terrace deposits is generally
massive and not well defined. Bedding within the Delmar Formation dips approximately
horizontally to 5 degrees to the south and southwest (Tan, 1986).
The Delmar Formation directly below the site is not well exposed because of the presence of
ground cover and the sea-wall. Therefore descriptions of the bedding and structure presented
above are taken primarily from. observations made on nearby properties to the north and south.
The Delmar Formation is known to contain sheared clay seams that occur parallel to the bedding.
Where present, the clay seams are the primary mechanism by which massive landsliding has
occurred on the bluff face south of the site near Beacons Beach. It is not known whether clay
seams are present on the bluff below the site because the Delmar Formation is obscured by the
sea-wall and vegetation. The closest known occurrence of a clay seam is several hundred south
of the site.
Groundwater:
Groundwater is presently seeping from the lower portion of the bluff along fractures in the
Delmar Formation and over the top of the sea-wall. This condition is common along the
Encinitas coastline. The source of water is believed to be rainfall and irrigation water that
percolates through the permeable sandstone of the terrace deposits to the contact with the Delmar
Formation. Once the water reaches the relatively impermeable geologic contact it travels
downdip to where it is visible as a linear spring usually in the lower portion of the cliff face near
MICHAEL W. HART, ENGINEERING GEOLOGIST
Residence, 1404 Neptune Avenue
File No. 363-98
' the contact between the terrace deposits and Delmar Formation. Based on the results of
observations made for this study, it is concluded that groundwater conditions on site are about
average compared to other sections of the Encinitas coast.
' Existing Bluff Conditions and Bluff Recession.
In order to estimate the future impact of erosion on the bluff and to provide estimates of bluff
' recession rates, several published and unpublished erosion studies were reviewed as well as
historical aerial photographs. The primary sources of.information concerning the rate of bluff-top
' recession and erosion utilized in the preparation of this report were the U.S. Corps of Engineers
Encinitas Shoreline Report (1996), and the Zeiser-Kling Beach Bluff Erosion Technical Report
' prepared for the City of Encinitas (1994). In addition, a previous report prepared by this office
for a property located at 1220 Neptune Avenue was reviewed. The report for 1220 Neptune
' Avenue indicated that no significant bluff top retreat occurred on that property between 1928 and
1981. The Corps of Engineers report (1996) indicates that the average long-term rate of bluff-top
recession in this area is approximately 0.15 feet per year. This site occurs within in Reach 1,
' the northernmost shoreline segment of the Encinitas shoreline which is described as follows in the
' Corps of Engineer report.
"Reach 1 is characterized by a narrow beach backed by high cliff. Development varies from
' multiple-family residential dwellings north of Grandview, and low-density private homes over the
remaining length. The seacliffs over this reach are relatively stable because of the erosional
' resistance of the basement material, flatter upper slope, extent of vegetative cover and presence of
abundant cobble that forms a continuous and protective toe berm. Natural seacliff toe protection
has been reinforced at some properties by privately constructed seawalls. The narrow beach has
been recently widened as result of upcoast sand nourishment that originated from Batiquitos
Lagoon restoration dredging. The extent of natural wave protection provided by the cobble berm
precludes all but the most severe storms from attacking the seacliff toe. Reducing the exposure to
marine erosion processes, and the already more stable configuration of the upper cliff slopes
translates to low seacliff recession and threat to development in the immediate future."
Major factors affecting future cliff retreat at the site are wave attack at the cliff base, continued
ground water seepage from the cliff face, and potential uncontrolled drainage problems along the
cliff top. The inspection of the bluff made for this study indicates that the localized near-vertical
portion of the upper bluff lying approximately 2/3 up from the base of the bluff may represent the
headscarp of an old slump within the terrace deposits. The date that this slumping occurred is not
known but based on the amount of vegetation at the base of slope it likely occurred more than 10
MICHAEL W. HART, ENGINEERING GEOLOGIST
Residence, 1404 Neptune Avenue
File No. 363-98
' years ago. This apparent slump did not likely affect the Eocene bedrock portion of the lower bluff
face that is currently protected from erosion by the sea-wall nor did it affect the current position
of the bluff edge.
' All of the referenced studies, which rely heavily on interpretation of historic aerial photographs
and topographic maps indicate that toe erosion and cliff or bluff-edge recession over the long term
' (one hundred years or more) should occur at essentially the same rate. For shorter periods of
time bluff edge erosion may exceed toe erosion. While the Corps of Engineers has estimated that
' erosion rates have been very low (approximately 0.15ft./yr.) along this section of the bluff since
1928, the average long term average rate of sea-cliff retreat for the entire Encinitas coastline, as
determined by Zeiser Kling (1994) is estimated at approximately 0.2 to 0.35 feet per year; a rate
that could be classified as moderate.
t
Toe erosion has been essentially halted at the base of the bluff by the concrete sea-wall. As long
t as this wall is properly maintained and assuming it was properly constructed with adequate
foundations, it should continue to provide protection from erosion occurring at wave level. The
' upper portion of the bluff, under most conditions, would continue to recede and erode at a much
reduced rate because of the effect of the sea-wall. As previously described, however, certain
' areas of the slope are oversteepened to inclinations varying from approximately 1:1 to nearly-
vertical and will be subject to future sloughage or shallow sliding. These oversteepened areas are
' shown on Figures 2 and 3. The size of future bluff failures can not be accurately predicted,
however, it is estimated that they could extend on the order of 5 feet into the rear yard.
4
' MICHAEL W. HART, ENGINEERING GEOLOGIST
I Residence, 1404 Neptune Avenue
File No. 363-98
I Conclusions and Recommendations:
1. Published sea-bluff erosion rates indicate that sea-cliff recession rates in this area of Encinitas
have been low to moderate. The average long term rate of bluff recession is estimated at
approximately 0.15 to 0.35 feet per year.
2. Currently, drainage in the rear yard is by sheet-flow directed over the top of the bluff.
Because the drainage is not concentrated over the bluff edge and the upper bluff face is protected
by a well established ground cover, the potential for accelerated erosion of the upper 12 to 15 feet
' of the bluff appears to be minimal. The central portion of the bluff is oversteepened and
essentially devoid of vegetation. Accordingly, this is the most likely location for future sloughage
or shallow landsliding. A combination of low retaining walls and landscaping may be effective in
reducing surface erosion in such areas. It is, therefore, recommended that consideration be given
to retaining a landscape architect knowledgeable with coastal conditions to provide specific
methods of reducing erosion.
' 3. The low sea-wall appears to have been effective in limiting erosion and toe recession at the
base of the bluff. Assuming that the foundations have adequate embedment into the formational
soils, the we should continue to provide erosion protection for the base of the bluff. Continued
' inspection of the wall and occasional maintenance will be required.
References:
Eisenberg, L.I., 1983, Pleistocene marine terrace and Eocene geology, Encinitas and Rancho Santa Fe Quadrangles, San
Diego County, California, Unpub. M.S. thesis, San Diego State Univ., pp 1-386.
Tan, S.S., 1986, Landslide hazards in the Encinitas Quadrangle, Calif. Div. Mines and Geology Open File Report 86-8
' LA.
U.S. Army Corp of Engineers, 1996, Reconnaissance Report, Encinitas Shoreline, San Diego County, CA, pub. by Los
' Angeles District, U.S. Army Corp of Engineers.
Woodward Clyde Consultants, 1980, Sea cliff study, Mullane Residence, 1220 Neptune Avenue, Leucadia, California.
Woodward-Clyde Consultants, 1981, Update sea cliff study, Mullane Residence, swimming pool addition, 1220
Neptune Avenue, Leucadia, California.
Zeiser Kling Consultants, Inc., 1994, Beach Bluff Erosion Technical Report, unpub. tech report submitted to City of
' Encinitas, pp. 1-98.
Aerial Photographs: San Diego County 1960 stereographic pairs; San Diego Historical Society, 1957., Neg. #54130.
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