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.f,J,. . File ..06.50-14
.V"f}RBGUB8T FOR COUNCIL ACTION
CITY OF SAN
From: ROGER G. HARDGRAVE
Preliminary Geologic Evaluation
Subject: of Potential Landslide Areas --
San Manuel Fire North of Patton
Hospi tal GARY S. RASMUSSEN
& ASSOCIATES
De~: Public Works/Engineering
Date: 5-30-90
Synopsis of Previous Council action:
02-05-90 --Resolution No. 90-44 adopted authorizing execution
of an Agreement for Professional Services, with Gary
S. Rasmussen & Associates.
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Recommendld motion:
1. That the Preliminary Geologic Evaluation of Potential Landslide
Areas in the San Manuel Fire Area, prepared by Gary S. Rasmussen
and Associates, be accepted and filed.
2. That the Director of Public Works/City Engineer be directed to
include a project for constructing a debris basin, along Bangor
Street, in the 1991 Capital Improvement Program.
cc: Marshall Julian
Supporting data attached:
Staff Report & StudY
Phone:
Ward:
5025
4
ContllCt penon:
Roqer G. Hardqrave
FUNDING REQUIREMENTS:
Amount:
N/A
,
Source: /Acct. No.!
/Acct. DescriDtionl
Finll1C8 :
Council No1lll:
75~0262
Agenda Item No .:5 3
c..rv OF "N -a..DlNO - RBlUUT L COUNCIL ACTION
STAFF REPORT
The u. S. Forest Service routinely prepares a Burn Area
Report for every major. fire. During the assessment of the
report for the San Manuel Fire, the Resource Geologist for the
U.S. Forest Service became concerned about potential stability
problems for fire areas within the burn. This concern prompted
the Geologist to write a letter advising the City of the
potential for landslides within these areas.
At the Council meeting of 2-5-90, authorization was
granted to execute an Agreement for Professional Services with
Gary Rasmussen and Associates. Under the terms of this
Agreement, an independent evaluation of the landslide potential
would be prepared by Gary Rasmussen and Associates.
The study, entitled "Preliminary Geologic Evaluation of
Landsliding, San Manuel Fire Burn, McKinley Mountain Area
Northeast of San Bernardino, California," has been completed.
This evaluation concludes, in part, that complete
failure of these older slides is not expected, and recommends
that a debris basin be provided for the central watercourse
(generally along Bangor Street). Based upon these fundings, we
recommend that no further action be taken, except to include a
debris basin in our next Capital Improvement Program.
We recommend that this evaluation be accepted and filed.
5-30-90
75..0264
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C I T Y
OF SAN BE R N A RoD I NO
INTEROFFICE MEMORANDUM
TO:
MARSHALL JULIAN, City Administrator
ROGER G. HARDGRAVE, Director of Public Works!
City Engineer
Item Bo. 20, COUDcil Agenda of 6-18-90
Preliminary Geoloqic Bval_tion of Potential
Landslide Areas
FROM:
SUBJECT:
DATE:
June 26, 1990
COPIES:
File No. 6.50-141 Reading File
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This item was continued to the Council meeting of 7-02-90, and
direction given to provide a report with cost estimates,
financing plan and phasing.
During preparation of the Burn Area Report, that was routinely
done for the San Manuel tire, the Resource Geologist for the
U.S. Forest Service became concerned about potential stability
problems. This concern was a result of a superficial
evaluation, consisting merely of reviewing aerial photos of
the burned area.
In order to take a fully responsible position, the City
entered into an Agreement with Gary Rasmussen , Associates to
conduct a more in depth investigation. This investigation
consisted of a geologic field reconnaissance of the burn area
and oblique stereoscopic aerial photographs, and a review of
pertinent geologic literature and maps. The Consultant was to
use this information to identify and accurately map previous
landslides, provide a preliminary evaluation of the age and
activity of the mapped landslides, as well as the potential
for future landslides to occur.
The above described duties were referred to as the initial
phase. If the preliminary evaluation indicated there isa
significant potential for landslide activity in any of the
fire areas, a more detailed investigation could be warranted.
The preliminary investigation does not recommend that a more
detailed investigation be conducted. ---
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!UUtSBALL JULIAN
Potential Landslide Areas
June 26, 1990
Page - 2 -
The Preliminary Geologic Evaluation has been completed, and
contains the following conclusions:
1. Previous historic fires have not significantly affected
the stability of the older and larger landslides.
2. Only Area No. 1 appears to have been recently active.
Continued failure of Area No.
contribute significant amounts
portion of the watercourse.
4. Some renewed activity of the older and larger landslides
could occur during an earthquake or erosion of the
landslide toe area, butc:iomplete failure of these older
slides is not expected due to their apparent stability
within at least the past several hundred years.
3.
1 is expected, and could
of debris into the upper
5. Rill erosion, shallow slope failures and debris flows will
contribute to increased sediment production.
6. Sediment yield is expected to increase significantly until
vegetation is re-established.
7. The sediment production rate of 700 cubic yards per acre
in 1980 from Harrison Canyon is probably atypical.
Recommendations set forth in the Preliminary Evaluation are:
1. Prepare topographic map to document existing conditions.
2. Consider a debris basin for the central un-named
watercourse (even though the geologic field reconnaissance
suggests that the majority of debris has been deposited
within the upper portions).
3. Evaluate the adequacy at the two existing debris basins
(on the Sand Creek and Small Canyon watercourses).
It should be noted that the landslide areas contributory to
the central watercourse are located about 3,300 feet north of
the reservation southerly boundary. The middle portion of
this watercourse is on a flat gradient, which results in the
deposition of a majority of debris produced.
A study of 56 separate burned watersheds in Los Angeles
County, during the 1978 and 1980 storm seasons, revealed
debris production of up to 227 cubic yards per acre, with a
median of 140 cubic yards. The volume of debris from Harrison
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MARSHALL JULIAN
Potential Landslide Areas
June 26. 1990
Page - 3 -
Canyon in
which is
atypical.
By using the maps included in the Preliminary Evaluation. it
is estimated that there are 11.5 acres contributory to the
watershed. The total amount of debris that can be expected is
1610 cubic yards. by assuming the median rate. A considerable
portion of this debris would settle out over the 3.300 feet
from the landslide areas to the reservation boundary.
1980 was estimated at 700 cubic yards per acre.
considered by Gary Rasmussen , Associates as
South of the reservation boundary. the watercourse traverses
the northwest corner of Tract No. 12949. known as "Amber
Hills." A small pond was developed during construction of the
improvements for this tract. This pond has a volume of 648
cubic yards. which would trap a portion of any debris that
reached this point.
Tract No. 13470. .north of Piedmont Drive and west of Bangor
Avenue. constructed a concrete flume to transition storm water
from the natural channel to the 48-inch storm drain.
Calculations submitted by C. P. Lange Engineers. the Civil
Engineer for Tract No. 13470. shows that the channel and flume
have 266% of the required capacity. of which 166\ is for
debris entrapment. The total capacity of the channel and
flume is 894 cubic yards. of which 558 cubic yards is for
debris.
In addition. block walls were contructed on each side of the
flume. These walls were designed for two feet of debris
loading. which can hold an additional 555 cubic yards.
The cross section of Bangor Avenue provides a capacity of 351
cubic yards. within the right of way.
Based upon these amounts. the total present storage available
for debris is 2.112 cubic yards (648 + 558 + 555 + 351).
Since this is greater than the projected debris amount of
1.610 cubic yards. the possibility of developed properties
being damaged is considered unlikely.
However. the relatively minor amounts of debris could settle
out in the 48-inch storm drain. A debris basin would clean up
the storm water and lessen the potential of maintenance
problems with the storm drain.
The normal location of a debris basis in at the downstream
limit of debris producing areas. Construction of a basin at
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MARSHALL JULIAN
Potential Landslide Areas
June 26, 1990
Page - 4 -
this location would entail development of 3,300 feet of road
on the reservation. Also, the settlement of debris over the
portion of the channel with a flat gradient would be lost.
In conclusion, the possibility of catastrophic failure appears
slim. However, a debris basin would protect against this
remote possibility and reduce maintenance problems. Based
upon our very preliminary review, we recommend that a debris
basis be constructed just north of Amber Hills, on the
reservation.
An inquiry has been submitted to the San Manuel Band of
Mission Indians about acquiring an easement for construction
of a debris basin. This inquiry will be considered at the
next tribal meeting_
In view of the time that will be required to acquire the
necessary easements, obtain environmental clearances and
engage a consultant, it is obvious that the debris basin will
not be able to be completed for this upcoming rainy season.
The preliminary estimate of cost for the debris basis is
$150,000. This amount could be included in the 1990/91 Storm
Drain Construction Budget, but this action would defer some
other storm drain project.
Please advise if you would like any additional information. A
copy of the map in the Preliminary Geologic Evaluation is
inc d for refere
ROGER G. HARDGRAVE
Director of Public Works/City Engineer
RGH:rs
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IINCUMnJU 1
INDEX IMP
aery S. .. fr . ~II_ IDe.
Ot,v of SaD BenuIIdIlIo - 1_1~ B9a1uatloB
SaD BerDardIDo, CaUfomJa
USGS Bale Map: 1" . 2000' scale
HarrIIaD MtD. '1.S' QuacIraIIIIe
TIN RSW See. IS. 1'1, 18, 19,20,21