HomeMy WebLinkAboutCT 00-16; Poinsettia Properties; Offsite Detention Basin; 2003-05-29PROJECTDESIGN CONSULTANTSPLANNING • ENVIRONMENTAL • ENGINEERING • SURVEY/GPS
File: 2068.00
May 29, 2003
Mr. John Maashoff
CITY OF CARLSBAD
1635 Faraday Avenue
Carlsbad, CA 92008
SUBJECT: Water's End - Offsite Detention Basin
Dear John,
Per your direction, ProjectDesign Consultants (PDC) has reviewed the drainage reports
prepared by O'Day Consultants (O'Day) concerning the function and design of the
existing desilting basin that discharges into Batiquitos Lagoon. From PDC's perspective,
and as shown in the O'Day report titled, "Avenida Encinas & Offsite Storm Drain-CT 94-
01-Hydrology Study", the basin was not designed to provide detention for the watershed
that drains to the lagoon. Instead, the basin was designed for water quality treatment and
open space sediment removal.
Based on previous conversations with you, it is PDC's understanding that the basin can be
used for detention as long as the original function of the basin is not compromised. Before
proceeding with PDC's evaluation of the original basin design, we should clarify PDC's
proposed detention analysis methodology and design approach:
Hydrology:
The existing conditions hydrology (i.e., detained target storm flow) should be the existing
conditions storm flow, prior to entering the basin. In effect, the existing basin should not
be considered as an existing conditions facility from a detention standpoint.
Basin Function:
The basin offers water quality treatment. However, its function as a desilting basin has
been greatly diminished due to the development of the watershed. It is the latter that PDC
has addressed in this letter, since the basin volume once occupied by the projected
sediment volume can now be used for detention.
R:\WP\LETTER\2000\2068DETBASDM.doc
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Mr. John Maashoff
May 29, 2003
Page 2
Sediment Yield Analysis - O'Day vs. PDC
O'Day used the Universal Soil Loss Equation (USLE) to determine the sediment volume
for the basin. The calculation is included in the O'Day report and was performed prior to
the development of the majority of the watershed. As such, it is PDC's contention that the
values O'Day originally assigned to the USLE equation parameters are conservative
relative to the current development of the watershed and approach used by PDC to better
define the parameters. The USLE is as follows:
A= RK(Ls)CP, where
A= computed soil loss in tons
R= rainfall erosion index
K= soil erodability factor
Ls=slope length and gradient factor
C= crop management factor
P= erosion control factor
The following table shows the USLE parameter values selected by O'Day and PDC, and
the acreages used to determine the sediment yield for the basin:
TABLE 1
O'Day
PDC
USLE Parameters and Values
R
25
25
K
0.24
0.10
Ls
2.16
0.10-0.35
C
0.01
0.1
p
0.8
1.0
Area
227.5
36.1
K Factor:
PDC determined the K factor using the Soil Conservation Service Soil Survey for San
Diego County. The soil classification for the watershed is primarily MIC, which reflects a
K factor of 0.10. Note that K factors are included in the draft Hydrology Manual for the
County of San Diego for the various soil classifications identified in the soil survey.
The K factor used by O'Day is 0.24, which was acquired based on a soils report and the
Triangular Nomograph for Estimating K. This information is contained in the O'Day
report.
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Mr. John Maashoff
May 29, 2003
Page 3
Ls Factor:
The length-slope factor (Ls) gave the largest discrepancy in the results of the PDC and
O'Day USLE calculations. Based on PDC's field investigation, the sediment producing
areas that drain to the desilting basin are fairly flat. Therefore, the Ls values for the PDC
computation ranged from 0.10 to 0.35 for each sediment sub-basin, respectively. See
Exhibits A and B for the sediment sub-basins.
A worst-case scenario was used in the O'Day study in the selection of the Ls factor. A
change in height of 23 feet and a length of 250 feet were used to get an Ls factor equal to
2.16. This factor was applied to the entire watershed, which is extremely conservative
under existing and current topographic conditions.
C and P factors:
PDC performed the USLE calculations using two C and P factor scenarios. Scenario 1
(SN1) was performed using the C and P factors used by O'Day, while Scenario 2 (SN2)
was performed using C and P factors based on PDC's field investigation and the San Diego
County Hydrology Manual. The SN1 analysis was based on the assumption the entire area
would be developed. As a result, reduced C and P factors of 0.01 and 0.8, respectively,
were used that are reflective of native vegetation. Note that the USLE is intended for
natural and graded construction site conditions where sheet and rill erosion may occur.
PDC used developed C and P factors of 0.1 and 1, respectively, commensurate with current
watershed developed conditions, i.e., only those undeveloped naturally vegetated
properties. The sediment yield per year was determined to be approximately 10 cubic-feet
(cf). Note that the field investigation led to an increase in both factors.
The following is a summary of the PDC field investigation:
The PDC field investigation showed the majority of the sediment producing
acreage is the undeveloped areas north of the desilting basin, bound by Carlsbad
Boulevard to the west, Batiquitos lagoon to the south, railroad tracks to the east,
and a housing development to the north and the area along the railroad tracks. The
rest of the drainage area is developed. See Exhibit A for the location of the
undeveloped areas and railroad acreages used in the USLE calculations.
The undeveloped areas are well vegetated and fairly flat, except for the southwest
edge of the area south of Avenida Encinas. A concrete ditch drains the southern
area prior to discharging down the hill to the desilting basin. The northern area
T:\WaterResources\2068-Poinsettia\Offsite Sediment\2068DETBASIN.doc
Mr. John Maashoff
May 29, 2003
Page 4
drains toward Ponto Drive prior to interception by inlets that drain to the desilting
basin.
The railroad track area is relatively unvegetated. However, since the track area is
flat and the storm water travels through vegetation prior to entering the mainline
storm drain system, it is unlikely that a significant amount of sediment will reach
the basin. Note, however, that PDC's sediment calculations include this acreage to
ensure that every possible area has been accounted for in the calculations.
Sediment Acreage:
PDC used the acreage that is currently undeveloped (i.e., acreage that is naturally vegetated
or uncovered) to determine the sediment yield per year. This methodology falls within the
assumptions that were used in the development of the equation. The sediment yield was
determined to be 10 cf/yr, which is insignificant given the current basin volume. The basin
acreage used by PDC was 36.1 acres as opposed to the 227.5 acres used by O'Day, which
is not reflective of current site conditions.
See Exhibits A and B for the O'Day and PDC drainage maps, respectively, and Table 2 for
the PDC USLE calculations. Additionally, photographs of the sediment producing areas
have also been included. The photograph locations are listed on Exhibit B.
PDC vs. O'Day Analysis Results and Conclusion
The O'Day study calculated a soil loss of 527 cf/yr. PDC's updated analysis, per current
watershed conditions, estimates a soil loss of 10 cf/yr. Therefore, it is PDC's contention
that the basin volume previously used for sediment removal can now be used for detention
and that the basin, from a hydrologic standpoint, can be treated as a proposed facility.
PDC would like to set up a meeting with you to go over the results of the analysis in more
detail, if you deem necessary, to clarify PDC's proposed detention basin and sediment
yield design and analysis methodology. If you wish, PDC can provide you with back-up
material concerning the use of the USLE to expedite your review. Please give Curt Turner
or myself a call upon the completion of your review.
Sincerely,
Adolph Lugo, PE
Director of Water Resources
T:\WaterResources\2068-Poinsettia\OffsiteSedimenl\2068DETBASIN.doc
Table 2A
Universal Soil Loss Equation
As = FTK*Ls*C*P
C = 0.1 P = 1.0(PDC Values)
110LB/CF
AREA
W
S
I
U
J-3
K-1
K-2
A-27
R
R
25
25
25
25
25
25
25
25
25
K
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
Ls
0.15
0.1
0.28
0.28
0.2
0.28
0.35
0.13
0.33
C
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
P
1
1
1
1
1
1
1
1
1
As
(TN/YR)
0.0375
0.025
0.07
0.07
0.05
0.07
0.0875
0.0325
0.0825
SOIL DENSITY
(CF/LB)
0.0091
0.0091
0.0091
0.0091
0.0091
0.0091
0.0091
0.0091
0.0091
SOIL LOSS
(CF/YR)
0.682
0.455
1.273
1.273
0.909
1.273
1.591
0.591
1.500
AREA
7.8
2.5
2.9
5.6
3.1
3.2
1.9
0.8
8.3
TOTAL 0.525 9.545 36.100
TABLE 2B
Universal Soil Loss Equation
As = R*K*Ls*C*P
C = 0.01 P = 0.8 (O'Day Values)
110LB/CF
AREA
W
S
I
U
J-3
K-1
K-2
A-27
R
R
25
25
25
25
25
25
25
25
25
K
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
Ls
0.15
0.1
0.28
0.28
0.2
0.28
0.35
0.13
0.33
C
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
P
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
As
(TN/YR)
0.003
0.002
0.0056
0.0056
0.004
0.0056
0.007
0.0026
0.0066
SOIL DENSITY
(CF/LB)
0.0091
0.0091
0.0091
0.0091
0.0091
0.0091
0.0091
0.0091
0.0091
SOIL LOSS
(CF/YR)
0.055
0.036
0.102
0.102
0.073
0.102
0.127
0.047
0.120
AREA
7.8
2.5
2.9
5.6
3.1
3.2
1.9
0.8
8.3
TOTAL 0.042 0.764 36.100
Photo 1
Southern Undeveloped Area
Location: Looking Northeast from the End of Ponto Drive.
Photo 2
Northern & Southern Undeveloped Areas
Location: Looking North along Carlsbad Boulevard.
Legend
O'Day Drainage Basin Boundary (227acres)
PDC Sediment Production Area Boundary (36 acres)
Note: See Exhibit A for the Sub-basin delineations
Exhibit B
0 800 1600
GRAPHICAL SCALE
Water fs End
Off site Sediment Study PROJEcrDmGN CONSULTANTS
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