SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Page:
1 of 14
1.0 PURPOSE
This design guide explains how to use the SPCC Oil Retention Basin Spreadsheet to analyze existing SPCC
containment systems and to design new containments. This supplements the SPCC Plans and Facility
Improvements Design Criteria Memorandum (DCM No. C-1.3 Rev. 1). SPCC facility improvements decrease the
probability of an oil spill discharging into navigable waters.
2.0 SCOPE
The SPCC Oil Retention Basin Spreadsheet (Spreadsheet) contains two worksheets, “Existing/New SPCC Basin
Storage” and “Existing/New SPCC Flow-Through Analysis”. “Existing/New SPCC Basin Storage” computes the spill
retention capacity in an existing basin and checks the storage adequacy for a new basin design. “Existing/New
SPCC Flow-Through Analysis” checks the storm water flow-through requirements. The application of this user’s
guide for specific projects shall be as directed by the Responsible Engineer (RE).
3.0 GLOSSARY OF TERMS
Above-base storage
Volume of oil stored above the highest floor elevation.
Allowable Oil Storage
Volume of oil that can be contained in the basin.
Back Wall
Back wall in weir system.
Base Storage
Volume stored below the highest floor elevation.
Basin Capacity
Volume of fluid that can be contained in the basin.
Basin Floor
Rectangular floor at bottom of basin.
Bermed Area
Area enclosed within the berm system.
Cut-off Wall
(see Skimmer Wall)
Highest Point
Highest basin floor elevation compared to top of weir elevation.
Lowest Point
Lowest basin floor elevation compared to top of weir elevation, typically at the weir
location.
Oil Specific Gravity
The specific gravity of the equipment mineral oil is set at 0.85, the nominal value
for transformer oils.
Overflow Weir
Wall that regulates the outflow rate from and the maximum water level within the
oil retention basin.
Perimeter Wall
Outermost wall of weir system.
Skimmer Wall
Wall that regulates the flow and separation of water and/or oil upstream of the
outlet structure.
Sluice Slot
Flow passage below the skimmer wall.
Spill Quantity
The volume of oil and/or water to be retained. It is equal to the maximum of the
following (based on reference 3, Section 5.2.3):
1) 110% of oil volume of largest container
2) 10% of total aggregate oil volume (<100,000 gal)
Weir
(see Overflow Weir)
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
© 2004 by Pacific Gas and Electric Company. All rights
SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Page:
2 of 14
4.0 ANALYSIS CASES
Note:
 Existing basins: Analyze Cases A and B below to determine the allowable oil storage of
the existing SPCC system based on site-specific conditions.
 New basins: Analyze all three cases.
 Refer to Sections 5.1.3.2 and 6.1.2 for definitions of variables in the figures below.
 Basins in areas of high water table should be checked for buoyancy.
4.1 CASE A: Oil spill into empty basin with no overflow
 Analyze/design the basin or berm system to contain the maximum of the following:
1) Spill quantity equal to 110% of oil volume in largest container
2) Spill quantity equal to 10% of total aggregate oil volume (<100,000 gal)
B'
Perimeter Wall
M
C
How
B
Skimmer
Upstream
Invert
A
D=How+C'
h
Weir
v
Back
Wall
C'
D'
F
Figure 4.1 Case A
B'
How
Hew
Z’
B
A
C' or Creq
Skimmer
D = d12
C'
4.2 CASE B: Oil spill when existing basin water level is at top of overflow weir
 With the basin full of water, analyze/design the basin or berm system to contain the spill
quantity listed in Section 4.1.
 Because the oil has a specific gravity of 0.85, the spilled oil remains behind the skimmer wall
and above the existing water level, which is to the top of overflow weir.
 This condition determines the upper limit of the underflow weir. To prevent the oil from mixing
with water, a pressure balance between the fluids must be maintained. The pressure balance is
defined by Z′= (d12-A- C ′)/0.85 + C′, where C′ is the user defined freeboard (refer to section
6.1.2 for definition of d12).
 The following two conditions are used to maintain the pressure balance:
1) Condition 1: If Z′< B – C′
2) Condition 2: If Z′ > B – C′, a required freeboard, Creq, must be provided to prevent oil from
flowing over the skimmer wall (refer to section 6.1.3 for definition of Creq).
Figure 4.2 Case B
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
© 2004 by Pacific Gas and Electric Company. All rights
SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Page:
3 of 14
B'
4.3 CASE C: Storm overflow only, no oil spill
 Design all basin or berm systems to pass the design storm (25-year return period) runoff in
combination with no oil.
 The design should include a basin or berm dewatering system. The dewatering system may be
gravity or pump operated. The system (pipes, valves, pumps, or other devices) should be
capable of dewatering a full basin or berm within one hour or less.
 The outlet flow capacity should exceed the design storm runoff (see Section 5.2.5 for outlet
variables).
d
a
A
F
C
b
Hinitial
D
B
E
D'
M
Figure 4.3 Case C
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
© 2004 by Pacific Gas and Electric Company. All rights
SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Page:
4 of 14
5.0 USER INPUT
Note: A star appears to the right of an input value when the input does not meet the normal minimum
requirement (see Section 6.1.3). A larger value must be entered.
A pound sign appears in the input cell or to the right of an input cell when an invalid number (either too
large or too small) is entered. Check to make sure that the correct input values were entered.
5.1 Sheet 1: Oil Storage Input
Note: Sheet 1, “Existing/New SPCC Basin Storage”, calculates the amount of oil to be contained and the allowable
oil storage.
5.1.1 Select Spreadsheet Type
Note: Check one of the following options. If both or none of the options have been selected, the
spreadsheet title will print “Check Spreadsheet Selection”.
Existing Basin Analysis = Select this option if you are analyzing an existing basin. The Sheet 1 title
will print “Existing SPCC Basin Storage” and Sheet 2 will print “Existing SPCC Basin Flow-Through
Analysis”.
New Basin Design = Select this option if you are designing a new basin. The Sheet 1 title will print
“New SPCC Basin Storage” and Sheet 2 will print “New SPCC Basin Flow-Through Analysis”.
5.1.2 Oil Input Data
Amount of Oil in Largest Container (gal) = If the amount of oil in the largest container is larger than
ten percent of total oil, input gallons of oil in largest container from Attachment #1 “Inventory and Spill
Prediction Table” of the specific substation SPCC Plan, or as provided by the RE. Otherwise, input
“N.A.” for not applicable or leave this cell blank.
Ten Percent of Total Aggregate (gal) = If ten percent of total oil is larger than the amount of oil in
largest container, input ten percent of the total aggregate from Attachment #1 “Inventory and Spill
Prediction Table” of the SPCC Plan. Otherwise, input “N.A.” for not applicable or leave this cell blank.
5.1.3 Basin Input Data
5.1.3.1 General Input
Average wall slope, h/v = Input the ratio of the horizontal length divided by vertical length of
sloped portion surrounding the basin (if applicable). If slopes differ, use the average value of all
side slopes. If the wall slope is vertical, enter “0”.
X (ft) = Longer inside basin dimension, based on a rectangular shape, as shown in Figure 5.1
Y (ft) = Shorter inside basin dimension, based on a rectangular shape, as shown in Figure 5.1
L (ft) = Inside width of Weir shown in Figure 5.1
 L is typically between 1 and 10 ft. For smaller weirs, L typically ranges from 1 to 3 ft.
 As L increases, the following decrease:
1) Minimum requirement for A (see Section 6.1.3)
2) Required head, E, from “Time of Concentration” (see Section 6.2.4)
X
d1
Y
L
L
d4
A
d3
d2
Figure 5.1 Basin Plan View
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
© 2004 by Pacific Gas and Electric Company. All rights
SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Page:
5 of 14
5.1.3.2 Elevation View Dimensions
Note: Refer to Figures 4.1 through 4.3 for illustrations of the following variables.
A (in) = Sluice slot height, usually 3” minimum
 Dependent on required head, E (refer to Section 6.2.4 for definition of E)
B (in) = Skimmer wall height
B′(in) = Height difference between top of perimeter and top of skimmer wall
 If no perimeter wall exists, enter “0” for B′.
F (ft) = Horizontal distance between weir and skimmer wall
 F is typically between 1.5 and 10 feet. For construction space requirements, F is usually
not less than 18 inches. For small weirs, with the approval of the RE, F may be less than
18 inches, but in no case shall F be less than A+3”.
 If the weir system is outside of the basin, enter a negative value so the spreadsheet can
either add or subtract this volume from the total volume (depending on analysis case).
D (ft) = Weir wall height
 Measured from top of weir wall to the basin invert at the weir
C′(in) = Freeboard from top of oil to top of weir or skimmer wall, typically 1”, or as provided by
the RE
M (ft) = Horizontal distance between weir wall and outlet pipe in the perimeter wall
 Measured from the inside of the perimeter wall to the upstream side of the weir wall
 If the weir system is outside of the basin, enter a negative value. The spreadsheet will
subtract this volume from the total volume, since this portion does not contain oil.
 If no back wall exists, enter “0” for M.
5.1.3.3 Depth Measurements
Note: The top of weir in this section is defined as the elevation at the top of the weir wall.
Depths d1 to d4 represent depths from the top of weir wall elevation to the measured ground
elevation below the top of weir wall elevation. Refer to Figure 5.1 for a plan view of basin
depths. For an example of how input depths d1 through d4 are calculated, refer to Figure 5.2
below.
If the ground elevation is flat, enter the same values for all depths (should be equal to the weir
wall height).
If only two depths on opposite ends of the basin are measured, enter them into cells d1 and d3. Since an average
depth will be taken from d1 and d2 (d12) and from d3 and d4 ( d34), you may also enter the same depth for d2 as d1 and
enter the same depth for d4 as d3.
d1 = Distance from top of weir elevation to lowest ground elevation in basin
 d1 = D = Top of weir elevation – Lowest basin elevation
d2 = Distance from top of weir elevation to second lowest ground elevation in basin
 d2 = Top of weir elevation – Second lowest basin elevation
d3 = Distance from top of weir elevation to second highest ground elevation in basin
 d3 = Top of weir elevation – Second highest basin elevation
A
d
Top of Weir
EL 100.00’
1
C
'
d4 = Distance from top of weir elevation to highest ground elevation in basin
 d4 = Top of weir elevation – Highest basin elevation
Third lowest
basin
EL 99.00’
d3 EL = 100.0’-99.0’ = 1.00’
Figure 5.2 Example of Depth Measurement, d3
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
© 2004 by Pacific Gas and Electric Company. All rights
SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Page:
6 of 14
5.2 Sheet 2: Flow-through Input
Note: Sheet 2, “Existing/New SPCC Basin Flow-through Analysis”, determines whether the basin flow
capacity meets the storm water flow-through requirements for Cases A and B for existing basins
and Case C for new basins.
5.2.1 Table 2. Drainage Parameters
5.2.1.1 Surface Type
Basin Surface Area (sq.ft) = Horizontal projected area of the SPCC containment basin
including side slopes
Foundation Area (sq.ft) = The total surface area of foundations within the upstream drainage
area
Paved Area (sq.ft) = The total area of paved surface within the drainage area
Gravel Area (sq.ft) = The total area of gravel surface within the drainage area
Soils Area (sq.ft) = The total area of soil surface within the drainage area
5.2.1.2 Runoff Coefficient (C)
Gravel = Input appropriate runoff coefficient, typically 0.75, or as provided by RE.
Soils = Input appropriate soil coefficient, typically 0.50, or as provided by RE.
5.2.2 Time of Concentration
Note: Refer to Section 6.2.3 for time variable definitions.
Tc (minutes) = Input closest time duration, TD CLOSE, computed at the end of Table 4.
 Tc is set equal to the rainfall duration interval,TD , closest to the corresponding total time, Ttot.
Ttot is the rainfall runoff travel time from the most remote part of the substation through the
basin and over the outflow weir.
 For Ttot values less than 5 minutes, set Tc equal to 5 minutes, the minimum duration interval.
 As Tc increases, the calculated minimum diameter, d min, and the design storm inflow, Qin des
decrease (refer to Section 6.2.5 for definitions of dmin and Qin des ).
Percent Full (%) = Percent of water already contained in the basin when the design storm begins.
 The RE should determine the Percent Full based on site specific conditions.
 0% means the basin is empty and 100% means the basin is full when rainfall begins.
 As Percent Full increases, the time to fill the basin, Tfill, and total time, Ttot, decrease.
5.2.3 Table 3. Time Constants for Flow to Basin
Overland Flow Length (ft) = Longest length between ridge and ditch flow
Ditch Flow Length (ft) = Longest total length of ditch flow from beginning of ditch to the basin
SG = Ground slope in feet per foot. SG is calculated by dividing the change in substation ground
elevation, ∆ZG, by the corresponding length, LG, which is equal to either Loverland or Lditch.
 SG = ∆ZG/LG
 SG varies on the order of 0.005 to 0.03, for both overland flow and ditch flow.
5.2.4 Table 4. Short Duration Rainfall
Blank Space next to Table 4. Title: Enter letter, number (under station bsn no) and station name
found on the “Precipitation Depth-Duration-Frequency” Table from the California Department of Water
Resources, Short Duration Precipitation Depth-Duration-Frequency Data Table (ref. 1). If no station
name is listed, use the station order number.
Storm Frequency (yr) = Input 25-year return period.
De (in) = Input the depth-duration data, for the corresponding return period from ref. 1.
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
© 2004 by Pacific Gas and Electric Company. All rights
SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Page:
7 of 14
5.2.5 Discharge Outlet
Discharge of Outlet = Select “Ditch”, “Pipe”, or “Pump” from the list box.
 Ditch: Fluid flows over the overflow weir, out of the weir system, and into a ditch or open
area located outside the weir system. No additional input is required. Delete any
entered input.
 Pipe: Fluid flows over the overflow weir, out of the weir system, and through a downstream
pipe. Enter all input described below in this section. Delete any input entered in the
cells below “Design storm inflow, Qin des”.
 Pump: Fluid flows over the overflow weir, out of the weir system, and through a downstream
pump system. Enter the manufacturer’s pump capacity, P cap (in gpm), or as
provided by RE. Delete all other entered input.
Manning’s roughness coefficient “n” (ref.2)
 Concrete/Steel:
n = 0.012
 PVC:
n = 0.010
 CMP:
n = 0.023
Drop of outlet pipe, a (ft) = Vertical distance between inverts at each end of outlet pipe
Run of Outlet pipe, b (ft) = Horizontal length of outlet pipe
Outlet Pipe Diameter, d (in) = Inner diameter of outlet pipe
Pump Capacity, Pcap (gpm) = Manufacturer’s pump capacity, or pump capacity as provided by RE
6.0 CALCULATED OUTPUT
Note: If a non-integer output (e.g. #NUM, ##, #VALUE!) appears in an output cell, an invalid input was entered.
Check to make sure that the correct input values were entered.
6.1 Sheet 1: Oil Storage Output
Note: Sheet 1, “Existing/New SPCC Basin Storage”, calculates the amount of oil to be contained and the
allowable oil storage.
6.1.1 Oil Output Data
Oil to be contained (gal) = The maximum of the following:
1) 110% of oil in the largest container
2) 10% of total aggregate oil volume
Oil to be contained (cu.ft) = Gallons of oil to be contained divided by 7.48 gallons per cubic foot
6.1.2 Basin Output Data
Note: Refer to Figures 4.1 to 4.3 from Section 4 for illustrations of the following variables.
C (in) = Height from inside bottom of weir to top of perimeter wall
 C = A + B + B′
D′ (in) = Height difference between top of perimeter wall and top of weir wall

D′ = C –D
d12 (in) = Average basin depth on weir side of basin. Takes the average of depths d1 and d2 and is
typically equal to the weir height (see Figures 6.1 to 6.4).
 d12 = (d1 + d2) / 2
or
d12 = d1 (if only two depths are measured)
d34 (in) = Average basin depth on opposing side of basin. Takes the average of depths d 3 and d4
(see Figures 6.1 to 6.4).
 d34 = (d3 + d4) / 2
or
d34 = d3 (if only two depths are measured)
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
© 2004 by Pacific Gas and Electric Company. All rights
SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Page:
8 of 14
6.1.3 Normal Minimum Requirements
Note: The minimum values calculated from the equations below are required for adequate
containment.
A (in) = Depth of flow over the weir plus freeboard
 A = E + C′ (refer to Section 6.2.4 for calculation of E)
B (in) = The maximum of the following:
1) The weir wall height from Cases A or
2) For Case B the maximum of the following two conditions are used to maintain a pressure
balance between the fluids. The variable Z′, where Z′= (d12-A- C ′)/0.85 + C′, defines the
pressure balance. The Z′ and Z′(condition 2) variables apply only to Case B.
a) Condition 1: If Z′ < B- C′, then B = Z′ + C′
b) Condition 2: If Z′ > B- C′, then B = Z′(condition 2) + Creq. For condition 2, a required
freeboard, Creq, must be calculated to prevent oil from flowing over the skimmer wall. In
addition, a new pressure balance, Z′ (condition 2), must be defined using the calculated
required freeboard.
 Creq
= (d12 – A - 0.85 x B + 0.85 x C′)/0.15
 Z′ (condtion 2) = (d12 – A - Creq)/0.85 + Creq
1) B = D

MAX
2) MAX a) B = Z′ + C′
b) B = Z′(condition 2) + Creq
F (ft) = One (1) foot plus sluice slot height
 F = 1’ + A
D (ft) = The maximum of the following:
1) The height of oily water from Case A, How Case A (see Section 6.1.4.1), plus freeboard or
2) The height of oily water from Case B, which equals the maximum of the following two
conditions used to maintain a pressure balance between the fluids.
The variable Z′, where Z′ = (d12 – A- C′)/0.85 + C′, defines the pressure balance.
a) Condition 1: If Z′ < B- C′, then D = ( A + C′ ) + ( Z′ - C′ ) x 0.85
b) Condition 2: If Z′ > B- C′, then D = ( A + Creq ) + ( Z′(condition 2) - Creq) x 0.85

1) D = How Case A + C′
MAX
2) MAX
a) D = ( A + C′ ) + ( Z′ - C′ ) x 0.85
b) D = ( A + Creq ) + ( Z′(condition 2) - Creq ) x 0.85
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
© 2004 by Pacific Gas and Electric Company. All rights
SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Page:
Condition 1
9 of 14
Condition 2
∆Zo > A
d34
A
A
∆Zo
D=d12
d34
A
A
D=d12
CASE A
C' C’
C' C’
∆Zo < A
∆Zo
Figure 6.2
Figure 6.1
Z’ < B – C’
Z’ > B – C’
d34
∆Zo
Z'
A
A
D=d12
d34
Z'
C' or Creq
C’
C’
C
'
C' or Creq
AA
D=d12
CASE B
C
'
∆Zo > A + C’ or A+Creq (sub-condition 2)
∆Zo < A + C’ or A + Creq (sub-condition 1)
∆Zo
Figure 6.3
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
Figure 6.4
© 2004 by Pacific Gas and Electric Company. All rights
SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Page:
10 of 14
6.1.4 Table 1. Oil Storage Summary
Note: The critical oil storage value has “GOVERNS” printed to the right.
6.1.4.1 Height of Oily Water in the Basin, How
Note: How is the height of oily water at the weir system location.
1) CASE A: Height of oily water equals the weir wall height, D, minus the user defined
freeboard, C′
 How = D – C′
2) CASE B: The height of oily water depends on the pressure balance and whether the oil
must be prevented from flowing over the skimmer wall. For condition 1, How is equal to Z′
minus the user defined freeboard C′. For condition 2, the pressure balance must be
maintained to prevent oil from spilling over the skimmer wall. Thus, How is equal to
Z′(condition 2) minus the calculated required freeboard, Creq.


Condition 1: If Z′ < B – C′, then How = Z′ – C′
Condition 2: If Z′ > B – C′, then How = Z′(condition 2) – Creq
6.1.4.2 Gross Oil Storage Amount
Note: The gross oil storage calculation is based on a prismatically shaped basin with constant
side slopes. The basin floor slope, ∆Zo, is calculated by taking the difference between
opposing average basin depths, d12 and d34, and is defined as ∆Zo = d12 - d34 (refer to Figures
6.1 through 6.4 for illustrations of ∆Zo). For a sloped floor, ∆Zo equals a non-zero value.
For a flat floor, ∆Zo equals zero.
6.1.4.2a Two Slope Conditions
Note: The gross oil storage for each case is determined by calculating a base and
above-base storage, both depend on the floor and sidewall slopes.
1) CASE A:
a) Condition 1: Defines a small floor slope. Difference between d12 and d34 is less
than or equal to sluice slot height.
 ∆Zo < A
b) Condition 2: Defines a large floor slope. Difference between d12 and d34 is
greater than the sluice slot height.
 ∆Zo > A
2) CASE B: Since Case B has two conditions to maintain the pressure balance behind
the skimmer wall, the following two slope sub-conditions are considered:
a) Condition 1: If Z’ < B – C′, then
i) Sub-condition 1: ∆Zo < A + C’
ii) Sub-condition 2: ∆Zo > A + C’
b) Condition 2: If Z’ > B – C′, then
i) Sub-condition 1: ∆Zo < A + Creq
ii) Sub-condition 2: ∆Zo > A + Creq
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
© 2004 by Pacific Gas and Electric Company. All rights
SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Page:
11 of 14
6.1.4.2b Base storage, Vbase (gal) = Calculates the volume of storage available below the highest
floor elevation. It is based on the floor area, floor slope, and side slope.

Vbase = Xo/∆Zo * [(Yo*∆Zo2)/2 + (S*∆Z3)/3] + (S*Yo*∆Z2)/2 + (2*S2*∆Z3)/3
1) CASE A: For both slope conditions defined in 6.1.4.2a, the lower limit is d 12, the
average basin floor depth at the weir side of the basin. The upper limit is ∆Z o,
the difference between d12 and d34.
2) CASE B:
a) Condition 1:
b) Condition 2:
i) Sub-Condition 1: No base storage is calculated
because the floor slope is small.
ii) Sub-Condition 2: The lower limit is zero. The upper
limit is A + C′. For simplicity, the overall base storage
is calculated by subtracting the base storage for Case B
from the base storage for Case A.
i) Sub-Condition 1: No base storage is calculated.
ii) Sub-Condition 2: The lower limit is zero. The upper
limit is A + Creq. For simplicity, the overall base storage
is calculated by subtracting the base storage for Case B
from the base storage for Case A.
6.1.4.2c Above-base Storage, Vabove-base or Va.b. (gal) = Calculates the storage above the base
storage. The volume equation is derived by integrating from zero to dz’:
Areaabove-base = (X + S x ∆Zo) x (Y + S x ∆Zo), where S = wallslope, ∆Zo = d12 – d34, and
dz’ = vertical height of oily water from ∆Zo or the entire height of oily
water
Basin
S x ∆Zo
S x ∆Zo
X
Y + S x ∆Zo
Figure 6.7 Above-base Area
1) CASE A: For both slope conditions defined in 6.1.4.2a, the lower limit is d34, the
higher basin elevation. The upper limit is the defined freeboard C'. The difference
between d34 and C' equals dz'.

Above-base storage, Vabove-base (gal) = (Xo + S x ∆Zo) x (Yo + S x ∆Zo) x dz'
+ S x (dz' 2) x [(Xo + S x ∆Zo) + (Yo + S x ∆Zo) + 4 x S x dz'/3]
–
0,
If M<0
dz’ x L x M, If M>0
“M-TERM”
–
F x L x dz’, If F<0
0,
If F>0
“F-TERM”
x 7.48 gal/ft3
M-Term: The portion of the weir system between the back and weir wall, (horizontal
distance M), which does not store oil. If M is outside of the basin or less than zero
(negative value), subtract zero since this volume does not contribute to the total oil
storage.
If M is inside the basin or greater than zero (positive value), this part of the weir system does not store oil, but still
occupies space in the basin. Thus, to reflect the correct oil storage in the basin, subtract dz’ x L x M from the total oil
storage.
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
© 2004 by Pacific Gas and Electric Company. All rights
SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Page:
12 of 14
F-Term: The portion of the weir system between the weir and skimmer wall,
(horizontal distance F), which can store oil. If F is outside of the basin or less than
zero (negative value), this part of the weir system can still store oil. Therefore, F x L x
dz’, must be added to the total oil storage.
If F is inside the basin or greater than zero (positive value), subtract zero since the
volume of oil in this part of the weir system is already accounted for in the total oil
storage.
2) CASE B: Only the F term changes from the equation used for Case A.

–
0,
L x dz’, If F>0
If M<0
―
a) Condition 1:
i) Sub-Condition 1: No base storage is calculated
because the ground elevation slope is small.
ii) Sub-Condition 2: The lower limit is C′. The upper limit
is Z′. The difference between Z′ and C′ equals dz'.
b) Condition 2:
i) Sub-Condition 1: No base storage is calculated.
ii) Sub-Condition 2: The lower limit is ∆Zo. The upper
limit is Z′(condition2). For this sub-condition, dz' equals ∆Zo
plus A minus Z′(condition2).
Above-base storage, Vabove-base (gal) = (Xo + S x ∆Zo) x (Yo + S x ∆Zo) x dz'
+ S x (dz' 2) x [(Xo + S x ∆Zo) + (Yo + S x ∆Zo) + 4 x S x dz'/3]
0,
If F<0
x 7.48 gal/ft3
“M-TERM”
dz’ x L x M, If M>0
Fx
“F-TERM”
M-Term: (same as Case A above)
F-Term: This portion of the weir system does not store oil. If F is outside the basin,
subtract zero since this part does not contribute to the total oil storage.
If F is inside the basin, this part of the weir system does not store oil, but still occupies space inside the basin.
Thus, to reflect the correct oil storage in the basin, subtract F x L x D from the total basin volume.
6.1.4.2d Gross Oil Storage , Vtot (gal)
1) CASE A: For both slope conditions, the gross oil storage equals the sum of the base
and above-base storage.
 Vtot = Vbase Case A + Vabove-base Case A
2) CASE B: For both conditions used to maintain the pressure balance, the gross oil
storage is calculated with the equations below. For sub-condition 2, the gross oil
storage is computed using values from Case A.


Sub-Condition 1: Vtot = Vabove-base Case A
Sub-Condition 2: Vtot = Vbase Case A – Vbase Case B + Vabove-base Case B
6.1.4.2e Percent Storage (%) = Percentage of oil volume in largest container
 Case A: 110%
 Case B: 110%
6.1.4.2f Allowable Oil Storage (gal): The allowable oil storage determines if the basin has
sufficient storage. Since the gross oil storage includes the corresponding percent storage
per Case, the allowable basin oil storage equals the gross oil storage divided by the
percent storage:
 Allowable Oil Storage (gal) = Gross Oil Storage (gal) ÷ Percent Storage/100
6.2 Sheet 2: Flow-Through Output
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
© 2004 by Pacific Gas and Electric Company. All rights
SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Note:
Page:
13 of 14
Sheet 2, “Existing/New SPCC Basin Flow-Through Analysis”, determines if the basin flow capacity
meets the storm water flow-through requirements for Cases A and B for existing basins, and Case
C for new basins.
6.2.1 Table 2. Drainage Parameters
CxA : Calculates values for each surface type by multiplying Runoff coefficient (C) times Drainage
Area (A)
Total CxA = Sum of CxA
6.2.2 Table 3. Constants for Flow Time to Basin
Manning’s “n” = An assigned constant dependent on surface roughness
 For overland flow, n = 0.03 typically.
 For lined ditch flow, n = 0.012 typically.
6.2.3 Table 4. Short Duration Rainfall
Time Duration Intervals, T D (minutes) = Time duration intervals (minutes) of rainfall concentration
taken from Ref. 2. The first 3 intervals are usually the most critical.
Rainfall intensity, i (in/hr) = The magnitude of rainfall measured in inches per hour. When D e input
values are not available, enter 0.00 inch. The intensity (I) is then extrapolated using the following
formulas:
 I (in/hr) = K/T
 log I (in/hr) = log K – nlogT (minutes)
Storm inflow, Qin (cfs) = Total CxA from Table 2, times the corresponding rainfall intensity, I, divided
by 43200 (from 60 sec/hr x 12 in/ft):
 Qin = Total CxA x I
43200
Time to Fill Basin, Tfill (minutes) = Time required to fill the basin. Takes the governing oil storage
volume from sheet 1, divides it by the storm inflow corresponding to the duration time, and then
multiplies it by the Percent Full. If the Percent Full amount equals 100%, the basin is full and the T fill
value will print as “---“.
 Tfill =
V (ft3)
x Percent Full
Qin (cfs) x 60sec/min
100
Time to Reach Basin, T to basin (minutes) = Time for drainage to flow from the furthest point in the
substation drainage area to the basin. The time to reach the basin equals the time for overland flow
plus the time for ditch flow.
 Tto basin = Toverland + Tditch,
where Toverland = Longest travel time required to flow between ridge and ditch flow,
and Tditch = Longest time required for ditch flow from beginning of ditch to basin
 Toverland = (0.67 x n x Loverland)/(SG1/2), SG = ∆ZG/L
[Kerby’s
equation]
 Tditch = Lditch/Velocity,
where Velocity = Q/A = Q/1.5yo2, Q = 1.486AR2/3SG1/2 / n (Manning’s equation),
A=
1.5 x yo2 and R = A/Pwet
Total Time, Ttot (minutes) = Sum of time to fill basin and time to reach basin
 Ttot = Tfill + Tto basin
Closest Time Duration, TD CLOSE (minutes) = Time duration (TD) closest to the total time of travel (Ttot)
from the most remote part of the substation drainage area through the basin and over the outflow weir.
This value equals the Tc input value in the “Time of Concentration” table described in Section 5.2.2.
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
© 2004 by Pacific Gas and Electric Company. All rights
SUBSTATION CIVIL ENGINEERING GUIDE
Subject:
CEG-3
Approved By:
Spill Prevention Control and Countermeasure (SPCC)
Oil Retention Basin Spreadsheet User’s Guide
1
Revision:
SED Civil
DFW
Effective Date:
02/10/05 Electronically Approved
Page:
14 of 14
6.2.4 Outflow Weir Head
Required Head, E (in) = Head necessary to pass the required design storm inflow, Qin des, over the
broad crested outflow weir:

E=
Qin des
3L
2/3
6.2.5 Discharge Outlet Type



Pipe: All output values described below are printed.
Pump: Only “Pump capacity” and “Design storm inflow” values are printed.
Ditch: Only “Design storm inflow” value is printed.
Slope of pipe (s) = Drop of outlet pipe, a, divided by run of outlet pipe, b:
Note: Pipe flow is based on the assumption of a free outlet (not submerged) and free inlet
(no head).
 s = a/b
Minimum outlet pipe diameter, Dmin (in) =
 Dmin = ( (n/0.093) x (48/d)1/6) x (8 x b x (Qin2) )1/5
3.14162 x 32.2 x a
Pipe outflow capacity, Qcap (cfs) =
 Qcap = 1.486 x s1/2 x 3.1416 x (d/12)2
n
4
x
x
d
48
12in/ft
2/3
Design storm inflow, Qin des (cfs) = Using the input time of concentration value, Tc, from the
“Time of Concentration” table (see Section 5.2.2), the corresponding Qin value is selected as
required flow capacity.
For Ditches:
 If Qcap > Qin des, “Outflow capacity OK” will print.
 If Qcap < Qin des, “Required Ditch Capacity Must Be > Qin des” will print.
For Pipes:
 If Qcap > Qin des, “Outflow capacity OK” will print.
 If Qcap < Qin des, “Overflow capacity insufficient” will print.
For Pumps:
 If Pcap > Qin des, “Outflow capacity OK” will print.
 If Pcap < Qin des, “Pump outflow capacity < Qin des” will print.
Pump capacity, Pcap (cfs) =
 Pcap = Pump Discharge (gpm)
448.8
7.0 REFERENCES
1.
California Dept. of Water Resources. (1988). Short Duration Precipitation Depth-Duration-Frequency Data
Table. Sacramento: California Department of Water Resources.
2. Linsley, R.K., Kohler, M.A., and Paulhus, J.L.H. (1975). Hydrology for Engineers. New York: McGraw-Hill.
3. Pacific Gas and Electric Company. (2004). Design Criteria Memorandum: Spill Prevention Control and
Countermeasure (SPCC) Plans and Facility Improvements, DCM No. C-1.3 Rev. 1. Oakland: Pacific Gas
and Electric Company.
4. Pacific Gas and Electric Company. (2004). Spill Prevention Control and Countermeasure (SPCC) Oil
Retention Basin Spreadsheet. Oakland: Pacific Gas and Electric Company.
SPCC Oil Retention Basin Spreadsheet User’s Guide.doc
reserved.
© 2004 by Pacific Gas and Electric Company. All rights