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RFI - Sediment basin calculations
SWMP Commentary, Detailed Calculations
Note: These "Detailed Calculation" spreadsheets relate only to high erosion hazard lands as identified in
figure 4.6 or where the designer chooses to use the RUSLE to size sediment basins. The "Standard
Calculation" spreadsheets should be used on low erosion hazard lands as identified by figure 4.6 and
where the designer chooses not to run the RUSLE in calculations.
1. Site Data Sheet
Site Name: Lot 1 Taylor Court, Cooroy
Site Location: Lot 1 Taylor Court, Cooroy
Precinct:
Description of Site: Industrial Lot Reconfiguration (4 Lots)
Sub-catchments
Site area Remarks
1
Total catchment area (ha) 1.136
Disturbed catchment area (ha) 1.09
Soil analysis (enter sediment type if known, or laboratory particle size data)
Sediment Type (C, F or D) if known: D
% sand (fraction 0.02 to 2.00 mm)
% silt (fraction 0.002 to 0.02 mm)
% clay (fraction finer than 0.002 mm)
Dispersion percentage
% of whole soil dispersible
Soil Texture Group D
Rainfall data
Design rainfall depth (days) 5
Design rainfall depth (percentile) 80
x-day, y-percentile rainfall event 34.6
Rainfall R-factor (if known)
IFD: 2-year, 6-hour storm (if known) 15
RUSLE Factors
Rainfall erosivity (R -factor) 5010 5010 5010 5010 5010 5010 Auto-filled from above
Soil erodibility (K -factor) 0.047
Slope length (m) 50
Slope gradient (%) 5.5
Length/gradient (LS -factor) 1.01
Erosion control practice (P -factor) 1.3 1.3 1.3 1.3 1.3 1.3
Ground cover (C -factor) 1 1 1 1 1 1
Calculations
Soil loss (t/ha/yr) 308
Soil Loss Class 3 See Section 4.3 and Table 4-2.
3
Soil loss (m /ha/yr) 237
Sediment basin storage volume, m3 44 See Sections 6.5.3 and 6.6.3
QLD - Sediment Basins- V2.xls 1
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SWMP Commentary, Detailed Calculations
2. Storm Flow Calculations
Peak flow is given by the Rational Formula:
Qy = 0.00278 x C10 x FY x Iy, tc x A
where: Qy is peak flow rate (m 3/sec) of average recurrence interval (ARI) of "Y" years
C10 is the runoff coefficient (dimensionless) for ARI of 10 years. Rural runoff
coefficients are given in Volume 2, figure 5 of Pilgrim (1998), while urban
runoff coefficients are given in Volume 1, Book VIII, figure 1.13 of Pilgrim
(1998) and construction runoff coefficients are given in Appendix F
Fy is a frequency factor for "Y" years. Rural values are given in Volume 1,
Book IV, Table 1.1 of Pilgrim (1998) while urban coefficients are given in
Volume 1, Book VIII, Table 1.6 of Pilgrim (1998)
A is the catchment area in hectares (ha)
Iy, tc is the average rainfall intensity (mm/hr) for an ARI of "Y" years
and a design duration of "tc" (minutes or hours)
Time of concentration (tc) = (107 n L0.333) / S0.2, in minutes - Friends Equation
or use table of standard inlet times (QUDM Table 4.06.1)
where: n is Hortons roughness value for the surface (similar to Mannings value)
L is length of flow path in metres
S is slope in %
n= 0.0275 L= 50 m S= 5.5 %
Time of concentration (tc) = 8 mins
Peak flow calculations, 1
A tc Rainfall intensity, I, mm/hr
Site C10
(ha) (mins) 1 yr,tc 5 yr,tc 10 yr,tc 20 yr,tc 50 yr,tc 100 yr,tc
1 1.136 10 97.5 148 172 196 225 247 0.9
Peak flow calculations, 2
Peak flows
Frequency
ARI
factor 1 Comment
(yrs)
(Fy)
(m3/s) (m3/s) (m3/s) (m3/s) (m3/s) (m3/s)
1 yr,tc 0.8 0.222
5 yr,tc 0.95 0.400
10 yr,tc 1 0.489
20 yr,tc 1.05 0.585
50 yr,tc 1.15 0.735
100 yr,tc 1.2 0.842
QLD - Sediment Basins- V2.xls 2
Page 3
SWMP Commentary, Detailed Calculations
4. Volume of Sediment Basins, Type D and Type F Soils
Basin volume = settling zone volume + sediment storage zone volume
Settling Zone Volume
The settling zone volume for Type F and Type D soils is calculated to provide capacity to contain all
runoff expected from up to the y-percentile rainfall event. The volume of the basin's settling zone (V)
can be determined as a function of the basin's surface area and depth to allow for particles to settle and
can be determined by the following equation:
V = 10 x Cv x A x Rx-day, y-%ile (m 3)
where:
10 = a unit conversion factor
Cv = the volumetric runoff coefficient defined
as that portion of rainfall that runs off as
stormwater over the x-day period
Rx-day, y-%ile = is the x-day total rainfall depth (mm) that
is not exceeded in y percent of rainfall
events. (See Sections 6.3.4(d), (e), (f),
(g) and (h)).
A = total catchment area (ha)
Sediment Storage Zone Volume
In the detailed calculation on Soil Loss Classes 1 to 4 lands, the sediment storage zone can be taken as
50 percent of the settling zone capacity. Alternately designers can design the zone to store the 2-
month soil loss as calculated by the RUSLE (Chapter 6.5.3(d). However, on Soil Loss Classes 5, 6 and
7 lands, the zone must contain the 2-month soil loss as calculated by the RUSLE (Chapter 6.5.3(d).
Place an "X" in the box below to show the sediment storage zone design parameters used here:
x 50% of settling zone capacity,
2 months soil loss calculated by RUSLE
. Average Depth of basin = 1 metres
Total Settling Sediment Total Basin shape
catchment zone storage basin
Site Cv Rx-day, y-%ile
area volume volume volume L:W Length Width
(ha) (m3) (m3) 3
(m ) Ratio (m) (m)
1 0.58 34.6 1.136 228 114 342 3 32.0 10.7
0.58 3
0.58 3
3
3
3
QLD - Sediment Basins- V2.xls 5