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FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.2 Runoff Coefficients
Page 4
3.2 Runoff Coefficients
Runoff coefficients used for the Rational Method are determined based on either overall land use or
surface type across the drainage area. For Overall Drainage Plan (ODP) submittals, when surface types
may not yet be known, land use shall be used to estimate flow rates and volumes. Table 3.2-1 lists the
runoff coefficients for common types of land uses in the City.
Table 3.2-1. Zoning Classification - Runoff Coefficients
Land Use Runoff Coefficient (C)
Residential
Urban Estate 0.30
Low Density 0.55
Medium Density 0.65
High Density 0.85
Commercial
Commercial 0.85
Industrial 0.95
Undeveloped
Open Lands, Transition 0.20
Greenbelts, Agriculture 0.20
Reference: For further guidance regarding zoning classifications, refer to the Land Use
Code, Article 4.
For a Project Development Plan (PDP) or Final Plan (FP) submittals, runoff coefficients must be based on
the proposed land surface types. Since the actual runoff coefficients may be different from those
specified in Table 3.2-1, Table 3.2-2 lists coefficients for the specific types of land surfaces.
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.2 Runoff Coefficients
Page 5
Table 3.2-2. Surface Type - Runoff Coefficients
Surface Type Runoff Coefficients
Hardscape or Hard Surface
Asphalt, Concrete 0.95
Rooftop 0.95
Recycled Asphalt 0.80
Gravel 0.50
Pavers 0.50
Landscape or Pervious Surface
Lawns, Sandy Soil, Flat Slope < 2% 0.10
Lawns, Sandy Soil, Avg Slope 2-7% 0.15
Lawns, Sandy Soil, Steep Slope >7% 0.20
Lawns, Clayey Soil, Flat Slope < 2% 0.20
Lawns, Clayey Soil, Avg Slope 2-7% 0.25
Lawns, Clayey Soil, Steep Slope >7% 0.35
3.2.1 Composite Runoff Coefficients
Drainage sub-basins are frequently composed of land that has multiple surface types or zoning
classifications. In such cases a composite runoff coefficient must be calculated for any given drainage
sub-basin.
The composite runoff coefficient is obtained using the following formula:
t
n
i
ii
A
xAC
C 1 Equation 5-2
Where: C = Composite Runoff Coefficient
Ci = Runoff Coefficient for Specific Area (Ai), dimensionless
Ai = Area of Surface with Runoff Coefficient of Ci, acres or square feet
n = Number of different surfaces to be considered
At = Total Area over which C is applicable, acres or square feet
3.2.2 Runoff Coefficient Frequency Adjustment Factor
The runoff coefficients provided in Table 3.2-1 and Table 3.2-2 are appropriate for use with the 2-year
storm event. For any analysis of storms with higher intensities, an adjustment of the runoff coefficient is
required due to the lessening amount of infiltration, depression retention, evapotranspiration and other
losses that have a proportionally smaller effect on high-intensity storm runoff. This adjustment is
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.3 Time of Concentration
Page 6
applied to the composite runoff coefficient. These frequency adjustment factors, Cf, are found in Table
3.2-3.
Table 3.2-3. Frequency Adjustment Factors
Storm Return Period
(years)
Frequency Adjustment
Factor (Cf)
2, 5, 10 1.00
25 1.10
50 1.20
100 1.25
3.3 Time of Concentration
3.3.1 Overall Equation
The next step to approximate runoff using the Rational Method is to estimate the Time of
Concentration, Tc, or the time for water to flow from the most remote part of the drainage sub-basin to
the design point under consideration.
The Time of Concentration is represented by the following equation:
Equation 5-3
Where: Tc = Total Time of Concentration, minutes
Ti = Initial or Overland Flow Time of Concentration, minutes
Tt = Channelized Flow in Swale, Gutter or Pipe, minutes
3.3.2 Overland Flow Time
Overland flow, Ti, can be determined by the following equation:
Equation 3.3-2
Where: C = Runoff Coefficient, dimensionless
Cf = Frequency Adjustment Factor, dimensionless
L = Length of Overland Flow, feet
S = Slope, percent
CXCF
PRODUCT OF CXCF
CANNOT EXCEED THE
VALUE OF 1
OVERLAND FLOW LENGTH
L=200’ MAX IN DEVELOPED AREAS
L=500’ MAX IN UNDEVELOPED
AREAS
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.4 Intensity-Duration-Frequency Curves for Rational Method
Page 7
3.3.3 Channelized Flow Time
Travel time in a swale, gutter or storm pipe is considered “channelized” or “concentrated” flow and can
be estimated using the Manning’s Equation:
Equation 5-4
Where: V = Velocity, feet/second
n = Roughness Coefficient, dimensionless
R = Hydraulic Radius, feet (Hydraulic Radius = area / wetted perimeter, feet)
S = Longitudinal Slope, feet/feet
And:
Equation 5-5
3.3.4 Total Time of Concentration
A minimum Tc of 5 minutes is required. The maximum Tc
allowed for the most upstream design point shall be
calculated using the following equation:
Equation 3.3-5
The Total Time of Concentration, Tc, is the lesser of the
values of Tc calculated using Tc = Ti + Tt or the equation
listed above.
3.4 Intensity-Duration-Frequency Curves for Rational Method
The two-hour rainfall Intensity-Duration-Frequency curves for use with the Rational Method is provided
in Table 3.4-1 and Figure 3.4-1.
TC
A MINIMUM TC OF 5
MINUTES IS REQUIRED IN
ALL CASES.
A MAXIMUM TC OF 5
MINUTES IS TYPICAL FOR
SMALLER, URBAN PROJECTS.
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.4 Intensity-Duration-Frequency Curves for Rational Method
Page 8
Table 3.4-1. IDF Table for Rational Method
Duration
(min)
Intensity
2-year
(in/hr)
Intensity
10-year
(in/hr)
Intensity
100-year
(in/hr)
Duration
(min)
Intensity
2-year
(in/hr)
Intensity
10-year
(in/hr)
Intensity
100-year
(in/hr)
5 2.85 4.87 9.95 39 1.09 1.86 3.8
6 2.67 4.56 9.31 40 1.07 1.83 3.74
7 2.52 4.31 8.80 41 1.05 1.80 3.68
8 2.40 4.10 8.38 42 1.04 1.77 3.62
9 2.30 3.93 8.03 43 1.02 1.74 3.56
10 2.21 3.78 7.72 44 1.01 1.72 3.51
11 2.13 3.63 7.42 45 0.99 1.69 3.46
12 2.05 3.50 7.16 46 0.98 1.67 3.41
13 1.98 3.39 6.92 47 0.96 1.64 3.36
14 1.92 3.29 6.71 48 0.95 1.62 3.31
15 1.87 3.19 6.52 49 0.94 1.6 3.27
16 1.81 3.08 6.30 50 0.92 1.58 3.23
17 1.75 2.99 6.10 51 0.91 1.56 3.18
18 1.70 2.90 5.92 52 0.9 1.54 3.14
19 1.65 2.82 5.75 53 0.89 1.52 3.10
20 1.61 2.74 5.60 54 0.88 1.50 3.07
21 1.56 2.67 5.46 55 0.87 1.48 3.03
22 1.53 2.61 5.32 56 0.86 1.47 2.99
23 1.49 2.55 5.20 57 0.85 1.45 2.96
24 1.46 2.49 5.09 58 0.84 1.43 2.92
25 1.43 2.44 4.98 59 0.83 1.42 2.89
26 1.4 2.39 4.87 60 0.82 1.4 2.86
27 1.37 2.34 4.78 65 0.78 1.32 2.71
28 1.34 2.29 4.69 70 0.73 1.25 2.59
29 1.32 2.25 4.60 75 0.70 1.19 2.48
30 1.30 2.21 4.52 80 0.66 1.14 2.38
31 1.27 2.16 4.42 85 0.64 1.09 2.29
32 1.24 2.12 4.33 90 0.61 1.05 2.21
33 1.22 2.08 4.24 95 0.58 1.01 2.13
34 1.19 2.04 4.16 100 0.56 0.97 2.06
35 1.17 2.00 4.08 105 0.54 0.94 2.00
36 1.15 1.96 4.01 110 0.52 0.91 1.94
37 1.16 1.93 3.93 115 0.51 0.88 1.88
38 1.11 1.89 3.87 120 0.49 0.86 1.84
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.4 Intensity-Duration-Frequency Curves for Rational Method
Page 9
Figure 3.4-1. Rainfall IDF Curve – Fort Collins
FORT COLLINS STORMWATER CRITERIA MANUAL Water Quality (Ch. 7)
5.0 Hydrologic Basis of the WQCV
5.0 Hydrologic Basis of the WQCV
Page 12
WQCV = a(0.91 1.19 + 0.78 ) Equation 7-1
Where: WQCV = Water Quality Capture Volume, watershed inches
a = Coefficient corresponding to WQCV drain time (Table 5.4-1)
I = Imperviousness (%/100)
Table 5.4-1. Drain Time Coefficients for WQCV Calculations
Drain Time (hrs)Coefficient (a)
12 0.8
40 1.0
Reference: The UD-BMP excel-based spreadsheet, RG and EDB tabs may be used to aid in
calculating WQCV.
Figure 5.4-1 WQCV Based on BMP Drain Time
Once the WQCV in watershed inches is found from Figure 3.2-12 or using Equation 3.2-1, the
required BMP volume in acre-feet can be calculated as follows:
Equation 7-2
Where: V = required volume, acre-ft
A = tributary catchment area upstream, acres
WQCV = Water Quality Capture Volume, watershed inches
1.2 = to account for the additional 20% of required storage for sedimentation accumulation
SAFL Baffle Research Summary
Four years of research was conducted to develop and test the SAFL Baffle. The research took place at
ry and was funded by the Minnesota Department
of Transportation. Links to SAFL Baffle project reports and publications can be found at the end of this
document. In this research summary, the following four topics will be discussed:
1. SAFL Baffle Performance
2. Effects of Trash and Vegetation
3. 90 Degree Outlet Sump Manholes
4. Sump Manholes with Inlet Grates and Inlet Pipes
SAFL Baffle Performance
SAFL Baffles are installed in existing or new construction sump manholes. Without a SAFL Baffle, sump
manholes capture sediment found in stormwater during rain storms through settling. During intense storm
events, however, this previously captured sediment can be washed out of the sump due to a circular
water flow pattern. With the SAFL Baffle installed in a sump manhole, water is unable to travel in a
circular pattern. During most low intensity storm events, slightly more sediment is captured in the sump
than without the SAFL Baffle. But during intense storm events, the SAFL Baffle prevents the circular
water flow pattern to form inside of the sump manhole. This prevents washout of sediment (Howard et al.
2010).
Figure 1: Sediment deposits in a scale model sump manhole after a high flow rate test. (Left)
Without a SAFL Baffle & (Right) With a SAFL Baffle (Howard et al. 2011).
Several stormwater treatment devices were tested at St. Anthony Falls Laboratory in addition to the SAFL
Baffle. The performance of all these devices was characterized using by measuring (1) how well the
device captures sediment and (2) how well it retains sediment at high flow rates. The first metric is called
Removal Efficiency, and can be characterized in terms of the Péclet number over the Froude number of
the inlet jet velocity versus the amount of sediment captured in terms of a fraction. And the second metric
is called Washout Performance, and can be characterized in terms of Péclet number over the Froude
number of the inlet jet velocity versus a dimensionless concentration number The Péclet
number, the Froude number of the inlet jet velocity and the dimensionless concentration number are
shown below. By using these dimensionless numbers, it is possible to compare the Removal Efficiency
and Washout Performance of different devices, different sized devices, different sediment particle sizes,
and different flow rates (McIntire, et al. 2012).
Where:
Where:
Where:
The Removal Efficiency of a sump manhole with and without a SAFL Baffle is shown below in Figure 2.
Tests were conducted by starting with an empty sump manhole, feeding set sediment sizes into the sump
at various flow rates with and without a SAFL Baffle, and measuring the amount of sediment captured. On
the figure, low Pe/Frj
2 values correspond to small sump manholes, experiencing high flow rates, and
receiving small sediment particles, and high Pe/Fr j
2 values correspond to large sump manholes,
experiencing low flow rates, and receiving large sediment particles. This means that a curve laying left of
another curve captures more sediment. Figure 2 shows that a SAFL Baffle installed in a sump manhole
will capture 10-15% more sediment than a sump manhole without a SAFL Baffle (McIntire, et al. 2012).
The Washout Performance of a sump manhole with and without a SAFL Baffle is shown below in Figure
3. Tests were conducted by starting with a sump manhole partially filled with sediment, increasing the
flow rate to match a storm flow rate, and measuring how much sediment was washed out of the sump. On
the figure, Pe/Fr j
2
correspond to high sediment effluent concentrations (the concentration of sediment leaving the sump).
Figure 3 shows that without a SAFL Baffle, previously captured sediment will wash out of the sump
manhole. With the SAFL Baffle, however, washout is significantly decreased to near negligible levels,
depending on flow rate (Howard et al. 2011).
The washout benefits of using a SAFL Baffle can be plainly seen in Figure 4. When a SAFL Baffle is not
installed in a sump manhole, washout increases exponentially with an increase in flow rate. Effluent
concentrations were measured as high as 800 mg/L at a flow rate of 16 cubic feet per second (cfs). With
a SAFL Baffle, washout dramatically decreases. At the same flow rate of 16 cfs, the effluent concentration
was measured was less than 50 mg/L. And with the SAFL Baffle, below 7 cfs, the effluent concentration
measured was negligible (McIntire, et al. 2012).
Figure 2: Removal Efficiency of a sump manhole with and without a SAFL Baffle (From Howard et
al. 2011)
Figure 3: Washout Performance of a sump manhole with and without a SAFL Baffle (From Howard
et al. 2011)
Figure 4: Washout Performance of a 6-ft diameter, 3-ft deep sump manhole with and without a
SAFL Baffle (From Howard et al. 2011)
Effects of Trash and Vegetation
Stormwater debris like trash and vegetation can affect all stormwater treatment devices. To understand
the effects of trash and vegetation on sump manholes equipped with the SAFL Baffle, a year of research
was conducted at St. Anthony Falls Laboratory. The research determined what makes up debris in
stormwater, and how will it affect the SAFL Baffle. Tests were completed by inundating a sump manhole
with debris, and measuring the effects on Removal Efficiency and Washout Performance (McIntire, et al.
2012).
Researchers concluded that sump manholes that are nearly as deep as they are in diameter will
experience no change in Washout Performance due to debris clogging. Sump manholes that are about
half as deep as they are in diameter will experience a decrease in Washout Performance due to debris
clogging. Figure 5 illustrates this point by showing two scale model sump manholes equipped with a
SAFL Baffle, inundated with debris, and the resulting washout of sediment. Both of the sump manholes
have depths equal to about half of their diameter, but the image on the left has a SAFL Baffle with hole
sizes equal to three inches, and the image on the right has a SAFL Baffle with hole sizes equal to five
inches. This indicates that the decrease of Washout Performance due to clogging on shallow sump
manholes can be mitigated by using a SAFL Baffle with larger hole sizes (McIntire et al. 2012).
Figures 6 and 7 show the effects of trash and vegetation on the Removal Efficiency and Washout
Performance of sump manholes equipped with SAFL Baffles. Figure 6 indicates that debris has little to no
effect on Removal Efficiency. Figure 7, however, shows that at high flow rates, clogging can create
washout of sediment. The results shown in Figure 7 indicate that deep sumps do not experience much
washout, even when clogged with debris. Shallow sumps, on the other hand, experience washout due to
clogging. By using a 5 inch hole diameter SAFL Baffle, this washout problem can be mitigated. The
results found during full scale testing match with the results found during scale model testing (McIntire, et
al. 2012).
Figure 5: Washout of sediment measured due to debris clogging the SAFL Baffle. (Left) A 3 inch
hole diameter SAFL Baffle installed in a sump manhole and (Right) a 5 inch hole diameter SAFL
Baffle installed in a sump manhole (From McIntire et al. 2012).
Figure 6: The effects of debris on Removal Efficiency. Debris has little to no effect on the Removal
Efficiency of sump equipped with a SAFL Baffle (From McIntire, et al. 2012).
Figure 7: Washout of sediment measured due to debris clogging the SAFL Baffle. Deep sump
manholes (ex. 6-ft diameter and 6-ft deep (6x6)) with a SAFL Baffle experience negligible washout,
and shallow sump manholes (ex. 6-ft diameter and 3-ft deep (6x3) with a SAFL Baffle experience
washout. Using a 5 inch hole diameter SAFL Baffle mitigates the washout problem for shallow
sump manholes (From McIntire et al. 2012).
90 Degree Outlet Sump Manholes
Not all sump manholes have an outlet pipe that is located 180 degrees to the inlet pipe. Some have outlet
pipes that are located 90 degrees to the inlet pipe (See Figure 8). Scale model tests were conducted at
St. Anthony Falls Laboratory to determine the optimum orientation of a SAFL Baffle in a 90 degree outlet
sump manhole. Next, Removal Efficiency and Washout Performance tests were conducted on a full
scale, 6-ft diameter by 6-ft deep sump manhole equipped with a SAFL Baffle oriented 113 degrees to the
inlet pipe.
Figure 8: A SAFL Baffle installed in a 90 degree outlet sump manhole at a 113 degree angle with
respect to the inlet pipe (From McIntire et al. 2012).
Scale model tests indicate that sump manholes with 90 degree outlet pipes will experience significant
washout during high flow rates. However, when a SAFL Baffle is installed at a 90 degree angle relative to
the inlet pipe, washout is negligible. Tests were completed at angles in between 90 and 180 degrees with
respect to the inlet pipe, under otherwise similar conditions. Figure 10 shows these scale model results.
The results indicate that washout of sediment is negligible when the SAFL Baffle is installed between 90
and 120 degrees.
Figure 9: The sediment bed after conducting a high flow rate Washout Performance test on a 90
degree outlet sump. (Left) Without a SAFL Baffle and (Right) with a SAFL Baffle installed at 90
degrees with respect to the inlet pipe (From McIntire et al. 2012).
Figure 10: Washout of sediment at high flow rates for a 90 degree outlet sump with a SAFL Baffle
installed at angles between 90 to 180 degrees with respect to the inlet pipe (From McIntire, et al.
2012).
Tests on a 6-ft diameter, 6-ft deep sump manhole were conducted with a SAFL Baffle installed at a 113
degree angle with respect to the inlet pipe. This is within the range of negligible washout as indicated by
the scale model testing. Figure 11 shows Removal Efficiency results, and indicates increased Removal
Efficiency when compared to a straight flow through sump manhole with a SAFL Baffle installed at a 90
degree angle with respect to the inlet pipe (also called Standard Sumps). Figure 12 shows the Washout
Performance results, and indicates that washout increases with flow rate. At a flow rate of 12 cfs, washout
is at a maximum of about 62 mg/L (McIntire et al. 2012).
Figure 11: Removal Efficiency results of a 6-ft diameter, 6-ft deep sump manhole with a 90 degree
outlet and a SAFL Baffle installed at a 113 degree angle with respect to the inlet pipe (McIntire et
al. 2012).
Figure 11: Washout Performance results of a 6-ft diameter, 6-ft deep sump manhole with a 90
degree outlet and a SAFL Baffle installed at a 113 degree angle with respect to the inlet pipe
(McIntire et al. 2012).
Sump Manholes with Inlet Pipes and Inlet Grates
Some sump manholes receive water from both an inlet pipe and an inlet grate from above. To know how
the inlet grate water will affect the Removal Efficiency and Washout Performance of the system, tests
were completed at St. Anthony Falls Laboratory. A test stand (see Figure 12) was built and included a 6-ft
diameter, 6-ft deep sump manhole equipped with a SAFL Baffle and a simulated road surface with an
inlet grate. Water could be sent through this system through the inlet pipe and the simulated road surface
simultaneously. The SAFL Baffle was installed traditionally, at a 90 degree angle with a respect to the
inlet pipe. The inlet grate was located such that half of it was upstream of the SAFL Baffle and half was
downstream. Removal Efficiency tests were completed by maintaining a constant inlet grate flow rate of
0.4 cfs through all of the tests, and varying the flow through the inlet pipe. Washout Performance tests
were completed by maintaining a constant inlet grate flow rate of 0.7 cfs through all of the tests, and
varying the flow rate through the inlet pipe.
Figure 12: A 3D rendering of the test setup used for testing a SAFL Baffle installed in a sump that
receives water from both an inlet pipe and an inlet grate (From McIntire et al. 2012).
Figure 13 shows the Removal Efficiency data for the inlet grate sump manhole testing. The results
indicate that this type of system will capture sediment as well as a Standard Sump manhole equipped
with a SAFL Baffle. However, if the flow through the inlet pipe was less than three times that through the
inlet grate, Removal Efficiency was decreased and was less than a Standard Sump manhole equipped
with a SAFL Baffle. The researchers theorized that water entering the sump through the inlet grate was
able to plunge deeper into the water below if flow rates through the inlet pipe were low. The plunging
reduced the ability of the sump & SAFL Baffle to capture sediment (McIntire, et al. 2012).
Figure 14 shows the Washout Performance data for the inlet grate sump manhole testing. The results
indicate that, if the inlet grate flow rate is held constant, washout decreases as the flow rate through the
inlet pipe increases. This matches with results found during the Removal Efficiency tests described
above. Water from the inlet grate plunges deeper into the sump when flows through the inlet pipe are low,
resulting in washout of sediment. Washout is negligible as long as the flow through the inlet pipe is three
times that of the flow through the inlet grate (McIntire, et al. 2012).
Figure 13: Removal Efficiency data from the inlet grate sump manhole (McIntire, et al. 2012).
Figure 14: Washout Performance data for the inlet grate sump manhole (McIntire, et al. 2012).
References
Howard, A., O. Mohseni, J.S. Gulliver, and H.G. Stefan. Assessment and Recommendations for the
Operation of Standard Sumps as Best Management Practice for Stormwater Treatment (Volume 1) (St.
Paul: Mn/DOT Research Services Report, Feb. 2011).
Howard, A., O. Mohseni, J.S. Gulliver, and H.G. Stefan. "SAFL Baffle Retrofit for Suspended Sediment
Removal In Storm Sewer Sumps," Water Research 45 (2011): 5895-5904.
McIntire, K., A. Howard, O. Mohseni, and J.S. Gulliver. Assessment and Recommendations for the
Operation of Standard Sumps as Best Management Practice for Stormwater Treatment (Volume 2) (St.
Paul: Mn/DOT Research Services Report, Feb. 2011).
Further Resources
http://www.dot.state.mn.us/research/TS/2011/201108.pdf
http://stormwater.safl.umn.edu/content/updates-december-2011
http://stormwater.safl.umn.edu/content/updates-december-2010