SW5200702_SCS Calculations - 1yr Peak_20200827

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SCS METHOD
Drainage Area A
Project Snow Hill Pump Station
Date 8/27/20
Name CEW
1. Runoff Curve Number (values from TR-55)
HSG
Cover description
CN
Area
CN x area
-
Impervious
98
3.14
307.7
B
Open Space, Good Condition
61
0.40
24.4
C
Open Space, Good Condition
74
0.78
57.7
B
Woods, Good Condition
55
0.00
0.0
C
lWoods, Good Condition
70
0.77
53.9
Totals
Weighted CN
5.09
443.7
87.2
2. Runoff
Q= (P - 0.2 S)Z / (P + 0.8S)
Rainfall, P (24-hr)
Runoff, Q
3. Graphical Peak Discharge Method
1. Data
Drainage area, Am
Runoff curve number, CN
Time of concentration, Tt
Rainfall distribution
Pond and swamp areas spread
throughout watershed
S = (1000/CN) -10
1-Year
inj 3.00
ini 1.75
mil (acres/640)
0.0080
(from step 1)
87.2
5-min minimum
0.17
(I, la, II, III)
II
(Table 4-2)
1
Storm 1
2. Frequency yr
3. Rainfall, P (24-hr)
4. Initial abstraction, la (Use CN with Table 4-1)
5. Compute la/P
6. Unit peak discharge, q„ (Use Tt and la/P with Exhibit 4-II)
7. Runoff, Q (From Step 2)
8. Pond and Swamp adjustment factor, Fp
9. Peak discharge, qp cfs
1
3.00
0.294
0.10
850
1.75
1
11.85
qp=quAmQFp
1.41
SCS Calculations.xlsx
8/27/2020

SCS METHOD
Drainage Area A
Project Snow Hill Pump Station
Date 8/27/20
Name CEW
1. Runoff Curve Number
HSG
Cover description
CN
Area
CN x area
-
Impervious
98
3.35
328.3
B
Open Space, Good Condition
61
0.40
24.4
C
Open Space, Good Condition
74
0.87
64.4
B
Woods, Good Condition
55
0.00
0.0
C
lWoods, Good Condition
70
0.63
44.1
Totals
Weighted CN
5.25
461.2
87.8
2. Runoff
Q= (P - 0.2 S)Z / (P + 0.8S)
Rainfall, P (24-hr)
Runoff, Q
3. Graphical Peak Discharge Method
1. Data
Drainage area, Am
Runoff curve number, CN
Time of concentration, Tt
Rainfall distribution
Pond and swamp areas spread
throughout watershed
S = (1000/CN) -10
1-Year
inj 3.00
ini 1.81
mil (acres/640)
0.0082
(from step 1)
87.8
TR-55 Spreadsheet
0.17
(I, la, II, III)
II
(Table 4-2)
1
Storm 1
2. Frequency yr
3. Rainfall, P (24-hr)
4. Initial abstraction, la (Use CN with Table 4-1)
5. Compute la/P
6. Unit peak discharge, q„ (Use Tt and la/P with Exhibit 4-II)
7. Runoff, Q (From Step 2)
8. Pond and Swamp adjustment factor, Fp
9. Peak discharge, qp cfs
1
3.00
0.286
0.10
850
1.81
1
12.59
qp=quAmQFp
Increase in Peak Discharge 0.704 cfs
6 /o
1.38
SCS Calculations.xlsx 8/27/2020

SCS METHOD
Drainage Area B
Project Snow Hill Pump Station
Date 8/27/20
Name CEW
1. Runoff Curve Number (values from TR-55)
HSG
Cover description
CN
Area
CN x area
-
Impervious
98
2.06
201.9
B
Open Space, Good Condition
61
2.44
148.8
C
Open Space, Good Condition
74
1.09
80.7
B
Woods, Good Condition
55
0.00
0.0
C
lWoods, Good Condition
70
4.72
330.4
Totals
Weighted CN
10.31
761.8
73.9
2. Runoff
Q= (P - 0.2 S)Z / (P + 0.8S)
Rainfall, P (24-hr)
Runoff, Q
3. Graphical Peak Discharge Method
1. Data
Drainage area, Am
Runoff curve number, CN
Time of concentration, Tt
Rainfall distribution
Pond and swamp areas spread
throughout watershed
S = (1000/CN) -10
1-Year
inj 3.00
ini 0.90
mil (acres/640)
0.0161
(from step 1)
73.9
5-min minimum
0.25
(I, la, II, III)
II
(Table 4-2)
1
Storm 1
2. Frequency yr
3. Rainfall, P (24-hr)
4. Initial abstraction, la (Use CN with Table 4-1)
5. Compute la/P
6. Unit peak discharge, q„ (Use Tt and la/P with Exhibit 4-II)
7. Runoff, Q (From Step 2)
8. Pond and Swamp adjustment factor, Fp
9. Peak discharge, qp cfs
1
3.00
0.707
0.24
660
0.90
1
9.59
qp=quAmQFp
3.53
SCS Calculations.xlsx
8/27/2020

SCS METHOD
Drainage Area B
Project Snow Hill Pump Station
Date 8/27/20
Name CEW
1. Runoff Curve Number
HSG
Cover description
CN
Area
CN x area
-
Impervious
98
2.46
240.6
B
Open Space, Good Condition
61
2.44
148.8
C
Open Space, Good Condition
74
1.19
88.0
C
Brush, good condition (replanted riparian areas)
65
1.33
86.5
C
lWoods, Good Condition
70
2.70
189.0
Totals
Weighted CN
10.11
752.9
74.4
2. Runoff
Q= (P - 0.2 S)Z / (P + 0.8S)
Rainfall, P (24-hr)
Runoff, Q
3. Graphical Peak Discharge Method
1. Data
Drainage area, Am
Runoff curve number, CN
Time of concentration, Tt
Rainfall distribution
Pond and swamp areas spread
throughout watershed
S = (1000/CN) -10
1-Year
inj 3.00
ini 0.93
mil (acres/640)
0.0158
(from step 1)
74.4
TR-55 Spreadsheet
0.25
(I, la, II, III)
II
(Table 4-2)
1
Storm 1
2. Frequency yr
3. Rainfall, P (24-hr)
4. Initial abstraction, la (Use CN with Table 4-1)
5. Compute la/P
6. Unit peak discharge, q„ (Use Tt and la/P with Exhibit 4-II)
7. Runoff, Q (From Step 2)
8. Pond and Swamp adjustment factor, Fp
9. Peak discharge, qp cfs
1
3.00
0.689
0.23
660
0.93
1
9.71
qp=quAmQFp
Increase in Peak Discharge 0.102 cfs /o 1
3.43
SCS Calculations.xlsx 8/27/2020