Interesting results in the design of KR-3:
Pond storage information
To develop the elevation storage curve, NRCS data was used. This data obtained from the NRCS
is listed in Table 1. By using this data, we determined the elevation storage values for KR-3 using an
Excel spreadsheet. The calculations and results are listed in the appendix (Table 1A). The same results
can be obtained by using SEDCAD, which automatically develops an elevation-storage curve given
elevation-area data. The elevation storage curve is shown in Figure 1.
Table 1. Elevation-area values for KR-3
Surface
Elevation Area
(ft)
(ac)
1092
0.14
1096
2.39
1100
11.39
1104
23.65
1108
36.50
1112
51.93
1116
66.12
1120
85.12
Figure 1. Elevation Storage Curve for Structure 6 (KR-3).
Structure Six (KR-3) Elevation Storage Curve
1000
900
800
Storage (ac-ft)
700
600
500
400
300
200
100
0
1092
1097
1102
1107
1112
1117
Elevation (ft)
To determine the volume of the sediment the NRCS Sediment Yield Curve, which is located in
the appendix (Figure 1A), was used. Land use curve three and a reservoir sediment trap efficiency of
98% were used, and the total sediment mass and volume were computed for a deposition period of 100
years. It was also assumed that 80% of the sediment is submerged and had a bulk density of 50 lb/ft3
and that 20% of the sediment was aerated and had a bulk density of 75 lb/ft3. With these assumptions
it was approximated that the bulk density was 60 lb/ft3. These calculations are listed in the appendix
(Table 4A). Our sediment storage for KR-3 was determined to be 117.71 ac-ft.
Spillway Design
The elevations corresponding to the crests of the principal spillway can be determined by using
the elevation-storage curve and the calculated sediment volume. The value found using this curve was
1104.7 ft. However, in our design the NRCS approximated value of 1108.5 ft was used, upon the
recommendation of an executive in our engineering firm.
The diameter of the principal spillway was designed by specifying an outflow of 100 cfs in
response to the 6.17 inch 100-yr, 24-hr rainfall event. Note that a further restriction in their design was
that the pipes needed to be in standard dimensions (six inch increments). This was determined by trial
and error in SEDCAD. The diameter of the riser was 48 inches and the diameter of the barrel was 30
inches.
The peak watershed surface elevation from the 6.17 inch 100-yr, 24-hr rainfall event
corresponds to the elevation of the emergency spillway. The crest of the emergency spillway elevation
was determined to be 1112.5 feet by simulating the event with SEDCAD.
Design of the emergency spillway was done using the 6 hour rainfall depth for a class C
structure. This is an NRCS design guideline. The calculation for this 6 hour rainfall depth is in the
appendix. This depth was used in a SEDCAD simulation to determine the peak water surface elevation.
This peak water elevation is used to design the emergency spillway embankment height. This elevation
was 1115.1 feet.
Elevation of the top of the dam was determined by using the probable maximum precipitation
for a 6 hour rainfall event. This rainfall depth was 23.7 inches. SEDCAD was again used to simulate the
event and the peak water surface elevation was used to determine the elevation of the dam. This
elevation was 1120.0 feet.
Generate a schematic
Hydrologic Impact
The peak outflow of the watershed was greatly reduced when the design of KR-3 was
implemented in a SEDCAD simulation. The peak outflow rate without the structure was 1829 cfs, and
once the dam was in place the peak outflow was 103 cfs. This is shown in Figure 2. This reduction will
help prevent flooding in Rochester, MN, and other nearby communities.
Figure 2. Outflow and Inflow at Structure 6.
Appendix
Calculations and Results for Elevation-Storage Curve:
Sample calculation:
𝑉𝑖 =
𝐴𝑖 + 𝐴𝑖−1
× (𝐸𝑙𝑖 − 𝐸𝑙𝑖−1 )
2
Calculations and Results for Structure 6 (KR-3):
Table 1A. Elevation-Storage Curve calculations for Structure 6 (KR-3).
Elevation
(ft)
Area
(ac)
1092
0.14
Structure 6 (KR-3)
Avg
ΔEL
Area
(feet)
(acre)
5.06
5.06
4
27.56
11.39
32.62
17.52
1104
4
2.39
6.89
1100
Storage
Vol
(acre-ft)
0
1.265
1096
Vol
(acre-ft)
4
70.08
23.65
102.7
30.075
4
120.3
1108
36.5
223
44.215
1112
4
51.93
399.86
59.025
1116
4
236.1
66.12
635.96
75.62
1120
176.86
4
302.48
85.12
938.44
Calculations and Results for Structure 1:
Table 2A. Elevation-Storage Curve calculations for Structure 1.
Elevation
(ft)
Area
(ac)
1150
0.96
Structure 1
Avg
ΔEL
Area
(feet)
(acre)
1170
1180
Storage
Vol
(acre-ft)
0
3.74
1160
Vol
(acre-ft)
10
37.4
6.52
37.4
12.805
10
128.05
27.97
10
279.7
19.09
165.45
36.85
445.15
Calculations and Results for Structure 3:
Table 3A. Elevation-Storage Curve calculations for Structure 3.
Elevation
(ft)
Area (ac)
1140
0.97
1150
12.12
Structure 3
Avg
ΔEL
Area
(feet)
(acre)
Storage
Vol
(acre-ft)
0
6.545
10
65.45
65.45
24.725
1160
Vol
(acre-ft)
37.33
Figure 1A. NRCS Sediment Yield Curve.
10
247.25
312.7
Calculations of sediment storage volume and mass:
𝑀𝑎𝑠𝑠 𝑑𝑒𝑝𝑜𝑠𝑖𝑡𝑒𝑑 = (𝐶𝑢𝑟𝑣𝑒 3 𝑣𝑎𝑙𝑢𝑒) × 𝑑𝑒𝑠𝑖𝑔𝑛 𝑙𝑖𝑓𝑒 × 𝐴𝑟𝑒𝑎 × 98%
𝑉𝑜𝑙𝑢𝑚𝑒 𝑑𝑒𝑝𝑜𝑠𝑖𝑡𝑒𝑑 =
𝑀𝑎𝑠𝑠 𝑑𝑒𝑝𝑜𝑠𝑖𝑡𝑒𝑑
𝑙𝑏
60𝑓𝑡
3
Table 4A. Sediment storage mass and volume, and principal spillway elevation.
Area
Structure (sq mi)
S1
0.21
S3
0.52
S6 (KR-3)
1.47
Curve 3
Value
Mass
Volume
P.S.
(tons/sq Deposited Deposited Elevation
mi/y)
(tons)
(ac-ft)
(ft)
1300
13477
10.31
1154.0
1100
28488
21.80
1144.5
1050
153825
117.71
1104.5
Calculations for the 6 hour rainfall depth for an NRCS class C structure:
𝐶𝑙𝑎𝑠𝑠 𝐶 𝑑𝑒𝑝𝑡ℎ = 𝑃100 + 0.26 × (𝑃𝑀𝑃 − 𝑃100 )
9.77 𝑖𝑛𝑐ℎ𝑒𝑠 = 4.88 𝑖𝑛𝑐ℎ𝑒𝑠 + 0.26 × (23.7 𝑖𝑛𝑐ℎ𝑒𝑠 − 4.88 𝑖𝑛𝑐ℎ𝑒𝑠)