Neuse_River_Overtopping_Flood_Study_Volume_III_Report

NEUSE RIVER BASINFLOOD STUDY
DAM OVERTOPPING STUDY
SUMMARY REPORT
VOLUME III
PREPARED BY:
NC Department of Enviromental QualityDivision of Energy, Mineral, and Land Resources
October 2020
Neuse River Basin
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Overtopping of embankment dams could lead to dangerous dam failures. The purpose of
the Neuse River Basin Dam Overtopping Flood Study, authorized in 2017, is to evaluate the
potential of overtopping failure for select dams, based on volume and height criteria, during
predetermined storm events (24‐hour durations with return periods of 25, 50, 100, 200 and 1,000
years). This study helps identify dams which pose a risk for a given storm; directly protects lives
through early preparation and/or evacuations; increases the resiliency of towns’ emergency
response, and reduces damages from flood events. NC Department of Environmental Quality’s
Division of Energy, Mineral and Land Resources (DEMLR) selected a total of 207 dams, from 15
NC counties, located within the Neuse River Basin to participate in the overtopping study. This
volume encompasses 50 dams. Dams chosen for the study have the largest potential downstream
impact during flood conditions and excluded structures that have marginal to negligible impacts.
Dams having a maximum storage capacity (at the dam crest) of greater than 100 acre‐feet were
included in the study. Additionally, impoundments with a structural height greater than or equal
to 25 feet and having a maximum storage capacity greater than or equal to 50 acre‐feet are
included in the study. The analyses presented in this report were conducted by staff of the Division
of Energy, Mineral, and Land Resources, and a subcontracted engineering consulting firm.
In order to evaluate the overtopping potential of the dams, simplified Hydrologic
Engineering Center – Hydrologic Modeling System (HEC-HMS) models were prepared for each
dam to estimate the hydrologic and hydraulic (H&H) performance of the dam. Data for the
analyses were gleaned from existing Dam Safety files, including as-built drawings, previous
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analyses, design reports, and inspections. When sufficient dam measurement data did not exist,
field surveys were performed to measure dam parameters necessary for the analyses. Watershed
parameters were estimated using StreamStats, a web-based tool designed by US Geologic Survey
(USGS). When necessary reservoir parameters were estimated using available historic data and
geospatial estimation from county LiDAR data. Precipitation was estimated using National
Oceanic and Atmospheric Administration (NOAA) Atlas 14 data. The selected precipitation
events include six frequency-based, 24-hour duration storms. The six precipitation events had
Annual Exceedance Probabilities (AEP) of 0.04, 0.02, 0.01, 0.005, 0.002, and 0.001. These
precipitation events are commonly referred by their average recurrence interval (25, 50, 100, 200,
500, and 1000 year return events, respectively). Using precipitation data and hydrologic runoff
calculations, as inputs for the HEC-HMS reservoir hydraulic routing, the maximum reservoir level
was estimated for each of the six hypothetical storm events.
Table 1 summarizes the HEC-HMS modeling results for 50 dams comprising Volume III
of the study. Table 2 shows additional dam information including name, location, structure height,
maximum impoundment capacity, hazard classification, the minimum spillway design flood
(SDF), and the flood event at which the dam initially experiences overtopping. In both Table 1
and 2, red cells indicate dam overtopping. The study dams range in structure height from 5.0 to
95.0 feet, in maximum impoundment capacity from approximately 20 to 33,300 acre-feet, and in
dam crest elevation from 72.5 to 653.7 feet, NAVD88 (Table 3). Table 4 shows the number of
dams in each potential hazard classification and each dam size classification. The Appendix
includes individual reports for each dam. Each report provides the dam watershed delineation,
watershed characteristics, input hydrology, dam outlet details, dam outlet rating curves,
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engineering assumptions, and results of the Hydrologic Engineering Center – Hydrologic
Modeling System (HEC-HMS) simulations.
Among the 50 dams presented in this report, HEC-HMS modeling shows 28 dams do not
experience overtopping under the study events (Figure 1). The remaining 22 dams experience
overtopping from at least one storm event (ranging from 24-hour 25-year return event to 24-hour
1000-year return event). The percentage of dams that experience overtopping during study flood
events varied by their potential hazards (low, intermediate, and high, as defined by 15A NCAC
02K.0105) and sizes (small, medium, and large, as defined by 15A NCAC 02K.0205). The
percentage of low, intermediate, and high potential hazard dams which experienced overtopping
during study flood event simulations was 35.0%, 81.8% and 36.8%, respectively. There are 57%
(16 out of 28) small-size; and 32% (6 out of 19) medium-size; and 33% (1 of 3) large-size dams
were predicted to overtop during the study events. Table 2 shows the regulatory minimum
spillway design flood (SDF) for the 50 dams ranges from a 50-year return period rainfall to the
probable maximum precipitation (PMP) event, according to the dam safety regulation 15A NCAC
02K.0205. However, 10 dams are predicted to start overtopping during the 24-hour 25-year
precipitation event (Figure 1), which means these dams are potentially hydrologically deficient in
passing the SDF.
The potential deficiency of the studied dams should sound an alarm to flood risk
management. Responding agencies will be able to identify measures needed for reducing
flooding, such as upgrading or modifying existing structures or building new structures.
Additionally, preventative practices such as drawdown of the dam’s reservoir level for at-risk
dams is recommended before a major storm event, rather than taking a reactive approach during
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the storm event. This study benefits not only the local community but also any residents that live
downstream or within the inundation area.
The reports for the 50 dams are available for download from the DEMLR Laserfiche site1.

1 : The Volume II report for Neuse River Basin Dam Overtopping Study could be downloaded through
this link through DEMLR Laserfiche site:
https://edocs.deq.nc.gov/EnergyMineralLandResources/Browse.aspx?dbid=0&startid=1310
Table 1: Neuse River Basin Overtopping Flood Study: HEC-HMS Modeling Results (Datum: NAVD88), the duration of storms in all the simulation are 24 hours.
25 50 100 200 500 1000 25 50 100 200 500 1000 25 50 100 200 500 1000
DURHA-046 5.95 6.66 7.38 8.12 9.12 9.91 364.1 359.6 361.3 361.3 362.2 363.4 364.3 -4.5 -3.6 -2.8 -1.9 -0.7 0.2
GRANV-004 5.94 6.65 7.37 8.11 9.12 9.91 283.0 285.2 285.7 286.1 286.6 287.3 287.8 2.2 2.7 3.1 3.6 4.3 4.8
GRANV-005 5.90 6.60 7.32 8.05 9.04 9.82 353.5 353.9 354.0 354.0 354.1 354.1 354.1 0.4 0.4 0.5 0.5 0.6 0.6
GREEN-002 7.25 8.53 9.97 11.60 14.10 16.30 80.0 78.9 79.4 79.8 80.2 80.6 80.9 -1.1 -0.6 -0.2 0.2 0.6 0.9
GREEN-003 7.26 8.55 9.99 11.60 14.10 16.40 102.2 99.9 100.4 100.9 101.3 101.9 102.4 -2.3 -1.8 -1.3 -0.9 -0.3 0.2
GREEN-004 6.58 7.67 8.87 10.20 12.10 13.80 104.5 99.1 99.6 100.0 100.4 100.9 101.2 -5.4 -5.0 -4.6 -4.2 -3.7 -3.3
GREEN-005 7.25 8.53 9.97 11.60 14.10 16.30 106.5 104.1 104.5 105.0 105.5 106.1 106.6 -2.4 -2.0 -1.5 -1.0 -0.4 0.1
JOHNS-003 6.28 7.13 8.01 8.93 10.20 11.20 210.4 210.0 210.5 210.7 210.9 211.0 211.1 -0.4 0.1 0.3 0.4 0.6 0.7
JOHNS-007 6.41 7.35 8.36 9.45 11.00 12.40 186.8 188.1 188.4 188.6 188.9 189.3 189.6 1.3 1.3 1.8 2.1 2.5 2.8
JOHNS-039 6.53 7.44 8.40 9.42 10.90 12.00 200.5 200.8 200.8 200.8 200.9 200.9 201.0 0.3 0.3 0.3 0.4 0.4 0.5
JOHNS-043 6.25 7.07 7.93 8.81 10.00 11.00 213.6 212.6 213.1 213.6 213.9 214.2 214.5 -1.1 -0.6 -0.1 0.3 0.6 0.9
JOHNS-079 6.78 7.81 8.92 10.10 11.90 13.40 170.4 167.7 167.8 167.9 168.1 168.2 168.4 -2.7 -2.6 -2.4 -2.3 -2.1 -2.0
JOHNS-086 6.34 7.23 8.17 9.17 10.60 11.80 301.6 300.2 300.4 300.6 300.9 301.0 301.2 -1.4 -1.2 -1.0 -0.7 -0.6 -0.4
JOHNS-093 6.64 7.62 8.66 9.78 11.40 12.70 190.3 189.5 189.6 189.8 190.0 190.2 190.3 -0.8 -0.7 -0.5 -0.3 -0.1 0.0
LENOI-005 7.36 8.66 10.10 11.80 14.30 16.50 82.7 82.8 83.0 83.1 83.3 83.5 83.6 0.2 0.3 0.5 0.6 0.8 0.9
LENOI-015 7.45 8.75 10.20 11.90 14.40 16.70 72.5 70.7 70.7 70.9 71.2 71.4 71.7 -1.8 -1.8 -1.6 -1.3 -1.1 -0.8
ORANG-005 5.88 6.58 7.29 8.02 9.00 9.77 625.0 617.2 617.6 618.1 618.5 619.1 619.5 -7.8 -7.4 -6.9 -6.5 -5.9 -5.5
ORANG-017 5.88 6.58 7.29 8.01 9.00 9.77 653.7 643.3 643.5 643.7 643.9 644.1 644.3 -10.4 -10.2 -10.0 -9.8 -9.6 -9.4
PITT-012 7.28 8.54 9.96 11.60 14.00 16.10 73.6 71.3 71.4 71.5 71.6 71.7 71.8 -2.2 -2.1 -2.0 -1.9 -1.8 -1.8
WAKE-010 6.11 6.91 7.74 8.61 9.82 10.80 348.8 348.0 348.1 348.2 348.3 348.4 348.5 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3
WAKE-014 5.88 6.61 7.35 8.10 9.13 9.93 265.4 260.0 260.6 261.2 261.7 262.4 262.9 -5.4 -4.8 -4.2 -3.7 -3.0 -2.5
WAKE-040 6.10 6.88 7.69 8.52 9.68 10.60 202.2 202.0 202.1 202.2 202.3 202.5 202.5 -0.2 0.0 0.1 0.2 0.3 0.4
WAKE-046 5.95 6.69 7.45 8.23 9.30 10.10 302.9 299.0 299.2 299.3 299.5 299.8 300.0 -3.9 -3.7 -3.9 -3.4 -3.1 -2.9
WAKE-053 6.10 6.88 7.68 8.51 9.65 10.60 309.0 310.4 310.9 311.3 311.7 312.3 312.7 1.4 1.9 2.3 2.7 3.3 3.7
WAKE-061 5.93 6.66 7.41 8.18 9.23 10.00 418.2 417.1 417.4 417.7 418.0 418.4 418.7 -1.1 -0.8 -0.5 -0.2 0.2 0.5
WAKE-072 6.25 7.16 8.13 9.17 10.70 11.90 257.0 255.0 255.2 255.4 255.6 255.9 256.1 -2.0 -1.8 -1.6 -1.4 -1.1 -0.9
WAKE-081 5.92 6.63 7.36 8.11 9.12 9.92 259.2 252.2 253.0 253.7 254.5 255.5 256.3 -7.0 -6.2 -5.5 -4.7 -3.7 -2.9
WAKE-118 5.93 6.67 7.42 8.19 9.24 10.10 379.0 375.5 375.6 375.7 375.7 375.8 375.9 -3.5 -3.4 -3.3 -3.3 -3.2 -3.1
WAKE-127 5.85 6.56 7.28 8.02 9.02 9.80 320.2 312.2 313.3 314.4 315.3 315.7 316.0 -8.0 -6.9 -5.8 -4.9 -4.5 -4.2
WAKE-132 5.87 6.59 7.32 8.07 9.09 9.89 409.7 409.8 410.0 410.1 410.2 410.3 410.4 0.1 0.3 0.4 0.5 0.6 0.6
WAKE-171 5.90 6.63 7.37 8.13 9.17 9.98 337.7 329.3 329.6 329.9 330.2 330.6 330.8 -8.4 -8.1 -7.8 -7.5 -7.1 -6.9
WAKE-176 5.96 6.71 7.47 8.26 9.33 10.20 303.2 298.2 298.7 299.1 299.6 300.1 300.6 -5.0 -4.5 -4.1 -3.6 -3.1 -2.6
WAKE-218 5.90 6.63 7.37 8.13 9.16 9.97 364.7 360.3 360.5 360.6 360.8 361.1 361.3 -4.4 -4.2 -4.1 -3.9 -3.6 -3.4
WAKE-228 5.96 6.71 7.46 8.24 9.31 10.10 440.5 439.3 439.6 439.8 440.0 440.3 440.5 -1.2 -0.9 -0.7 -0.5 -0.2 0.0
WAKE-317 5.90 6.63 7.37 8.13 9.17 9.98 417.2 415.0 415.2 415.3 415.5 415.7 416.0 -2.1 -2.0 -1.8 -1.7 -1.4 -1.2
WAKE-358 5.94 6.68 7.43 8.21 9.27 10.10 445.0 436.6 436.7 436.8 436.8 436.9 437.0 -8.4 -8.3 -8.2 -8.2 -8.1 -8.0
WAKE-362 5.93 6.67 7.42 8.19 9.24 10.10 441.6 439.3 439.4 439.5 439.6 439.7 439.7 -2.3 -2.2 -2.1 -2.0 -1.9 -1.9
WAYNE-003 7.14 8.40 9.82 11.40 13.90 16.00 109.2 106.1 106.9 107.7 108.6 109.5 109.7 -3.1 -2.3 -1.5 -0.6 0.3 0.5
WAYNE-009 7.07 8.27 9.61 11.10 13.40 15.40 81.5 79.5 79.6 79.7 79.8 79.9 80.1 -2.0 -1.9 -1.8 -1.7 -1.6 -1.4
WAYNE-012 7.24 8.52 9.97 11.60 14.10 16.30 118.2 112.2 113.6 114.5 114.8 115.1 115.4 -6.0 -4.6 -3.7 -3.4 -3.1 -2.8
WAYNE-021 7.28 8.56 10.00 11.70 14.20 16.40 99.6 96.7 96.9 97.1 97.3 97.6 97.8 -2.9 -2.7 -2.5 -2.3 -2.0 -1.8
WAYNE-022 7.09 8.31 9.67 11.20 13.50 15.50 88.3 84.3 84.6 85.0 85.3 85.8 86.2 -4.0 -3.7 -3.3 -3.0 -2.5 -2.1
WAYNE-023 7.33 8.62 10.10 11.70 14.20 16.40 80.3 77.9 78.2 78.5 78.9 79.3 79.6 -2.4 -2.1 -1.8 -1.5 -1.0 -0.7
WAYNE-028 7.18 8.41 9.78 11.30 13.70 15.70 137.4 136.1 136.3 136.4 136.5 136.8 137.0 -1.3 -1.1 -1.0 -0.9 -0.6 -0.4
WAYNE-031 7.02 8.19 9.50 11.00 13.20 15.10 78.7 78.7 78.7 78.7 78.8 78.8 78.8 0.0 0.0 0.0 0.1 0.1 0.1
WAYNE-032 7.03 8.21 9.53 11.00 13.20 15.20 78.7 78.4 78.6 78.7 78.7 78.7 78.8 -0.3 -0.1 0.0 0.0 0.0 0.1
WAYNE-033 7.03 8.21 9.52 11.00 13.20 15.10 78.5 77.1 77.4 77.8 78.1 78.5 78.5 -1.4 -1.1 -0.7 -0.4 0.0 0.0
WILSO-007 6.68 7.84 9.15 10.60 12.90 14.90 118.9 120.7 121.2 121.6 122.1 122.7 123.2 1.8 2.3 2.7 3.2 3.8 4.3
WILSO-009 6.62 7.76 9.05 10.50 12.70 14.60 135.6 136.6 137.0 137.4 137.8 138.3 138.7 1.0 1.4 1.8 2.2 2.7 3.1
WILSO-010 6.59 7.64 8.79 10.10 12.00 13.60 163.4 150.3 150.9 151.6 152.3 153.4 154.2 -13.1 -12.5 -11.8 -11.1 -10.0 -9.2
Dam State ID Dam Crest
Elevation, feet
Cumulative Precipitation, inches Peak Reservoir Elevation, feet Overtopping Depth, feet
Table 2: Supporting information for the dams in the Volume III report
DURHA-046 Little River Dam (36.1135, -78.8685)95.0 18000 High Large 3/4 PMP 24-hour 1-in-1000 year event
GRANV-004 Lake Rogers Dam (36.13, -78.705)20.0 900 Intermediate Medium 1/3 PMP 24-hour 1-in-25 year event
GRANV-005 Norwood Dam (36.1867, -78.6961)27.0 43 Intermediate Small 100 year 24-hour 1-in-25 year event
GREEN-002 Turnage Millpond Dam (35.414, -77.73)13.9 120 High Small 1/3 PMP 24-hour 1-in-200 year event
GREEN-003 Cobb Lake Dam (35.397, -77.798)16.2 900 Low Medium 100 year 24-hour 1-in-200 year event
GREEN-004 Phelps Lake Dam (35.384, -77.795)19.3 465 Low Small 50 year No overtopping under study Events
GREEN-005 Whitley Lake Dam (35.4048, -77.804)21.0 700 High Small 1/3 PMP 24-hour 1-in-1000 year event
JOHNS-003 Langdon Lake Dam #1 (35.5688, -78.5724)20.0 130 Low Small 50 year 24-hour 1-in-50 year event
JOHNS-007 Cattail Lake Dam (35.677, -78.274)17.0 940 Intermediate Medium 1/3 PMP 24-hour 1-in-25 year event
JOHNS-039 Charles Byrd Dam (35.434, -78.543)23.0 187 Intermediate Small 100 year 24-hour 1-in-25 year event
JOHNS-043 Lake Eva Marie (35.5756, -78.6239)18.0 0 Low Small 50 year 24-hour 1-in-25 year event
JOHNS-079 Lee Lake Dam (35.3431, -78.3878)28.0 67 Low Small 50 year No overtopping under study Events
JOHNS-086 Delmer Flowers Dam (35.668, -78.381)25.0 156 Low Small 50 year No overtopping under study Events
JOHNS-093 Temple Pond Dam (35.4322, -78.4431)25.0 0 Low Small 50 year 24-hour 1-in-100 year event
LENOI-005 Waters Millpond Dam (35.306, -77.76)15.0 200 Intermediate Small 100 year 24-hour 1-in-25 year event
LENOI-015 Neuse Regional Water Treatment Plant Dike (35.2485, -77.707)27.0 92 Intermediate Small 100 year No overtopping under study Events
ORANG-005 Lake Orange Dam (36.1486, -79.1714)40.0 1640 High Medium 1/2 PMP No overtopping under study Events
ORANG-017 Hillsborough Water Supply Dam (35.5376, -77.4876)64.8 24061 High Large 3/4 PMP No overtopping under study Events
PITT-012 Worthington Farms Dam (35.825, -78.457)17.1 142 Low Small 50 year No overtopping under study Events
WAKE-010 Robertson Lake Dam (35.84, -78.745)20.0 133 Low Small 50 year No overtopping under study Events
WAKE-014 Reedy Creek Lake Dam (35.802, -78.531)32.0 96 Low Small 50 year No overtopping under study Events
WAKE-040 Milburnie Fish Club Lake Dam (35.9566, -78.509)18.0 139 Intermediate Small 100 year 24-hour 1-in-200 year event
WAKE-046 Holding Lake Dam (35.652, -78.788)35.0 145 High Medium 1/2 PMP No overtopping under study Events
WAKE-053 Sunset Lake Dam (35.783, -78.805)25.0 750 High Medium 1/2 PMP 24-hour 1-in-1000 year event
WAKE-061 Edgehill Farm Dam (35.829, -78.263)28.0 81 Intermediate Small 100 year 24-hour 1-in-500 year event
WAKE-072 Privette Lake Dam #2 (36.025, -78.688)40.0 125 Low Medium 100 year No overtopping under study Events
WAKE-081 Beaverdam Creek Lake Dam (35.912, -78.597)40.0 33300 Low Large 1/3 PMP No overtopping under study Events
WAKE-118 EM Johnson Alum Sludge Lagoon Dam (35.8594, -78.8253)41.0 108.3 High Medium 1/2 PMP No overtopping under study Events
WAKE-127 Crabtree Creek W/S #1 (PL-566)(35.8959, -78.7114)33.0 480 High Small 1/3 PMP No overtopping under study Events
WAKE-132 Springdale Estates Lower Dam (35.943, -78.63)30.0 190 Intermediate Small 100 year 24-hour 1-in-25 year event
WAKE-171 Coachman Trail Lake Dam Lower (35.9708, -78.4894)35.0 93 High Medium 1/2 PMP No overtopping under study Events
WAKE-176 Wake Forest Water Supply Dam (35.9419, -78.6342)36.4 945 High Medium 1/2 PMP No overtopping under study Events
WAKE-218 Coachman Trail Lake Dam Upper (35.798, -78.717)37.0 180 High Medium 1/2 PMP No overtopping under study Events
WAKE-228 State Fair H & L Dam (35.804, -78.799)25.0 78 High Small 1/3 PMP 24-hour 1-in-1000 year event
WAKE-317 Gallop Dam (35.9111, -78.5964)26.0 42 Low Small 50 year No overtopping under study Events
WAKE-358 E.M. Johnson Water Plant B (35.9117, -78.5972)42.0 383 High Medium 1/2 PMP No overtopping under study Events
WAKE-362 E.M. Johnson Plant A Dam (35.457, -77.849)40.0 110 High Medium 1/2 PMP No overtopping under study Events
WAYNE-003 Wayne County Wildlife Pond Dam (35.381, -78.085)24.0 900 High Medium 1/2 PMP 24-hour 1-in-500 year event
WAYNE-009 H.F. Lee Power Station Cooling Lake Dam (35.388, -77.893)17.0 5446 High Medium 1/2 PMP No overtopping under study Events
WAYNE-012 Wills Pond (35.3661, -77.8539)19.0 768 Low Medium 100 year No overtopping under study Events
WAYNE-021 Lunker Lake (35.3791, -78.0698)26.0 125 Low Small 50 year No overtopping under study Events
WAYNE-022 H.F. Lee Active Ash Pond (35.236, -77.885)17.0 2720 High Medium 1/2 PMP No overtopping under study Events
WAYNE-023 Cliffs Of Neuse State Park (35.2331, -78.1209)36.6 125 Low Medium 100 year No overtopping under study Events
WAYNE-028 Doug Jernigan Farms Lagoon Dike (35.3813, -78.1102)18.0 615 Low Small 50 year No overtopping under study Events
WAYNE-031 H.F. Lee Ash Pond 1 (Inactive)(35.3828, -78.1041)5.0 460 Low Small 50 year 24-hour 1-in-25 year event
WAYNE-032 H. F. Lee Ash Pond 2 (Inactive)(35.3758, -78.1069)5.0 384 Low Small 50 year 24-hour 1-in-100 year event
WAYNE-033 H. F. Lee Ash Pond 3 (Inactive)(35.7889, -77.9206)5.0 453 Low Small 50 year 24-hour 1-in-500 year event
WILSO-007 Lake Wilson (35.802, -77.949)19.7 998 High Medium 1/2 PMP 24-hour 1-in-25 year event
WILSO-009 Silver Lake (35.691, -78.12)13.1 538 Intermediate Small 100 year 24-hour 1-in-25 year event
WILSO-010 Buckhorn Lake (35.6919, -78.1178)20.0 0 Intermediate Small 1/3 PMP No overtopping under study Events
Potential
Hazard Class
(according to
Dam Size
Regulatory
Minimum
Spillway Design
Begin OvertoppingDam State ID Dam Name Coordinates Structure
Height, ft
Maximum
Impoundment
Capacity, Acre-
Table 3 Statistical summary for dams included in the volume I, II, and III reports
Parameters
Report volumes I II III I II III I II III
Minimum 14.8 8.0 5.0 18 20 0 72.5 22.0 72.5
Mean 30.6 23.4 26.8 1372 12596 1993 239.3 281.3 239.3
Median 26.0 25.0 25.0 147 143 189 206.3 291.6 206.3
Maximum 92.0 92.0 95.0 18660 1128100 33300 653.7 530.1 653.7
Table 4 Total number of dams in different potential hazard classes and dam sizes, included in volume I, II, and III reports
Volume I Volume II Volume III Volume I, II,
& III
Very Large 0 1 0 1
Large 6 0 2 8
Medium 11 2 13 26
Small 11 31 4 46
Large 2 0 0 2
Medium 3 1 2 6
Small 6 15 9 30
Large 0 0 1 1
Medium 4 0 4 8
Small 23 41 15 79
Summary 66 91 50 207
Potential hazard classes
100 year
50 year
Regulatory minimum spillway
design flood (SDF)
High
PMP
3/4 PMP
1/2 PMP
1/3 PMP
Structure height (feet)Maximum impoundment capacity (Acre-Ft)
Dam sizes
Total number of dams in each category
Dam crest elevation (feet)
Intermediate
Low
1/2 PMP
1/3 PMP
100 year
1/3 PMP
28 dams (56%) would
not overtopping under
study events 10 dams (20%) start overtopping
under 25 year return storm
1 dams (2%) start overtopping under
50-year return storm
3 dams (6%) start
overtopping under 100-
year return storm
2 dams (4%) start overtopping under
200-year return storm
2 dams (4%) start overtopping under
500-year return storm
4 dam (8%) start overtopping under
1000-year return storm
22 dams (44%) would
start overtopping under
study events
Figure 1. Predicted dam overtopping status under study storm events. Note: a. All the storm events last for 24 hours. b. The 100-year storm is
the storm with a 1/100 or 1% chance of occurrence in any given year in tha dam area.
Total number of dams in the volume III
report is 50
22 dams would start overtopping under study events
6
7
2
0
3
3
10
2
4
6
8
10
12
14
16
18
Small Medium Large
Figure 2. Number of dams that is prediced to start overtopping under study events, categorized by the dam sizes (small,
medium, and large) and potential hazard classes (low, intermediate, and high)
Low Intermediate High
Number of dams