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1 Memo Date: Wednesday, April 11, 2018 Project: To: From: Subject: City Lake Dam Engineering Services Carl Brooks, P.E. David Wiseman, P.E. City Administrator s Report HDR Engineering has concluded the engineering services for the City Lake Dam evaluation. Final, sealed versions of the Hydrologic and Hydraulic Analysis and Downstream Flood Hazard Analysis Reports have been transmitted to the City for its use. The Hydrologic and Hydraulic Analysis identified principal and auxiliary spillway remedial alternatives to prevent overtopping the dam in critical frequency storm events. The Downstream Flood Hazard Analysis Report classified the Peculiar Reservoir currently in the low hazard class category as defined by the State of Missouri. The breach analysis does not indicate a significant damage hazard to residences, main highways or minor railroads, or risk to interruption of use or service of relatively important public utilities. Estimated costs for recommended spillway improvements were furnished to the City Engineer for comparison to repair work that could be contracted.

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3 List of Contents Introduction... 1 Project Information... 2 Project Location & Information... 2 FEMA Flood Insurance Rate Map... 4 Digital Elevation Model... 6 Existing Condition Discussion... 7 Hydrologic Analysis... 7 HEC-GeoHMS... 8 Watershed Delineation... 8 Curve Number... 9 Time of Concentration HEC-HMS Sub-Basin Characteristics Channel Routing Precipitation Depths and Temporal Distribution HEC-HMS Peak Flows Hydraulic Analysis Existing Conditions Cross Sections Manning s n Values Design and Performance Criteria for New Dams Proposed Alternative Improvements Alternative 1: Principal Spillway Capacity Designed for 100YR Event Alternative 2: Principal Spillway Capacity Designed for 25YR Event Alternative 3: Principal Spillway Capacity Designed for 10YR Event Alternative 4: Restore Dam and Spillway Crest to Original Design and Widen Spillway to Convey 25 YR Event Summary i

4 List of Figures Figure 1: Peculiar City Reservoir Project Location... 2 Figure 2: Map of Existing Dam Features... 3 Figure 3: Map of FEMA Zones A and AE for Wolf Creek... 5 Figure 4: Map of Publicly Available DEM Files Downloaded for this Project... 6 Figure 5: Peculiar Reservoir Contributing Watersheds and Rivers... 8 Figure 6: Curve Number Grid Figure 7: Longest Flow Paths for Contributing Watersheds Figure 8: HEC-HMS Model View Schematic Figure 9: HEC-RAS Geometry and Terrain for Peculiar Reservoir Figure 10: HEC-RAS Cross Sections Figure 11: Inline Structure Representing the Dam Crest and Spillways Figure 12: Overgrown Vegetation Blocking Spillway Figure 13: GeoRAS Bank Lines and Flow Paths Figure 14: Peculiar Dam Cross Section and Various Recurrence WSELs Figure 15: Peculiar Dam Profile with Multiple Return Interval Profiles Figure 16: Table 1-1 from TR-60, August 2011: Minimum Principal Spillway Hydrologic Criteria Figure 17: Table 2-2 from TR-60, August 2011: Minimum Auxiliary Spillway Hydrologic Criteria Figure 18: Proposed Alternative 1, widening the principle spillway to convey the freeboard hydrograph Figure 19: Stepped Concrete Armoring of Downstream Embankment (image courtesy of 29 Figure 20: Proposed Alternative 2, convey the 25 yr flood in the principle spillway and armor the downstream embankment slope Figure 21: Proposed Alternative 3, Designing Principal Spillway to the 10 YR Interval 31 Figure 22: Proposed Alternative 4, Restoring Dam and Spillway Crest Elevations and Widening Spillway to capture 25 YR Storm ii

5 List of Tables Table 1: Land Use and Curve Number Generation Table 2: TR-55 Curve Number Percentage Summary for Entire Project Area Table 3: Time of Concentrations Table 4: Sub-Basin Hydrologic Parameters used in HEC-HMS Table 5: NOAA Atlas 14 Precipitation Depths for Peculiar, MO Table 6: HEC-HMS Peak Flows for 1YR, 2YR, 5YR, 10YR, 25YR, 50YR, 100YR, and 500YR Intervals Table 7: Existing Conditions WSEL's at Face of Peculiar Dam Table 8: Proposed Alternative Summaries List of Appendices Appendix A: Copy of Missouri Department of Natural Resources Inspection Report of Peculiar Reservoir and Dam, Dated August 30, iii

6 Introduction HDR was hired by the City of Peculiar to evaluate the hydrologic and hydraulic capacity of the Peculiar City Reservoir. Two known overtopping s of the dam occurred during the summer months of 2017 which cause extensive damage. A dam safety inspection was performed by HDR and findings are presented in a separate technical memorandum. In general, the findings of the inspection indicated: The auxiliary spillway channel showed extensive signs of erosion and scouring with steep vertical cuts on the order of 8-10 feet. Construction debris including concrete and asphalt were scattered as debris in the auxiliary spillway channel. Accumulated debris including concrete and asphalt at the auxiliary channel exit. Undercutting of the principal chute spillway exit with signs of concrete deterioration and exposed rebar on both retaining walls and the sloping concrete. Erosion behind the left and right retaining walls of the principal chute spillway. This technical memorandum determines the frequency of storm that can be passed through the principal chute spillway and auxiliary vegetated spillways in addition to the frequency of storm that inundates the dam crest under existing conditions. Multiple alternate principal and auxiliary spillway remedial designs will be presented as potential solutions to safely pass a critical frequency storm event and prevent overtopping of the dam. 1

7 Project Information Project Location & Information The Peculiar City Reservoir is located approximately 1.5 miles south of Peculiar, MO along Missouri C, see Figure 1. Figure 1: Peculiar City Reservoir Project Location The Reservoir and spillways were designed in 1959 by Associated Municipal Engineers of Kansas City by Earle R. Beckner, P.E., and construction was assumed shortly thereafter. Record drawings indicate the Reservoir was constructed with clay soils and on the upstream face with rip rap shoreline protection from elevation ft. (NGVD29). The dam crest is approximately 12 ft. wide at the top and stretches 685 ft. in length. The height, measured from the dam crest to the lowest channel elevation on the downstream side of the principal spillway is approximately 30 per the record drawings. Embankments less than 35 feet tall are not considered regulatory dams by the State of Missouri. 2

8 Based on the record drawings a principal chute spillway is located on the south side of the dam and the crest control section consists of a 30 foot wide by 8 foot tall reinforced concrete with vertical side walls. The length of the chute from upstream toe to downstream tow is approximately 84 feet long and the control section is 48 feet long. Downstream erosion protection is minimal and consists of remnant rock rip rap mixed with concrete chunks. Figure 2 presents a map of the existing dam features including dam crest, principle, and auxiliary spillway. Figure 2: Map of Existing Dam Features Topographic survey was completed in the fall of 2017 and targeted critical dam components such as principal spillway, auxiliary spillway, dam crest, fore and back slopes, reservoir pool elevation, and downstream channel bottom. The information provided by the survey is used in the hydraulic analysis performed in HEC-RAS as well as to compare key reservoir components to the record drawings. For simplicity, the record drawing 3

9 elevations have been converted to NAVD88 elevations by adding 0.39 feet to the NGVD29 values. Significant discrepancies between record drawings and topographic survey information has been identified for a couple of major dam features. First, the dam crest design elevation is ft. (NAVD88) while the survey shows it generally around ft. (+/- 2 feet lower than the original design). Another discrepancy is the normal operating pool elevation which was designed to be ft. (NAVD88) but is surveyed at ft. (0.65 feet higher than the original design). City personnel believes the increase in the pool level to be due to work done on the spillway in the 1970 s which resulted in an increased normal pool elevation of the reservoir. Current dam configurations suggest there is roughly 4.4 ft. of storage between normal pool elevations and the dam crest versus the original design of 7 ft. The reduction in storage has significant hydraulic implications that are examined in the hydraulic analysis section. FEMA Flood Insurance Rate Map Federal Emergency Management Agency (FEMA) Flood Insurance Study (FIS) number 29037CV001B and 29037CV002B effective January 2, 2013 includes data for Wolf Creek from approximately 1,170 feet upstream of 233 rd Street (upstream) to approximately 1,300 feet downstream of Cowger Road. The limits of coverage may be viewed in Figure 3 which is a snapshot of the Flood Insurance Rate Map (FIRM). The FIRM shows Zone A classification for some segments upstream of Peculiar Reservoir. Zone A is defined by FEMA as the 1-percent-annual-chance floodplains that are determined by approximate methods. No base (1-percent-annual-chance) flood elevations (BFEs) or depths are shown within Zone A. Zone AE, flood insurance rate zone that corresponds to the 1-percent-annual-chance floodplains that are determined in the FIS report by detailed methods, are found along most of Wolf Creek (between Cowger Rd. and just upstream of Peculiar Reservoir). Peak discharges presented in the FIS were calculated using the USGS regression equations for urban areas and rural areas. USGS Regression equations are a quick way to estimate flows but have a wide error band and generally can be 20% to 30% higher or lower than the calculated flow Regression equations also only provide the peak flow and will not allow provide enough information to perform the reservoir storage calculations. For this technical memorandum, new flows using Soil Conservation Service (SCS) Curve Number (CN) method are developed and presented in the Hydrologic Analysis section. 4

10 Figure 3: Map of FEMA Zones A and AE for Wolf Creek 5

11 Digital Elevation Model Publicly available DEM data collected by the USGS dated 2006 was downloaded from the Missouri Spatial Data Information Service (MSDIS) to create the topographic mapping for project area not covered by topographic survey. Tiles (CASS_MO_DEM_1.2192m_UTM15N83_TIF_ D_03/04 and E_03/04) in CASS County, Missouri provided complete coverage of the design area (refer Figure 4) with the majority of the drainage area falling in tiles D-03 and D-04. Figure 4: Map of Publicly Available DEM Files Downloaded for this Project The DEM was transformed from Universal Transvers Mercator (UTM) Zone 15 into State Plan Missouri West Coordinate System, North American Datum (NAD) 1983 and the vertical datum is based on NAVD 88. It has a horizontal accuracy of 1.0 meter, vertical accuracy of 18.5 cm root mean square error (RMSE), and a point spacing of 1.4 meter. The DEM is used as the surface for Hydrologic and Hydraulic analyses that require surface data outside of the topographic limits. The topographic survey was limited primarily to the major dam features such as the spillways and crest and therefore was not integrated into the DEM. Cross sections and dam features are manually modified to reflect data provided in the topographic survey in areas within the topographic limits. 6

12 Existing Condition Discussion The Missouri Department of Natural Resources, Dam and Reservoir Safety Program performed a site visit on August 25, 2017 to inspect Peculiar City Reservoir Dam per the Cities request. They reported the dam is listed in the National Inventory of Dams (MO20305) as 28 feet high having a lake area of 27 acres and a watershed area of 2,420 acres. Since the embankment is less than 35 ft. in height, it is not subject to the permit requirements of chapters through of the Revised Statutes of Missouri. The following summarizes some of their major findings from the dam safety inspection and the entire document may be found in Appendix A. The principal spillway concrete apron has been damaged and erosion has been observed underneath the foundation. Extensive erosion damage in areas on the downstream face. Embankment is overgrown with excessive trees and woody vegetation. The auxiliary spillway on the north side is blocked by heavy growth of trees and brush that greatly reduces the spillway capacity. Inadequate spillways capacity, the dam was overtopped multiple times this summer. The training berm is not functioning as designed and low areas were observed on the dam crest next to the concrete chute wing walls. They emphasized that because the dam is less than 35 ft. high, it is exempt from the requirements of the Missouri dam safety laws. However, they explain the exemption does not protect the owner from legal liability should failure of the structure cause harm or property damage downstream. Hydrologic Analysis U.S. Army Corps of Engineers Hydraulic Modeling System (USACE HEC-HMS) version 4.2 build 1668 dated 12 Aug 2016 was selected as the modeling software for the hydrologic analysis. The Geospatial Hydrologic Modeling Extension (HEC-GeoHMS) uses ArcGIS and the Spatial Analyst extension to develop a number of hydrologic modeling inputs for HEC-HMS. By analyzing the digital terrain data, HEC-GeoHMS transforms the drainage paths and watershed boundaries into a hydrologic data structure that represents the drainage network. 7

13 HEC-GeoHMS HEC-GeoHMS generated watershed boundaries, river/stream reaches, and weighted curve numbers. Time of concentrations calculations were developed in an excel spreadsheet following TR-55 methodology. Figure 5 presents an overview of the watersheds and their hydrologic characteristics, the tools and methods used to develop the characteristics are discussed in detail in the following sections. Figure 5: Peculiar Reservoir Contributing Watersheds and Rivers Watershed Delineation Four watersheds were delineated using HEC-GeoHMS tools and the DEM. They are identified by the direction from which they contribute: SB-SW, SB-W, SB-N, and SB-NE. The majority of the contributing area comes from the west and north basins with the combined total of all four equaling 3.9 square miles. 8

14 Curve Number Design guidelines and suggestions found in the NRCS TR-55 Urban Hydrology for Small Watersheds and the NRCS National Engineering Handbook part 630, Chapter 4, were followed when developing composite curve numbers and time of concentrations. A curve number grid was generated for the contributing drainage areas and this was accomplished by merging soil information downloaded from the web soil survey and 2011 National Land Cover Database (NLCD) land cover. Table 1 presents the original 16-class land cover classifications that have been applied across the United States at a spatial resolutions of 30 meters as well as the associated TR-55 classifications and associative curve numbers based from soil class. The soils classes A, B, C, and D were defined from the web soil survey with the majority of the soils as class C and D for the contributing drainage areas. TR-55 assigns curve numbers based on hydrologic soil conditions (poor, fair, and good) which are defined as having cover less than 50% for poor condition, cover between 50% and 75% for fair condition, and cover greater than 75% for good conditions. Cover type hydrologic conditions have been determined through aerial imagery examination and applied to each cover type. The condition selected for each cover type is seen in the TR- 55 classification description in Table 1. A sensitivity analysis was performed for wooded/herbaceous land types to determine if curve number interpolation was sensitive to cover. Results indicate that raising the curve number to 100% cover (12.3% increase from 62.5% cover) only changed the total curve number by 0.3% thus showing the end product is not sensitive to varying wooded/herbaceous percent cover. Therefore, CN values were not interpolated between various cover densities and were left grouped into the poor, fair, and good conditions described above. 9

15 Table 1: Land Use and Curve Number Generation Original NCLD Classification A B C D TR-55 Classification Open Water Water Woody Wetlands Emergent herbaceous wetlands Woody Wetlands - Assumed to be saturated Emergent Herbaceous Wetlands - Assumed to be saturated Developed, open space Open Space - good Developed, low intensity Residential districts (2 acre) Developed medium intensity Residential districts (1 acre) Developed, high intensity Deciduous forest Evergreen forest Mixed forest Barren land Shrub/scrub Grassland/herbaceous Pasture/hay Cultivated crops Residential districts (1/2 acre) Other agricultural lands: Woods - fair Other agricultural lands: Woods - fair Other agricultural lands: Woods grass combination - fair Other agricultural lands: Pasture, grassland or range - poor Other agricultural lands: Pasture, grassland or range - poor Other agricultural lands: Pasture, grassland or range - poor Other agricultural lands: Pasture, grassland or range - good Cultivated Agricultural Lands: Row crop contoured - poor The curve number grid is visualized in a polygon shapefile presented in Figure 6. Notice that Harper Lake and Peculiar Reservoir are classified as water with a 100 curve number. It is assumed that there will be 100% runoff in these locations. Harper Lake is located upstream of Peculiar Reservoir. To be conservative, it is assumed Harper Lake would not 10

16 significantly reduce the peak flow reaching Peculiar Reservoir. This is a conservative approach and there is likely some amount of attenuation. Given the small amount of watershed it controls, it is not likely to alter the findings. Further, it is recommended a conservative approach is taken there is potential for future changes to the reservoir which could reduce any existing storage effects. Table 2 breaks down the percent of each TR-55 classification defined within the total project area. Notice, pasture and open space account for 73.5% of the total area. Table 2: TR-55 Curve Number Percentage Summary for Entire Project Area TR-55 Classification Percent Water 2.4% Open Space - good 19.7% Residential districts (2 acre) 9.4% Residential districts (1 acre) 1.2% Residential districts (1/2 acre) 0.1% Other agricultural lands: Woods - fair 1.9% Other agricultural lands: Pasture, grassland or range - poor 0.4% Other agricultural lands: Pasture, grassland or range - good 53.8% Cultivated Agricultural Lands: Row crop contoured - poor 11.0% Woody Wetlands - Assumed to be saturated 0.2% 11

17 Figure 6: Curve Number Grid Time of Concentration TR-55 defines travel time as the time it takes water to travel from one location to another in a watershed and it is a component of time of concentration which is the time for runoff to travel from the hydraulically most distant point of the watershed to the point of interest within the watershed. The time of concentration is the sum of sheet flow, shallow concentrated flow, and open channel flow. The longest flow paths used for the time of concentration calculations are presented in Figure 7 for each contributing watershed. 12

18 Figure 7: Longest Flow Paths for Contributing Watersheds SHEET FLOW Sheet flow is flow over plane surfaces whose Manning s n is an effective roughness coefficient that includes the effect of raindrop impact and drag over the plane surface for very shallow flow depths of about 0.1 foot or so. A Manning s n value of 0.24 (representing dense grasses) was selected for these watersheds. A distance of 300 ft. is assumed to be sheet flow and TR-55 recommends using Manning s kinematic solution (Overtop and Meadows 1976) to compute the sheet flow travel time, see Equation 1. The 2-Year 24 hour rainfall depth used was 3.64 in. with its source being NOAA Atlas 14 for Peculiar, MO. 13

19 Equation 1: Manning s Kinematic Solution for Sheet Flow TT tt(ssssssssss) = (nnnn)00.88 (PP 22 ) ss Where: TT tt(ssheeeeee) = sheet flow travel time nn = Mannings roughness coefficient LL = flow length (ft.) PP 2 = 2-year, 24-hour rainfall (in) ss = slope of hydraulic grade line (land slope, ft./ft.) SHALLOW CONCENTRATED FLOW Sheet flow usually becomes shallow concentrated flow after a maximum of 300 ft. in rural areas and in large residential acreages. This flow is concentrated but not yet in an open channel configuration. The time is calculated using the flow length divided by an average velocity that was estimated using the slope, Equation 2. The average velocity was estimated using National Engineering Handbook (NEH) Part 630 Chapter 15 Table 15-3 for short-grass pasture (V=6.962*s 0.5 ). Equation 2: Shallow Concentrated Flow Travel Time TT tt(ssssssssssssss cccccccccccccccccccccccc) = LL Where: TT tt(sshaaaaaaaaaa cccccccccccccccccccccccc) = shallow concentrated flow travel time LL = flow length (ft.) VV = average velocity (ft. /s) 3600 = conversion factor from seconds to hours OPEN CHANNEL FLOW Open channels are assumed to begin where channels are visible on aerial photographs, or USGS indicates streams, or where surveyed cross section information defines the channel. Channels were identified using a combination of USGS information and aerial imagery. TR-55 recommends determining open channel flow travel time using Equation 2 but requires the designer to estimate average velocity using Manning s equation. Estimating the average velocities can be iterative as the 2 year flow must first be estimated before the velocity can be calculated. 14

20 USGS regression equations for Region 1 as defined in Methods for Estimating Annual Exceedance-Probability Discharges and Largest Recorded Floods for Unregulated Streams in Rural Missouri were used to estimate the 2 year flow based off the longest flow path, area, and basin shape (longest flow path squared divided by drainage area). Velocities were then extracted from Manning s equation using cross sections cut through each reach. After the velocities were estimated, open channel flow time was calculated using Equation 2. Table 3 summarizes the sheet flow, shallow concentrated flow, and channel flow travel time for each contributing sub-basin. Time of concentration is the summation of all the travel times and is presented in the last column. Table 3: Time of Concentrations Watershed Sheet Flow Tt (hr) Shallow Concentrated Flow (hr) Channel Flow (hr) Time of Concentration SB-SW SB-W SB-N SB-NE HEC-HMS Pre-processing parameters drainage area, weighted curve number, channel cross sections, and time of concentrations computed in HEC-GeoHMS were exported to HEC- HMS for hydrologic computations. Figure 8 presents a schematic of the HEC-HMS model interface. 15

21 Figure 8: HEC-HMS Model View Schematic Sub-Basin Characteristics The SCS Curve Number loss method was utilized for modeling purposes, which requires NRCS Runoff Curve Number, initial abstraction, and impervious percentage for input parameters. The SCS unit hydrograph was selected for the transform method, which requires a lag time input. The initial abstraction was left blank, which defaults the program to 0.2*(CN/ ) and the initial abstraction was taken over the entire sub-basin area. In addition, the percent impervious input was left zero because the curve numbers selected accounts for pervious and impervious areas. Table 4 presents watershed characteristics that include the lag times, curve numbers, and drainage areas that were input into the HEC-HMS model. Table 4: Sub-Basin Hydrologic Parameters used in HEC-HMS Sub-Basin Lag Time (min) Curve Number Area (sq-mi) SB-SW SB-W SB-N SB-NE

22 Channel Routing There is minimal channel routing in the HEC-HMS model due to the sub-basin configuration. Most sub-basins discharge directly into Peculiar Reservoir and their routing was accounted for in the open channel travel time computations. However there is an exception for the west and southwest sub basins. The southwest sub-basin combines with the west sub-basin approximately 675 ft. upstream of Peculiar Reservoir. Muskingum- Cunge Eight Point channel routing was chosen to model the reach between the junction and Peculiar Reservoir. Precipitation Depths and Temporal Distribution Precipitation depths for the 1YR, 2YR, 5YR, 10YR, 25YR, 50YR, 100YR, and 500YR were obtained for Peculiar, MO (Latitude: , Longitude: ) via NOAA Atlas 14, Volume 8, Version 2 and are presented in Table 5. Spatial variation within the watershed was determined to be within 1/1,000 th and considered negligible for this analysis. Therefore, the precipitation values shown below are representative of the entire contributing area. A nested frequency storm was developed in accordance with NEH chapter 4 draft guidelines and a duration of 24 hours was selected as it is typical of the area. The nested frequency storm is also one of the more conservative methods. The HEC-HMS Technical Reference Manual (March 2000) references the World Meteorological organization (1994) when stating depth area reductions are not to be used with areas less than 9.6 square miles and therefore was not used in this hydrologic analysis. Table 5: NOAA Atlas 14 Precipitation Depths for Peculiar, MO Average Recurrence Interval (YR) Duration min min hr hr hr hr hr hr

23 HEC-HMS Peak Flows Utilizing HEC-GeoHMS and HEC-HMS, entire storm runoffs were calculated as the total inflow into Peculiar Reservoir. The 24 hour hydrographs with a time of increment of 5 minutes were copied into a.dss file and read into HEC-RAS for unsteady flow computations through the reservoir. Table 6 presents the peak inflows for multiple recurrence intervals at each sub-basin as well as the total inflow into Peculiar Reservoir. Table 6: HEC-HMS Peak Flows for 1YR, 2YR, 5YR, 10YR, 25YR, 50YR, 100YR, and 500YR Intervals Average Recurrence Interval (YR) Sub- Basin/Feature SB-SW ,154 SB-W ,268 1,685 2,022 2,368 3,213 SB-N ,100 1,323 1,553 2,120 SB-NE Total Inflow 1,000 1,380 2,060 2,680 3,560 4,280 5,030 6,870 Hydraulic Analysis The purpose of the hydraulic analysis is to determine the water surface elevation utilizing the flows determined within HEC-HMS. The hydraulic analysis evaluates the current reservoir storage and outlet works as well as several alternatives that would reduce the overtopping frequency. A breach analysis was also performed to aid in evaluating the downstream flooding hazards and the results are presented in a separate memorandum. The hydraulic model chosen for the analysis is the Hydrologic Engineering Center s River Analysis System (HEC-RAS) Version September 2016 in the one dimension unsteady flow routing mode because it efficiently is able to calculate the storage calculations, the downstream floodplain elevations and is able to perform the breach routing. HEC-GeoRAS is an ArcGIS extension specifically designed to process geospatial data for use with the HEC-RAS. Using HEC-GeoRAS geospatial tools, stream centerlines and cross sections were cut and processed against the DEM discussed previously. An HEC- RAS import file containing the aforementioned was created and imported into HEC-RAS. The field topographic survey was used to set the outlet works elevations and sizes. 18

24 Existing Conditions Analysis of the existing conditions will evaluate what recurrence interval the auxiliary spillway is activated and what interval the dam crest is overtopped. Other variables such as freeboard between any given interval the dam crest will be useful when evaluating the long term stability of the embankment. Cross Sections Figure 9 presents an image of the HEC-RAS cross section locations in relation to the total contributing watershed area. The upstream extents HEC-RAS model was developed to the extents of the sub-basin outfalls. The downstream extents of the HEC-RAS model extends slightly beyond the Zone AE study limits in the FIS. The downstream cross sections are utilized as part of the dam breach analysis. Figure 9: HEC-RAS Geometry and Terrain for Peculiar Reservoir Figure 10 presents a closer look at the cross sections and reach layout. In accordance with the HEC-RAS hydraulic reference manual, the dam is represented as an inline structure. 19

25 Figure 10: HEC-RAS Cross Sections Figure 11 presents inline structure cross section with the spillways and dam crest called out. The elevations of the inline structure are taken from topographic survey. The survey is given priority above the record drawings because they have been determined outdated due to modifications since construction. Figure 11: Inline Structure Representing the Dam Crest and Spillways Approximately 75% of the auxiliary spillway is blocked off in the model to represent existing blockage/roughness due to tree and vegetation debris in the spillway. Figure 12 is of a 20

26 picture taken looking upstream in the spillway, significant erosion and overgrown vegetation is observed. Figure 12: Overgrown Vegetation Blocking Spillway The cross sections were cut using the DEM as described in earlier sections and this introduces some inconsistencies with reality. First, the DEM data does not penetrate water which means the true channel invert is not available. Other discrepancies can arise from the accuracy of the DEM. It is possible for the DEM to completely miss the channel low point due to horizontal spacing of the shots. This can lead to downstream cross sections appearing to be higher than upstream ones. There are two problems with this, first, it is misrepresenting the actual physical characteristics of the channel and second, it causes extreme instability in the unsteady one dimension HEC-RAS model. To remedy this, cross section channel sections were slightly modified such that there are straight slope segments between adjacent sections. This was accomplished by slightly raising or lowering the existing channel inverts for sections outside of the reservoir. Cross sections downstream of the reservoir were modified using the built in channel design modification tool in HEC-RAS. This tool requires a template and a small one foot wide rectangular section was selected for the modifications of these sections. The primary purpose of this modification is for stability within the model. 21

27 Cross sections were modified within the channel reservoir using grading plans provided in record drawings. This modification will allow the separate dam breach analysis to account for the reservoir volume. For this analysis, permanent ineffective flow areas are set in the reservoir at the normal lake elevation of NAVD88. By doing this, the model assumes there is no available storage beneath the normal pool, thus only storage above the normal pool is considered available. Manning s n Values Manning s n values were selected in accordance with Table 3-1 in HEC-RAS 2.0 Reference Manual. Generally speaking, the overbanks consist of dense weeds, brush, and trees while the channel is expected to have a clean non-maintained channel bottom. Therefore to lean conservative, Manning s n values of 0.08 and 0.06 were selected for the overbanks and channel sections, respectively. However, Manning s n were reduced to for the channels cut through Peculiar Reservoir to simulate the runoff flowing over a water surface. Figure 13 presents the bank lines and flow paths created in GeoRAS. The overbanks described above are defined by the bank lines in the figure. Figure 13: GeoRAS Bank Lines and Flow Paths Figure 14 presents water surface elevations (WSELs) for multiple recurrence intervals plotted against Peculiar Dam cross section. 22

28 Figure 14: Peculiar Dam Cross Section and Various Recurrence WSELs Figure 15 presents a close up profile plot of Peculiar Dam and multiple return interval profiles. Figure 15: Peculiar Dam Profile with Multiple Return Interval Profiles Refer to Table 7 for WSELs at the face of Peculiar Dam for the 1YR, 2YR, 5YR, 10YR, 25YR, 50YR, 100YR, and 500 YR return intervals. Freeboard from the WSELs to the dam crest is also present in Table 7. 23

29 Table 7: Existing Conditions WSEL's at Face of Peculiar Dam Recurrence Interval (YR) Face of Dam (ft.) Freeboard from Dam Crest (ft.) Design and Performance Criteria for New Dams Because the dam is not regulated under the Missouri dam safety laws, there are not clearly defined design and performance criteria that apply to the site. Rather than strict design standards, references to other standards can be used to gauge customary levels of performance that have a long history of adequate performance. The first design standards considered were found in the National Resources Conservation Service (NRCS) Conservation Practice Standard POND Code No. 378 dated September 2015 (Pond Standards). The Pond Standards are applicable to small low hazard dams that meet the following criteria: The failure of the dam will not result in the loss of life, damage to homes, commercial or industrial buildings, main highways or railroads, or in interruption of the use or service of public utilities. The product of the storage times the effective height of the dam is less than 3,000 acre-feet 2. The effective height of the dam is 35 ft. or less. Failure of the dam is not anticipated to result in the loss of life or damages to habitable structures and the extents of a dam failure are examined in a breach analysis technical memo. The product of the storage times the effective height of the dam however is estimated to be roughly 7,000 acre-feet 2 which is larger than the Pond Standards allow. For this reason, HDR recommends the Pond Standards level of performance should be the 24

30 lowest level of performance considered and preference given to higher levels of performance. The next design publication of design practices considered is NRCS s Technical Release No. 60 Earth Dams and Reservoirs dated August Per Table 2-1 in the manual, a low class dam involving municipal water and a product of effective height and storage of less than 30,000 acre-feet 2 shall have a minimum principal spillway capacity of the 25 year recurrence interval. Also, the technical release requires a minimum auxiliary capacity of the 100 year recurrence interval. Figure 16 shows the Minimum Principal Spillway Hydrologic Criteria and Figure 17 shows the Minimum Auxiliary Spillway Hydrologic Criteria both of which are set forth by TR-60 dated August Per these tables, the principal spillway shall handle the 25 year storm and the auxiliary spillway shall be sized for the freeboard hydrograph, which is 11.2 inches for Cass County. Figure 16: Table 1-1 from TR-60, August 2011: Minimum Principal Spillway Hydrologic Criteria 25

31 Figure 17: Table 2-2 from TR-60, August 2011: Minimum Auxiliary Spillway Hydrologic Criteria Recall Missouri DNR stated this dam is not a regulatory structure due to effective height being less than 35 ft. The NRCS criteria (Pond Standards and TR-60) only applies to dams designed by the NRCS which this one was not. Therefore, the listed design criteria s may be considered as guidelines moving forward. The next section will present various alternatives that improve the existing dam s performance. Proposed Alternative Improvements All return intervals analyzed in the previous section exceed the principal spillways capacity and the auxiliary spillway capacity is exceeded somewhere between the 2 and 5 year intervals. The permanent pool elevation is only 1.2 feet below the auxiliary spillway crest and even minor storms would raise the lake elevation and begin flowing water down the vegetated spillway. Frequent inundation of a vegetated spillway will lead to erosion and concentration of flow which further exacerbates the rate of erosion. A vegetated spillway at the current elevation with the permanent pool at the current elevation is not viable. Spillway armoring would be needed to sustain the frequent inundation and armoring at the current elevation will essentially create a new principle spillway 1.2 feet higher than the current one. Two principle spillways would lead to more cost than a single spillway given the configuration of the dam and the receiving stream. All of the alternatives evaluated involved filling in the current auxiliary spillway and widening the principle spillway to convey large flows. Also, it is assumed the top of dam cannot be raised due to upstream land uses, roadway overtopping and lack of available ponding easements. 26

32 Three of the following alternatives consider leaving the principal spillway and dam crest elevations constant while widening the principal spillway to increase capacities. The last alternative explores restoring the dam and spillway crest elevations to their original design intent which increases the available storage in the lake but decreases the pool elevation. The results are summarized in Table 8 and each alternative is discussed in greater detail below. Table 8: Proposed Alternative Summaries Alternative #1 #2 #3 #4 Description Widen the Principal Spillway to Convey the Freeboard Hydrograph Defined in TR-60 Widen the Principal Spillway to Convey the 25 Year Flood Hydrograph and Armor the Downstream Embankment Slope Widen the Principal Spillway to Convey the 10 Year Flood Hydrograph and Armor the Downstream Embankment Slope Restore the Dam and Spillway Crest Elevations to Original Design and Widen the Spillway to Capture the 25 Year Flood Hydrograph Principal Spillway Width (ft.)

33 Alternative 1: Principal Spillway Capacity Designed for 100YR Event The first alternative examined involves modifying the principal chute spillway by widening it to increase the capacity such that it may pass the TR-60 recommended freeboard hydrograph (11.2 depth) without needing a separate auxillary spillway and without needing to raise the top of dam. The permanent pool elevation was not modified. In order to meet these requirements, the principal spillway was widened to a total width to 250 and the auxillary spillway filled in. Figure 18 shows a cross section of the dam and principal spillway configuration, notice the 100 year WSEL ( ft.) is less than the crest ( ft.). This alternative meets the design standards for low hazard class dams set forth in TR-60. This is also the most conservative alternative as the freeboard hydrograph is very similar to the 500 year hydrograph. Figure 18: Proposed Alternative 1, widening the principle spillway to convey the freeboard hydrograph. 28

34 Alternative 2: Principal Spillway Capacity Designed for 25YR Event Alternative 2 involves increasing the principal spillway capacity to contain the 25 year storm, filling in the auxiliary spillway and armoring the downstream face of dam to handle overflows. The dam downstream embankment could be graded to a flatter slope and possibly lined with turf reinforcement mat or a similar erosion control product/material. Alternatively, the downstream embankment could remain steep and lined with reinforced concrete steps. The principle spillway would need to be reinforced concrete and although the auxiliary spillway channel entrance has been removed, the remnant channel could be destabilized such that it can handle flows coming over the top of dam. One option involves armoring the downstream embankment with stepped concrete similar to that presented in Figure 19 Figure 19: Stepped Concrete Armoring of Downstream Embankment (image courtesy of 29

35 Under this scenario, the principal spillway would require widening to a minimum of 125. Figure 20 presents the dam and principal spillway internal section for this alternative, notice the 25 year WSEL is contained within the principal spillway. Figure 20: Proposed Alternative 2, convey the 25 yr flood in the principle spillway and armor the downstream embankment slope. 30

36 Alternative 3: Principal Spillway Capacity Designed for 10YR Event Alternative 3 involves increasing the principal spillway capacity to contain the 10 year storm and let everything above the 10 year crest the top of the dam which has been retrofitted to handle such overflows much like that presented in Alternative 2. Although this alternative does not meet the Pond Standards or TR-60 design criteria, it should be considered a significant improvement over existing conditions. Under this scenario, the principal spillway would require widening to a minimum of 80. Figure 21 presents the dam and principal spillway internal section for this alternative, notice the 10 year WSEL is contained within the principal spillway. Figure 21: Proposed Alternative 3, Designing Principal Spillway to the 10 YR Interval 31

37 Alternative 4: Restore Dam and Spillway Crest to Original Design and Widen Spillway to Convey 25 YR Event The record drawings show the top of dam elevation should have been ft. (NAVD88) which is 2.0 higher than the survey reported elevation ( NAVD88). Another difference is the principal spillway crest elevation and pool elevation. The record drawings indicate the crest should have been ft. which is 0.65 lower than the survey reported ( ft. NAVD88). In summary of the differences, the normal pool was raised 0.65 while the top of dam decreased 2.0 resulting is a large loss of storage. According to GIS data, the 920 contour is roughly 29 acres so it is anticipated that approximately (estimated from GIS data) 75 acft. of storage has been lost between the original design and present day configuration. In alternative 4, the dam and spillway crest elevations are restored to their original design intent while the principal spillway is widened in order to convey the 25 year storm (Figure 22). Under these conditions, the spillway only is required to be 42 ft. wide versus 125 ft. if the existing dam and spillway elevations are kept as Alternative 2 showed. Figure 22: Proposed Alternative 4, Restoring Dam and Spillway Crest Elevations and Widening Spillway to capture 25 YR Storm 32

38 Summary Recent overtopping events of Peculiar Reservoir Dam the City requested a hydrologic and hydraulic study of the dam. Using HEC-GeoHMS and HEC-HMS, four contributing subbasins were delineated and their flows determined. Hydrographs representing each of the four basins were input to an unsteady HEC-RAS model for hydraulic analysis of the reservoir pool and outlet works. Hydraulic analysis of the existing system indicates the principle concrete chute spillway plus the vegetated auxiliary spillway only has enough capacity for the two year recurrence intervals before the crest of the dam is overtopped Results agree well with recent observed overtopping events (two in summer 2017) and visible erosion and undercutting of the dam embankment and auxiliary spillway channel. Missouri Department of Natural Resources recently performed a site visit in response to the recent overtopping s and the City of Peculiar s request. They noted a number of flaws such as structural damage to the principal spillway, erosion behind and beneath the principal spillway foundation/walls, extensive erosion caused by overtopping of the embankment, auxiliary spillway is blocked by overgrown trees and vegetation, and inadequate spillway capacity to name a few of their listed discrepancies. The report also emphasized that the dam is less than 35 feet high which makes it exempt from the requirements of the Missouri dam safety laws but does not protect the owner from legal liability should failure of the structure cause harm or property damage downstream. Since the structure is not regulated under State criteria, other relevant design criteria are referenced for comparison to accepted standards. NRCS Pond No. 378 applies to low hazard dam less than 35 feet tall and as those whose product of effective height and available storage is less than 3,000 acre-feet 2. The Peculiar Reservoir meets the height limitation but is estimated to have roughly 7,500 acre-feet 2 of storage. Next, NRCS TR-60 Earth Dams and Reservoirs was referenced for design guidance and they indicate a low hazard dam design as being applicable for reservoirs with less than 30,000 acre-feet 2. Under a low hazard dam design standard, principal spillways should have a 25 year capacity with no flow through the auxiliary spillway. With both the principle spillway and auxiliary spillway flowing, it should be able to pass the 100 year flood without overtopping the dam. The State and NRCS design standards for new dams do not apply to this structure however they are useful guidance documents. Based on this guidance, the reservoir outlet works are greatly undersized. The frequency flows pass down the vegetated auxiliary spillway are practically problematic. Four alternatives are presented to increase the principle spillway and decrease the damage from overtopping the dam. 33

39 It is recommended to remove the auxiliary spillway entrance under most alternatives as there is only 1.2 ft. between it and the normal pool elevation. It would be better to have it removed than to have it activated multiple times per year. Potentially it may remain in service if the normal pool was lowered and the dam raised but more analysis would be required to verify it. If the principle spillway is not sized to carry the full 100 year event, steps can be taken to protect the downstream embankment and dam crest that will help stabilize it during overtopping. Stabilization methods could include turf reinforcement mats, rock-riprap, and concrete. Some armoring options will require the downstream slope to be flattened using a large volume of earth fill. The conclusions and recommendations in this report are based on the conditions of the project site and the associated watershed at the time of this study. Any modifications to the site, man-made or natural, could alter the analysis, findings, and recommendations contained herein and could invalidate the analysis, findings, and recommendations. Site conditions, completion of upstream or downstream projects, upstream or downstream land use changes, climate changes, vegetation changes, maintenance practice changes, or other factors may change over time. Additional analysis or updates may be required in the future as a result of these changes. Other alternatives that involve retrofitting the existing auxiliary spillway will be completed at a later time after discussing potential alternatives with the City. 34

40 Appendix A: Copy of Missouri Department of Natural Resources Inspection Report of Peculiar Reservoir and Dam, Dated August 30,

41

42

43

44 Contents Introduction... 1 Downstream Flood Hazard Analysis... 1 Overview of Dam Breach Model... 2 Flood Routing Analysis... 5 Cross Section Development... 6 Dam Breach Analysis Results... 8 Discussion Figures Figure 1: Froehlich's (2008) Regression Equations for Average Breach Width and Failure Time... 3 Figure 2: HEC-RAS Dam Breach Data... 5 Figure 3: Image Capture of Flood Insurance Rate Map at Wolf Creek Cowger Rd Crossing6 Figure 4: HEC-RAS Cross Section Extents... 7 Figure 5: Sunny Day and 100-YR Dam Breach Hydraulic Profiles... 8 Figure 6: Hydrograph Comparison for 100 YR Storm Breach and 100 YR Storm without Breach... 9 Figure 7: HEC-RAS Cross Section near FIS Cross Section H Tables Table 1: Froehlich (2008) Calculated Breach Parameters... 4 Table 2: FIS and HEC-RAS Peak Flow Comparison Table 3: FIS and HEC-RAS Water Surface Elevation at Lettered Cross Sections Exhibits Exhibit 1: Overview of Breach Inundation Limits, Covering FIS Cross Sections I-A Exhibit 2: Breach Inundation Limits, Cowger Rd. Close-up i

45 Introduction The Peculiar City Reservoir dam embankment was overtopped twice the summer of 2017 and has caused erosion of the auxiliary spillway and erosion of the dam embankment. A dam safety inspection performed by the Missouri Department of Natural Resources (MDNR) and identified several concerns over the current condition of the dam. The dam however is less than 35 feet tall which means it is not a regulated dam under the State of Missouri statues. The City of Peculiar hired HDR to perform a Hydrologic and Hydraulic Analysis (H&H Study) which discussed the existing reservoir storage and the capacity of the outlet facilities to convey flood events. The H&H analysis was in agreement with observations that the combination of the loss in storage and the undersize outlet works is causing frequent overtopping and the results are presented in a separate document. This report evaluates the potential inundation zone downstream of the dam in the event of a dam breach. The inundation zone will assist the City in evaluating the downstream flood hazards and assist in identifying the dam hazard class. Downstream Flood Hazard Analysis As mentioned previously, this embankment is below the height threshold of a regulatory dam within the State of Missouri. If this was a regulatory dam, a hazard class would be assigned based on the following definitions: a) Class I High Hazard: A Class I dam s downstream zone contains 10 or more permanent dwellings or any public building. b) Class II Significant Hazard: A Class II zone contains one to nine permanent dwellings or one or more industrial buildings, or one or more campgrounds with permanent water, sewer and electrical services. c) Class III Low-Hazard: A Class III dam is everything else that is not Class I or II. On regulatory dams, the State of Missouri indicates breach analysis should be used to evaluate the appropriate Hazard Class. Sunny day breach, dam failure with the reservoir at the dam crest, is the State standard per Engineering Analysis of Dams (MDNR Aug 1989). Since the facilities are undersized and frequent dam inundation occurs, a 100 year (also known as 1% annual chance) breach can be considered a worst case scenario for this breach analysis as the reservoir level will likely be higher than the Sunny Day approach. Breach analysis of storms in excess of 100 year were not considered because FEMA only publishes detailed information for the 100 year events and there would be no reliable way to compare results of larger occurrences. 1

46 Results of this dam breach analysis are to be considered approximate due to the uncertainty in hydrologic, hydraulic, and breach failure assumptions. No allowance is made for possible channel enlargement or temporary damming effects, both of which could affect the resulting magnitude and timing of flooding to some extent. An actual dam breach failure may produce varying results than predicted in this analysis. The intent of this study is to provide a basis for delineating the approximate inundation area from a breach and to evaluate potential risks to downstream structures, recognizing that specific characteristics of an actual dam failure may be different than those modeled. This memo presents the modeling methodology and results of the computer simulation for peak flood stage from a potential breach scenario. This information provides a basis for determining the potential inundated area downstream of the dam in the event of a breach compared to the FEMA regulatory 100-year flood extent. Overview of Dam Breach Model Analysis of the hypothetical dam breach scenario was conducted using the U.S. Army Corps of Engineers (USACE) HEC-RAS computer model (Version 5.0.3, September 2016). Input for the HEC-RAS computer model includes a description of the breach geometry and anticipated duration of the breach formation. As the breach develops, the contents of the basin become suspended and are released in the form of breach discharge. Hypothetical dam failures often use a sunny-day approach which calculates the outflow from a failure of the embankments when the water level is equal to the low point in the existing dam embankment (923.4 NAVD88) which follows the accepted practice for regulatory dams within the State of Missouri. This approach assumes there is no additional Stormwater runoff into the ponds or downstream of the ponds. Based on the lack of storage and the depth of embankment overtopping during the 100 year storm, a second dam breach analysis was performed. This second analysis started with the pool at normal elevation and then routes the 100 year runoff through the pond. The 100 year hydrographs were developed in a separate hydrologic analysis using a nested frequency storm of 24 hour duration. The breach is initiated during the peak of the 100 year event which is most conservative. This approach adds storage behind the reservoir as well as the remaining 100 year flow. Input parameters for a breach analysis on an earthen embankment include type of breach (overtopping or piping), when or at what elevation the breach begins, the expected growth rate of the breach, and the shape of the breach as it grows. The breach formation continues until either there is not enough water to continue the breach growth or the breach has reached resilient surfaces. For this evaluation the breach growth parameters 2

47 were based on Froehlich (2008) regression equations described in the USACE Training Document # 39, Using HEC-RAS for Dam Break Studies, dated August The Froehlich (2008) regression equations utilized 74 earthen, zoned, earthen, earthen with a core wall (i.e., clay), and rock fill data sets to predict average breach width, side slopes, and failure times. The height of dams in the study ranged from ( feet) with over 81% of the widths being less than 45 feet. The volume of water at the time of the breach in the study ranged from 11.3 acre feet to 535,000 acre feet. Figure 1 presents the regression equations as presented in the USACE training document. Figure 1: Froehlich's (2008) Regression Equations for Average Breach Width and Failure Time The height of the breach is defined as the difference in elevation of the dam crest to the bottom of the breach formation which in this case would be feet feet, or 25.4 feet. The bottom elevation of feet is the design elevation listed in record drawings. The available storage at the top of the dam is estimated to be 270 acre feet and is derived from a combination of grading identified in record drawings and 1 meter 2006 digital elevation model. The Froehlich (2008) equations are deemed appropriate as the parameters for Peculiar Reservoir fall within the ranged criteria of the study. Table 1 presents the calculated breach parameters of Froehlich (2008) regression equations. A breach formation time of 0.42 hours was calculated for the dam and the side slopes were assumed to be 1:1 for the breach shape, which resulted in an average width of 73 feet and a bottom width of 48 feet at the downstream flowline. 3

48 Table 1: Froehlich (2008) Calculated Breach Parameters Parameter B ave B final Value 73 ft. 48 ft. K V w h b 271 ac-ft ft. g 32.2 ft./s 2 t f 0.42 hr Figure 2 shows the HEC-RAS Dam Breach Data input window which includes the parameters selected for the breach analysis. One parameter not already discussed is the breach centerline station which was selected to be station 1200 because the breach area is fully captured by the upstream cross section and should be most conservative. A sensitivity analysis was performed for breach weir coefficients which are suggested to range between 2.6 and 3.3 for earthen clay dams. Results concluded a 15% change in weir coefficient (2.6 to 3.0) yielded an 11% change in peak flow during the breach. This relationships suggests the model to be sensitive to the breach weir coefficient. A coefficient of 3.0 was selected for this analysis as it splits the suggested range. 4

49 Figure 2: HEC-RAS Dam Breach Data Flood Routing Analysis Peak water surface elevations from the hypothetical dam breach were analyzed in HEC- RAS and ArcMap. The model was used to route the breach wave through the downstream reach of Wolf Creek. Generally, as the floodwave moves downstream, the peak flow is reduced as the valley is filled in with water. This peak flow attenuation is calculated within the model and the flows can be routed downstream to a point when the peak water surface elevation of the breach is less than the FEMA 100 year flood elevation. Figure 3 is an image of Flood Insurance Rate Map (FIRM) map number 29037C0162F (revised January 2, 2013) which shows the location near the Cowger Rd. and Wolf Creek crossing. There are two dwellings near the Cowger Rd. and Wolf Creek crossing, specifically one dwelling to the southwest and one to the northeast (both of which are called out in the image). 5

50 Figure 3: Image Capture of Flood Insurance Rate Map at Wolf Creek Cowger Rd Crossing The breach routing extended past the FEMA detailed study limits (AE limits). Downstream of the AE limits, a Zone A has been delineated by FEMA. Zone A is defined by FEMA as the 1-percent-annual-chance floodplains that are determined by approximate methods. No base (1-percent-annual-chance) flood elevations (BFEs) or depths are shown within Zone A. Since elevations are not available, the horizontal floodplain limits were used to determine if the breach was at or below the regulatory floodplain elevation. Cross Section Development Cross sections used for the breach analysis are shown on Figure 4 and were developed from a 2006 digital elevation model. In addition, HDR performed a field survey November 2017 to collect the top of road profile and the necessary culvert information for the modeling. Additional information on the cross section development and selected 6

51 parameters are detailed in the City of Peculiar Dam, Hydrologic and Hydraulic Analysis technical report. Figure 4: HEC-RAS Cross Section Extents 7

52 Dam Breach Analysis Results Water surface elevations were generated at each cross section in HEC-RAS for both the Sunny-Day and 100-YR breaches to evaluate which condition resulted in the highest downstream flood elevations. Figure 5 shows the Sunny-Day and 100-YR breach hydraulic profiles. The blue line represents the 100-YR breach and it is consistently higher than the Sunny-Day run by a range typically between 0.5 and 1 foot. Therefore, the downstream flood hazard analysis will move forward using the 100-YR breach run as the worst case scenario. 930 Peculiar_MO_Dam_Breach Plan: 1) DB-100YR 1/9/2018 2) DB-Sunny 1/9/2018 Wolf Creek Wolf Creek Lege nd WS Max WS - DB-100YR WS Max WS - DB-Sunny Ground Elevation (ft) End of Breac... FIS CX-A FIS CX-B Cowger Rd. Twin 12x8 RCB FIS CX-D FIS CX-E FIS CX-F FIS CX-G FIS CX-H FIS CX-I Peculiar Dam Spillway and Crest Main Channel Dis tance (ft) Figure 5: Sunny Day and 100-YR Dam Breach Hydraulic Profiles 8

53 Figure 6 presents a hydrograph comparison between the 100 year storm with and without activating the dam breach option. The peak flow with the breach is 18,800 cfs and without the breach is 4,470 cfs, a difference of 14,300 cfs. The reason for the increase in discharge with a breach is that the stored water (approximately 400 ac-feet) is released suddenly and adds the 100 year runoff. Figure 6: Hydrograph Comparison for 100 YR Storm Breach and 100 YR Storm without Breach 9

54 The FIS used regional regression equations to develop 100-YR peak flows and they reported flows at the base of Peculiar Dam and at Cowger Rd. (near the end of Zone AE limits). Table 2 compares the flows used in the FIS and those produced in HEC-RAS during the dam breach 100-YR run. Notice the 100-YR hydrograph without breach (4,740 cfs) is higher than the FIS peak flow (3,040 cfs). The difference is due to the hydrology methods used, rural regression (FIS) versus SCS Runoff Curve Number (HEC-RAS). The flows developed for this study are assumed to be more representative of the physical characteristics of the Peculiar Dam contributing drainage area than the regression equations. Table 2: FIS and HEC-RAS Peak Flow Comparison 100-YEAR Peak Q (cfs) Location FIS Dam Breach At Cowger Road 3,590 15,000 At base of Peculiar Dam 3,040 18,800 Table 3 presents comparisons between the FIS reported lettered cross section 1% water surface elevations and the HEC-RAS breach water surface elevations at corresponding cross sections. The last column presents the difference in water surface elevation with positive numbers representing the breach being higher than the FIS. The model reports a 30 minute delay between the start of the breach formation and the peak flow reported at Cowger Rd. This delay/warning time should be incorporated into emergency action planning in anticipation of a breach. 10

55 Table 3: FIS and HEC-RAS Water Surface Elevation at Lettered Cross Sections FIS Cross Section HEC-RAS CX FIS 1% WSEL (feet NAVD88) RAS 1% Breach WSEL (ft. NAVD88) Difference (feet) NA/Zone A* A B C D E F G H I *Breach does not tie into Zone AE study limits. Breach ran until it tied into the horizontal Zone A limits. The indicated FIS 1% WSEL is estimated based on where the Zone A limits ends compared to digital elevation model topographic data. Exhibit 1 and Exhibit 2 present the breach analysis inundation mapping limits plotted against the Flood Insurance Rate Map (FIRM). Exhibit 1 is an overview and covers the entire study area while Exhibit 2 zooms into the Cowger Rd. crossing and calls out the two nearby farmsteads/residences. The orange fill on the outside of the FIRM 1% limits represent the areas inundated by the breach in excess of the regulatory floodplain limits. Many sections along Wolf Creek between Peculiar Dam and Cowger Rd. have steep side slopes which means large changes in elevation may not translate to large changes in horizontal limits when comparing FIS and breach WSELs. Figure 7 shows cross section (near FIS lettered cross section H) which illustrates the steep side slopes. 11

56 Peculiar_MO_Dam_Breach Plan: Peculiar_DAM_Breach_100YR 1/9/2018 River = Wolf Creek Reach = Wolf Cr eek RS = Lege nd WS Max WS Ground Bank Sta Elevation (ft) Station (ft) Figure 7: HEC-RAS Cross Section near FIS Cross Section H The FIS Zone AE study ended approximately 1,000 feet downstream of Cowger Rd. near HEC-RAS cross section Referring to Table 3 above, the WSEL at HEC-RAS cross section 6640 is 0.9 feet higher than that reported for FIS lettered cross section A. The breach analysis was extended downstream past Zone AE into Zone A until the breach WSEL is determined less than the FIS Zone A limits. Zone A does not report lettered cross sections so the floodplain limits were compared to breach limits. Based on this comparison, the breach analysis is terminated at HEC-RAS cross section 3511 as it is estimated to be lower than the estimated Zone A WSEL. Runoff in excess of the twin 12 x8 RCB capacities at the Cowger Rd. crossing will traverse south and pool in a flat field until it eventually raises high enough to cross the road over an existing low point. The model indicates frequent overtopping which could activate during storms equal to or in excess of the 5 year. A county representative estimated the average daily traffic (ADT) to be 132 which is low and should not cause significant disruption if forced to close do to inundation. Discussion The purpose of the Peculiar Reservoir dam breach analysis is to identify locations that a breach will result in a water surface greater than the FEMA regulatory floodplain. The analysis will help determine if the increase in flood stage caused by a potential breach would impact structures or residences that are above the floodplain. 12

57 There are two existing farmsteads with residences located just northeast and southwest of the Cowger Rd. culvert, both locations are called out in Exhibit 1 and Exhibit 2. The farmstead on the northeast is on a bluff which is well above the breach inundation limits and the farmstead on the southwest resides on a peninsula that is roughly two to three feet above the surrounding breach and FIS Zone AE and A inundation limits. The breach analysis indicates the breach water surface elevation (WSEL) to be roughly 2.5 feet higher at Cowger Rd. than the 1% WSELs published in the FIS. However, results do not indicate inundation of the existing farmsteads/residences. The north farmstead should still have access to and from the north part of Cowger Rd. with the road being closed shortly south of their driveway. The farmstead to the south is shown to be completely cut off from Cowger Rd. access and is surrounded by flooded areas. Likewise, the floodplain extents published in the FIRM also show the property surrounded by flooding and cut off from Cowger Rd. access. The breach analysis shows the inundation limits further up their driveway with more of the land under water but it does not indicate inundation of any physical property. Although the water surface elevation is greater as a result of a breach, significant changes in conditions between the breach and the 100-year are not anticipated at this residence. Per TR-60 and State of Missouri definitions, Peculiar Reservoir currently fits the low hazard class category. Low hazard dams are defined as dams located in rural or agricultural area where failure may damage farm buildings, agricultural land, or township county roads. The breach analysis does not indicate a significant damage hazard to residences, main highways or minor railroads, or cause interruption of use or service of relatively important public utilities. The conclusions and recommendations in this report are based on the conditions of the project site and the associated watershed at the time of this study. Any modifications to the site, man-made or natural, could alter the analysis, findings, and recommendations contained herein and could invalidate the analysis, findings, and recommendations. Site conditions, completion of upstream or downstream projects, upstream or downstream land use changes, climate changes, vegetation changes, maintenance practice changes, or other factors may change over time. Additional analysis or updates may be required in the future as a result of these changes. 13

58 Exhibit 1: Overview of Breach Inundation Limits, Covering FIS Cross Sections I-A Exhibit 2: Breach Inundation Limits, Cowger Rd. Close-up 14

59 COWGER RD E. 233RD ST E. 237TH ST PECULIAR RESERVOIR DAM PECULIAR RESERVOIR FIS CROSS SECTION A, END OF ZONE AE STUDY WOLF CREEK EXISTING FARMSTEAD/RESIDENCE 3 HEC-RAS CROSS SECTION 3511, END OF BREACH ANALYSIS LIMITS 7352 EXISTING FARMSTEAD/RESIDENCE WOLF CREEK Esri, HERE, DeLorme, MapmyIndia, OpenStreetMap contributors, and the GIS user community, Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, 2830 CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community PECULIAR RESERVOIR DAM O 0 PATH: C:\USERS\NPARKS\DOCUMENTS\PROJECTS\PECULIAR_RAS\GEO-RAS\PECULIAR_MO_GEORAS.MXD - USER: NPARKS - DATE: 1/9/ COWGER RD. XSCutLines 100YR_INUN_Weir3_0_Clip Legend Miles 0.2 BREACH ANALYSIS INUNDATION LIMITS EXHIBIT 1: FIS CROSS SECTIONS I-A

60 EXISTING FARMSTEAD/RESIDENCE COWGER RD COWGER RD Legend 8028 EXISTING FARMSTEAD/RESIDENCE XSCutLines YR_INUN_Weir3_0_Clip Esri, HERE, DeLorme, MapmyIndia, OpenStreetMap contributors, and the GIS user community, Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community PECULIAR RESERVOIR DAM O 0 PATH: C:\USERS\NPARKS\DOCUMENTS\PROJECTS\PECULIAR_RAS\GEO-RAS\PECULIAR_MO_GEORAS.MXD - USER: NPARKS - DATE: 1/9/2018 Feet 320 BREACH ANALYSIS INUNDATION LIMITS EXHIBIT 2: COWGER RD CLOSE UP

61 GPR Investigation of Concrete Slab Prepared For: HDR Prepared By: Terry Jeffries Project Manager-Kansas City 12/16/2017

62 December 16 th, 2017 HDR Attn: Scott Brand Site: Peculiar Spillway We appreciate the opportunity to provide this report for our work completed on 12/6/17 at the above site in Peculiar, MO. PURPOSE The purpose of the project was to identify any problematic areas that could be found using the rebar deterioration mapping process and to identify the presence of any voids. EQUIPMENT 1600 MHz GPR Antenna. The antenna is approximately 6 x9 and rolls over the surface. The antenna needs a reasonably smooth, unobstructed surface for scanning so we would not be able to scan within 3 of obstructions such as walls and metal tracks unless they are removed prior to our work. The data is displayed on a screen during the scanning and marked on the surface in real time. GPR works by sending pulses of energy into a material and recording the strength and the time required for the return of the reflected signal. Reflections are produced when the energy pulses enter into a material with different electrical properties from the material it left. The strength of the reflection is determined by the contrast in signal speed between the two materials. The total depth achieved can be as much as 18 or more with this antenna but can vary widely depending on the conductivity of the materials and other factors such as the spacing of the reinforcing. No harmful radiation is emitted and the work can be performed at any time with people in close proximity. For more information, please visit: Link PROCESS Our process begins with collecting scans with GPR up the spillway in equally spaced intervals. The scans were collected at 2 intervals. The data from our scan was then sent to our office for processing and interpretation. LIMITATIONS Please keep in mind that there are limitations to any subsurface investigation. The equipment may not achieve maximum effectiveness due to conditions in the concrete or soil such as moisture content, age of the concrete, reinforcing size and spacing, and a variety of other factors. No subsurface investigation or equipment can provide a complete image of what lies below. Our results should always be used in conjunction with as many methods as possible such as consulting existing plans and drawings, visual inspection of above ground features, drilling or cutting as far as possible from all of our markings, etc. FINDINGS The processed data concluded that welded wire fabric (wwf) was present, not rebar. Due to this a deterioration mapping would not work. The depths need to be consistent for that type of investigation. Deeper depths will give lower amplitude which will appear to be deterioration. Between the wwf, and multiple depth changes the deterioration mapping was not an option. With the data we collected we were able to identify areas where possible voids are present which could be of concern. The following pages will provide photos and further explanation of our findings. Page 1 of 7

63 Photo 1- Image from the bottom of the spillway looking up. Photo 2- Image from the top of the spill way looking down. Photo 3- Image of the concrete present at the bottom of the spillway. Photo 4- Image of the concrete present at the top of the spillway. Photo 5- Visible holes and cracking in the spillway. Photo 6- Photo taken inside of one of the holes present on the spillway. GPR Data Screenshots and Photos Peculiar Spillway Page 2 of 7

64 GPR data screenshot- The depth scale is on the left and the distance of the scan is across the top, forming a cross section view of the subsurface. The red dots I indicate areas where voids are present. Peculiar Spillway Page 3 of 7

65 GPR data screenshot- The depth scale is on the left and the distance of the scan is across the top, forming a cross section view of the subsurface. The red dots indicate areas where voids are present. Peculiar Spillway Page 4 of 7

66 GPR data screenshot- The depth scale is on the left and the distance of the scan is across the top, forming a cross section view of the subsurface. The red dots indicate areas where voids are present. Peculiar Spillway Page 5 of 7

67 Y X The above image is an overlaid map of the combined data shots within the spillway. The vertical axis is labeled as y- axis and the horizontal axis is labeled as x-axis. The areas with the red are areas where voids were present. Peculiar Spillway Page 6 of 7