Ash Hollow Dry Dam Table of Contents

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2 Ash Hollow Dry Dam Table of Contents Page No. Executive Summary... ES 1 SECTION 1 Project Background & Purpose Introduction Purpose... 1 Figure 1-1 FEMA Regulatory Floodplain... 2 Figure 1-2 Location Map... 3 SECTION 2 Environs Evaluation Collection & Use of Best Available Data... 4 SECTION 3 Regulatory Issues Review Preliminary Functional Stream Assessment Preliminary Wetlands Analysis Department of Natural Resources Lancaster County Coordination... 6 SECTION 4 Preliminary Dam Design Introduction Proposed Dam Location Dam-Design Hydrology Reservoir Storage Capacity... 7 Figure 4-1 Elevation vs. Surface Area Plot... 7 Figure 4-2 Elevation vs. Storage Volume Principal Spillway Preliminary Design Embankment & Auxiliary Spillway Preliminary Design Dry Dam Detention-Storage Evaluation Figure 4-3 Computed Reservoir Storage Duration Figure 4-4 Rainfall (Inches) vs. Storage (Days) Figure Year Peak Timing (East Tributary vs. Reservoir) Figure Year Peak Timing (East Tributary vs. Reservoir) Preliminary Geotechnical Design Preliminary Geotechnical Evaluation Future Geotechnical Design Considerations Preliminary Hydrogeologic Evaluation Preliminary Hydrogeologic Evaluation for Dry Dam Figure 4-7 Cross Sections Map Figure 4-8 Cross Sections Map... 17

3 Figure 4-9 Cross Sections Map Future Hydrogeological Considerations Concept Evaluation of Potential for Future Dam/Reservoir Modification Amberly Road Ditch Reversal Evaluation SECTION 5 Preliminary Economic Evaluations Existing Flooding & Average Annual Damages Table 5-1 Existing Conditions Structural Damages Table 5-2 Existing Conditions Damages Table 5-3 Existing Conditions Annual Damages Post Project Flooding & Average Annual Damages Figure 5-1 Post Project Inundation Map Preliminary Opinion of Probable Costs Table 5-4 Opinion of Probable Development Costs Preliminary Benefit/Cost Evaluations Table 5-5 Cash Flow Stream Potential Funding Assistance Appendices Appendix A Preliminary Design Plans Appendix B Memo Ash Hollow Dry Dam Layout Refinement

4 EXECUTIVE SUMMARY The 2010 City of Waverly Watershed Master Plan identified improvements to reduce flooding damages along Ash Hollow Creek. The drainage basin for Ash Hollow includes approximately 3370 acres, including urban and agricultural land uses. It is approximately 5.3 miles long and drains an area from Prairie Home south of Interstate 80 through Waverly and into Salt Creek to the north of Waverly. The highest priority proposed in the master plan is to construct a dry dam upstream of Interstate 80, south of Waverly. The Lower Platte South Natural Resources District and City of Waverly have decided to undertake a preliminary design for this top priority flood control project. The preliminary design of Ash Hollow Dry Dam includes evaluation of the potential project utilizing best available information. The environs evaluation of this project included compiling available information and performing a site assessment of potential conflicts. From an environmental permitting perspective, steam and wetland impacts from this project should be minor enough to fall under current limits for a Nationwide Permit through the US Army Corps of Engineers. Comprehensive and master plan evaluations show that the future sanitary sewer alignment will need to be considered if the dry dam were to be constructed. Other elements of the comprehensive and master plans would not be impacted by this project. Four centerline locations upstream of Interstate 80 were analyzed to find the optimum location for the dam. The selected location, just upstream of McKelvie Road, was chosen for a number of reasons. It has minimal environmental impacts, lower embankment fill volumes, doesn t significantly back water through the N. 148 th Street culvert and has potential for a future permanent pool modification. This location also accommodates a dam embankment and spillway layout that avoids impact to the center pivot travel path on nearby irrigated farm land. Based on the preliminary design evaluation, hydraulically a dry dam could be constructed. A small permanent pool would be unavoidable; however the permanent pool would be within the Ash Hollow channel banks. The preliminary dam centerline selected provides a good opportunity for a future permanent pool; however detailed design and analysis would need to be completed if future dam/reservoir modifications were to be considered. The initial preliminary hydrogeologic evaluations indicate that there would not be impacts to groundwater for the proposed dry dam. Based on these preliminary design evaluations, a preliminary plan set was developed; see Appendix A. Land requirements for the proposed site and structure include approximately 141 acres. Of this total, approximately 30 acres of land will be impacted for construction and land rights for the dam and auxiliary spillway area. Around111 acres will be required for flood pool land rights. The preliminary economics evaluation utilized the Nebraska Resources Development Fund Guidelines, to determine the current damages to structures and contents inundated in various storm events. Only areas inundated by Ash Hollow Creek were considered. Areas determined in the Watershed Master Plan to be affected by Salt Creek and End Run backwater were not included in the calculation of flood damages. A preliminary post-project inundation map was developed using the updated hydrologic and hydraulic analysis of Ash ES - 1

5 Hollow with the dry dam in place. The preliminary inundation map shows that areas used in the economics evaluation (that currently are subject to flood damage from Ash Hollow and End Run) would be outside of the 100-year flood area if the dry dam was in place. This means that previously calculated damage amounts can be counted as flood reduction benefits. A preliminary opinion of probable costs was determined and compared to the benefits. The total anticipated project costs including construction, land rights, feasibility, engineering and inspection are estimated at approximately $2,785,000. The total estimated annual benefits are around $234,500. The estimated annual operation, maintenance and repair costs are approximately $11,000. Over a 50-year life cycle the total benefit to cost ratio was computed to be 3.34:1, with a rate of return of 7.40%. In summary, the findings of the preliminary design and study report indicate that this project would provide great flood reduction benefits to the City of Waverly. The next steps in the process, should this project move forward, would be to: Prepare a Conditional Letter of Map Revision to submit to FEMA. Identify funding sources and develop an application. Prepare a detailed geotechnical evaluation. Obtain a detailed topographical survey of the dam embankment and spillway area. Prepare detailed ground water study. Prepare final design plans and specifications. Obtain land rights and permits. Bid and construct the project. ES - 2

6 SECTION 1 Project Background & Purpose 1.1 Introduction The Lower Platte South Natural Resources District (LPSNRD) and City of Waverly (City) developed a Watershed Master Plan in The primary catalyst for these planning efforts was the 2008 release of the FEMA Preliminary Flood Insurance Study. Updated FEMA hydrologic and hydraulic modeling shows that there were significant increases in the Base Flood Elevations through the City. As such, a large portion of Waverly is within the regulatory floodplain, as shown in Figure 1-1. The master plan identified a system of improvements to manage and reduce flood damages caused by drainage through Waverly. The LPSNRD and City selected the highest priority proposed in the Master Plan for preliminary design. This flood reduction measure, identified as Priority Project 1 Ash Hollow Dry Dam, includes construction of a dry dam across what the FEMA study refers to as Ash Hollow Creek near the southwest edge of Waverly (Figure 1-2). The project also includes a design evaluation for potential improvements along Amberly Road. 1.2 Purpose The project goal is to complete a preliminary design of a dry dam, based on the use of available data. During the preliminary design process, the potential for a dam with a future permanent pool was studied. A secondary goal is to determine if revising the grade and direction of flow in the south ditch of Amberly Road has an impact on reducing flooding. The purpose of this summary report is to document the preliminary design process, the preliminary economic evaluations, and evaluation of potential funding options. 1

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9 SECTION 2 Environs Evaluation 2.1 Collection & Use of Best Available Data The first step in the process was to perform an environs evaluation which included on-theground observation, gathering best available existing information, and building a database. Geotechnical information was compiled from previous projects with similar soil types, as well as, information from the Lincoln Water System transmission main project along Interstate 80. Existing geologic profiles and ground water data in and around the project location was compiled. Much of Ash Hollow Creek channel was walked, and the National Wetland Inventory Maps were reviewed to identify potential environmental concerns. The Lancaster County GIS dataset was utilized to gather property information. The City provided building elevation certificates, utility plans, the comprehensive development plan, future land use plan, comprehensive sewer plan, and water study. Current LIDAR topographic information was utilized, along with, published FEMA Flood Insurance Study and Flood Insurance Rate Maps, and hydraulic models. Available bridge plans were acquired from Nebraska Department of Roads and the Lancaster County Engineer. In addition to the available information, our survey crew obtained elevations on several critical roadway culverts and commercial buildings. All of this information was gathered in order to complete the preliminary design. The comprehensive plans and studies were reviewed to identify any potential conflicts that this project may cause with the long range plans for the City. According to the Future Land Use Plan, areas directly downstream of the proposed dry dam are identified for parks and recreation use. Areas between McKelvie Road and Interstate 80 are identified as low density and mixed density residential. Based on the dam site selected, these uses won t change. In order to serve future property in and around this project area, the 2005 Comprehensive Sewer Plan identifies a future sanitary sewer system running through Waverly along the Ash Hollow Creek, under Interstate 80 and continuing south. It is important to note that this sewer has yet to be designed. In general, the construction of a dry dam would alter the alignment of the future sanitary sewer through this section. Ideally, sanitary sewers are located along draws to save on depth. The sewer main would have to be designed to go around the dam embankment which would add length and depth to the construction. This may require tunneling for a portion of the future trunk sewer. The future sewer main alignment could potentially run along the flood pool contour or parallel to Alvo Road and N. 148 th Street to avoid the dam embankment. A sewer alignment study may be necessary during the final design stages. The 2005 Water Study indicates no future conflicts with proposed dam location and planned well locations or transmission lines. SECTION 3 Regulatory Issues Review 3.1 Preliminary Functional Stream Assessment The draft Nebraska Stream Assessment Protocol (NSAP) methodology being developed by the United States Army Corps of Engineers Omaha District Nebraska Regulatory Office (Corps) was used to quantify functions of the proposed impact site. The Corps determined scores that are to be assigned based on different existing and proposed conditions. A preliminary evaluation of existing conditions (Pre-Project) and proposed conditions (Post- Project) was conducted using the above protocol to determine if there would be a net loss of 4

10 aquatic functions, based on the preliminary site walk-through. Initial indications are that there will likely be a minimal effect to the stream function. Excessive erosion occurring within the drainageway is due to the apparent straightening of the channel (which increased the associated gradient), the undulating topography in the watershed, the incised drainage itself, and the cultivation of crops within five feet of the drainage. Several ancillary drainages (farmed drainages) through the adjacent agricultural fields direct surface flow into the channel. As the agricultural fields appear to be tilled, surface flow likely moves soil to the drainage, which negatively affects the water quality. 3.2 Preliminary Wetland Analysis The proposed placement of the dry dam will impact approximately 250 linear feet of existing stream channel. The preliminary walk through, conducted on January 31, 2012, found no evidence of wetlands along the top of bank or buffer, which was approximately five feet wide, and found only minimal hydrophytic vegetation within the drainage. It is recommended that delineation be conducted, prior to final design, during the approved growing season, which is from May 1 st through October 31 st. Several braided drainages through the agricultural fields were checked and no wetlands were associated with these either. There was evidence of sloughing in the upper portion of the stream and these sloughed areas were the primary locations of hydrophytic growth. These areas appeared to have Phalaris arundinacea (reed canary grass) growing and some dead vegetation, likely Conium maculatum (poison hemlock), from the previous year. The site is devoid of wetland habitat to the south of the proposed dam. Fringe wetlands appear to be growing within the drainage as it gets closer to Interstate 80 (I-80). The forested area immediately south of I-80 does not appear to be a forested wetland. Based on the existing flow paths, there appears to be no potential for change in surface runoff to areas of potential saline wetlands located northwest of Waverly. Runoff that currently overflows out of the Ash Hollow Creek banks to End Run eventually flows back into Ash Hollow Creek at a confluence point just north of Cornhusker Highway. This confluence location is upstream of the potential saline wetland areas; therefore, the route the runoff takes to get to the confluence point should cause no impact. 3.3 Department of Natural Resources Construction of a dam will require approval from the Nebraska Department of Natural Resources (DNR) Dam Safety Division. In general, DNR requires that all dams located within the zoning jurisdiction of cities be classified as high-hazard potential dams. The proposed dry dam will need to be designed to comply with high-hazard potential dam criteria. Final design plans and specifications for the dam will need to be approved by DNR before construction could proceed. A storage permit would also be required from DNR if permanent storage of water would be 15 acre-feet or more. As part of the preliminary design process, a Pre-Design Conference was held with the DNR Dam Safety Engineers. They did not see any reason, at this preliminary stage in the design process, that the project couldn t meet safe dam design criteria. 5

11 3.4 Lancaster County Coordination The Lancaster County Engineer was contacted to review the preliminary design of this project and discuss potential impacts to the county right-of-way. Initial conversations about potential vacation of McKelvie Road right-of-way indicated that it should not be a problem; however, future development may require a road through that general area. Also, the county did not see any concerns that would prevent the construction of the proposed dry dam. They did; however, indicate that a future permanent pool could cause concerns due to potential saturation of the 148 th Street roadway embankment. SECTION 4 - Preliminary Dam Design 4.1 Introduction Four potential dam-centerline locations were evaluated before the proposed dam location was selected for the preliminary design. Those centerline options were described in the March 15, 2012 Memo, included in Appendix B. The Ctl B dam alignment described in that Memo was the preferred alignment at that time. 4.2 Proposed Dam Location The three-square-mile drainage area to be controlled by the dam is depicted on Sheet 1. The proposed location for the dam and its auxiliary spillway is shown in greater detail on Sheet 2, along with the approximate boundaries of the flood-pool area upstream of the dam that would be inundated by a 500-year frequency storm event. To avoid a conflict between the dam s auxiliary spillway and two existing pivot irrigation systems, the west end of the Ctl B alignment for the dam was rotated about 15 degrees counter-clockwise to arrive at the preliminary-design dam location shown on the plan sheets. The preliminary design for Ash Hollow Dam is illustrated by the six preliminary plan sheets for the dam that are included in Appendix A of this summary report. 4.3 Dam-Design Hydrology The DNR Rules for the Safety of Dams and Reservoirs classifies dams in accordance with their hazard potential. Hazard potential refers to the degree of adverse consequences from a failure or mis-operation of a dam and does not reflect on the condition of the dam. Dams located such that a failure or mis-operation of the dam could result in the probable loss of human life are classified as high-hazard potential. Ash Hollow Dam, located above a City and a major highway, is classified as high-hazard potential. Such dams must be designed, constructed, maintained to safely contain (and/or pass through spillways) the runoff from a probable maximum precipitation (PMP) storm event without the dam being overtopped. For the City of Waverly vicinity, such a storm event would produce 23.8 inches of rainfall in 24 hours. In addition, a high-hazard potential dam must be designed so runoff from at least a 100-year storm event can be contained without the detention storage level rising above the crest elevation of the auxiliary spillway. For the City of Waverly vicinity, such a storm event would produce 6.7 inches of rainfall in 24 hours, or 10.5 inches of rainfall in ten days. 6

12 Due to its close proximity to the City of Waverly, rather than the minimum 100-year requirement, the preliminary design for Ash Hollow Dam has been based on containing runoff from a 500-year storm event (8.2 inches of rainfall in 24 hours) before flow through the auxiliary spillway would occur. See Sheet 1 of the preliminary plans for a summary of the hydrologic information used for the preliminary design of Ash Hollow Dam. 4.4 Reservoir Storage Capacity The storage reservoir of a dam is the volume of upstream space, below the top-of-dam elevation, that could potentially be filled with water. For the proposed dam location, the stage-storage relationship between the elevation contours in the reservoir area of the proposed dam and the volume of water that could be stored is indicated by the Preliminary Reservoir Capacity Table on Sheet 1, in Appendix A. The graph in Figure 4-1, below, is a plot of the elevation versus surface area relationship indicated in the Preliminary Reservoir Capacity Table on Sheet 1. Figure 4-2 is a plot of the elevation versus storage volume. 200 Elevation vs. Surface Area Surface Area (Acres) Elevation VS Surface Area Elevation (Feet) Figure 4-1 Elevation vs. Surface Area Plot 7

13 Elevation vs. Storage Volume Storage Volume (Acre-Feet) Elevation VS Storage Volume Elevation (Feet) Figure 4-2 Elevation vs. Storage Volume 4.5 Principal Spillway Preliminary Design Earthen dams typically have a principal spillway (usually a pipe conduit) that serves as the low-elevation path for release of water from the reservoir, and an auxiliary spillway (commonly an excavated, vegetated-earth overflow at one end of the dam) that serves as the high-elevation release path to prevent dam overtopping during extremely intense storm events. Since Ash Hollow Dam is intended to have a normally dry reservoir, the principal spillway conduit needs to be as low in the site terrain as possible, while still hydraulically operating properly. On Sheet 3 of the preliminary plans, the principal spillway conduit is proposed to be installed within the existing creek channel in order to drain the storage reservoir area down as low as can be workable for proper hydraulic operation. Before the principal spillway conduit can be installed at the location shown, that portion of the creek channel will need to be excavated several feet below the existing creek flow line and all sides of the excavation sloped back to a stable configuration. Once that creek area has been excavated to provide a sound foundation for construction, controlled embankment fill material will need to be placed and compacted to provide a stable subgrade for the principal spillway construction. A profile view of the preliminary design for the principal spillway is shown on Sheet 5. A 30- inch diameter principal spillway conduit is proposed (that is the smallest principal spillway 8

14 size allowed for a high-hazard potential dam). To avoid scour degradation of the downstream creek channel, an impact basin structure is planned at the outlet end of the principal spillway conduit to provide energy dissipation before water is released to the creek. For the principal spillway and impact basin to hydraulically operate properly, the outlet flow line of the principal spillway conduit cannot be as low as the creek channel flow line. For this dam and channel configuration, the outlet flow-line of a 30-inch diameter spillway needs to be at least 1.5 feet above the creek channel flow line. This is necessary to avoid the principal spillway conduit outlet being unacceptably surcharged by backwater conditions from the flowing water in the downstream channel. From an outlet elevation set adequately above the creek flow line, the principal spillway conduit must slope upstream steep enough so positive drainage toward the outlet will be maintained after allowing for settlement to occur. Preliminary analysis, based on available existing geotechnical information from similar projects in Lancaster County, indicates 1 to 2 feet of foundation settlement should be anticipated due to the weight of the dam embankment. Based on the information available for this preliminary design, it s likely the flow line of the upstream end of the principal spillway conduit would need to be constructed at Elevation feet or higher, as shown on Sheet 5. The principal spillway inlet flowline would be approximately 4 feet above the existing creek flow line, and the lower half of the existing creek channel could not drain dry. There would be a normal water surface within the creek channel upstream of the dam 4.6 Embankment & Auxiliary Spillway Preliminary Design The proposed dam location is about 800 feet north of, and has more potential storage capacity than, the dam location shown in the master plan. As indicated earlier, the west end of the dam was rotated counterclockwise to fit the auxiliary spillway (AS) between the travel circles of two existing irrigation systems. The preliminary-design layout for grading of the Ash Hollow Dry Dam is shown on Sheets 2 and 3 of Appendix A. The dam would be approximately 2350 long, with the AS excavated around the west end of the dam embankment. As shown on Sheet 4, the AS is planned to have a bottom width of 200 feet with the side slopes excavated and graded at a ratio of 3:1 (3 horizontal to 1 vertical). The control crest is planned to be 30 feet in length, at Elevation feet, with the outlet slope graded away from the crest at a 3.64% slope. As shown on Sheet 5, the general cross-section of the embankment is planned to have a total height, from the creek flow line, of approximately 30 feet. The embankment top width is planned to be 14 feet (the minimum top-width required per USDA TR-60 design criteria for dams of this height). Based on preliminary geotechnical analyses, using available existing soils data from other projects in Lancaster County, embankment side slopes having a ratio of 3:1 should provide adequate stability factors of safety (per USDA TR-60 design criteria) for a dry dam. Before final design of the dam could be done, site-specific geotechnical borings, analyses, and design would be necessary at the proposed location. 9

15 Based on available existing soils information, modeling with SITES software (developed by USDA NRCS) indicated an AS width less than 150 feet would not have good stability against erosive scour during storm events that would cause substantial flow through the AS. Modeling also indicated an AS width of either 150 or 200 feet would be adequate for stability purposes. For this preliminary dry dam design, it has been anticipated that borrowing upstream of the dam to achieve an earthwork balance would not be acceptable. Since excavating a 150 feet wide AS would not provide enough earth material to construct the dam as planned, a 200 feet wide AS is planned to achieve an earthwork balance. Hydraulic routings of required design hydrographs (with allowance for recommended longterm sediment accumulation) indicate a high-hazard-potential dry dam could have a 500- year frequency, 200-feet wide AS crest at Elevation , with top of dam at Elevation Sheets 3, 4, and 5 indicate the preliminary design for the dry dam is based on a higher AS crest at Elevation , and a top of dam at Elevation This top of dam elevation would be about as high as could be workable with the lowest existing centerline elevation of 148 th Street, and it would be essentially equal to the lowest top of dam elevation that could work for a dry dam at the location proposed in the master plan. Constructing the AS crest at Elevation , rather than at , could provide approximately 117 acre-feet of additional storage volume below the AS crest. This spare storage volume may leave an option for future decision-makers to consider designing and constructing modifications to the dry dam in order to potentially impound a future permanent pool. Whether or not that potential could be realized in the future would be greatly dependent on future permitting and regulatory requirements. If permitting for such a conversion seems to be possible in the future, then geotechnical investigations, analyses, and design of appropriate modifications would be needed at that time before applying for necessary agency approvals to permit construction of modifications to the dry dam. 4.7 Dry Dam Detention-Storage Evaluation The reservoir stage/storage relationship for the proposed dam location was discussed earlier in Section 4.4 Reservoir Storage Capacity. Construction of the dry dam would throttle existing storm runoff peak flow rates along the Ash Hollow channel, downstream of the dam, down to the conveyance capacity of the 30- inch principal spillway conduit for all storm events, up to a 500-year frequency storm. As indicated on Sheet 1 of the preliminary plans, discharge to the Ash Hollow Creek channel from the dam s 30-inch principal spillway would range up to about 113 cubic feet per second (cfs) for a 500-year storm event. Under existing conditions, peak flow rates at the dam location range from approximately 600 cfs from a 1-year frequency storm, to approximately 3700 cfs for a 100-year storm, and approximately 4900 cfs for a 500-year storm. Throttling the creek flows through the 30-inch diameter principal spillway will result in temporary detention storage of water in the reservoir area upstream of the dam. The 10

16 elevation, area, volume, and timing of detention storage will vary with the size(s) of storm events experienced. The graph in Figure 4-3, below, is a plot of reservoir storage duration for various frequent storm events. The inundated surface area and duration can be identified by finding the peak elevation from Figure 4-3, below, and comparing it with Figure 4-1, above. For instance, the 1-year event elevation is approximately 1154 feet. This equates to approximately 26 acres inundated for 38 hours. Elevation (Feet) Computed Reservoir Storage Duration Storm Event 1Yr-24Hr (2.5 in) 2Yr-24Hr (3.0 in) 5Yr-24Hr (4.0 in) 10Yr-24Hr (4.7 in) Time (Hours) Figure 4-3 Computed Reservoir Storage Duration 11

17 Another more simplified way to look at this is to use a common measurement of rain fall compared to how many days it will take the reservoir to draw down. Since most people understand rainfall events in inches, the graph in Figure 4-4, below, plots rain events in inches Hour Rainfall (Inches) Rainfall (Inches) vs. Storage (Days) Storage Duration (Days) Figure 4-4 Rainfall (Inches) vs. Storage (Days) As shown on Sheet 2 of the preliminary plans, the reservoir high-storage boundary (where the level water would begin escaping across the proposed Elevation AS crest) would extend to Alvo Road, on the south line of Section 28, and 148 th Street, on the east line of the section. At the location on Alvo Road, there is a large existing box culvert (12 feet by 8 feet) with a flow-line at Elevation The center of Alvo Road is at approximately Elevation The box culvert flow-line is 1 foot above the 100-year temporary storage level for the dry dam, but would be 3.5 feet below the AS crest. At the location on 148 th Street, there is an existing box culvert (10 feet by 5 feet) with a flowline at Elevation The center of 148 th Street is at Elevation The flow-line of the box culvert is essentially at the 10-year temporary storage level, and 4 feet below the 100- year temporary storage level for the dry dam. 12

18 A tributary area of approximately 345 acres is drained to Ash Hollow, through 148 th Street, by that box culvert. Analysis was done to compare timing of peak flows from the tributary with timing of peak storage levels in the reservoir. The analysis indicated flow peaks from the tributary will pass through the 148 th Street box culvert before the reservoir storage level rises above the box culvert flow-line; therefore, temporary storage upstream of the dry dam will not impede peak flows through 148 th Street from the tributary. The timing analysis also indicated that headwater elevations, at the east end of the 148 th Street box culvert, resulting from peak flows on the tributary would be higher than the reservoir peak storage elevations for storm events of corresponding frequencies; therefore, the proposed dry dam would not cause drainage deficiencies for 148 th Street, or for properties east of 148 th Street, as they currently exist. See Figure 4-5 and Figure 4-6. Reservoir Elevation Year - 24Hour Routings Reservoir East Trib Time (Hours) Figure Year Peak Timing (East Tributary vs. Reservoir) 13

19 Reservoir Elevation Year - 24 Hour Routings Reservoir East Trib Time (Hours) Figure Year Peak Timing (East Tributary vs. Reservoir) 4.8 Preliminary Geotechnical Design Preliminary Geotechnical Evaluation Based on available existing geotechnical information, a preliminary geotechnical evaluation was completed for the proposed Ash Hollow Dam. The preliminary evaluation included a settlement, slope stability, and seepage analyses. Soils information was used from nearby projects in order to estimate the consolidation, shear strength, and seepage parameters. The dam was assumed to be a dry dam with no permanent pool impounded. Based on our analyses, the maximum settlement near the existing stream channel is estimated to range from one to two feet. Slope stability was analyzed for end of construction, rapid drawdown, steady-state, and steady-state seismic conditions. The embankment was assumed to a 14-foot wide crest with an elevation of feet (msl) and the upstream and downstream embankment slopes were assumed to have a 3(H):1(V) slope. Preliminary results, based on available existing data, indicate that the analyzed conditions would meet minimum factor of safety requirements according to USDA Soil Conservation Service Technical Release No. 60 (TR-60), Earth Dams and Reservoirs. The rapid drawdown condition was analyzed with a reduction in pool elevation from 1155 to 1143 over a period of 40 to 45 hours. Based upon the results of the preliminary slope stability and seepage analyses, a drainage system within the proposed dry dam embankment would not be necessary. Assuming that storm runoff would be impounded at the dam for a relatively short duration, along with the dam embankment material likely having a relatively low permeability, a phreatic line would not be able to develop in the embankment during brief periods of detention storage and therefore a drainage system would not be needed. 14

20 Before final design of the dry dam, much more detailed geotechnical site investigations, testing and analyses specific to the final dam and spillway location(s) would be necessary Future Geotechnical Design Considerations If modification of the planned dry dam to be a dam with a permanent pool is ever considered at some point in the future, detailed additional geotechnical investigation, analyses, and design for permanent-pool conditions would be necessary at that time. Modification to impound a permanent pool would likely require the installation of a toe drain and the upstream face of the dam would likely need to be reconstructed to a 4:1 slope to reestablish adequate stability. 4.9 Preliminary Hydrogeologic Evaluation Preliminary Hydrogeologic Evaluation for Dry Dam To assess potential impacts to current groundwater elevations, the following evaluation was completed. Initially, geologic profiles in and around the proposed Ash Hollow Dry Dam location were completed. As illustrated in Figure 4-7, the location of the University of Nebraska-Lincoln Conservation Survey Division (UNL-CSD) test holes were used to develop four cross sections across the project area. 15

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22 The cross sections presented in Figures 4-8 and 4-9 illustrate that the subsurface geology in the area consists of alluvial silt, clay, sand, and gravel deposits underlain by bedrock consisting of limestone, sandstone, and shale. Also illustrated on the cross sections is the 1995 groundwater elevation, which is the most recent water table configuration map, published by UNL-CSD in this area. 17

23 As seen in cross section C-C on Figure 4-9, groundwater is approximately 30 feet below ground surface at the proposed dam location. The depth to groundwater becomes shallower to the northeast (towards Waverly) and deeper to the southeast (toward the uplands). 18

24 Specifically, the water levels fluctuate between 6 and 14 feet below ground surface in Waverly at USGS monitoring well ( The water table fluctuates due to precipitation and groundwater pumping in the area. Because of the high water table conditions within and around Waverly, Olsson developed an analytical groundwater model to assess the potential for a water table elevation rise in and around the proposed dry dam. The model was constructed using the software package AQTESOLV for Windows (version 4.5). The code simulates water table mounding beneath rectangular reservoirs using the Hantush 1967 equations. According to the engineering calculations discussed earlier in this report, during precipitation events the dry dam would rise and then be drawn down in 2 to 4 days, depending on the size of the event. Several model simulations were run to estimate the change in water table conditions for the various events. The analytical model simulation indicates that the water table could rise up to approximately 0.5 feet directly beneath the reservoir during the largest events; however, a negligible rise would be apparent at the edges of the reservoir. Based on this analysis, the calculations indicate that the dry dam would not adversely impact the water table conditions in Waverly because the rise in water table conditions is isolated to the area beneath the proposed reservoir Future Hydrogeological Considerations If the dry dam is reconfigured to sustain a permanent pool, a new simulation of the potential impacts to water table conditions in and around the reservoir must be completed. The evaluation should include development of both an analytical model (as completed in Section 4.9.1), along with a more robust numerical model developed with the USGS groundwater modeling code MODFLOW. MODFLOW is the industry standard for numerical groundwater modeling and should be used to develop a detailed assessment of potential impacts to the residents of Waverly and the surrounding area. The numerical model will provide a method to assess the changes in water level conditions coupled with climatic changes over a time period of interest, which, in this case, could be the expected life time of the Ash Hollow Dam Structure Concept Evaluation of Potential for Future Dam/Reservoir Modification The objectives of LPSNRD and the City are for Ash Hollow Dam to be a storm water detention structure with no normal-pool retention of water for other purposes. Even so, it s possible that other purposes might be envisioned by future decision-makers, or owners, as Waverly continues to grow. For that reason, the contract scope of services for preliminary design of the dam included limited technical evaluations toward facilitating options that potentially might be considered by others sometime in the future. Therefore, initial hydraulic routings performed for Ash Hollow Dam have briefly considered, at a concept level, how the dry dam might need to be different if a permanent pool of water was an objective of the current project. A key design consideration for dams, especially high-hazard potential dams, is that the planned flood-storage volume exist long term (and not reduced by unplanned storagevolume loss due to sediment accumulation). Dam designs typically include storage volume to anticipate at least 50 years of sediment accumulation during the dam s service life. The initial permanent water pool of many dams is equal to the sediment accumulation 19

25 anticipated by the dam design, so elimination of the normal water pool, due to sediment accumulation over time, is often part of the design-life plan for a flood-reduction dam. Initial hydraulic routings for this preliminary design project determined that the dam location depicted in the Master Plan would need to have its top at least as high as Elevation 1174, even if little or no volume was planned for sediment accumulation. Subsequent evaluations of potential downstream sites led to the proposed dam location, as shown in the preliminary plans. Hydraulic analyses were done to determine how much spare storage volume could be held at the proposed location, if a top of dam at Elevation 1174 was used. It was determined by hydraulic routings that an adequate flood storage volume could exist between reservoircontour Elevations 1174 and about 1159, if the dam had a 200 feet wide AS crest at Elevation Therefore, a lake surface of perhaps 50 to 55 acres, at about elevation 1159, might be possible, if appropriate modifications to the dry dam could be designed, permitted, and constructed at some future time. If it were certain that maximizing potential for permanent pool upstream of the dam would never be an objective in the future, a dry dam with minimal allowance for long-term sediment accumulation could be constructed with top of dam at Elevation 1173 (maybe ) and an AS crest at Elevation (or maybe as low as ). Raising the dam top and AS crest (especially the AS crest) 1 to 1.5 feet would be substantially more difficult (and costly) in the future than building the dry dam to that height. The relatively small difference (1 to 1.5 feet ) in critical elevations between a dry dam and a dam with permanent pool is because 1.5 feet at the top of the reservoir area, amounts to approximately 268 acre-feet of volume (essentially the same volume that is currently below Elevation 1159 in the lower portions of the reservoir area). Potential future alteration of the dry dam for permanent pool impoundment would still require other substantial dam modifications. A potential conversion including toe drains and stability berms would still likely be needed (and costly) if necessary permits could be obtained in the future. However, building the top of dam and AS crest about a foot higher initially should make it somewhat easier for future decision-makers and designers to consider modifying the dry dam. Water budgeting for a potential future permanent pool was conceptually evaluated based on the potential ratio of watershed area to potential permanent pool area, as it compares to similar projects within the State, such as Walnut Creek in Papillion. However, detailed hydrogeologic and stream flow analysis would be necessary in the future to determine if the watershed could sustain a permanent pool Amberly Road Ditch Reversal Evaluation Another component of the top priority project was to consider reversing the south side ditch in Amberly Road, to take flows from the Waverly Heights subdivision. The runoff currently flows from east to west. The idea from the Watershed Master Plan was to reverse this flow to aid in flood reduction benefits to the drainage system known as End Run. This in turn would benefit the industrial / commercial area. Upon review of the ditch grades, it is very difficult to reverse the grade much farther west than 140 th Street. In order to get the most benefit, the aluminum arch culvert and storm sewer inlets would need to be reconstructed to 20

26 drain to the east. The ditch would need to be re-graded to a tenth of a percent slope to the east. Approximately 18.4 acres could be drained to the east which would be around 81 cfs for a 100-year event. If a smaller area was taken east, to avoid reconstruction of the storm sewer facilities, around 13.8 acres or 59 cfs in a 100-year event could be directed east. In either case, there are various utilities such as gas, electric and communications lines that run along this ditch and cross under it. Utility relocations and would most likely have to be completed in order to re-grade the ditch. Based on the findings from modeling of the dry dam in place, the flood reduction goals are realized by keeping Ash Hollow from split flowing into End Run. The potential benefits of reversing the ditch flow are minor in comparison to the overall project and therefore it wasn t evaluated any further. SECTION 5 Preliminary Economic Evaluations 5.1 Existing Flooding & Average Annual Damages The FEMA Digital Flood Insurance Rate Map (DFIRM) indicates that large portions of Waverly would be inundated by a 100-year flood, as shown in Figure 1-1 above. Existing U.S. Army Corps of Engineers (USACE) Hydrologic Engineering Center s River Analysis System (HEC-RAS) models from FEMA and from the Waverly Watershed Master Plan were reviewed. Based on this review, it was determined that the modeled overbank cross sections were not based on current LIDAR information. Estimated annual flood reduction benefits for the Ash Hollow Dry Dam Project were analyzed according to methodologies presented in the 2000 Nebraska Resources Development Fund (NRDF) Guidelines. To calculate average annual damages, information was gathered for all parcels and structures that appear in the Ash Hollow and End Run portions of the regulatory floodplain. A spreadsheet was populated with data including the address, assessed value and structure type. Available elevation certificates were used to identify first floor elevation, along with lowest adjacent grade and garage elevations. Contours generated from 2010 LIDAR were used for parcels that did not have elevation certificates completed. The contour closest to the structure was selected to identify the grade to the nearest foot. Then photos from the County Assessor s website were reviewed for each structure to estimate the elevation difference between the contour and the first floor. This additional height was added to the contour elevation. For two critical commercial structures, our survey crew visited the sites and obtained first floor elevations. Water surface elevations were interpolated for each structure using the latest FEMA flood maps, flood profiles, cross sections, and water surface elevations. From these numbers a flood depth was calculated at each structure for the 10- year, 50-year, and 100-year storm events. Depth / damage factor tables from the NRDF Guidelines were used to calculate percent damage. These factor tables utilize the structure type and the flood depth to calculate the percent of damage for both structures and contents. For residential structures, the structure type is selected based on if the structure is a split level, one story or multiple story and if it has a basement. Damage calculations for residential structures began at 6 below the first floor. For commercial structures, the structure type is selected based on if the structure is brick, metal, frame or concrete block, and damages start on the first floor. 21

27 The value of each structure was taken from Lancaster County s assessed values. Content values were calculated as 50 percent of the structure value. For commercial structures the method is the same, except in the case of large businesses. For large business, interviews were performed to obtain values. The County Assessor had no values for the Waverly School District s Bus Barn. The school district was contacted and values for the structure and contents were obtained by interview. LENCO is a large plastic manufacturing facility that was also interviewed. Due to the nature of the manufacturing facility, there is a high damage potential for the equipment, raw materials and finished products. Using this methodology, damage costs were calculated for three storm events. The number of damaged structures for the 100-year, 50-year, and 10-year events were 108, 101 and 68 respectively. Based on the frequency of the storms, average annual damages for existing conditions were calculated for the structures and contents, as shown in Table 5-1 and Table 5-2. The total calculated average annual damage for the existing conditions is shown in Table 5-3. Table 5-1 Existing Conditions Structral Damages Average Total Total Ave. Annual Recurrence Annual Structural Structural Structural Interval Probability Value Damage Damage 10-yr 0.1 $37,487,206 $257,342 $20, yr 0.02 $37,487,206 $438,666 $7,255 $5, yr 0.01 $37,487,206 $528,708 Total Annual Structural Damage = $33,423 Table 5-2 Existing Conditions Contents Damages Average Total Total Ave. Annual Recurrence Annual Contents Contents Contents Interval Probability Value Damaged Damaged 10-yr 0.1 $38,331,500 $1,202,285 $104, yr 0.02 $38,331,500 $2,273,278 $51, yr 0.01 $38,331,500 $4,561,179 $45,612 Total Annual Contents Damage = $201,137 Table 5-3 Existing Conditions Annual Damages Average Annual Recurrence Annual Total Total Total Interval Probability Value Damaged Damaged 10-yr 0.1 $75,818,706 $1,459,628 $125, yr 0.02 $75,818,706 $2,711,944 $58, yr 0.01 $75,818,706 $5,089,887 $50,899 Total Annual Damage = $234,560 22

28 5.2 Post Project Flooding & Average Annual Damages For post project flooding, the HEC-RAS model from the watershed master plan was utilized. The model was updated using the 2010 LIDAR contours to revise the overbank cross sections. In addition, hydrologic analysis was performed on the areas between the proposed Ash Hollow Dry Dam location and the confluence of Salt Creek. Runoff was computed using SCS TR-55 methodology and the USACE Hydrologic Modeling System (HEC HMS). This new hydrologic information along with the hydrograph from the preliminary dam design hydrology was used to calculate peak flows. The flow data was entered into the updated HEC-RAS model for analysis. A preliminary inundation map was prepared based on the dry dam being in place (Figure 5-1). The updated hydraulic modeling of the channel provides updated water surface elevations which are then mapped over the LIDAR contours. This updated inundation map indicates that all structures previously considered damaged are now free of inundation, up to the 100-year storm event; therefore, all of the calculated existing average annual damages are considered the project benefits. Since Salt Creek was not remapped, the mapping considered flooding on the Ash Hollow tributary only. Areas determined in the Watershed Master Plan to be affected by Salt Creek backwater were not included in the damage assessment and were not remapped. The backwater area identified in the master plan was traced onto the preliminary post-project inundation map. These areas will still experience flooding from Salt Creek as indicated in the master plan. As shown in the preliminary inundation map, there are still areas south of Cornhusker Highway and west of Ash Hollow Creek that experience nuisance flooding. This is caused by water coming out of the banks at the highway bridge and spreading to the west. This flooding however is shallow enough that the previously flooded commercial structures are now shown above of the 100-year water surface elevation. 23

29 5.3 Preliminary Opinion of Probable Costs The quantities for the proposed dry dam structure were taken from the preliminary design plans and unit costs were obtained by using bid tabulations from recently bid projects. The preliminary opinions of costs are presented in the following Table

30 Table 5-4 Opinion of Probable Development Costs Item Unit Quantity Unit Cost Item Cost CONSTRUCTION Mobilization LS 1 $200, $200, Construction Staking LS 1 $15, $15, Clearing & Grubbing LS 1 $10, $10, SWPPP LS 1 $7, $7, Salvage and Spread Topsoil SY 76,000 $1.50 $114, Earth fill, class A CY 124,000 $3.25 $403, Common Excavation CY 5,000 $3.50 $17, R.C. for P.S. energy dissipation impact basin LS 1 $25, $25, R.C. for P.S. intake headwall LS 1 $15, $15, " LCP principal spillway conduit w/ beveled-spigot wall fitting LF 200 $ $50, Concrete bedding for principal spillway conduit CY 30 $ $15, Metal fabrication for intake headwall trash rack and impact basin railing LS 1 $15, $15, Rock riprap TN 50 $65.00 $3, Turf reinforcement mat SY 18,000 $9.00 $162, Seeding and mulching AC 30 $ $22, Cover crop seeding AC 30 $ $4, Rock construction entrance LS 1 $2, $2, Temporary sediment basin EA 6 $1, $7, Drainage swale grading LF 7500 $3.00 $22, Hay-bale sediment checks EA 50 $25.00 $1, Silt Fence LF 1100 $4.00 $4, Temporary Creek Crossing EA 2 $3, $6, Fencing LF 6500 $8.00 $52, Total $1,174, Contingency 25% $293, Total Estimated Construction Costs $1,467, FEASIBLITY, ENGINEERING & INSPECTION Unit Quantity Unit Cost Item Cost Preliminary / Feasibility LS 1 $100, $100, Funding App., CLOMR, Geotechnical, Geohydrology, Survey, Permitting, Engineering, Bidding, Inspection & Testing LS 1 $317, $317, Total Feasibility, Engineering & Inspection Costs $417, LAND ACQUISITION Unit Quantity Unit Cost Item Cost Easements - Dam / Spillway AC 30 $6, $191, Easements - Flood Pool AC 111 $6, $709, Total Estimated Land Acquisition Costs $900, TOTAL ANTICIPATED PROJECT COSTS $2,785, ANNUAL O, M & R COST Unit Quantity Unit Cost Item Cost Operation, Maintenance and Repair (3/4% of Construction Costs) LS Per Year 1 $11, $11, Total Estimated O, M & R Costs $11,

31 Operation, maintenance, and repair costs are based on NRDF Guidelines Attachment 21. These costs are calculated as 0.75% of the construction costs for floodwater retarding structure. Farmland values are taken form the NRDF Guidelines Attachment 20. This attachment is indicated in the guidelines and is revised annually; however, the latest revision from DNR is from Since there is current moratorium on these funds, the attachment hasn t been updated. The Nebraska Department of Natural Resources (DNR) was contacted to obtain current average land values. The DNR provided the source that they used when creating Attachment 21. The source is a publication called Cornhusker Economics, published by the University of Nebraska-Lincoln Extension Office. Within that publication, is Table 1. This table is the average reported value of Nebraska farmland for different land types and substate regions. Land values are preliminary from February 1, The land value for dryland cropland with irrigation potential in the east district of the state was selected. The average reported value of Nebraska farmland was listed as $6,390 per acre. To be conservative in the preliminary estimation process, the average value was used for all land rights for the project. The land value for fee, title, and easements will be different, and an appraisal process will have to be followed at final design in order to determine the actual values for each. 5.4 Preliminary Benefit / Cost Evaluations The total calculated average annual damages for the existing conditions, as shown in Table 5-3, were subtracted from the total average annual damages for the post-project conditions to determine the average annual project benefit. The project benefits versus project costs were used to calculate the benefit cost ratio. In this case, all pre-project damages calculated become post-project benefits. In addition to structure and contents damages, the Flood Insurance Study (FIS) was reviewed to determine if there are any public benefits from the flood reduction project. The FIS and regulatory flood maps show that currently, during a flood event, various streets are inundated. These include Canongate Road, Amberly Road, N. 140 th, Deer Park Road and several other residential streets. Interstate 80 and Highway 6 are not overtop and in a 100- year flood event. The flood reduction benefits to the public infrastructure are difficult to quantify in dollars, however there would be significant benefits due to flood reduction by the drydam. These include reductions to post flood cleanup and repair of the streets. There is also a benefit to emergency response and the traveling public by not having to detour around flooded streets. In accordance with NRDF Guidelines, comparison of project benefits and costs to determine economic feasibility was done by preparing a cash flow stream for the flood control project. The cash flow stream is based on a project lifespan of fifty years. Table 5-5 includes the cash flow stream for the project and shows the proposed timing of expenditures and receipt of benefits. The ratio of benefits and costs (B/C Ratio) for the project is 3.34:1. The rate-ofreturn on investment for the project, which is the discount rate at which the net present worth of the project incremental cash flow is zero, is 7.40%. 26

32 Table Cash Flow Stream (Rate of Return = 7.40%, B/C Ratio = 3.34) Feasibilty Total Value Project Engineering Capital Associated Gross Of Project Incremental Accumulative Year Year Inspection Items OM&R Costs Costs (Gross Benefits) Benefits Benefits $100, $0.00 $0.00 $0.00 $100, $0.00 -$100, $100, $317, $0.00 $0.00 $0.00 $317, $0.00 -$317, $417, $0.00 $900, $0.00 $0.00 $900, $0.00 -$900, $1,317, $0.00 $1,467, $0.00 $0.00 $1,467, $0.00 -$1,467, $2,785, $0.00 $0.00 $11, $0.00 $11, $234, $223, $2,562, $0.00 $0.00 $11, $0.00 $11, $234, $223, $2,338, $0.00 $0.00 $11, $0.00 $11, $234, $223, $2,114, $0.00 $0.00 $11, $0.00 $11, $234, $223, $1,891, $0.00 $0.00 $11, $0.00 $11, $234, $223, $1,667, $0.00 $0.00 $11, $0.00 $11, $234, $223, $1,444, $0.00 $0.00 $11, $0.00 $11, $234, $223, $1,220, $0.00 $0.00 $11, $0.00 $11, $234, $223, $997, $0.00 $0.00 $11, $0.00 $11, $234, $223, $773, $0.00 $0.00 $11, $0.00 $11, $234, $223, $550, $0.00 $0.00 $11, $0.00 $11, $234, $223, $326, $0.00 $0.00 $11, $0.00 $11, $234, $223, $102, $0.00 $0.00 $11, $0.00 $11, $234, $223, $120, $0.00 $0.00 $11, $0.00 $11, $234, $223, $344, $0.00 $0.00 $11, $0.00 $11, $234, $223, $567, $0.00 $0.00 $11, $0.00 $11, $234, $223, $791, $0.00 $0.00 $11, $0.00 $11, $234, $223, $1,014, $0.00 $0.00 $11, $0.00 $11, $234, $223, $1,238, $0.00 $0.00 $11, $0.00 $11, $234, $223, $1,461, $0.00 $0.00 $11, $0.00 $11, $234, $223, $1,685, $0.00 $0.00 $11, $0.00 $11, $234, $223, $1,908, $0.00 $0.00 $11, $0.00 $11, $234, $223, $2,132, $0.00 $0.00 $11, $0.00 $11, $234, $223, $2,356, $0.00 $0.00 $11, $0.00 $11, $234, $223, $2,579, $0.00 $0.00 $11, $0.00 $11, $234, $223, $2,803, $0.00 $0.00 $11, $0.00 $11, $234, $223, $3,026, $0.00 $0.00 $11, $0.00 $11, $234, $223, $3,250, $0.00 $0.00 $11, $0.00 $11, $234, $223, $3,473, $0.00 $0.00 $11, $0.00 $11, $234, $223, $3,697, $0.00 $0.00 $11, $0.00 $11, $234, $223, $3,920, $0.00 $0.00 $11, $0.00 $11, $234, $223, $4,144, $0.00 $0.00 $11, $0.00 $11, $234, $223, $4,368, $0.00 $0.00 $11, $0.00 $11, $234, $223, $4,591, $0.00 $0.00 $11, $0.00 $11, $234, $223, $4,815, $0.00 $0.00 $11, $0.00 $11, $234, $223, $5,038, $0.00 $0.00 $11, $0.00 $11, $234, $223, $5,262, $0.00 $0.00 $11, $0.00 $11, $234, $223, $5,485, $0.00 $0.00 $11, $0.00 $11, $234, $223, $5,709, $0.00 $0.00 $11, $0.00 $11, $234, $223, $5,932, $0.00 $0.00 $11, $0.00 $11, $234, $223, $6,156, $0.00 $0.00 $11, $0.00 $11, $234, $223, $6,380, $0.00 $0.00 $11, $0.00 $11, $234, $223, $6,603, $0.00 $0.00 $11, $0.00 $11, $234, $223, $6,827, $0.00 $0.00 $11, $0.00 $11, $234, $223, $7,050, $0.00 $0.00 $11, $0.00 $11, $234, $223, $7,274, $0.00 $0.00 $11, $0.00 $11, $234, $223, $7,497, $0.00 $0.00 $11, $0.00 $11, $234, $223, $7,721, TOTAL: $417, $2,368, $517, $0.00 $3,302, $11,024, $7,721,

33 5.5 Potential Funding Assistance Several potential funding sources exist to provide potential grant or cost sharing opportunities. The funding sources provide varying levels of financial assistance to projects and the application processes vary in complexity and processing time. These sources availability is always changing. We have provided a brief summary of potential funding sources below. The Nebraska Resources Development Fund (NRDF) program is a state grant program that could be used to fund the proposed flood control measures. The fund is administered by the Nebraska Department of Natural Resources, was established in 1974, and has since provided more than $80 million in financial assistance over the years. The fund has been used to support more than 70 projects across the state, including LPSNRD Stevens Creek Farm Ponds and Supplemental Culvert and Antelope Valley. NRDF assistance can be provided in the form of a grant or a loan and funding is available for up to 75% of the project cost. The local partners would be required to provide the remaining portion of the project cost. Applicants must prove that the flood reduction benefits for the proposed project exceed the project costs and provide at least a 3% rate of return. The application process can be extensive and can take up to a year or more to complete. Applications are reviewed on a competitive basis. Because of current financial commitments to ongoing projects, funds from NRDF may not be available for the foreseeable future. The Community Development Block Grant (CDBG) program is a federal grant program that could be used to fund the proposed flood control measures, if the City is eligible. CDBG could cover up to 75% of the total cost of the project and the local partners would be required to match 25% of the project cost. CDBG funds are distributed through the Nebraska Department of Economic Development. The application process can be extensive but can typically be completed within six to 12 months. Funds available vary from year to year. Communities with relatively high populations of low to moderate income residents typically receive high priority for CDBG funds. Waverly does not meet the low to moderate income criteria. Although rare, funding can also be awarded on a critical needs basis and the proposed project may qualify on that basis; however, an economic justification may be required to prove the critical need. At times, based on Federal Disaster Declarations, special Disaster Recovery funding has been made available for specific types of projects. In 2008, funding was made available based on a flood-related disaster (Disaster Declaration 1770) and flooding and drainage projects were eligible for funding. Disaster Recovery funding is typically more widely available to communities without high populations of low to moderate income residents than regular CDBG funding. The FEMA Hazard Mitigation Grant Program (HMGP) provides grants to state and local governments for long term hazard mitigation. The program is administered by the Nebraska Emergency Management Agency (NEMA) and funds are provided through the Federal Emergency Management Agency (FEMA). Similar to CDBG, HMGP will cover up to 75% of the total cost of the project. The remaining portion of the project cost must come from the grantee or other non-federal sources and can include in-kind services. The application process for HMGP grants is extensive and often takes several years. Applicants must prove the flood reduction benefits outweigh the project costs. Furthermore, funding for HMGP projects is only available on a disaster-related basis. Whenever a Federal Disaster 28

34 Declaration is made, 15 percent of the total disaster cost is set aside for HMGP funding. Typically, applications for projects are accepted for up to two years after a disaster occurs. The applications are ranked by NEMA and funded according to ranking. Projects not funded under a particular disaster declaration are still considered for funding when future disaster declarations are made and go through the same ranking process again. Typically, HMGP funds can only be used for projects that CAN NOT be funded through other sources. Therefore, all other funding options must be exhausted prior to applying for HMGP funds. This option has been researched for dry dam projects and currently HMGP funds are not allowed for dam or levee projects. The U.S. Department of Agriculture - Natural Resources Conservation Service (NRCS) specializes in small watershed dams and can also provide technical or administrative assistance. NRCS programs, such as the PL 566 program for small dams, were used to fund small dams similar to the dam proposed in this report. However, funding for small dams through the PL 566 program has waned over the years and probably is not available for this project due to a shift in NRCS funding priorities to land treatment and conservation measures. The Nebraska Clean Water State Revolving Fund (CWSRF) program is a state grant and loan program that could be used to fund the proposed flood control measures. The fund is administered by the Nebraska Department of Environmental Quality (NDEQ). The fund is for green or clean water projects. They can be used for watershed management projects, as was recently done for Lake Wanahoo, in Wahoo, NE. The funds provide low interest (2%) loans over a 20 year term. The NDEQ charges an additional one percent for an administration fee. The project has to have a primary environmental component to qualify. This requirement could potentially be achieved by including water quality wetlands and vegetated buffers along Ash Hollow. A water quality comprehensive plan should be developed for the City. This comprehensive plan should be developed in close conjunction with NDEQ staff and could include various green streets initiatives as well as environmental quality projects along Ash Hollow. By developing a plan and identifying water quality projects, money spent on the Ash Hollow Dry Dam and environmental quality components could count towards the 40% cost share for Nonpoint Source Water Quality Grants (Section 319). The CWSRF program also provides facility planning grants for communities with populations less than 10,000. A planning grant could be sought to help fund the water quality comprehensive plan. In this way, the CWSRF and Section 319 Grant could be combined to provide the greatest funding potential. 29

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43 MEMO Overnight Regular Mail Hand Delivery Other: _ Preliminary Centerline Evaluation TO: Lower Platte South NRD, City of Waverly PHONE: FROM: Don R. Day, PE, Bob Wolf, PE, Dennis Hurtz, PE RE: City of Waverly Priority 1 Ash Hollow Dry Dam Layout Refinement DATE: 3/22/2012 PROJECT #: PHASE: 2 NOTES: For the purpose of refining the Ash Hollow Dam location, we ve initially evaluated four potential dam centerlines. Our primary goal was to identify a location that will allow the detention storage reservoir to be as dry as possible. A secondary goal was to facilitate a potential for future modification to provide a permanent pool within the detention reservoir. We ve considered the four potential dam centerline locations for various reasons including: Centerline M is essentially as proposed in the Master Plan and it is the most upstream location considered. Per a suggestion at the February 1 open-house meeting, an alternative dam centerline as near as practical to Interstate 80 was considered. This is identified as Centerline I- 80 Centerline A was considered because it could collect runoff from additional subwatershed areas that drain to Ash Hollow from east of 148 th Street. Centerline B was considered because it could provide more storage volume than Centerline M, but would allow a shorter dam length than Centerline A Lincoln Mall, Suite 111 TEL Lincoln, NE FAX

44 Based on our analysis of the four centerlines, we recommend centerline B with an approximate location as shown on the attached exhibit. The main reasons for recommending Centerline B are: Overall, Centerline B would have less impact than Centerline M on the Lancaster County Right-of-Way for both N. 148 th Street and Alvo Road. o Shifting the dam alignment somewhat downstream would reduce the potential for backwater conditions from the dam through a box culvert. o Centerline B could have a 500-year water storage surface at about elevation 1165; this is approximately 4 feet lower than the critical lowest road elevation of In comparison, the 500-year storage level of Centerline M would be at about the center of N. 148 th Street. The channel invert is approximately 5 feet lower at Centerline B than at Centerline M. This would allow limited potential for modification if a small permanent pool were wanted for some reason in the future. Centerline M has no potential for a future permanent pool. The length of dam at Centerline B would be approximately 600 feet (1/3) longer than Centerline M, but about 500 feet shorter than at Centerline A. Centerline B has less environmental impacts than that of the A or I-80. This is due to the trees along the channel downstream of Centerline B. Permitting requirements are more difficult when projects impact trees along a channel. Construction and final design costs would be substantially higher for shifting the centerline closer to I-80. This location would require a structural spillway which would nearly double the cost of the dam. With a dry dam at any of the four centerline locations evaluated there would be approximately 6 feet of water permanently stored within the channel banks. At Centerline B the permanent water that is held would be approximately 0.5 feet lower, with respect to the channel banks, than that of Centerline M. In addition, we recommend doing the design with a 500-year auxiliary spillway crest because it is only 2 feet higher than a 100-year auxiliary spillway crest provides a higher level of protection for Waverly and provides flood protection in the event that FEMA moves towards regulation of the 500-year floodplain in the future. F:\Projects\ \Documents\Reports\MEMO_Dam Layout.doc

45 Appendix A Exhibit 1 - Centerline Location Options

46 I-80 N 148TH STREET DAM CL 80 DAM CL A DAM CL B DAM CL M PROJECT NO: DRAWN BY: DATE: AMM 03/14/12 CENTERLINE LOCATION OPTIONS LPSNRD ASH HOLLOW DRY DAM 1111 Lincoln Mall, Suite 111 P.O. Box Lincoln, NE TEL FAX EXHIBIT 1