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DRAFT ENVIRONMENTAL ASSESSMENT

South Western Avenue Drainage Improvements Weir Modification

Minnehaha County, South Dakota

U.S. Army Corps of Engineers Northwest Division Omaha District

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District TABLE OF CONTENTS 1.0 INTRODUCTION ...... 1 1.1 Proposed Action ...... 1 1.1 Purpose of and Need for the Proposed Action ...... 2 1.2 Authority ...... 4 1.3 Prior Reports ...... 4 1.4 NEPA Scoping ...... 4 2.0 ALTERNATIVES...... 4 2.1 Alternative 1 – No-Action ...... 5 2.2 Alternative 2 – Weir Modification (Preferred Alternative) ...... 5 3.0 AFFECTED ENVIRONMENT AND ENVIRONMENTAL CONSEQUENCES ...... 8 3.1 Environmental Setting ...... 8 3.2 Subject Headings Eliminated from Environmental Consequences Analysis ...... 9 3.3 Relevant Resources ...... 10 3.3.1 Water Quality ...... 10 3.3.2 Wetlands ...... 11 3.3.3 Threatened and Endangered Species ...... 16 3.3.4 Floodplain Resources ...... 18 3.3.5 Cumulative Impacts ...... 18 4.0 COORDINATION ...... 19 5.0 MITIGATION...... 19 6.0 COMPLIANCE WITH ENVIRONMENTAL LAWS AND REGULATIONS ...... 19 7.0 CONCLUSION ...... 19 8.0 PREPARED BY...... 20 9.0 LITERATURE CITED ...... 21

FIGURES Figure 1. 2005 EA Location and Vicinity Map ...... 2 Figure 2. Weir Removal Location and Detailed Drawing ...... 3 Figure 3. National Wetlands Inventory Wetlands within the Weir Modification Project Area ... 12 Figure 4. Desktop-Determined Wetland Areas within the Weir Modification Project Area...... 12 Figure 5. Percent of Time Elevation is Exceeded ...... 14

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District i Figure 6. Oxbow Detention Characteristics With and Without the Weir Modification for a 100-year Storm Event ...... 15 Figure 7. Oxbow Detention Characteristics With and Without the Weir Modification for a 1-inch Storm Event ...... 16

TABLES Table 1: Summary of Potential Effects by Alternative ...... 6 Table 2: Federally Listed and Candidate Endangered and Threatened Species in the Weir Modification Project Area, Minnehaha County, South Dakota ...... 17 Table 3: Threatened and Endangered Species Effect Determinations ...... 17

APPENDICES Appendix A 2005 FRRP Supplemental EA Appendix B FRRP Wetland Mitigation Plan Appendix C South Western Avenue Drainage Analysis Appendix D Agency Correspondence Appendix E Environmental Compliance

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District ii LIST OF ACRONYMS AND ABBREVIATIONS

2005 EA Supplemental Environmental Assessment for the Sioux Falls Flood Protection Project CEQ Council on Environmental Quality CFR Code of Federal Regulations EA Environmental Assessment EC Engineering Circular EO Executive Order ER Engineering Regulation FONSI Finding of No Significant Impact FRRP Big Sioux River and Skunk Creek Flood Risk Reduction Project HGM hydrogeomorphic IPaC Information for Planning and Consultation NEPA National Environmental Policy Act of 1969 PBO Programmatic Biological Opinion SDDENR South Dakota Department of Environmental and Natural Resources U.S.C. United States Code USACE U.S. Army Corps of Engineers USFWS U.S. Fish and Wildlife Service

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District iii ENVIRONMENTAL ASSESSMENT South Western Avenue Drainage Improvements Weir Modification MINNEHAHA COUNTY, SOUTH DAKOTA

1.0 INTRODUCTION

The City of Sioux Falls, South Dakota, the local sponsor that maintains and operates the congressionally authorized and federally constructed Sioux Falls Big Sioux River LB South levee system along the Big Sioux River, is requesting an alteration of a weir that is part of the Big Sioux River and Skunk Creek Flood Risk Reduction Project (FRRP). The U.S. Army Corps of Engineers (USACE), Northwest Division, Omaha District, has prepared this Environmental Assessment (EA) to evaluate the potential impacts of the Western Avenue Weir Modification. This EA has been prepared in accordance with the National Environmental Policy Act of 1969 (NEPA) and the Council on Environmental Quality (CEQ) regulations (40 Code of Federal Regulations [CFR] 1500–1508), as reflected in the USACE Engineering Regulation (ER) 200-2-2. This EA provides sufficient information on the potential adverse and beneficial environmental effects to allow the District Commander, USACE, Omaha District, to make an informed decision on the appropriateness of an Environmental Impact Statement or a Finding of No Significant Impact (FONSI). The finding of the EA determines whether an Environmental Impact Statement is required. If the EA indicates that no significant impact is likely, then the agency can publish a FONSI and proceed with the proposed action. This EA incorporates by reference the Supplemental Assessment for the Sioux Falls Flood Protection Project (2005 EA) and supporting documents (see Appendix A) per Engineering Circular (EC) 1165-2-220. The FRRP provided a 100-year level of protection from Skunk Creek flooding, including installation of an inflatable dam above the confluence of Skunk Creek and the Big Sioux River, raising of existing levees in several locations, and replacement of the 41st Street Bridge at a higher elevation (see Figure 1).

1.1 Proposed Action

The proposed action consists of modifying an existing concrete weir structure located in an oxbow wetland near 50th Street and Elmwood Avenue (see Figure 2). The structure was constructed as part of wetland mitigation for the Sioux Falls FRRP, Phase II C Improvements and is located approximately 150 feet from the top of the adjacent levee. The existing weir is a rectangular notch weir that would be altered to create a wider base trapezoidal weir. The existing concrete wing walls would be saw cut to the concrete sill, which is at ground level, to increase the flow width across the sill from 4 feet to 10 feet. The base elevation of the existing weir would not change. Removal of the wing walls would eliminate the capability to add stop logs to vary the weir crest elevation. The exposed rebar in the concrete sill would be covered with High Performance Epoxy (100 percent solid performance coating) to seal and prevent oxidation of the metal. The proposed work does not involve any grading operations or filling activities.

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 1

Figure 1. 2005 EA Location and Vicinity Map

The weir modifications are anticipated to take less than a day. Equipment would include a concrete saw, a small loader to transport concrete wing wall pieces out of the wetland, and a truck to remove material from the site. The proposed project is located within the City of Sioux Falls in Township 101 North, Range 49 West, Section 31. The weir is located near 50th Street and Elmwood Avenue, and is approximately 150 feet from the adjacent Big Sioux River levee (see Figure 1).

1.1 Purpose of and Need for the Proposed Action

The purpose of the proposed action is to improve flow through the oxbow wetland, improve interior drainage, and reduce flooding of buildings adjacent to the FRRP. Sioux Falls has received multiple major rain events in recent years that have resulted in flooding impacts on private properties in the city, including landward buildings adjacent to the FRRP. Hydraulic analysis completed in the area shows that reducing the peak water surface elevation and flood duration by over 1 foot for a 100-year flood event would result in a more efficient storm sewer system and would reduce flooding in the area adjacent to the FRRP (DGR Engineering 2016).

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 2 WEIR REMOVAL TOP VIEW ' ..

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Figure 2. Weir Removal Location and Detailed Drawing

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 3 1.2 Authority

The Sioux Falls levee system was originally constructed in 1961 and was enhanced per the 1993 feasibility study and integrated EA, and the 2005 EA. USACE has the authority to grant permission for permanent alterations to federally authorized civil works projects under Section 14 of the Rivers and Harbors Act of 1899, codified at 33 United States Code (U.S.C.) 408 (Section 408). EC 1165-2-220 provides Section 408 guidance for addressing the weir modification request by the City of Sioux Falls.

1.3 Prior Reports

FRRP Supplemental EA: USACE prepared a supplemental EA for changes to the proposed Phase II FRRP improvements located within the Sioux Falls Big Sioux River LB South FRRP operated and maintained by the City of Sioux Falls. USACE completed a supplement to the 1993 original EA to address modifications to the proposed Phase II improvements to the FRRP and to satisfy the requirements of NEPA (USACE 2005). The 2005 EA is incorporated herein by reference and can be found in Appendix A. Wetland Mitigation Plan for FRRP Impacts: Following completion of the 2005 EA, a wetland mitigation plan for the Phase II FRRP improvements was developed by the USACE, Planning Branch and submitted to the USACE Regulatory Branch, Omaha District, as part of the Section 404 permit application for 2.30 acres of wetland impacts associated with completion of the FRRP. Mitigation included creating and conserving 9.0 wetland acres in an existing oxbow wetland adjacent to the Big Sioux River. The wetland mitigation plan can be found in Appendix B. South Western Avenue Drainage Analysis for the City of Sioux Falls (DGR Engineering 2016): A drainage study was completed by DGR Engineering for the City of Sioux Falls. The study determined the need to improve flow through the oxbow wetland to allow proper drainage of the South Western Avenue area. The drainage study can be found in Appendix C.

1.4 NEPA Scoping

The City of Sioux Falls requested authorization for the weir modification from USACE on January 31, 2018. USACE comments on the request are addressed in this EA. The U.S. Fish and Wildlife Service (USFWS) Information for Planning and Consultation (IPaC) was used to generate a threatened and endangered species list and a verification letter that the proposed action is consistent with the activities analyzed in the USFWS Programmatic Biological Opinion (PBO) for the northern long-eared bat. Coordination letters concerning the Fish and Wildlife Coordination Act were mailed by HDR on April 24, 2019, to the USFWS Ecological Services Office in Pierre, South Dakota. USACE will issue a public notice on the availability of the EA pursuant to EC 1165-2-220.

2.0 ALTERNATIVES

Two alternatives are being considered: Alternative 1 – No-Action and Alternative 2 – Weir Modification (Preferred Alternative). Alternative 2 is the proposed action.

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 4 2.1 Alternative 1 – No-Action

Under the No-Action Alternative, the proposed action would not be completed. The weir would remain as it currently is, and drainage would be unchanged. Flood elevations and durations would continue to impact homes and roads in the vicinity. The No-Action Alternative would not impact the wetland mitigation site developed as part of the FRRP.

2.2 Alternative 2 – Weir Modification (Preferred Alternative)

As discussed in Section 1.1, Proposed Action, flooding on the landward side of the levee from storm events is anticipated to continue; however, the duration would be reduced by altering the weir under this alternative. Removing the wing walls would reduce flood elevations by 1.25 feet, and flooding duration would be reduced by up to 2 hours in the oxbow wetland under a 100-year storm event scenario while the flooded elevation would only be reduced by 0.3 feet under a more frequent 1-inch storm event. The outlet elevation would not change. The size and function of the existing wetland mitigation site created for the FRRP would not be adversely impacted, as discussed in Section 3.3.2. Table 1 summarizes the effects of implementing the alternatives. Chapter 3.0 discusses in detail the resources in the affected area and the potential impacts on each resource.

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 5 Table 1: Summary of Potential Effects by Alternative

Resource Alternative 1 – No-Action Alternative 2 – Weir Modification (Preferred Alternative) Air quality No effect Minor, short-term emissions would be caused by equipment used for removing concrete wing walls.

Water quality No effect Peak flood elevations would be reduced by 1.25 feet, and inundation duration would be reduced by 1 to 2 hours. The permanent ponding elevation within the oxbow would not be affected.

Wetlands No effect Negligible changes to surface hydrology during storm events would occur; no adverse effect on wetlands would be expected.

Aquatic resources/fisheries No effect No effect

Terrestrial resources No effect Temporary trampling of grasses and forbs would occur during removal of concrete wing walls.

Wildlife resources No effect Minor, temporary noise effects would occur during removal of concrete wing walls.

Threatened and endangered No effect No effect species

Cultural resources No effect No effect

Recreation resources No effect No effect

Farmland resources No effect No effect

Floodplain resources No effect Interior drainage within the oxbow wetland would be improved, and flood risk to adjacent landward properties would be minimized, with no adverse effects on floodplain resources.

Aesthetics No effect Removal of wing walls would make the wetlands appear more natural and would improve aesthetics.

Socioeconomics Flooding would continue for Adverse effects and damages caused by flooding of adjacent landward structures would adjacent landward structures. be minimized.

Environmental justice No effect No effect

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 6 Table 1: Summary of Potential Effects by Alternative

Resource Alternative 1 – No-Action Alternative 2 – Weir Modification (Preferred Alternative) Noise No effect Minor, temporary noise effects from construction activity would occur during removal of the concrete wing walls.

Hazardous, Toxic and No effect None observed during wetland mitigation activities Radioactive Waste

Land Use Surrounding area is developed No change to the potential impacts documented in the 2005 EA. and adjacent landward structures may continue to be flooded during storm events

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 7 3.0 AFFECTED ENVIRONMENT AND ENVIRONMENTAL CONSEQUENCES

This chapter presents an analysis of each resource topic identified as having a potential to be affected by implementation of the proposed action. Each section describes the environmental setting as it relates to the specific resource topic; the direct and indirect effects that could result from implementation of the proposed action; and mitigation measures that would avoid, reduce, or compensate for substantial adverse effects of the proposed action. The relevant resources section of this chapter presents the adverse and beneficial environmental effects of Alternative 1 – No-Action and Alternative 2 – Weir Modification (Preferred Alternative), which is the proposed action. The section is organized by resource category, and presents the existing conditions of the resource and effects of each of the alternatives on the resource. Impacts are quantified whenever possible. Qualitative descriptions of impacts are explained by accompanying text where used. See Table 1 for a summary of potential impacts by alternative for each resource. “Significance” has been analyzed in this document in terms of both context (sensitivity) and intensity (magnitude and duration), as follows: • Context – sensitivity o An impact’s significance is influenced by the importance of the resource or value being impacted, the geographic location and timing, and other relevant factors that provide context for more fully understanding the severity of the impact. o The context in which impacts are considered should generally be specific to the area and resources being affected. For most actions, this means considering impacts in the context of the affected locale as opposed to the state, country, or world as a whole. In many cases, however, it is also appropriate to consider the role of an area in the larger landscape. • Intensity – magnitude o No effect – The resource would not be measurably impacted. o Minor – There would be noticeable impacts on the resource in the project area, but the resource would still be mostly functional. o Moderate – The resource would be impaired, so that it cannot function normally. o Major – The resource would be severely impaired so that it is no longer functional in the project area. • Intensity – duration o Short term – Temporary effects would be caused by the construction and/or implementation of a selected alternative. o Long term – Effects would be caused by an alternative after the action has been completed and/or after the action is in full and complete operation.

3.1 Environmental Setting

The project area is located near the modified stretch of the Big Sioux River and within the Sioux Falls Big Sioux River LB South levee system. The South Western Avenue Drainage Basin draining into the existing wetland area drains approximately 1,220 acres of urbanized and mostly residential areas of Sioux Falls. The surface geology is composed of lower proterozoic Sioux

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 8 Quartzite, which is an iron-stained orthoquartzite with minor metamorphosed conglomerate and mudstone layers (Tomhave and Schulz 2004). Sioux Falls has average daily temperatures between 79 and 84 degrees Fahrenheit during the summer months and average daily highs of between 26 and 43 degrees Fahrenheit during the winter months. Most of the annual precipitation is received between April and August, with an average of almost 4 inches of precipitation received in June, which is the month that receives the most rain on average. During these months, flooding can be an issue near creeks and rivers.

3.2 Subject Headings Eliminated from Environmental Consequences Analysis

The resources in the following list have been considered and found not to be affected by the proposed alternatives (either by not being present or not being measurably affected), or impacts on these resources were previously discussed in the 2005 EA and are not greater or different than what was documented in the 2005 EA. Where there were no potential effects identified or the impacts were previously covered and there are no substantive changes, the resource itself has been eliminated from further evaluation and analysis. As noted in Section 1.0, the 2005 EA is incorporated by reference; a summary for eliminated subject headings follows: • Air Quality – Sioux Falls is within an attainment area, and real-time monitoring data shows the air quality index within the good range (South Dakota Department of Environmental and Natural Resources [SDDENR] 2019). The potential temporary construction impacts would be less than documented in the 2005 EA. • Aquatic Resources/Fisheries – There is no change in the affected environment, and potential impacts would be less than those documented in the 2005 EA because all work would be done on the landward side of the levee. • Terrestrial Resources –Tree losses associated with the FRRP were mitigated per the mitigation plan; there would be no change in the potential temporary impacts documented in the 2005 EA. • Wildlife Resources – There would be no change in the affected environment and potential impacts documented in the 2005 EA. • Cultural Resources – Alternative 2 is within an area that was previously cleared with the 1993 and 2005 projects. No further investigation of cultural resources is required for the proposed weir modification. • Recreation Resources – There would be no change in the affected environment from the 2005 EA, and no designated recreation facilities would be affected. • Farmland Resources – Farmlands were not considered in the 2005 EA because the project is in an urban area. No farmlands are present in the project area. • Aesthetics – Aesthetics were not considered in the 2005 EA. Modification of the weir would occur within a wetland mitigation site. The removal of concrete would enhance the natural aesthetics of the mitigated wetland.

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 9 • Socioeconomics – The population of Sioux Falls has continued to grow, but the demographic trends are similar to those in the 2005 EA. Implementation of the weir modification would reduce flooding for adjacent businesses. • Environmental Justice – The population of Sioux Falls has continued to grow, but the demographic trends are similar (US Census Bureau 2010). There would be no change in potential impacts documented in the 2005 EA. • Noise – There has been no change in the affected environment, and potential temporary construction impacts would be less than documented in the 2005 EA. • Hazardous, Toxic, and Radioactive Waste – This resource was not considered in the 2005 EA. Modification of the weir would occur within the wetland mitigation area. No hazardous, toxic, or radioactive waste materials were identified during wetland mitigation activities. • Land Use – Sioux Falls has adopted a new land use plan, Shape Sioux Falls 2040 (City of Sioux Falls 2016). The objectives of the plan are similar to the previous land use plan analyzed. There would be no change to the potential impacts documented in the 2005 EA.

3.3 Relevant Resources

This section describes the relevant resources that could be impacted by the project. The important resources described in this section are those recognized by laws, executive orders (EOs), regulations, and other standards of national, state, or regional agencies and organizations; technical or scientific agencies, groups, or individuals; and the general public.

3.3.1 Water Quality

Existing Conditions

The SDDENR has developed surface water quality standards for all waters of the state, as required by the Clean Water Act, as amended. The water quality standards establish the use or uses to be made of a water body, set criteria necessary to protect the uses, and establish policies to maintain and protect water quality. South Dakota’s water quality standards are designed to protect public health and welfare, enhance the quality of water, and uphold the goals of the federal Clean Water Act. The 2005 EA describes the water quality for the project area. In a letter dated July 27, 2004 (Appendix A), the SDDENR stated that the proposed projects would take place within a segment of the Big Sioux River that is classified by the South Dakota Surface Water Quality Standards and Uses Assigned to Stream for the following beneficial uses: (1) domestic water supply waters; (2) warm water semipermanent fish life propagation waters; (3) immersion recreation waters; (4) limited contact recreation waters; (5) fish and wildlife propagation, recreation, and stock- watering waters; and (6) irrigation waters. Also in its letter, the SDDENR stated that because of the beneficial uses assigned to the Big Sioux River in the proposed project area, special construction measures may have to be taken to ensure that the total suspended solids standard of 90 mg/L for the Big Sioux River and 150 mg/L for Skunk Creek is not violated. These designated uses documented in the 2005 EA have not changed, as noted in the 2018 South Dakota Integrated Report for Surface Water Quality Assessment (SDDENR 2018).

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 10 Alternative 1 – No-Action

Without implementation of the proposed action, water quality would not change. Discharge of water from the wetland mitigation area would continue to occur through the weir and to the existing 72-inch culvert, which outlets into the Big Sioux River. The duration and elevation of floodwaters would continue at their current levels.

Alternative 2 – Weir Modification (Preferred Alternative)

With implementation of the proposed action, water quality is not expected to be impacted. Minor and short-term ground disturbance is anticipated as part of the project, but negligible discharge into the Big Sioux River from construction is anticipated. Project activities are anticipated to last less than 1 day, reducing the chance of unexpected discharges. Section 401 Water Quality Certification is not anticipated to be required for this project due to its short-term temporary impacts. No material would be purposely placed into wetlands or waterways. The wing walls, after removal, would be removed from site and disposed of appropriately. The exposed concrete and rebar would be sealed so that corrosion of the concrete and steel would not occur. After modification of the weir, water would discharge from the wetland mitigation area approximately 2 hours faster, and flood elevations would be reduced by approximately 1 foot for a 100-year storm event. This minor change in flood duration and elevation is not expected to impact water quality.

3.3.2 Wetlands

Existing Conditions

The project area is within a wetland mitigation site constructed to offset impacts caused by the FRRP. The mitigation site was constructed by expanding existing wetlands (creation) and conserving existing wetland areas with an expected total wetland size of 9 acres. According to the 2005 EA, these wetlands can be described as palustrine emergent wetlands and forested wetlands. The USFWS National Wetlands Inventory characterizes the wetlands as scrub shrub and emergent wetlands (see Figure 3). A desktop evaluation of current wetlands was conducted and shows approximately 12 acres of wetlands, or 3 acres more than expected under the mitigation plan (see Figure 4).

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 11

Figure 3. National Wetlands Inventory Wetlands within the Weir Modification Project Area

Desktop Determined Wetlands

Figure 4. Desktop-Determined Wetland Areas within the Weir Modification Project Area

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 12 Alternative 1 – No-Action

Without implementation of the proposed action, no change to wetland type, area, or function would occur. Flood waters would be retained within the oxbow wetland at current rates; although adjacent businesses are being flooded, this has not resulted in a different wetland type from the mitigation plan. The concrete wing walls would not be removed, and the ability to vary the weir crest elevation by adding or removing stop logs above the sill would be maintained.

Alternative 2 – Weir Modification (Preferred Alternative)

No permanent or temporary activities requiring dredge or fill would occur with implementation of the proposed action. The concrete wing walls would be removed from the site and disposed of appropriately. The ability to vary the weir crest by adding stop logs above the sill would not be maintained. Since installation of the weir in the oxbow wetland, the City, which maintains the wetland mitigation site, has not used stop logs in the weir to vary the weir crest elevation (City of Sioux Falls 2019). Altering the weir is not anticipated to affect the wetland type, area, or function. According to the FRRP wetland mitigation plan, there was adequate runoff to support the mitigated wetlands because the mean annual runoff into the site substantially exceeded the pool level of the wetlands and is wasted through a 72-inch outlet pipe. As shown in Figure 5, the surface elevations of the existing wetland exceeded the outlet elevation during storm events, which occur less than 2 percent of the time; the site was at or below the outlet elevation the remaining time. Approximately 50 percent of the time, surface elevations are the weir height. According to the FRRP wetland mitigation plan, the weir was not expected to increase the percentage of time the wetland surface elevation exceeded the outlet elevation. The South Western Avenue Drainage Analysis, which was prepared to analyze the feasibility of improvements for reducing risk of water damage from major storm events, determined that removal of the wing walls would reduce flood duration in the mitigated wetland by approximately 2 hours for 100-year storm event and would reduce flood elevations by approximately 1.25 feet (see Figure 6). In looking at a more frequent 1-inch storm event, DGR Engineering found that there would be a 0.3-foot difference between the in-place weir conditions and the proposed weir alteration at the peak of the storm (see Figure 7). As shown, there is a small reduction in peak flood elevation and duration in the wetland if the weir is modified. This, combined with the annual pool elevation and amount of time the pool elevation exceeded the outlet during storm events, indicates that only short-term, minor changes to hydrology would be expected. These changes would not affect the ability of the wetland to retain hydric soil and hydrophytic vegetation conditions. To analyze the potential effects of the proposed weir alteration on the function of the mitigation wetland, a hydrogeomorphic (HGM) analysis on the floodplain and wetland hydrology alterations, characterized under the Vhydalt variable of the analysis, was completed. In consultation with the South Dakota USACE Regulatory Branch on the best approach to analyze potential impacts, HDR focused on the Vhydalt indicator in the Interim Hydrogeomorphic Functional Assessment Model for Low-Gradient, Fine Substrate Riverine Wetlands with Defined Channels and Intermittent (Seasonal) Flow Regimes in Eastern South Dakota to determine if there was a functional change as the moderation of flow into the river is the function that would

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 13 be altered (USACE Regulatory Branch 2019). The analysis evaluated what would happen if the Vhydalt indicator was changed from the best case (pristine wetland in a non-urban area or score = 1) to worst case. The functional capacity index decreases from 9 to 6.43, or approximately a 28 percent loss in function for pristine wetland in a non-urban area. The current wetland is approximately 12 acres but was designed to sustain 9 acres for mitigation, which is 33 percent more wetland area than required. Even under the maximum possible reduction in the Vhydalt variable, total function would be reduced by 28 percent and the site would still have a higher wetland function than required for the mitigation. Under a more likely scenario, the Vhydalt variable would result in a maximum reduction of up to 0.25 in the score or 4 percent loss in function of the wetland. Overall, there would be changes to the short-term hydrological dynamics of the wetland during infrequent storm events; however, the hydrology of the site would be expected to continue to support the oxbow wetland. The function of the wetland could be decreased, but under the worst- case scenario as shown in the HGM analysis, the wetland would be expected to function above the mitigation requirements.

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Source: FRRP Wetland Mitigation Plan Figure 5. Percent of Time Elevation is Exceeded

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 14

Oxbow A Detention Characteristics

- Modify Weir Struaure - Existing Conditions

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Source: DGR Engineering Figure 6. Oxbow Detention Characteristics With and Without the Weir Modification for a 100-year Storm Event

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 15 Oxbow A Detention Characteristics - 111 Rainfal l

- Modify Weir Structure - Existing Conditions

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Source: DGR Engineering Figure 7. Oxbow Detention Characteristics With and Without the Weir Modification for a 1-inch Storm Event

3.3.3 Threatened and Endangered Species

Existing Conditions

An official species list was generated for the project from the USFWS IPaC website (accessed 4/12/2019). The official species list can be found in Appendix D. Based on this list, three species would be reasonably expected to occur within the project area: northern long-eared bat, rufa red knot, and western prairie fringed orchid. Table 2 summarizes the likelihood of occurrence and preferred habitat for each species.

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 16 Table 2: Federally Listed and Candidate Endangered and Threatened Species in the Weir Modification Project Area, Minnehaha County, South Dakota

Status Common Name Likelihood of Preferred Habitat (Scientific Name) Occurrence Threatened Northern long-eared bat Unlikely The northern long-eared bat roosts within trees (Myotis septentrionalis) greater than 3 inches in diameter with peeling bark and/or cavities. Hibernacula consists of caves or mines. Threatened Rufa red knot (Calidris Unlikely South Dakota would be stopover habitat during canutus rufa) migration. Stopover habitat includes sandbars along rivers or large streams. Threatened Western prairie fringed Unlikely The orchid resides in mesic to wet unplowed orchid (Platanthera tallgrass prairies and meadows, but have been found praeclara) in old fields and roadside ditches.

Alternative 1 – No-Action

Without implementation of the proposed action, no temporary or long-term disturbance would occur to threatened species.

Alternative 2 – Weir Modification (Preferred Alternative)

Habitat for the rufa red knot and western prairie fringed orchid does not occur within the project area. Therefore, the proposed action would have “No Effect” on those species. Roosting trees for the northern long-eared bat occur near and within the project area, but no trees would be removed for the project. It is possible that noise from removal of the weir wing walls may disturb roosting bats near the project area, but the disruption would be less than 1 day. The determination key for northern long-eared bat consultation and 4(d) rule consistency on IPaC was used. Based on the IPaC submission, the proposed action is consistent with activities analyzed in the USFWS PBO for northern long-eared bat. The proposed action may affect the northern long-eared bat; however, any take that may occur as a result of the proposed action is not prohibited under the Endangered Species Act Section 4(d) rule adopted for this species at 50 CFR 17.40(o). A verification letter can be found in Appendix D, which verifies that the PBO satisfies and concludes the responsibilities for the proposed action under Endangered Species Act Section 7(a)(2) with respect to the northern long-eared bat. Table 3 summarizes the effect determinations for the threatened and endangered species in the area.

Table 3: Threatened and Endangered Species Effect Determinations

Common Name (Scientific Name) Effect Determination Reason for Determination Northern long-eared bat May Affect Possible roosting trees are near project area. (Myotis septentrionalis) Noise may disturb roosting bats. Rufa red knot No Effect Lack of habitat. (Calidris canutus rufa) Western prairie fringed orchid No Effect Lack of habitat. (Platanthera praeclara)

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 17 3.3.4 Floodplain Resources

Existing Conditions

The USACE Omaha District’s Flood Plain Management Services Section reviewed the proposed project and considered EO 11988, Floodplain Management. EO 11988 is applicable to all planning, design, and construction civil works projects, to activities under the operation and maintenance program, and to the real estate program (ER 1165-2-26). The project is outside the Federal Emergency Management Agency’s regulated floodway. The FRRP has helped reduce flood risk in the area, but as noted in the South Western Avenue Drainage Analysis, major storm events have resulted in flooding damage to commercial properties landward of the levee in the area of South Western Avenue and West 50th Street (DGR Engineering 2016).

Alternative 1 – No-Action

Without implementation of the proposed action, stormwater is expected to continue to damage existing commercial properties during major storm events. No changes to the area protected by the FRRP would be expected.

Alternative 2 – Weir Modification (Preferred Alternative)

With implementation of the proposed action, the permanent ponding elevation within the oxbow wetland would not be affected. The USACE Omaha District’s Flood Plain Management Services Section has reviewed the project and has determined that the alteration would meet the intent of EO 11988. Alteration of the weir would improve interior drainage within the oxbow wetland and would reduce flood risk to properties landward of the levee. No adverse effects on flood risk reduction provided by the FRRP have been identified, and the project would not induce any development in the floodplain. Potential impacts on the wetland area from changes to the internal drainage in the oxbow wetland are discussed in Section 3.3.2.

3.3.5 Cumulative Impacts

The CEQ regulations (40 CFR 1500–1508) implementing the procedural provisions of NEPA, as amended (42 U.S.C. 4321 et seq.), define cumulative effects as “the impact on the environment which results from the incremental impact of the action when added to other past, present, or reasonably foreseeable future actions regardless of what agency (Federal or non-Federal) or person undertakes such other actions. Cumulative impacts can result from individually minor but collectively significant actions taking place over a period of time” (40 CFR 1508.7). The proposed weir alteration is in a developed area of Sioux Falls. The drainage subbasin consists of commercial properties, a manufactured housing development, and the City recreation offices. The urban development has resulted in conversion of natural resources to urban uses. Water is maintained through the City’s stormwater system and the FRRP. The weir was installed to satisfy the FRRP wetland mitigation plan. The wetland provides wildlife habitat and restored wetland functions along the river. No future development is expected is the area near the project.

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 18 More detailed descriptions of past and future actions are described in the cumulative impacts section of the 2005 EA. Impacts of the weir alternation that were evaluated during the preparation of this EA on the natural environment are minor in magnitude and short-term in duration, and should not have any significant adverse cumulative effects on the South Western Avenue drainage subbasin or the Big Sioux River watershed.

4.0 COORDINATION

Coordination with resource agencies was completed as part of the 2005 EA and is discussed in relevant sections of this EA. Preparation of this EA is being coordinated with USFWS in accordance with the Fish and Wildlife Coordination Act. USFWS responded that they had no additional comments on the project on June 25, 2019.

5.0 MITIGATION

The appropriate application of mitigation as defined by CEQ regulations is to formulate an alternative that first avoids adverse impacts, then minimizes adverse impacts, and lastly, compensates for unavoidable impacts. The impacts associated with altering the weir would be negligible and short-term. No mitigation is recommended. However, the City of Sioux Falls, which is the local sponsor that maintains and operates the levee system and oxbow wetland, will continue to protect the wetland area and will notify USACE if changes to the wetland are observed.

6.0 COMPLIANCE WITH ENVIRONMENTAL LAWS AND REGULATIONS

The proposed action is in compliance with federal and state environmental statutes. Appendix E details how altering the weir is in compliance with applicable laws and regulations.

7.0 CONCLUSION

The proposed action consists of removing the concrete wing walls from a weir located in a mitigated wetland near the Big Sioux River to improve internal drainage within the oxbow wetland on the landward side of the levee. This office has assessed the environmental impacts of the proposed action and has determined that the proposed action would have no adverse or beneficial impact on cultural resources and no adverse impacts on aquatic resources and fisheries; land use; recreation resources; hazardous, toxic, or radioactive waste; farmland resources; socioeconomics; or environmental justice populations. Minor, short-term impacts associated with construction on water quality, wetlands, floodplain resources, air quality, noise, terrestrial resources, threatened and endangered species, and wildlife resources would be expected. The proposed action would not adversely affect the function of the mitigated oxbow wetland and would be consistent with EO 11988 on floodplain management. No cumulative impacts are expected to result from the proposed action.

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 19 8.0 PREPARED BY

This action is being completed in accordance with CEQ regulations in Section 1506.5(a) and 1506.5(b), which allow an applicant to prepare an EA for federal actions. USACE has independently evaluated and verified the information and analysis undertaken in this EA and takes full responsibility for the scope and content contained herein. The EA and the associated draft FONSI were prepared by Laura Lutz-Zimmerman, Jill Rust, and Kendall Vande Kamp of HDR for USACE. Quality control review was completed by Brian Goss of HDR.

______Signature – Reviewed by Date Aaron Quinn Environmental Resource Specialist

______Signature – Accepted by Date Eric Laux Chief, Environmental Resources and Missouri River Recovery Program Plan Formulation Section

Draft EA U.S. Army Corps of Engineers June 2019 Omaha District 20 9.0 LITERATURE CITED

City of Sioux Falls. 2016. Shape Sioux Falls 2040: Sioux Falls Comprehensive Development Plan. September. Accessed on May 6, 3019. https://www.siouxfalls.org/planning- dev/planning/shape. City of Sioux Falls. 2019. Personal communication between Andrew Berg, City of Sioux Falls, and Kendall Vande Kamp, HDR. April 17. DGR Engineering. 2016. Southwestern Avenue Drainage Analysis. Prepared for the City of Sioux Falls, SD. September. SDDENR. 2018. The 2018 South Dakota Integrated Report for Surface Water Quality Assessment. SDDENR, Pierre, South Dakota. SDDENR. 2019. Real Time Air Quality Map. Accessed on April 12, 2019. https://denravweb.sd.gov/AirVision/default.aspx. Tomhave, D.W., and L.D. Schulz. 2004. Bedrock Geologic Map Showing Configuration of the Bedrock Surface in South Dakota East of the Missouri River, South Dakota Geological Survey, GM 9, scale 1:500,000. USACE. 2005. Supplemental EA for the Sioux Falls Flood Protection Project, Sioux Falls, South Dakota. July 2005. USACE Regulatory Branch. 2019. Personal communication between Nathan Morey, USACE Regulatory Branch, and Kendall Vande Kamp, HDR. April 1. U.S. Census Bureau. 2010. Selected tables for Sioux Falls and Minnehaha County, South Dakota. Accessed on May 6, 2019. https://factfinder.census.gov/faces/nav/jsf/pages/index.xhtml.

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