APPENDIX H: SFEIS FLOODPLAIN IMPACT ANALYSIS SECTION

Transcription

1 TH 36/STH 64 St. Croix River Crossing 2005 Water Resources Preliminary Design Report APPENDIX H: SFEIS FLOODPLAIN IMPACT ANALYSIS SECTION

2 within the project area. Other methods of attaining this reduction to the St. Croix River may be through phosphorus banking opportunities, where a project reduces phosphorus levels in a tributary to the river. Even though a TP reduction is achieved for the entire project area, discussions have and will take place with regulatory agencies during final design to determine if mitigation is needed FLOODPLAINS Figure 10-3a of the SDEIS illustrates the existing boundaries of the St. Croix River 100-year floodplain, elevation feet (1998 datum), based on data provided by the U.S. Army Corps of Engineers. The existing floodplain is broad and wide on the Minnesota side of the river where terrain is relatively flat, and narrower on the Wisconsin side of the river where the terrain is more steep. The flood profile of this stretch of the St. Croix River is governed by hydraulic structures on the Mississippi River. This section addresses the impacts to floodplain from the Preferred Alternative. In accordance with Executive Order 11988, Minnesota Statute 103F F.155, and Wisconsin Statute 87.30, an analysis of floodplain impacts that would result from the Preferred Alternative was completed. Under these statutes, the Preferred Alternative will be determined to have no adverse floodplain impacts if: No significant potential for interruption of a transportation facility which is needed for emergency vehicles or provides a community s only evacuation route will occur; No significant impact on natural and beneficial floodplain values will occur; No significant increased risk of flooding will result; and The project will not support and/or result in incompatible floodplain development. Refer to Section of this SFEIS for a discussion of Preferred Alternative floodplain impacts Information from the SDEIS that Remains Unchanged The information presented in Section 10.3 of the SDEIS regarding floodplains has not changed. Estimates of floodplain fill and excavation activities associated with the Preferred Alternative were provided to the NPS for completion of the Section 7(a) Evaluation for the project Changes in the Setting or Technical Analysis Since the SDEIS Since release of the SDEIS, the Preferred Alternative design for TH 36 (TH 5 to Osgood Avenue) has been revised to incorporate the design approved with the 1995 FEIS Preferred Alternative. Refer to Chapter 3 of this SFEIS for a detailed description of the Preferred Alternative TH 36 design through the Oak Park Heights area. This segment of the project is located in the Minnesota upland area, west of the St. Croix River, and will not result in any floodplain impacts. St. Croix River Crossing Project Supplemental Final EIS

3 There have been no changes to the proposed floodplain impacts or technical analysis since the SDEIS Impacts Associated with the Preferred Alternative A floodplain assessment analyzed flooding risks, longitudinal/transverse impacts and activities that will occur in the floodplain as a result of the Preferred Alternative. The assessment identified the area of any permanent impacts between the 100-year flood elevation and normal water elevation. Encroachments on the floodplain were also identified. Impacts (longitudinal and transverse) to the 100-year floodplain (elevation = feet) of the St. Croix River from the Preferred Alternative have been evaluated and summarized in Table Figure 10-3 illustrates the Preferred Alternative alignment in relation to the 100-year floodplain. Figure 10-3a illustrates the loop trail alignment in relation to the 100-year floodplain. Impacts resulted from the following sources: Preferred Alternative River Crossing and Roadways The Minnesota approach bridge piers will require fill in the 100-year floodplain. The extradosed bridge piers (4 to 6 piers anticipated in the river) will also require fill in the 100- year floodplain. One extradosed bridge pier is anticipated to be located along the Wisconsin bluff. Because the 100-year floodplain is quite narrow along the Wisconsin shoreline (compared to the Minnesota shoreline), it is possible that this pier along the Wisconsin bluff will not be located in the 100-year floodplain. Exact pier location will be determined through the visual quality planning process and final bridge design. Piers are not expected to influence the river hydraulics, because the river at flood stage flows at a very low velocity due to the lake characteristics found here. The Preferred Alternative will result in longitudinal disturbances for the construction of the Stillwater Municipal Barge Facility property access road and loop trail. One wet detention pond (Riverside Pond; see Figure 10-3 of this SFEIS) will be located partially in the 100-year floodplain. Impacts to the floodplain have been included as pond construction will require a berm between the pond and the river. Preferred Alternative Mitigation Items Completion of the loop trail mitigation item (see Section of this SFEIS) will result in fill and excavation in the 100-year floodplain in the Stillwater Municipal Barge Facility property. A small portion of the loop trail will be located below the 100-year floodplain elevation on the Lift Bridge causeway in Wisconsin. St. Croix River Crossing Project Supplemental Final EIS

4 TABLE 10-4 PREFERRED ALTERNATIVE FLOODPLAIN IMPACTS (1) Floodplain Encroachment (Below Elev feet) Minnesota Wisconsin Longitudinal Transverse Longitudinal Transverse Preferred Alternative Loop trail (2) 2,500 (3) (4) (1) Includes floodplain impacts from all items listed above in Section , except impacts from construction of approach bridge and extradosed bridge piers. (2) Refer to Figure 15-2 in Chapter 15 of this SFEIS for an illustration of the loop trail location. (3) Approximately 2,700 feet of the pedestrian trail portion of the loop trail system through the Stillwater Municipal Barge Facility property will be partially located below the 100-year floodplain elevation. (4) Approximately 350 feet of the loop trail will be located below the 100-year floodplain elevation on the Wisconsin causeway east of the Lift Bridge abutment. This portion of the loop trail will be located within the existing STH 64 road right-of-way. The NPS draft Section 7(a) Evaluation discusses an estimate of the amount of fill and excavation below the 100-year floodplain elevation level with the project. The NPS concluded in their draft Section 7(a) Evaluation that the project would have no measurable influence on flood elevations, the velocity of the river, and water depth upstream. Refer to the Section 7(a) Evaluation in Appendix F of this SFEIS for additional information Potential for Interruption of a Transportation Facility No potential for interruption of a transportation facility will result from the Preferred Alternative because all roadway grade elevations will be above the 100-year floodplain elevation of feet Impacts on Natural and Beneficial Floodplain Values No substantial impacts on natural and beneficial floodplain values will result from the Preferred Alternative. Temporary and permanent erosion control measures will be used where appropriate and will be designed to meet Mn/DOT and MPCA guidelines and applicable Wis/DOT and WDNR Administrative Codes. (A temporary disruption of the navigational and recreational use of the river may occur during bridge construction, as described in Chapter 12 of this SFEIS). Activities within the Lower St. Croix Riverway are subject to regulation under Wisconsin Statute 30.27, Lower St. Croix River Preservation, Minnesota Statute , Lower St. Croix National Scenic Riverway, and the amendments to the Wild and Scenic Rivers Act (Public Law ). Appendix E includes the Final Section 4(f) Evaluation for the Lower St. Croix National Scenic Riverway. Both beneficial and adverse impacts on the following areas are discussed in Chapter 9 and Chapter 12 of this SFEIS. The topics discussed include fisheries, plants, public access (boat St. Croix River Crossing Project Supplemental Final EIS

5 and/or canoe), channel changes, boat passage, threatened and/or endangered species, and various temporary and permanent erosion control measures Flooding Risks No increase in flooding, change in the river profile, or increase in the 100-year and 500-year floodplain elevations is expected as a result of the Preferred Alternative. The project encroachment into the floodplain will be limited to the items described above in Section of this SFEIS. Based on discussions with the U.S. Army Corps of Engineers, the flood stage through this part of the St. Croix River is flat and flood stages in this reach are controlled by hydraulic structures on the Mississippi River. Given the lake-like nature of this stretch of the river, velocities at the bridge crossing average less than one foot per second at flood stage. Mn/DOT completed a bridge hydraulics study in 1999 which concluded that the new bridge construction considered at that time would result in a maximum stage increase of 0.01 feet. A similar, or possibly less impact, would be anticipated with the Preferred Alternative extradosed bridge (4 to 6 piers in the river). Consequently, it is anticipated that the project will not increase flood stage in the area and the proposed floodplain encroachments will not affect the flood profile Incompatible Floodplain Development No incompatible floodplain development (i.e., any development that is not consistent with a community s floodplain development plan) will result from the Preferred Alternative. The cities of Stillwater and Oak Park Heights in Washington County and St. Croix County in Wisconsin have zoning ordinances regulating floodplain development. Development within the Lower St. Croix National Scenic Riverway is subject to regulation under the statutes noted above in Section Appendix E of this SFEIS includes the Final Section 4(f) Evaluation for the Lower St. Croix National Scenic Riverway Preferred Alternative Mitigation Measures The Preferred Alternative has been designed to limit floodplain impacts to the maximum extent practicable. Impacts due directly to roadway construction have been minimized to the extent possible. Impacts as a result of approach bridge and extradosed bridge construction will be refined with final bridge design. The preliminary designs for the wet detention ponds included minimization of floodplain encroachment by locating the ponds above the 100-year floodplain where possible. In addition, waterway crossings (including the proposed bridge and any other localized water crossings or localized flood area crossings that may be identified during final design) will comply with both Minnesota and Wisconsin regulations, and will minimize the headwater and tailwater impacts of the structures. The Preferred Alternative will not substantially increase flooding risks, impact the floodplain s natural and beneficial values, or support incompatible floodplain development. The Preferred Alternative will eliminate any potential for disruption of a transportation facility due to flooding. The Preferred Alternative includes measures to minimize floodplain impacts and preserve the natural and beneficial floodplain values. Therefore, no substantial impacts on the floodplain will result from the Preferred Alternative. St. Croix River Crossing Project Supplemental Final EIS

6 10.4 GROUNDWATER Any type of development, including roadway construction, can affect groundwater in a number of ways. First, the groundwater elevation can be temporarily or permanently altered due to dewatering efforts. Second, an increase in impervious surfaces can reduce movement of precipitation to the groundwater tables. Third, filling of wetlands and ponds can also reduce the migration of surface water to the underlying layers. Fourth, groundwater quality can also be affected by contaminated surface water migrating through the surficial layers to groundwater aquifers, or migrating directly to the aquifers via bedrock outcropping. This section summarizes any impacts to groundwater from the Preferred Alternative, as well as mitigation measures proposed Information from the SDEIS that Remains Unchanged Groundwater data provided in the SDEIS is accurate, and this information remains unchanged. Section of the SDEIS described the deep groundwater aquifers in the project area: the Prairie Du Chien and Jordan aquifers. Depth of these aquifers varies greatly throughout the corridor. In Minnesota, depths of the water table drops from approximately 175 feet in the upland areas to approximately 20 feet at TH 95. In Wisconsin, depth of the water table varies from approximately 105 feet along STH 35 to 10 feet (or less) near the eastern project terminus. Information was also presented on wells and groundwater recharge in the project area. Well depths in Minnesota range from 70 to 650 feet, with a majority of wells drawing from the Jordan aquifer. Residential wells in Wisconsin draw water from at least 100 feet, with a majority of wells having depths greater than 150 to 250 feet below ground level. An in-depth analysis of groundwater issues, Analysis of Highway Stormwater Runoff Impacts to Potable Groundwater Aquifers, (April 13, 2004), guided conclusions in the SDEIS. No changes to this analysis have occurred since the SDEIS. This report concluded that it is not expected that the existing wells in the project area will experience heavy metal contamination from highway stormwater drainage. It is possible that there will be some occasional and temporary increases in salt content, but it is expected that these concentrations would be infrequent (spring season) and within acceptable State water quality health standards Changes in the Setting or Technical Analysis Since the SDEIS Since release of the SDEIS, the Preferred Alternative design for TH 36 (TH 5 to Osgood Avenue) has been revised to incorporate the design approved with the 1995 FEIS Preferred Alternative. Refer to Chapter 3 of this SFEIS for a detailed description of the Preferred Alternative TH 36 design through the Oak Park Heights area. This segment of the Preferred Alternative will largely be constructed at existing roadway grades. While the amount of impervious surface will increase with the Preferred Alternative in this segment of the project (see Section 10.1 of this SFEIS), the amount is less than proposed with the Preferred Alternative (Alternative B-1) as described in the SDEIS. St. Croix River Crossing Project Supplemental Final EIS