————————————————————————————————— ————————————————————————————————— DEADMANS RUN LINCOLN, NEBRASKA SECTION 205 – FLOOD RISK MANAGEMENT FEASIBILITY REPORT APPENDIX C – GEOTECHNICAL ENGINEERING, SOILS AND GEOLOGY MARCH 2018 DEADMANS RUN – LINCOLN, NE SECTION 205 - FEASIBILITY STUDY APPENDIX C – GEOTECHNICAL ENGINEERING, SOILS AND GEOLOGY TABLE OF CONTENTS 1. INTRODUCTION 1.1 Purpose and Scope of Appendix III ………………………….............. C-1 2. EXISTING CONDITIONS 2.1 Study Area …………………………………........................................ C-1 2.2 Site Description ………………………………………………………. C-2 2.3 Physiography, Topography and Drainage ……………………………. C-4 2.4 Geology ………………………………………………………………. C-4 2.5 Available Soil Survey and Boring Information in Project Area ……… C-9 3. PROBLEM IDENTIFICATION & FUTURE W/O PROJECT CONDITIONS 3.1 General ……………………………………………………………….. C-13 4. ALTERNATIVES EVALUATED/SELECTED ALTERNATIVE 4.1 General ……………………………………………………………….. C-13 4.2 Site Selection and Project Development ……………………………… C-13 4.3 Description of Alternatives Evaluated ………………………………... C-13 4.4 Preliminary Design of Channel Alternative …………………………... C-21 4.5 Update to Tentatively Selected Plan…………………………………... C-21 5. FURTHER STUDIES, TESTING, & ANALYSES AFTER FEASIBILITY 5.1 Engineering During Preconstruction Engineering and Design Phase …C-23 6. REFERENCES ………………………………………………………………… C-25 FIGURES Figure 1 – Deadmans Run Watershed Location Figure 2 – Typical Deadmans Run Channel Photo Figure 3 – Lancaster County Geologic Cross Sections Locations Figure 4 – Interpretive Geologic Cross Section – A-A’ Figure 5 – Interpretive Geologic Cross Section – B-B’ Figure 6 – Geologic Time Scale Figure 7 – Subsurface soils encountered within Well G-131749H Figure 8 – Subsurface soils encountered within Well G-137203C Figure 9 – Alternative #1 Overview Figure 10 – Alternative #2 Overview Figure 11 – Alternative #3 Overview Figure 12 – Alternative #4 Overview Figure 13 – West Tributary Detention Basin Figure 14 – Detention Basin Embankment at Drainage Ditch EXHIBITS Exhibit C-1 - NRCS Soil Survey Maps – Deadmans Run Study Area APPENDIX III i GEOTECHNICAL ENGINEERING, SOILS AND GEOLOGY DEADMANS RUN, LINCOLN, NEBRASKA SECTION 205 – FEASIBILITY STUDY APPENDIX III – GEOTECHNICAL ENGINEERING, SOILS AND GEOLOGY 1. INTRODUCTION. 1.1 Purpose and Scope of Appendix III. The purpose of this appendix is to present the geotechnical and geological investigations/studies conducted for the Section 205 Feasibility Study, Deadmans Run (DMR), Lincoln, Nebraska. Also presented is limited information relating to Civil Design (selected alternative grading & impacted utilities) and available survey information. Appendix III consists of: (1) an introduction, (2) a discussion of existing conditions in the study area (3) a discussion of site geology, (4) available information from geotechnical investigations of the subsurface soils and bedrock formation, (5) an evaluation of the spoil bank materials, (6) a summary of the problem identification and future without project conditions, (7) a discussion of the evaluated alternatives, and (8) a discussion of further studies, testing, and analyses needed after the feasibility study phase. This geotechnical and geological analysis was prepared by the Geotechnical Engineering & Sciences Branch of the U.S. Army Corps of Engineers-Omaha District. 2. EXISTING CONDITIONS. 2.1 Study Area. Deadmans Run, a right bank tributary of Salt Creek, begins in the eastern part of the city of Lincoln near intersection of 84th and A streets. It flows northwesterly 7 miles to enter Salt Creek just below the Chicago and Northwestern Railway bridge in North-Central Lincoln. The watershed is bounded by Stevens Creek basin on the north and Antelope Creek basin on the south. Its 9.6 square miles of significantly urbanized drainage area lies entirely within Lancaster County and within the Lincoln City limits. The area discussed in this study extends from the Cornhusker Highway bridge on the downstream end to just upstream of the 48th Street bridge on the upstream end. Figure 1 is a map showing the location of the study area along DMR. APPENDIX C C-1 GEOTECHNICAL ENGINEERING, SOILS AND GEOLOGY Figure 1. Deadmans Run Watershed Location 2.2 Site Description. The DMR has a well maintained channel in general. The main channel is protected by a layer of fabric formed concrete mat lining for most of the stream except for the reach downstream of the Cornhusker Highway bridge to the confluence of the DMR with Salt Creek., and at the agricultural test field of University of Nebraska campus. Though somewhat aged, the fabric- formed concrete lining is still in good condition for most of the channel. Upward from the main channel, the embankments are typically covered by the fabric formed concrete mat up to the top edge of the main channel or higher. Then one or more steps of gabions cover the bank above the main channel. The rest of the embankment is typically covered by well-maintained grasses. The channel changes width from upstream to downstream. The unprotected channel just downstream of 48th St Bridge has been widened into a pool by erosion on the banks and the channel bed. The embankments in this segment of stream have severe erosion on both sides of channel. Sediments and debris block the flow in this stretch of the stream. Both banks are covered by dense woods and bushes. Below is a summary of stream conditions at structures in the study area. Summaries are taken from URS Group’s site visit report in Deadmans Run Hydrologic Analysis, April 2015. 1) Upstream the Cornhusker Highway Bridge. The channel bed is covered by fabric formed concrete linings up to the edge of main channel. Above the linings, the bank slope is covered by two steps of gabions and small bushes and grass up to one-quarter of slope. Then the slope APPENDIX C C-2 GEOTECHNICAL ENGINEERING, SOILS AND GEOLOGY continues with fabric formed concreted linings on the banks up to three-quarter of the slope. Dense grasses cover the rest of the slope to the top of embankment. No visible sign of erosion on the bank or channel. 2) Railroad Bridges. Downstream: Channel is covered by fabric formed concrete linings and bank is protected by 2-step gabions on each side along the main channel. Channel is clean. Minor growth of vegetation on the bank slope. Upstream: Fabric formed concrete linings and bank is protected by 2-step gabions on each side along the main channel. Channel is clean. Minor growth of vegetation on the bank slopes. Channel becomes wider just upstream of the bridge and continues downstream. 3) North 33rd Street and Baldwin Road Box Culvert. Upstream: Channel is covered by fabric formed concrete linings on bed and both banks. No visible erosion. Downstream: Channel is protected by fabric formed concrete linings on bed and banks. No visible erosion. 4) Huntington Avenue Bridge, west of 35th Street. Upstream: Vertical erosion caused by high flood can be seen on banks above gabions at about 500 feet upstream of the bridge. One extra step of new gabion (< 1 year) was placed on top of the original gabion on both banks from 50 feet away from the bridge abutment toward upstream for about 60 feet long. Channel was widened by flow from about 250 feet upstream of bridge toward downstream. There are some deposit of sediment under the bridge. The deposit is about 6 inches deep in average. There is no sediment deposit upstream of the bridge. There is one culvert output right on the upstream edge of the bridge on south side. It is suspected that the sediment could be from the bank just upstream of the bridge or from the culvert. But the pipe in culvert is clean. It doesn't look like that it is from the culvert. There are some sign of erosion on the north bank just upstream of the abutment. It is the most likely source of sediment. Downstream: Downstream has no sign of erosion. Some sediment deposited on the channel bed downstream from the bridge. The deposit is about 6 inches deep in average. 5) East Campus Bridge (North 38th Street). Upstream: No erosion sign near the bridge. Fabric form mat protected channel and gabions. Stream upstream of DMR030 and below DMR035 has a confluence from a tributary. The channel in this part suffers severe erosion on earth banks. No protection measure exist. Ripraps were applied to two location on the north bank and south banks. Pools formed in the channel near the downstream of DMR035. Downstream: No erosion sign near the bridge. Mat and gabion. 6) North 48th Street Bridge, between Garland and Francis Streets. Downstream: Downstream channel suffered severe erosion on bed and the banks. The embankments retreated to form a pool in the channel downstream from the bridge about 250 feet long. The maximum channel width at this location is about 4 times of the original width. Further downstream, there is no protection on the channel bad and embankments. The embankment is covered by dense bushes. Sign of erosion is widespread in this reach all the way down the stream until the upstream of structure DMR030. Upstream: Upstream channel is covered by fabric formed concrete mat. After decades, the mat is still in a good condition. The embankment is covered by 3 steps of gabions APPENDIX C C-3 GEOTECHNICAL ENGINEERING, SOILS AND GEOLOGY above the tope edge of the mat. No sign of erosion. Above the gabion, the embankment is covered by short grasses. On January 19, 2016, the USACE Project Development Team (PDT) conducted a site visit to the study area. In general, the PDT concurred with the description of the study area as described above by URS Group. The existing Deadmans Run Channel is approximately 100 feet in width throughout the study area. Exceptions to this width occur at the box culvert at 33rd Street and Baldwin Avenue, where the width is approximately 35 feet; and at the two railroad bridges, where the width is approximately 90 feet. Throughout most of the study area, the main channel is protected by a layer of fabric formed concrete mat lining for most of the stream except at locations near the downstream, between the Deadmans Run mouth and Cornhusker Highway, and at the agricultural test field of the University of Nebraska campus.