Old Channel of the Comal River Bank Stabilization Draft

Transcription

1 Old Channel of the Comal River Bank Stabilization Draft Prepared for: City of New Braunfels Prepared by: REESE AND NICHOLS, INC Jollyville Rd, Bldg 4, Suite 100 Austin, Texas NEB13133

2 Old Channel of the Comal River Bank Stabilization Prepared for: City of New Braunfels DRAT THIS DOCUMENT IS RELEASED OR THE PURPOSE O INTERIM REVIEW UNDER THE AUTHORITY O M. LESLIE BOYD, P.E., TEXAS NO ON 06/11/2013. IT IS NOT TO BE USED OR CONSTRUCTION, BIDDING OR PERMIT PURPOSES. Prepared by: REESE AND NICHOLS, INC. Texas Registered Engineering irm Jollyville Rd, Bldg 4, Suite 100 Austin, Texas NEB13133

3 TABLE O CONTENTS 1.0 INTRODUCTION Background PROJECT APPROACH Geomorphological Evaluation Riparian Vegetation Plan Bank Stabilization Evaluation STABILIZATION CONCEPT DEVELOPMENT Background Initial Stabilization Alternatives Selected Concepts RECOMMENDATIONS COST OPINIONS CONCLUSION Table of Illustrations Photo No. 1 Sediment Island and Overgrowth... 1 Photo No. 2 Leaning Tree at the Top Edge of Bank... 2 Photo No. 3 Invasive Elephant Ear Thicket... 2 Photo No. 4 Washed Bank above Sediment Island... 7 Photo No. 5 TRM Installation on Steep Bank...13 Photo No. 6 Tree Roots Clinging to Steep Bank...18 igure 1.1 Existing Conditions Map... 4 igure 3.1 Slope Reducing Alternative: Typical Section igure 3.2 Waler and Anchor System: Typical Section igure 3.3 Waler and Anchor System Conceptual Sketch Table of Tables Table 5.1: POPCC for TRM and Anchored Waler Stabilization Table 5.2: POPCC for Slope Reduction with TRM Stabilization Appendices Appendix A Geomorphologic Site Assessment Appendix B Riparian Vegetation Plan Appendix C Topographic Survey Appendix D Geotechnical Exploration Report Appendix E Conceptual Drawings of Proposed Stabilization Alternatives

4 1.0 INTRODUCTION This report addresses the study and evaluation of possible bank stabilization alternatives for the Old Channel reach of the Comal River in Landa Park, New Braunfels, Texas. The reach under study extends from the culverts on the east side of Landa Lake to the culverts beneath Golf Course Road near the golf clubhouse. This report is the third component of the reese and Nichols, Inc. (NI) assignments addressing this reach of the river. 1.1 BACKGROUND This reach of the Old Channel of the Comal is home to several endangered species, especially the fountain darter. Surface runoff and flooding have steepened the north bank of the river and resulted in the deposition of a sediment island (Photo No. 1). The sediment island reduced the available area for fountain darters and was removed in March Large trees are clinging precariously onto the top edge of the bank (Photo No. 2). Invasive species of plants and trees have invaded the reach and further reduced the preferred habitat of the darters (Photo No. 3). Photo No. 1 Sediment Island and Overgrowth

5 Photo No. 2 Leaning Tree at the Top Edge of Bank Photo No. 3 Invasive Elephant Ear Thicket

6 The City of New Braunfels in conjunction with the Edwards Aquifer Recovery Implementation Program (EARIP) have embarked upon conserving the endangered species habitat in Landa Park and have engaged a variety of consultants and contractors to provide services on numerous areas within the Landa Habitat Conservation Plan (HCP). The City has developed a plan for this specific reach of the Comal which includes modifying the culverts on the east side of Landa Lake (Halff Associates), removing the sediment island (D&M Construction), replanting native plants upon removal of the sediment island (Bio-West), and developing bank stabilization concepts along with riparian restoration for the reach (reese and Nichols, Inc). Henceforth, this report will present the reese and Nichols, Inc. work efforts on this reach of the Comal River. The project map is shown in igure 1.1 and depicts existing conditions at the site with notations.

7 IGURE acing upstream - Banks are laid back, sparsely vegetated with few locations of minor undercut banks. 8. acing left bank - Vegetation is providing toe protection. 10. acing left bank - Overland sheet flow from the golf course flows down the bank eroding the soil, exposing roots, and forming rills and gullies. 12. acing left bank - Steep, near vertical bank with large void spaces in the bank profile. Banks are undercut and channel bed is covered in about 8 inches of sediment with vegetation growing on top. ILE NAME: Ex 3 Channel Assessment- ANSI D.mxd COORDINATE SYSTEM: NAD 1983 StatePlane Texas Central IPS 4203 eet SCALE: 500 DRATER: JJC DATE: May, 2013 Observed Water Level Bank Erosion Surface low from Golf Course 0 4. acing upstream - Minor undercut toe along both banks near the beginning of a riffle-pool sequence. Minor undercut banks with exposed roots make good aquatic habitat Jollyville Rd. Bldg. 4, Suite 100 Austin, Texas Phone - (512) acing downstream - Main outfall from the lake to the stream. Rock riprap lined for bed and bank protection. NI NO. : NEB13133 City of New Braunfels Ba n k 100 Right "Old Channel" of the Comal Bank Stabilization Left Ban k Areas of Interest N 11. acing downstream - Mass wasting on left bank has led to tree fall. 9. acing left bank - Steep, bare ground, gully erosion, and approximately 2-ft undercut bank. eet 7. On left bank - Vegetative ground cover can help stabilize banks. Deer may prevent or slow the establishment of ground cover. 5. acing left bank - Overland sheet flow from the golf course On left bank - Seepage spring 1. acing upstream - The toe of the slope was undercut, exposing roots.

8 2.0 PROJECT APPROACH NI developed a three stage approach to evaluate this reach of the river: Perform geomorphological evaluation Perform riparian vegetation plan Perform bank stabilization evaluations Each of these will be discussed in brief details below. 2.1 GEOMORPHOLOGICAL EVALUATION The geomorphological evaluation was performed to provide information on the erosive forces acting upon the reach and to observe the stability of the existing channel and banks. The existing river model was obtained from Halff Associates and modified as necessary to provide a model for use in the geomorphological assessment. The assessment evaluated the shear stresses acting upon the stream bottom and banks. Incipient motion evaluations and bend scour characteristics were also evaluated. This information was then utilized to establish which areas in the reach could be stabilized with planting and other non-mechanical stabilization methods. The geomorphological evaluation was performed first in order to delineate the limits where mechanical stabilization was deemed appropriate. Recommendations were set forth in the Geomorphic Site Assessment which is included herein as Appendix A. igure 3.8 therein, indicates the areas where shade tolerant planting, toe coir rolls, berms, and mechanical stabilization are recommended. Cost opinions will be presented herein in subsequent sections. 2.2 RIPARIAN VEGETATION PLAN The riparian restoration plan was developed on the concept of identifying all invasive plants and determining acceptable replacement plantings of native origin. The riparian restoration plan included the entire reach. It identified several invasive plants, primarily elephant ears and canes that should be removed. The riparian restoration plan is included herein as Appendix B. It was prepared subsequent to the geomorphological assessment in order to benefit from the zone delineations of the geomorphological assessment. Cost opinions will be presented herein in subsequent sections.

9 2.3 BANK STABILIZATION EVALUATION The bank stabilization evaluation of alternatives was developed following the geomorphological assessment and the riparian vegetation plan. Initial work included establishing sub-contracts to perform survey work and geotechnical investigations on the reach under study. The Schultz Group of New Braunfels performed the topographical survey for the project. Their survey was utilized in all three segments of work in this project. The topographic maps are included in Appendix C. Brierley Associates performed the geotechnical investigation of the site from their Austin office. Six borings were completed for the project. our holes were on the north bank near the golf course and two holes were on the south bank in the flat area adjacent to the sediment island. The geotechnical report is also included herein as Appendix D. The results of both reports will be discussed in more detail in subsequent sections.

10 3.0 STABILIZATION CONCEPT DEVELOPMENT 3.1 BACKGROUND In initial meetings with the City of New Braunfels (CONB) representatives noted that they were interested in stabilizing the bank, but they wanted the solution to be attractive. CONB representatives ruled out the use of gabions and sheet piling. NI agrees that the high visibility of this project warrants an attractive solution. When geotechnical borings were selected for the site and the drill rig was ready to mobilize, there were some anxious moments before authorizing the drilling to proceed. CONB expressed the high archaeologically importance of this site. Review of archaeological mapping indicated the entire study area has a high probability of cultural resource deposits. The drilling was allowed to proceed only due to the minute surface area being affected. The sediment island which had formed at the bend in this reach was the result of sediment deposition either from sediment transport or bank erosion from surface runoff. Initial observance by the geomorphological team indicates that it is most likely caused primarily by surface erosion. Proof of this is visible in the washed portion of the bank above the sediment island (Photo No. 4). Photo No. 4 Washed Bank above Sediment Island

11 Cross sections taken from the topographical mapping indicate that the top of bank is approximately twenty (20) feet above the normal water surface in the reach near the sediment island and downstream therefrom. The edge of bank offset horizontally from the water s edge is typically fifteen (15) to twenty (20) feet. The steepness of the slope severely hampers the options available to correct the bank without encroaching onto the nearby golf course. The geotechnical results indicate alluvial clays in the upper segments of the bank and they are underlain by gravelly clays and clays having both gravel and cobbles in their matrix. The interface between the clays and gravelly clays was found to be near elevation 617 on the four (4) borings on the north bank. The interface occurred near elevation 611 on the two (2) borings on the south. All of the above background information pieces served to direct the NI team in development of stabilization alternatives and will be discussed below as various stabilization alternatives are discussed. 3.2 INITIAL STABILIZATION ALTERNATIVES The portion of bank requiring mechanical bank stabilization is approximately three hundred (300) feet in length. It stretches from approximate station (just upstream of the sediment island) to station (near the south end of golf clubhouse). There are numerous methodologies available for stabilizing earthen streambanks. Some of the most common include: Concrete retaining walls Steel sheetpile walls Crib walls Mechanically stabilized earth slopes (MSE) Rock gabions Soil nail walls Turf mat reinforced slopes Tie-back walls Rock riprap slope paving Concrete slope paving Grassed slopes Concrete secant pile wall

12 The following considerations were considered most significant in selecting a proposed bank stabilization method. Minimal excavation due to archaeological concerns Appearance Resistance to erosion Cost MSE walls were initially a top contender for stabilization. However, MSE walls require approximately 70 percent of their height as a minimum base width. This would result in very wide cuts into the existing bank and MSE was thus dismissed. The concerns were archaeological as well as space constraints and near vertical cuts approaching 20 feet in height. Additionally, the golf clubhouse is situated at the edge of the top of bank, which prevents extensive excavation of the slope in this area. The next favored contender was soil nail walls. They can be constructed from the top down and can be artistically treated to resemble stones, rock layers or any other number of pleasant looking architectural features. The preferred height was approximately 10 feet for the soil nail wall, but stability issues were identified at the toe of the wall due to the gravelly material below the toe. Additionally, there is no bank to cut into at the location where the wall was desired. Thus, all soil nailed walls would have to be formed and backfilled for their entire height. This configuration limits the incremental benefits gained from the soil nail walls over other alternatives. Another benefit of the soil nail walls was their ability to produce a tree well around some of the edge hanging trees. The soil nail solution is not considered further at this time, but could be resurrected if interest in tree wells prevails. Crib walls are often found in bank stabilization, but they require approximately the same excavation as MSE walls for stability. Concrete retaining walls would also fall into the excessive excavation category because their typical base to height ratio is also around 70%. Concrete secant piles and concrete slope paving were considered to be as poor in appearance as gabions and steel sheetpile walls, which were explicitly ruled out on grounds of appearance by CONB. These four (4) were summarily abandoned from consideration. Grassed slopes and rock rip rap slope paving were dismissed from consideration due to the steep bank being almost 1H:1V.

13 We are left with turf mat reinforced slopes and tie back walls. These two concepts are further developed in the following section. 3.3 SELECTED CONCEPTS We believe that turf reinforcing mats (TRM) represent a reasonable bank stabilization method for the slopes in our study reach. They are resistant to velocities beyond those indicated in the hydraulic models. We would expect good service from TRM especially when protected from excessive overland flows by constructing a diversion berm at the top of slope as discussed in the geomorphologic assessment. There are essentially two different types of material to be protected on the banks, i.e., clays and gravelly clays. The geotechnical report indicates that the gravelly clays are stable at slopes just slightly above 2H:1V and the clays are stable at slopes up to 1H:1V. A majority of the existing slopes in our 300 feet length to be mechanically stabilized exceed these maximums. Where this occurs, we can either excavate back to a lesser slope or mechanically anchor the bank with a system of tie back anchors and walers. Excavating back to a lesser slope is indicated in igure 3.1. One drawback with this solution is that, due to the extensive excavation, there exists a high possibility of unearthing archaeological remnants. This could represent a significant cost and delay in the project. urther, review of the cross sections in Appendix E associated with this alternative indicates that significant encroachment onto the golf course can occur. Also, any trees within the excavation would be lost. These are significant shortcomings for this solution. The system of TRM with tie back anchors and walers is indicated in igure 3.2. This solution would essentially dress the slope to its present configuration and add anchors and walers as indicated to provide stabilization to the already steep slopes. The addition of anchors with walers will significantly increase the project cost, but would not require extensive excavation. This stabilization system will give the sloped bank a terraced appearance, and the walers can be made to look like a natural feature (e.g. limestone outcrop, old rock wall, etc.). It is possible that the final solution to stabilization would be one where the appropriate method was field selected on the basis of existing slope conditions. A conceptual sketch of the waler and anchor system is presented in igure 3.3. A TRM installation on a steep bank (without anchors and walers) is shown in Photo No. 5 for visualization purposes. TRM can be cut and molded around trees, so the reach s existing trees can be incorporated into the final visual appearance of the rehabilitation of the bank. Appendix E shows conceptual layouts and sections for each of the two options discussed herein.

14 igure 3.1 Slope Reducing Alternative: Typical Section igure 3.2 Waler and Anchor System: Typical Section

15 igure 3.3 Waler and Anchor System Conceptual Sketch

16 Photo No. 5 TRM Installation on Steep Bank

17 4.0 RECOMMENDATIONS Based upon the assessments performed for this project, we respectfully recommend the following: Geomorphological (Ref. igure 3.8, Appendix A) 1. Provide shade tolerant plants along the north bank (left) from the Landa culverts to approximately 400 feet downstream. 2. Provide a grassed earthen diversion berm beginning near the north end of the golf clubhouse and extending for approximately 500 feet upstream along the top edge of the north bank. 3. Provide core roil toe protection for approximately 125 feet downstream of Landa culverts on both the left and right bank. 4. Remove invasive plants along the north (left) toe from the end of the core roils to the start of the bank stabilization. Replant native species along this toe. 5. Remove invasive plants along the right toe from the end of the core roils to the culverts at Golf Course Road. Replant native species along this toe. Riparian Vegetation (Ref. Appendix B) 1. Remove invasive plants from Herbs 1 through 13 as indicated on Attachment 1 of Riparian Vegetation Plan. 2. Replant native vegetation in Herbs 1 through 13 in accordance with Section 3.2 of Riparian Vegetation Plan, page 7 and 8. Bank Stabilization 1. Stabilize the existing bank on the north side of the river from the south end of the golf clubhouse to approximately 300 feet upstream. 2. Minimize excavation of existing slopes by clearing existing slopes to present grade. 3. Stabilize cleared slopes with TRM and provide three rows of anchored walers as indicated on igure Trim TRM to fit around all trees chosen to remain.

18 5.0 COST OPINIONS We have prepared preliminary opinions of probable construction cost (POPCC) for the recommendations set forth in Section 4.0. In addition, a POPCC has been prepared for bank stabilization for TRM reinforcement of slopes as shown in igure 3.1. Table 5.1 presents the POPPC for the TRM/anchor/waler bank stabilization applied to the existing slopes. Table 5.2 presents the POPCC for the TRM reinforcement on trimmed back slopes for cost comparison. Table 5.1 City of New Braunfels Old Channel of the Comal Landa Park Preliminary Opinion of Probable Construction Cost Bank Rehabilitation - Waler and Anchor Slope Dressing ESTIMATOR GSH CHECKED BY MLB ACCOUNT NO. NEB13133 ITEM DESCRIPTION QUANTITY UNIT UNIT PRICE TOTAL 1 Mobilization 1 LS $ 20,000 $ 20,000 2 Site preparation and maintenance 1 LS $ 10,000 $ 10,000 3 Permanent turf reinforcement mat 2,250 SY $ 8 $ 18, " ground anchors 150 EA $ 400 $ 60,000 5 Cast-in-place concrete walers 1,000 L $ 35 $ 35,000 6 Coir rolls 250 L $ 24 $ 6,000 7 Toe plantings 1,500 L $ 24 $ 36,000 8 Shade tolerant plants 1 LS $ 5,000 $ 5,000 9 Remove invasive plants and replant native vegetation 9,000 S $ 3 $ 27, Earthen diversion berm 1 LS $ 4,000 $ 4,000 SUBTOTAL: $ 221,000 PROJECT SUBTOTAL: $ 221,000 CONTINGENCY: 30% $ 66,000 PRELIMINARY OPINION O PROBABLE CONTRUCTION COST $ 287,000 THIS DOCUMENT IS RELEASED OR PURPOSE O INTERIM REVIEW. IT IS NOT TO BE USED OR CONSTRUCTION OR BIDDING PURPOSES.

19 Table 5.2 City of New Braunfels Old Channel of the Comal Landa Park Preliminary Opinion of Probable Construction Cost Bank Rehabilitation - Slope Reduction ESTIMATOR GSH CHECKED BY MLB ACCOUNT NO. NEB13133 ITEM DESCRIPTION QUANTITY UNIT UNIT PRICE TOTAL 1 Mobilization 1 LS $ 13,000 $ 13,000 2 Site preparation and maintenance 1 LS $ 6,000 $ 6,000 3 Permenant turf reinforcement mat 2,500 SY $ 8 $ 20,000 4 Excavation 1,200 CY $ 20 $ 24,000 5 Coir rolls 250 L $ 24 $ 6,000 6 Toe plantings 1,500 L $ 24 $ 36,000 7 Shade tolerant plants 1 LS $ 5,000 $ 5,000 8 Invasive plant removal 9,000 S $ 3 $ 27,000 9 Earthen diversion berm 1 LS $ 4,000 $ 4,000 SUBTOTAL: $ 141,000 PROJECT SUBTOTAL: $ 141,000 CONTINGENCY: 30% $ 42,000 PRELIMINARY OPINION O PROBABLE CONTRUCTION COST $ 183,000 THIS DOCUMENT IS RELEASED OR PURPOSE O INTERIM REVIEW. IT IS NOT TO BE USED OR CONSTRUCTION OR BIDDING PURPOSES.

20 6.0 CONCLUSION During our site assessments, our staff visited with local residents familiar with this study reach of the Comal River. Residents indicated that for more than 20 years they have not noted any significant changes in the banks in this area and that the trees on the edge of the north bank have successfully survived. In the outset of this project, we had hoped to build tree wells around selected trees. We have since concluded that the trees are taking changes in stride and developing a root system which keeps them well founded (Photo No. 6). The geomorphological assessment indicated that the majority of this reach can be stabilized with natural means of construction such as shade tolerant plants, core roils, and native plants to reinforce the toes. The riparian site assessment identified 13 different areas affected by invasive species and recommends their removal and replacement with native plants. The bank stabilization is recommended to be primarily of TRM. The TRM will reinforce the slopes against velocities far greater than any indicated in our analyses. The area in a large flood is more a backwater effect than a raging river. We have endeavored to select stabilization methods which will minimize excavation and similarly reduce the likelihood of encountering artifacts which could severely impact the project schedule and cost. The POPCC for the recommended stabilization, i.e. TRM with anchors and walers is $287,000 (Table 5.1). The POPCC for the stabilization without anchors and walers is $183,000 (Table 5.2). We recognize this is a significant difference. However, the lower cost alternative will encroach into the golf course and require the removal almost all of the trees on the upper bank. In our opinion the TRM with anchors and walers represents the better stabilization alternative. We are of the opinion that these recommended improvements will preserve and protect this reach of the Comal River for many years. It will require that a continual maintenance effort be extended to keep invasive plants from recurring and once again spoiling the fountain darter habitat.

21 Photo No. 6 Tree Roots Clinging to Steep Bank