C04_Att 5_ Design Rpt Cedar Creek 16A_.pdf

DESIGN REPORT

For

CEDAR CREEK WATERSHED

SITE 16A

REMEDIAL REPAIR

DECEMBER 2024

TABLE OF CONTENTS

CEDAR CREEK WATERSHED, SITE 16A

REMEDIAL REPAIR

CONTENTS PAGE NO.

Project Description

General Design of Existing Structure

Survey

Repair Work Design

Cultural Resources

Environmental Considerations

Construction Review

Construction Specifications and Bid Schedule

Construction Schedule

Operation and Maintenance

Appendices:

A. Attachment to the Operation and Maintenance Plan

CEDAR CREEK WATERSHED

SITE 16A

PROJECT DESCRIPTION

The National Inventory of Dams (NID) identification for Cedar Creek Site No. 16A is TX00790.

The structure is a single-purpose, earthen dam designed in 1968 for flood prevention. Construction was completed in 1969. The structure is an integral part of the Cedar Creek Watershed (a sub-watershed of the Trinity River Watershed). Site No. 16A is approximately 10 miles southeast of the city of Rockwall, in Rockwall County, Texas. The site is situated within the Blackland Prairie physiographic area of Rockwall County, Texas.

The site was originally designed and built by the Soil Conservation Service, now Natural Resources Conservation Service (NRCS), in cooperation with the Kaufman – Van Zandt Soil and Water Conservation District. It was designed as a single-purpose, low hazard, flood retarding structure located in a predominantly agricultural area. It is currently classified as “High” hazard by NRCS and the Texas Commission on Environmental Quality (TCEQ). “After field review, at least 4 homes are present downstream of the dam in the potential breach inundation area.” (NRCS Dam Safety Inspection Report, 5/21/2018).

Substantial erosion of the front dam embankment slope and the earthen plunge basin was documented in a 2018, NRCS Dam Safety Inspection Report. In 2019, a committee was organized to conduct a remedial repair investigation.

According to the USDA-NRCS Abstract of Engineering Report dated July 12, 2019:

“The excessive erosion to the wave berm and front slope was due to a design deficiency.

A vegetative wave berm was designed in an area that experiences fluctuating water levels, and an appropriate stand of vegetation was not established and maintained on the wave berm. A northern facing shoreline and a longer effective fetch length also contributed to the erosion. Scarps of up to 4 feet are present on the front slope for the entire length of the normal water line (approximately 1,050 feet).”

and

“The plunge basin side slopes have eroded due to a design deficiency. The original plunge basin should have been armored. The principal spillway conduit foundation has not been affected at this time. However, continued degradation could eventually impact the stability of the conduit and the toe of the embankment.”

and

“An unauthorized construction modification has occurred to the end of the embankment opposite the auxiliary spillway. Approximately 400 feet (station 39+00 to 43+00) has been removed and/or altered by the development of a residential and recreational area.

The modification could result in damage to the embankment and surrounding properties if the auxiliary spillway were to flow.”

A site inspection was conducted by the design engineer along with the project sponsor on September 27, 2022; this was to establish the repair area extents, fencing needs, the preferred construction access road, borrow, waste, and campsite areas.

This repair project is funded through the Remedial Repair Program of the NRCS. The remedial repair will consist of:

Reshaping and armoring the plunge basin in accordance with NRCS Design Note 6, “Riprap Lined Plunge Pool for Cantilever Outlet” and “Riprap Design for Pipe Spillway Outlets - o by Rice and Kadavy, and

Rebuilding the upstream slope and wave berm, with rock riprap armoring in accordance with NRCS Technical Release 69, “Riprap for Slope Protection Against Wave Action.”

Regarding the unauthorized construction modification to the end of the embankment (between approximate station 39+00 to 43+00), the Design staff and Engineering management concur with the conclusion that the Remedial Report authors make: the modification could result in damage to the embankment and surrounding properties.

The recent survey for the remedial repair documents the natural ground in this area to be approximately 6-inches higher than the auxiliary spillway crest but 2.8-feet below its original top of dam elevation. The unauthorized concrete roadway is approximately 2-feet above the auxiliary spillway crest. The required repairs fall outside the scope of qualifying remedial repair funding criteria. Consequently, these repairs will not be addressed in this project. The sponsor will be encouraged to pursue available options that would return this section of the dam to its original dimensions.

GENERAL DESIGN OF EXISTING STRUCTURE

Site No. 16A is a high hazard potential class structure with an effective height of 21.5 feet. Per the original design, the drainage area is approximately 3,155 acres and total storage capacity is approximately 2,011 acre-feet.

The dam embankment was designed with 2.5:1 slopes (front and back slopes) and a 14-foot-wide crest at the top of dam (TOD). The TOD elevation is 517.6 after allowance for settlement. An unauthorized modification was made to the dam sometime during 2005 (year based on aerial imagery). The original overall length of the dam embankment (approximately 3,676 feet) was shortened by approximately 400 feet; this occurred with the removal of the original earthen embankment between Sta. 39+00 to Sta. 43+00.

The auxiliary spillway (AS) is an 80-foot-wide, vegetated spillway constructed on a 2.71 percent grade (from the crest down to Sta. 14+00) and a 1.52 percent grade (from Sta. 14+00 to Sta.

20+00). It outlets into a natural channel. The AS crest elevation is 513.5 (between Sta. 10+00 and Sta. 10+50). The AS is contained on the left (looking downstream) by a diversion/dike combination and a dike on the right. Two embankment stub diversions are located on the front embankment slope at approximately Sta. 10+00 and Sta. 24+00.

The principal spillway (PS) is 150 feet of 30-inch I.D., A.W.W.A. C-301 standard pipe (prestressed, concrete lined, steel cylinder). Three (3) anti-seep collars are located along the PS with center-to-center distance at 20 feet. The PS is set on shaped, Class 2500 concrete bedding.

The 20-foot outlet section rests on a reinforced concrete pipe support. The inlet riser is a standard, open riser, (30-inch by 90-inch by 5 feet) with a metal trash guard. The riser crest elevation is

506.0. Four (4), riser ports (8-inch by 10-inch), were placed on each side of the riser at elevation 504.5; this elevation constitutes the normal pool elevation.

The PS discharge is into an earthen plunge basin with a bottom width of 20 feet, 2:1 side slopes, a

2:1 exit slope, and a 2.5:1 upstream slope. There is a 12-foot-wide, pipe crossing berm on the front slope at elevation 506.0 with a: 2.5:1 toe slope, flat earth fill area at elevation 501.0 upstream the riser, and 3:1 slope into the reservoir. An 18-foot-wide berm is located on the downstream slope at elevation 504.5; it transitions on a 4:1 slope to elevation 503.0 at each end of the berm and on a 2.5:1 toe slope into the earthen plunge pool.

SURVEY

A repair survey was performed by the NRCS Watershed Construction Office during September of 2022. A global positioning system (GPS) survey instrument calibrated to the State Plane Coordinate System (SPCS) was employed for the survey. Rockwall County is in the Nor th Central Zone of the SPCS. The GPS data was then processed using Online Positioning User Service (OPUS). OPUS processes GPS data files with the same models and tools which help manage the nation's Continuously Operating Reference Station (CORS) network. This results in highly accurate and consistent coordinates that can be utilized among varying project stakeholders.

The existing, as-built elevation of the reservoir riser crest is 506.0 and has been OPUS corrected to elevation 506.04; this is a negligible difference of 0.04 feet. This can be attributed to a change in datum from NGVD 29 to NAVD 88. All elevations in the repair construction drawings and in the “Repair Work Design” section of this report reflect the OPUS adjusted elevations. Survey data collected was used to calculate approximate earth and rock quantities required for the project repair.

It should be noted that no change in water surface elevation is planned as part of this repair work design.

REPAIR WORK DESIGN

The scope of the repair is to reshape the existing front embankment wave berm and the existing plunge pool/outlet channel and to armor these areas with rock riprap. Barbed wire fence will be removed to access the repair areas and reconstructed to preconstruction alignments.

The original Geological Report (October 12, 1967) indicates: the fill material used to construct the original dam embankment was CL to CH, “Borrow soils consist of silty to very silty sometimes slightly gyppy clods (CH, CL-CH, & CL) 5-feet to 12-feet thick overlaying residual shaly CH”, and the shaly CH was not used in the project fills (per report note).

The upstream slope repair consists of:

1. Drawing down the reservoir to a level below the repair elevations shown on the drawings.

2. Stripping and stockpiling the topsoil from the upstream embankment slope.

3. Excavating the upstream slope.

4. Rebuilding the slope with compacted earthfill.

5. Laying geotextile.

6. Pacing rock riprap armoring on the slope.

7. Placing topsoil on exposed earthfill above the normal water level.

8. Vegetating the topsoil.

Per Construction Specification 21, Excavation, the lower limits of the excavation will be inspected by the assigned construction engineer. The limits of excavation will be increased if uncompacted earth or objectionable materials are encountered below the lower limit of excavation shown on the drawings. The engineer shall verify and approve these excavations before placement of earthfill.

The earthfill to reconstruct the berm and slope will be placed with compaction in accordance with Construction Specification 23, Earthfill.

Calculations were performed to determine the upper and lower extents for placement of rock riprap using criteria established in Technical Release (TR-69), “Riprap for Slope Protection Against Wave Action”. Multiple, recent NRCS wave berm repair projects within the vicinity of site 16A used climatic wind data from nearby Dallas, TX. The Method 1 wind speed data of 61.67 mph is taken from that data. The effective fetch length of 1,772 feet was calculated. Averaging wind speed Methods 1 through Method 3 and the Max PGU (peak gust) resulted in a design wind velocity of 67 mph and an adjusted overland wind velocity of 71 mph. The results indicate a significant wave height of 1.9 feet.

TR-69 analysis shows a minimum upper riprap elevation 3.26 feet above the normal pool level and a minimum lower riprap elevation of 2.86 feet below the normal pool elevation. The top and bottom elevations of the rock will be placed at elevation 507.8 (3.26 feet above the normal pool, post construction) and elevation 501.6 (2.86 feet below the normal pool, post construction) respectively. The rock will be placed on a 2.5H:1V slope. The embankment slope above the rock will be 2.5H:1V or flatter and blend to the existing embankment where the excavation cut daylights with the existing embankment slope.

The minimum W50 rock weight determined from TR-69, is 34 pounds. A 35-pound spherical rock with a specific gravity of 2.6 has an approximate diameter of 9.0 inches. The selected rock size for this project is ASTM D6092 R-150. R-150 rock has a D50 size of 10.5 inches which: is larger and more conservative than the minimum required diameter of 9.0 inches, is widely available to contractors, and has been used effectively in several similar NRCS repair projects. Further, the larger stone size makes it more difficult for vandals to deface and remove the improvements as designed. The placement depth of the rock normal to the slope will be 2.0 feet.

PS outlet flow was calculated using the “Principal Spillway Rating Data (Routing)” spreadsheet.

With a 150-foot long, 2.5-foot I.D. PS and an AS crest elevation of 513.5, approximately 84.5 cubic feet per second (cfs) will flow.

The existing plunge pool will be reshaped to install a rock-lined plunge basin and outlet channel designed for low tailwater (TW) conditions. Low TW conditions were determined by using recent survey data taken from the natural earthen channel (taking a cross-section of Sta. 5+00 at elevation 494.7) and processing the cross-section data to determine the TW elevation capable of conveying a minimum flow of 84.5 cfs. The resulting TW was calculated at elevation 497.5 which is 1.50 feet below the outlet invert elevation of 499.0 (TW=-1.5).

With TW=-1.5, the tailwater to pipe diameter ratio (TW/Do) equals -0.6. Consequently, the Rice and Kadavy paper, “Riprap Design for Pipe Spillway Outlets - o , was consulted to derive the minimum dimensions for a new rock-lined plunge basin. The “Manning’s Equation Analysis: Cross-Section Data” (Version 4, Jan 2008) spreadsheet was also used in the design of the plunge basin and channel. Given the resulting output, a rock-lined plunge basin was designed.

The basin design dimensions as shown in the drawings exceed the minimum calculated dimensions for a basin constructed of R-150 rock riprap approximately 2.0 feet thick (normal to the excavated subgrade); the design is a best fit into the existing plunge pool and natural channel footprint. For constructability advantages and to coincide with the existing plunge pool and channel footprint, the design team elected to utilize a 20-foot bottom width (instead of the minimum calculated design width (Ws) of 10 feet) and a longer armored basin length. Compaction will be utilized for all earthfill activities in the plunge pool area.

A rock beaching check calculation in the NRCS Design Note 6 spreadsheet, “Riprap Lined Plunge Pool for Cantilever Outlet”, confirmed the rock riprap sizing suitability and a “Beaching Controlled” outcome (based on a 10.5-inch, approximate D50 size of R-150 riprap).

Approximately 70 feet of the outlet channel will be reshaped and armored with rock riprap to control erosion along curves and bends. Channel capacities were checked using the “USDA-NRCS Hydraulics Formula” program. The capacity for a 20-foot bottom-width, rock-riprap armored trapezoidal channel with a: 0.007 ft/ft slope, Manning’s “n” of 0.05, 3:1 side slopes, and depth of

2.2 feet is 209 cfs; this is much greater than the projected PS flow of 84.5 cfs. For the earthen channel that follows, a 12.9-foot bottom-width, earthen trapezoidal channel with: a 0.0058 ft/ft slope, a Manning’s “n” of 0.035, 3:1 side slopes, and a depth of 2.4 feet resulted in a calculated earthen channel capacity of 224 cfs; this is much greater than the projected PS flow of 84.5 cfs.

The allowable velocity for a vegetated (Bermuda grass) earthen channel with slopes less than 5 percent is 6 feet per second (ft/sec) for easily eroded soils and 8 ft/sec for erosion resistant soils;

both are greater than the 4.6 ft/sec calculated in the hydraulic model for the proposed earthen channel.

The purpose of the geotextile design used in the rock repair areas is to separate the embankment materials, some of which will be underwater, and to provide permanent erosion control. The strength property and survivability class requirements are specified according to guidance in “AASHTO M288 Geotextiles”.

The selection and placement of the geotextile designed for material separation and permanent erosion control purposes (and not for the purposes of subsurface drainage, stabilization, sediment control, or enhancement) will reduce:

drainage induced failures resulting from clogging or displacement and damages from surging conditions along wave berms and plunge basins.

The success of this geotextile design can be observed in similar NRCS repair and rehabilitation rock armoring projects in Texas where it has been installed.

Given the prevalence of the site’s CL and CH material found at all depths and as indicated on the “Geologic Cross Sections of Borrow Area” bore hole data in the as-built drawings (sheet 11 of 13), it can be assumed more than 50 percent of the embankment and plunge basin subgrade materials will pass the number 200 sieve. For these conditions, a Class I, non-woven geotextile is specified which should also meet the “Requirements for non-woven geotextile” specified in Material Specification 592, Geotextile.

Note that Class I, non-woven product is also recommended by the AASHTO M288-21 Selection Guide as conducive for subsurface drainage and stabilization applications where the percent fines are greater than 50 percent. Consequently, the new rock riprap wave berm, plunge basin, channel and anti-burrowing extension will be underlain with Class I, non-woven geotextile.

The geotextile will be placed between the prepared earth subgrade materials and the rock in accordance with:

Construction Specification 94, Contractor Quality Control, section 10, Items of work and construction details, a.(8)(f), Geotextile, Construction Specification 95, Geotextile, section 5, Placement, and section 7, Items of work and construction details, a.(2) and a.(4).

The geotextile shall be keyed into the subgrade at all ends by a minimum of 18 inches.

A calculated rock placement depth normal to the geotextile covered subgrade (designed to a minimum two times the D50 size) is specified.

The repair areas from the edges of all rock armoring and above, as designated in the drawings, will be backfilled with earthen materials, covered with a layer of topsoil, and vegetated.

Earthwork and rockwork quantities were calculated by taking area quantities from AutoCAD section drawings and inputting them into a quantity spreadsheet using the average end method to arrive at quantity totals. A 1.25 factor is applied to the eartfill totals to account for shrinkage and compaction of the soil when it is placed and compacted. A 1.75 conversion factor is applied to rock riprap cubic yard quantities to arrive at a quantity measurement in tons. The factor accounts for variation in the specific gravity of rock sourced from varying quarries, subgrade variations, final grade placement variations, and rock riprap waste materials.

It is assumed that all fill areas will be supplied by the soil from the cut and borrow areas. Assuming 25 percent shrinkage approximately 5,896 cubic yards of fill is required for the repair and approximately 6,083 cubic yards is available from the excavations. This leaves a borrow requirement of zero cubic yards although not all front embankment excavation may be reusable.

According to the as-built drawings and the recent site visit with the project sponsor, borrow material of suitable quantity and quality appears to be available on-site (just beyond original borrow area Grids A, B, and C and along the right bank, facing upstream of the reservoir). The required borrow materials will be excavated from one borrow area of approximately 1.0 acre, as designated in the drawings. Reserving the top foot of this area for topsoil, approximately 3,227 cubic yards of borrow material is available with two additional feet of excavation.

Detailed geometry and construction information can be obtained from the drawings. Topsoil will be placed over all the repaired areas and vegetation will be established.

CULTURAL RESOURCES

A TX-NRCS Cultural Resource Specialist (CRS) gathered information concerning previously recorded historically or culturally significant sites at Cedar Creek 16A (located at Lat/Long 32.821 -96.312) by searching the database of the Texas Historic Sites Atlas (https://atlas.thc.texas.gov/). In addition, other relevant databases were searched including the National Register of Historic Places (https://www.nps.gov/subjects/nationalregister/index.htm) and the National Association of Tribal Preservation Offices (http://nathpo.org/wp/thpos/find-a-thpo/), to identify historically significant sites within the boundaries of the project and Native American interests in the area. Based on a review of these databases, historical imagery, topographic maps, and the Web Soil Survey (WSS) (https://websoilsurvey.sc.egov.usda.gov), there are no previously recorded cultural resources or historically significant sites that may be impacted by the proposed project. Because the project repairs will occur within the same footprint of previous disturbance, no fieldwork is needed at this time. From a cultural resource perspective, there is no reason to delay construction.

The project engineer and/or inspector will monitor for previously undiscovered cultural resources during construction, including prehistoric and historic artifacts and features that may appear as a result of excavation. The TX-NRCS CRS will be notified if cultural resources are encountered in the work areas. Construction activities in the area of the discovery will be suspended until the Cultural Resources Specialist provides clearance to proceed.

ENVIRONMENTAL CONSIDERATIONS

Environmental Evaluation Worksheets (Form NRCS-CPA-52) have been started. Under current regulations of the National Environmental Policy Act (NEPA), Cedar Creek Site 16A is required to have one completed by an interdisciplinary team to determine the level of NEPA documentation required. It is not anticipated that the repair construction of the structure will have any detrimental effects on the environment. The construction specifications call for control of pollution in any form resulting from construction operations. All exposed/disturbed areas not covered with rock riprap will be vegetated to control erosion and provide wildlife food and cover. Vegetative plantings will be part of this contract and shall be done in accordance with the Storm Water Pollution Prevention Plan (SWP3) by the Contractor. The SWP3 also includes the discussion of erosion control measures. The rounding of the edges associated with the various slopes will occur during normal construction operations and this will aid in the blending of the site with the surrounding landscape.

CONSTRUCTION REVIEW

It should be noted that subgrade excavations in the vicinity of the principal spillway conduit shall be carefully monitored to ensure the underlying pipes are not damaged or disturbed. The excavation should be monitored to assure a minimum of 2 feet of cover is maintained over the pipe for 10 feet left and right of the principal spillway centerline.

The embankments bordering the waste areas shall be shaped, smoothed, and revegetated at the end of construction.

The State Design Engineer will be notified of any significant discontinuities in the embankment.

A course of action will be decided upon by consultation with other personnel as necessary and by field review if required.

Policy by the Texas Commission on Environmental Quality (TCEQ) requires new permanent reference markers (PRMs) to be placed on reconstructed dams. The old PRMs shall be located and removed and then replaced with new markers.

Elevation data must be recorded using a survey instrument or survey grade global positioning system (GPS) receiver.

Horizontal and vertical measurements recorded with a GPS receiver must be based on:

The North American Datum of 1983 and

The North American Vertical Datum of 1988.

Latitude and longitude measurements must be provided in decimal degree to six places.

Control points for the layout of the embankment centerline will be established by the NRCS.

CONSTRUCTION SPECIFICATIONS AND BID SCHEDULE

Standard NRCS Construction Specifications have been used to specify the work items to be accomplished during the repair of these sites. The various items included in the bid schedule delineate the significant items of work to be accomplished. All subsidiary items are included and discussed in the construction specifications. The bid schedule is concluded to be fair and equitable division of the work for bidding purposes.

A dewatering plan will be specified in the Construction Specifications. Rapid drawdown, exceeding more than 1 foot of reservoir elevation drawdown per day, is to be avoided.

In accordance with Construction Specifications 21, 23, 61, 94 and established construction best practices:

The excavated foundation for all rock riprap shall be examined to ensure all low-density materials have been removed, A stable foundation exists on which to lay geotextile and place rock riprap, The construction engineer shall adjust the excavation limits as necessary to remove the materials if low density materials are still present, Open excavations should be backfilled in a timely manner to ensure the density and moisture requirements for the material are maintained at specific levels, All excavated subgrade or earthfill materials that exhibit drying cracks, slaking, and other evidence of being unsuitable shall be removed or reworked prior to placement of fill, At the discretion of the construction engineer, Bid Item No. 8 unit rates for excavation will be paid to the contractor for the removal of saturated cut material.

CONSTRUCTION SCHEDULE

There may be shutdowns and delays due to unforeseen weather events. Approximately 185 calendar days (holidays and weather days not included) will be required to complete these repairs.

OPERATION AND MAINTENANCE

Operation and maintenance of this site is the responsibility of the Sponsoring Local Organization (SLO) and will be subject to periodic inspection and review by the NRCS. An unsigned copy of the standard operation and maintenance agreement (O&M) is included in Appendix A. The signed version of the O&M will remain on file with the NRCS State Office.

The Sponsors shall include periodic inspections of the repaired areas for indication of potential slides, wave erosion, rilling, and deteriorated and/or displaced rock riprap, and damaged or displaced vegetation.

A good vegetative cover shall be established and maintained on the site (particularly on the repaired areas). Remove excessive and/or undesirable vegetative growth; remove excessive debris; restore slopes and berms; and reestablish vegetative cover as needed.

Designed by: Concurred with:

____________________ Date _______ ___________________ Date _______

GARY E. GERACI, E.I.T. R. BRIAN WENBERG, P.E.

Design Engineer State Conservation Engineer NRCS, Temple, Texas NRCS, Temple, Texas

GARY GERACI

Digitally signed by GARY

GERACI

Date: 2024.12.16 11:30:10 -06'00'

APPENDIX A

CEDAR CREEK WATERSHED SITE 16A

ATTACHMENT TO THE OPERATION AND MAINTENANCE PLAN

Operation and maintenance of these structures will be the responsibility of the SLO and subject to periodic inspection and review by the NRCS. A copy of the O&M agreement is on file in the NRCS State Office and is a Standard agreement for operation and maintenance.

a. Inspection

(1) Rock armoring along the wave berm and within the plunge pool/outlet channel-check for deterioration or displacement of rock, settlement of earthfill, quality of vegetation stand, needed woody brush/weed control, and needed debris removal along the rock-lined basin.

(2) Embankment slopes – check for cattle induced erosion and back-slope toe erosion caused by neighboring residential improvements.

(3) Fences - check for: deterioration of coatings and/or materials; bent or overturned line, brace, stub out, corner, and gate posts; and adequate tension of line and brace wire. Inspect swinging gates for proper opening and closing.

b. Maintenance

(1) Rock armoring along the wave berm and within the plunge pool/outlet channel-remove excessive accumulations of debris; remove undesirable vegetative growth; replace or repair displaced rock; and replant poor vegetation stands within the vegetated areas indicated on the drawings.

(2) Embankment slopes – repair trail erosion caused by cattle traffic and ensure cattle do not loiter on the dam; add and maintain rock riprap along the back toe of the dam where neighboring residential improvements have caused erosive channel flow during rain events; and replant poor vegetation stands within the vegetated areas indicated on the drawings.

(3) Fences - replace line, brace, stub-out, corner, and gate posts that are excessively deteriorated, damaged, bent, or overturned; replace excessively deteriorated wire and stays; and restore adequate tension of line and brace wire. Replace or repair any damaged swinging gate components that prohibit proper closure and securing of gates.

(4) Control beaver/rodent activity at the site on a regular basis.

Prepared by:

____________________ Date _______

GARY E. GERACI, E.I.T.

Design Engineer NRCS, Temple, Tex

GARY GERACI Digitally signed by GARY GERACI Date: 2024.12.16 11:30:52 -06'00'