Hydro_NE_FLAP_7136(1)_MONTROSE_BRIDGE.pdf

Technical Memorandum

Subject: Montrose Bridge Replacement Hydraulic Recommendations

Date: December 18, 2017

Project Number: NE 7136(1), Montrose Bridge Replacement

Prepared by: Brian Campbell, P.E.

Hydraulics Engineer FHWA Central Federal Lands Lakewood, CO

Through: Veronica Ghelardi, P.E.

Hydraulic Team Leader FHWA Central Federal Lands Lakewood, CO

Prepared for: Jeff Jerome Project Manager

Introduction The purpose of this memo is to provide hydraulic recommendations for the replacement of bridge structure number

C008301205P.

Summary

Foundation Design

- Foundation will be socketed into bedrock

Riprap Armor at Southeast Abutment

- Class III riprap

- Place riprap per Federal Highway Administration (2014) “Standard Specifications for Construction of Roads and

Bridges on Federal Highway Projects FP-14”. This is referred to as FP-14.

- Inspect riprap per FP-14 method B (see section 251).

- Riprap shall not be placed steeper than a 1.5H:1V.

- Minimum thickness is 2 feet.

- High flows returning to the main channel are causing a new side channel to form upstream of the southeast abutment. The extents of the riprap armor placed at southeast abutment will arrest the uncontrolled development of this channel and prevent further degradation of the channel bank in this area. The footprint extends approximately 16 feet upstream from the southern wingwall and approximately 20 feet down the channel bank where it intersects a series of piles put in place to stabilize the channel bank.

Project Location The structure is located in Sioux County on Hat Creek, approximately 13 miles from Ardmore, South Dakota, see Figure

1. The project location can be found by referencing the following geographic coordinates: Latitude: 42.922376° N and

Longitude: -103.742550° W. Hat Creek flows from south to north and is a tributary to the Cheyenne River.

Figure 1:Site Location

Project Background The existing bridge is a single span, 102 foot long truss bridge built in 1915. The timber deck is 15.7 feet wide. The abutments consist of 5 steel piles with horizontal timber backing and detached timber wings, each with two steel soldier piles. The timber is rotting. A recent flood event in 2015 inundated and washed out the approach road, see Figure 2.

The structure load rating has been lowered to 12 tons preventing larger vehicles from using it. The existing bridge has reached the end of its useful life, and is functionally obsolete and structurally inadequate. A new bridge will be designed and constructed to current live load standards and crash tested railings and transitions, with a larger width to provide passage of local farm machinery. The projected ADT for Montrose Road is 53.

Figure 2: 2015 Flooding

Bridge Design Criteria Per the 2012 PDDM, CFLHD design standards for bridged waterways are a design capacity for 50-year flood with a minimum of 2 feet of freeboard below the low chord. The scour design flood standard is the 100-year flood and the scour check flood is the 200-year flood. However, if there is a flood event less than the scour check flood standard that causes greater scour at the bridge, it should be used as the scour check flood. This is the case for the proposed project.

The 100-year runoff rate causes a greater scour depth at the bridge than the 200-year runoff rate. This is because during the 200-year runoff event, the bridge approach section to the east of the bridge is overtopped by a significant amount. This overtopping changes the hydraulics of the channel downstream of the bridge and ultimately reduces the unit discharge through the bridge opening to below the amount during the 100-year runoff.

Hydrology The drainage basin was delineated using Aquaveo WMS version 10.1.

Figure 3: Drainage Basin

Hat Creek at the structure of interest has a drainage area of 222.4 square miles. The site is not in a FEMA regulated floodplain. Historic data for runoff events is limited. There is a Nebraska Department of Natural Resources stream gage located at the structure, however, conversations with personnel from DNR concluded the gage is not calibrated and should not be used.

The next available stream gage on Hat Creek is USGS 06400000 has a drainage basin of 959 square miles and multiple tributaries. Although this gage is useful to identify when Hat Creek could potentially have peaked at the site of interest, the data was not used to determine peak flow rates at the site of interest.

Peak flow estimates for the site were developed using regional regression equations from the 1999 water-resources investigation report 99-4032 “Peak-Flow Frequency Relations and Evaluation of the Peak-Flow Gaging Network in

Nebraska”. The site is in the region identified as the Northern and Western Region.

The design hydrology is shown in Table 1.

Project Site

North

Table 1: Peak Discharge

Return Period (years) Peak Discharge (cfs)

2 630

5 1775

10 2815

25 4315

50 5660

100 9210

200 12195

Bridge Hydraulic Analysis

Software Hydraulic modeling was performed using Aquaveo SMS version 12.2.7. Many calculations were done with Microsoft

Excel 2016.

Source of Mapping CFL survey staff conducted a topographic survey in April of 2017. However, the initial model iterations identified that the extents of the topographic survey did not capture the boundary of the floodplain. At the time of the analysis, available elevation data was limited to 10-meter DEM. Therefore, the “stamping” feature available in SMS was used along with aerial imagery to extend data upstream and downstream of the channel alignment. This was done by projecting the last cross section available along the channel alignment, as outlined using an aerial background, at the average slope of the channel.

This edited surface model was checked during a field visit in May of 2017. Additional points were collected at key locations along the roadway and in the floodplain using a level, level rod, and survey wheel. The level was initially set up on a project control point set by CFL survey personnel. The final model is in the project SMS file.

Figure 4: Site Map

Figure 5: Additional Surface Information Added to Model

Existing Conditions for Channel and Floodplain Floodplain

There are no FEMA mapping studies for Sioux County near this structure. The 100-year floodplain width around Hat

Creek is approximately 1200 feet wide and composed of dense grasses. Few trees are in the area. Hydraulic modeling of the area shows water can begin to inundate the floodplain at about the 10-year storm, 2815 cfs. The bridge is located at the furthest west point of the floodplain. The approach road from the east encroaches on approximately 1100 feet of floodplain width.

Channel

Hat Creek is a highly meandering stream, however, the bridge is located on a straight section of the meander and is considered laterally stable. The channel banks consist of short grasses while the normal low flow area is mostly clay bottom. The top width of the main channel near the bridge is approximately 100 feet, with bank heights

Bridge Site

North

Projected

Channel

Level and Rod

Survey Points approximately 20 feet tall. Flow depths range from 2 feet for normal low flows to 17 feet during a 100-yr event.

The channel slope is approximately 0.0011 ft/ft or 0.11%. The channel bed is clay with a d50 of 0.04 mm.

The east side embankments are showing sloughing of the soils upstream and downstream of the bridge and directly in front of the east abutment. Debris on the underside of the bridge as well as along the embankments indicate that there is the potential for substantial flows under this structure. However, the stream does not carry high debris flows.

Figure 6: Image looking Downstream

Figure 7:East Bank Sloughing/Scarp

Proposed Bridge Condition The proposed structure is a 106 foot single span structure. The proposed design will utilize the existing piles, in addition to new piles, for some support. Therefore, the distance between the sloping abutments is constrained to 98 feet.

Looking downstream, the left abutment is approximately 2.5H:1V and the right abutment is approximately 1.8H:1V. A bridge width of 20 feet is proposed with a gravel surface. The low chord elevation is 3601.6 feet. The approach road is a gravel surface, 20 feet wide tapering to the existing approach width of 12 feet wide with a gravel surface.

Hydraulic Model The main channel was modeled with a Manning’s n value of 0.035. The floodplain was modeled with a Manning’s n value of 0.048 and the gravel approach roads were modeled with a Manning’s n value of 0.025.

Downstream boundary conditions were calculated from normal depth for the runoff event being analyzed at the channel slope of 0.11%.

Results

Results were calculated from a cross section centered under the proposed bridge structure.

Table 2: Hydraulic Model Results

Return Period Peak Discharge (cfs)

Water Surface Elevation (feet)

Average Velocity (cfs)

50 Year 5,660 3598.93 6.46

Incipient Overtop of East Approach

Road

7,050 3599.37 6.82

100 Year 9,210 3600.24 6.80

200 Year 12,195 3601.53 6.40

Discussion

The analysis shows the bridge will pass the 50-year return period with 2 feet or more of freeboard. The calculated freeboard value is 3.27 feet.

Figure 8 shows a drop in water surface elevation going through the bridge for the 100-year and 200-year return periods.

The water surface elevation in a subcritical flow regime is expected to drop through a horizontal contraction. The abutments contract the channel width at the bridge and in response, the water surface elevation is lowered.

The 200-year return period is under the low chord elevation of 3601.6 by approximately 0.07 feet. Pressure scour was not considered.

The analysis identified the east approach road begins overtopping at approximately 7050 cfs. Armoring the approach road from the east is not within the project scope.

Figure 8: Profile at Bridge

Scour Analysis Site Geology

The channel material under the abutments is classified as a low plasticity clay with a D50 of approximately 0.04mm.

Approximately 3.5 feet below this layer is medium plasticity clay identified from data collected for the geotechnical analysis. This combined information was used to identify a critical shear stress value used in the contraction scour calculations.

Bridge Low

Chord 3601.6’

Flow Direction

Horizontal Stability and Vertical Long Term Degradation

The County inspection records for the site are minimal dating back to 2010. The records do not document changes in the thalweg elevation. The County bridge inspector, Stacy Swinney, was contacted to discuss bridge records and site history. Mr. Swinney has been inspecting this structure since the early 1980’s. Mr. Swinney stated older inspection records are maintained by the State. In his years, he has not seen any change in channel thalweg elevation.

Reviewing aerial imagery from 1953, 1974, 1993, and current, the channel appears to be horizontally stable. There is one section to the east of the bridge where the channel can be seen encroaching on the roadway section.

The channel is considered vertically stable.

Contraction Scour

The parameters used in the contraction scour equations were determined from the SMS 2D hydraulic model. Scour depth was estimated using methods outlined in HEC 18 (2012), section 6.7, “Contraction Scour in Cohesive Material”.

Equation 6.6 was used to determine the ultimate scour depth. Figure 6.11 in HEC 18 was used to determine the critical shear stress for the material. The site geology information collected was used to identify a critical shear stress value of the material as approximately 0.42 lbs/ft².

Abutment Scour

The NCHRP approach to abutment scour was used. The proposed abutments are a spill through configuration integrated within the main channel. The configuration is categorized as scour condition (a). Figure 8.9 in HEC 18 was used to determine the amplification factor. Because the abutments are integrated within the main channel, and the channel bottom width is relatively short, 15 feet, the resulting abutment scour depth is applied to the channel bottom.

Although the initial overtop occurs at a flow rate of 7050 (see table 2), iterations of overtopping were analyzed on 500 cfs increments to the 200-year event. This was done to identify the worst case overtopping condition. The analysis shows the 100-year event is the worst-case event. The results are shown in Table 3 and Figure 9.

Table 3: Scour Analysis Results

Return Period

Contraction Scour (ft)

Amplification Factor

Abutment Scour (ft) Total Scour (ft)

Approximate Thalweg Elevation below Centerline of Bridge (ft)

Scour Elevatio n (ft)

Right Left

Incipient Overtopping (9000 cfs)

3.77 1.65 6.22 6.22 6.22 3584.20 3577.98

100-year 3.79 1.65 6.26 6.26 6.26 3584.20 3577.94

200-year 3.55 1.3 4.62 4.62 4.62 3584.20 3579.58

Figure 9: Results of Worst-Case Analysis

Armoring Protection at the Abutments The design for the bridge foundation designates the supporting H-piles will be driven into bedrock. The driven H-piles will require a minimum of 5-foot socket into the Pierre Shale bedrock to resist the lateral bridge loads at an elevation

(3,565 feet) well below the estimated worst-case scour elevation (3,577.9 feet). Since the tip foundation elevation is approximately 13 feet below scour, the bridge stability will not be effected should the worst-case scour occur.

However, there is still a concern about the right (east) abutment’s embankment sloughing during high flow events. This sloughing is due to the reduction in shear strength of the clay materials when saturated. This sloughing upstream and downstream of the bridge and through the bridge is believed to be caused by the turbulence resulting from water returning to the channel mixing with water in the channel during high flow events. This turbulence combined with the clay soils becoming saturated are causing the sloughing. A geotechnical solution is proposed to stabilize the embankment by arresting the slip plaby driving 10-foot long piles into the embankment soils. It should be noted that this sloughing does not endanger the bridge stability, but it will affect the embankment and the approach road stability.

The high flows returning to the channel are also causing a channel to form upstream of the right abutment. Riprap will be required on the embankment to arrest this new channel development. A Class III riprap per the specifications in table 705-1 in the Federal Highway Administration (2014) “Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects FP-14” (Table 4) will be placed to prevent further degradation of the embankment in this area.

Table 4: Riprap Gradation Specifications per Table 705-1 in FP-14

3577.5

3578.5

3579.5

Sc o u r

El e va ti o n

Runoff Event

Chart Title

The extents of the riprap armor placed at southeast abutment will arrest the uncontrolled development of this channel and prevent further degradation of the channel bank in this area. The footprint extends approximately 16 feet upstream from the southern wingwall and approximately 20 feet down the channel bank where it intersects a series of piles put in place to stabilize the channel bank. Due to the steep slope of the existing channel bank, riprap shall not be placed steeper than a 1.5H: 1V. Minimum thickness is 2 feet.

The final layout is shown in Figure 10.

Figure 10: Riprap Armor Layout

Recommendations

Foundation Design

- Foundation will be socketed into bedrock

Riprap Armor at the Abutments

- Class III riprap

- Place riprap per Federal Highway Administration (2014) “Standard Specifications for Construction of Roads and

Bridges on Federal Highway Projects FP-14”. This is referred to as FP-14.

- Inspect riprap per FP-14 method B (see section 251)

- Riprap shall not be placed steeper than a 1.5H:1V.

- Minimum thickness is 2 feet.

- The footprint extends approximately 16 feet upstream from the southern wingwall and approximately 20 feet down the channel bank where it intersects a series of piles put in place to stabilize the channel bank.

Southeast

Wingwall

References Federal Highway Administration (2012) “Project Development and Design Manual” Federal Lands Highway Division. US

Department of Transportation. Chap 7.

Federal Highway Administration HEC-18, Evaluating Scour at Bridges, Fifth Edition, April 2012.

Federal Highway Administration HEC-23, Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design guidance-third Edition, Volume2, September 2009.

Federal Highway Administration (2014) “Standard Specifications for Construction of Roads and Bridges on Federal

Highway Projects FP-14”

Aquaveo SMS version 12.2.7

Aquaveo WMS version 10.1

Federal Highway Administration Hydraulic Toolbox version 4.3

US Geological Survey (1999) “Peak-Flow Frequency Relations and Evaluation of the Peak-Flow Gaging Network in

Nebraska” Water-Resources Investigation Report 99-4032

Appendix A: Montrose Riprap Position Memo, Submitted August 30, 2017

Technical Memorandum

Subject: Montrose Bridge Replacement Riprap Protection

Date: August 30, 2017

Project Number: NE 7136(1), Montrose Bridge Replacement

From: Veronica Ghelardi, P.E.

Hydraulics Engineer

To: Jeff Jerome

Project Manager

The hydraulics recommendations memo dated June 23, 2017 recommended extensive riprap be placed around the bridge and across the channel for scour protection. Since that memorandum was issued, it was decided that the bridge foundation will be driven into bedrock. The driven H-piles will require a minimum of 5-foot socket into the Pierre Shale bedrock to resist the lateral bridge loads at an elevation (3,565 feet) well below the estimated worst-case scour elevation (3,577.9 feet). Since the tip foundation elevation is approximately 13 feet below scour, the bridge stability will not be effected should the worst-case scour occur. Therefore, the original scour protection riprap design is not needed.

However, there is still a concern about the right (east) abutment’s embankment sloughing during high flow events. This sloughing is due to the reduction in shear strength of the clay materials when saturated. We believe this sloughing upstream and downstream of the bridge and through the bridge is being caused by the turbulence resulting from water returning to the channel mixing with water in the channel during high flow events. This turbulence combined with the clay soils becoming saturated are causing the sloughing. We propose to stabilize the embankment with a geotechnical solution that will provide resistance forces by driving 10-foot long piles into the embankment soils. It should be noted that this sloughing does not endanger the bridge stability, but it will affect the embankment and the approach road stability.

The high flows returning to the channel are also causing a channel to form upstream of the right abutment. Riprap will be required on the embankment to arrest this new channel development. A Class III riprap per the specifications in table 705-1 in the Federal Highway Administration (2014) “Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects FP-14” (Table 1) will be placed to prevent further degradation of the embankment in this area.

Figure 11 Riprap Gradation per Table 705-1, FP-14