8436_Bridge_Hydraulic_Study_PRELIMINARY_Full-TM_20140418.pdf

DEVELOPING INNOVATIVE DESIGN SOLUTIONS

Tahoe Transportation District

P R E L I M I N A RY B R I D G E H Y D R A U L I C S T U D Y

for State Route 89/Fanny Bridge Community Revitalization Project in Placer County on State Route 89 from

SR-89 - PM 7.5/9.4 to

SR-28 - PM 0.0/0.1

03-ED-89

03-0A7600

April 2014

PRELIMINARY BRIDGE HYDRAULIC STUDY

FOR STATE ROUTE 89/FANNY BRIDGE COMMUNITY REVITALIZATION

APRIL 18, 2014

ii

TABLE OF CONTENTS

INTRODUCTION

BACKGROUND

PURPOSE

HYDRAULIC DESIGN STANDARDS AND METHODS

WATERSHED DESCRIPTION

PROJECT DESCRIPTION

EXISTING/PREVIOUS STUDIES

HYDROLOGY

SURVEYS AND TOPOGRAPHY

HYDRAULIC ANALYSIS

Surface

Hydraulic Model Construction

Manning’s Roughness Coefficients

In-line Structures

Contraction and Expansion Coefficients

Ineffective Flow Areas

Upstream Boundary

Downstream Boundary

Bridge Modeling

Results

SCOUR AND EROSION

Aggradation and Degradation

Contraction Scour

Pier Scour

Abutment Scour

Results

Scour Counter Measure

OTHER/SPECIAL CONSIDERATIONS

Caltrans

Recreational Use

CONCLUSIONS AND RECOMMENDATIONS

iii

EXHIBITS

1 Photo taken from downstream face of Existing Fanny Bridge, Facing Downstream 8

2 Photo taken from Upstream Face of Southwest Pacific Sewer Crossing, Facing Upstream 8

3 Photo taken from Upstream of Existing Fanny Bridge, Facing Downstream 11

4 Existing Fanny Bridge as Displayed in HEC-RAS 12

5 Proposed Truckee River Bridge as Displayed in HEC-RAS 13

6 Contraction, Pier, and Abutment Scour Input Parameters at the Existing Fanny Bridge 17

7 Contraction, Pier, and Abutment Scour Input Parameters at the Proposed Truckee River Bridge 18

8 Scour Results at the Existing Fanny Bridge 20

9 Scour Results at the Proposed Truckee River Bridge 22

TABLES

1 In-line Structure Locations and Descriptions 9

2 Water Surface Elevation Comparison (NGVD 29) 14

3 Hydrologic and Hydraulic Summary 14

4 Bridge Freeboard Results 15

5 Scour Results for the Existing Fanny Bridge 19

6 Scour Results for the Proposed Truckee River Bridge 21 iv

FIGURES

1 Project Location Map

2 Site Map

3 Proposed Truckee River Bridge Plan and Profile

4 Survey Sources

5 Composite DEM

6 Existing Fanny Bridge Survey

7 Proposed Truckee River Bridge Alignment and Survey

8 HEC-RAS Water Surface Elevation Profiles

APPENDICES

A FEMA Flood Insurance Rate Map (FIRM)

B FEMA Models (Digital Files Only)

C GIS Files (Digital Files Only)

D Site Photos (Digital Files Only)

E Record Drawings (Digital Files Only)

F Survey Notes

G USGS Stream Gage Information (Digital Files Only)

H HEC-RAS Model (Digital Files Only)

I Riprap Design Calculations (Digital Files Only)

Prepared for: Tahoe Transportation District

Prepared by: Mr. Michael C. Nowlan, P.E., CFM Mr. Allan Laca, P.E.

Date: April 18, 2014

INTRODUCTION

The area known as Tahoe City in the state of California is located along the northwestern shore of Lake Tahoe, where the Truckee River flows downstream from Lake Tahoe through Lake Tahoe Dam, toward the city of Truckee and into Nevada. From the western shore of Lake Tahoe, State Route (SR) 89 carries travelers north toward Tahoe City, California (City) and intersects with SR 28, just west of the City (see Figure 1). This roadway connection has been identified by the Tahoe Transportation District as requiring improvements. The Tahoe Transportation District recently was awarded funding by the Federal Lands Access Program (FLAP) to construct the Fanny Bridge Community Revitalization Project.

BACKGROUND

The project being proposed, to re-route California State highway traffic and local traffic in the vicinity of Tahoe City, has been in the planning stages for several years, and several project alternatives have been considered and evaluated. Previous studies have evaluated roadway and drainage configurations, bridge alternatives, and river analyses, which were associated with the respective project roadway configurations. The current project configuration has now been funded through FLAP and is under final design.

PURPOSE

The purpose of the project is to improve traffic flow and public safety for motorists, local inhabitants, and visitors in the vicinity of the SR 89 and SR 28 connection. The current highway intersection was studied and has been shown to experience local traffic congestion with the potential risk for vehicular accidents. The proposed project will correct the known deficiencies while accounting for storm water quality and flood conveyance issues.

The purpose of this study is to evaluate the bridge hydraulics within the affected Truckee River corridor in order to ensure that any proposed bridge design will properly convey design flooding levels with sufficient freeboard and scour/erosion protection.

HYDRAULIC DESIGN STANDARDS AND METHODS

The current project is funded through the federal government, but also involves the California State Department of Transportation (Caltrans), the Tahoe Transportation District, and Placer County.

The bridge hydraulic design should evaluate each proposed structure to have a minimum of zero feet of freeboard (water surface to bridge soffit) for the 1% Chance Annual Event (100-year event), or two feet of freeboard for the 2% Chance Annual Event (50-year event), in accordance with Caltrans standards.

To model the Truckee River and its bridges, Wood Rodgers, Inc. (Wood Rodgers) is utilizing the HEC-RAS Version 4.1 software program, which was developed by the United States Army Corps of Engineers (USACE) Hydrologic Engineering Center (HEC), based in Davis, California.

This program can dynamically compute water surface elevations for rivers and streams with complex bridges and culverts under a wide variety of flow conditions. The program also provides scour estimates for piers, abutments and contractions, and is sufficient to evaluate the impacts of larger flood events on proposed and existing structures.

WATERSHED DESCRIPTION

The affected portion of the Truckee River (just downstream of Lake Tahoe) is fed by an alpine watershed of approximately 505 square miles located in eastern California and western Nevada.

This high altitude watershed receives a combination of rain and snow precipitating over large wooded areas around the perimeter of Lake Tahoe, which itself has a surface area of approximately 192 square miles. The elevation of the Truckee River within the project area is approximately 6,250 feet above sea level, with all contributing land in the upstream watershed ranging from 6,250 feet to 10,878 feet. Temperatures range from average summertime highs in the 80s to wintertime lows in the 10s. Much of the watershed is frozen and snowbound in the winter months, with snow melt greatly affecting upstream creek and lake levels. In extreme climatic conditions, such as the 1997 flood event, atmospheric river rainfalls, falling primarily as rain due to warmer air, can precipitate severe runoff into the lake.

While there is a small portion (approximately 1.3 square miles) of the watershed downstream of the Lake Tahoe Dam that could be contributing to the Truckee River conditions within the project area, the direct storm runoff of this drainage area is not considered coincident with the upstream watershed runoff and discharge timing through the lake. The magnitude of local runoff during a large event is considered to be significantly less than the maximum outflow from Lake Tahoe.

PROJECT DESCRIPTION

The proposed project known as the Fanny Bridge Community Revitalization Project consists of roadway reconstruction, bike path construction, pedestrian improvements, water quality treatment, and bridge construction, as shown on Figure 2. The highways are being realigned and reconfigured to carry traffic through roundabout intersections, with reconstructed and widened segments of roadway. The new SR 89 roadway alignment bypasses Tahoe City and includes a new bridge crossing over the Truckee River, downstream of the existing foot bridge and just east of the existing Caltrans maintenance yard. The existing Fanny Bridge crossing, just downstream of Lake Tahoe Dam, is proposed to be rehabilitated, leaving the existing pier and abutment profile in place within the flood conveyance portion of the river channel. The existing Fanny Bridge will be utilized primarily to carry local traffic into Tahoe City from northbound SR 89 and to carry local traffic onto southbound SR 89.

To represent the impacts of the project bridges on riverine conditions, as well as the hydraulic impacts of the river on the bridge designs associated with this roadway project, the Truckee River must be modeled from the Lake Tahoe Dam to approximately 0.40 miles downstream, which is approximately 310 feet downstream of the new bridge crossing.

A cross-section and plan view of the proposed new bridge crossing is provided on Figure 3.

EXISTING/PREVIOUS STUDIES

Previous studies include an older Federal Emergency Management Agency (FEMA)-accepted study, which was used as the basis of Flood Insurance Rate Mapping (FIRM) along the Truckee River (Appendix A). The HEC-2 models developed for the FEMA study were requested by Wood Rodgers in 2011 and were provided via FEMA mapping contractor Michael Baker Corporation on July 1, 2011. The basis of the hydrologic information contained in the hydraulic modeling was not provided. The peak flow in the model is applied from the Lake Tahoe Dam several miles downstream to a major tributary confluence. To be conservative, FEMA studies typically calculate downstream peak flows and project them upstream. The peak flow utilized for the 100-year storm in the models provided is 3,350 cfs. The HEC-2 models have been included in Appendix B.

As part of the development of alternatives for the Fanny Bridge project, a previous study was developed by Quincy Engineering, entitled “Truckee River Bridge (Bridge No. 19-0033) Administrative Draft Design Hydraulic Study”, dated October 7, 2005. In this report, the existing Fanny Bridge was evaluated as well as the channel immediately upstream and downstream of the bridge. The peak flow was calculated using statistical methods four different ways, as cited in their report. The flow cited in the appendices for computations is 76.2 cms, which is equivalent to 2,689 cfs. This flow is listed as the “flood of record” occurring in 1997, and was utilized as the design flood by Quincy Engineering.

HYDROLOGY

For purposes of this study, Wood Rodgers evaluated the previous studies and analyzed the available data. As reported in the previous study, there is an existing stream gage just downstream of the existing Fanny Bridge. According to discussions with the Bureau of Reclamation (Reclamation), the gage (10337500) is operated by the United States Geological Survey (USGS) and serves as a flow measurement, monitoring the outflow of Lake Tahoe through Lake Tahoe Dam. Wood Rodgers downloaded the entire gage record from the USGS website for analysis, for the years of record from 1895 to 2014 (missing the period from 1896 to 1909). While 2013 and 2014 data was available, it was listed as provisional (subject to change) and was not utilized for purposes of this study.

Consistent with the previous Quincy Study, there were two occurrences (June 1969 and January 1997) when the maximum release of 2,690 cfs from the dam occurred without overtopping the dam. Two other large dam release occurrences of approximately 2,300 cfs were recorded in March of 1986 and December of 1983, and both were considered large flood events in California’s recorded history.

The Lake Tahoe Dam, construction of which began in 1909, has been in operation since 1913.

Therefore, the stream gage measurements just downstream of the dam were made almost entirely during the time the dam was in place and releases of flow were controlled by humans. While the geology of the lake and outlet could be considered a “natural” dam and spillway prior to Lake Tahoe Dam’s construction, the installation of the dam altered the releases from the watershed.

While previous studies have attempted to evaluate the watershed’s measured stream flow using statistical methods, it is not considered appropriate to attempt to fit a distribution such as Log-Pearson Type III to analyze a gage record data set which is so heavily manipulated by human intervention. The flow in the stream is no longer dictated by the natural watershed’s configuration, but is controlled by the opening and closing of gates on the dam. The frequency and magnitude of the discharge are decided for other reasons beyond the natural response of the watershed. Considering that the dam was not overtopped during the largest recorded event, it appears that the lake storage and outlet structures were able to accommodate the largest historical volume and peak flow events over the last 100 years.

It is possible that a more detailed estimate of 100-year outflow could be derived if the effects of lake storage and the dam were properly accounted for. The lake level and outlet would need to be re-analyzed as if the dam were never in place, which would require analyzing detailed lake level data at the dam over the entire period of record. The historical record would need to be translated into inflow to the lake, with a long-term simulation performed to see how the natural outlet would have allowed the lake to drain over the last 100 years. A statistical analysis could then be performed, as the natural lake attenuation is present, and is “constant” during the entire representation of the distribution. The results of the analysis would then need to be re-regulated to adjust for the presence of the dam on the natural distribution.

For purposes of this study, it is considered sufficient to utilize the maximum historical outflow from the dam (controlled release) as the maximum flood that a downstream bridge will encounter with a 1% Chance Annual Occurrence. Considering this same maximum flow has occurred twice in the last 100 years, with two other occurrences nearly as large, Wood Rodgers considers it appropriate to apply this design flow as a “50-year” event with the associated design freeboard considerations.

SURVEYS AND TOPOGRAPHY

Topographic survey information was obtained from aerial surveys performed by Aerotech Mapping to represent the terrain outside of the wet channel areas. Wood Rodgers performed supplemental field surveys of the channel bathymetry and the hydraulic structures present in the channel, including the existing Fanny Bridge, the downstream foot bridge, and a sewer main crossing. Figure 4 shows the extent and source of each survey.

The terrain and channel information was merged to create the basis for cross-sections that were used as input for HEC-RAS modeling of the Truckee River in the project area.

HYDRAULIC ANALYSIS

The extent of the model is from the Lake Tahoe Dam to the Southwest Pacific sewer pipe crossing approximately 0.40 miles downstream of the Dam. The model and the report reference the California State Plane II (NAD 83) horizontal datum and the National Geodetic Vertical Datum (NGVD 29). Based on the survey information, the model uses ground coordinates instead of grid coordinates. The information necessary to convert the horizontal coordinates from ground to grid is listed below. The difference between ground and grid coordinates is +/- 2 feet. Also included below is the vertical conversion equation to convert elevation values from NGVD 29 to North American Vertical Datum of 1988 (NAVD 88).

Horizontal:

Centroid:

N=2,190,291.257 feet

E= 7,088,539.918 feet

Ground coordinates to Grid coordinates scale factor = 0.99962189

Vertical:

Conversion: NAVD 88 = +4.042 feet +NGVD 29

Surface

Topographic lines were created with photogrammetric surveys performed by Aerotech Mapping in August of 2010 and provided in the form of a .dwg file. The .dwg file was then used to create a triangulated irregular network (TIN) surface. Due to limitations inherent with photogrammetric surveys, the inundated portion of the channel bottom was not captured.

Additional field surveys were performed by Wood Rodgers in March of 2014 to measure the bathymetric portions of the channel. The bathymetry information and the photogrammetric surveys were integrated with GIS software to create a composite Digital Elevation Model (DEM) raster. The composite DEM reflects the photogrammetric information in the over-bank portion of the channel and the additional field surveys in the bathymetry portion of the channel, as provided in Appendix C and shown in Figure 5.

Hydraulic Model Construction

The composite raster was used with HEC-GeoRAS 10.1 to develop an HEC-RAS model that best represents the hydraulic capacity of the Truckee River. The standard HEC-GeoRAS input was defined by Wood Rodgers for river, reach, and station identifiers; cross-sectional cut lines;

cross-sectional surface lines; cross-sectional bank stations; and downstream reach lengths for the over-bank, main channel, and right over-bank.

Manning’s Roughness Coefficients

The Manning’s “n” values were manually entered into HEC-RAS. The USGS Water-Supply Paper 2339 was referenced to select the appropriate “n” values for the model. For the channel portion of the Truckee River, the calculation for the “n” value is indicated below:

Channel “n” value = (nb + n1+ n2 + n3 + n4)m = 0.0350

Where:

Base “n” value (nb) = 0.027

Surface irregularities (n1) = 0.00125

Variation in shape and channel cross-section (n2) = 0.000

Obstructions (n3) = 0.005

Vegetation and Flow Conditions (n4) = 0.002

Meandering (m) = 1.00

For the over-bank portion of the Truckee River, the calculation for the “n” selection is indicated below:

Over-bank “n” value = (nb + n1+ n2 + n3 + n4)m = 0.040

Where:

Base “n” value (nb) = 0.027

Surface irregularities (n1) = 0.007

Variation in shape and channel cross-section (n2) = 0.000 (assumed for all over-banks)

Obstructions (n3) = 0.005

Vegetation and Flow Conditions (n4) = 0.001

Meandering (m) = 1.00 (assumed for all over-banks)

This portion of the Truckee River is uniform, and the calculated “n” values for the channel and over-banks were used for the entire length of the HEC-RAS model. Exhibits 1 and 2, below, illustrate the channel conditions to justify the calculations of the channel and floodplain “n” values. Additional site photos can be found in Appendix D.

Exhibit 1 – Photo taken from Downstream Face of Existing Fanny Bridge, Facing Downstream

Exhibit 2 – Photo taken from Upstream Face of Southwest Pacific Sewer Crossing, Facing Upstream

In-line Structures

There are two existing bridge crossings, one existing sewer pipe crossing, and one proposed bridge. For purposes of this study, all in-line structures were modeled as bridges. Table 1 (below) summarizes all the bridges in the HEC-RAS model. Record drawings are included in Appendix E. Survey field notes and sketches are included in Appendix F.

Table 1

In-line Structure Locations and Descriptions

River River Mile Description

Truckee River 0.463 Existing Fanny Bridge

Truckee River 0.258 Pedestrian Bridge

Truckee River 0.149 Proposed Truckee River Bridge

Truckee River 0.084 Southwest Pacific sewer pipe crossing

The inclusion of the sewer pipe crossing was necessary to capture any backwater effect it may have on the Proposed Truckee River Bridge.

Contraction and Expansion Coefficients

The default values of 0.1 for contraction and 0.3 for expansion were used. These values assume gradual transitions between cross-sections. The bridges present in the river do not significantly contract normal bank-to-bank flow widths.

Ineffective Flow Areas

Ineffective flow locations are generally used to define portions of the channel cross-section that do not significantly convey flow due to downstream obstructions. They describe water that will pond, with velocity, in the downstream direction, close to or equal to zero. Ineffective flow areas were manually entered into HEC-RAS to both banks of cross-sections immediately upstream and downstream of bridges. Ineffective flow areas were also added to the left bank of cross-sections upstream of the Southwest Pacific sewer crossing.

Upstream Boundary

The only flows contributing to the upstream portion of the Truckee River are governed by the releases from the dam, as described in the Hydrology section above. The maximum flow of 2,690 cfs from the stream gage was used in the model. The USGS stream gage data is included in Appendix G.

Downstream Boundary

Wood Rodgers converted the FEMA HEC-2 model to an HEC-RAS model for ease of use. The hydraulic downstream boundary condition was set to normal depth, with slope = 0.000943 ft/ft as measured from this HEC-RAS model. This HEC-RAS model is included with this submittal in Appendix B.

Bridge Modeling

Record drawings for the existing Fanny Bridge and the Southwest Pacific sewer pipe crossing were obtained. Additional surveys were performed at the existing Fanny Bridge, the pedestrian bridge, and the Southwest Pacific sewer pipe crossing to obtain the high and low chord (highest and lowest structural member) information, pier widths, abutment locations, and other information necessary to construct the bridges in HEC-RAS.

Existing Fanny Bridge Rehabilitation: The piers at the existing Fanny Bridge measured

2.2 feet. The piers of the Rehabilitated Fanny Bridge, along with the pier location, pier spacing, and deck thickness, were assumed to have the same dimensions and measurements. For low-flow computation method, the Energy, Momentum, and Yarnell Methods were selected because losses due to piers should be accounted for. The program was set to choose the highest energy answer among the three low-flow methods. For high-flow computation method, the Energy option was selected because the flows in the simulation do not over-top the bridge deck, so there is no need to model weir and pressure flows through this structure. Exhibit 3 (below) shows the existing Fanny Bridge. Figure 6 contains the elevations and locations of survey points used to construct the high and low chords of the bridge deck. Exhibit 4 (below) shows the existing Fanny Bridge as displayed in the bridge editor of HEC-RAS.

Exhibit 3 – Photo taken from Upstream of Existing Fanny Bridge, Facing Downstream

Exhibit 4 – Existing Fanny Bridge as Displayed in HEC-RAS

Proposed Truckee River Bridge: The location of the proposed bridge is approximately

0.30 miles downstream of the existing Fanny Bridge. Figure 7 shows the proposed alignment and the location of the channel bottom surveys. The high and low chord information used to construct the proposed bridge in HEC-RAS, as well as the pier location, deck thickness, and abutment locations, were taken from the Planning Study drawing developed by Wood Rodgers.

Figure 3 shows the Planning Study drawing. For the proposed bridge, the Energy low-flow computation method was selected. The Momentum and Yarnell Methods were not selected because the majority of the energy losses through the bridge are not associated with the piers.

The Energy option was selected for the high-flow computational method because the flows in Truckee River do not over-top the bridge deck. Exhibit 5 (below) shows the Proposed Truckee River Bridge as displayed in the bridge editor of HEC-RAS.

Exhibit 5 – Proposed Truckee River Bridge as displayed in HEC-RAS

Results

An existing and proposed conditions model was developed to show the impacts of the improvements at Fanny Bridge and the Proposed Truckee River Bridge. A profile of the existing and proposed water surface elevations has been included in Figure 8. The HEC-RAS model has been included under Appendix H. Table 2 (below) indicates the differences in water surface elevations between the existing and proposed conditions. The maxim increase to the water surface elevations due to the proposed improvements is 0.01 foot at the cross-section upstream of the proposed bridge.

Table 2

Water Surface Elevation Comparison (NGVD 29)

River Mile Current Conditions Proposed Conditions Differences

(feet) W.S. Elevation (feet) W.S. Elevation (feet)

0.471 6225.99 6225.99 0.00

0.470 6225.99 6225.99 0.00

0.457 6225.90 6225.90 0.00

0.393 6225.44 6225.44 0.00

0.331 6225.31 6225.31 0.00

0.261 6224.78 6224.78 0.00

0.255 6224.77 6224.77 0.00

0.187 6224.42 6224.42 0.00

0.160 6224.18 6224.19 0.01

0.138 6224.13 6224.13 0.00

0.114 6224.06 6224.06 0.00

0.085 6223.85 6223.85 0.00

0.083 6223.73 6223.73 0.00

A summary of the hydrology and the hydraulic results for the existing Fanny Bridge and the Proposed Truckee River Bridge have been included in Table 3 (below).

Table 3 Hydrologic and Hydraulic Summary

Drainage Area: 505 Square Miles

Design Flood Base Flood Overtopping

Flood/Flood of Record

Frequency (years) 50-yr 100-yr NA

Discharge (cfs) NA NA 2,690.00 Water Surface Elevation at Existing Fanny Bridge (feet, NGVD 29)

NA NA 6,226.00

Velocity at Existing Fanny Bridge (ft/s) NA NA 2.56

Water Surface Elevation at Proposed Truckee River Bridge (feet, NGVD 29)

NA NA 6224.20

Velocity at Proposed Truckee River Bridge (ft/s) NA NA 4.81

NOTE: Flood plain data are based upon information available when the plans were prepared and are shown to meet federal requirements. The accuracy of said information is not warranted by the Tahoe Transportation District, Central Federal Lands, State, and County and interested or affected parties should make their own investigation.

The resulting freeboard have been summarize for the existing Fanny Bridge and the Proposed Truckee River Bridge in Table 4 (below).

Table 4 Bridge Freeboard Results

Bridge

WSE

(feet, NGVD 29)

Minimum Soffit Elevation

(feet, NGVD 29) Freeboard (feet)

Existing Fanny Bridge 6226.0 6229.6 3.6 Proposed Truckee

River Bridge 6224.2 6237.0 12.8

Existing Fanny Bridge Rehabilitation: The results show that the improvements at this bridge will result in no increase to the water surface elevation at Fanny Bridge. The Caltrans requirement to pass the 50-year storm plus two feet of freeboard has been met.

Proposed Truckee River Bridge: The results show a 0.01-foot increase to the water surface elevation at the Proposed Truckee River Bridge. The bridge will meet the requirement to pass the 50-year storm plus two feet of freeboard. The bridge will also meet the recommended six-foot clearance during summer flows for recreational uses.

SCOUR AND EROSION

Aggradation and Degradation

Aggradation and degradation are long-term streambed elevation changes due to natural or man-induced causes that can affect the reach or the river on which the bridge is located.

Aggradation involves the deposition of material eroded from the channel or watershed upstream of the bridge, but is not considered a component of total scour. Degradation involves the lowering or scouring of the streambed over relatively long reaches due to a deficit in sediment supply from upstream, and degradation contributes to total scour. Publication HEC-18 was referenced to determine the effects of long-term aggradation and degradation at this location of the Truckee River.

Evaluation of the historical stream gage discharge and stage relationships obtained from USGS does not indicate progressive aggradation or degradation. In addition, long-term aggradation or degradation is not evident at neighboring bridges. Furthermore, there are no plans to change the local land use in the watershed. The forested areas of the watershed are government-owned and regulated to prevent wide-spread fire damage. Based on these observations, and due to the lack of other possible impacts to the river reach, the channel is relatively stable vertically. These observations indicate that future aggradation or degradation of the channel, due to changes in sediment delivery from the watershed, are minimal.

Contraction Scour

Existing Fanny Bridge: The D50 size is defined as the mean particle size of the bed material.

For contraction scour, a D50 size of 0.25 mm was used based on the Foundation Report for the Proposed Truckee River Bridge done by Wood Rodgers. The Clear-Water Contraction Scour equation was used at all portions of the cross-sections at the existing Fanny Bridge due to low velocity conditions (critical velocity > mean velocity).

Proposed Truckee River Bridge: The D50 size is defined as the mean particle size of the bed material. For contraction scour, the same D50 size of 0.25 mm was used. The Clear-Water Contraction Scour equation was used at the over-bank portions of the cross-sections at the Proposed Truckee River Bridge due to low velocity conditions (critical velocity > mean velocity). The Live-Bed Contraction Scour equation was used at the channel portion of the cross-section.

Pier Scour

Existing Fanny Bridge: For pier scour, the D50 size indicated above was used, and a D95 of 25 mm was initially assumed (equivalent to a one-inch-sized particle). The angle of flow is measured as zero degrees because flow is perpendicular the piers. The existing Fanny Bridge has square-nose shaped piers. The Colorado State University (CSU) Pier Scour equation was used for model computations.

Proposed Truckee River Bridge: For pier scour, the D50 size indicated above was used, and the same D95 of 25 mm was used. Angle of flow is measured as zero degrees because flow is perpendicular. The proposed bridge has a group of cylinders for its pier shape. The CSU Pier Scour equation was used for model computations.

Abutment Scour

Existing Fanny Bridge: Vertical abutments were indicated as the abutment type at the existing Fanny Bridge. The Froehlich equation was used for model computations.

Proposed Truckee River Bridge: Vertical abutments were indicated as the abutment type at the proposed Truckee River Bridge. The Froehlich equation was used for model computations.

Exhibits 6 and 7 (below) show the input parameters for calculating contraction, pier, and abutment scour at the existing Fanny Bridge and the Proposed Truckee River Bridge, respectively.

Exhibit 6 – Contraction, Pier, and Abutment Scour Input Parameters at the Existing Fanny Bridge

(Note that all values in green are automatically populated.)

Exhibit 7 – Contraction, Pier, and Abutment Scour Input Parameters at the Proposed Truckee River Bridge (Note that all values in green are automatically populated.)

Results

Existing Fanny Bridge Rehabilitation: The result for the scour analysis for the existing Fanny Bridge is summarized in Table 5 (below). The results show that contraction scour does not occur at the existing Fanny Bridge. The scour depth due to contraction is 0.41 feet. The scour depth due to piers is 3.82 feet. The scour depths due to the left and right abutments are

1.50 feet and 7.19 feet, respectively. Exhibit 8 (below) shows the results of the scour analysis.

Table 5

Scour Results for the Existing Fanny Bridge

Contraction Scour Left Bank Channel Right Bank

Ys (ft): 0 0.41 --

Minimum Elevation (ft): -- 6215.32 --

Vc (ft/s): 1.01 1.49 --

Equation: Clear Live --

Pier Scour

Ys (ft): -- 3.82 --

Minimum Elevation (ft): -- 6211.60 --

Froude #: -- 0.16 --

Equation: -- CSU equation --

Abutment Scour

Abutment Ys (ft): 1.50 -- 7.19

Minimum Elevation (ft): 6224.22 -- 6216.56

Ve= 0.44 -- 1.68

Froude #: 0.09 -- 0.18

Equation: Froehlich -- Froehlich

Combined Scour Depths

Pier Scour + Contraction Scour Ys (ft): -- 4.24 --

Pier Scour + Contraction Scour Minimum Elevation (ft):

-- 6211.60 --

Abutment + Contraction Scour Ys (ft): 1.50 -- 7.60

Abutment + Contraction Scour Minimum Elevation (ft):

6224.22 -- 6216.56

Exhibit 8 – Scour Results at the Existing Fanny Bridge

Proposed Truckee River Bridge: The result for the scour analysis for the Proposed Truckee River Bridge is summarized in Table 6 (below). The proposed bridge experiences only contraction and pier scour. There is no scour as a result of the abutments. The scour depth due to contraction is 0.45 foot in the channel portion of the bridge and 0.30 foot on the left bank of the bridge. The scour depth due to piers is 7.75 feet. Since the contraction and pier scour areas overlap, the combined scour is 8.05 feet on the left bank. Exhibit 9 (below) shows the results of the scour analysis.

Left Bank Right Bank

Table 6

Scour Results for the Proposed Truckee River Bridge

Contraction Scour Left Bank Channel Right Bank

Ys (ft): 0.30 0.45 0

Minimum Elevation (ft): 6220.51 6215.40 --

Vc (ft/s): 1.15 1.43 1.15

Equation: Live Live Live

Pier Scour

Ys (ft): 7.75 -- --

Minimum Elevation (ft): 6215.18 -- --

Froude #: 0.35 -- --

Equation: CSU equation -- --

Combined Scour Depths Pier Scour + Contraction Scour Ys (ft): 8.05 0.45 --

Pier Scour + Contraction Scour Minimum Elevation (ft): 6215.18 6215.40 --

Exhibit 9 – Scour Results at the Proposed Truckee River Bridge

Scour Counter Measure

Existing Fanny Bridge Rehabilitation: Based on the procedure outlined in HEC-23 Volume 2, Design Guideline 14, the riprap size that would be required for the abutments will need to be Class I (D50 value of 1.5 inches). Based on the procedure outlined in Design Guideline 11 of the same publication, the riprap size that would be required for the piers will need to be Class I, as well (D50 value of 4.38 inches).

Propose Truckee River Bridge: Based on the procedure outlined in Design Guideline 11 of the same publication, the riprap size that would be required for the piers will need to be Class IV (D50 value of 17.11 inches). Since there is no abutment scour at the Proposed Truckee River Bridge, no riprap recommendations were made.

See Appendix I for the above calculations.

Left Bank Right Bank

OTHER/SPECIAL CONSIDERATIONS

Caltrans

The proposed bridge complies with the Caltrans design standards of two feet of freeboard for the 50-year storm (Highway Design Manual Section 821.3 Selection of Design Flood). Hydraulic design of the proposed new bridge and bridge improvements did not account for seismic seiche wave activity, as such a wave event and flow impact cannot be modeled with any associated flow frequency and requires extensive lake research and modeling beyond the design considerations for riverine bridges.

Recreational Use

Special considerations for summer flows were taken into account. During the warmer seasons, the Truckee River serves as a recreational water body. Raft launches from the Truckee River Raft Company are located on the right bank of the Truckee River, approximately 300 feet downstream of Fanny Bridge. Since the existing Fanny Bridge is upstream of the raft launch locations, the clearance for recreational use was not taken into consideration for the improvements at the existing bridge. To accommodate recreational uses on the river, a six-foot clearance during the maximum summer flows at the proposed bridge is recommended. While rafting safety issues may preclude river access during a maximum outflow of 2,690 cfs, freeboard for the proposed bridge exceeds six feet.

CONCLUSIONS AND RECOMMENDATIONS

The existing Fanny Bridge structure has sufficient freeboard to safely convey the design flood flow of 2,690 cfs. Debris (floating or submerged) is not expected to be present due to Lake Tahoe and the Lake Tahoe Dam immediately upstream capturing all potential debris loading. Due to the low velocities in the channel, contraction scour is effectively zero, with pier and abutment scour potential only slightly below the existing channel bottom at elevation 6,214.3 feet. Wood Rodgers recommends armoring the abutments to help prevent scour erosion from forming. All foundation design should consider support of the structure with exposure down to the provided abutment and pier scour elevation.

The new proposed bridge structure has sufficient freeboard to safely convey the design flood flow of 2,690 cfs. With a minimum of two feet of freeboard required, the actual freeboard during maximum flow is 16.0 feet. The configuration of the structure accommodates a bike path which has significantly increased the space for water to flow through. While summertime flows can vary widely, safety is a major concern when it comes to rafting operations which are conducted only between the months of June and September. Rafts are accessing the river downstream of the existing Fanny Bridge, so safety clearance should not be a concern for the existing structure.

F ig ur es

Figures

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Truckee River

Bear Creek

Tr uc ke e R ive r

§̈¦80§̈¦80

ST89

ST267

ST28

ST431

ST28

ST89

ST89

ST28

ST89

ST28

ST89

ST89

ST89

P l a c e rP l a c e r C o u n t yC o u n t y

E l D o r a d oE l D o r a d o C o u n t yC o u n t y

N e v a d aN e v a d a C o u n t yC o u n t y

W a s h o eW a s h o e C o u n t yC o u n t y

D o u g l a sD o u g l a s C o u n t yC o u n t y

C a r s o n C i t yC a r s o n C i t y C o u n t yC o u n t y

Eldorado National Forest

Toiyabe National Forest

0 2 41

Miles

PRELIMINARY

NORTH

J:\Jobs\8436_Tahoe\20_FannyBridge_PSE\GIS\Tasks\Figures\Fig1_Project_Location.mxd 4/18/2014 9:16:14 AM alaca FIGURE 1

PROJECT LOCATION

LAKE TAHOE DAM

TAHOE TRANSPORTATION DISTRICT

STATE ROUTE 89/FANNY BRIDGE

COMMUNITY REVITALIZATION PROJECT

PROJECT LOCATION MAP

PLACER COUNTY, CALIFORNIA

APRIL, 2014

Ø Streams Counties State Parks and Forests Water Bodies

Roads Major Highways Highways Major Roads

Existing Southwest Pacific Sewer Pipe Crossing

End Improvements

End Improvements

Begin Improvements

N LAKE BLVD

L A

K E

T A

H O

E

W RIVER RD

W

LA

KE

B

LV

D

LO

WER

TRUCKEE

RIVER

Existing Fanny Bridge

Existing Pedestrian Bridge

Proposed Bridge

RA

WH

ID

E D

R

VIR

GI

NI

A D

R

TAHOE TAVERN RD

MA

CK

INA

W

RD

KI

MB

ER

LY

D R

DIRT RD

MT

W

AT

SO

N

RD

GRANLIBAKKEN RD

FAIRWAY DR

AÄ

AÄ

AÄ

AÄ

End Improvements

Begin Improvements

0 150 30075

Feet

TAHOE TRANSPORTATION DISTRICT

STATE ROUTE 89 / FANNY BRIDGE

COMMUNITY REVITALIZATION PROJECT

SITE MAP

PLACER COUNTY, CALIFORNIA

APRIL, 2014

NORTH

J:\Jobs\8436_Tahoe\20_FannyBridge_PSE\GIS\Tasks\Figures\Fig2_Site_Map.mxd 4/18/2014 9:51:52 AM alaca FIGURE 2

Roadway, Bike, Pedestrian, and Bridge Improvements Striping

PRELIMINARY

TAHOE TRANSPORTATION DISTRICT

STATE ROUTE 89/FANNY BRIDGE

COMMUNITY REVITALIZATION PROJECT

PROPOSED TRUCKEE RIVER BRIDGE PLAN AND PROFILE

PLACER COUNTY, CALIFORNIA

APRIL, 2014

J:\Jobs\8436_Tahoe\04_FannyBridgeSR89PAED\GIS\Tasks\ReportFigures\Fig4_HEC-RAS_FEMA_WSE_Profiles_20120123_V2.mxd 5/9/2012 2:18:38 PM jbuchanan FIGURE 3

Proposed Bridge

Existing Pedestrian Bridge

Existing Fanny Bridge

AÄ

AÄ

AÄ

AÄ

LO

WER

TRUCKEE

RIVER

W

LA

KE

B

LV

D

W RIVER RD

L A

K E

T A

H O

E

N LAKE BLVD

Existing Southwest Pacific Sewer Crossing

0 150 30075

Feet

TAHOE TRANSPORTATION DISTRICT

STATE ROUTE 89 / FANNY BRIDGE

COMMUNITY REVITALIZATION PROJECT

SURVEY SOURCES

PLACER COUNTY, CALIFORNIA

APRIL, 2014

NORTH

J:\Jobs\8436_Tahoe\20_FannyBridge_PSE\GIS\Tasks\Figures\Fig4_Survey_Information.mxd 4/18/2014 9:52:21 AM alaca FIGURE 4

Survey Sources Aerotech Mapping Survey Wood Rodgers, Inc. Survey

Proposed Bridge

Existing Pedestrian Bridge

Existing Fanny Bridge

AÄ

AÄ

AÄ

AÄ

LO

WER

TRUCKEE

RIVER

W

LA

KE

B

LV

D

W RIVER RD

L A

K E

T A

H O

E

N LAKE BLVD

Existing Southwest Pacific Sewer Crossing

0 150 30075

Feet

TAHOE TRANSPORTATION DISTRICT

STATE ROUTE 89 / FANNY BRIDGE

COMMUNITY REVITALIZATION PROJECT

COMPOSITE DEM

PLACER COUNTY, CALIFORNIA

APRIL, 2014

NORTH

J:\Jobs\8436_Tahoe\20_FannyBridge_PSE\GIS\Tasks\Figures\Fig5_Composite_DEM.mxd 4/18/2014 9:55:04 AM alaca FIGURE 5

Composite DEM 6,215 - 6,220 6,221 - 6,224 6,225 - 6,227 6,228 - 6,230 6,231 - 6,233 6,234 - 6,237 6,238 - 6,242 6,243 - 6,247 6,248 - 6,251 6,252 - 6,254 6,255 - 6,257 6,258 - 6,260 6,261 - 6,264 6,265 - 6,268 6,269 - 6,272 6,273 - 6,277 6,278 - 6,280 6,281 - 6,284 6,285 - 6,287 6,288 - 6,291 6,292 - 6,295 6,296 - 6,300 6,301 - 6,304 6,305 - 6,307 6,308 - 6,311 6,312 - 6,315 6,316 - 6,319 6,320 - 6,323 6,324 - 6,327 6,328 - 6,331

6232.6 ft

6232.45 ft

6232.31 ft

6230.4 ft

6216.33 ft

6217.34 ft

6217.35 ft

6215.94 ft

6217.49 ft

6220.06 ft

6228.13 ft

6220.05 ft

6227.89 ft

6219.68 ft6221.93 ft

6219.97 ft

6230.35 ft

6220.27 ft

6223.06 ft

6225.16 ft

6217.49 ft

6218.14 ft

6215.93 ft

6217.24 ft

6219.36 ft

0 25 5012.5

Feet

TAHOE TRANSPORTATION DISTRICT

STATE ROUTE 89 / FANNY BRIDGE

COMMUNITY REVITALIZATION PROJECT

EXISTING FANNY BRIDGE SURVEY

PLACER COUNTY, CALIFORNIA

APRIL, 2014

NORTH

J:\Jobs\8436_Tahoe\20_FannyBridge_PSE\GIS\Tasks\Figures\Fig6_Existing_Fanny_Bridge.mxd 4/18/2014 9:56:20 AM alaca FIGURE 6

Survey Points Channel Bridge Deck

LO

WE

R T

RU

CK

EE

RIV

ER

0 25 5012.5

Feet

TAHOE TRANSPORTATION DISTRICT

STATE ROUTE 89 / FANNY BRIDGE

COMMUNITY REVITALIZATION PROJECT

PROPOSED TRUCKEE RIVER BRIDGE ALIGNMENT AND SURVEY

PLACER COUNTY, CALIFORNIA

APRIL, 2014

NORTH

J:\Jobs\8436_Tahoe\20_FannyBridge_PSE\GIS\Tasks\Figures\Fig7_Proposed_Bridge_Alignment.mxd 4/18/2014 10:06:17 AM alaca FIGURE 7

Survey Points Channel

Proposed Improvements Roadway Improvements Striping

0.1 0.2 0.3 0.4 0.5

Main Channel Dis tance (mi)

Ele va tio n -

N

GV

D2 9 D atu m

(ft)

Legend WS PF 1 - Proposed Cond.

WS PF 1 - Current Cond.

Ground

Truckee River Upper Reach

TAHOE TRANSPORTATION DISTRICT

STATE ROUTE 89/FANNY BRIDGE

COMMUNITY REVITALIZATION PROJECT

HEC-RAS WATER SURFACE ELEVATION PROFILES

PLACER COUNTY, CALIFORNIA

APRIL, 2014

J:\Jobs\8436_Tahoe\20_FannyBridge_PSE\GIS\Tasks\Figures\Fig8_HEC-RAS Profiles.mxd 4/18/2014 9:59:31 AM alaca FIGURE 8

EX

IST

IN

G

FA

NN

Y B

RI

DG

E

PE

DE

ST

RA

IN

B

RI

DG

E

PR

OP

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ED

B

RI

DG

E

SO

UT

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ES

T P

AC

IFI

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W

ER

PI

PE

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IN

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A p p en d ic es

Appendices

FEMA Flood Insurance Rate Map (FIRM)

FEMA Models (Digital Files Only)

GIS Files (Digital Files Only)

Site Photos (Digital Files Only)

Record Drawings (Digital Files Only)

Survey Notes

USGS Stream Gage Information (Digital Files Only)

HEC-RAS Model (Digital Files Only)

Riprap Design Calculations (Digital Files Only)

A

B

C

D

E

F

G

H

I

P

P D

D

D

D

D

D

D

D

,AQ

,AQ

,AR

,AS

Burton Creek

Truckee River

LAKE

TAHOE

P28

P89

P89

WEST LAKE BOULEVARD

P28

KIMBERLY DRIVE

BONANZA DRIVE

OLYMPIC DRIVE

BUNKER DRIVE

NORTH STREET

SIERRA

TERRACE

ROAD

CAMPGROUND ROAD

MACKINAW ROAD

TAHOE TAVERN

ROAD

CEDAR COURT

TAHOE TAVERN

ROAD

ROCKY RIDGE

ROAD

FAIRWAY DRIVE

PIONEER WAY

GROVE STREET

JACKPINE STREET

TA

VE

RN

DR

IVE

GRANLIBAKKEN ROAD

Tahoe City Trailer Park Road

Lake Tahoe Outlet Structure

Placer County Unincorporated Areas

060239

Tahoe National Forest

R.

E

R.

E

R.

E

R.

E

T. 15 N.

T. 16 N.

T. 15 N.

T. 16 N.

JOINS PANEL 0335

JO

IN

S P

AN

EL

JO

IN

S P

AN

EL

JOINS PANEL 0344

FO

REST B

OUNDARY

ZONE X

! ZONE X

!ZONE X

ZONE X

ZONE AE

ZONE A

ZONE A

ZONE A

ZONE X

ZONE X

ZONE X

ZONE X

39°09'22.5'' 120°09'22.5''

39°11'15'' 120°09'22.5''

39°11'15'' 120°07'30''

39°09'22.5'' 120°07'30''

KS0305

KS0284

KS0286

KS0282

KS0309

KS0307

KS0283

6227 62

PROFILE BASE LINE

7085000 FT

7090000 FT

2190000 FT

2195000 FT

746000mE 747000mE

748000mE

4339000mN

4340000mN

4341000mN

FIRM

FLOOD INSURANCE RATE MAP

(SEE MAP INDEX FOR FIRM PANEL LAYOUT)

C ONTAINS:

C OMMUNITY NUMBER PANEL SUFFIX

PL ACER COUNTY 060239 0342 H

N ot ice to User: The Map Number shown below should be used when placing map orders; the Community Number shown above should be used on insurance applicat ions for the subject community.

MAP NUMBER

MAP REVISED

Federal Emergency Management Agency

LEGEND

SPECIAL FLOOD HAZARD AREAS SUBJECT TO INUNDATION

BY THE 1% ANNUAL CHANCE FLOOD

The 1% annual flood (100-year flood), also known as the base flood, is the flood that has a 1% chance of being equaled or exceeded in any given year. The Special Flood Hazard Area is the area subject to flooding by the 1% annual chance flood. Areas of Special Flood Hazard include Zones A, AE, AH, AO, AR, A99, V, and VE. The Base Flood Elevation is the water-surface elevation of the 1% annual chance flood.

ZONE A No Base Flood Elevations determined.

ZONE AE Base Flood Elevations determined.

ZONE AH Flood depths of 1 to 3 feet (usually areas of ponding); Base Flood

Elevations determined.

ZONE AO Flood depths of 1 to 3 feet (usually sheet flow on sloping terrain); average depths determined. For areas of alluvial fan flooding, velocities also determined.

ZONE AR Special Flood Hazard Area formerly protected from the 1% annual chance flood by a flood control system that was subsequently decertified. Zone AR indicates that the former flood control system is being restored to provide protection from the 1% annual chance or greater flood.

ZONE A99 Area to be protected from 1% annual chance flood by a Federal flood protection system under construction; no Base Flood Elevations determined.

ZONE V Coastal flood zone with velocity hazard (wave action); no Base Flood

Elevations determined.

ZONE VE Coastal flood zone with velocity hazard (wave action); Base Flood Elevations determined.

FLOODWAY AREAS IN ZONE AE

The floodway is the channel of a stream plus any adjacent floodplain areas that must be kept free of encroachment so that the 1% annual chance flood can be carried without substantial increases in flood heights.

OTHER FLOOD AREAS

ZONE X Areas of 0.2% annual chance flood; areas of 1% annual chance flood with average depths of less than 1 foot or with drainage areas less than 1 square mile; and areas protected by levees from 1% annual chance flood.

OTHER AREAS

ZONE X Areas determined to be outside the 0.2% annual chance floodplain.

ZONE D Areas in which flood hazards are undetermined, but possible.

COASTAL BARRIER RESOURCES SYSTEM (CBRS) AREAS

OTHERWISE PROTECTED AREAS (OPAs)

CBRS areas and OPAs are normally located within or adjacent to Special Flood Hazard Areas.

Floodplain boundary

Floodway boundary

Zone D boundary

CBRS and OPA boundary

Boundary dividing Special Flood Hazard Area zones and boundary dividing Special Flood Hazard Areas of different Base Flood Elevations, flood depths or flood velocities.

Limit of Moderate Wave Action

Base Flood Elevation line and value; elevation in feet*

Base Flood Elevation value where uniform within zone; elevation in feet*

Cross section Line

Transect line

87°07'45'', 32°22'30'' Geographic coordinates referenced to the North American Datum of 1983 (NAD 83), Western Hemisphere

2476000mN 1000-meter Universal Transverse Mercator grid values, zone

600000 FT

DX5510 [

Bench mark (see explanation in Notes to Users section of this FIRM panel)

M1.5

River Mile

MAP REPOSITORY

Refer to listing of Map Repositories on Map Index

EFFECTIVE DATE OF COUNTYWIDE

FLOOD INSURANCE RATE MAP

EFFECTIVE DATE(S) OF REVISION(S) TO THIS PANEL

For community map revision history prior to countywide mapping, refer to the Community Map History table located in the Flood Insurance Study report for this jurisdiction.

To determine if flood insurance is available in this community, contact your Insurance agent or call the National Flood Insurance Program at 1-800-638-6620.

PANEL 0342H

MAP SCALE 1" = 500'

FEET

250 0 500 1000

METERS

150 0 150 300

(EL 987)

AWAW

S23S23

* Referenced to the North American Vertical Datum of 1988

NAD 1983 UTM Zone 10N 5000-foot grid values: California State Plane coordinate system, zone II (FIPSZONE 0402), Lambert Conformal Conic projection

June 8, 1998

November 21, 2001 - to change/ add Base Flood Elevations, to update corporate limits, to change floodway, to incorporate previously issued Letters of Map Revision, to add roads and road names, to change Special Flood Hazard Areas, to change zone designations

PLACER COUNTY,

CALIFORNIA

AND INCORPORATED AREAS

PANEL 342 OF 1060

06061C0342H

NOTES TO USERS

This map is for use in administering the National Flood Insurance Program. It does not necessarily identify all areas subject to flooding, particularly from local drainage sources of small size. The community map repository should be consulted for possible updated or additional flood hazard information.

To obtain more detailed information in areas where Base Flood Elevations (BFEs) and/or floodways have been determined, users are encouraged to consult the Flood Profiles and Floodway Data and/or Summary of Stillwater Elevations tables contained within the Flood Insurance Study (FIS) report that accompanies this FIRM. Users should be aware that BFEs shown on the FIRM represent rounded whole-foot elevations. These BFEs are intended for flood insurance rating purposes only and should not be used as the sole source of flood elevation information. Accordingly, flood elevation data presented in the FIS report should be utilized in conjunction with the FIRM for purposes of construction and/or floodplain management.

Coastal Base Flood Elevations shown on this map apply only landward of 0.0' North American Vertical Datum of 1988 (NAVD 88). Users of this FIRM should be aware that coastal flood elevations are also provided in the Summary of Stillwater Elevations tables in the Flood Insurance Study report for this jurisdiction.

Elevations shown in the Summary of Stillwater Elevations tables should be used for construction and/or floodplain management purposes when they are higher than the elevations shown on this FIRM.

Boundaries of the floodways were computed at cross sections and interpolated between cross sections. The floodways were based on hydraulic considerations with regard to requirements of the National Flood Insurance Program. Floodway widths and other pertinent floodway data are provided in the Flood Insurance Study report for this jurisdiction.

Certain areas not in Special Flood Hazard Areas may be protected by flood control structures. Refer to Section 2.4 "Flood Protection Measures" of the Flood Insurance Study report for information on flood control structures for this jurisdiction.

The projection used in the preparation of this map was Universal Transverse Mercator (UTM) zone 10. The horizontal datum was NAD 83, GRS80 spheroid.

Differences in datum, spheroid, projection or UTM zones used in the production of FIRMs for adjacent jurisdictions may result in slight positional differences in map features across jurisdiction boundaries. These differences do not affect the accuracy of this FIRM.

Flood elevations on this map are referenced to the North American Vertical Datum of 1988. These flood elevations must be compared to structure and ground elevations referenced to the same vertical datum. For information regarding conversion between the National Geodetic Vertical Datum of 1929 and the North American Vertical Datum of 1988, visit the National Geodetic Survey website at http://www.ngs.noaa.gov or contact the National Geodetic Survey at the following address:

NGS Information Services

NOAA, N/NGS12

National Geodetic Survey

SSMC-3, #9202

1315 East-West Highway Silver Spring, Maryland 20910-3282

(301) 713-3242

To obtain current elevation, description, and/or location information for bench marks shown on this map, please contact the Information Services Branch of the National Geodetic Survey at (301) 713-3242, or visit its website at http://www.ngs.noaa.gov.

Base map information shown on this FIRM was provided in digital format by Placer County Department of Public Works.