CA_FLAP_SR89(1)_DrainageReport-Final.pdf

D R A I N A G E R E P O R T – F I N A L S U B M I T T A L

State Route (SR) 89 / Fanny Bridge Community Revitalization Project

Prepared for

Central Federal Lands - Highway Division

June 2016

Contents Section Page

Acronyms and Abbreviations .............................................................................................................. v

Project Overview ........................................................................................................................... 1-1

1.1 Introduction ..................................................................................................................... 1-1

1.2 Project Description .......................................................................................................... 1-1

Project Background ........................................................................................................................ 2-2

2.1 Existing Drainage Facility Summary ................................................................................. 2-2

2.2 Floodplains and Bridges ................................................................................................... 2-3

2.3 Field Review ..................................................................................................................... 2-3

2.4 Historic Issues .................................................................................................................. 2-5

2.5 Regulatory Requirements ................................................................................................ 2-5

Hydrology ...................................................................................................................................... 3-1

3.1 General ............................................................................................................................. 3-1

3.2 Methods and Recurrence Intervals.................................................................................. 3-1

3.3 Basin Characteristics ........................................................................................................ 3-1

3.4 Precipitation ..................................................................................................................... 3-3

3.5 Debris and Bulking Potential ............................................................................................ 3-3

3.6 Travel Time and Time of Concentration .......................................................................... 3-3

Hydraulics ...................................................................................................................................... 4-1

4.1 General ............................................................................................................................. 4-1

Stormwater Quality........................................................................................................................ 5-1

5.1 General ............................................................................................................................. 5-1

5.2 Water Quality Treatment Option and Sizing ................................................................... 5-1

5.3 Portions of Alignments A and the Wye Roundabout ....................................................... 5-1

5.4 Alignments B, D, and East Roundabout ........................................................................... 5-2

5.5 Alignment C and West Roundabout ................................................................................ 5-2

5.6 Storm Water Data Report ................................................................................................ 5-2

References

Appendixes

A Location Maps and Field Photos B Hydrology C Hydraulics D Floodplain Hydraulic Study

Table(s)

Table 1. Hydrologic Discharges

Table 2. Wood Rogers Existing Culvert Inspection

III

Acronyms and Abbreviations BMP Best Management Practice

CFL Central Federal Lands cfs cubic feet per second

CP concrete pipe

CSP corrugated steel pipe

DSN Drainage System Number

FEMA Federal Emergency Management Agency

FHWA Federal Highway Administration

GIS Geographic Information Systems

HDM Highway Design Manual msl mean sea level

No. Number

NOAA National Oceanic and Atmospheric Administration

NRCS Natural Resources Conservation Service

NWS National Weather Service

PDT Project Development Team

PPDG Project Planning and Design Guide

Project SR 89 Fanny Bridge Community Revitalization Project

RCP Reinforced Concrete Pipe

Regional Board Lahontan Regional Water Quality Control Board

SEZ stream environmental zone

SR State Route

SWMM Placer County Stormwater Management Manual

TMDL Total Maximum Daily Load

TRPA Tahoe Regional Planning Agency

TST Traction Sand Traps

USACE U.S. Army Corps of Engineers

USGS United States Geological Survey

WQF Water Quality Flow

WQV Water Quality Volume

WSEL Water Surface Elevation

V

SECTION 1

Project Overview

1.1 Introduction

This drainage report summarizes the proposed hydrologic and hydraulic design conditions for the State Route (SR) 89 Fanny Bridge Community Revitalization Project. The project is located in Tahoe City, California on the west shore of Lake Tahoe. Figure 1, Appendix A provides the project location map.

Preliminary design was performed by Wood Rodgers and is documented in the Intermediate Drainage Report, April 2014 (Reference 1).

1.2 Project Description

The existing SR 89 will be realigned and a new bridge crossing the river will be constructed downstream from the existing Fanny Bridge. The West River Road portion of SR 89 at the southwestern end of the project area will cross the Truckee River over the new Truckee River Bridge structure just east of an existing Caltrans Maintenance Yard. SR 89 will then continue southeast on a new alignment, across US Forest Service property and will connect to the existing alignment of the West Lake Boulevard portion of SR 89, north of Granlibakken Road. The realignment will include a roundabout at the southwest end of the project and a roundabout at the southeast end of the project connecting West River Road with West Lake Boulevard. The West Lake Boulevard portion of the existing SR 89 will become a Placer County road and minor improvements will be made to the existing roadway including streetscape improvements (curb, gutter, sidewalk, and landscaping features), parking, and restriping. Fanny Bridge will be replaced to address existing structural deficiencies and to ensure the structure meets current seismic standards.

Northwest of the Fanny Bridge, a new wye roundabout with parking facilities will join the West River Road portion on SR 89 and West Lake Boulevard. Figure 2, Appendix A provides the roadway layout.

Existing drainage facilities will be adjusted and/or upgraded as necessary to accommodate the proposed improvements. Best Management Practices (BMP) will be utilized to treat storm water runoff from impervious surfaces to limit impacts to the water quality to the Lower Truckee River. The Lower Truckee River has a Total Maximum Daily Load (TMDL) for sediment that was adopted by the Lahontan Regional Water Quality Control Board (Regional Board) in 2008. The project is within the Tahoe Regional Planning Agency’s (TRPA) jurisdiction and is required to meet TRPA Code Chapter 60.4 BMP requirements.

1-1

SECTION 2

Project Background

2.1 Existing Drainage Facility Summary

SR 89, West River Road and SR 89, West Lake Boulevard is a two-lane conventional highway, serving local and tourism-related traffic. The roadway, within the project limits, is characterized by gentle grades and limited shoulder widths, varying in elevation from 6,240 feet to approximately 6,235 feet above mean sea level (msl).

SR 89 West Lake Boulevard accommodates offsite watersheds which drain through the roadway to the Lower Truckee River near the existing Fanny Bridge. Existing drainage facilities for SR 89 West Lake Boulevard are localized to the roadway and nearby parking lots. Modifications to the existing storm drain facilities will be required to accommodate the new roadway improvements.

The existing drainage facilities on West River Road, near the north end of the project, include an existing drainage system (consisting of culverts that range in size from 24 to 48 inches) collecting runoff from the Tahoe City Urban Improvement Project. This drainage system was constructed in 1999 and sized for a 25-year flow capacity. The basin is drained by two 54-inch culverts and ultimately discharges to the Lower Truckee River. A drainage study, entitled Tahoe City Urban Improvements Project, completed by KB Foster for the Placer County Department of Public Works, dated 1998, provides the details for the existing drainage system. This proposed project will not utilize the existing drainage basin facility;

therefore, no evaluation of the offsite watershed or existing basin storage volume or the outflow drainage system to the Truckee River is included in this report (Wood Rodgers, 2014).

According to the Tahoe City Public Utilities District, an existing wetlands area accommodates the offsite watershed drainage immediately upstream of local businesses located on the northeast corner of the intersection of Fairway Drive and West River Road. The wetlands convey runoff through an existing culvert with headwall at the north side of West River Road located immediately to the northeast of the existing Chevron service station property. This offsite culvert ultimately discharges to the Truckee River.

The surrounding businesses at the northeast corner of Fairway Drive and West River Road have existing drainage facilities and are assumed to provide onsite drainage accommodations that ultimately discharge to the Truckee River through this offsite culvert or other nearby drainage facilities. Within the watershed of this existing wetlands area are other upstream drainage facilities and structures that direct runoff to the wetlands. The Tahoe City Golf Course and its drainage facilities are part of the offsite system tributary to the existing wetlands area. The proposed drainage system will function separately from this offsite wetland facility and contributing watershed; therefore, no evaluation of the offsite watershed or drainage system to the Truckee River is included in this report.

SR 89 West River Road accommodates other offsite watersheds which drain through the roadway via existing pipes to the Truckee River. The Tahoe City Public Utilities District has identified existing offsite drainage systems with storm drain pipes currently draining through the proposed project limits near the existing intersection of West River Road and West Lake Boulevard at the north end of the project. These systems and flow patterns are intended to be perpetuated through the proposed project limits. The existing discharge location at the west bank of the Truckee River immediately south of the Fanny Bridge will be maintained. A map provided by the Tahoe City Public Utilities District for these existing storm drainage facilities is included in Appendix A.

The Lower Truckee River begins at the outlet of the Lake Tahoe Dam and extends through the project area, paralleling the West River Road section of SR 89. The Fanny Bridge crosses the Truckee River, just to the south of the SR 89/SR 28 intersection and about 100 feet downstream of the Lake Tahoe Dam.

Another 1,100 feet downstream of the Fanny Bridge a pedestrian bridge crosses the river. A sanitary

2-2

SECTION 2 – PROJECT BACKGROUND

sewer pipe crosses the river via a bridge approximately 300 feet downstream of the proposed Truckee Bridge.

2.2 Floodplains and Bridges

Regulatory floodplains are delineated in the effective Federal Emergency Management Agency (FEMA) Flood Insurance Study for Placer County (06061CV001, revised November 21, 2001), and shown on the associated FEMA Flood Insurance Rate Map 06061C0203F (effective date of June 8, 1998). This portion of the Truckee River is located within a mapped Special Flood Hazard Area of Zone AE (which includes Base Flood Elevations).

A January 22, 2016 addendum was prepared by CH2M to accompany the original Bridge Hydraulic Study prepared by Wood Rodgers in April 2014. Together, the Wood Rodgers report and CH2M addendum cover the following topics in more detail, which are only summarized as follows:

• Proposed bridge design and geometry for both the new Fanny and SR-89 bridge crossings.

• Hydrologic assessment, including 50-year flood of record, FEMA discharges (10, 50, 100, and 500-year), and a discharge rate from the Bureau of Reclamation. See Table 1 Hydrologic Discharges.

Table 1. Hydrologic Discharges Hydrologic Discharges Used for the River and Bridge Assessment

Flood Discharge Rate

(cfs)

FEMA 10-year 850

FEMA 50-year 2,650

Flood(s) of Record (“50-year” from 1969 and 1997) 2,690

FEMA 100-year 3,350

Reclamation 3,530

FEMA 500-year 4,350

• Hydraulic modeling, performed in HEC-RAS, and which shows no adverse impact as a result of the proposed Fanny and SR-89 bridges. The modeling also determined available bridge freeboard as compared to requirements from Placer County, Central Federal Lands, and Caltrans.

• Bridge scour, which was assessed per the NCHRP 24-20 method for both the 100 and 500-year discharges. This determined key structural/footing elevations.

• Bridge countermeasure design (rock riprap), including rock sizing, depth, and geometry. Riprap is also shown on the bridge construction plans.

A Floodplain Hydraulic Study was prepared by CH2M and is included in Appendix D.

2.3 Field Review

Wood Rodgers conducted a site visit within the project limits on June 20-21, 2011, which included field notes on pipe sizes, locations, and photos of the existing system facilities; site visit information is documented in the Intermediate Drainage Report, April 2014 (Reference 1) and was used as a reference for locating and understanding the existing drainage facilities. CH2M also conducted a field visit in

2-3

SECTION 2

September 2015. Appendix A provides key photos collected from this field visit. In conjunction with the Caltrans culvert summary presented in Table 2, the existing system was reviewed by Wood Rogers to determine if any portions require replacement, based upon age and condition (Wood Rodgers, 2014).

Based upon field review and the Caltrans evaluation, the system appears to be in relatively good condition, and the sand traps are relatively new, with the exception of System 9; which is buried and in relatively poor condition. Table 2 summarizes the Caltrans culvert inventory between Post Mile (PM) 8.29 and PM 9.05, dated September 7, 2011 (Wood Rodgers, 2014).

Summary of the existing facility was reviewed for the following:

• If the pipe entrance was in poor condition, the pipe was determined to be in overall poor condition and identified for lining/replacement.

• If the pipe entrance was in good condition, perform an additional field review to safely attempt to access the pipe outlet down slope.

• Where pipe outlets cannot be safely accessed, review information on the age of the pipe from the reference plans. For pipes that are greater than 40 years old, these pipes can be identified as needing lining/replacement.

• For newer pipes (less than 20 years old) with observable good pipe condition at the entrance, these pipes are likely adequate.

• For moderately aged pipes (20 to 40 years old) and which appear to be in fair to good condition from entrance observations, contact Caltrans for a potential culvert video (Wood Rodgers, 2014).

Table 2. Wood Rogers Existing Culvert Inspection

System1 Appr.

Post Mile

Inspec.

Date

Culvert Size (in) Material

Treatment Est.

Service

Life

Work Nec. Remarks

Inlet Outlet

17 9.05 15Jul98 24 PVC CSP

FES

Proj.

Barrel 10 Yrs ------ Good Pick Test

20 9.03 15Jul98 18 PVC Conc.

HW

Proj.

Barrel 10 Yrs ------ Fair Pick Test

NA 8.94 15Jul98 18 CSP DI CSP FES ------- ------ Good Pick Test

25 8.76 15Jul98 18 PVC DI ------ 30 Yrs ------ Good Condition

26 8.71 22Oct98 18 CSP ----- ------ ------ ------ DI on Both Side of

HWY

26 8.69 15Jul98 24 CSP DI ------ ------ ------ Outlet to Truckee River

35 8.51 16Jul98 18 CSP DI ----- ------ ------ Outlet to Truckee River

1 8.46 06Jul08 18 CP DI Buried ------ Replace Looks bad

1 8.35 06Jul08 18 CP DI DI >10 Yrs. ___ Good Condition

1 8.29 06Jul08 18 CP DI DI >10 Yrs. ___ Good Condition

Source: Wood Rodgers, 2014 1 System number updated from Intermediate Drainage Report April 2014

2-4

SECTION 2 – PROJECT BACKGROUND

2.4 Historic Issues

Wood Rodgers collected historical information for consideration in the overall design of the Project.

Available information, specific to the project, which was reviewed, includes: the existing culvert inventory, recorded historical floodplain information, flood records provided by Caltrans maintenance personnel and property owner observations (Wood Rodgers, 2014).

According to correspondence with Caltrans District 3, this Project area has had no significant flood history, even with the recorded flood-of-record storm in January 1997. Even though Lake Tahoe Dam released the highest recorded flow in 1997 and Fanny Bridge experienced erosion through the bridge structure, the roadway remained open during the high peak flow releases. Significant flooding occurred to existing homes and businesses on the Lower Truckee River, but the locations were well downstream of this proposed Project (Wood Rodgers, 2014).

2.5 Regulatory Requirements

A number of agencies will be directly and indirectly involved in the design review through various permitting processes. Due to construction within the Truckee River and impacts to wetlands, the following are the agencies that will be involved:

• Bureau of Reclamation – An encroachment permit and coordination will be required to address considerations that need to be provided for the Lake Tahoe Dam and to complete construction activities within the Bureau’s easement for the dam.

• California Department of Fish and Wildlife – A Streambed Alteration Agreement Permit (Section 1601) will be required to address the required construction within the Truckee River.

• Caltrans District 3 – will provide review of the drainage report for compliance with their highway design manual.

• Federal Highway Administration (FHWA), Central Federal Lands Highway Division – will provide review of the drainage report for compliance to their design and construction requirements.

• Placer County – An encroachment permit will be required for any work that is performed within the County right-of-way.

• Regional Board, Lahontan Region – The Regional Board will require a Water Quality Certification (Section 401) to certify the required 404 Permit and to certify that the project will not exceed water quality standards. Construction of the project will also require coverage under the General Permit for Discharges of Storm Water Associated with Construction Activity Construction General Permit Order 2009-0009-DWQ.

• Tahoe Regional Planning Agency (TRPA) - TRPA is represented on the Project Development Team (PDT) and as such, is actively involved by providing input on roadway design and stormwater treatment strategies for the project. Specifically, TRPA regulates: development within a stream environmental zone (SEZ), tree removal, impervious surface coverage, and a soil hydrologic analysis in areas where infiltration facilities are proposed to ensure there is 1-foot separation between the invert of the infiltration facility and the seasonally high groundwater elevation. Soil hydrologic information was obtained from project geotechnical boring samples.

• U.S. Army Corps of Engineers (USACE) – A 404 Permit will be required to address impacts to jurisdictional wetlands and due to the required construction activities within the Truckee River.

• USDA Forest Service– A special use permit will be required to complete any work that occurs within the limits of the Forest Service property (Wood Rodgers, 2014).

2-5 http://www.waterboards.ca.gov/water_issues/programs/stormwater/constpermits.shtml

SECTION 3

Hydrology

3.1 General

This section provides an overview of the input parameters, calculations, and peak flow results for both the proposed drainage system networks and proposed roadside channels. The offsite and onsite watershed hydrology has been analyzed using methods outlined in the Caltrans Highway Design Manual (HDM) (Reference 2).

3.2 Methods and Recurrence Intervals

Rational Method hydrologic calculations were performed to determine the surface runoff tributary to the proposed drainage system. Rational Method runoff coefficients given in Figure 819.2A and Table 819.2B of the HDM were used to calculate a weighted composite runoff coefficient for each tributary area. The undeveloped portion of each tributary area was assigned a pervious runoff coefficient using the method described in Figure 819.2A.

For this method, each undeveloped portion of a drainage tributary area was matched with four different basin characteristics of relief, soil infiltration, vegetal cover, and surface storage. The relief or slope of the undeveloped surface was determined by using existing and proposed topographic elevation data.

The soil infiltration characteristic was determined by matching the existing hydrologic soil group with the soil infiltration classifications found in Figure 819.2A. Existing soils that were mapped by the Web Soil Survey (WSS) Soil Data (Reference 3) as hydrologic soil group A were characterized as high infiltration or well drained soils. For existing soil groups mapped as hydrologic soil group B and C, the infiltration classification was matched with Figure 819.2A as having normal and slow infiltration soils, respectively. Existing soils that were mapped as hydrologic soil group D were characterized as having poor infiltration capacity. WSS Soil Data is provided in Appendix B. Vegetal cover for each tributary area was determined from aerial imagery of each basin. The last basin characteristic of surface storage was also determined by using aerial imagery and field visit photography.

Tributary areas with impervious surfaces such as sidewalks, parking lots, roof tops, paved roadways and pathways were assigned a runoff coefficient of 0.95. The pervious runoff coefficient and pervious area was used with the impervious runoff coefficient and impervious area of each tributary area to calculate a weighted composite runoff coefficient. This weighted composite runoff coefficient was used with the Rational Method formula to generate a peak flow for each tributary area.

The hydrologic calculations for the proposed drainage systems were evaluated with the 10-year storm event in accordance with the HDM Table 831.3 for highways with the design speed limit of 45 miles per hour and under. The proposed drainage systems were also evaluated for the 100-year storm event to check for objectionable backwater as requested by Caltrans.

3.3 Basin Characteristics

The proposed drainage basins contributing runoff to the roadway improvements along with a detailed summary of hydrology calculations for each area are presented in Appendix B. Drainage basin boundaries were delineated using a combination of proposed roadway improvement contour data and existing topographic project data including an InRoads digital terrain model. Outside of the project limits, drainage basin boundaries and land slopes were delineated using United States Geological Survey (USGS) Quadrangle data and Lake Tahoe Basin LiDAR data for the Lake Tahoe area collected in August 2010 by Watershed Sciences, Inc.

3-1

Project roadway alignments used to locate drainage facilities correspond with the four main roadways for this project:

• Alignment A – West Lake Boulevard,

• Alignment B – SR 89, the new south connector road between West Lake Boulevard and West River Road,

• Alignment C – SR 89, West River Road,

• Alignments D2 and D3 – the new driveway access roads that connect the existing Tahoe City Transit Center with West Lake Boulevard

Proposed drainage facilities are separated into drainage networks that are identified with a Drainage System No. (DSN). Each drainage network is labeled on the construction plans and in this report. In addition to the Drainage System No., each proposed structure also has a structure identification number also referred to as Structure ID. The Structure ID is comprised of the nearest roadway station followed by a structure type label and a roadway offset identifier. Proposed downstream pipes from proposed structures have been labeled with the same Structure ID as its upstream structure. Hydrology basin boundaries assigned to proposed structures also contain the name of the proposed Structure ID that they are assigned for flow calculations. Some existing structures that are included in the construction documents and drainage calculations have been assigned Structure IDs and follow the same naming convention as the proposed structures; however, the existing Structure ID contains the letters “EX” as a prefix.

The north portion of the Alignment A roadway will be drained by a storm drain system (DSN 1-6) consisting of inlets and pipes that discharge to an outfall location at the east bank of the Truckee River immediately north of the Fanny Bridge. Existing storm drain systems (DSN 4) that currently drain parking lots and driveways east of the northern portion of Alignment A will be connected into this proposed storm drain system.

The project area consisting of the south portion of Alignment A, East Roundabout, and the Alignment B connector roadway, will be drained by a storm drain system (DSN 8-15) consisting of inlets, pipes, ditches, and culverts that discharge to an outfall location at the east bank of the Truckee River immediately north of the proposed Truckee River Bridge location.

A small portion of the project north of Alignment D3, near the Tahoe City Transit Center will drain through a proposed culvert (DSN 7) and continue downstream to the Truckee River.

At the southwestern most location of the project, the Alignment C roadway currently is drained by an existing drainage system consisting of inlets and pipes. This existing system will be adjusted to accommodate the proposed roadway and bike trails in this area. Adjustments include existing pipe extensions and proposed storm drain structures and pipes (DSN 16-20) in this area to maintain existing drainage patterns for roadway improvements. The storm drain systems in this area will continue to drain to the existing storm drain system within the Caltrans Maintenance Facility. This facility ultimately drains to the Truckee River at the west bank adjacent to the Caltrans property.

The project area immediately surrounding the West Roundabout will be drained by a storm drain system (DSN 21-24) consisting of inlets and pipes that will discharge to an outfall location at the west bank of the Truckee River.

From the West Roundabout to approximately 300 feet south of Fairway Drive along Alignment C, a proposed storm drain system (DSN 25) consisting of inlets and pipes will be used to provide drainage and discharge to an outfall at the west bank of the Truckee River. Existing inlets (DSN 42) that can remain in place with minor adjustments will be preserved and will function separately from the

3-2

SECTION 3 – HYDROLOGY

proposed drainage system pipe network. Large existing offsite areas to the west of Alignment C are included in the tributary area draining to the proposed storm drain system and will be conveyed through this system to the Truckee River.

Approximately 160 feet south of Fairway Drive along Alignment C, the proposed low point of the roadway will be drained by providing minor adjustments to the existing storm drain system (DSN 26, 41) consisting of inlets and pipes that currently discharge to existing locations along the west bank of the Truckee River. Large existing offsite areas to the north and west of Alignment C are included in the tributary areas to this storm drain system and will be conveyed to the Truckee River.

Existing inlets that provide onsite and offsite drainage for the area surrounding the intersection of West River Road and Fairway Drive will be protected in place (DSN 39, 40). Inlets located within the existing curb and gutter to remain have been analyzed to ensure they meet project requirements for spread.

The Wye Roundabout and surrounding parking areas will be drained by providing minor structure elevation adjustments and new storm drain pipe (DSN 35, 36) to the existing storm drain system that currently discharges to an existing location at the west bank of the Truckee River. A proposed storm drain system (DSN 27-38) will provide drainage to accommodate proposed roadway and parking improvements and will also discharge to the west bank of the Truckee River immediately south and north of the Fanny Bridge.

3.4 Precipitation

National Oceanic and Atmospheric Administration (NOAA) Atlas 14 (Reference 4) data were used to determine point precipitation and intensity duration frequency curves. This data is included in Appendix B.

3.5 Debris and Bulking Potential

The debris and bulking potential, or likelihood of large volume stormwater runoff to entrain large volumes of sediment, increases the volume and rates of runoff within watersheds and causes the capacity of facilities, designed for clear-water flow, to be exceeded in large storm events. The facilities throughout the project site do not have a history of lack of conveyance due to large amounts of debris and there are no major reports of flooding areas. The upstream areas draining to the project area have no history of large-scale burns and do not have a large sediment supply and, for the most part, runoff travels to the roadway in concentrated flow. Any sediment clogging roadway facilities appears to be from onsite flows from cut slopes and traction sand. This has been addressed through the incorporation of sand traps and water quality structures in the existing system, which have been sized to collect traction sand applied to onsite watersheds. The application of a bulking factor is typically appropriate for areas where very high sediment generation is anticipated. All runoff contains some portion of entrained sediment, but for most typical hydrologic analyses it is neglected. In the case of the Tahoe Basin, standard hydrologic analysis is very imprecise (and typically conservative) due to the ability of forested areas to absorb and attenuate runoff below established runoff coefficients. The volume of the limited sediment generated, falls within the accuracy of the standard methods to predict runoff.

Because the runoff reaching the project area is predominantly offsite flows with relatively small sediment generation potential, bulking potential was neglected in the evaluation of the existing system and in the sizing of any proposed culverts for the project (Wood Rodgers, 2014).

3.6 Travel Time and Time of Concentration

The proposed time of concentrations for the hydrology basin boundaries were calculated according to the HDM, Section 816.6. The time of concentration was broken into segments of sheet flow, shallow concentrated flow, and channel flow for the maximum flow length through a watershed. The maximum

3-3 flow length and slope were determined from the most hydraulically distant point of the watershed to the concentration point using existing and proposed topography. Sheet flow and shallow concentrated flow travel times were determined with the equations and parameters outlined in the HDM. Channel velocities/travel times were determined using the Manning’s Equation with slopes determined by offsite or onsite topography (depending upon the location of the channel) and roughness factors and depths determined by field or photo observation. Maximum lengths of approximately 100-feet for unpaved areas and 300-feet for paved areas was used for sheet flow segments. Flow was considered shallow concentrated from the end of the sheet flow to the roadway unless there is an established creek or stream evident on the topography. A minimum time of concentration of 5 minutes was used as recommended in the HDM. Where tributary areas consisted of mostly pervious surfaces, a minimum time of concentration of 10 minutes was used. InRoads Storm and Sanitary was used to determine drainage network concentration times as calculations proceeded downstream through the drainage network. For hydrology basin boundaries where the time of concentration was calculated to be greater than the recommended minimum of 5 or 10 minutes, a time of concentration calculation has been included in Appendix B.

3-4

SECTION 4

Hydraulics

4.1 General

The 10-year design storm with InRoads Storm and Sanitary was used to size and design the storm drain pipe networks, culverts, and ditches throughout the project per HDM Table 831.3. System capacity was designed to keep the hydraulic grade line (HGL) below the elevation of the top of pipe for the 10-year storm event. System capacity was also designed to keep the HGL below the rim elevation of drainage structures for the 100-year storm event. The 10-year design storm with Bentley Flowmaster was used to evaluate the storm drain inlet capacity and spread calculations based on methods described in FHWA Hydraulic Engineering Circular (HEC) 22 (Reference 5). A 33% clogging factor was used for inlets on grade and a 50% clogging factor was used for inlets within sag locations. Inlet locations and roadway improvements were made to keep the 10-year storm event spread within the roadway shoulder and out of the traveled way. Where the roadway section does not include a shoulder width, spread was mitigated to keep a 12 foot clear lane open within the travel way.

All proposed roadway inlets are depressed 1 ¼ -inches to avoid catching the blade of snow plows.

Roadway inlets are combination curb opening with grates to assist debris to pass through the inlet.

Caltrans Maintenance was concerned with the plow blade catching a curb opening for inlets along the inner gutter of the roundabouts so these inlets have a grate opening only. Type 24-12X grates were also selected for the roadway inlets to increase interception capacity and accommodate bicycle traffic.

The 10- and 100-year design storm with Hydraulic Design Series 05 (Reference 6) was used to evaluate culvert headwater depths to keep headwater below an elevation that would cause objectionable backwater depths. Ditch capacity and lining calculations with a 10-year design storm were performed with Bentley Flowmaster and FHWA Hydraulic Toolbox, version 4.2.

For pipe networks that discharge to the Truckee River, the 10-year water surface elevation (WSEL) of the river was used as the tailwater elevation for the pipe network analysis within InRoads Storm and Sanitary. Based on Frequencies for Coincidental Occurrence from FHWA Hydraulic Engineering Circular No. 22, Third Edition (Revised August 2013) Urban Drainage Design Manual the 10-year river WSEL is the appropriate tailwater condition because of the difference in drainage areas between the project and the Lake Tahoe basin. The dam further increases the time of concentrations between the project and lake.

Riprap aprons are located at the outlets of the storm drain pipe networks and culverts following CFL and Caltrans District 3 standards. Riprap apron rock sizes at culvert outlets have been sized using methods outlined in the Caltrans Rock Energy Dissipator at Culvert Outlet document provided by Caltrans and procedures outlined in FHWA HEC-14 (Reference 7). The maximum rock size calculated for all the riprap aprons is 5.5 inches; the minimum rock size chosen for riprap aprons for this project has a D50 of 9 inches. These calculations and procedures are included in Appendix C.

Roadside ditches will convey offsite and roadway runoff throughout the project. Stable ditch linings have been determined using methods outlined in HEC-15 (Reference 8) and calculated using the FHWA Hydraulic Toolbox, version 4. Calculation output showing stable vegetated and rock linings are included in Appendix C.

The following hydraulic calculations have been included in Appendix C:

• 10- and 100-year hydraulic summary for pipes with HGLs, generated by InRoads Storm and Sanitary

• Inlet capacity and spread calculations for the 10-year design storm using Bentley Flowmaster

4-1

SECTION 4

• Culvert inlet headwater and hydraulic calculations

• Rock Energy dissipator at culvert outlet calculations

• Ditch hydraulic and lining calculations

4-2

SECTION 5

Stormwater Quality

5.1 General

CH2M reviewed the Caltrans PPDG for Stormwater Quality and TRPA’s Code of Ordinances to determine storm water related requirements for the project. The project incorporates BMPs for design pollution prevention, permanent treatment, temporary construction site, and maintenance to control the water quality discharge of storm water. Treatment BMPs are designed to handle the discharge from impervious surfaces generated by the 20-year, 1-hour storm (1 inch of rainfall in the Lake Tahoe Basin).

The treatment strategy relies upon use of existing and proposed BMP including: biofiltration strips (biostrips), permanent check dams, traction sand traps (TST), and a Jellyfish Filter. The project area can be separated into three distinct segments with respect to water quality improvements: Alignment A and the Wye Roundabout; Alignments B, D and the East Roundabout; and Alignment C and the West Roundabout.

5.2 Water Quality Treatment Option and Sizing

CH2M utilized previous design and record information to develop the BMP strategy with the goal of reusing existing BMP where possible. Below is a description of the selected BMP and their functions:

Biostrips / Check Dams: Biostrips are vegetated slopes adjacent to impervious areas where storm water sheet flows through the vegetation. Vegetation removes sediment through filtration. The biostrips are a function of the width of pavement draining to the slope, runoff coefficient, design intensity, Manning’s n and slope. Permanent check dams have been provided in the roadside ditches along the new SR 89 alignment to provide storage, reduce sediment loads, and increase the time of concentration to the Truckee River.

TST: This project location incurs snow throughout the winter months. At the proposed roundabouts, Caltrans will apply traction sand or other roadway abrasives to increase driver safety. This sand will collect along the roadway until a snow melt or rain event washes the sand off the roadway. TSTs capture the sand before it discharges into storm drain systems which drain to the Truckee River. After discussions with TRPA, Lahontan RWQRB, and Caltrans, TST will be used at all inlet and manhole locations, where feasible, on the project. For this project, the Traction Sand Trap, Pipe Inlet – CIP Type will be used. This TST type also has the benefit of infiltration through the weep holes on the bottom of the inlet. The TST volume is located below the lowest invert of the outlet pipe to a maximum depth of 4 feet. Where a depth of 4 feet was not practical due to limiting constraints of depth or possible groundwater, the TST depth was restricted to only 2 feet below the lowest pipe outlet invert.

Calculations for traction sand trap sizing were done for the inlet with the highest sand volume based on contributing tributary area and used as the basis for design for all TST volumes. These calculations are included in Appendix C.

Jellyfish Filter: Jellyfish Filter removes sediment from the WQF in a large manhole with tentacle like filtration cartridges. The size and configuration of the Jellyfish Filter is a function of the WQF and peak flows, if the system is online or offline, and the filtration required. This filtration device is approved by and will be maintained by Placer County.

5.3 Portions of Alignments A and the Wye Roundabout

There are two storm drain systems that drain Alignment A to the Truckee River near the Fanny Bridge.

The storm drain system south of the Fanny Bridge, drains Alignment A from Station 116+00 to the

5-1

SECTION 5

Truckee River. A Jellyfish Filter will be used south side of the Fanny Bridge. The Jellyfish Filter has been designed by Contech. TST will be used for all inlets and manholes except for 122GTL1 which functions as the bypass structure for the Jellyfish Filter.

The proposed storm drain system draining the Wye Roundabout and portions of Alignment C from Station 319+50 to the north will outlet on the northwest side of the Fanny Bridge. TST will be used at each inlet. Manholes will also have sumps except near the river because of potential groundwater elevation. Manholes that will be installed to connect to the existing system will also not include sumps.

5.4 Alignments B, D, and East Roundabout

TSTs will be used at all inlets and manholes for the East Roundabout and adjacent portions of Alignment A. These systems will be routed directly to the Truckee River. Curb has been removed on West Lake Road from Station 110+40 to 114+30 on the left side and on both sides of Alignment D. Biostrips are used to treat the storm water sheet flowing off the road.

As Alignment B traverses through undeveloped U.S. Forest Service property, biostrips will be used to filter the storm water sheet flowing off the road to minimize impacts to the existing trees throughout this property. Permanent check dams will be used in the roadside ditches to provide storage, reduce sediment loads, and increase the time of concentration.

Manholes in the shoulder of Alignment B will also have sumps with the exception of 206MHL1. This manhole is located next to guardrail. A sump was not included for this manhole since maintenance parking would be limited next to the guardrail. Remaining drainage structures to the west do not have sumps since maintenance access would be difficult for these locations well off of the Alignment B roadway. These areas are already treated by the biostrips discussed above.

5.5 Alignment C and West Roundabout

Alignment C from approximately Station 301+40 to Station 308+80 reuses existing storm drains and TSTs as well as extending storm drains at Station 301+40, 303+50 and 306+25. The system located near Station 301+40 drains to the west and ultimately to the Truckee River. The other two systems drain towards the existing Caltrans maintenance facility which has three existing TST vaults the storm water will be routed through before entering the Truckee River. No new BMPs are proposed in this section.

A new storm drain system is proposed on Alignment C from Station 308+80 to the West Roundabout.

Existing TSTs are recommended for reuse. TSTs are used for all new inlets and manholes except for 315MHR1. This deep manhole is near the river and could have potential groundwater issues.

5.6 Storm Water Data Report

A Storm Water Data Report (SWDR) has been prepared for the project. The report incorporates the rationale for the selection of BMPs for design pollution prevention, permanent treatment, temporary construction site, and maintenance to control the discharge of storm water. The SWDR also documents design considerations, environmental concerns, and Caltrans regional and local requirements for the project.

5-2

References

1. Wood Rodgers Intermediate Drainage Report, April 2014

2. Caltrans Highway Design Manual, July 2015

3. Web Soil Survey Soil Data produced by the National Cooperative Soil Survey

4. National Oceanic and Atmospheric Administration Atlas 14 Precipitation Data

5. FHWA Hydraulic Engineering Circular 22 (Urban Drainage Design Manual, 2001)

6. Hydraulic Design Series 05, May 2005

7. FHWA Hydraulic Engineering Circular 14, July 2006

8. FHWA Hydraulic Engineering Circular 15, September 2005

Appendix A Location Maps and Field Photos

Project Location

CA CA FLAP SR 89 (1)

PROJECT

Location Map

Project

Figure 1

STATE

10 miles5 miles0

Scale in Miles

I-80

S R

I-80

S R 9 S

R

SR-89

US-50

U S

U S

US-50

50,000'25,000'0

Scale in Feet

S R

FLAG POLE

DI

DI

DI

DI DI DI

DI

DI

DI DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DIDI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI DI DI

DI

DIDIDI DI

DI

DI DI DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

DI

W

W

W

West Roundabout

Wye Roundabout

"B" Alignment

"D3" Alignment

"D2" Alignment

Maintenance Yard

Existing Caltrans

Bridge

Truckee

Proposed

Center

Transit

Tahoe City

Lake Tahoe Dam

Lake Tahoe

Bridge

Fanny

Roundabout

East

F a irw a y D riv e T ru ck e e R iv e r

(West River Blvd.)

"C" Alignment

(West Lake Rd)

"A" Alignment

Truckee River

31+87

105+00

110+00

115+00

120+00

5+

0+

320+

325+

31+87

105+00

110+00

115+00

120+00

5+

0+

320+

325+

320+

325+

5+

120+00

115+00

31+87

110+00

105+00

0+

CA CA FLAP SR 89 (1)

PROJECT

Roadway Layout

Proposed

Figure 2

STATE

500'250'0

Scale in Feet

Photo 1. West Bank of Truckee River, Outfall location of proposed Drainage System No. 1

Photo 2. South side of existing Fanny Bridge showing existing 30” CMP storm drain discharge location to the Truckee River. Proposed outfall location of Drainage System No. 27 near the west bank of the Truckee River.

Photo 3. Existing storm drain outfall near Caltrans Maintenance Yard; pipe to be extended as part of proposed Drainage System No. 20

Photo 4. Existing drainage ditch north of Caltrans maintenance yard and downstream of proposed Drainage System No. 20

Photo 5. Existing inlet near northwest corner of Fanny Bridge to be reconstructed for Drainage System No. 37.

TCGC Drainage

SHEET:

1P.O. BOX 5249

TAHOE CITY, CA. 96145

(530) 583-3796

TAHOE CITY PUBLIC UTILITY DISTRICTNOTES:

DATE PRINTED:

MAP FILE NAME:

MXD LOCATION:

MAP SCALE:

DRAWN BY:

CHECKED BY:

KH

KH

Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community

1 inch = 96 feet

OF 01

TCGC_Drainage map docs/mxd/

3/23/2016

DRAFT

3/23/2016

Data Incomplete µ

") Inlet

!. Manhole

#* Treatement

Culvert

TCUIP Culvert

Ditch

Line

Spring

Appendix B Hydrology

Map 1 Map 2

Map 3

CA CA FLAP SR 89 (1)

PROJECTSTATE

1,800'900'0

Scale in Feet

Keymap

Basin Boundaries

Drainage

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D

ID

ID

I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

W

W

W

0+

200+00

A201G0R1 A201G0L1

A317G4L1 A318G4L1

A310G0L1

A203G0R1 A203G0L1

AEX321G4L2

AEX322G4L1

AEX321G4L3

AEX321G4L1

A319GOL1

A319GOR1 AEX321GR1

A311G4L1

AEX316GL1

A318G0R1

A312G0R1

A311G0R1

A311G0L3

AEX318GL1

Roundabout

East

(West River Blvd.)

"C" Alignment

A314G0R1

A309G0L2

A309G0L1

A311G0L1

A311G0L2 A314L1

A314L2

A314G0R2

AEX319GR1

AEX319GR2

AEX322GL1

AEX323GL1

A306R1

A305G0R1

(West River Blvd.)

"C" Alignment

AEX322GR1

300'150'0

Scale in Feet

Time of concentration flow path

Existing storm drain network

Proposed storm drain network

Proposed major basin boundary

Proposed minor basin boundary

CA CA FLAP SR 89 (1)

PROJECTSTATE

Outfall location

Boundaries - Map 1

Drainage Basin

See Map 2

S e e M a p

Basin Boundary ID

Drainage System No.

A311G0L1

300'150'0

Scale in Feet

Time of concentration flow path

Existing storm drain network

Proposed storm drain network

Proposed major basin boundary

Proposed minor basin boundary

CA CA FLAP SR 89 (1)

PROJECTSTATE

Outfall location

Boundaries - Map 2

Drainage Basin

See Map 1 See Map 3

Drainage System No.

Basin Boundary ID

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

A311G0L1

120+

30+00

0+

5+

120+

30+00

0+

5+

A201G0L1

A122GOL1

A121GOL1

AEX121G4R1

AEX120G4R1

A119GOL1

A117G4L1

A110G0R1

A213G0L1

A213G0R1

A113FL1

A114G0R1

A113G4R1

A203G0R1 A203G0L1

A119GOR1

A121G4R2

A122GOR1

A203G4L1

A203FR1

A106L1

A213FR1

A108FR1

A212G0L1

"B" Alignment

"D3" Alignment

"D2" Alignment

(West Lake Rd)

"A" Alignment

Roundabout

West

A114G4R1

A113G0R1

A203G4L1

A111G0R1

A114GOL1

A31FR1

A120GOL1

A118GOL1

A117GOL1

A122GOR2

A115GOL1

A122GTL1

AEX117G4R1

A106R1

A107R1

A107L1

A121GOR1

A314G0R1

A108R1

A211R1

A211L1

A206L1

A206R1

A203L1

A203R1

A203R2

FLA

G P

O

LE

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I

D I D I

D I

D I

D I

125+00

A122GOL1

A333G0L1

A334G0R1

A332G0R2

A126G4R1

A126G0R1

A124G0L1

A127G0L1

A329G0R1 A331G0R1

A331G0L1

A331G0L2

A126G0R3

A126G0R2

Wye Roundabout

(West River Blvd.)

"C" Alignment

A329G0L1

A331G0R2

A332G0R1

A333G0R1

A127G0L1

AEX322GR1

A124G0R1

AEX323GL1

Time of concentration flow path

Existing storm drain network

Proposed storm drain network

Proposed major basin boundary

Proposed minor basin boundary

CA CA FLAP SR 89 (1)

PROJECTSTATE

Outfall location

300'150'0

Scale in Feet

Boundaries - Map 3

Drainage Basin

Drainage System No.

Basin Boundary ID

See Map 2

S e e M a p

A311G0L1

Truckee River Project - Proposed 10-year Hydrology Summary

System No.

Tributary Area

ID

Relief Descrip-tion (HDM Figure 819.2A)

Relief Runoff

Coefficient, C1

Soil Infiltration

Descrip-tion (HDM

Figure 819.2A)

Soil Infiltration

Runoff Coefficient, C2

Vegetal Cover

Descrip-tion (HDM

Figure 819.2A)

Vegetal Cover Runoff

Coefficient, C3

Surface Storage Descrip-tion (HDM Figure 819.2A)

Surface Storage Runoff

Coefficient, C4

Pervious Runoff

Coefficient, C

Pervious Tributary

Area (acres)

Impervious Runoff

Coefficient, C

Impervious Tributary

Area (acres)

Frequency Factor, Cf

Basin Weighted

Runoff Coefficient, C

Time of Concen-tration, Tc

(min)

Rainfall Intensity (in/hr)

Total Tributary

Area (acres)

Rational Method Runoff

(cfs)

Bypass or Flow

From Up-stream (cfs)

Total Flow to

Structure (cfs)

Assigned Structure to

Receive Flow

Alignment A - Outfall to Truckee River Near Fanny Bridge, East Bank

1 A 114G4R1 Flat 0.11 Normal 0.07 Sparse 0.14 Negligible 0.11 0.43 0.12 0.95 0.00 1.0 0.43 10 2.69 0.12 0.14 0.00 0.14 114G4R1 1 A 114GOL1 Flat 0.11 Normal 0.07 Sparse 0.14 Negligible 0.11 0.43 0.00 0.95 0.09 1.0 0.95 5 3.76 0.09 0.32 0.00 0.32 114GOL1 1 A 115GOL1 Flat 0.11 Normal 0.07 Sparse 0.14 Negligible 0.11 0.43 0.00 0.95 0.07 1.0 0.95 5 3.76 0.07 0.25 0.00 0.25 115GOL1 1 A 117GOL1 Flat 0.11 Normal 0.07 Sparse 0.14 Negligible 0.11 0.43 0.00 0.95 0.01 1.0 0.95 5 3.76 0.01 0.04 0.00 0.04 117GOL1 1 A 118GOL1 Flat 0.11 Normal 0.07 Sparse 0.14 Negligible 0.11 0.43 0.00 0.95 0.08 1.0 0.95 5 3.76 0.08 0.29 0.00 0.29 118GOL1 1 A 119GOL1 Flat 0.11 Normal 0.07 Sparse 0.14 Negligible 0.11 0.43 0.00 0.95 0.15 1.0 0.95 5 3.76 0.15 0.54 0.00 0.54 119GOL1 1 A 120GOL1 Flat 0.11 Normal 0.07 Sparse 0.14 Negligible 0.11 0.43 0.00 0.95 0.09 1.0 0.95 5…