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Home My WebLink AboutSWP273205(3)1-405, Renton Nickel Improvement Project, 1-5 to SR 169 Congestion Relief S Bus Rapid Transit Projects FLOODPLAINS DISCIPLINE REPORT October 2005 1-405 Project Area 524 Bothell. I 522 Y ' Kirkland 9os 520 1 7 ahe wosninyroo ' ! tBl,e�l lI e v u e ' 7 a Renton Nickel' a Improvement Project �Renton� i 169 - �Iel 167 - -__ C 5i5� - Arterial Road Lake — Freeway Park rJ+ Municipality AdElk MWashington State fa APDepartmentof Transportation U.S. Department of Transportation Federal Highway Administration FLOODPLAINS DISCIPLINE REPORT 1-405, Renton Nickel Improvement Project Prepared for Washington State Department of Transportation Urban Corridors Office And Federal Highway Administration Prepared by Bruce Jensen and Karen Comings, DMJM Harris October 27, 2005 i�- 1tun_- M/l WSDOT ensures full compliance with Title VI of the Civil Rights Act of 1964 by prohibiting discrimination against any person on the basis of race, color, national origin or sex in the provision of benefits and services resulting from its federally assisted programs and activities. For questions regarding WSDOT's Title VI Program, you may contact the Department's Title VI Coordinator at 360. 705.7098. Americans with Disabilities Act (ADA) Information If you would like copies of this document in an alternate format —large print, Braille, cassette tape, or on computer disk, please call 360.705.7097. Persons who are deaf or hard of hearing, please call the Washington State Telecommunications Relay Service, or Tele-Braille at 7-1-1, Voice 1.800.833.6384, and ask to be connected to 360.705.7097. TABLE OF CONTENTS Glossary............................................................................................................................................................................. in Acronyms and Abbreviations Used in this Report ......................................................................................................... v Introduction........................................................................................................................................................................1 What is the Renton Nickel Improvement Project?...........................................................................................................1 Whatis the No Build Alternative?................................................................................................................................ 2 Whatis the Build Alternative?..................................................................................................................................... 2 How will stormwater from the project be managed?.................................................................................................12 What environmental and utilities issues influenced the project design and what was done to avoid and minimize project effects?....................................................................................................................................13 What is planned for wetland and stream mitigation?.....................................................................................................16 What benefits will the project provide?......................................................................................................................17 How will the project incorporate community design preferences?.............................................................................17 How will the project be constructed?.........................................................................................................................18 Why do we consider floodplains as we plan for this project?.........................................................................................19 Whatare the key points of this report?.......................................................................................................................... 20 ExistingConditions......................................................................................................................................................... 22 What information was collected to identify floodplains?................................................................................................. 22 What floodplains are present in the study area?............................................................................................................ 22 What floodplain regulations apply to this project?.......................................................................................................... 25 PotentialEffects...............................................................................................................................................................27 What methods were used to evaluate effects on floodplains?....................................................................................... 27 Which floodplains could be affected by the project?...................................................................................................... 27 How will project construction temporarily affect the Springbrook Creek floodplain? Will the project permanently affect floodplains?....................................................... What indirect effects result from the direct effects on floodplains?.......................... 28 28 29 Measures to Avoid or Minimize Project Effects............................................................................................................ 30 What has been done to avoid or minimize negative effects to floodplains?................................................................... 30 How will the project compensate for unavoidable negative effects to floodplains?........................................................ 30 References........................................................................................................................................................................ 31 PublishedDocuments.................................................................................................................................................... 31 Websites........................................................................................................................................................................ 31 Renton Nickel Improvement Project Floodplains Discipline Report TABLE OF CONTENTS EXHIBITS Exhibit1. Project Vicinity Map..........................................................................................................................................1 Exhibit 2. Project Overview Section 1..............................................................................................................................3 Exhibit 3. Project Overview Section 2..............................................................................................................................4 Exhibit 4. Project Overview Section 3..............................................................................................................................5 Exhibit5. Project Overview Section 4..............................................................................................................................6 Exhibit6, Project Overview Section 5..............................................................................................................................7 Exhibit 7. Project Overview Section 6..............................................................................................................................8 Exhibit8. Project Overview Section 7..............................................................................................................................9 Exhibit 9, Project Overview Section 8............................................................................................................................10 Exhibit 10. Floodplains within Renton Nickel Improvement Project Study Area.............................................................23 Renton Nickel Improvement Project ii Floodplains Discipline Report GLOSSARY 100-year flood event A flood having a 1-percent chance of occurring in any given year. Base flood A flood having a 1-percent chance of occurring in any given year; also called the 100-year flood. Basin planning programs Basin planning programs contain land use recommendations, regulations, capital projects, and programs to reduce and prevent flooding, erosion, and preserve salmonid habitat in a particular basin. Box culvert A concrete box structure that drains open channels, swales, or ditches under a roadway or embankment. Compensatory floodplain The removal of material from a site in the same floodplain and at the same elevation to storage compensate for the placement of any fill within the limits of the regulatory floodplain. Critical areas Critical areas include both hazard areas (such as floodplains and steep slopes) and environmentally sensitive areas (like wetlands and streams). Critical areas also include areas that are important for protecting groundwater. The state Growth Management Act requires counties to protect the "functions and values" of critical areas. Examples of wetland functions are filtering out pollutants, providing wildlife habitat, controlling floods, and recharging groundwater. Falsework The temporary frame that supports the weight of a bridge or other structures during construction. Fill Fill refers to materials placed to construct structures within waters of the United States, including streams and wetlands. Fill materials can be rock, sand, or dirt. Flood Insurance Rate Maps The insurance and floodplain management map produced by the Federal Emergency Management Agency. These maps identify the areas subject to flooding during a 1-percent- annual-chance (100-year) flood event in a community. Flood insurance risk zones, which are used to compute actuarial flood insurance rates, also are shown. Floodplain The total area subject to inundation by a flood, including the floodway. Floodway The channel of the river or stream, and those portions of the adjoining floodplains that are reasonably required to carry and discharge the base flood flow. The "reasonably required" portion of the adjoining floodplains is defined by flood hazard regulations. Flow rate The volume of water that moves by a particular point in one second. The flow rate is measured in cubic feet per second. Indirect effects Indirect effects are defined in the WSDOT Environmental Procedures Manual as the "effect caused by the proposed action that is later in time or farther removed in distance, but still reasonably foreseeable. Indirect effects may include growth -inducing effects and other effects related to induced changes in the pattern of land use, population density or growth rate, and related effects on air and water and other natural systems, including ecosystems." Interlocal agreements An agreement between local agencies such as cities and counties. Levee system A manmade structure, usually an earthen embankment, designed and constructed in accordance with sound engineering practices to contain, control, or divert the flow of water so as to provide protection from temporary flooding. Ordinary High Water Mark The line on the shore established by the change in water levels and indicated by physical characteristics such as a clear, natural line impressed on the bank or the presence of litter and debris. The presence and action of water generally leaves an impression on the stream bed and banks that makes a distinct separation between the stream and the adjacent areas and indicates the location of the ordinary high water mark. Renton Nickel Improvement Project Floodplains Discipline Report iii GLOSSARY Peak flows The maximum instantaneous rate of flow during any given storm. For this report, peak flow refers to the 100-year storm event. Piers A vertical support or substructure unit that supports the spans of an elevated structure such as a bridge. Pump station A pump station controls flows to one body of water from another. For this project, the Black River pump station controls flows to the Green River from Springbrook Creek. During flooding, the pump station reduces pumping rates from Springbrook Creek and may suspend pumping entirely if flows in the Green River exceed 12,000 cubic feet per second. Side channel This is a secondary stream that splits off the main channel. Stormwater detention Storing stormwater in manmade facilities such as ponds and releasing the stormwater at a controlled rate. This process helps control how much and how fast stormwater enters streams and rivers. Controlling the flow of stormwater helps maintain existing base flood levels and minimizes erosion of stream banks. Tributaries A stream or other body of water that contributes its water to another stream or body of water. Wetland and Habitat Mitigation A mitigation bank is a project constructed to enhance wetlands and their associated habitat. bank The concept is that these are completed in advance of planned transportation projects to mitigate for unavoidable effects on wetlands and habitat within the project area. Because these banks are generally sized to provide sufficient mitigation for several projects in one location, the mitigation bank creates more useable habitat and higher functioning wetlands than may be possible on an individual project scale. Zero -rise requirement The zero -rise requirement prohibits projects in Flood Hazard Areas that cause any rise in the base floodplain elevation. Renton Nickel Improvement Project iv Floodplains Discipline Report ACRONYMS AND ABBREVIATIONS USED IN THIS REPORT BMP Best Management Practice cfs cubic feet per second CSS Context Sensitive Solution EA Environmental Assessment EEI Early Environmental Investments EIS Environmental Impact Statement FEMA Federal Emergency Management Agency FIRM Flood Insurance Rate Map GP General-purpose lane HOV High -Occupancy Vehicle HPA Hydraulic Project Approval OHWM Ordinary High Water Mark WSDOT Washington State Department of Transportation Renton Nickel Improvement Project Floodplains Discipline Report v This page intentionally blank. Renton Nickel Improvement Project vi Floodplains Discipline Report INTRODUCTION What is the Renton Nickel Improvement Project? The Renton Nickel Improvement Project is a highway expansion project that will improve mobility and safety through Tukwila and Renton. On 1-405, this project begins just east of the 1-5/1-405 interchange in Tukwila and extends north past the Cedar River to the SR 169 (Maple Valley Highway) interchange. The project will build an additional lane both northbound and southbound between 1-5 and SR 169. On SR 167, the project will extend the southbound high -occupancy vehicle (HOV) lane north to 1-405 and add a southbound auxiliary lane from 1-405 to the SW 41st Street off -ramp. These limits comprise the study area for the project. Prior to planning this specific project, WSDOT created the 1-405 Corridor Program. This program provides a comprehensive strategy to reduce congestion and improve mobility throughout the 1-405 corridor. The corridor begins at the I-5 interchange in the city of Tukwila and extends northward 30 miles to the 1-5 interchange in the city of Lynnwood. The program's purpose is to provide an efficient, integrated, and multimodal system of \IV ' j7X �'� : 1 Norihcrn Projcct Limit at Sk 169 c t SW 716 St 0 ?' 1-405 Sauthem ` N unit ^�.1 sW Wait t• /� R E N T O N atI-S TUKWILa t : SW 23rd St �`� ■ to r\—T—� -: __:; _ sw 27tn s1167 I 9 SW 341n SI �• L i sW 4 at St -- Arterial Road Lake SR 167 sauthem y i j — Park Freewayroje Pct Limit' — — Trail [, �� Municipality at SW 41st 511 at � 1 Stream Exhibit 1. Project Vicinity Map Renton Nickel Improvement Project Floodplains Discipline Report transportation solutions. Using the 1-405 Corridor Program's Selected Alternative as the Master Plan to improve 1-405, WSDOT developed relatively low cost, congestion relief roadway improvements as an interim step in achieving the Master Plan. As part of this effort, WSDOT began to define the Renton Nickel Improvement Project. The Renton Nickel Improvement Project was developed as part of a first step in providing a focused strategy to improve 1-405 between 1-5 in Tukwila and SR 169 in Renton and SR 167 southbound from 1-405 to SW 41st Street, see Exhibit 1. This discipline report analyzes two project alternatives: the No Build Alternative and the Build Alternative. INTRODUCTION What Is an auxiliary lane? An auxiliary lane is a lane added between interchanges —from one on -ramp to the next off -ramp. It is dedicated to traffic entering and leaving the freeway and provides motorists with more time and extra room to accelerate or decelerate and merge when getting on and off the freeway. O Existing On -ramp On -ramp with proposed auxiliary lane What is the No Build Alternative? The No Build Alternative assumes that only routine activities such as road maintenance, repair, and safety improvements would take place over the next 20 years. This alternative does not include improvements to increase roadway capacity or reduce congestion. For these reasons, it does not satisfy the project's purpose —improve 1-405 between 1-5 in Tukwila and SR 169 in Renton and SR 167 southbound from 1-405 to SW 41st Street. The No Build Alternative has been evaluated in this discipline report to establish a baseline for comparing the effects associated with the Build Alternative. What is the Build Alternative? The new lanes that will be built under this project are: ■ An 1-405 northbound general-purpose (GP) lane from 1-5 to the SR 167 off -ramp. ■ An 1-405 northbound auxiliary lane from the SR 167 to 1-405 on -ramp to the SR 169 off -ramp. ■ An 1-405 southbound auxiliary lane from the SR 169 to 1-405 on -ramp to the SR 167 off -ramp. ■ An 1-405 southbound GP lane from the SR 167 to 1-405 on -ramp to the 1-5 off -ramp. ■ A SR 167 southbound auxiliary lane from 1-405 to the SW 41 st Street off -ramp. Also, the existing inside HOV lane will be extended north to 1-405 from its present starting point in the vicinity of SW 21 st Street. See Exhibits 2 through 9 show the project features. In addition to adding lanes to 1-405 and SR 167, this project will provide the following improvements. Improve Interchanges Minor modifications will be made to the ramps at the SR 167 interchange: ■ The one -lane ramp from northbound 1-405 to SR 167 will be widened to a 2-lane off connection, which provides a dedicated lane to southbound SR 167 and a dedicated lane to northbound Rainer Avenue. See Exhibit 5. ■ Traffic from two consecutive single -lane on - ramps from southbound 1-405 to SR 167 will be separated by a concrete barrier. This will provide a smoother transition to the mainline and reduce congestion on the on -ramps. Renton Nickel Improvement Project Floodplains Discipline Report INTRODUCTION I-405 NORTHBOUND Existing AW Proposed • a ..vtw�F4. ;� Existing Proposed LIn Cottage Creek Y f� hie • I-405 - Southern Gilliam Creek Project Limit , T,ukwila.Parkway at I-5 .i .,..�.. s t� o °� .. Westfield771 Shoppingtown Mall Open River/Creek Channel Piped River/Creek Channel ♦Mw+YM Ecology Embankment Retaining Wall Stormwater Flow Control Facility New Pavement Areas of Construction Easement Acquisition . Parcel Acquisition OExisting ROW ® New ROW I-405 Wrthboundbound Improvements: A general-purpose lone will be added by restriping the existing pavement and adding pavement up to 15 feet to the outside at some locations. I-405 Southbound Improvements: A general-purpose lane will be added by restriping the existing pavement and adding pavement up to 15 feet to the outside at some locations. Exhibit 2. Project Overview Section 1 0 250 500 Feet t Renton Nickel Improvement Project Floodplains Discipline Report 3 INTRODUCTION I-405 SOUTHBOUND 9 "� C \•� '•i Existing F Proposed w� Gib +less . ej 9�e ly Bridge Rail Replacement: - f� s � � 7 ' c � {� I co fi I-405 NORTHBOUND Existing Proposed Open River/Creek Channel Piped River/Creek Channel Bridge Rail Replacement Bridge Restripe Only � + v w. ,G tip. G r e 181 e r r • .+++.. Ecology Embankment I-405 Northbound Improvements: Retaining Wall A general-purpose lane will be added by restriping the existing OStormwater Flow Control pavement and adding pavement up to 15 feet to the outside at Facility some locations. New Pavement Areas of Construction I-405 Southbound Improvements: A general-purpose lane will be added by restriping the existing Easement Acquisition pavement and adding pavement up to 15 feet to the outside at - Parcel Acquisition some locations. OExisting ROW ® New ROW Exhibit 3. Project Overview Section 2 1 RRENTON1/ �C C Sec.lSeclSac. .�=o7 pp sec. I•L -Se1C TUKNIILA 6♦-�.... 0 250 500 Feet wMrx Renton Nickel Improvement Project 4 Floodplains Discipline Report I,v'FaODW-410.v 11,te h ""'� �I f/`!,S .p �t Bridge Replacement • �� .� CtadY --�� _ a •* r " Bridge Replacement k L sw-z _...._. _ istn St Potential Staging Areaall 6. h i 4 N' I-405 NORTHBOUND A ` Existing J £ Proposed I-405 SOUTHBOUND � " 1 Existing aw s Proposed ,je F r-� lift .Y S Open River/Creek Channel I-405 Northbound Improvements: Piped River/Creek er/Creek Channel A general-purpose lane will be added by restriping the existing ++��+ Ecology Embankment pavement and adding pavement up to 15 feet to the outside at Retaining Wall some locations. The existing Springbrook Creek and Oakesdale CStormwater Flow Control Avenue bridges will be replaced and the existing culvert will be Facility removed. New Pavement Areas of Construction I-405 Southbound Improvements: A general-purpose lane will be added by restriping the existing Easement Acquisition pavement and adding pavement up to 70 feet to the outside at . Parcel Acquisition some locations. The existing Springbrook Creek and Ookesdale Existing ROW Avenue bridges will be replaced and the existing culvert will be New ROW removed. Exhibit 4. Project Overview Section 3 v ; 1 Sec. s- RENTON .SKI t� 1 A=C c Sec; Sec.'4 1 2 'J iei S TUKMLA '. 0 250 500 Feet can Renton Nickel Improvement Project Floodplains Discipline Report 5 INTRODUCTION �Cli f III Ni54tMr� #1S.'vi?' 15i ------------ ---- s b 167 1 � •S "A j Renton Rot ing Hills Creek doom < ;Cinema Noise Wall Potential 3! � r SW_16th St �ilt- Area fD I-405 NORTHBOUND r i s c Existing Cn Proposed SR 167 SOUTHBOUND ; I-405 SOUTHBOUND ...•:rTlx�T1�a: .f: .•r � I Existing , Existing - Proposed W e t l 8 Il d S Proposed _- Open River/Creek Channel Piped River/Creek Channel I-405 Northbound Improvements: A general-purpose lane will be added up to the SR 167 interchange and an ouxdiary lane Proposed Noise Wall will be added from the SR 167 to I-405 on -ramp north. These lanes will be added by Ecology Embankment restriping the existing povement and adding pavement up to 15 feet to the outside at a •- some lomt,ons. �. RENTON Retaining Wall 7 Sto,ter Flow Control O ailitymwa F-il I-405 Southbourd Improvements: hoc An auxiliary will be added up to the I-405 to SR 167 on -ramp and a general-purpose Sac. 1 2 '2ye _., New Pavement lane will be added south of the interchange. These lanes will be added by restr,ping the existing pavement and adding pavement up to 15 feet to the outside at some locations. iei t1 5 TUKWIUI Areas of Construction Easement Acquisition -of Parcel Acquisition OExisting ROW ® Ncw ROW 5R 167 Southbound Improvements: An auxiliary lane will be added by restriping existing povement and adding up to 19 feet povement at the outside at some locations. The existing HOV lane will be extended north from SW 21st Street to the interchange with I-405. 0 250 500 Feet Exhibit 5. Project Overview Section 4 Renton Nickel Improvement Project 6 Floodplains Discipline Report INTRODUCTION ""� IL SR 167 SOUTHBOUND _ Al Existing M Proposed his Potential Staging Area Open River/Creek Channel Piped River/Creek Channel SR 167 Improvements: raxrwer Ecology Embankment In addition to extending the HOV lane north from SW 21st ae� Retaining Wan Street, an auxiliary lane will be added by restriping the existing RENTON �f t00 Stormwater Flow Control pavement and adding pavement up to 19 feet to the outside at Eye. 1} ' esc. C Facility some locations. 1 2 a srtc New Pavement iet I 5 sac. Areas of Construction Easement Acquisiton Parcel Acquisition Existing ROW New ROW V1J 1UKVNU "8 o 250 500 � Feet Exhibit 6. Project Overview Section 5 Renton Nickel Improvement Project Floodplains Discipline Report 7 INTRODUCTION SR 167 SOUTHBOUND Existingrc7i 167 w ; Rt Proposed Jn, , 1SW 33rd St It con rr a, . fx. `. a. _.yo•.. 4 ��; ai},..,, , l " '" s 71mL; n = 4- I. � rt Open Riper/Creek Channel �'4I iW-41 st 1 SR 167 �r Southern Project Limit at SW 41st St i b k k Piped RiMer/Creek Channel SR 167 Improvements: wwww Ecology Embankment An auxiliary lane will be added by restriping the existing Retaining Wall pavement and adding pavement up to 19 feet to the outside at some locations. The new lane will tie into the existing ramp Stormwater Flow Control Facility connection to SW 41st Street. New Pavement Areas of Construction Easement Acquisition Parcel Acquisition Existing ROW New ROW Exhibit 7. Project Overview Section 6 a Ml �\\ RENTON1 seta Its c T 3 -r Sec. Is TUKIMIA _.. 0 250 500 Feet I Renton Nickel Improvement Project 8 Floodplains Discipline Report INTRODUCTION I-405 NORTHBOUND Existing •.k� }R�" .�! iiiu.liVjL •'..,fir - Proposed ..4. M- ��;�i. I-405 SOUTH BOUND Jill Existing Proposed s G. i Fn Thunder Hills Creek �� Potential Staging Area T r i Replace Bridge ` L S Bridge Widening Rolling Hills CreekIle• Existing Bridge to be Demolished r <<— n t Benson Rd 5 `= Noise Wall 1� 4� .x.�c `� eti" r ° Open River/Creek Channel Piped River/Creek Channel I-405 Northbound Improvements: Proposed Noise Wall An auxiliary lane will be added by restriping the existing pavement r '' ...... Ecology Embankment and adding pavement up to 15 feet to the outside at some locations. 8: '` ONI Retaining Wall RENTi691 f. Stormwater Flow Control Sec I-405 Southbound Improvements: Facility An auxiliary lane will be added by restriping the existing pavement y ; ,.- �_�se New Pavement and adding pavement up to 24 feet to the outside at some locations+ s ier Areas of Construction T yyt- Benson Rd 5 Improvements: Easement Acquisition The Benson Rd 5 overpass will be replaced and realigned to the Parcel Acquisition west of its current location. The new overpass will have 2 lanes with 0 250 500 Existing ROW 5-foot bike lanes on both sides and a 6-foot sidewalk on the west side. Feet 01 New ROW Exhibit 8. Project Overview Section 7 Renton Nickel Improvement Project Floodplains Discipline Report 9 INTRODUCTION 1-7 I-405 NORTHBOUNDWier 30M 51ML Existing , •J 1!7� ,'1"fp•,• ~ R �a� Proposed I-405~ ' 4, r; Northern I-405 SOUTHBOUND._`_ - Project Limit • - at SR 169 Existing Proposed -. 'c Pack dA 90 /P • / \ %� Cedar Py t 'vt r Park y i . � � t � + � � f P �� ,,y r � A• , 169 ,,, 111 .r • �Restripe Only y C'e :. ' ti st 1 Open River/Creek Channel Piped River/Creek Channel w11*01 Ecology Embankment I-405 Northbound Improvements: Retaining Wall An auxiliary lane will be added by restriping the existing CStormwater Flow Control pavement and adding pavement up to 15 feet to the outside Facility at some locations. New Pavement Areas of Construction I-405 Southbound Improvements: An auxiliary lane will be added by restriping the existing Easement Acquisition pavement and adding pavement up to 15 feet to the outside - Parcel Acquisition at some locations. OExisting ROW ® New ROW Exhibit 9. Project Overview Section 8 8 sqc: 169 r e.Sec. sec. Sec_T r t►►2 3 _ 1 Sec. M1 J � TUKYNLA Renton Nickel hnprovement Project 10 Floodplains Discipline Report INTRODUCTION Why rebuild Benson Road on a new alignment over 1-405? By building the new overpass to the west on a new alignment, the new structure can be constructed while the existing structure remains open to traffic. Traffic can then be shifted onto the new structure, while the old overpass is demolished. What does a "rail" replacement involve? Typically, a bridge rail replacement project consists of making minor adjustments to the width of the bridge deck and replacing the guard rail or barrier. This type of project does not include adding new bridge columns or footings. Renton Nickel Improvement Project Floodplains Discipline Report Improve Benson Road The Benson Road overpass will be replaced and realigned to accommodate the southbound auxiliary lane on 1-405 as well as future improvements to 1-405 as shown on Exhibit 8. Improvements on Benson Road include a 6-foot sidewalk on the west side and 5-foot bike lanes on both sides. Widen and replace bridges Several bridges within the study area will be widened or replaced based on present location, cost, and existing soil conditions. To construct the new lanes, the project will: ■ Widen Talbot Road Bridge on both the northbound and the southbound sides. See Exhibit 8. ■ Replace Springbrook Creek Side Channel Bridge and Oakesdale Avenue Bridge with new southbound and northbound structures and remove the Springbrook Creek box culvert. See Exhibit 4. ■ Replace the rail on the 1-405 bridges over SR 181 and the Union Pacific and Burlington Northern Santa Fe railroads. The project will not affect the 1-405 bridges over the Green River, Lind Avenue, or the Cedar River. The project will also not affect the Cedar Avenue or Renton Avenue overpasses. The roadway will be restriped in these areas to accommodate the new lanes. Use retaining walls Widening 1-405 and SR 167 will require retaining walls to minimize the construction footprint and right-of-way acquisition. Retaining walls will also help avoid and minimize effects to wetlands and other sensitive areas. Improve culverts WSDOT anticipates that construction will affect some existing stormwater cross culverts and one stream culvert. Associated culvert improvements include extending the existing structures due to widening the roadway and stabilizing culvert ends with rock or retaining walls. The 1-405 Team will conduct a hydraulic analysis of the culverts to ensure that the modifications will have no effect on the base flood elevations. See the Fisheries and Aquatic Resources Discipline Report for detailed discussion on fish passage. 11 INTRODUCTION What are the guidelines for stormwater management facilities? Water quality treatment will be provided for an area equal to the new impervious surfaces created on the project. Impervious surfaces, such as pavement, are those that do not allow water to penetrate into the ground. Stormwater from new impervious surfaces or an equal area will be controlled in detention facilities. This process allows water to be held (detained) and thus released at rates that are equal to existing conditions. Build a noise wall One noise wall will be built on the northbound side of the freeway as shown on Exhibits 5 and 8. The wall will begin at the intersection of South 14th Street and South 15th Street and follow South 14th Street east to Talbot Road. This wall will be approximately 2,150 feet long and 18 feet tall. How will stormwater from the project be managed? Stormwater from the project will be managed for both quality and peak flows using currently accepted best management practices (BMPs). The 1-405 Team has designed the stormwater management facilities to comply with the following guidelines and procedures: ■ WSDOT Highway Runoff Manual M 31-16 ■ WSDOT Hydraulics Manual M 23-03 Stormwater treatment facilities The project will add new impervious surface within the study area, most of which will be within the Springbrook Creek basin. This project will treat runoff for an area equal to 100 percent of these new surfaces. The project will use BMPs that the HRM lists as enhanced treatment facilities. The 1-405 Team has proposed that stormwater be treated using a combination of these facilities. In most of the study area, ecology embankments will be used to capture runoff from the edge of the pavement and provide water Nrap Filter Fabric _Seed, Fertilize, Antch to Erosion Blanket— Mulch and Second Mulch Erosion Control ROADWAY SHOULDER Blanket 6�cvei Borrow y I 4 7 Ecology Mix 12 MI Filter Fabric % y Depth Depends on Storage Requirements Grovel Backfill quality treatment. Ecology embankments also serve to convey treated runoff to receiving waters or to flow control facilities as required. The project also includes a combined stormwater quality wetland and detention facility that addresses water quality and flow control in one facility. Exhibits 2 through 9 show the location of stormwater facilities Ecology Embankment Cross -Section that will be built for this project. Renton Nickel Improvement Project 12 Floodplains Discipline Report INTRODUCTION Drainage Collection and Conveyance Some changes to existing drainage will be necessary Mat are detention facilities? to provide flow control and water quality treatment to These facilities control stormwater runoff address the new impervious area added by the so that it can be released at a controlled project. However, existing storm drainage systems rate. Two types are commonly used: will be kept to the greatest extent possible and existing ■ ponds. flow patterns will be maintained. Where roadway widening affects drainage ditches that convey water ■ Vaults. Similar to a pond, but with a hard -sided construction. These from adjacent private properties, the project design will concrete structures function like a assure that existing conveyance capacities are pond but also provide detention maintained. storage. What environmental and utilities issues influenced the project design and what was done to avoid and minimize project effects? Throughout the development of the Renton Nickel Improvement Project design, environmental elements were reviewed and design features were modified to avoid or minimize negative effects to the environment. Influence on the project design came from: ■ Soil Conditions: the soils in the project area are highly prone to accentuate earthquake shaking, which influences how bridges can be widened or replaced. ■ Noise: highway noise in the project area already exceeds acceptable levels, which means that including noise walls as part of the project had to be considered. ■ Wetland Locations: many wetlands are located along the edges of the highway, which influence whether the widened sections will use retaining walls or fill slopes. ■ Historical Sites: some historic sites exist within the study area, so the project design was coordinated to avoid these properties. Because the 1-405 Team planned for these environmental considerations, several design features have the benefit of avoiding or minimizing potential effects due to the project. These design features are described from south to north below. I-405, I-5 to SR 167 WSDOT will construct a retaining wall from west of the 68th Avenue structure over 1-405 at Tukwila Parkway Renton Nickel Improvement Project Floodplains Discipline Report 13 INTRODUCTION The proposed design modifications allow the additional lanes to be added over the Green River by restriping instead of bridge widening. This avoids effects on the river, stream habitat, floodplain, and Interurban Trail. What is an exit gore? An exit gore is a roadway feature that separates an exiting lane from the main lanes. An exit gore can be defined either by paint stripes, raised buttons, physical barriers, or a combination of these. to the Green River. This wall avoids the need to construct a fill slope that would extend into Gilliam Creek. See Exhibit 2. WSDOT will provide a narrower outside shoulder on northbound 1-405 at the Green River Bridge. The shoulder will vary from 10 to just over 3 feet at the west abutment of the existing bridge. Narrowing the shoulder avoids modifications to the existing bridge. As a result, the design also avoids effects to the river, the 100-year floodplain, the ordinary high water level, and adjacent riparian zones. At the SR 181 interchange, the bridge and ramp will be restriped to provide the new general-purpose lane and ramp improvements. This approach minimizes the need to widen the existing SR 181 Bridge, reconstruct the SR 181 interchange, or modify the Southcenter Boulevard crossing of the Green River. This in turn avoids relocating or diverting the Interurban Trail, which goes under the bridge. See Exhibit 3. Near the Westfield Shoppingtown Mall, a large Seattle Public Utilities water transmission line parallels 1-405. WSDOT will line this pipe so that is can support the loads from the new roadway embankment. This approach allows the line to stay in its present location. WSDOT will remove the existing 1-405 bridges over the Springbrook Creek side channel and Oakesdale Avenue and replace them with a single northbound and a single southbound bridge. This approach will allow for the removal of the Springbrook Creek box culvert. Construction of the new bridges will be phased with the southbound bridge built slightly to the north of the existing roadway. This phasing minimizes the need to construct temporary roadway to maintain traffic operations. WSDOT also evaluated the location of the new bridge piers and selected locations that will minimize the effect on the existing stream, stream buffer, and trail that crosses under the bridge. WSDOT will construct a narrower exit gore from 1-405 to the northbound ramp at the SR 167 interchange as shown in Exhibit 5. By building a narrower exit gore, the project can be constructed within the existing right- of-way. This has the benefit of avoiding right-of-way acquisition, avoiding effects to the wetland outside the right-of-way, and avoiding effects to the existing Lind Avenue Bridge. Renton Nickel Improvement Project 14 Floodplains Discipline Report INTRODUCTION Retaining walls will help to avoid and minimize effects on the Panther Creek wetlands along SR 167 What is a recovery lane? A recovery lane is a paved area adjacent to an off -ramp. This area gives drivers, who find themselves exiting the freeway unintentionally, room to maneuver back onto the freeway. Renton Nickel IniprovementProject Floodplains Discipline Report SR 167, southbound from I-405 to SW 41st Street WSDOT will build a retaining wall along a large portion of the west edge of SR 167 southbound instead of an earth fill slope. See Exhibits 6 and 7. The retaining wall minimizes effects on three wetlands. The retaining wall has the added benefit of minimizing right-of-way needs and reduces the effect on existing utility crossings, in particular, the City of Seattle's 60- inch water line and Olympic Petroleum's two high pressure pipelines, which all cross under SR 167. I-405, SR 167 to SR 169 WSDOT will add a lane by restriping 1-405 northbound next to the Talbot Hill retaining wall immediately east of the SR 167 interchange. Restriping instead of widening avoids the need to reconstruct the existing Talbot Hill retaining wall and avoids effects on properties south of 1-405 in this area. Between Talbot Road and the "S-Curves", northbound 1-405 will be widened to achieve standard lane and shoulder widths. Most of this length will be supported by retaining walls to minimize effects to Thunder Hills Creek, adjacent properties, and the existing cut slope south of 1-405. To support the fill required to widen the roadway on the north side of 1-405 next to the outfall for the original Rolling Hills Creek culvert, the design uses a retaining wall. By using the retaining wall, the project improvements at this location can be constructed without affecting the existing culvert. WSDOT will use a non-standard design for the 1-405 to SR 167 exit ramp. The changes from the design standards include not providing a recovery lane, narrowing the distance between the through lane and ramp, and providing narrower shoulders. While these changes deviate from WSDOT design standards they are an improvement over existing conditions. These features will avoid effects to the existing Rolling Hills Creek/Thunder Hills Creek channel located between 1-405 and the Renton Cinema complex as shown in Exhibit 5. Using retaining walls along the west side of Benson Road avoids effects to Rolling Hills Creek and the wetlands east of Talbot Road. WSDOT will use retaining walls to support widening southbound 1-405 south of the Cedar Avenue overpass. Using retaining walls versus a fill slope, avoids encroaching on Cedar Avenue and Main Avenue in Renton. 15 INTRODUCTION Renton Coal Mine Hoist Foundation site looking west WSDOT also plans to replace the existing Benson Road overpass on a new alignment. The new bridge will be located slightly to the west of the existing bridge. This will allow traffic to continue to use the existing overpass until the new one is completed. This will minimize disruption for local traffic and to emergency response vehicles. Where northbound and southbound 1-405 passes under the Renton Avenue and Cedar Avenue overpasses, WSDOT will add lanes by restriping. This design avoids replacing the two overpasses; however, the available area does not allow the standard shoulder and lane widths. WSDOT shifted a proposed stormwater facility to avoid effects to the existing Renton Coal Mine Hoist Foundation site south of Benson Road. This site is on the Washington Historic Register. What is planned for wetland and stream mitigation? �h s� GIs" We SSW 16n1 3 � SW 231d St 1 ,SW 271h St �S 341h St I 1 w a,st st I81 I _ --' ---------- Legend veven a T-1 100 Year Rwdpi- -1 Road - 5 year FlOod0— P -1 Road Spnr g—k Creek W .. ® — Freeway a Habitat Mipga— Bank R--Creek Channel Park w 0 e35 OS SNdy Area L-a, Rehm Tukvnla Springbrook Creek Wetland and Habitat Mitigation Bank WSDOT will compensate for unavoidable effects to wetlands with credits from the Springbrook Creek Wetland and Habitat Mitigation Bank. Mitigation is needed for 1.66 acres of wetlands. The Springbrook Creek Wetland and Habitat Mitigation Bank is being developed as a joint effort between WSDOT and the City of Renton. This 'bank' will construct a new high quality wetland complex that will serve to replace other wetlands that are filled in by projects such as the Renton Nickel Improvement Project. The location of the bank is shown to the left. In addition to wetland mitigation, the site will also provide flood storage mitigation. The Springbrook Creek Wetland and Habitat Mitigation Bank will be one of the first urban mitigation banks to be certified in Washington. To mitigate project effects on streams, WSDOT will remove the existing Springbrook Creek box culvert. With the new 1-405 southbound and northbound bridges that will span both Springbrook Creek and Oakesdale Avenue, the box culvert is no longer needed. After the new bridges are in place, the box culvert will be removed and the streambed in that area will be restored. This will improve fish habitat within Springbrook Creek. Any additional stream mitigation required to offset project effects will be accommodated within the project vicinity. Renton Nickel Improvement Project 16 Floodplains Discipline Report INTRODUCTION FrMw'" "'"-4EFF This rendering shows the new Benson Road overpass with the CSS Guidelines applied What benefits will the project provide? The Build Alternative will benefit the area by reducing congestion at chokepoints, reducing the duration of congestion during peak commuter travel hours, and improving freight movement. This section of 1-405, from the 1-5 interchange to SR 169, is congested due to large traffic volumes and merging and diverging traffic. The new lanes will help relieve congestion by adding roadway capacity. This in turn will improve safety by providing drivers with more time and extra room to accelerate or decelerate and move into and out of the stream of traffic when getting on and off the freeway. This provides a smoother transition for motorists as they get on and off 1-405 in Tukwila and Renton and helps decrease year- end and sideswipe collisions. The project reduces congestion approaching the SR 167 interchange, and it complements the completed southbound 1-405 to southbound SR 167 flyover ramp. This project will construct one noise wall along northbound 1-405 from the intersection of South 14th Street and South 15th Street east to Talbot Road. This wall will benefit residents in that area by lowering the overall noise levels. Another benefit of this project is that it continues the application of the Context Sensitive Solutions (CSS) design choices made by the communities within the 1-405 corridor. The Benson Road realignment will reflect the most comprehensive application of these design choices as explained further in the next section. How will the project incorporate community design preferences? The Renton Nickel Improvement Project is being planned, developed, and designed according to CSS guidelines. These guidelines establish the community design preferences used to design the project features. Working within the framework for the overall 1-405 corridor, the Urban Design Guidelines will be adapted to incorporate the communities' design preferences. These preferences will be included in the contract documents prepared for the Renton Nickel Improvement Project. The selected 1-405 theme of "Culture, Nature, and Progress," with nature Renton Nickel Improvement Project Floodplains Discipline Report 17 INTRODUCTION At -grade construction for this project will likely be staged similar what is shown above. Here, the southbound lanes of 1-5 were shifted toward the median and a concrete barrier closed off the shoulder to provide crews a safe work area. being the dominant theme, will be carried into corridor - wide and local 1-405 designs. The new Benson Road overpass is the main project feature that will receive CSS treatment. The new southbound and northbound bridges over Springbrook Creek and Oakesdale Avenue will also receive CSS treatments. The rest of the project elements will be designed to match in color and vegetation type only, as many of these elements will be affected by construction of future Master Plan projects. During future Master Plan phases for the overall 1-405 corridor, the approved CSS guidelines will be applied throughout. How will the project be constructed? Construction of the entire Renton Nickel Improvement Project is expected to take two years, beginning in early 2008 and being completed in late 2010. However, construction activity will not be constant for the entire study area throughout this time, and in some locations, the work will take substantially less time than two years. Construction will pose some minor inconveniences because of localized travel delays due to temporary lane closures and narrowed lanes and shoulders. At -grade construction At -grade construction, which occurs on the same elevation as the existing lanes, will be staged to minimize traffic delays and detours. Typically, lanes are shifted toward the median. WSDOT then places a concrete barrier to close off the shoulder. Staging allows construction to occur safely without closing lanes for the duration of construction. Access to construction areas will occur from the roadway side to minimize property effects. Bridge construction Construction of the 1-405 bridges will occur in multiple stages to minimize traffic delays and detours. The following describes typical staging for bridge construction. As the first stage, traffic is shifted toward the 1-405 median and the existing lanes and shoulders are narrowed slightly to allow widening of the existing structure or construction of the new bridge depending on the design. In the next stage, traffic is shifted onto the new bridge area. If the bridge is being replaced rather than simply widened, the old structure is demolished after traffic is shifted to the new bridge. Renton Nickel Improvement Project 18 Floodplains Discipline Report INTRODUCTION The new Benson Road overpass will also be staged. The new structure will be built to the west, while the existing overpass remains in service. After traffic has been shifted onto the new overpass, the existing structure will be demolished. Staging areas Construction staging areas along 1-405 and SR 167 will be within the WSDOT right-of-way. Potential staging areas have been identified as shown on Exhibits 2 through 9. Traffic control Detour agreements with the local agencies will be obtained after WSDOT awards the contract. A traffic control plan will be approved by WSDOT prior to starting construction. The plan's primary objectives will be to provide a safe facility, to streamline the construction schedule, and to minimize reductions to existing traffic capacity. To lessen effects on traffic, the duration of activities will be minimized and reductions in capacity will be limited and will be targeted to a period when they will have the least effect. Why do we consider floodplains as we plan for this project? Floodplains are important because they convey and store flood water and minimize flood risks during large storm events. These functions reduce flood losses, maintain clean and plentiful water supplies, and generally enhance quality of life in communities. Recognizing the importance and the sensitive nature of these areas is an important first step in planning for this project. By adapting to the natural phenomenon of flooding rather than trying to control floodwaters, we can reduce the loss of life and property, protect critical natural and cultural resources, reduce maintenance and repair costs, and contribute to the sustainable development of our communities. This philosophy is reflected in the Executive Order 11988 that requires agencies to avoid adverse effects and incompatible development in floodplains thereby reserving floodplains as natural floodwater storage areas. Renton Nickel Improvement Project Floodplains Discipline Report 19 INTRODUCTION What are Flood Insurance Rate Maps (FIRMS)? The Federal Emergency Management Agency produces maps for insurance and floodplain management. These maps identify the areas subject to flooding during a 1-percent-annual-chance (100- year) flood event in a community. These areas are based on detailed or approximate analyses. Flood insurance risk zones, which are used to compute actuarial flood insurance rates, also are shown on the maps. In areas studied by detailed analyses, the FIRM shows Base Flood Elevations to reflect the elevations of the 100-year flood event. Floodplain management is influenced by federal, state, and local regulations or guidance. However, counties and cities bear the primary responsibility for regulating the activities allowed in floodplains. For example, the cities of Tukwila and Renton have developed specific programs to manage floodplains. Both cities have floodplain restrictions in their zoning and building codes as well as in their sensitive area ordinances. These codes prohibit projects that cause any rise in the base flood elevation for the 100-year flood event within the 100-year floodplains as identified on the Flood Insurance Rate Maps (FIRMs). As the study area has several 100-year floodplains, these regulations will apply to the project. What are the key points of this report? What Is Springbrook Early Environmental Investments (EEI)? The Springbrook Creek Wetland and Habitat Mitigation Bank is being developed as a joint effort between WSDOT and the City of Renton. This 'bank' will construct a new high quality wetland complex that will serve to replace other wetlands that get filled in by projects such as the Renton Nickel Improvement Project. In addition to wetland mitigation, the Springbrook EEI site will also provide flood storage mitigation. The Springbrook Creek Wetland and Habitat Mitigation Bank will be the first urban mitigation bank to be certified in Washington State. What are Early Environmental Investments (EEI)? EEI is part of WSDOT's commitment to the corridor -wide environmental program. EEI is mitigation in advance of constructing the transportation project. It is a corridor -wide process to identify, evaluate, rank, select, design, and permit environmental investment opportunities prior to construction. The EEI focus is to identify watershed opportunities for improving aquatic resources, fish, wetlands, water quantity, and floodplains. EEI is a cooperative effort, typically requiring information sharing and partnering among agencies and jurisdictions with an interest in the 1-405 project improvements or affected area. 20 This report presents two key points: This project will not raise the existing 100-year floodplain level because any filling within the floodplain will be mitigated for by excavation within the same floodplain, and at the same elevation, at the Springbrook Early Environmental Investments (EEI) site. ■ Some temporary piles and falsework may need to be placed in the floodplain during construction. However, any change in elevation due to these structures will also be mitigated for by the excavation at the Springbrook EEI site. The Federal Emergency Management Agency (FEMA) identified several 100-year floodplains in the study area along the Green River, Springbrook Creek, Panther Creek, Rolling Hills, and the Cedar River. The project's main construction activity that will affect floodplains is the replacement of the Springbrook Creek and Oakesdale Avenue bridges. The exact construction methods and amounts of fill needed for this project element will be determined prior to construction. Regardless of the selected construction method, excavation at the Springbrook Creek Wetland and Habitat Mitigation Bank will provide sufficient flood storage to compensate for the fill needed for bridge construction. WSDOT will perform a hydraulic analysis prior to beginning construction to confirm that the Springbrook site provides adequate mitigation. Renton Nickel Improvement Project Floodplains Discipline Report INTRODUCTION How are floodplains affected? Direct changes to floodplains occur when a project physically encroaches on the floodplain or makes flooding worse by generating increased runoff during storm events. Encroachment can be due to placing a bridge pier or column, abutment, or other fill material within the floodplain. When fill or objects encroach on the floodplain, the area available for storing flood waters decreases and flood elevations become higher and have adverse social and ecological effects. In addition to the effects on the Springbrook floodplain, some minor fill will also occur along the south side of I-405 between Oakesdale Avenue and Lind Avenue and on the west side of SR 167 at SW 23rd Street, where the Panther Creek tributary to Springbrook Creek crosses the highway. At this location, the project design will use conventional engineering techniques to achieve the goal of a zero rise in the 100-year flood elevation. Where encroachment into the floodplain or floodway is unavoidable due to technical or physical constraints, localized loss of flood storage will be alleviated by designing additional flood storage within the affected floodplain. Renton Nickel Improvement Project Floodplains Discipline Report 21 EXISTING CONDITIONS What information was collected to identify floodplains? To identify floodplains, the team used the Flood Insurance Rate Maps (FIRM) prepared by the Federal Emergency Management Agency (FEMA). We used these maps to evaluate the extent and elevation of the floodplain. The team also reviewed other related reports and documents. These included a report on the performance of the Black River Pump Station (which regulates the flow out of lower Springbrook Creek during flood events) prepared for the City of Renton, and an on -going study to remap the Springbrook Creek floodplain. The floodplain remapping study uses updated information. One key change is the Black River pump station is assumed to operate at higher flow rates than was used for the existing FIRM panel. Since the pumping rate controls the floodplain elevation on Springbrook Creek, this remapping study is expected to show a somewhat lower base flood elevation. Therefore the analysis we present in this report is conservative as it is based on the existing mapping. What floodplains are present in the study area? What Is a F000dplain? A floodplain is the level area bordering river channels that become inundated during flood level flows. These portions of river valleys are frequently defined in terms of the likelihood of flooding in a given year. Hence, the "100-year" flood event is the flood having a 1% chance of occurring during any given year. This flood is considered the base flood. FIRM maps identify the 100-year floodplain and are used to determine the risk of flooding for a given area. The floodplains identified in the FIRM maps are associated with the Green River, Springbrook Creek, Panther Creek, and Rolling Hills Creek. At the north end of the study area, 1-405 also crosses the Cedar River floodplain. Exhibit 10 shows that the primary floodplain within the study area is associated with the Green River and Springbrook Creek and its tributaries. Renton Nickel Improvement Project 22 Floodplains Discipline Report EXISTING CONDITIONS r \` 1 I I-405 I I, + Northern Prot1ect Limit \ n at SR 169 \ 1 4. ♦ 4-1 1 , ,h.a, _ P. rx ' n4 .,. I v ` h St ; IN R E N ON ^` W 16th St 1 1 m _1 I-405 Southern T U K W I L A y i xl oject Limit 1 s1 at I-5 y n, h . r x •fl.n4• QD W 23rd St W 27th SI 34th St 41.t St SR 167 Southern Project Limit at SW 41st 51 / Legend Trail n Local Road Arterial Road Freeway Open River/Creek Channel Piped River/Creek Channel Study Area Limits 100 Year Floodplain - 500 Year Floodplain ®Springbrook Creek Wetland & Habitat Mitigation Bank i Park Renton Tukwila 0 025 05 � I We, Exhibit 10. Floodplains within Renton Nickel Improvement Project Study Area Source: FEMA maps published March 15, 1996 Multiple efforts have been made over the years to control flooding in the Green River Valley, especially as development has occurred. In late 1961, the U.S. Army Corps of Engineers completed the Howard Hanson Dam, near the headwaters of the Green River in King County, to control flooding. This dam was designed to limit peak flows to about 12,000 cubic feet per second (cfs), which is about a 25-year storm event. A levee system was also constructed on the river. Although the levee reduces the frequency of floods, it eliminates the natural floodwater storage that floodplains typically provide. Renton Nickel Improvement Project Floodplains Discipline Report 23 EXISTING CONDITIONS The Black River pump station controls flows to the Green River from Springbrook Creek. Springbrook Creek side channel has piers in the floodway. What Is the Ordinary High Water Mark? The line on the shore established by the change in water levels and indicated by physical characteristics such as a clear, natural line impressed on the bank or the presence of litter and debris. The presence and action of water generally leaves an impression on the stream bed and banks that makes a distinct separation between the stream and the adjacent areas and indicates the location of the ordinary high water mark. 24 Springbrook Creek drains into the Green River; however, its flow is controlled by the Black River pump station located north of the project. Operation of the pump station is determined by interlocal agreements and basin planning programs. The pump station can pump as much as 1,700 cfs during high flow events. This pumping rate exceeds the estimated peak flows on Springbrook Creek during a 100-year flood event, which is 1,307 cfs for current land use conditions. However, during flooding, flows on the Green River rise, reducing its capacity to receive additional flows. This decrease in capacity during floods requires that the pumping rates from Springbrook Creek be reduced. The pump station starts reducing pumping rates when the Green River reaches flows of 9,500 cfs. If flows in the Green River exceed 12,000 cfs, then the pump station may stop operating completely until flows in the Green River go down. The main channel of Springbrook Creek crosses under 1-405 through a 61-foot-wide box culvert. This culvert is divided into 5 cells, each approximately 11 feet wide. Adjacent to the box culvert, Springbrook Creek has a side channel (Springbrook Creek Side Channel) that flows under 1-405. 1-405 crosses this channel with a bridge. This bridge has four parallel rows of 13-inch piers within the floodway. Flooding along Springbrook Creek affects its tributaries. This is because the tributaries are not able to flow freely into Springbrook Creek during a flood and water backs up in the tributaries. In particular, flooding affects Panther Creek, which ponds on both sides of SR 167. Businesses west of East Valley Road experience parking lot flooding in this area. Another tributary to Springbrook Creek is Rolling Hills Creek. This creek has a FEMA-mapped floodplain on the north side of 1-405 east of SR 167. In this area, flooding is caused by undersized pipes not because the area is in a natural floodplain. When flooding occurs, it is limited to a large parking lot. The Cedar River flows beneath two 1-405 bridges at milepost 3.51. The east bridge has two columns within the 100-year floodplain and two columns within the ordinary high water mark (OHWM). The west bridge has three columns within the 100-year floodplain and one column within the OHWM. A pedestrian bridge spans the Cedar River beneath the east highway bridge, connecting the Renton Community Center on the right bank with the Cedar Renton Nickel Improvement Project Floodplains Discipline Report EXISTING CONDITIONS River Trail on the left bank. Two columns are located within the 100-year floodplain, with one column within the riverbed and floodway. What f loodplain regulations apply to this project? What Is a zero -rise requirement? Zero -rise requirements prohibit any activity in the floodplain that would raise the level of the floodplain. Projects that add material to a floodplain, either structures or fill, must remove an equivalent volume from the same floodplain at the same elevation so that the flood level does not increase. What Is backwater? Backwater refers to the body of water created during a flood. In a flood, water "backs up" because it is unable to flow downstream. This condition causes deeper water at this point in the channel and can contribute to flooding. Renton Nickel Improvement Project Floodplains Discipline Report Floodplain management within the Renton Nickel Improvement Project area is influenced by federal, state, and local regulations or guidance. However, counties and cities bear the primary responsibility for regulating activities allowed in floodplains. The City of Tukwila addresses floodplain regulations in Municipal Code 16.52 Floodplain Management. The City of Renton addresses floodplain regulations in Municipal Code [4-3-050] Critical Areas Regulations. These municipal codes help the Cities of Tukwila and Renton manage their floodplains. These codes prohibit projects within the 100-year floodplain that cause any rise in the base floodplain elevation. This is known as the zero -rise requirement. As several 100- year floodplains occur within the study area, these regulations apply to the project. These codes also dictate that projects must not reduce the amount of flood storage volume on a floodplain. For any fill placed in the floodplain, the project must compensate by removing equal volumes of fill at equivalent elevations. The cities of Tukwila and Renton will review the project to ensure that these provisions are met. The Floodplain Management guidance, Presidential Executive Order 11988 signed May 24, 1977, directs federal agencies to avoid development that will affect floodplains. If development will have significant encroachment on the floodplain, then the guidance requires a specific finding that the proposed development is the only practicable alternative. If the project encroaches on a regulatory floodway, then the document should also discuss the consistency of the action with the regulatory floodway. If a floodway revision is necessary, then the document should include evidence from FEMA and local or state agencies indicating that such revision is acceptable. WSDOT's Hydraulic Manual provides guidance on how to conduct engineering, hydrologic, and hydraulic studies to determine 100-year flood elevations, velocities, and backwater conditions that are expected to occur under existing conditions and with the conceptual designs proposed for the Build Alternative. 25 EXISTING CONDITIONS Although these studies have not yet been completed, the analyses will help determine the effects of encroachment on floodplains, including what compensatory storage will need to be created within the floodplain to meet the cities of Tukwila and Renton regulatory requirements and ensure no adverse effects to flood levels or floodplains. The project will also require a Hydraulic Project Approval (HPA) permit, as well as Shoreline Permits. The HPA is issued by the Washington State Department of Fish and Wildlife. The permit will place timing limitations on any construction work that will affect streams. The cities of Tukwila and Renton will issue shoreline permits once they determine that the project is consistent with the goals and policies of the Shoreline Master Program (see the Land Use Plans and Policies Discipline Report for more information). Renton Nickel Improvement Project 26 Floodplains Discipline Report POTENTIAL EFFECTS What methods were used to evaluate effects on floodplains? The team compared existing conditions with the proposed widening to determine where temporary or What Is meant by fill? permanent construction will require fill to be placed or The term 'fill' in this report means where bridge piers will be built in the floodplain. materials such as rock, sand, or dirt that will be placed within the floodplain to To determine how much fill the project will place within support new construction. the floodplain, the team used the floodplain elevation on the map prepared by FEMA. To maintain the 100- year floodplain elevation, the project design will allow for an equal or greater amount of excavation from What Is floodplain storage capacity? within the floodplain at the same elevations. Floodplain storage capacity is the volume Hydraulic principles were used to evaluate whether of water that can be contained within the the floodplain will be affected by the project. To floodplain during a flood. If the storage capacity changes because part of the evaluate the effect on floodplain storage, the team floodplain is filled in, then the floodplain compared how much material is being placed in the will expand outward into places that floodplain to how much material is being removed. If previously did not flood. there is no net gain in floodplain fill, then the project will not affect floodplain storage capacity. Which floodplains could be affected by the project? The bridges over Springbrook Creek and Oakesdale Avenue will be replaced to accommodate the new lanes on 1-405. The existing bridges will be replaced with new southbound and northbound bridges that span not only the side channel but also span the main channel and Oakesdale Avenue. These bridges cross a 100-year floodplain and construction will require placing new piers and fill in the floodplain. In addition to the fill for the new bridges over Springbrook Creek and Oakesdale Avenue, some minor fill will also occur along the south side of 1-405 between Oakesdale Avenue and Lind Avenue and on the west side of SR 167 at SW 23rd Street where Panther Creek, a tributary to Springbrook Creek, crosses the highway. The other floodplain crossings at the Green River, Rolling Hills Creek, and the Cedar River will not be affected by this project. Renton Nickel Improvement Project Floodplains Discipline Report 27 POTENTIAL EFFECTS How will project construction temporarily affect the Springbrook Creek floodplain? What Is FalseworkT Falsework is the temporary structure erected to support work during the process of construction. During construction, temporary piles and falsework could be placed in the Springbrook floodplain. These structures provide work platforms and support the new structures as they are built. The replacement of the bridges over Springbrook Creek and Oakesdale Avenue may use these techniques. The exact construction methods will be determined by the contractor that WSDOT selects for this project. Depending upon how long it takes to construct the bridge, some of the temporary piles and falsework may need to remain in place through the winter. The Springbrook Creek Wetland and Habitat Mitigation Bank is an Early Environmental Investments (EEI) Project. This project will compensate for this temporary construction, because greater volumes of material will be removed from within the Springbrook floodplain at the same elevation in advance of any construction activities. The excavation, which is required to construct the wetlands, will provide sufficient floodplain storage to compensate for placing temporary piles and falsework in the area of the Springbrook Creek Side Channel and Springbrook Creek culvert. Because of the compensatory storage provided by the mitigation bank, the temporary piles and falsework are neither anticipated to affect the hydraulics of winter flows under the bridge, nor raise the floodplain elevation. Will the project permanently affect f loodplains? Replacing the bridge over Springbrook Creek will add approximately 11,200 cubic yards of fill to the floodplain. In addition, some piers will need to be placed in the floodplain to support the bridge. Compensation for these additions will come from the Springbrook Creek Wetland and Habitat Mitigation Bank. Excavation of material from this site will be more than 11,200 cubic yards and will therefore compensate for the bridge fill by removing more material from the floodplain at the same elevation in advance of any construction activities. Prior to final design and construction, WSDOT will analyze the predicted backwater conditions to verify that replacing the bridges over Springbrook Creek and Oakesdale Renton Nickel Improvement Project 28 Floodplains Discipline Report POTENTIAL EFFECTS Avenue will not affect the 100-year floodplain elevation. The box culvert that currently exists in the Springbrook main channel will no longer be necessary once the new bridge is in place. WSDOT will remove this culvert from the channel, which will be counted as additional compensatory storage for the fill and new piers. What indirect effects result from the direct effects on floodplains? There are no indirect effects to floodplains anticipated as a result of this project. What are Indirect Effects? Indirect effects are defined in the WSDOT Environmental Procedures Manual as the "effect caused by the proposed action that is later in time or farther removed in distance, but still reasonably foreseeable. Indirect effects may include growth -inducing effects and other effects related to induced changes in the pattern of land use, population density or growth rate, and related effects on air and water and other natural systems, including ecosystems." Were cumulative effects looked at for this discipline? The team did not evaluate cumulative effects for this discipline report. A report of cumulative effects is not needed for every discipline studied for NEPA and SEPA documentation. The disciplines that were studied for cumulative effects are Air Quality, Surface Water and Water Quality, Fish and Aquatic Habitat, and Wetlands. The cumulative effects for these disciplines are presented in the Cumulative Effects Analysis Discipline Report. Renton Nickel Improvement Project Floodplains Discipline Report 29 MEASURES TO AVOID OR MINIMIZE PROJECT EFFECTS What has been done to avoid or minimize negative effects to floodplains? In addition to providing compensatory floodplain storage, stormwater detention will also be provided for drainage from new impervious surfaces. Detaining stormwater will help minimize changes to flow patterns of inlet sources to the floodplain. See the Surface Water and Water Quality Discipline Report for more information on stormwater facilities. Bridge piers placed within the floodplain will be designed to minimize hydraulic disturbance to flow. This can be achieved by designing piers that are all the same size and placed in lines parallel to the flow path. How will the project compensate for unavoidable negative effects to floodplains? Adding fill to the floodplain of Springbrook Creek is an unavoidable negative effect. Compensation for this fill comes from the Springbrook Creek Wetland and Habitat Mitigation Bank, an Early Environmental Investments (EEI) Project where large volumes of material will be removed in order to construct that project. The excavation at the EEI site, which is required to construct the wetlands, will provide sufficient floodplain storage to compensate for the fill placed within the floodplain during construction of the new southbound and northbound bridges over Springbrook Creek and Oakesdale Avenue. WSDOT will hydraulically analyze the effectiveness of the fill mitigation to confirm that no rise will result in the 100-year floodplain. If the Springbrook EEI is demonstrated to be inadequate for mitigation, then another site will be found for compensatory storage. In addition, WSDOT will evaluate headloss at stream crossings prior to construction. Renton Nickel Improvement Project 30 Floodplains Discipline Report Published Documents Northwest Hydraulic Consultants, Inc. 2003 RW Beck 2004 WSDOT 2004 2001 Websites REFERENCES Hydrology Analysis for Floodplain Mapping Study of Springbrook Creek, King County Washington East Side Green River Watershed Plan Technical Memorandum Supplement Environmental Procedures Manual. September 2004. 1-405 Corridor Program NEPA/SEPA Final Environmental Impact Statement http://www.ci.renton.wa.us http://www.ci.tukwila.wa.us http://www.metrokc.gov/ddes http://www.fema.gov http://www.nwd-wc.usace.army.mil Renton Nickel Improvement Project Floodplains Discipline Report 31