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HomeMy WebLinkAboutRS_Conceptual_Drainage_Technical_Memorandum_180108_v1PSE Energize Eastside – Renton Segment 1 Conceptual Drainage Technical Memorandum Technical Memorandum To: Brad Strauch, PSE From: John Erickson P.E., HDR Date: January 2018 Project: PSE Energize Eastside – Renton Segment Subject: Conceptual Drainage Technical Memorandum 1.0 Introduction The purpose of this technical memorandum is to facilitate the permit coordination and approval process between the City of Renton (City) and Puget Sound Energy, Inc. (PSE) related to the storm drainage design for the portion of the Energize Eastside Project (Project) that is within the City of Renton. The information provided within summarizes Design Team’s understanding of the stormwater requirements applicable to the Project based on the Conceptual Design. The stormwater design developed in accordance with the 2017 City of Renton Surface Water Design Manual (SWDM), which is has been approved as an equivalent to the 2016 King County Surface Water Design Manual with revisions to reflect specific requirements of the City. 2.0 Project Overview PSE proposes to construct a new 230 kV to 115 kV substation (Richards Creek Substation in Bellevue) and to upgrade approximately 18 miles of existing 115 kV transmission lines located within an approximately 100-foot wide regional utility corridor to accommodate 230 kV. The Project will improve reliability and supply the needed electrical capacity for anticipated growth and development on the Eastside including the City of Renton (City). For the purposes of this report the “Corridor” is limited to the segment of the proposed Energize Eastside Corridor within Renton. The corridor runs from the north Renton city limits south of SE 95th Way to the existing Talbot Hill Substation in the vicinity of Beacon Way S, a distance of approximately 4 miles, shown in the Vicinity Map (Figure 1). The Corridor is generally limited to the area within the boundaries of an approximately 100-foot wide regional utility corridor, except between the Talbot Hill substation and Shadow Hawk neighborhood where the study area was widened to capture additional area based on refined design parameters. The Project corridor is primarily located in the Cedar-Sammamish Watershed (WRIA 8). Within WRIA 8 the Project area lies within three drainage basins. The north end of the study area is within the May Creek basin, the middle portion is within the East Lake Washington basin, and the southern portion is in the Lower Cedar River drainage basin. No wetlands or streams were identified in the East Lake Washington basin. At the southern end of the Corridor, near the Talbot Hill Substation, is in Black River basin of the Duwamish-Green Watershed (WRIA 9). The Project corridor is located in Township 23N, Range 05E, and Sections 4, 9, 16, 20, and 21. The proposal improvements will include removal of approximately 144 existing poles, installation of approximately 41 new poles, and stringing new transmission cables. Although the Corridor is approximately 4 miles long and 100 feet wide, the areas where there will be new impervious surface and PSE Energize Eastside – Renton Segment 2 Conceptual Drainage Technical Memorandum land disturbing activity will be limited to isolated work areas associated with each pole removal or installation areas. Figure 1 Vicinity Map PSE Energize Eastside – Renton Segment 3 Conceptual Drainage Technical Memorandum 3.0 Existing Conditions When the corridor was constructed in the late 1920s and early 1930s, the entire corridor was cleared. Construction activities resulted in a compacted subsurface in those areas where the poles were installed. Since that time, the corridor has been continually maintained by PSE through easement rights. Using existing access routes/paths, poles have been replaced and vegetation has been managed. To do so, vehicles and equipment (such as cranes) have been used in the corridor. Over time, development has occurred adjacent to and within the corridor, including residential development, roads, parking lots, commercial development, and the establishment of trails (using overgrown access routes). The Talbot Hill area is utilized by an extensive number of co-located critical utilities, which include power substations (owned PSE and Bonneville Power Administration), watermain transition lines (owned by Seattle Public Utilities, Olympic Pipeline, and fiber optic communications lines. Most of the developed portions of the Corridor that are vegetation in residential, commercial, and industrial areas can be generally described as maintained yards or landscaped. On parcels that have not been developed as commercial or residential property, vegetated areas are often dominated by invasive plants including Himalayan blackberry and reed canary grass. Forested patches are limited to topographically low regions near the Cedar River and Honey Dew Creek. The stormwater runoff from the developed portions of the Corridor are managed through the combination of local parking lot/private property drainage system and roadway drainage systems. The existing systems are comprised of a variety of system that include closed pipe conveyance systems, ditches, and culverts. The stormwater runoff from the undeveloped portions of the Corridor are very limited, relying on occasional ditch and culvert systems that ultimately drain to the adjacent roadway drainage systems, disperse into natural areas, or directly to a downstream water body. PSE Energize Eastside – Renton Segment 4 Conceptual Drainage Technical Memorandum 4.0 Proposed Conditions The proposed site and drainage conditions will result in only minor permanent changes from the existing conditions. This is due to the small footprint of the proposed poles and through the use of construction methods that are intended to minimize the impacts to the site. The proposed work elements and there impacts to the site are described below. The Corridor falls within seven threshold discharge areas (TDAs), which are used for assessing stormwater requirements and sizing stormwater facilities. The TDAs and their respective drainage basins are summarized in Table 1 and shown in Figure 2. The assumptions for determining the new impervious areas and land disturbing activities are discussed Section 5.0. Table 1 TDA and Drainage Basin TDA Drainage Basin 1 Disturbed Land Area 2 (SF) New Impervious Area (SF) 1 May Creek 25,000 158 2 May Creek via Honey Creek 40,000 237 3 East Lake Washington 10,000 28 4 East Lake Washington 10,000 28 5 Lower Cedar River 22,500 100 6 Lower Cedar River 17,500 93 7 Lower Cedar River 52,500 323 Project Total = 177,500 969 Notes: 1) Drainage Basin per the City of Renton Basin Locations Map (10/16/2009) 2) Disturbed Land Area includes both permanent improvements and areas that will be temporarily disturbed during construction. 5.0 Proposed Work Elements The proposed work elements, which include pole replacement, access routes, stringing sites, and vegetation management are discussed in the following sections. Pole Replacement Existing H-frames (consisting of 2 or 3 poles) will be replaced with new monopoles (i.e., a single pole) or 2 steel poles in the case of turning structures. Approximately 144 existing poles will be replaced with 41 new poles. The existing poles will be cut off approximately 1-2 feet below the ground surface and removed from the site. The below ground portion of the pole will be abandoned in place. The surface will be restored with mulch and seeding. In general, relocation activities will occur in close proximity to the existing H-frames, but some of the replacement poles will be moved to accommodate landowner preferences, due to easement considerations, and to minimize impacts to critical areas. The new and replaced impervious surfaces at each site will be limited to the pole diameter, footing, and any additional existing impervious areas that will need to be replaced due to installation activities. The poles will be approximately 3 feet in diameter (direct embedded) and some poles will require a 6-foot by 6-foot concrete foundation. The disturbed land activity at each location has been assumed to be between 2,500 and 5,000 square feet depending on the type of installation. These numbers are PSE Energize Eastside – Renton Segment 5 Conceptual Drainage Technical Memorandum expected to be conservatively high because a number of the pole will be able to be accessed from paved areas, in which case there would be minimum land disturbing activity outside of the immediate pole location. Included in the work areas discussed above, will be a 6-foot buffer around the poles where installed in vegetated areas that will be restored with vegetation that will have a max mature height of 15 feet and maintain 20 of vertical clearance from the transmission wires. In some cases these area will cause a conversion from forested to landscaped land cover. The majority of pole construction work area and pole buffer impacts are expected to be temporary due to the existing use and management of the corridor (i.e., lack of trees) and the existing groundcover will be restored or regenerate on its own within one growing season. BMPs will be used to minimize impacts resulting from pole replacement activities. In critical areas or buffers, mats will be placed over existing vegetation where possible. Typically, crushed vegetation rebounds within one growing season resulting in only temporary impacts to vegetation. Post construction, all disturbed areas will be re-vegetated, if necessary, and left to return to their natural state. Access Routes Access to poles located in the transmission corridor will generally occur using existing, partially vegetated access (established during original construction and re-used over time to maintain the corridor). In critical areas or buffers, mats will be placed over existing vegetation where possible. Typically, crushed vegetation rebounds within one growing season resulting in only temporary impacts to vegetation. Where access route alignment requires tree removal, impacts will be characterized as a land cover conversion (New or replaced pervious surfaces). Post construction, all disturbed areas will be re-vegetated, if necessary, and left to return to their natural state in compliance with vegetation management requirements. Based on the existing conditions, proposed construction BMPs, and post-construction methods, disturbance associated with access in the transmission corridor will predominantly be temporary. Stringing Sites In order to replace the transmission conductor, stringing and tensioning equipment will be staged near new steel poles at specific locations along the corridor in preparation for the stringing of new wire. The disturbance area associated with the equipment and materials to restring the conductor wire will be isolated from wetlands and streams to the extent feasible. In critical areas and buffers, mats will be placed over existing vegetation where possible to allow access to poles for stringing activities. Typically, crushed vegetation rebounds within one growing season resulting in only temporary impacts to vegetation. Tree removal activities necessary for the stringing of new wire (in the wire zone) will be performed in a manner to minimize impacts to underlying shrubs, groundcover and other trees, without disturbance to soil. The various techniques utilized to string the wire will not result in surface disturbance (i.e., shooting the wire past obstacles, pulling it along established guide wire, etc.). Each stringing site will be approximately 7,500 square feet of disturbance. Similar to pole construction work areas, the shape of the stringing site will depend upon the presence of adjacent critical areas, existing land conditions, and area needed for equipment staging based on the necessary angle needed to string the conductor. In many areas, this disturbance will overlap with various impacts quantified for proposed access, pole installation, and vegetation management. While impacts have not been quantified for stringing sites, stringing sites are expected to largely overlap other work areas and are not expected to require additional tree removal. Any additional impacts resulting from stringing sites will be PSE Energize Eastside – Renton Segment 6 Conceptual Drainage Technical Memorandum temporary in nature; temporary impact areas will be re-vegetated and left to return their natural state or enhanced following construction. Consistent with federal standards, vegetation in the wire zone must have a mature height of no greater than 15 feet, unless the topographic change is sufficient to allow a 20-foot vertical clearance between the power lines and the mature height of trees under the power lines. The same vegetation requirement was applied to the managed ROW zone. The legal ROW is composed of existing and proposed easements; its width is approximately 100 feet through Renton. The area outside of the managed ROW, but still within the legal ROW, is also subject to select clearing of trees that pose a risk of damaging the lines. MAP CREATED BY: Date: 1/5/2018Path: G:\Projects\Washington\Puget_Sound_Energy_007022\EnergizeEastsideMapbook_10059878\7.2_WP\Map_Docs\Landuse_CUP\Renton\Version_N\PSE_Renton_Index.mxdTopo Basemap - ESRI Online,Transmission Line - PSE ST169 ST900 §¨¦405 Renton Renton Newcastle KingCounty I0 0.25 0.5Mile RENTON CLEARING AND GRADING PLAN INDEX SOURCES: APPLICANT:Brad StrauchPuget Sound EnergyP.O. Box 97034, Bellevue WA 98009-9734(425) 456 - 2556 PSE Energize Eastside – Renton Segment 9 Conceptual Drainage Technical Memorandum 6.0 Stormwater Requirements Assessment The stormwater requirements for the project were determined based on the methodology provided in Chapter 1 of the 2017 City of Renton Surface Water Design Manual. The type of review and applicability of the core and special requirements were determined by evaluating the amount of new and replaced impervious surfaces, land disturbing activity, and total amount of new impervious surfaces in a hydraulically connected subbasin. This project will create less than 2,000 square feet of new plus replaced impervious surface, and create less than 50 acres of new impervious surface within a hydraulically connected subbasin. However, it will require greater than 7,000 square feet of land disturbing activity and therefore will be subject to a Full Drainage Review. As a result all nine Core Requirements and all five Special Requirements are required to be evaluated by for each Project TDA area. The Core and Special Requirements that apply to this project and justifications for requirements that are not applicable to the project are discussed as in the remainder of this section. Core Requirement #1 – Discharge at the Natural Location Core Requirement #1 will be satisfied by maintaining the existing drainage patterns of the site and discharging to existing downstream drainage systems. The proposed improvements will be limited to the transmission pole installations with footprints that range between approximately 7 and 36 square feet. Where the poles will be installed in vegetated areas, the surrounding areas will be designed to allow the stormwater to runoff in a dispersed manner. The surface restoration around poles that will be installed in paved areas will be graded to drain to the existing drainage system matching the existing drainage patterns. Core Requirement #2 – Offsite Analysis The Project is exempt from Core Requirement #2 based on the TDA level analysis summarized above in Table 1. The analysis shows that all seven TDAs will have less than 2,000 square feet of new impervious surface, less than ¾ acres (32,670 square feet) of new pervious surface, and will not construct or modify drainage pipes or ditches. Additionally the work sites are not located in a landslide hazard, steep slope hazard areas, or erosion hazard area as defined in RMC 4-3-050. Core Requirement #3 – Flow Control The Project is exempt from Core Requirement #3 based on the criteria in the formal Basic Exemption, when analyzed on the TDA level. The proposed improvements in all seven TDAs will create less than 5,000 square feet of new plus replaced impervious surfaces and less than ¾ acres (32,670 square feet) of new pervious surface, as shown in Table 1. Core Requirement #4 – Conveyance System Core Requirement #4 does not apply to the Project because no pipe conveyance, ditch, or culvert systems will be constructed or modified. Core Requirement #5 – Erosion and Sediment Control Core Requirement #5 will be satisfied by the Erosion and Sediment Control (ESC) Plan and Construction Stormwater Pollution Prevention (CSWPP) Plan that will be developed in accordance with the City of Renton stormwater requirements. PSE Energize Eastside – Renton Segment 10 Conceptual Drainage Technical Memorandum Core Requirement #6 – Maintenance and Operations Core Requirement #6 does not apply to the Project because there are no formal drainage facilities being proposed by the project. Core Requirement #7 – Financial Guarantees and Liability Core Requirement #7 will be satisfied by providing the financial guarantee and liability information as required by the City. Core Requirement #8 – Water Quality The Project is exempt from Core Requirement #8 based on the criteria in the formal Surface Area Exemption. The proposed improvements in all seven TDAs will create less than 5,000 square feet of new plus replaced pollution generating impervious surfaces and less than ¾ acres (32,670 square feet) of new pollution generating pervious surface, as shown in Table 1. Core Requirement #9 – On-Site BMPs All Project TDAs will trigger Core Requirement #9 by disturbing greater than 7,000 square feet of land (primarily temporary construction impacts), but will create less than 2,000 square feet of new plus replaced impervious surface. The requirement will be satisfied using basic dispersion and soil amendment BMP (Appendix C, Section C.2.13 of the 2017 City SWDM) where applicable. Special Requirement #1 – Other Adopted Requirements This requirement will be satisfied by incorporating the requirements of all adopted areas-specific regulations that apply to the project. Special Requirement #2 – Flood Hazard Area Delineation This requirement does not apply to the project as it is not within a flood hazard area. Special Requirement #3 – Flood Protection Facilities Special Requirement #3 does not apply to the Project because the proposed improvements do not rely on an existing or proposed flood protection facility (such as a levee or revetment) for protection against hazards posed by erosion or inundation. Special Requirement #4 – Source Control Special Requirement #4 does not apply to the Project because the proposed improvements does not require a commercial building or commercial site development permit. Special Requirement #5 – Oil Control Special Requirement #5 does not apply to the Project because it will not be classified as a new high- use site. Where poles will be installed on existing high-use sites there are already oil control facilities in place. Additionally, the propose improvements do not warrant an increase in oil protection. Special Requirement #6 – Aquifer Protection Area Special Requirement #6 is applicable to the Project because it walls within Aquifer Protection Area Zone 1 per the Groundwater Protection Areas Map (Reference 15-B in the 2016 City of Kent Surface Water Design Manual). The Special Requirement will be satisfied by not constructing open stormwater facilities (i.e., flow control or water quality ponds, stormwater wetlands, or infiltration PSE Energize Eastside – Renton Segment 11 Conceptual Drainage Technical Memorandum facilities), On-site BMPs that rely on infiltration (except dispersion of non-pollution generating area), or open conveyance systems.