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Sherwood Elem-Stormwater response.pdfDate: June 3, 2015 To: City of Edmonds From: Alex Sundell, KPFF Engineers Subject: ESD Sherwood — Stormwater Comment Responses ff iii The following Stormwater comments were received from Jerry Schuster on May 06, 2015. The responses to the comments are based on a discussion with Jerry on May 11, 2015. 1) Provide an exhibit that shows the extent of land disturbing activity for the project. Note that land disturbing activity is defined as: 'Any activity that results in a change in the existing soil cover (both vegetative and non -vegetative) and/or the existing soil topography. Land disturbing activities include, but are not limited to clearing, grading, filling, and excavation. Compaction that is associated with stabilization of structures and road construction shall also be considered a land disturbing activity. Vegetation maintenance practices, including landscape maintenance and gardening, are not considered land -disturbing activity. Stormwater facility maintenance is not considered land disturbing activity if conducted according to established standards and procedures. " Please review the comment for plan sheet 04.01 prior to producing this exhibit. Per discussion on May 11, 2015, an exhibit showing the extent of all land disturbing activity wiF ce added as an Appendix to the Drainage Report. 2) Provide a table with the following information for the post -development scenario: Per discussion on May 11, 2015, a table with the above information for the post -developed scenario will be added to the Drainage Report. 3) The Drainage Report from 1995 when existing sub -basin B was constructed has 1.37 acres (0.85 acres impervious plus 0.52 acres pervious) going to the existing swale and the detention facility (available in City files). The Drainage Report for this proposed project has 0.99 acres existing going to the swaleldetention. Please reconcile. The 0.99 acres only represents the area within the limit of work. The total 1.37 acres includes additional area outside the limits of work for this project. Per discussion on May 11, 2015, a description of this case will be added to the Drainage Report. 4) Infiltration is the preferred method of stormwater management for the City. Sherwood Elementary is located in the area of the City that has the best soils for infiltration. Since the infiltration rate is favorable and the School District has the space, the City would prefer 100% infiltration up to the 50 year event. Note that the City plans to construct an infiltration gallery in the summer of 2015 on 228th St SW near the proposed entrance to the school to infiltrate runoff from the adjacent neighborhood. Per discussion on May 11, 2015, the infiltration systems are designed to the Edmonds Stormwater Supplement requirements for a Small Site Project. The preference of Edmonds School District is to minimize the extent of playfield area taken for infiltration facilities. It is expected that 100% infiltration up to the 50 year event would require additional infiltration facilities in the playfield area, which is not preferable to the School District. Infiltration above the Edmonds Stormwater Supplement requirements for a Small Site Project is not proposed for this project. 5) Per volume V, Section 4.1 of the 2005 Ecology Stormwater Management Manual for Western Washington, "If runoff from non -pollution generating surfaces reaches a runoff treatment BMP, flows from those areas must be included in the sizing calculations for the facility. Once runoff from non -pollution generating areas is mixed with runoff from pollution -generating areas, it cannot be separated before treatment. " Rain Gardens I & 2 are modeled for treatment only using the PGIS, when the flows to these facilities insides non-PGIS. Remodellresize for the combined flows. Per discussion on May 11, 2015, the rain gardens are revised to include treatment for both PGIS and non-PGIS areas. Revised model outputs are included in the Drainage Report to ,eflect the revisions. 6) The modeling needs a few modifications to represent what is designed. The bioretention cells (rain gardens) in Appendix G needs the following changes: The surface areas of the rain garden needs to be subtracted from the tributary area in the post -development run. 1 1! 1111 1 111 1 1 1 1 #11 f from the facility. Per discussion on May 11, 2015, an explanation of the model will be added to the Drainage Report, see response to comment V. Bioretention soil porosity should be 40% per the LID Technical guidance manual. Per discussion on May 11, 2015, bioretention soil porosity is revised to be 40%. An underdrain is not present. An underdrain is present in the model. See response to comment #7 for a full description of the model. Bioretention cells treat runoff by passing it vertically through the bioretention soil mix (BSM) and infiltrating the water, not filling up the pond and overflowing it to the layer below. The runs show 0% infiltrated. Add the 2.5 ft of gravel below the BSM and the design infiltration rate of I inch per hour. The model accounts for both treated and untreated flows distributed to the infiltration basin. See response to comment #7 for a full description of the model. A circular riser 12 inches in diameter is modeled. The plan shows the riser is a Type I CB with a beehive grate. Dimensions of the overflow structure are changed to match the dimensions of a Type 1 Catch Basin. 7) The Infiltration Basins modeled in Appendix H needs the following changes: It does not appear that the MGS Flood model is capable of modeling the proposed bioretention/infiltration/overflow system. Runoff gets to the infiltration layer by two routes from the bioretention layer 1) infiltration through the bottom and 2) via the overflow structure. Please verify that the model accounts for this. If this cannot be accurately modeled in MGS Flood, WWHM2012 or WWHM4 can do this. Per discussion on May 11, 2015, the MGSFlood model accurately reflects the design of the rain garden/infiltration systems and accounts for all treated and untreated waters flowing fro the rain garden layer to the infiltration basin. • The rain garden/infiltration facilities are designed to receive stormwater flows from upland basins via a pipe outlet from the proposed stormwater systems. This is modele,�. in MGSFlood by a link connection from the post -developed runoff cell to the rain garden cell. • The rain garden facilities are designed to infiltrate treated stormwater through the bottom of the rain garden layer to the infiltration basin below. This is modeled in MGSFlood through an underdrain that directs all treated flows from therain garden cell to the infiltration cell in the model. The rain garden cell is linked to the downstream infiltration trench cell to model flows moving from the rain garden facility to the infiltration basin. The 0% infiltration that is shown in the model for the rain garden cell IT 'T 1111111111111111H 1111111H11111 kpff indicates that no treated waters are infiltrating to native soil. All treated waters are modeled as being collected by an underdrain and directed to the infiltration cell. The model properly accounts for all treated waters passing from the rain garden facility to the infiltration basin with the underdrain. The rain garden facilities are designed to pass all untreated flows in excess of the rain garden capacity through an overflow structure (Type 1 Catch Basin) to the infiltration basin below. The infiltration basin is designed with a series of 6 -inch diameter perforated pipes that evenly distribute untreated waters from the rain garden layer that pass through the overflow structure. This is modeled in MGSFlood through the overflows structure that directs all untreated flows from the rain garden cell to the infiltration cell in the model. The rain garden cell is linked to the downstream infiltration trench cell to model flows moving from the rain garden facility to the infiltration basin. The model properly accounts for all untreated waters passing from the rain garden layer to the infiltration basin through the overflow structure. The infiltration facilities are designed to receive stormwater form the rain garden through infiltration from the bottom of the rain garden layer (treated flows) and through the overflow structure (untreated flows). This is modeled in MGSFlood by linking the upstream rain garden cell to the downstream infiltration trench cell. Treated flows from the rain garden cell are collected by the underdrain and untreated flows are collected by the overflow structure. The link in the model directs flows from the underdrain and overflow structure to the infiltration trench cell. The model properly accounts for all waters moving from the rain garden to the infiltration basin as it is designed. The infiltration basins are designed to discharge flows in excess of infiltration capacity by means of a storm drain pipe with the invert at the top of the infiltration basin layer. This is modeled in MGSFlood through an overflow structure. The point of compliance in the model is set at the outflow of the infiltration trench, which properly accounts for all flows in excess of infiltration that are leaving the system. The model reports indicate the 2 -yr, 10 -yr, and 100 -yr flows that leave the system, and a description of flow control compliance for the requirements of a Small Site Project per the Edmonds Stormwater Supplement is provided in the report. A detailed description of how the model accounts for the design of the rain garden/infiltration basins will be added to the Drainage Report. Snapshots of the model layout will be provided in an Appendix to the Drainage Report. The bioretention soil saturated hydraulic conductivity should be 9.5 in/hr The saturated hydraulic conductivity of the rain garden soil is updated in the models to be 1.5 in/hr. The bioretention soil porosity should be 40% per the LID Technical guidance manual. Per discussion on May 11, 2015, bioretention soil porosity is revised to be 40%. An underdrain is not present. An underdrain is present in the model. See above comment response for a full description of the model. NTI 'I' [Ofl, lq:yff The infiltration trench does not appear to be a "Trench on Embankment Slope." Choosing the "Trench Located Beneath Ditch" option may be more representative. Per discussion on May 11, 2015, the model is updated to include the "Trench Located Beneath Ditch" option. The model reports indicate "Trench Located at Toe of Embankment" when this option is chosen. A circular riser 12 inches in diameter is modeled. The plan shows an 8" pipe outlet The outlet from the system is updated in the model to be an 8" overflow orifice. 8) Sheet C3.01— The storm drainage crossing downstream of the proposed SDCB#6 has incorrect elevations. Per discussion on May 11, 2015, the elevations are updated to the correct values. 9) Sheet C3.01 -Verify that the existing SDCB#9 connects to the existing 18 -inch diameter storm drain. It appears to be a tee connection. Verify that this is an operational connection. Per discussion on May 11, 2015, the as -built records indicate that this is a tee connection to the 18 -inch storm drain. Edmonds School District is not concerned about maintenance of this connection. The School District has a maintenance staff that is capable of maintaining this connection if it becomes clogged. 10) Sheet C3.02 — It is unclear the purpose of Detail 2 on this sheet (very poor quality reproduction as well) The swale adjacent to the new bus lane is a replace -in-kind system. This swale provides infiltration for field drainage. Per discussion on May 11, 2015, this detail is reused from as -built drawings. Reuse of this detail indicates that the system is not a proposed means of flow control for the playfield, but instead a relocation of the existing field drainage system. 11) Sheet C3.02 — Detail 2 — Inlets to bioretention/rain gardens should have 3 to 4 inches of "catch" at the inlet pipe to collect sediment. Fer aisclssion on ifially, s orm pipe outiets to rain garoens are revisea to inci=uez _nches of "catch" above the bottom of the rain garden facilities. 12) There should be 18 inches of bioretention soil mix for treatment, not 15 inches plus the mulch layer. 13) A geotextile should not be placed under rain garden soil (See LID guidance), they tend to clog. A solution used by Seattle is Seattle mineral aggregate type 26 between the bioretention soil and the gravel. It is a Y4" washed sandy gravel. 1(p III ff I Per discussion on May 11, 2015, mineral aggregate type 26 (3/4" washed sandy gravel) is now included in the rain garden soil profile, in place of geotextile fabric between the infiltration basin and bioretention soil. 14) Geotextile on the bottom of an infiltration trench tends to clog (top and sides OK) Per discussion on May 11, 2015, geotextile is removed from the bottom of the infiltration trench. The only geotextile now included in the profile is on the walls of the infiltration basin. 15) Sheet C4.01 - The constructability of paving what shown on this drawing will be difficult. The amount of saw cutting and sealing just to have so many areas of existing remain pavement seems unwarranted. The site has more than enough room to manage the stormwater from the areas that are current plan to remain in place. Sealing so many edges is never 100% and the long-term viability of this paving plan is not very good. The site will also look a lot better if less of the existing pavement remains. Per discussion on May 11 2015, the goal of this project is to remain under 1 ac of land disturbing activity to qualify as a Small Site Project per the Edmonds Stormwater Supplement. Fully replacing all asphalt pavements would put the project in a different category for storm water requirements, which is not desirable for this project (see response to comment #4). Only those areas of pavement needing replacement for grading or utility cuts will be fully replaced. Smaller localized areas that were not previously included in full depth pavement replacement are now included and the total disturbed area quantity is updated. An additive alternative for a 1 -inch grind and overlay and crack sealing is now included in the set. The full site asphalt grind and overlay will increase pavement life and provide better aesthetics, without a full depth replacement of all pavements.