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DRAINAGE MEMO (2)Imm engineering pllc Memorandum Date: January 21, 2020 To: City of Edmonds Public Works & Utilities Department 121 5 h Avenue N, 2nd Floor Edmonds, WA 98020 Attn: Natalie Griggs From: Jennifer Argraves, P.E. I Andrew Wong, E.I.T. 1932 Ist Avenue, Suite 201 Seattle, WA 98101 p. 206.725.121 1 f. 206.973.5344 Ipdengineering.com Subject: Edmonds Woodway High School Synthetic Turf Conversion (#bld20l80708) Drainage Memorandum This memorandum outlines the drainage impacts associated with construction changes for the Edmonds Woodway High School Synthetic Turf Conversion project. The drainage change involved removing the underdrain trenches from a portion the pervious concrete area between the field and the tennis courts, east of the field. Originally, runoff from this area was intended to combine with the baseball field underdrain system, and thus be tributary to its flow control structure. However, due to grading concerns, this area was sloped toward the tennis courts. Therefore, this portion of pervious concrete walkway would become part of the StormTech facility sub -basin. This area is approximately 2,393 (0.055 acres), and is shown on the revised Figure 5 — Proposed Conditions Modeled Areas (attached). An updated MGS Flood model was run to account for the revised sub -basin totals. Note that per Appendix III-C of the 2014 DOE Manual, permeable pavement without underdrains would be modeled as 100% grass. However, to be more conservative, we assumed the same 50% impervious/50% grass condition that was used for the permitting process. The model passed with same size detention facilities and control structures as originally proposed during permit. See Graph 1 below showing flow control compliance. MGS Flood Report is attached with this memorandum. Graph 1— Revised Flow Duration Plot 1I1 ox ox on 1>x-0r 11 Q50 02 50%Q2 la-M la-M la_oc 1i1e-M la-M 1a-01 Ia+00 Exceedance Probability if Predeveloped f Postdeveloped Imm engineering pllc January 21, 2020 Edmonds Woodway HS — Drainage Memorandum Page 2 Additionally, it was asked that we evaluate the drainage capabilities of the existing tennis courts north of the field. Due to the new tennis court additions, the tennis courts north of the field are no longer in use. Only the northern half is visible, while an asphalt transition has been installed on the south side, connecting from the new pervious concrete walkway. In the existing conditions (pre -project construction), the two courts were sloped to drain toward a trench drain between them. The trench drains conveyed water to a Type-1 catch basin in the middle of the courts. From here, an 8-inch piped outlet headed to the east, continuing on to the downstream drainage course. As a result of this project, the southern half of the trench drain was paved over with the asphalt transition. However, the Type-1 catch basin and its outlet pipe remain. The full tennis court area (pre -project) is approximately 13,000 square feet. The new asphalt transition will slope north toward the existing catch basin, but the permeable concrete walkway that has replaced a portion of the court will reduce the amount of tributary area to the existing system. Not only is the surfacing permeable, but the trench beneath hydraulically connects with the baseball field underdrain system. This area is accounted for in the flow control design of the baseball field system, and thus is no longer tributary to 8-inch system. The approximate amount of area draining to the system now is 5,800 square feet. This is less than the predeveloped tributary area, and therefore this conveyance system will not be adversely affected. Thank you for your time. Please let us know if you have any questions or need further clarification. Attachments: • Figure 5 — Proposed Conditions Modeled Areas (revised) • MGS Flood Report — Flow Control (Update) • Exhibit — North Tennis Court Basin FIELD FOOTPRINT 116,969 SF (2.685 AC) BYPASS NEW PLUS REPLACED HARD SURFACE EX UNDISTURBED GRASS AREA SD SYSTEM RE-ROUTED THROUGH FLOW CONTROL STRUCTURE (DOES NOT REQUIRE MITIGATION) FIELD DETENTION BASIN 177,553 SF (4.076 AC) it - . �'. • 1 .• PIP ME I 2. �... � ql� �•� ►uuuu•uuuuu•1, ®� ��� �'•ui'•uuuu•••uu•uua � � !, � ■ •i I•�Y ��•••••••!•••••�►•�!a 11 III �'_=� _ - _ Vie- = = - • • "�t3� `bIl�' legism jPR MP 1111 No I-Sh O0 rt STRUCTURE STORMTECH CHAMBER TENNIS COURT DETENTION SYSTEM DETENTION BASIN 40,765 SF (0.936 AC) ©2020 LPD Engineering PLLC I Bypass Basin I ^Y T T T I ++ IMPERVIOUS 0.173 AC GRASS 0.000 AC PERMEABLE PAVEMENT AS 50Z IMPERVIOUS, 50Z GRASS TOTAL BYPASS AREA (FORESTED IN PREDEVELOPED) 0.173 AC 19321st Ave., DESCRIPTION JANUARY 21, 2020 FIGURE Suite 201, Seattle, PROPOSED CONDITIONS p. 206.725.1211 f. 206.973.5344 MODELED AREAS 5 engineering PIIC www.lpdengineering.com MGS FLOOD PROJECT REPORT - FLOW CONTROL (UPDATE) Program Version: MGSFlood 4.46 Program License Number: 201410003 Project Simulation Performed on: 08/30/2019 11:05 AM Report Generation Date: 08/30/2019 11:14 AM Input File Name: Const. Update - COMBINED detention model.fld Project Name: Edmonds Woodway HS Analysis Title: Flow Control Comments: Both basins - with "converted" pervious PRECIPITATION INPUT — Computational Time Step (Minutes): 60 Extended Precipitation Time Series Selected Climatic Region Number: 14 Full Period of Record Available used for Routing Precipitation Station : 96003605 Puget East 36 in_5min 10/01/1939-10/01/2097 Evaporation Station 961036 Puget East 36 in MAP Evaporation Scale Factor 0.750 HSPF Parameter Region Number: 1 HSPF Parameter Region Name : USGS Default ********** Default HSPF Parameters Used (Not Modified by User) *************** ********************** WATERSHED DEFINITION *********************** Predevelopment/Post Development Tributary Area Summary Predeveloped Post Developed Total Subbasin Area (acres) 5.185 5.185 Area of Links that Include Precip/Evap (acres) 0.000 0.000 Total (acres) 5.185 5.185 ----------------------SCENARIO: PREDEVELOPED Number of Subbasins: 1 ---------- Subbasin Till Forest Till Grass ------------------------- Subbasin Total Subbasin 1--------- ------- Area (Acres) 4.246 0.939 5.185 ----------------------SCENARIO: POSTDEVELOPED Number of Subbasins: 3 ---------- Subbasin : Field Basin --------- ------- Area (Acres) Till Grass 2.608 Impervious 1.468 Subbasin Total 4.076 ---------- Subbasin : Bypass ---------- ------- Area (Acres) Impervious 0.173 Subbasin Total 0.173 ---------- Subbasin : Tennis Court Basin -------Area (Acres) Till Grass 0.142 Impervious 0.794 Subbasin Total 0.936 ************************* LINK DATA ******************************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 0 ************************ LINK DATA ******************************* ----------------------SCENARIO: POSTDEVELOPED Number of Links: 3 Link Name: Field Detention Link Type: Structure Downstream Link Name: New Copy Lnk2 User Specified Elevation Volume Table Used Elevation (ft) Pond Volume (cu-ft) 100.00 0. 100.30 1. 100.60 2. 100.90 3. 101.05 4. 101.35 6. 101.65 30. 101.95 67. 102.25 94. 102.55 118. 102.85 141. 103.15 167. 103.45 273. 103.75 627. 104.05 1247. 104.35 2292. 104.65 3095. 104.86 3569. 104.96 7078. 105.06 10587. 105.16 14096. 105.26 17605. 105.36 21114. 105.46 24623. 105.56 28132. 105.71 33396. 105.81 36905. Constant Infiltration Option Used Infiltration Rate (in/hr): 0.00 Riser Geometry Riser Structure Type Riser Diameter (in) Common Length (ft) Riser Crest Elevation Hydraulic Structure Geometry Number of Devices: Circular 12.00 0.000 105.71 ft ---Device Number 1 --- Device Type Circular Orifice Control Elevation (ft) 100.00 Diameter (in) 0.76 Orientation : Horizontal Elbow : No ------------------------------------------ Link Name: New Copy Lnk2 Link Type: Copy Downstream Link: None Link Name: StormTech Link Type: Structure Downstream Link Name: New Copy Lnk2 User Specified Elevation Volume Table Used Elevation (ft) Pond Volume (cu-ft) 100.00 0. 100.08 281. 100.17 563. 100.25 844. 100.33 1126. 100.42 1407. 100.50 1689. 100.58 2285. 100.67 2881. 100.75 3472. 100.83 4061. 100.92 4647. 101.00 5228. 101.08 5804. 101.17 6377. 101.25 6945. 101.33 7507. 101.42 8066. 101.50 8617. 101.58 9162. 101.67 9700. 101.75 10231. 101.83 10755. 101.92 11272. 102.00 11779. 102.08 12277. 102.17 12764. 102.25 13239. 102.33 13701. 102.42 14150. 102.50 14586. 102.58 15003. 102.67 15398. 102.75 15766. 102.83 16087. 102.92 16391. 103.00 16680. 103.08 16961. 103.17 17243. 103.25 17524. 103.33 17806. 103.42 18087. 103.50 18368. 103.58 18650. 103.67 18931. 103.75 19213. 103.83 19494. 103.92 19776. 104.00 20057. Massmann Infiltration Option Used Hydraulic Conductivity (in/hr) 0.00 Depth to Water Table (ft) : 100.00 Bio-Fouling Potential : Low Maintenance : Average or Better Riser Geometry Riser Structure Type : Circular Riser Diameter (in) : 12.00 Common Length (ft) : 0.000 Riser Crest Elevation : 103.50 ft Hydraulic Structure Geometry Number of Devices: 2 ---Device Number 1 --- Device Type Circular Orifice Control Elevation (ft) 100.00 Diameter (in) 0.55 Orientation : Horizontal Elbow : Yes --- Device Number 2 --- Device Type : Horizontal Rectangular Orifice Control Elevation (ft) 102.75 Length (in) 0.25 Height (in) 9.00 Orientation : Horizontal Elbow : Yes **********************FLOOD FREQUENCY AND DURATION STATISTICS******************* ----------------------SCENARIO: PREDEVELOPED Number of Subbasins: 1 Number of Links: 0 ----------------------SCENARIO: POSTDEVELOPED Number of Subbasins: 3 Number of Links: 3 ***********Compliance Point Results ************* Scenario Predeveloped Compliance Subbasin: Subbasin 1 Scenario Postdeveloped Compliance Link: New Copy Lnk2 *** Point of Compliance Flow Frequency Data *** Recurrence Interval Computed Using Gringorten Plotting Position Predevelopment Runoff Postdevelopment Runoff Tr (Years) Discharge (cfs) Tr (Years) Discharge (cfs) ------------------------------------------------------------------------------------------------------------ 2-Year 0.103 2-Year 8.592E-02 5-Year 0.199 5-Year 0.105 10-Year 0.255 10-Year 0.131 25-Year 0.359 25-Year 0.213 50-Year 0.455 50-Year 0.334 100-Year 0.556 100-Year 0.367 200-Year 0.754 200-Year 0.389 ** Record too Short to Compute Peak Discharge for These Recurrence Intervals **** Flow Duration Performance **** Excursion at Predeveloped 50%Q2 (Must be Less Than or Equal to 0%): -4.6% PASS Maximum Excursion from 50%Q2 to Q2 (Must be Less Than or Equal to 0%): -4.6% PASS Maximum Excursion from Q2 to Q50 (Must be less than 10%): 7.1 % PASS Percent Excursion from Q2 to Q50 (Must be less than 50%): 4.8% PASS MEETS ALL FLOW DURATION DESIGN CRITERIA: PASS Exhibit - North Tennis Court Basin A V� � 1 .,e•'• U A- • • Y CATCH / STAND Predeveloped tennis court Y I RIM 4 sub -basin 13,000 SF OW LINE OF A10 H 8" CM \ w •4 o TICAL CUR °D 8" CM �w _� AREA -BRAID \ � s ,6y —� I E 4 " ADS 415- A_Z cD _ 21. Al Aryblu, UYU IIGH FENCE VSHD LIJ o w Asphalt Transition J U 411- U Z y, CA 0*Q: U LwL p16 A CD CATCH BAE STANDARD RIM 416.0 N CATCH BASIN 12" CONIC STANDARD GRATE 4" PVC E 8" DI W 8" DIE 13.96 IE 4" ADS N 4" PVC S 414.79 IE 4" PVC N 414.78 IE 4" ADS N 414.29 IE I kSIN 7 V11 I I I v, GRATE ,°°; 59 = I J 419.49 IE U w m y p16 � Ag W Z _.p10... .. .. ;ATCH BASIN �� o+ STANDARD GRATE L.)w bo 1IM 420.47 U li A� . A� ' A� A�6 •. t" PVC NW 419.57 TOP.. A 3" PVC NW 418.87 IE AREA DRAIN W /./RIM. 05; 71 Crm CD(FULL . OF GRAVEL) P \ UU U W W \ y y 9A A 416.79 • 9 c CB 416.14 Postdeveloped tennis TOP A1y co rt sub -basin 5,800 416.0' 4, q 9Ss O CI'�7s Area that is now \ s� underdrained (field & ED , A pervious concrete walkway). I a Going to different drainage eRFe nReTN sub -basin.