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Home My WebLink AboutBLD2011-00272 - 05 SW PLAN S CEOVIE iI Pokorny Mitigation Plan MAR 2 4 2014 Performance Standards for Enhancing the Buffer(1,000 sq ft) JEFFERSON COUNTY Parcel#801 212 004 DEPT.OF COMMUNITY DEVELOPMENT 28.)The applicant shall submit a mitigation plan that addresses the performance standards for the enhancing the buffer post-construction, maintenance and monitoring. The buffer area will be enhanced by replacing pasture grasses with native trees,shrubs and plants after the building project and final grading are complete. Monitoring and weed control will be for five years. Year 1 Plants will be installed,according to the attached planting plan,during the late winter/early spring following construction and mulched and watered.Total number of plants and livestakes is 24¢1.Source of plants will primarily be the annual Jefferson County Conservation District native plant sale. ... . ._...._..._...._.. .. Plants will include(#of plants): Big leaf maple Acer Macrophyllum(3) Pacific rhododendron Rhododendron macrophyllum(6) Swordfern olystichum munitum(25) Sand strawberry Fragaria chiloensis(50) Bitter cherry Prunus Emarginata(1) Quaking Aspen Populus Tremula(5) Salal Gaultheria Shallon(50) Snowberry Symphoricarpos albus (50) Red flowering current Ribes Sanguineum(3) Red osier dogwood livestakes Cornus Stolonifera(25) Pacific dogwood Cornus nuttallii(3) Vine maple Acer circinatum (25) i3a fa k`l" rose. F0.5,v r'►vtoca rpm_ C/5 ) Year 2 Plants be w ante ed and weeds weed,serrnharlic Ily controlled.Survival will exceed 95%. "rl✓a-sr ti Year 3 tom _ / Plants will be watered and weeds mechanically controlled and mulch supplemented. Survival will exceed 90%. we-{- a ca��v�a .e 44,117 h e <<,-' • Pr• o lb Year 4 Weeds will be mechanically controlled and mulch ' supplemented.Suvival will exceed 85%.7 ✓ust we a z w to 7. Year 5 Weeds will be mechanically controlled and mulch supplemented.Survival will exceed 80%. 1----14 v"K S C�'`rc rve,c Car Psta�c LtE Monitoring:A monitoring report will be submitted by Oct.31 of each year. ECERVE li MAR 242014 b , JEUiµ - A r.--OEPt Of COMMUNITY ffERSON cO DEVELOPMEiVf `P X IN --.", rJ t , «a ,a. . . . +. S IN i 3 s. i : \ N.- s i i , Z.t 1 Za i'l _ - ,3 ,� 1 �T ''• c n v• N • • 3.1.2 Downspout Dispersion Systems (BMP T5.10B) Downspout dispersion systems are splash blocks or gravel-filled trenches, which serve to spread roof runoff over vegetated pervious areas. Dispersion attenuates peak flows by slowing the runoff entering into the conveyance system, allowing some infiltration, and providing some water quality benefits. Applications & Downspout dispersion may be used in all subdivision lots where Limitations downspout full infiltration, full dispersion, and bioretention/rain gardens are not feasible. Flow Credit for In WWHM3, roof areas may be modeled as grassed surfaces if roof runoff Roof Downspout is dispersed according to the requirements of this section on lots greater Dispersion than 22,000 square feet, and the vegetative flow' path is 50 feet or larger through undisturbed native landscape or lawn/landscape area that meets BMP T5.13. This is done in WWHM on the Mitigated Scenario screen by entering the roof area into one of the entry options for dispersal of impervious area runoff. For WWHM 2012, see Appendix III-C in this Volume. Design Criteria 1. Use downspout trenches designed as shown in Figures 3.1.5 and 3.1.6 for all downspout dispersion applications except where splash blocks are allowed below. 2. Splash blocks shown in Figure 3.1.7 may be used for downspouts discharging to a vegetated flowpath at least 50 feet in length as measured from the downspout to the downstream property line, structure, slope over 15%, stream,wetland, or other impervious surface. Sensitive area buffers may count toward flowpath lengths. 3. Cover the vegetated flowpath with well-established lawn or pasture, landscaping with well-established groundcover, or native vegetation with natural groundcover. The groundcover shall be dense enough to help disperse and infiltrate flows and to prevent erosion. 4. If the vegetated flowpath(measured as defined above)is less than 25 feet, a perforated stub-out connection per Section 3.1.3 may be used in lieu of downspout dispersion. A perforated stub-out may also be used where implementation of downspout dispersion might cause erosion or flooding problems, either on site or on adjacent lots. For example,this provision might be appropriate for lots constructed on steep hills where downspout discharge could culminate and might pose a potential hazard for lower lying lots, or where dispersed flows could create problems for adjacent off-site lots. This provision does not apply to situations where lots are flat and on-site downspout dispersal would result in saturated Vegetative flow path is measured from the downspout or dispersion system discharge point to the downstream property line,stream,wetland,or other impervious surface. Volume III—Hydrologic Analysis and Flow Control BMPs—August 2012 3-11 • • yards. Perforated stub-outs are not appropriate when seasonal water table is<1 foot below trench bottom. Note:For all other types of projects, the use of a perforated stub-out in lieu of downspout dispersion shall be as determined by the Local Plan Approval Authority. 5. For sites with septic systems,the discharge point of all dispersion systems must be downgradient of the drainfield. This requirement may be waived if site topography clearly prohibits flows from intersecting the drainfield. Design Criteria for Dispersion Trenches 1. A vegetated flowpath of at least 25 feet in length must be maintained between the outlet of the trench and any property line, structure, stream, wetland, or impervious surface. A vegetated flowpath of at least 50 feet in length must be maintained between the outlet of the trench and any slope steeper than 15%. Sensitive area buffers may count towards flowpath lengths. 2. Trenches serving up to 700 square feet of roof area may be simple 10- foot-long by 2-foot wide gravel filled trenches as shown in Figure 3.1.5. For roof areas larger than 700 square feet, a dispersion trench with notched grade board as shown in Figure 3.1.6 or alternative material approved by the Local Plan Approval Authority may be used. The total length of this design must not exceed 50 feet and must provide at least 10 feet of trench per 700 square feet of roof area. 3. Maintain a setback of at least 5 feet between any edge of the trench and any structure or property line. 4. No erosion or flooding of downstream properties may result. 5. Have a geotechnical engineer or a licensed geologist, hydrogeologist, or engineering geologist evaluate runoff discharged towards landslide hazard areas. Don not place the discharge point on or above slopes greater than 15%or above erosion hazard areas without evaluation by a geotechnical engineer or qualified geologist and jurisdiction approval. 6. For purposes of maintaining adequate separation of flows discharged from adjacent dispersion devices,the outer edge of the vegetated flowpath segment for the dispersion trench must not overlap with other flowpath segments, except those associated with sheet flow from a non- native pervious surface. Volume III—Hydrologic Analysis and Flow Control BMPs—August 2012 3-12 • • •° De. p °9f level outlet w p • , °°o° 9eon, . ° 4 •p V 44 4 odoo yn4•dA ,0 4• ° o0490 • 175, o q4 0••• - o)no ;6nth o °; ;na eA . ., to 4*pelf pipe 18*min o ago oaopa°400°0. oo° V 094°n°o4a`dam?-; 11/2*-e/4•washed rock od p ed.a o o�e•oa•GaO�e oee° 4 4 0veAeeQa n°c 24*min TRENCH X-SECTION PITS slope--► smal catch basin or yard drain n 12'x 10' level trenches 5700 sq.ft. TypeICB >700 sq.ft. standard u dispersion trench w/notched grade board length 10'per 700 sq.ft. of roof area. PLAN VIEW OF ROOF I Figure 3.1.5-Typical Downspout Dispersion Trench Source: King County Volume III—Hydrologic Analysis and Flow Control BMPs—August 2012 3-13 • . y. pipe O.D. 1'-0 1'-0 inch rtda a rxi cap or plug bay to second dispersal trench clean out wye from pipe if necessary le ye% 4"or5"perbrared pipe lad flatievel notched . grade board 2'X2'notches type I C B w i s e r lid cover(l o c k i n g) II iB'O.C. A " type I i I uukalid c over iiiiiiiiit 44— b ■ a infiuent pipe (max design la I 1 fbw50.5CFS per trench) -q 4• 1 i 4 • r clean outw1e from pipe i il lit fbw to other branching CB's P LAN as necessary NTS pipe OD. I 18)0.C. I galvanized bolts 1`-0 1`.0 2" 14* Mill n .- 'eat., r . 2' grade 2'x12' ` , ` r p1essure *- / 11 , 'a ¢ board ca tches 2" treated grade . "`-`., — • • wore?'perforated board N ; 0 4 ,, N pipe laid flat ,' ' „e s, NOTES : 4"x 4 support past it. 451 dean((�ii nes) 1.This trench shall be constructed so X 11 s • 4 6 a 1'/ '•'ck"washed rock as t7 prevent point dab barge andb r **r�:r 0,4 4' �;"N erosion. fitter is�bric a - 2.Trenches may be placed no cbser than 50 feet b one another.(100 feet "15% rnexf r flow contnoltwaker quelity abng fbwGne) treatment in m ai areas. 3.Trench and grade de board must be ieuel.Align to follow contours of si4e. SECTION A.A 4.Support post spacing as required by NTS sal condit ans to ensure grade board remains level. Figure 3.1.6-Standard Dispersion Trench with Notched Grade Board Volume III—Hydrologic Analysis and Flow Control BMPs—August 2012 3-14 I . /' • • Design Criteria for Splashblocks A typical downspout splashblock is shown in Figure 3.1.7. In general, if the ground is sloped away from the foundation and there is adequate vegetation and area for effective dispersion, splashblocks will adequately disperse storm runoff. If the ground is fairly level, if the structure includes a basement, or if foundation drains are proposed, splashblocks with downspout extensions may be a better choice because the discharge point is moved away from the foundation. Downspout extensions can include piping to a splashblock/discharge point a considerable distance from the downspout, as long as the runoff can travel through a well-vegetated area as described below. The following apply to the use of splashblocks: 1. Maintain a vegetated flowpath of at least 50 feet between the discharge point and any property line, structure, slope steeper than 15%, stream, wetland, lake, or other impervious surface. Sensitive area buffers may count toward flowpath lengths. 2. A maximum of 700 square feet of roof area may drain to each splashblock. 3. For purposes of maintaining adequate separation of flows discharged from adjacent dispersion devices,the vegetated flowpath segment for the splashblock must not overlap with other flowpath segments, except those associated with sheet flow from a non-native pervious surface. 4. Place a splashblock or a pad of crushed rock(2 feet wide by 3 feet long by 6 inches deep)at each downspout discharge point. 5. No erosion or flooding of downstream properties may result. 6. Have a geotechnical engineer or a licensed geologist, hydrogeologist, or engineering geologist evaluate runoff discharged towards landslide hazard areas. Do not place Splashblocks on or above slopes greater than 15%or above erosion hazard areas without evaluation by a professional engineer with geotechnical expertise or a licensed geologist, hydrogeologist, or engineering geologist, and approval by the Local Plan Approval Authority. 7. For sites with septic systems, the discharge point must be downslope of the primary and reserve drainfield areas. This requirement may be waived if site topography clearly prohibits flows from intersecting the drainfield or where site conditions(soil permeability, distance between systems, etc)indicate that this is unnecessary. Volume III—Hydrologic Analysis and Flow Control BMPs—August 2012 3-15 • • I house II I 1 Roof W I. I downspout F I serves up to I I 700 sf. Of roof I..... 50' min. 1 Vegetated ° °�......) flow path 1.1111 i J Splash 1 1 r block I 4ti - 5. :1 _ ti1 r Downspout extension NTS Splash block Figure 3.1.7-Typical Downspout Splashblock Dispersion IMITITIMIWITEIrrit Vc vine III—Hydrologic Analysis and Flow Control BMPs—August 2012 3-16