BIO. Post Construction Stormwater Control Practices. PTP-10 Bioretention Systems. Bowling Green, KY Stormwater Best Management Practices BIO

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1 3.4 POST CONSTRUCTION STORMWATER CONTROL FACT SHEETS (PTP) Pst Cnstructin Strmwater Cntrl Practices PTP-10 Biretentin Systems BIO Symbl BIO TSS Reductin: 80% Descriptin Biretentin systems are structural water quality cntrl devices that capture and temprarily stre, treat, and release strmwater runff. A prperly designed area will replicate a small, dense frest flr. Biretentin systems cnsist f tw main cmpnents: a pretreatment area and filtratin chamber. The pretreatment area remves flatable materials and heavy sediments, and helps reduce flw velcities. The filtratin chamber traps and strains pllutants, and allws the micrbial remval f pllutants. Target pllutants fr biretentin systems include suspended slids, suspended particulates, bichemical xygen demand (BOD), fecal clifrm bacteria, and thers. Biretentin systems emply rganic materials such as peat r cmpst cmbined with sand, and plantings and mulch n the surface layer. This allws additinal pllutant remval via bacterial decmpsitin and vegetatin uptake f nutrients. The tw main structures f biretentin systems (the pretreatment area and filtratin area) may include r be enhanced by the fllwing cmpnents: Grass filter strip Sand bed Pnding area r pretreatment basin Organic layer Planting sil layer Plant material Underdrain/cllectin system 3-356

2 Applicatins Biretentin systems are ften used t manage strmwater runff frm urban areas where space is limited, and can be applied t areas where retrfit is needed, and are typically suitable in the fllwing applicatins: Small stabilized drainage areas Drainage areas with high impervius cver Off-line facilities adjacent t parking lts Alng rad drainage swales Within larger landscaped pervius areas Landscaped islands in impervius r high-density envirnments (i.e. parking lts) Retrfitting exiting parking lt islands/ff-line facilities Biretentin systems are nt suitable in the fllwing applicatins: Within drainage areas that have nt been stabilized Areas with mature trees Adjacent t areas with slpes greater than 5:1 (H:V) Areas that experience cntinuus flw frm surface water, grundwater, sump pumps, r ther surces Biretentin systems shuld nly be applied t stabilized drainage areas, as heavy sediment lads frm cnstructin areas will clg and disable it. Likewise, they shuld nt be used in areas where strmwater has ptential fr high silt r clay cntent, and areas with a high water table. As a guide, sites implementing biretentin systems shuld have ver 50% impervius cver in the drainage area. Biretentin systems shuld typically be designed fr ff-line use t capture the first flush f runff. A diversin structure such as a flw splitter r weir may be necessary t separate and rute the first flush t the biretentin system fr water quality cntrl, and rute the remaining strmwater t a water quantity cntrl device dwnstream. Other ptins include an verflw structure than carries flws larger than the water quality treatment requirement. Biretentin systems are mst effective when turbulent flw is minimized and the flw is spread unifrmly acrss the surface area. Biretentin is best emplyed clse t the surce f runff generatin and is ften lcated in the upstream prtin f the strmwater treatment train, with additinal strmwater BMPs fllwing dwnstream. Strng cnsideratin shuld be given t multiple smaller biretentin system rather than ne large biretentin system

3 Biretentin System Variatins FILTRATION/PARTIAL RECHARGE FACILITY Figure PTP-10-1 Filtratin/Partial Recharge Facility Surce, Minnesta Strmwater Manual This type f facility is suitable fr areas where high filtratin and partial recharge f runff wuld be beneficial. This facility is designed with an under-drain at the invert f the planting sil mix t ensure that the facility drains at a desired rate. The facility allws fr partial recharge, as an impervius liner is nt used. The depth is als shallw (2.5 ) t allw the facility t handle high capacity flws if necessary. Siting f this perfrmance type is suitable fr visually prminent r gateway lcatins in a cmmunity. The facility type is suitable fr areas and land uses that are expected t generate nutrient and metals ladings (residential, business campus, r parking lts). Attentin t mulch type and amunt will ensure the adequate treatment f the anticipated ladings. The facility shwn abve incrprates a filter material between the gravel blanket arund the under-drain and the planting sil abve. The filter fabric des nt need t extend t the side walls. The filter fabric may be installed hrizntally abve the gravel blanketextending just 1-2 feet n either side f the under-drain pipe belw. D nt wrap the underdrain with filter fabric. Instead f using a filter fabric, the designer may pt t utilize a pea gravel diaphragm ver the under-drain gravel blanket. This type f facility is als recmmended fr tight impermeable sils where infiltratin is limited. Sme vlume reductin will be seen frm evaptranspiratin

4 Biretentin System Variatins INFILTRATION/FILTRATION/RECHARGE FACILITY Figure PTP-10-2 Infiltratin/Filtratin/Recharge Facility Surce, Minnesta Strmwater Manual This type f facility is recmmended fr areas where higher nutrient ladings (particularly nitrates) are anticipated. The facility is designed t incrprate a fluctuating aerbic/anaerbic zne belw the raised under-drain discharge pipe. This fluctuatin created by saturatin and infiltratin int the surrunding sils will achieve de-nitrificatin. With a cmbinatin f a fresh mulch cvering, nitrates will be mitigated thrugh the enhancement f natural denitrificatin prcesses. This type f facility wuld be suitable fr areas where nitrate ladings are typically a prblem (residential cmmunities). The raised under-drain has the effect f prviding a strage area belw the invert f the under-drain discharge pipe. This area prvides a recharge zne and quantity cntrl can als be augmented with this strage area. The strage area is equal the vid space f the material used

5 Biretentin System Variatins FILTRATION ONLY FACILITY Figure PTP-10-3 Filtratin Only Facility Surce, Minnesta Strmwater Manual This type f facility is recmmended fr areas that are knwn as ptential strmwater htspts (gas statins, transfer sites, and transprtatin depts). An imprtant feature f this type f facility is the impervius liner designed t reduce r eliminate the pssibility f grund water cntaminatin. The facility prvides a level f treatment strictly thrugh filtratin prcesses that ccur when the runff mves thrugh the sil material t the underdrain discharge pint. In the event f an accidental spill, the under-drain can be blcked and the bjectinable materials siphned thrugh the bservatin well and safely cntained

6 Maintenance Maintenance access shuld be prvided fr apprpriate equipment, vehicles, and persnnel. Mnthly Remve trash r debris Inspect the biretentin system fr clgging Pruning and weeding t maintain appearance. Mulch replacement when ersin is evident. Semi-annually The planting sils shuld be tested fr ph t establish acidic levels. If the ph is belw 5.2, limestne shuld be applied. If the ph is abve 7.0 t 8.0, then irn sulfate plus sulfur can be added t reduce the ph. Annually Remve sediment as necessary Repair r replace any damaged structural parts Stabilize any erded areas Replace mulch ver the entire area. Replace gravel diaphragm if warranted every 2 t 3 years. As Needed Inspect inflw pints fr clgging (ff-line systems). Remve any sediment. Inspect grass filter strip/grass channel fr ersin r gullying. Re-seed r sd as necessary. Trees and shrubs shuld be inspected t evaluate their health and remve any dead r severely diseased vegetatin. Pnding f water n the surface fr mre than 48 hurs indicates that the filtering capacity is substantially diminished. Replace mulch layer by remving the tp few inches that cntain sediment. Cre aeratin r cultivating f unvegetated areas may als be required t ensure adequate filtratin. The remved sediment shuld be dispsed f prperly, such as in a landfill. Silt r sediment shuld be remved frm the biretentin system at the accumulatin f apprximately 2 inches. Prperly dispse f any material generated during maintenance activities

7 Inspectin Checklist Mnthly Cntributing area, facility, inlets, and utlets are clear f debris Cntributing area is stabilized and mwed, with clippings bagged r remved Treatment area is nt clgging als inspect after mderate/majr strm events Activities in the drainage area minimize il/grease and sediment entering the system Standing water is nt present N ersin is present in the biretentin system Pretreatment area shws n evidence r ersin Depsitin f sediment shuld be n mre than 2 inches befre it is cleaned ut Annually Treatment area cntains n mre than 2 inches f sediment N evidence f deteriratin, spalling, r cracking is present n cncrete, if present Inspect grates, where applicable Inlets, utlets, and verflw spillways r diversin structures shw n evidence f ersin r deteriratin Flw is nt bypassing the biretentin system Wetland vegetatin is nt present in the biretentin area (signifies pr drainage) Figure PTP-10-4 Biretentin System Surce, Maryland Department f Natural Resurces,

8 Criteria The size f the drainage area typically dictates the size f the biretentin practice. These areas shuld be limited t a maximum cntributing drainage area f five (5) acres. One-half (0.5) t tw (2) acre drainage areas are preferred. Multiple biretentin areas may be required fr larger drainage areas. N mre than 50 percent f the drainage area can be pervius. Slped areas immediately adjacent t the biretentin system shuld be n greater than 5:1 (H:V) nr less than 1% t prmte psitive flw tward the system. Biretentin systems shuld be sized based n the principles f Darcy s Law, as shwn in the Prcedures sectin. Hwever, the minimum size f a biretentin system is 200 square feet (equivalent t 10-feet wide and 20 feet lng). The biretentin system surface slpe shuld nt exceed 1%, t prmte even distributin f flw thrughut the system. The maximum side slpes fr a biretentin system is 3:1 (H:V). Planting sils shuld cntain less than 5% clay by vlume. Additinal specificatins fr sils are utlined in the Cmpnents sectin. Where feasible pnding depths shuld be n greater than 6 inches. The maximum allwable pling depth is 18 inches. The biretentin system shuld be designed such that it is drained within 48 hurs frm the peak water level in the system. Biretentin systems require pre-treatment and as many pretreatment cmpnents as feasible shuld be incrprated. Pretreatment cmpnents are described belw. Fr applicatins where runff enters the biretentin system thrugh sheet flw, such as frm parking lts, r residential back yards, a grass filter strip with a gravel diaphragm is the preferred methd f pretreatment. Fr applicatins where cncentrated (r channelized) runff enters the biretentin system, such as thrugh a sltted curb pening, a grassed channel with a gravel diaphragm is the preferred methd f pretreatment. Underdrains are required in biretentin systems t carry flw t anther cnveyance element. The underdrains shuld be equipped with a minimum 8-inch perfrated PVC pipe surrunded by a 12-inch thick gravel layer. The underdrain can be installed at the bttm f the strage area r at an elevatin abve the bttm f the strage area, depending n the treatment gals fr the system. When designing the underdrain, infiltratin f the in situ sils shuld nt be cnsidered. Zer drawdwn thrugh the in situ sils shuld be assumed. The underdrain system must be sized t drain the entire water quality vlume (WQ v) within 48hrs The elevatin difference frm the inflw t the utflw must be 4-6 feet. A minimum f 3 feet (5 feet recmmended) f separatin must be prvided between the bttm f the biretentin system and seasnally saturated sils. Ptential fr ersin f stabilized areas and the biretentin system shuld be evaluated. Biretentin systems must have a detailed landscaping plan

9 Cmpnents Pre-treatment Pre-treatment areas capture and remve carse sediment particles frm runff prir t discharging int the biretentin area. Incrpratin f pretreatment cmpnents helps t reduce the maintenance burden f biretentin, and reduces the likelihd that the planting sil layer will clg ver time. Gravel Diaphragm Lcated at the beginning f the grass buffer strip t reduce velcity f runff, filter particles in the strmwater, and spread flw acrss the grass buffer strip. Grass Buffer Strip/Grass Channel Reduces velcity f runff and filters particles in the strmwater. The length f the grass buffer strip depends n the drainage area, imperviusness, and the buffer strip slpe. When biretentin is used t treat runff frm parking lts r radways that are frequently sanded during snw events, grass buffer strips shuld be a minimum f 10 feet lng and grass channels a minimum f 20 feet lng. Cncrete Frebay r Curb Often biretentin areas are incrprated int parking lts and ther highly impervius areas. Curbs and/r cncrete frebays can be cnstructed t slw runff and allw larger slids t settle befre reaching the biretentin area. Curbs can simply have a lip set 1-2 inches abve the parking lt elevatin and effectively settle large particles. A cncrete frebay can als be installed t aid maintenance and cleaning. Pnding Area r Pretreatment Basin Runff is detained t settle particulates suspended in strmwater. Treatment Surface/Pnding Area The surface area f all infiltratin based biretentin systems is a functin f the infiltratin capacity f the underlying sils. The surface area f all filtratin based biretentin practices is a functin f the filtratin capacity f the sil medium and underdrain. Pnding depths shuld be kept t a minimum t reduce hydraulic verlad f in-situ/planting sils and t maximize the surface area t system depth rati, where space allws. It is recmmended that apprximately 5-10% f the tributary impervius area be dedicated t the biretentin system ftprint. Organic Layer A layer f mulch filters pllutants ut f the strmwater and prtects sil frm erding. The layer can als sustain a nutrient rich envirnment with micrbes that can break dwn petrleum-based cntaminants. The layer shuld cntain apprximately 2 t 3 inches fresh shredded bark mulch, when pssible, t maximize nitrgen retentin. If aged mulch is used, use the shredded type instead f the chip variety t minimize flating actin. T much mulch can restrict xygen flw t rts

10 Cmpnents Planting Sil Layer This layer is used t prvide nutrients and stre water fr the area s plantings. The planting sil shuld be a well blended, hmgenus mixture f 50-60% cnstructin sand, 20-30% tp sil, and 20-30% rganic leaf cmpst. This blend is necessary t prvide a planting sil layer with a high infiltratin/filtratin capacity. Field experiments shw that pllutant remval is accmplished within the tp 30 f sil depth with minimal additinal remval beynd that depth (Prince Gerge s Cunty, 2002). Therefre, the recmmended depth f the prepared sil is 30 inches. Hwever, if large trees are preferred in the design, a sil depth f shuld be utilized. The sil depth generally depends upn the rt depth f the prescribed vegetatin and cntent f underlying sils. Clay material can absrb heavy metals, hydrcarbns and ther pllutants. Hwever, clay shuld be mixed with sand r tpsil such that the planting sil layer has a clay cntent f less than 5%. Additinally, the design permeability rate thrugh the planting sil bed shuld be high enugh t fully drain the strmwater quality design strm runff within 48hrs. It is recmmended that this permeability rate be determined by field testing. The planting sil shuld have a ph ranging frm 5.5 t 6.5. Plant Material Cnsider surrunding envirnment, climate, maintenance requirements and types f pllutants that the plants must withstand and treat, while maintaining a psitive aesthetic enhancement. Underdrain/Cllectin System Necessary t cllect and send flws t a strmwater cnveyance system. This system shuld cntain a minimum 8-inch perfrated PVC pipe surrunded by a 12-inch thick gravel layer. The gravel shall be washed and 1-1/2 in size. Increasing the diameter f the underdrain makes freezing less likely, and prvides a greater capacity t drain standing water frm the system. Pipe perfratins shuld be sized apprximately 3/8 inch in diameter spaced at 6-inch intervals n center. At a minimum, 4 hles per rw shuld be used, and pipe grade placement shuld be at least 0.5%. Pipes shuld be spaced n mre than 10 feet n center. The prus gravel layer prevents standing water in the system by prmting drainage. Gravel is als less susceptible t frst heaving than finer grain media. A pea gravel diaphragm and/r permeable filter fabric shuld be placed between the gravel layer and the planting sil layer

11 Landscaping Impervius area cnstructin must be cmpleted and a dense and vigrus vegetative cver shuld be established ver the cntributing pervius drainage areas BEFORE runff can be accepted int the biretentin system. Cnsult with a landscaping prfessinal t select vegetatin which fits int the landscape, is apprpriate fr the hardiness zne, and can tlerate cnditins fund in biretentin areas (shrt duratins f 6 inch pnding water). Vegetatin shuld be selected based n specified zne f hydric tlerance. The biretentin area shuld be vegetated t resemble a terrestrial frest ecsystem, with a mature tree canpy, sub canpy f under stry trees, shrub layer, and herbaceus grund cver. Three species each f bth trees and shrubs are recmmended t be planted. Many biretentin systems feature wild flwers and grasses in additin t trees and shrubs. Other typical landscape plants can be used, such as day lilies, landscape grasses, r ther native plantings. The tree-t-shrub rati shuld be 2:1 t 3:1. On average, the trees shuld be spaced 8 feet apart. Plants shuld be placed at regular intervals t replicate a natural frest. Wdy vegetatin shuld nt be specified at inflw lcatins. Trees shuld nt be planted directly ver tp f underdrains and may be best lcated alng the perimeter f the system. After the trees and shrubs are established, the grund cver and mulch shuld be established. Mulch shuld nt be munded arund the base f plants since this encurages damage frm pests and diseases. Salt resistant vegetatin shuld be used in lcatins with prbable adjacent salt applicatins, i.e. radside, parking lt, etc. Chse plants based n factrs such as resistance t drught and inundatin, cst, aesthetics, maintenance, etc. Native plant species shuld be specified ver nnnative species. Fluctuating water levels fllwing seeding (prir t germinatin) can cause seed t flat and be transprted. Seed is als difficult t establish thrugh mulch, a cmmn surface cmpnent f biretentin systems. It may take up t tw grwing seasns t establish the functin and desired aesthetic f mature vegetatin via seeding. Therefre mature plantings are recmmended ver seed. If a minimum cverage f 50% is nt achieved after the first grwing seasn, a reinfrcement planting is required. Biretentin system lcatins shuld be integrated int the site planning prcess, and aesthetic cnsideratins shuld be taken int accunt in the siting and design

12 Prcedure Step 1 Make a preliminary judgment as t whether site cnditins are apprpriate fr the use f a biretentin system, and identify the functin f biretentin in the verall treatment system. Cnsider basic issues fr initial suitability screening, including: Site drainage area Site tpgraphy and slpes Sil infiltratin capacity Site lcatin/minimum setbacks Presence f active karst features Determine hw the biretentin system will fit int the verall strmwater treatment system. Decide whether the biretentin system is the nly BMP t be emplyed, r if there are ther BMPs addressing sme f the treatment requirements. Decide where n the site the biretentin system is mst likely t be lcated. Step 2 Cnfirm design criteria, site cnstraints, and applicability. Determine the design criteria that will be used. Determine any cnstraints the site will place n the biretentin system such as: High pervius area in the drainage area Limited amunt f surface area available fr treatment High water table Water surface elevatin in any dwnstream treatment practices r cnveyance Ensure that strmwater runff frm impervius surfaces is being treated t the 80% TSS reductin standard. The equatin fr determining the weighted TSS reductin fr a site with multiple utlet pints is belw. Where: % TSS 1 ( TSS A TSS n n ( A A A... TSS 2... A ) n n A ) TSS 1 = TSS reductin by BMP prviding treatment fr A 1 A 1 = area 1, (acres) TSS 2 = TSS reductin by BMP prviding treatment fr A 2 A 2 = area 2, (acres) n Where ne BMP discharges int anther, the treatment train TSS reductin can be fund by the fllwing equatin: ( A B) TSStrain A B

13 Prcedure Where: TSS train = ttal TSS reductin thrugh successive BMPs A = TSS reductin thrugh first BMP B = TSS reductin thrugh secnd BMP Step 3 Perfrm field verificatin f site suitability. Determine the depth t grundwater. A minimum f 3 feet f separatin between the bttm f the biretentin system and seasnally saturated sils (r frm bedrck) is required (5 feet recmmended). The field verificatin shuld be cnducted by a qualified getechnical prfessinal. If the initial evaluatin indicates that a biretentin practice wuld be a gd BMP fr the site, it is recmmenced that sil brings r pits be dug (in the same lcatin as the prpsed biretentin practice) t verify sil types and infiltratin capacity characteristics and t determine te depth t grund water and bedrck. The number f sil brings shuld be selected as needed t determine lcal sil cnditins. It is recmmended that the minimum depth f the sil brings r pits be five feet belw the bttm elevatin f the prpsed biretentin system. Step 4 Cmpute runff cntrl vlumes and peak flws. Calculate the Water Quality Vlume (WQ V), peak runff fr the 25 year strm (Q P25), and the peak runff fr the 100 year strm (Q P100). Refer t Sectin 2 fr mre infrmatin strmwater quantity design. The required water quality treatment vlume is 1.1 inches f runff frm the new impervius surfaces created frm the prject. Determine Water Quality Vlume (WQ V). Where: P R V A WQ v = [P R v)(a)]/12 = is the average rainfall, (inches) = (I), where I is the percent impervius cver = the area f imperviusness, (acres) Calculate the peak flws fr Q P25 and Q P100 t meet detentin requirements

14 Prcedure Nte: Steps 5-8 are iterative Step 5 Determine biretentin type and size. Select type f biretentin basin after cmpletin f the previus steps the designer shuld knw the depth t the water table, bedrck r ther impermeable layers, and the cntributing drainage area. Determine Water Quality Vlume (WQ V) fr biretentin system. If part f the verall WQ V is t be treated by ther BMPs, subtract that prtin frm the WQ V t determine the part f the WQ V t be treated by the biretentin system. If the biretentin system has an underdrain the vlume f vids in the underdrain system shuld be subtracted frm the WQ v. The vlume f vids shuld be estimated at 35% f the ttal vlume f the underdrain system. Based n the knwn WQ V, infiltratin rates f the underlying sils and the knwn existing ptential pllutant lading frm prpsed/existing landuse select the apprpriate biretentin type (see Sectin 2.6). Size Biretentin System With An Underdrain The biretentin surface area is cmputed using the fllwing equatin, fr thse systems that are designed with an underdrain: Where: A f = (WQ V x d f) / [k x (h f + d f) x t f] A f = surface area f biretentin system, (ft 2 ) WQ V = water quality vlume, (ft 3 ) d f = filter bed depth, (ft) k = cefficient f permeability f filter media, (ft/day) (0.5 ft/day is the recmmended k fr planting medium / filter media sil. This value is cnservative t accunt fr clgging assciated with accumulated sediment.) h f = average height f water abve filter bed, (ft) t f = design filter bed drain time, (days) (48 hurs is the required maximum t f fr biretentin) STEP 6 Size utlet structure and/r flw diversin structure, if needed. It is required that a secndary utlet be incrprated int the design f a biretentin system t safely cnvey excess strmwater. Strmwater quantity requirements can be fund in Sectin Ptential fr ersin t stabilized areas and biretentin system shuld be evaluated and the design shuld incrprate ways t mitigate ersive flws

15 Prcedure STEP 7 Determine pre-treatment vlume and design pre-treatment measures. Sme frm f pre-treatment is required prir t the discharge f strmwater int the biretentin system, t remve any sediment and fines that may result in clgging f the sils in the treatment area. Grass filter strips shuld be sized based n the fllwing table. Table PTP-10-1 Grass Filter Strip Sizing (Minnesta Strmwater Manual) Parameter Impervius Parking Lts Residential Lawns Maximum Inflw Apprach Length (ft) Filter Strip Slpe 2% >2% 2% >2 2% >2% 2% >2% Filter Strip Minimum Length Figure PTP Filter Strip Parameters Grass channels shuld be a minimum f 20 feet in length and designed accrding t the fllwing. Parablic r trapezidal crss-sectin with bttm widths between 2 and 8 feet. Channel side slpes n steeper than 3:1 (H:V) Flw velcities limited t 1 ft per secnd r less fr peak flw assciated with the water quality event strm. Flw depth f 4 inches r less fr peak flw assciated with the water quality event strm. STEP 9 Prepare vegetatin and landscaping plan Prepare vegetatin and landscaping management plan based n the guidance given in the Landscaping Sectin. STEP 10 Prepare peratins and maintenance plan Prepare peratins and maintenance plan based n the guidance given in the Maintenance Sectin

16 Prcedure STEP 11 Cmplete the Summary Table. Parameter WQ v Underdrain strage, Su Treatment area, Af Pnding depth Treatment area (LxW) Required Size Actual Size 3-371

17 Example Prpsed develpment f an undevelped site int an ffice building and assciated parking. Base Data Ttal Drainage Area = 5 ac Site Area = 3.54 ac Sils Type C Pre-Develpment Impervius Area = 0 ac; r I = 0% Meadw (CN = 71) Pst-Develpment Impervius Area = 1.72 ac; r I = 1.72/3.54 = 49% Open Space, Fair (CN = 79) Paved parking lts, rfs, driveways, etc. (CN =98) Hydrlgic Data Pre Pst CN WQ v Depth = 1.1 in Precipitatin 2yr, 24hr 3.54 in 25yr, 24hr 5.88 in 100yr, 24hr 7.43 in 3-372

18 Example This example fcuses n the design f a biretentin facility t meet the water quality treatment requirements f the site. In general, the primary functin f biretentin is t prvide water quality treatment and nt large strm attenuatin. As such, flws in excess f the water quality vlume are typically ruted t bypass the facility r nn-ersively pass thrugh the facility. Where quantity cntrl is required, the bypassed flws can be ruted t cnventinal detentin basins (r sme ther facility such as undergrund strage vaults). Prblem: a water quality treatment plan fr this site. A dry detentin pnd will be cnstructed t meet the required detentin standards and will prvide 60% TSS reductin fr the site. The ttal drainage area t the pnd is 5 ac. Include multiple biretentin systems in the landscape islands in the parking area t meet the water quality gal f 80% TSS reductin. Step 1 Make a preliminary judgment as t whether site cnditins are apprpriate fr the use f a biretentin system, and identify the functin f biretentin in the verall treatment system. Cnsider basic issues fr initial suitability screening, including: The site has type C sils There are n minimum setbacks There are active karst areas n the site. Biretentin systems will nt be lcated clse t the sinkhle. Determine hw biretentin system will fit int the verall strmwater treatment system. Biretentin systems will be cnstructed in cmbinatin with a dry detentin pnd fr water quality and quantity cntrl n the site. f the dry detentin pnd can be fund in Sectin 4.8. Landscaping islands in the parking lt are likely spaces fr biretentin systems. The treated water quality vlume will be cllected by an underdrain and ruted t the dry pnd lcated in the nrthwest crner f the site fr water quantity cntrl. Flws greater than the water quality vlume will bypass the biretentin systems and be ruted t the dry pnd fr water quantity cntrl and treatment prir t discharging. Step 2 Cnfirm design criteria, site cnstraints, and applicability. The fllwing minimum criteria will be used in the design. Minimum 200 sq ft f surface area Maximum 6 in pnding depth Maximum 48hr drain time frm peak water level Minimum 8 in underdrain envelped in a 12 in gravel layer Minimum 3 ft separatin frm bttm t seasnally saturated sils Determine any cnstraints the site will place n the biretentin system: D nt place biretentin near sinkhle 3-373

19 Example Ensure that strmwater runff frm impervius surfaces is being treated t the 80% TSS reductin standard. Determining the weighted TSS reductin. Biretentin Systems have an 80% TSS reductin, and all strmwater runff frm impervius surfaces flw thrugh ne f three biretentin facilities. Therefre, a weighted TSS calculatin is nt necessary. Determine the treatment train TSS reductin. After the water quality vlume is treated by biretentin, it is then treated in the dry pnd befre leaving the site. Biretentin Systems have an 80% TSS reductin. Dry pnds have a 60% TSS reductin. TSStrain Where: A = 80% B = 60% TSStrain TSStrain 92% ( A B) A B 100 (80 60) Step 3 Perfrm field verificatin f site suitability. The biretentin systems will be designed as filtratin BMPs, with the full WQv discharging within 48 hrs thrugh the underdrain. Therefre, the nly field testing required is t determine the high water elevatin under the BMP lcatins. Field sil tests shw the high water elevatin t be 8 feet r mre belw the parking lt. Step 4 Cmpute runff cntrl vlumes and peak flws. Calculate the Water Quality Vlume (WQ V), Peak Flw Vlume (V P25), and the Extreme Fld Vlume (V P100). Where: WQv = [P Rv)(A)]/12 P = 1.1 inches Rv = (I) I = 49 Rv = (49) = A = 1.72 acres Ttal Water Quality Vlume: WQv = (1.1in x x 1.72ac)/12 = acre-ft = 3372 ft

20 Example The pre- and pst develpment vlumes fr bth 25-yr and 100-yr 24-hur return frequency strms shuld be calculated t determine the required water quantity cntrls. See Appendix B fr mre infrmatin regarding detentin and quantity design. Calculate the pre- and pst-develpment peak flws fr 25 yr (Q p25) and 100 yr (Q p100) strms fr the design f flw diversins, utlet structures, and verflw structures

21 Example Nte: Steps 5-8 are iterative Step 5 Determine biretentin type and size. Select type f biretentin basin The biretentin system will treat the entire water quality vlume. The biretentin system will include an underdrain/cllectin system. Determine Water Quality Vlume (WQ V) fr biretentin system. Biretentin System 1 A 1 = 0.70 ac; I = 100; WQ v = 3655 ft 3 Biretentin System 2 A 2 = 0.91 ac; I=100; WQ v = 3452 ft 3 Biretentin System 3 A 3 = 0.11 ac; I=100; WQ v = 417 ft 3 Size Biretentin System With An Underdrain Set pnding depth at 6 inches (h f) Set depth f the filter bed at 5 ft (d f) t drain in 48 hurs (t f) Assume 35% strage (S u) f WQ v in underdrain gravel layer Cmputed surface area Biretentin System 1 Assume Underdrain 70 ft lng S u = 0.35 x 70 ft x [0.5 x 1 ft x (8 ft +2 ft)] = ft 3 A f = [(WQ V - S u) x d f] / [k x (h f + d f) x t f] = [(3655 ft ft 3 )x 5 ft] / [0.5 ft/day x (0.5 ft + 5 ft) x 2 days) = 3211 ft 2 Biretentin System 2 Assume Underdrain 75 ft lng S u = 0.35 x 75ft x [0.5 x 1 ft x (8ft +2 ft)] = ft 3 A f = [(WQ V - S u) x d f] / [k x (h f + d f) x t f] = [(3452 ft ft 3 )x 5 ft] / [0.5 ft/day x (0.5 ft + 5 ft) x 2 days) = 3019 ft

22 Example Biretentin System 3 Assume Underdrain 25 ft lng S u = 0.35 x 25 ft x [0.5 x 1 ft x (8 ft +2 ft)] = ft 3 A f = [(WQ V - S u) x d f] / [k x (h f + d f) x t f] = [(417 ft ft 3 )x 5 ft] / [0.5 ft/day x (0.5 ft + 5 ft) x 2 days) = 339 ft 2 Biretentin System Crss Sectinal View 6 in - Pnding depth 5 ft - Planting layer (filter layer) Side Slpes 3:1 12 in - Gravel layer 2 ft Determine Dimensins f Biretentin System Biretentin System 1 A f = 3211 ft 2 Length = 120 ft; Width = 27 ft Biretentin System 2 A f = 3019 ft 2 Length = 80 ft; Width = 40 ft Biretentin System 3 A f = 339 ft 2 Length = 30 ft; Width = 12 ft STEP 7 Size utlet structure and/r flw diversin structure, if needed. A secndary utlet shuld be designed fr the biretentin systems t safely cnvey excess strmwater. Ptential fr ersin t stabilized areas and biretentin system shuld be evaluated and the design shuld incrprate ways t mitigate ersive flws

23 Example STEP 8 Determine pre-treatment vlume and design pre-treatment measures. Sme frm f pre-treatment is required prir t the discharge f strmwater int the biretentin system, t remve any sediment and fines that may result in clgging f the sils in the filtratin area. Biretentin System 1 will use curb cuts with lips raised 2 inches abve the pavement elevatin. Biretentin System 2 will use a Grass Filter Strip that is sized based n the Table PTP10-1. The maximum inflw length f 205 feet and the filter strip slpe is less than 2%. The grass filter strip shuld be 25 feet lng. There will als be a gravel diaphragm prir t the grass filter strip as an additinal pretreatment measure. Biretentin System 3 will use curb cuts with lips raised 2 inches abve the pavement elevatin. STEP 9 Prepare vegetatin and landscaping plan Prepare vegetatin and landscaping management plan based n the guidance given in the Landscaping Sectin. STEP 10 Prepare peratins and maintenance plan Prepare peratins and maintenance plan based n the guidance given in the Maintenance Sectin. STEP 11 Cmplete the Summary Table Biretentin Area 1 Required Size Parameter WQ v 3655 ft 3 Underdrain strage, Su Treatment area, Af Pnding depth Treatment area (LxW) ft 3 Actual Size 3211 ft ft 2 6 inches 120 x

24 Example Biretentin Area 2 Required Size Parameter WQ v 3452 ft 3 Underdrain strage, Su Treatment area, Af Pnding depth Treatment area (LxW) ft 3 Actual Size 3019 ft ft 2 6 inches 80 x40 Biretentin Area 3 Required Size Parameter WQ v 417 ft 3 Underdrain strage, Su Treatment area, Af Pnding depth Treatment area (LxW) ft 3 Actual Size 339 ft ft 2 6 inches 30 x