Bioretention Design, Installation, and Maintenance

Transcription

1 City of Atlanta Workshop August 16, 2013 Bioretention Design, Installation, and Maintenance Ryan Winston, M.S., P.E. Extension Associate Biological and Agricultural Engineering NC State University Atlanta, Georgia Hydrologic Cycle mwater Urban Water Cycle 1

2 Why Do We Care about Stormwater Management? Flooding: threatens human life & property Increased stormwater volume: accelerates stream erosion Increased pollutant load: impairs water quality for humans & other organisms Matt Hood 2008, with US permission EPA, 2011 Pace of Construction has Slowed The Water Budget: Predevelopment ET = 50% Runoff = 5% Infiltration = 45% Source: Swift et al

3 Postdevelopment(Urban) Hydrology ET = 30% Runoff = 55% IMPERVIOUS Infiltration = 15% Source: Tourbierand Westmacott 1981 What Can We Do? Enter the Stormwater Control Measure (SCM) Flooding Detainrunoff and release at controlled rate Increased stormwater volume Retainrunoff by infiltration or ET Increased pollutant load Treatcaptured runoff physiciochemically Permeable Pavement Bioretention Low Impact Development Principles 3

4 Rain garden Cistern r Pervious pavement LID Site: Commercial Bioretention Processes Source: NSCC Bioretention Guidelines (2008) Where can you find Bioretention/ Rain Gardens? 4

5 Huntersville, NC - Residential Residential North Shore Fat Street 5

6 8/18/2013 Skinny Street Rain Gardens Integrated throughout - Seattle In a Round-a-bout, Kapiti Coast, NZ 6

7 Filterra Bioretention System Ready Made Rain Garden A ready made example in Auckland NC DOT - Transportation 7

8 Prince George s County, MD Louisburg, NC - Joyner Park SE Queensland, Australia 8

9 Glencoe School Parking Lot retrofit - before Glencoe Parking Lot Infiltration landscape New River MCAS (NC) Officer s Club 9

10 Rain Garden installed into a planter box on a 100% impervious coverresidential mid-rise project in Old Town Alexandria. Roof-drains are within building façade. Integrating into High $ Landscapes Seattle, Washington Charlotte, NC - Downtown 10

11 Take Home Point Bioretention has a wide variety of applications from medium density residential to ultra urban Most applications treat either roof or vehicular (impermeable) surfaces Question: How does Bioretention Work? Bioretention Schematic Vegetation on Surface Runoff Permeable Media Fill Often Sandy Underdrain System 11

12 Bioretention Drawdown 2 ft -Draw down bowl in 12 hours -Draw Water Table Down 2 ft below surface in maximum of 2 days + 4 Hours +14 Hours Bioretention Design Objectives Peak Discharge Control 1-, 2-, 10-, 15-, 100-year storms Bioretention may provide part or all of this control Water Quality Control ½, 1 or 2 rainfall most frequently used Bioretention can provide 100% control Ground water recharge Many jurisdictions now require recharge ( e.g., MD, PA, NJ, VA, NC LID Guide) 12

13 Seasonally High Water Table Depths & Bioretention Seasonally High Water Tables. A Problem? (Coastal Areas and in Floodplains) Depends on Depth of Bioretention area Recommend: No W.T. within 1-2 ft of bottom B-R Area Q: How does Bioretention work: Pollutant Removal Mechanisms Sedimentation Trash, TSS, Phosphorus Microbial Processes Nitrogen Chemical Processes & Media Filtration Metals, Phosphorus Exposure to Sunlight & Dryness Pathogens, Oil & Grease Infiltration Where are pollutants removed? TSS TP Temp TN Pathogens Metals Oil & Grease 13

14 Q: How does Bioretention work: Hydrologic (Flow) Control Temporary surface storage Slow flow through porous media (peak flow control). Media with good field capacity means volume control, whether or not exfiltration is possible. Especially effective for small(ish), frequently occurring storm events typically little to no system discharge! Common Design Questions Question: Ponding Depths What is this depth? 14

15 Ponding? Maximum Ponding Depths NC DOT rest stop BR area, near Hickory, NC Design Point: Average Ponding Depth NCSU suggests 9-12 with inspection/ maintenance 12 is NC DENR Max 9 is NC DENR preferred 18 ponding depths have been used in some jurisdictions, primarily for peak flow control more research needed 15

16 A Comment on Deep Ponding Depths 36 ponding depth in interchange in Raleigh, NC Bowl Depth Particularly Important. Determines S.A. of BRC Bioretention Surface Area Surface area typically calculated based upon first flush volume (1 event) and average ponding depth Surface Area = Volume / Depth This does not include the forebay or pretreatment area 16

17 Question: Can Bioretention Areas be Grassed? Yes. May or may not function as well as shrub/mulch systems Questions to answer: Does a grass system supply enough organic material for microbial action to occur? Will a grass bioretention compact more quickly? Graham High School NC State University: Integrating into Landscape Best Grass Choices: Centipede (warm season) We Fescue/Bluegrass Bring Engineering to Life Mix (cool season) 17

18 Drive-Thru Events! (particularly with grassed bioretention) Q: How much Fill Soil Media Needed? Media Depth Major Cost Consideration What is this depth? 18

19 Fill Media Depth Predicated Upon 3 Factors Vegetation Health Hydrologic Goals Water Quality Needs Perhaps the most restrictive goal dictates design Bioretention Soil Depth: Vegetation Health Vegetation Depth (ft) Comments Grass 2.0 Minimum Shrubs/Trees 3.0 Minimum Shrubs/Trees Optimum Some native grasses may require deeper rooting depths. Some thoughts 1. Deeper cells may provide moisture reserves for extended dry periods. 2. Deeper cells provide runoff VOLUME reduction, regardless of in-situ soil condition or lining. Literature/Research Justification for Minimum Media Depths: WQ Pollutant Depth (ft) Studies TSS 1 Diblasi et al. 2009, Li et al Metals 1 Li and Davis 2008, Hatt et al Oil & Grease 1 Diblasi et al Phosphorus 2 (min); 3 (conservative) Hatt et al. 2009, Hsieh and Davis 2007, Passeport et al Nitrogen 3 Passeport et al Temperature 3 (min); 4 (optim) Jones and Hunt 2009 DON T FORGET HYDROLOGY deeper cells = greater potential for volume control 19

20 Media Selection Question: What s the Ideal Fill Media Composition? Infiltration Rate: Varies: Contact Time Temperature reduction and N removal More Contact Time 1-2 in/hr TSS, Fecal Coliform, High Sat K is OK > 6 in/hr Question: What s the Ideal Fill Media? Do you worry about Phosphorus? Soil P-Index: Important for Phosphorus Removal Range from (12-36 mg/kg P) Can be tested by NCDA for a nominal fee 20

21 Effluent TP Concentration vs. P-Index Site P-Index Depth (in) Outflow (mg/l) C L L O W L G G Take Home Point: Phosphorus Proper Media Selection is Critical With good media, TP sequestration is high. Most appears to occur in the upper layers of media With poor media selection, TP concentrations will increase through the BRC NC Fill Media Specification Ideal Mix 85-88% Sand 8-12% Fines (Clay+Silt) 3-5% Organics 21

22 Soil Texture 12% Fines (Clay) This relates to the recipe 8% Fines Underdrain Needed? Yes, in all locations outside Coastal Plain or Sandhills Yes, in most Coastal Plain & Sandhill Locations No, if in situ soil K > 4 in/hr Underdrain Configuration: Including an Upturn 22

23 New Design Guidance Retrofit Ease / Cost Savings Retrofit Ease / Cost Savings 23

24 How is an Internal Water Storage (IWS) Zone Created? Elevating the outlet with a 90 o PVC elbow to force water to be ponded in bottom layer. Looking down into the Overflow Basin The best $25 you ve ever spent! Rocky Mount BRCs 24

25 Water Balance Reduced Performance in Clayey insitu Soils, but still not bad Rocky Mount (sand): Upper Coastal Plain Greensboro (clay): Piedmont Graham: (N) loamy-clay & (S) sandy-loam Site # Events Monitored # Events w/ Outflow Media Depth (ft) IWS Depth (ft) RM Grass RM Mulched Greensboro Greensboro No IWS Graham (N) Graham (S) Revised Credits: Using IWS Layer Use IWS Layer in Coastal Plain & Sandhillswith A/B HSG soil 60% TN and 60% TP Removal Use IWS Layer in Piedmont & Mountains with B/C HSG soil 40% TN and 45% TP Removal 25

26 Media Depth & Separation: IWS Minimum Media Depth: 3 ft Separation: bottom of bowl top of IWS 12 in A, A/B Soils 18 in B/C, C Soils Modest benefit of IWS in HSG D Soils Should the Sides & Bottom of Bioretention Cells be Lined? Not usually, except Brownfield Construction Foundation preservation Research Maybe, if SHWT a concern Question: What Pretreatment is Needed? Bioretention NEEDS pretreatment, but not necessarily forebays. Limit velocity of inflow to 1 ft/sec 26

27 BRC Forebays Forebaysin Clay Soil: Line them Unlined Forebays provide Short Cut around treatment Pretreatment: Swales 27

28 Pretreatment? Take Home Design Point: Pre-treatment Include pretreatment (forebay, swale or filter strip) with Rain Garden Mulch Matters! 28

29 Double- or Triple-shredded Hardwood is Best Provides Moisture for Plants Benefits of Mulch? Pollutant Removal (hydrocarbons, metals, etc) Food Source for Microbes Aesthetic Appeal (if maintained) No Wood-based Float-proof mulch exists! Dense planting helps. Charlotte NC Bioretention Cell 29

30 Proper mulch thickness is 3! Do not over-mulch as it takes up bowl storage Take Home Point The Mulch and Upper Media Layers are critical for removal of Metals Hydrocarbons TSS Performance (for the above pollutants) appears to be independent of type of media used Excavation Technique to Enhance Infiltration Scoop vs. Rake For final 1 ft of excavation, depth most affected by compaction 30

31 Studies on Hydrology (mostly long-term) Mecklenburg Co. Hal Marshall Bioretention Cell ( ) First BRC in America studied for Fecal Coliform & E.Coli Soil 80% Mason Sand 20% Fines + Compost 4 ft (1.2 m) Depth Designed & Built by Mecklenburg Co. BIORETENTION HYDROLOGY (Hunt et al., 2008) 93 31

32 Swale and Rain Garden Treatment Train at Landcare Research: Hydrologic Performance Courtesy of Dr Sam Trowsdale, Landcare Research, March 2008 From an Individual Event Perspective, we see Peak Flow Mitigation (nearly all events) Total Volume Reduction (up to design storm ~1 inch) This Translates to Significant Long Term Hydrologic Benefits 10 NC & MD Data (Li et al., 2009) CP SS G1 G2 LB1 LB2 Volume Discharge Ratio, fv or fv Proportionally Smaller Cells Proportionally Larger Cells We Bring Engineering to Life Exceedance Probability

33 Three More Examples of Volume Abatement: MD, NC, PA 1 Volume Discharge Ratio, f v SS Villanova NC Exceedance Probability Jones and Hunt, Temperature ( C) /01/06 08/09/06 11/17/06 02/25/07 06/05/07 09/13/07 Median Inlet Median Outlet Max Inlet Max Outlet Ability of bioretention to exfiltrate water leads to reductions in thermal load Effluent reductions were greatest for bioretention media volumes larger with respect to their watershed Percentage of Watershed Area Events with Outflow Asheville 16% 12% Lenoir 4% 79% Brevard East 7% 76% Brevard West 11% 27% 33

34 Take Home Point From a Long Term Hydrology Perspective, Bioretention Cells Convert Lots of Runoff to Infiltration & Evapotranspiration Often more than 50% Depends on Several Factors Underlying Soil Media Volume & Type Relative Surface Area 140 Can Long Term Hydrologic Performance be Predicted? Silver Spring Outflow Volume (m^3) UMD 1:1 Regression UB LB Regression Slope = 0.72 x_intercept = % CI = Inflow Volume (m^3) Outflow Volume (m^3) Can Long Term Hydrologic Performance be Predicted? VTI 1:1 Regression UB LOW BAV AVE BAV HIGH BAV Inflow vs. Outflow for All Storm Events Villanova University Regression Slope = 0.96 x_intercept = % CI = Inflow Volume (m^3) 34

35 Outflow Volume (m^3) NC Grassed 1:1 Regression UB LB LOW BAV AVE BAV HIGH BAV NC Long Term Hydro Performance Inflow vs. Outflow for All Storm Events North Carolina Grassed Cell Regression Slope = 0.41 x_intercept = % CI = Inflow Volume (m^3) In Each Case, there is a rather clear Breakpoint where storms of a certain size are completely captured (no outflow) This volume is termed the Bioretention Abstraction Volume (BAV) Inflow vs. Outflow for All Storm Events Silver Spring Outflow Volume (m^3) UMD 1:1 Regression UB LB LOW BAV AVG BAV HIGH BAV Regression Slope = 0.72 x_intercept = % CI = Inflow Volume (m^3) 35

36 Another Way of Assessing Performance is Relating to Landscape Conditions In other words how much runoff does a given/target landscape produce? Characterizing the Landsape: Curve Numbers (USDA-NRCS) Soil Group Land Use A B C D Paved Parking Lots; Roofs Commercial & Bus. Distr Townhouses Residential Lot (1/2 AC) Residential Lot (1 AC) Open Space: grass > 75% Translating Rain Gardens into Landscape Detention: NC Outflow Volume (m^3) NC Grassed Cell Pavement Woods B (CN 55) Woods C (CN 70) Regression (CN 79) Rainfall (cm) 36

37 Outflow Volume (m^3) 8/18/2013 Translating Rain Gardens Into Landscape Detention: PA VU Bioinfiltration BMP Pavement Woods C (CN 70) Regression (CN 78) Rainfall (cm) Relating Rain Gardens to Landscapes: MD 350 UMD SS Silver Spring Outflow Volume (m^3) Pavement Woods C (CN 70) Regression (CN 75) Rainfall (cm) Effective Curve Numbers for the Three Sites Examined North Carolina Catchment CN Equivalent CN (post BRC) Target CN Maryland Pennsylvania

38 Studies on Water Quality Biochemical Pollutant Removal Mechanisms in a Bioretention Cell See bioretention info sheets on Cecil, US EPA BMP Design Guide Vol 2 Sect 7 (note error in equation 7-1: follow TP-10 formula), TP-10 Chp 7 Water Quality College Park Silver Spring Input Output Input Output 38

39 College Park (CP) Site Watershed Anacostia Year Built 2004 Watershed Size Surface to Drainage Area Ratio General Shape Ponding Depth Fill Media Depth Soil Texture b 0.26 ha 6% Trapezoidal 15 cm m Sandy loam Underdrain discharge to Stream Silver Spring (SS) Site Underdrain discharge to Storm Drain to Stream Watershed Anacostia Year Built 2006 Watershed Size Surface to Drainage Area Ratio General Shape Ponding Depth Fill Media Depth Soil Texture 0.45 ha 2% Triangle 30 cm 0.9 m Sandy clay loam Bioretention TSS (CP & SS) CP in CP out SS in SS out 200 TSS EMC (mg/l) No Flow/Below Limit Exceedance Probability Excellent treatment of TSS Li & Davis, J. Env. Eng

40 Bioretention PAH Field (CP) PAH concentration (ug/l) PAHs in dissolved phase CPI CPO 0 Naphthalene Acenaphthalene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benz(a)anthracene Chrysene Benzo(b+k)fluoranthene Benzo(a)pyrene Indeno91,2,3-cd)pyrene Benzo(g,h,i)perylene Dibenz(a,h)anthracene Diblasi, Li, Davis & Ghosh, Environ. Sci Technol., 2009 Lead (CP & SS) CP in CP out SS in SS out Lead EMC (ug/l) MD Chronic Fresh Water 2.5 C 2.5 C No Flow/Below Limit Toxicity Limit Exceedance Probability Lead is very well removed as evidenced by field bioretention data. Li & Davis, J. Env. Eng Zinc (CP & SS) 400 CP in CP out 400 Zinc EMC (ug/l) SS in SS out 200 MD Acute/ 120 Chronic Fresh 100 Water Toxicity Limit No Flow/Below Limit Exceedance Probability Zinc is very well removed as evidenced by field bioretention data. Li & Davis, J. Env. Eng

41 Copper (CP & SS) CP in CP out SS in SS out Copper EMC (ug/l) A 13 A C 9C MD Acute & Chronic Fresh Water Toxicity Limit No Flow/Below Limit Exceedance Probability Copper is removed, but not to the extent as Zinc and Lead. May be related to natural soil background levels or mobility of Cu by organic matter Li & Davis, J. Env. Eng Take Home Point The Mulch and Upper Media Layers are critical for removal of Metals Hydrocarbons TSS Performance appears to be independent of type of media used Not-so-Great Coastal signs 41

42 Mecklenburg Co. Hal Marshall Bioretention Cell ( ) Soil: 80% Mason Sand 20% Fines + Compost P-Index = 6 4 ft (1.2 m) Depth Hunt et al Bioretention Indicator Bacteria Indicator Bacteria - Charlotte Indicator Species Geometric Mean Inlet (# / 100 ml) Geometric Mean Outlet (# / 100 ml) Concentration Reduction (%) Fecal Coliform E. coli USEPA fresh waters Fecal coliform: 200 / 100 ml E. coli: 126 / 100 ml Hathaway et al. (2009) 42

43 Wilmington Bioretention Shallow = 1 ft media depth Deep = 2 ft media depth Cumulative Probability Inflow 0.2 Outflow - Deep Outflow - Shallow ,000 10, ,000 E. coli (MPN/100 ml) Performance Analysis E. coli enterococcus Location Concentration Reduction (%) Concentration (#/ 100ml) Concentration Reduction (%) Concentration (#/ 100ml) Inlet Bioretention-D Bioretention-S

44 Hydrology Shallow Cell Bigger Flux of H 2 O Take Home Points Soil depth matters Cost vs. performance balance Bioretention design can be manipulated for indicator bacteria removal Bioretention shows promise There are minimum bioretention depths 2 ft or greater seems safe Initial Research Focused on Nutrient Removal/Sequestration Particularly Important to Piedmont, but also to Coastal Plain Mass (Kg) Greensboro Data Hunt et al., TN NO3-N TKN TP In Out 44

45 Chapel Hill Cell, C1 STP/WS = 0.14 Conventional Drainage Annual Loadings ( ) Chapel Hill Cell C1 - Hunt et al., 2006 Mass (Kg) TN NO3-N TKN TP In Out Initial NCSU Research Relationship between P-Index (Soil Test P) and TP outflow load. Greensboro Chapel Hill TP +240% - 65% P-Index P-Index : High P- Index 0-25: Low (Hunt 2003) 45

46 Blame it on the Media Phosphorus Index (P- Index) is a measure of how much phosphorus is already in the soil media. Low P-Index: Can capture more phosphorus High P-Index: Soil is saturated with phosphorus Very High: > 100 High: Medium 25-50Low: 0-25 Mecklenburg Co. Hal Marshall Bioretention Cell ( ) Soil 80% Mason Sand 20% Fines + Compost P-Index = 6 4 ft (1.2 m) Depth TP Charlotte, NC ( ) Hunt et al., 2008 [TP] in mg/l Concentration Reduction = 31% Load Reduction 50% 1/1/04 7/1/04 12/30/04 6/30/05 12/29/05 Date TP-In TP-Out 46

47 Louisburg Bioretention Cells Soil Media: Nominally 0.75 m Deep 60% Sand 40% Ballfield Mix Low PI (1-2) fill 85% Sand 10% Fines 5% Organics Constructed Spring 2004 Load Reductions: Louisburg Cell TN TP L-1 (unlined) L-2 (lined) 64% 66% 68% 22% June February 2005 Overall Results: Phosphorus 100 Phosphorus 80 % Removal Box S1 Box S2 Box L Greenbelt Landover Bioretention Depth (cm) Phosphorus removal increases gradually with bioretention depth. Davis, Shokouhian, Sharma, Minami, Wat. Environ. Res

48 Mass Loads (kg/ha/yr) CP SS In Out In Out TSS Chromium ~0.007 Copper Lead ~0.005 Zinc Chloride TN Nitrate ~0.19 TKN TP TOC Li & Davis, J. Env. Eng Take Home Point: Phosphorus Proper Media Selection is Critical With good media, TP sequestration is high. Most appears to occur in the upper layers of media Nitrogen Removal ( ) Chapel Hill Cell C1 - Hunt et al., 2006 Mass (Kg) TN NO3-N TKN TP In Out 48

49 Graham High School ( ) Passeport et al Watershed area = 0.69 ha Bioretention Cells Area = 204m 2 Fill Media/ Soil 90% Expanded Slate Byproduct 10% Top Soil P-Index: Low 0.6 m & 0.9 m (2 & 3 feet) depth Both Cells Covered in Turf (Hybrid Bermuda) TN concentrations: Grassed Graham HS Bioretention (2006) TN Concentration (mg/l) Event N S IN 49

50 Mass Loads (kg/ha/yr) CP SS In Out In Out TSS Chromium ~0.007 Copper Lead ~0.005 Zinc Chloride TN Nitrate ~0.19 TKN TP TOC Li & Davis, J. Env. Eng Nitrate with Depth % Removal c. Nitrate Bioretention Depth (cm) Take Home Points: Nitrogen Proper Amounts of Organic Matter is Critical With good media, TN sequestration is high. Somewhat deeper media (>= 3 ft) may be best (esp. for NO 2-3 -N), with application of internal water storage zone. 50

51 Where are pollutants removed? TSS TP Temp TN Pathogens Metals Oil & Grease Literature/Research Justification for Minimum Media Depths: WQ Pollutant Depth (ft) Studies TSS 1 Diblasi et al. 2009, Li et al Metals 1 Li and Davis 2008, Hatt et al O&G 1 Diblasi et al. 2009, Phosphorus 2 (min); 3 (conserv) Hatt et al. 2009, Hsieh and Davis 2007, Passeport et al Nitrogen 3 Passeport et al Temperature 3 (min); 4 (optim) Jones and Hunt 2009 DON T FORGET HYDROLOGY deeper cells = greater potential for volume control Undersized BRC Study Site Interstate 540, Knightdale, NC Northbound Bridge Deck Southbound Bridge Deck Average Annual Daily Traffic ~ 34,000 vehicles/day (URS Corporation, 2010) 51

52 Objectives Direct comparison of undersized bioretention cell to standard-sized bioretention cell in their treatment of bridge deck runoff Bioretention Cell Characteristics Contributing drainage area: 1.29 acres Centipede grass sod Rip-rap forebay Standard Cell Surface Area: 2000 ft 2 Undersized Cell Surface Area: 1000 ft 2 Bioretention Sampling Schematic 52

53 Bioretention Hydrology Cumulative Volume Reduction Large Cell Small Cell n=47 n=51 30% 20% 0.1 to 4.8 inch monitored rainfall depths Both cells achieved statistically significant volume reductions Bioretention Hydrology Volume Reductions Based on Storm Size Large Cell Rainfall Depth (in) < > 2 n Vol Reduction 65.2% 16.2% 24.0% Small Cell Rainfall Depth (in) < > 2 n Vol Reduction 36.2% 17.1% 14.1% Better performance for smaller storm events Captured and stored in bowl and media Nutrient Concentrations Concentration (mg/l) Bioretention Inlet Undersized Outlet Standard Outlet 0.0 TKN NO2,3-N TN NH4-N TP 53

54 Sediment Concentrations Concentration (mg/l) TSS Bioretention Inlet Undersized Outlet Standard Outlet Bioretention Water Quality Mass Load Reductions Constituent Percent Reduction (%) p value Small Cell (n=25) TKN NO 2,3 -N 58 < * TN * NH 4 -N * TP TSS * Large Cell (n=21) TKN * NO 2,3 -N 68 < * TN * NH 4 -N * TP TSS * Asterisk indicates significant difference between locations Summary of Findings - Bioretention Larger Cell Works Better Pollutant Loads Volume Reduction Effluent Concentrations Undersized cell worked proportionately better than the large cell Internal water storage an easy improvement (promotes denitrification) 54

55 Final Take Home Points: Bioretention Research Improve Hydrology Modest Peak Flow Mitigation Long-term Hydrology Balance Leads to Pollutant Load Reduction Reduce Pollutant Concentrations / Release Low Pollutant Concentrations TSS Metals & Hydrocarbons TP & TN Bacteria Consider IWS! But, must be careful with Media Selection. CASE STUDY: BIORETENTION CELL RETROFIT AT LOUISBURG HIGH SCHOOL Project Objectives Grant funded in 2004 to build bioretention cell to treat runoff from Louisburg High School Construction completed in Summer 2012 Remove 18 reinforced concrete stormwater pipe Stabilize eroding ditch to reduce sediment loss Provide an outdoor classroom to LHS 55

56 Existing Conditions: Watershed Bioretention Location Watershed = 6 ac Existing Site Conditions Existing ditch/swale was the original impetus for the project Lower section (at left) had a low slope and held water constantly Soils clayey with some saprolite present Existing Conditions - Looking Upslope Weed and vegetation control consisted primarily of spraying Roundup Heavy erosion occurring on channel banks during each rain event Abandoned asbestos sewer pipe controlled grade in the swale design 56

57 Existing 18 Sewer Pipe Pipe Undersized and would flow full during intense rainfall Existing Conditions Continued Existing Yard Inlet Existing Yard Inlet 57

58 Bioretention Plan View Bioretention Cell Design Bioretention Cell: Ponding Depth = 1 ft Surface Area = 9500 ft 2 Media Depth = 3 ft Rock Lined Inlet Broad Crested Weir Outlet Vegetation = Grass + Trees Removal of Existing SW Pipe 58

59 Start of Construction: Excavation Excavation Continued 2 Feet of Cut Completed Existing 18 RCP Yard Inlet 59

60 Digging to 4 Foot Depth Direct rainfall would infiltrate within 12 hrs of the end of rainfall Existing 18 RCP Excavation Around Pipe Excavation Continued Excavation continued smoothly, with nearly the entire basin being dug out to a 4 ftdepth On 5/24/2012, Louisburg received 3.75 inches of rainfall in two hours Resulted in periods of pressurized flow through 18 RCP 60

61 Lake Louisburg Resulting in a Change Order Lost 10 sections (3 ftlong apiece) of RCP Change order totaled $7,300: Purchase of a pump to dewater hole Silt bags to remove sediment from pumped water Time & effort to operate the pump each time rainfall occurred Resulted in extension of construction window by ~30 days Long-Arm Excavator Used to Install Media Skid steer loaders used for final grading 61

62 Compaction vs. Infiltration Rate Outlet Structure: Broad Crested Weir Outlet Weir Design 62

63 Completed Bioretention Cell Storm Event Photo Outlet Structure 63

64 Swale (First Effort) Rock Lined (Final) Swale Design Rock Lined Swale: Length = 235 ft Depth = 1.33 ft Trapezoidal Cross Section 3:1 Side Slopes Bottom Width = 5 ft Top Width = 20 ft Slope ~ 4% Fi Final Swale Installation Grass never grew under TRM Erosion occurred beneath TRM Installed rip-rap lined channel Resulted in $18,830 change order 64

65 Costs for Bioretention Cell Original contract sum - $97,500 Change orders - $29,200 Total cost w/ change order -$126,700 Initial Cost per ft 2 = $10.25 Final cost per ft 2 = $13.33 Lessons Learned: LHS Provide a bypass for stormwater around stormwater practice while you are building it Sometimes rock is needed for conveyance in a swale As a designer, attempt to foresee potential change orders, as they will add a significant cost to construction Construction Concerns With BRC Despite popular myths... NOT a panacea 65

66 Never-Had-a-Chance Bioretention Take Home Construction Point: Sequencing is Essential BRCs are not sediment basins Install as close to End of Project Stabilize Watershed 66

67 NCSU: Serious About Bioretention Questions? 67