Eric Konzelmann, CPESC, CPSWQ E&S Technician

Transcription

1 Eric Konzelmann, CPESC, CPSWQ E&S Technician Berks County Conservation District Special thanks to Domenic Rocco, DEP SERO STRIVING TO MAKE STORMWATER GREEN

2 Effects from Conventional Thinking Rate control only. Focus on large storm. No volume control. No water quality. Compounding effects of extended peaks within watershed.

3 Things that we need to understand Nobody has all the answers. SWM is going through a renaissance and is still evolving quickly. SWM has become a multidisciplinary program. We need to make judgments based on the best information available. The BMP manual is not a bible. It is a tool to help us make a decision. There are other tools out there and many still yet to be developed. PG Eco? PE SS

4 Why focus on Volume? When you control volume, you often also deal with: Flow Rate Water Quality Stream Bank Protection Flooding Temperature Groundwater Recharge It is a way of feeling secure that watershed protection goals are being addressed.

5 What do we mean by Volume Control? BMPs Non-Structural* Chapter 5 Preventative Measures Structural* Chapter 6 Mitigative Measures It s not just Infiltration!! Processes: 1. Infiltration 2. Capture and Reuse 3. Vegetated Systems w/ ET PRV = Perm. Removed Volume. This is the portion of the runoff volume being controlled. It does not get discharged from the site. Infiltration It is anticipated that many of the stormwater management systems in PA will include one or more of these methods

6 Bio-Retention/Bio-Infiltration

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8 Versatility of Bioretention Office Complex Roadway Residential Commercial

9 Case Study: Villanova University More research info available. This one built in Subject of many studies. Long term infiltration rates sustained, though seasonally variable. Peak late summer Valley mid Winter

10 Risk Based Approach Elements of risk should be assessed for any project, including infiltration. High Risk Moderate Risk Low Risk No pretreatment Large Subsurface Seepage Beds High Pollutant Areas Low Permeability Utilized for E&S During Construction Pretreatment Provided Medium Subsurface Seepage Beds Moderate Pollutant Areas Moderate Permeability Small Subsurface Seepage Beds Surface (Vegetated) BMPs Low Pollutant Areas High Permeability

11 Follow Protocols in BMP Manual Design for volumes up to delta 2-year 24-hr storm Soil Infiltration Testing: Avoid using the perc test Use infiltration test that determines hydraulic conductivity. Use a minimum Factor of Safety of 2 Avoid hydraulic head > 2. Dewatering time < 72 hrs. Encased Borehole Double Ring Infiltrometer

12 Computation for Infiltration Infiltration BMP Sizing: To expedite process, follow recommended loading ratios*. Reasonable exceedances may be acceptable. Based on site-specific conditions and science Need to pass the stink test. Challenging sites will likely require further studies, engineering justification and the seal of appropriate professionals. (PE, PG, SS)

13 Loading Ratio A rule of thumb approach for sizing infiltration BMPs. Based purely on drainage area and BMP area. Soils and Geology not considered. Useful for planning purposes and those small projects where detailed studies are costprohibitive. Not intended to replace site-specific studies. BMP Manual has two ways of considering loading ratio. Imperv. DA: BMP Area = 5:1 BMP Area needs to be 20% of Total DA Total DA: BMP Area = 8:1 BMP Area needs to be 12.5% of Total DA

14 Infiltration BMP w/ Loading Ratio BioInfiltration Traffic Island (BTI) at Villanova Univ. Double the Recommended Maximum Total DA = 1.3 Ac 46% Impervious 10:1 IDA:IA

15 Volume Reduction Calculation Storage volume is the sum of 1 and the smaller of 2a or 2b.surface storage should be at least 50% 1. Surface storage = Bed area (sq ft) x water depth 2a. Infiltration volume = bed bottom area (sq ft) x infiltration design rate (in/hr) x infiltration period (hr) x 1/12 2b. Volume = bed bottom area (sq ft) x soil mix bed depth x void space

16 Sample Calculation Appendix D Rain Gardens Storage Volume = Surface Storage + Soil Storage* = (Area x Depth) + (Area x Soil Depth x 10%) = (1,820 ft 2 x 1.0 ft) + (1,820 ft 2 x 1 ft x 10%) = 2,002 ft 3 Infiltration Volume for Volume Abstraction in Routing Process: = Infiltration Rate x Infiltration Area x Infiltration Period (assume 6 hours) = 1/2 in/hour x 1,820 ft 2 x 6 hr x (1/12) ft/in = 455 ft 3

17 Poured Concrete Aprons Ensure no concrete lip!! Provide at least 6 of fall Consider vegetation rising

18 Plantings. The more the better!!

19 Proactive Approach: Used Sod Watered Sod Inlet tops and all joints must be sealed Bio-Retention / Rain Gardens

20 Seed Mixes Year 1 Companion crop for stabilization Spring Oats w/ernst Rain Garden Mix and plenty of straw mulch

21 Year 2 Ernst Rain Garden Seed Mix (2-3 year grow-in period)

22 Year 3

23 Repair requires removing and reinstalling plantings

24 BMP Manual Design Considerations Ponding depth max flexible dewater in 72 hours not flexible Planting soil depth min. 18 more is better At least 3 varieties of NATIVE trees & shrubs 700 shrubs/acre 300 trees/acre 2-3 of shredded mulch or leaf compost Under drains? Volume credits are lost

25 Infiltration in Karst Areas Can it be done? Yes It happens naturally. Key is to keep BMPs shallow and loads low. Is it more difficult? Yes Need to perform detailed geologic investigation. Decision Factors: Thickness of soil layer Degree of weathering of underlying rock formations. Existence of sinkholes.

26 Lining rain gardens has resulted in numerous failures

27 IMPORTANT POINTS: BMPs spread across the site. (each lot) Separate BMPs for the road. No Detention Basin. Lot owners responsible for maintenance. Some rain gardens have stone bed too (slow rates/extra volume?). Source: RGS Associates

28 Infiltration Berms /Bio-Detention runoff

29 SWM BMP that intercepts runoff and provides shallow ponding in a dished mulched area around the tree or shrub. Extend the mulched area to the tree dripline Tree and Shrub Pits

30 Why use Vegetated BMPs? They do the best job in mimicking natural hydrology Can provide multi-functional benefits beyond hydrology and water quality improvements: Wildlife habitat birds & butterflies Provide longer period and more choices for pollinators Provide carbon sequestration Provide temperature mitigation for air and runoff Can provide unique aesthetic benefits Allows for creative liberties They can add property value.

31 Vegetated Systems fit in with landscape More likely to receive adequate maintenance as well!

32 Importance of what lies beneath Rhizosphere: Living soil includes roots, viruses, bacteria, fungi, algae, protozoa, mites, nematodes, worms, ants, maggots, other insects and insect larvae (grubs), earthworms and rodents. In fact, the volume of living organisms below ground is often far greater than the volume living above ground (Clapperton, circa 2000).

33 DEEP-ROOTED PERRENIAL GRASSES Turf Grass

34 Discharges to Non-Surface Waters Legal Issues Common Law Easement* Express Easement Technical Issues Dealing with concentrated flows especially where they did not previously exist. Level spreader failures are very common Convey (to a receiving water or storm sewer) Disperse (through level spreading device) Also see DEP Fact Sheet 3930-FS-DEP4124

35 COMMON LAW RULE Because water is descendible by nature, the owner of a dominant tenement has an easement in the servient tenement for the discharge of all waters which by nature rise in or flow or fall upon the superior. Dom. tenement cannot create a nuisance (e.g. increase in runoff) or may be considered a trespass issue. Kauffman v. Griesemer, 26 Pa. 407 (1856). COMMON LAW EASEMENT Servient Tenement House Dominant Tenement pond stream House pond

36 Storage Volume Volume Reduction Achievable Volume Reduction Achievable w/ more in-depth analysis Not Total Storage Should only be Permanently Removed Volume. Should not include any volume to be discharged. This has been corrected in NOI application, but not in BMP Manual.

37 Info that s not in a Worksheet!! Stormwater BMP Information Chart 5.B Infiltration Information Drainage Information BMP Information BMP Proposed Infiltration BMP(s) (site specific) Pervious Pvmnt w. Infilt. Bed Elevation Soils, Water Table, Total Drainage Area to Total Impervious Drainage Infiltration BMP Surface Total Drainage Area Loading Infil. Elevation Top of Bed/ Infil. Elevation Bottom of Bed/ Elevation E & S Sediment Basin Bottom (if Volume to be Infiltrated or Permanently removed***** Measured Infiltration Rate Factor of Safety Design Infiltration Rate Dewatering Time* Rock** BMP Area to BMP Area Ratio Impervious Area Loading Ratio Basin*** Basin*** Elevation Infiltration Test**** applies) in./hr. 2 min. in./hr. hrs. sq. ft sq. ft. sq. ft. 8:1 Max 5:1 Max cf BMP BMP Infiltration Basin Subsurface Infiltration Bed INFILTRATION INFORMATION DRAINAGE INFORMATION BMP INFORMATION BMP Infiltration Trench BMP Rain Garden/Bioretention BMP Dry Well / Seepage Pit BMP BMP BMP BMP Constructed Filter Vegetated Swale Vegetated Filter Strip Infilt. Berm & Ret. Grading All information to be based on the 2-year 24-hour storm only. Provide page numbers from the stormwater narrative identifying the location of the above information. * Include active infiltration time, maximum 6 hrs. ** Depth to limiting zones must be >2 ft below infiltration testing elevation/proposed infiltration elevation. ***If greater than 2 feet of Hydraulic head then a soil scientist justification is required. ****Provide supporting field notes/documenation. *****Permanently Removed Volume includes water that is captured and not released. (reused or reduced by evapotranspiration.) Worksheet 5b