Post Construction BMP Inspection. Scott Taylor, P.E. Stormwater

Transcription

1 Post Construction BMP Inspection Scott Taylor, P.E.

2 Learning Objectives To Understand: Types of post construction BMPs Typical construction process of BMPs Construction errors that impact BMP operation Operation and Maintenance problems that impact BMP operation

3 Overview of Most Common Post Construction BMPs Per MRP Bioretention Water harvesting Porous pavers/pervious pavement Vegetated swales

4 Old and New BMPs Previous standard was construction of grey infrastructure: end of pipe BMPs New standard is green infrastructure: site based BMPs Within 5 years most NPDES Permits will only allow green infrastructure: Infiltration Harvest and use Bioretention and biofiltration

5 Detention Basins Volume: Designed for specific runoff volume Access: All weather access road around perimeter and to invert Detention Time: Relatively longer time provided to allow particles to settle out of the water column as compared to flood control basin detention time

6 Design Overview Detention Time (con t): Goal is average detention time (residence time) of 24 hours. Provide a full basin draw-down time of 96 hours, no more than 50% of volume in first 24 hrs. Basin Geometry: Maximize hydraulic flow length. L:W ratio of 1.5:1 or greater is desirable but not mandatory. Depths up to 7 ft (2 m), 2-5 feet optimum. Basins operate in batch rather than plug flow mode

7 Outlet Configurations

8 Extended Detention Basin

9 EDB - Operation

10 Common Design Flaws

11 Common Design Flaws

12 Infiltration Basins Site Investigation In-drill Hole Permeability Test A Must! Place Test Zone at Invert of Basin or in Lower Portion of Trench Locate Groundwater Table Elevation (Season High WT) Measure Permeability (3 Tests) Review Site Soil Lithology Soil Characteristics: Organic Content; Ph; and Cation Exchange Capacity

13 Design Overview Minimum Infiltration Rate is 0.5 in/hr or 13 mm/hr Calculate Basin Volume - Design Storm Size Basin to Have Maximum 96 Hour Drain Time (120 in Bay area) Six Hours is Recommended in Many References Recommended Depth 2 Ft (0.7 m) Maximum T = d/f (d Is Design Depth, in, f is Infiltration Rate, in/hr, T is Time, hrs) Trench - Assume Fill Material Has 35% Voids, Sides are ineffective in infiltration

14 Cautions Up to 50% of Infiltration Facilities Fail in First 5 Years (High Ground Water) Groundwater Table Can Be Variable, Take Readings Over 1 Year Period if GW is close to 10 Ft (3 m) of Device Invert Use Lowest of Field Permeability Readings Do Not Use Sites in Engineered Fill Exclude Sites With More Than 30% Clay Content unless geotechnical engineer provides recommendation Exclude Sites With urban runoff

15 Infiltration Basin

16 Failed Infiltration Basin

17 What Can Go Wrong Soil borings are not representative Groundwater rise Excess sediment load

18 Bioretention Vegetation with specific soil mix Plant palette will be specified Soil mixture will be specified May have underdrains BMP design will be specified in the plans How does it work? Water flows in on the surface, percolates through the vegetation and soil and either infiltrates or flows out the under drain

19 Bioretention What does it look like?

20 Rainwater Harvesting Cisterns or rain barrels are used to collect stormwater for future use

21 Porous Pavement Types Permeable pavers Porous concrete Porous asphalt Benefits Allows water to infiltrate instead of running off the site Allows water storage under or within the pavement Maintenance concerns Clogging, overcompaction, damage due to high traffic

22 Porous Concrete

23 Porous Asphalt

24 Permeable Pavers

25 Vegetated Swales Features Concentrated flow conveyance Flat bottom channel Vegetation must be 2-6 inches Designed for surface flow rather than infiltration

26 Vegetated Swale

27 POST CONSTRUCTION BMPS DURING CONSTRUCTION: BIORETENTION

28 Excavate and Place Underdrain

29 Backfill with Engineered Soil

30 Add Plants Note: Inlet is Blocked during Construction

31 Scour Protection at Inlet

32 Completed BMP

33 CONSIDERATIONS FOR INSPECTION OF POST CONSTRUCTION BMPS

34 Is the BMP there? Verify installation of a BMP The vent pipe is all that was visible We had to accept the contractor s word that there was a BMP here

35 Does Water Flow to the BMP? Parking lot was graded to drain down the middle Drainage never reached the BMP in the corner Site grading plan was different from the final site condition

36 Was the BMP Protected During Construction?

37 Was the Correct Soil Mixture Used? Engineer will specify a soil mixture For example, 50% compost 25% sand 25% soil MRP has a specific soil mix for bio units

38 Is the Correct Outlet Structure Used?

39 Is an Overflow Outlet Provided?

40 Are Underdrains Installed Per Plan?

41 Does the BMP Receive Excess Nuisance Flow?

42 Is the Correct Plant Palette in Place?

43 Could the BMP Clog?

44 Is Irrigation Needed to Establish Vegetation?

45 Are There Fencing Requirements?

46 Does the BMP Drain Within Five Days?

47 Other BMPs Many Proprietary/Non-proprietary that pond: Delaware sand filter CDS Most vault systems Cisterns and underground storage systems Key to inspection of these facilities is to ensure they are installed per plan, and that they are completely clean and free of debris when signed off.

48 Inspection Exercise Glorietta Bay Coronado, CA Permeable pavement

49 Inspection Exercise

50 Inspection Exercise Southwest College San Diego, CA Infiltration Basin

51 Inspection Exercise Juvenile Hall Parking Lot San Diego, CA Bioretention & flow through planters

52 Inspection Exercise

53 Inspection Exercise UCSD Parking Lot and Bldg San Diego, CA Extended detention basin

54 Inspection Exercise Miramar College San Diego, CA Underground water quality vault, CDS Unit

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56 Inspection Exercise Family Resource Center on Market Street San Diego, CA Flow through planters (bioretention cells)

57 Inspection Exercise Kaiser Permanente Anaheim, CA Bio-swale

58 Inspection Exercise Scripps Ranch Middle School Scripps Ranch, CA Vegetated Swale

59 Inspection Exercise Community Health Center 52 nd Street and University Avenue, San Diego CA Porous Asphalt