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“Landfill Final Cover Stormwater Drainage” Mark A. Taylor, PE EcoLogic Associates, P.C.

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Presentation on theme: "“Landfill Final Cover Stormwater Drainage” Mark A. Taylor, PE EcoLogic Associates, P.C."— Presentation transcript:

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2 “Landfill Final Cover Stormwater Drainage” Mark A. Taylor, PE EcoLogic Associates, P.C.

3 Purpose To Review the Design of Stormwater Management Features for a Landfill Final Cover and Propose a Design Strategy

4 Presentation Overview Final Cover Design Considerations Stormwater Drainage Considerations and Design Guidance Removal of Stormwater Runoff - H&H Analyses Other Drainage Considerations Conclusions

5 Final Cover Design Considerations - Functions Waste and By-products Containment Rainfall Infiltration Barrier Landfill Gas Barrier Odor and Vector Control Aesthetics (“Reclamation”)

6 Consequences of Failure Cover Damage (Structural) Erosion and Sedimentation Releases to Surface Water and Air Health & Safety (Vectors, Fire, etc.) Compromise of Leachate Collection and Landfill Gas Systems Increased Post-Closure Maintenance Costs

7 Final Cover Design Considerations - Owner Expectations Frequency of Failure Maintenance Requirements Cost (Construction & Post-Closure)

8 Final Cover Design Considerations - Design Factors Final Configuration and Post-Closure Use Removal of Stormwater Durability Stability Erosion and Sediment Control Constructability and Maintainability Cost

9 Principle Stormwater Drainage Considerations Removal of Stormwater Runoff Cover Stability Erosion & Sediment Control Durability, Constructability, and Maintainability

10 Deficiencies in Design Guidance How to handle Long, Steep Slopes in general Criteria for Diversion Channel Design on Side Slopes (longitudinal slope, spacing, freeboard) Design Storm Selection Accommodation of Differential Settlement Failure Criteria and Replacement Frequency

11 Hydrologic & Hydraulic Analyses Layout the System, Calculate Runoff Size Diversion Channels and Berms Select Channel Lining (Iterative) Design Downslope Conveyances (Slope Drains, Inlets and Conduits) Adjust the Layout as Necessary

12 System Layout Do Conceptual Layout of Diversion Channels (Berms) and Downslope Conveyances (e.g., Slope Drains) Experience in Temperate, Piedmont Mid- Atlantic Region where Annual Rainfall  40” Diversion Spacing = feet Downslope Conv. Spacing = feet

13 Calculate Runoff Rational Formula Q = CiA (cfs) C = Runoff Coeff. (Use 0.6 or higher for Side Slopes, 0.45 for 5%-10% Top Slopes) i = Rainfall Intensity (Use 10 Yr., 5 Min. Storm, T c less than 1 Min.) A = Drainage Area (A max is typically  2 acres)

14 Diversion Channels and Berms TRIANGULAR CHANNEL FLOW Manning’s Eqn. V=1.486/n x R 2/3 x S 1/2 (fps) Continuity Eqn. Q=AV (cfs) Solve for d = Flow Depth (ft) and V (fps)

15 Diversion Channels and Berms Assume values for ‘n’, Consult Mfrs. specs and adjust for impediments to flow Recommended range of slope,S = 3-5% Continue with Iterative Design, varying n, S, and layout as needed for workable ‘d’ with freeboard (minimum 1 foot recommended)

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17 Channel Lining Calculate Tractive “Force”, T T = y x d x S (psf) For low V,T Use a temporary erosion control mat For high V,T Use a non-degradable turf reinforcement mat (TRM) For very high T Use rip-rap or SI’s Pyramat®, a permanent erosion and reinf. matrix

18 Slope Drains Corrugated HDPE with Sewer-type, ‘Soil- tight’ Connections are Recommended Inexpensive; Ready to Install; Quick Construction; Place Directly on Prepared Subgrade, Geosynthetics, or Infiltration Layer Pleasing Aesthetics, Ease of Maintenance

19 Slope Drains - Inlet Design Assume Inlet is a Sharp-crested Weir Weir Eqn. Q = 3.33LH 3/2 (cfs) Solve for L based on flow from one direction For Circular Inlet, L max =  D/2, Use L min =D TYPICAL RESULTS D min = 2 Ft. (Side Slopes), 3 Ft. (Top) Use a Splitter-Type Anti-Vortex Device

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21 Slope Drain Inlets Check for Hydraulic Jump where channel widens at Inlet Check Critical Depth, d c, to see if flow will go from supercritical to subcritical When Specific Energy is at its minimum (Critical Depth), d c = 2 x V 2 /g (ft) Assume a Hydraulic Jump, Calculate d 2 by trial and error (Formula in paper)

22 Slope Drain Conduits PROPOSED DESIGN CRITERIA Pipe should not flow full to avoid “slugging” Assume/assure exit is “free” (Unsubmerged) Assume/assure all flow is Supercritical (pipe slope exceeds critical slope) Assume entrance is submerged (Orifice entrance condition)

23 Slope Drain Conduits Orifice Eqn. Q = C d x A x (2g(h-a)) 1/2 (cfs) Use C d = 0.6 and h from Inlet Design, then Simplify and Solve for Dia., d (2a or (4A/  ) 1/2 ) by trial and error. TYPICAL RESULTS Use 15- and 18-inch Diameter Pipe

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25 Slope Drain Conduits Check Flow in Corrugated/Smooth HDPE Pipe Avoid disruptions to flow where flows from inlets converge; thus, use Wye connections Check Pipe Capacity as Flows Accumulate Specify Watertight Connections and Provide Anchorage Provide robust Energy Dissipation and Outlet Protection at the outlet

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27 OTHER DRAINAGE CONSIDERATIONS

28 Drainage Effects on Veneer Stability Calculate Maximum Spacing of Drainage Outlets (Diversion Channels): L max = k 2 /k 1 (sin  )(h 2 ) (After Thiel and Stewart (1993)) Note that, for a given L, the ratio of Drainage Layer permeability, k 2, to Cover Soil permeability, k 1, has a Minimum Value in order to Maintain Stability

29 Erosion & Sediment Control Careful and accurate grading Pre-vegetation protection (critical) Liberal use of geosynthetics Vegetation selection and soil analysis Stormwater detention, sediment storage Proper and regular Maintenance

30 Durability, Constructability, and Maintainability Durability (Geosynthetics (TRM’s), Drought-resistant Vegetation, Flexibility (Soft Armor, Slope Drains)) Constructability (Uniform Slopes, Slope Drains, Geosynthetics (RECP’s)) Maintainability (Smooth Slopes for Mowing, RECP’s reduce Repairs and Sediment Removal, Clog-free Slope Drain Inlet Grates) Good Layout and CQA Testing and Surveying

31 CONCLUSIONS Landfill Final Cover Design is a Complex Process Involving Many Factors Landfill Owners need to Recognize Challenges and Should Establish Clear Design Goals and Realistic Budgets Erosion and Sediment Control is a Critical Item in Post-Closure Maintenance

32 CONCLUSIONS Short- and Long-term Cost-effectiveness are Potential Benefits of Improved Stormwater Management A Rigorous Design Approach to Stormwater Management and Erosion & Sediment Control is Recommended

33 QUESTIONS?


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