Design Methodology for Alternative Covers Craig H. Benson, PhD, PE Geo Engineering Program University of Wisconsin-Madison Madison, Wisconsin USA 53706 P: (608) 262-7242 F: (608) 263-2453 benson@engr.wisc.edu
Definition: - An alternative cover is a soil cover consisting of one or more layers that is used in place of a conventional cover. - Also called ET covers, water balance covers, store-andrelease covers. - Generally required to be equivalent to conventional cover. Equivalency: - Percolation rate for alternative < conventional. - Erosion rate for alternative < conventional.
Water Balance Principle Balance the storage capacity of finer textured soil with the water removal capabilities of evaporation & transpiration. ET Precipitation L Infiltration Runoff Sponge capacity defined by soil properties Percolation (if storage capacity of sponge is exceeded).
Cumulative Precipitation and Evapotranspiration (mm) 2000 1500 1000 500 USEPA s Alternative Cover Assessment Program (ACAP) Site in Marina, CA Storage Capacity = 300 mm Soil Water Storage Evapotranspiration Precipitation Missing Data Percolation No Surface Runoff 0 0 1/31/00 7/31/00 1/30/01 7/31/01 1/30/02 8/1/02 1/30/03 8/1/03 1/31/04 Percolation occurs every year when storage capacity is exceeded. 500 400 300 200 100 Cumulative Percolation, Soil Water Storage, and Surface Runoff (mm)
1. Site Characterization Five-Step Approach - climatic conditions (suitable location, design record) - available borrow soils (storage capacity) - suitable vegetation (growing season, root depth, coverage) 2. Preliminary Design Calculations - defining storage capacity of soils - sizing storage layer - monolithic cover or a capillary barrier 3. Numerical Modeling -- Session 3 - select a numerical model (UNSAT-H, HYDRUS, Vadose/W) - design verification and refinement
4. Design Details -- Sessions 1, 3, & 5 - surface water management - erosion, desiccation and frost effects - biota intrusion, fire - soil placement and re-vegetation 5. Performance Evaluation & -- Session 4 Monitoring - verify that cover performs as intended - lysimeter - water content and matric potential sensors - combinations of lysimeters and sensors - data needs and evaluation criteria
Site Characterization - climate - soils - vegetation
Climate 160 160 Average Annual Percolation (mm/yr) 140 120 100 80 60 40 20 ACAP Field Data Success Likely Marina Sacramento Success Unlikely ) Average Annual Percolation (mm / yr 140 120 100 80 60 40 20 ACAP Field Data Marina Sacramento Success Likely Success Unlikely 0 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0 200 400 600 800 1000 1200 1400 P/PET Average Annual Precipitation Suitable Sites: P/PET < 0.6, P < 700 mm/yr Contingent on proper cover design
Frequency & Type of Precipitation Th i ckness (m) for Percolat i on = 10 mm /yr 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0 Infil tration ( mm / yr ) 450 400 350 300 250 200 150 < 30% Snow > 30% Snow 100 0.2 0.4 0.6 0% 10% 20% 30% 40% 50% 60% Fraction of Days Precipitation Event Occurs Snow Precentage of Annual Precipitation - frequency and intensity of precipitation - % precipitation that is snow
Daily Infiltration/Precipitation 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Precipitation Intensity 0 20 40 60 80 100 Daily Precipitation (mm) More infiltration occurs (and thicker covers are required) as the precipitation intensity decreases More frequent and less intense precipitation events generally are MORE IMPORTANT than large thunderstorms.
Location & Climate Cover Thickness Contours (m) for 10 mm/yr Climate and location are closely related Thicker covers required in cooler and wetter regions
Key Characteristics: Soil Properties - Water retention characteristics (high, finer textured) - Saturated hydraulic conductivity (low, finer textured soil) - Shrink-swell potential (low, modest clay fines, well graded, appreciable coarse fraction) - Erosivity (low, well graded, blend of clay and silt fines) - Shear strength (high, well graded) - Sufficient volume and close proximity - Agronomic properties
Water Retention: Soil Water Characteristic Curve Volumetr ic Water Content 0.35 0.30 0.25 0.20 0.15 0.10 0.05 θ fc θ wp = 0.24 = 0.03 θ fc = 0.24 θ wp = 0.03 θ u = θ fc θ wp = 0.21 0.00 1 10 100 1000 10000 Unit storage capacity = field capacity = θ fc Unit available storage (θ u ) = available volume per volume of soil 1 m cover conceptually can store & release 210 mm. Finer-textured soils have higher θ u than clean coarse textured soils (loamy sand - 0.3%, loam - 7%, silty Suction (kpa) clay loam - 14%) ASTM D 6836
Net Infiltration Rate (mm/yr) 600 500 400 300 200 100 Soils & Percolation Saturated Hydraulic Conductivity (cm/sec) 10-3 10-4 10-5 SL Bismarck L SCL SiL non-vegetated vegetated CL SiCL(1) Albuquerque 0 0.0 0.1 0.2 0.3 0.4 Volumetric Water Content - Field Capacity Finer textured covers transmit less percolation by enhancing runoff & retention
Soils & ET Saturated Hydraulic Conductivity (cm/s) 100% 10-3 10-4 10-5 Transpiration/Potential Transpiration (% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Bismarck Albuquerque L SL SCL SiL CL SiCL(1) 0.0 0.1 0.2 0.3 0.4 Volumetric Water Content - Field Capacity Type of cover soil affects efficiency of transpiration
Vegetation Key Characteristics: - Warm and cool seasons - Deep and shallow rooted vegetation - Readily established and low maintenance - Native to location - Disease resistant
Vegetation is Essential Maximum Annual Percolation Rate (mm/yr) 100 10 0.1 0.01 Root Zone Non-vegetated Altamont, CA Entire record - In many climates, vegetation is key to reducing percolation - Vegetated covers Vegetated generally can be 1 thinner than non- vegetated covers relying on evaporation - Water is removed 0.0 0.5 1.0 1.5 2.0 2.5 directly from the root Thickness of Cover (m) zone, rather than cover surface
Coverage and Density 0.8 0.6 Requ ired Cover Th i ckness (m ) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Percolation 0.5 0.1 Rate ( mm/yr) 0.1 0.4 1.0 1.0 10 0.3 10 Percolation Rate ( mm/ yr) 0.2 0.1 Altamont Altamont 0.0 1.00 0.75 0.50 0.25 0.00 No Veg. Low LAI Typical LAI High LAI Fraction Unvegetated Requ i red Cover Th i ckness (m ) Model predictions suggest that having vegetation is more important than details of vegetation.
Preliminary Layer Sizing & Configuration - layer thickness - monolithic vs. capillary barrier
Design Approach 1. Define quantity of water to store. - wettest year on record - precipitation received outside the growing season (i.e., when ET is low), P o 2. Select layer thickness and/or configuration to provide adequate storage P L o F θu F = scaling factor. Assuming runoff = 0.
Lab-to-Field Scaling Annual Percolation Rate (mm/yr) 250 Storage capacity (S c ) of ACAP Prefential ACAP field covers computed assuming S c = Lθ Flow data fc, where L = cover 200 thickness. Conceptually, percolation 150 All should be negligble if peak soil Humid water storage < storage Sites 100 capacity. S p = 0. 7 S c Data suggests that percolation 50 can be appreciable at 70% θ fc. 0 L 0.0 0.4 0.8 1.2 1.6 Peak Soil Water Storage Soil Water Storage Capacity P o 0. 7 θ u P o = precipitation outside the growing season.
Capillary Barrier Design 10 7 Suct ion, ψ (cm) 10 6 10 5 10 4 10 3 Finer Textured Soil Without Break Khire et al. 2000. J. of Geotech. and Geoenvironmental Eng., 126(8), 695-708.a 10 ψ b With Break 10 1 Coarse Soil 10 0 0 0.1 0.2 0.3 0.4 0.5 Finer-Grained Soil Clean Coarse Soil Capillary break can enhance storage capacity of the finer textured layer. Capillary barrier effect is larger for non-plastic finer textured soils. L S CB = θ( z ψ 0 Volumetric Water Content, θ θ fc θ bf In this case, capillary break increases storage (θ θ + ) dz + ) L capacity by ~ 80 mm of T bf b water per 1 m of cover 2
Numerical Modeling Purpose: Numerical modeling is used to refine the design (make more efficent) and check the design (against percolation criterion). 1. Model should have been compared against field data, notably fluxes (percolation, runoff, ET) 2. Model should include a rigorous algorithm for the soil-plant-atmosphere continuum and effects of water availability on transpiration. 3. Measured parameters may need to be scaled before being used in models
Numerical Modeling (con t.) 4. Model predictions should be checked against typical ranges observed in the field (e.g., runoff < 10% of precipitation). 5. Model predictions should be reasonably consistent with preliminary design calculations.
Summary 1. Five-step procedure for designing alternative covers intended to be equivalent to conventional covers. 2. Be realistic about suitability of site. Equivalent alternative covers are not practical at all sites. 3. Essential to locate a soil with sufficent storage capacity that also satisfies all other engineering and agronomic criteria.
Summary 4. Account for scaling in design calculations. Field conditions often differ from laboratory measurements. 5. Check the design using verified models. Use justifiable input parameters and check the output against field data for reasonableness. 6. Be prepared to verify that the design functions as intended. This criterion is characteristic of any new environmental technology, even if conventional technology is not proven to be effective.
Acknowledgements Sponsors: NSF, USEPA, USDOE, Landfill Industry People: Glendon Gee, Bill Albright, Dave Carson, Steve Rock, my students, many other colleagues