slide1 n.
Skip this Video
Download Presentation
Designing Water Balance Covers (ET Covers) for Landfills and Waste Containment by

Loading in 2 Seconds...

play fullscreen
1 / 45

Designing Water Balance Covers (ET Covers) for Landfills and Waste Containment by - PowerPoint PPT Presentation

  • Uploaded on

Designing Water Balance Covers (ET Covers) for Landfills and Waste Containment by Craig H. Benson, PhD, PE , DGE, NAE Geological Engineering University of Wisconsin-Madison Madison, Wisconsin 53706 USA

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about 'Designing Water Balance Covers (ET Covers) for Landfills and Waste Containment by' - kayla

Download Now An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

Designing Water Balance Covers (ET Covers) for Landfills and Waste Containment


Craig H. Benson, PhD, PE, DGE, NAE

Geological Engineering

University of Wisconsin-Madison

Madison, Wisconsin 53706 USA

covers waste containment
Covers & Waste Containment

Gas vent or collection well

Cover system

Groundwater monitoring well


Native soil


Leachate collection system

Barrier system


Cover Strategy - Conventional vs. Water Balance Covers

Conventional Cover

Water Balance Cover


Cover Profiles for Water Balance Covers

Monolithic barrier: thicker layer of engineered fine-textured soil – “storage layer.”

Capillary barrier: fine-textured soil “storage layer” over coarse-grained capillary break.


What Drives Interest: Cost Savings

Subtitle D composite at site: 450 mm fine-grained soil < 10-5 cm/s, 1-mm geo-membrane, drainage layer, and 300 mm surface layer.

> 64% cost savings with water balance cover


Water Balance Covers: How They Work






S = soil water storage

Sc= soil water storage capacity

Percolation if S > Sc


The Balance in Water Balance Covers

Storage capacity of cover, Sc

Natural water storage capacity of finer textured soils.

Water removal by evaporation and transpiration.

Key: Design for sufficient storage capacity to retain water accumulating during periods with low ET with limited or desired percolation. Need to know required storage, Sr.


Soil Water Retention In Unsaturated Soil


Suction, y




Wilting point

Suction, y







Field capacity










Volumetric Water Content, q



As the soil becomes drier, the water filled pathways become narrower and more tortuous





unsaturated hydraulic conductivity
Unsaturated Hydraulic Conductivity

Water retreats into smaller pores as suction increases, causing water content (q) to diminish and hydraulic conductivity to drop.

unsat hydraulic conductivity suction
Unsat. Hydraulic Conductivity & Suction

Water retreats into smaller pores as suction increases, causing water content (q) to diminish and hydraulic conductivity to drop.

Coarser soil becomes less permeable than drier soil when suction is high enough




evaporation and transpiration et
Evaporation and Transpiration (ET)

PET = potential evapotranspiration = max ET for given meteorological condition


Potential Evapotranspiration (PET)

FAO Penman-Monteith Reference Evaporation (PET) in mm/d

e = atmospheric vapor pressure at 2 m (= saturated vapor pressure x relative humidity), [kPa]

es = saturated vapor pressure [kPa] of air at 2 m at air temperature Ta [oC]

U = wind velocity at 2 m above ground surface [m/s]

x = psychometric constant [kPa/oC]

D = slope of curve relating es and T [kPa/oC]

Rn = net radiation [MJ/m2-d] = net solar radiation (Rns) – long wave radiation (Rnl)

G = soil heat flux [MJ/m2-d]

T = atmospheric temperature (oC)

1 MJ/m2-d of energy = 0.408 mm/d water evaporation.


Design Process

  • Define performance goal (e.g., 3 mm/yr)
  • Evaluate local vegetation analog
    • Species distribution and phenology
    • Coverage
    • Leaf area index
    • Root depth and density
  • Evaluate candidate borrow sources
    • What types of soils?
    • How much volume?
    • Uniform?
    • Blending required or helpful?

Design Process - 2

  • Laboratory analysis on borrow source soil
    • Particle size analysis
    • Saturated hydraulic conductivity
    • Soil water characteristic curve
    • Shrink-swell, wet-dry, pedogenesis
  • Preliminary design computations
    • Required storage
    • Available storage and required thickness
  • Water balance modeling
    • Typical performance
    • Worst-case performance
    • What if scenarios?

Design Process - 3

  • Final Design
    • Geometric design
    • Surface water management
    • Gas management
    • Erosion control strategies
    • Specification preparation
  • Regulatory approval
  • Bid preparation & contractor selection

 Design Questions for Step 5

  • How much water needs to be stored?
  • Identify critical meteorological years
  • Define precipitation to be stored
  • How much water can be stored?
  • Define the storage capacity
  • Compute required thickness
  • Can water can be removed?
  • Define wilting point
  • Determine available capacity

Required Storage: Design Year

  • Typical Design Scenarios:
  • Wettest year on record
  • 95th percentile wettest year
  • Typical year
  • Wettest 10 yr period
  • Entire record
  • Year with highest P/PET
  • Snowiest winter
  • Combinations
water accumulation when how much
Water Accumulation: When & How Much

Example: for fall-winter months at sites without snow, water accumulates in the cover when monthly P/PET exceeds 0.34.

1. Determine when water accumulates.

2. Define how much water accumulates.

thresholds for water accumulation
Thresholds for Water Accumulation

Examined P, P/PET, and P-PET as indicators of water accumulation and found P/PET threshold works best.

Data segregated into two climate types (with & without snow and frozen ground) and two periods in each year (fall-winter and spring-summer).

Water accumulates when P/PET threshold exceeded.

Fall-winter = September - February

Spring-summer = March - August

how much water accumulates
How Much Water Accumulates?
  • 1. Use water balance approach: ΔS = P – R – ET – L – Pr
  • Δ S = change in soil water storage
  • R = runoff
  • P = precipitation
  • ET = evapotranspiration
  • L = lateral internal drainage (assume = 0)
  • Pr = percolation
  • 2. ET is unknown, but is a fraction (β) of PET: ET = βPET
  • 3. R, L, and Pr can be lumped into losses (Λ)
  • Simplify to obtain: ΔS = P – βPET – Λ
  • 4. Equation used to compute monthly accumulation of soil water storage if P, PET, β, and Λare known.
parameters for water accumulation equation
Parameters for Water Accumulation Equation

Δ S = P – βPET – Λ


Two sets of βand Λparameters (fall-winter & spring-summer) for a given climate type.

monthly computation of required storage
Monthly Computation of Required Storage





Sr = required storage

Δ Sr m= monthly water accumulation

hm= monthly index for threshold (0 = below, 1 = above)

If Δ Sr m< 0, set Δ Sr m= 0.

computing required storage
Computing Required Storage

Fall-Winter Months

Spring-Summer Months

If argument < 0, set = 0

  • βFW= ET/PET in fall-winter
  • βSS= ET/PET in spring-summer
  • ΛFW= runoff & other losses in fall-winter
  • ΛSS= runoff & other losses in spring-summer
  • Pm = monthly precipitation
  • PETm = monthly PET
example idaho site snow frozen ground1
Example: Idaho Site (snow & frozen ground)

For months below threshold, set ΔS = 0

Δ S = P – 0.37*PET


β= 0.37, Λ= 0

Store 97mm for typical year, 230mm for wettest year

example texas site no snow frozen ground
Example: Texas Site (no snow & frozen ground)

For months below threshold, set ΔS = 0

ΔS = (P – 0.37*PET)-27


β= 0.3, Λ = 27

Store 188 mm for 95th percentile year,

548 mm for wettest year

predicted and measured s r
Predicted and Measured Sr

Good agreement computed & measured required storage.

monolithic covers storage capacity
Monolithic Covers: Storage Capacity

What is the storage capacity (Sc)?


qc = water content when percolation transmitted.

monolithic covers storage capacity1
Monolithic Covers: Storage Capacity

What is available storage (Sa)?

qmin = lowest water content achieved consistently.


soil water characteristic curve swcc
Soil Water Characteristic Curve (SWCC)



  • qfc = field capacity water content, qfc at 33 kPa suction (use for qc).
  • qwp = wilting point water content, q at 1500 kPa. Arid regions 4000-6000 kPa. (use for qmin).
  • qwp= qfc-qwp=unit storage capacity.



qfc = 0.26, qwp = 0.01, qu = unit storage = 0.26-0.01 = 0.25

pedogenesis hysteresis

Lab curve on small compacted specimen, typically ASTM D6836

  • Field curve has lower air entry suction and steeper slope.
  • qs = saturated q
  • qr = residual q
  • a = shape parameter controlling air entry suction
  • n = shape parameter controlling slope
Pedogenesis & Hysteresis
compare field to lab
Compare: Field to Lab

Create field SWCC by adjusting lab-measured SWCC:

  • a adjustment
    • Plastic soils = 13x
    • Non-plastic soils = none
  • n = no adjustment
compare field measured computed storage capacities from acap
Compare Field-Measured & Computed Storage Capacities from ACAP

Good agreement between computed and field-measured storage capacities.

Need to account for effect of pedogenesis on soil properties.


Why model water balance covers?

  • Predict performance relative to a design criterion and/or refine design
  • Sensitivity analysis to determine key design parameters
  • Comparison between conventional and alternative designs.
  • “What if?” questions.

For these purposes, model MUST capture physical and biological processes controlling behavior (e.g., unsaturated flow, root uptake)


Output: Water Balance Quantities

  • Precipitation: water applied to the surface from the atmosphere (unfrozen and frozen)
  • Evaporation: water discharged from surface of cover to atmosphere due to gradient in vapor pressure (humidity)
  • Transpiration: water transmitted to atmosphere from the soil via plant root water uptake
  • Evapotranspiration: evaporation + transpiration
  • Infiltration: water flowing into soil across the surface

Sample Output

Predictions for 2001-2003 for a site in Altamont, CA using LEACHM

Predictions appear realistic, but are not real. All models are mathematical abstractions of reality. Apply appropriate skepticism to predictions.


PET Calculations - 1

  • Data for Input:
  • Air temperature at 2 m (daily minimum, Tmin, and maximum, Tmax), oC
  • Solar Radiation, Rs (MJ/m2-d)
  • Daily average wind velocity, U, at 2 m (m/s)
  • Daily average relative humidity, RH (%)
  • Soil heat flux, G ~ assume = 0

PET Calculations - 2

  • x = 0.665x10-3 P where P is atmospheric pressure in kPa
  • P = 101.3 [(293-0.0065z)/293]5.26 , P in kPa and z in m above mean sea level
  • T = mean daily air temperature (oC) at 2 m, [Tmin+Tmax/2]
  • es = 0.6108exp[17.27 T/(T+237.3)] in kPa and T in oC
  • {compute as average of es determined for Tmin and for Tmax}
  • e = es(RH/100), in kPa, where RH is relative humidity in %
  • D = 4098{0.6108exp[17.27 T/(T+237.3)]}/(T+237.3)2 , kPa/oC

PET Calculations - 3

Rns = net solar radiation = Rs (1-a)

a = albedo (fraction of solar energy reflected)

Rnl = net long wave radiation (emitted from earth)

h = Stefan-Boltzman constant (4.903x10-9 MJ/K4-m2-d)

Tmin and Tmax in oK

Rso = clear-sky solar radiation


PET Calculations - 4

with z in m above mean sea level

Ra = extraterrestrial radiation (MJ/m2-d):

J = Julian day (1-365 or 366)

For latitude: