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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 [email protected]

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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

[email protected]



Covers waste containment
Covers & Waste Containment www.asce.org or amazon.com

Gas vent or collection well

Cover system

Groundwater monitoring well

Waste

Native soil

Groundwater

Leachate collection system

Barrier system


Cover Strategy - Conventional vs. Water Balance Covers www.asce.org or amazon.com

Conventional Cover

Water Balance Cover


Cover Profiles for www.asce.org or amazon.com 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 www.asce.org or amazon.comSavings

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: www.asce.org or amazon.comHow They Work

Precipitation

Evapotranspiration

Infiltration

“Sponge”

L

S = soil water storage

Sc= soil water storage capacity

Percolation if S > Sc


The Balance www.asce.org or amazon.comin 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.


Real Data More Complex – But Predictable www.asce.org or amazon.com


Soil Water www.asce.org or amazon.comRetention In Unsaturated Soil

0

Suction, y

1

-

+

Wilting point

Suction, y

0

2

5

-

+

4

Field capacity

0

3

3

-

+

2

0

1

4

Volumetric Water Content, q

-

+

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

0

+

5

-


Unsaturated hydraulic conductivity
Unsaturated Hydraulic Conductivity www.asce.org or amazon.com

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 www.asce.org or amazon.com

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

WET

SOIL

DRY SOIL


Evaporation and transpiration et
Evaporation and Transpiration (ET) www.asce.org or amazon.com

PET = potential evapotranspiration = max ET for given meteorological condition


Potential Evapotranspiration (PET) www.asce.org or amazon.com

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

http://www.fao.org/docrep/x0490e/x0490e07.htm#solar%20radiation

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 www.asce.org or amazon.com

  • 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 www.asce.org or amazon.com

  • 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 www.asce.org or amazon.com

  • 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 www.asce.org or amazon.com

  • 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 www.asce.org or amazon.comStorage: 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 www.asce.org or amazon.com

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 www.asce.org or amazon.com

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


Example idaho site snow frozen ground
Example: Idaho Site (snow & frozen ground) www.asce.org or amazon.com

Mar-Aug:

0.51

Sept-Feb

0.32


Example texas site no snow frozen grd
Example: www.asce.org or amazon.comTexas Site (no snow & frozen grd.)

Mar-Aug:

0.97

Sept-Feb

0.34


How much water accumulates
How Much Water Accumulates? www.asce.org or amazon.com

  • 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 www.asce.org or amazon.com

Δ S = P – βPET – Λ

0

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


Monthly computation of required storage
Monthly Computation of Required Storage www.asce.org or amazon.com

Fall-Winter

Months

Spring-Summer

Months

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 www.asce.org or amazon.com

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) www.asce.org or amazon.com

For months below threshold, set ΔS = 0

Δ S = P – 0.37*PET

(Fall-Winter)

β= 0.37, Λ= 0

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


Example texas site no snow frozen ground
Example: www.asce.org or amazon.comTexas Site (no snow & frozen ground)

For months below threshold, set ΔS = 0

ΔS = (P – 0.37*PET)-27

(Fall-Winter)

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 S www.asce.org or amazon.comr

Good agreement computed & measured required storage.


Monolithic covers storage capacity
Monolithic Covers: Storage Capacity www.asce.org or amazon.com

What is the storage capacity (Sc)?

Area

qc = water content when percolation transmitted.


Monolithic covers storage capacity1
Monolithic Covers: Storage Capacity www.asce.org or amazon.com

What is available storage (Sa)?

qmin = lowest water content achieved consistently.

Area


Soil water characteristic curve swcc
Soil Water Characteristic Curve (SWCC) www.asce.org or amazon.com

4000

kPa

  • 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.

33

kPa

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


Pedogenesis hysteresis

Pedogenesis & Hysteresis


Compare field to lab
Compare: Field to Lab www.asce.org or amazon.com

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.


Design Step 6 – Water Balance Modeling ACAP

Webinar March 14

P

E

T

R

q

Pr

z


  • Why model ACAPwater 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 ACAPBalance 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 ACAP

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.


Appendix
Appendix ACAP


PET Calculations - 1 ACAP

  • 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 ACAP

  • 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 ACAP

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 ACAP

with z in m above mean sea level

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

J = Julian day (1-365 or 366)

For latitude: http://www.bcca.org/misc/qiblih/latlong.html


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