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

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]


For more details please see our book which is available at www asce org or amazon com

For more details, please see our book, which is available at www.asce.org or amazon.com


Covers waste containment

Covers & Waste Containment

Gas vent or collection well

Cover system

Groundwater monitoring well

Waste

Native soil

Groundwater

Leachate collection system

Barrier system


Designing water balance covers et covers for landfills and waste containment by

Cover Strategy - Conventional vs. Water Balance Covers

Conventional Cover

Water Balance Cover


Designing water balance covers et covers for landfills and waste containment by

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.


Designing water balance covers et covers for landfills and waste containment by

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


Designing water balance covers et covers for landfills and waste containment by

Water Balance Covers: How They Work

Precipitation

Evapotranspiration

Infiltration

“Sponge”

L

S = soil water storage

Sc=soil water storage capacity

Percolation if S > Sc


Designing water balance covers et covers for landfills and waste containment by

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.


Designing water balance covers et covers for landfills and waste containment by

Real Data More Complex – But Predictable


Designing water balance covers et covers for landfills and waste containment by

Soil Water Retention 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

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

WET

SOIL

DRY SOIL


Evaporation and transpiration et

Evaporation and Transpiration (ET)

PET =potential evapotranspiration = max ET for given meteorological condition


Designing water balance covers et covers for landfills and waste containment by

Potential Evapotranspiration (PET)

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.


Designing water balance covers et covers for landfills and waste containment by

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?


Designing water balance covers et covers for landfills and waste containment by

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?


Designing water balance covers et covers for landfills and waste containment by

Design Process - 3

  • Final Design

    • Geometric design

    • Surface water management

    • Gas management

    • Erosion control strategies

    • Specification preparation

  • Regulatory approval

  • Bid preparation & contractor selection


Designing water balance covers et covers for landfills and waste containment by

 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


Designing water balance covers et covers for landfills and waste containment by

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


Example idaho site snow frozen ground

Example: Idaho Site (snow & frozen ground)

Mar-Aug:

0.51

Sept-Feb

0.32


Example texas site no snow frozen grd

Example: Texas Site (no snow & frozen grd.)

Mar-Aug:

0.97

Sept-Feb

0.34


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

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

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

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

(Fall-Winter)

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

(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 Sr

Good agreement computed & measured required storage.


Monolithic covers storage capacity

Monolithic Covers: Storage Capacity

What is the storage capacity (Sc)?

Area

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.

Area


Soil water characteristic curve swcc

Soil Water Characteristic Curve (SWCC)

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

  • 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.


Designing water balance covers et covers for landfills and waste containment by

Design Step 6 – Water Balance Modeling

Webinar March 14

P

E

T

R

q

Pr

z


Designing water balance covers et covers for landfills and waste containment by

  • 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)


Designing water balance covers et covers for landfills and waste containment by

  • 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


Designing water balance covers et covers for landfills and waste containment by

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.


Appendix

Appendix


Designing water balance covers et covers for landfills and waste containment by

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


Designing water balance covers et covers for landfills and waste containment by

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


Designing water balance covers et covers for landfills and waste containment by

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


Designing water balance covers et covers for landfills and waste containment by

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


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