INFILTRATION

Infiltration is a key component in the water cycle; it brings water from the surface to the subsurface.  The quantity and quality of the water infiltrating into the ground is often monitored for flood forecasting, groundwater recharge calculations, or for contaminant migration.  In some cases the infiltrating water can become contaminated as it flows through contaminated media, such as landfills and mine tailings.  The creation of this contaminated water can be monitored and predicted using groundwater models such as Visual HELP, which is used for landfill design, monitoring and management.

In this newsletter other properties of infiltration will be discussed, such as:

1.  What causes infiltration?

2.  What affects infiltration?

3.  How can infiltration been quantified?

4.  What methods are available for calculating infiltration?

 

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1.  What causes infiltration?

Infiltration is caused when a tension is exerted between the soil particles and the water, this tension draws the moisture downwards into the ground through capillary passages.

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2.  What affects infiltration?

Infiltration is affected by many things.  The properties of infiltration vary from soil to soil, and from one moisture content to another in the same soil.  In general, infiltration is dependent on soil physical properties (such as porosity and soil type), vegetative cover, soil moisture conditions, rainfall intensity and the surface slope.  For example, conditions that may encourage high infiltration rates would include coarse soils, well-vegetated land and low initial soil moisture.

 

 

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3.  How can infiltration be quantified?

Infiltration can be quantified in a variety of ways.  For example, the infiltration capacity of a soil determines how much water the soil can take, the infiltration rate is how fast the water is infiltrating into the soil, and the cumulative infiltration is how much water has infiltrated into the soil up to a point in time.  The calculations for the variety of infiltration parameters vary in complexity from using reported average rates for soil classes, to the solution of differential equations governing flow in unsaturated media.

 

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4.  What methods are available for calculating infiltration?

As previously mentioned, there are several methods available for calculating infiltration.  One of the most popular is the Green-Ampt model, developed initially in 1911.  This is an approximate infiltration model based upon Darcy's Law.  This method requires an iterative process between the infiltration rate and the cumulative infiltration:

The Green-Ampt model has been adapted several times since 1911 to fit certain sites and conditions.

There are many more other types of infiltration models.  These include the SCS Runoff Curve model developed by the Soil Conservation Service (SCS).  In this method, the effects of land use and treatment are taken into account.  This method was empirically developed from studies of small agricultural watersheds, and was not designed to estimate infiltration capacity.

Other infiltration models include the Mein-Larson model, which uses the Green-Ampt formulation, the Huggins-Monke model, in which time dependencies avoided by introducing soil moisture as the dependent variable, and several more including Holtan (1961), Philip (1957) and Morel-Seytoux (1973). 

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There are many resources available for more information on infiltration.  For this newsletter the following textbooks were used:

Fetter, C.W. (1988).  Applied Hydrogeology, Third Edition.  Published by Prentice-Hall, Inc., New Jersey, USA.

Freeze, R.A., and J.A. Cherry (1979).  Groundwater.  Published by Prentice-Hall, Inc., New Jersey, USA.

Julien, P.Y. (2002).  River Mechanics.  Published by Cambridge University Press. Cambridge, UK.

Trindall, J.A. and J.R. Kunkel (1999).  Unsaturated Zone Hydrology for Scientists and Engineers.  Published by Prentice-Hall, Inc., New Jersey, USA.

Viessmann Jr., W. and G.L. Lewis (1996).  Introduction to Hydrology, Fourth Edition.  Published by HarperCollins College Publishers, New York, USA.

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VISUAL HELP

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Visual HELP is a fully developed modelling environment for evaluating and optimizing hydrologic landfill design.  It is capable of designing landfills, predicting leachate mounding and evaluating potential leachate seepage to the groundwater table.

The HELP model (Hydrologic Evaluation of Landfill Performance) was developed to help hazardous waste landfill designers and regulators to evaluate the performance of proposed landfill designs.  It was developed by the U.S. Army Engineer Waterways Experiment Station for the U.S. Environmental Protection Agency.  The original version of HELP was developed in 1984, and many advances in technology and scope have been incorporated into the code since.  Visual HELP provides a user-friendly interface to this model.

This newsletter will describe some of the capabilities of Visual HELP, in addition to some of the new tools and enhancements available in the newest version of the model.

 

What are some of the capabilities of

Visual HELP?

1.  Easy to Use

2.  Project Wizard

3.  Automatic Report Generator

4.  Weather Generator

5.  Teamwork

 

What are some of the new tools and enhancements available?

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1.  Easy to Use

The familiar windows based design makes Visual HELP easy to use.  The program includes three views:  the Project Tree View, which displays the Visual HELP project you are working on, the names of each model profile and each layer in the active model profile; the Profile View, which provides a graphical representation of the landfill profile; and the output view, which allows the user to display all of the generated weather and modelling data.  The simultaneous presentation of all three views aid in the ease of use of this model.

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2.  Project Wizard

Visual HELP includes a project wizard which provides the user with a step-by-step wizard to guide them through the process of creating a model.

 

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4.  Automatic Report Generator

Visual HELP also contains an automatic report generator which can quickly produce a rich text document with tabulated input data, profile graphics and a display of results.

 

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3.  Weather Generator

Visual HELP comes equipped with the WHI Weather Generator.  This weather generator is a built in component for generating statistically reliable weather for almost any location in the world for up to 100 years.  This generator uses more than 3000 international weather stations, and the database is also customizable.  The weather generator uses the weather generation algorithm developed by the Agricultural Research Service of the U.S. Department of Agriculture.

 

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5.  Teamwork

Visual HELP is fully equipped to handle several users working on the same project.  All the project files can be run over a networked system to make sharing and working in teams much easier.

 

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What are some of the new tools and enhancements available?

The HELP program is being continuously updated, and thus so is Visual HELP.  Some of the new tools and enhancements available in the most recent version include:

• the default soil material texture list has been expanded to contain additional waste materials, geomembranes, geosynthetic drainage nets and compacted soils

• computations of leachate recirculation and groundwater drainage into the landfill have been added

• leakage through the geomembranes due to manufacturing defects (pinholes) and installation mishaps (tears and punctures) is accounted for

• a frozen soil model has been added to improve infiltration and evapotranspiration predictions in cold regions.

• and many, many more!

 

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