HELP Model Description

 

The HELP model requires daily climatologic data, soil characteristics, and design specifications to perform the analysis. Daily data may be input by the user, generated stochastically, or taken from the model's historical database.

 

Necessary soil data

        Porosity

        Field capacity

        Wilting point

        Saturated hydraulic conductivity

        Soil Conservation Service (SCS) runoff curve number for antecedent moisture condition II.

 

The model contains default soil characteristics for 42 material types for use when measurements on site-specific estimates are not available.

 

Design specifications

        Slope and maximum drainage distance for lateral drainage layers

        Layer thickness

        Leachate recirculation procedure

        Surface cover characteristics

        Information about geomembrane

 

Figure 1. Schematic Profile View of a Typical Solid Waste Landfill

 

Synthetic Weather Generation

The HELP user has the option of generating synthetic daily precipitation data rather than using default or user-specified historical data. Similarly, the HELP user has the option of generating synthetic daily mean temperature and solar radiation data rather than using user-specified historical data. The generating routine is designed to preserve the dependence in time, the correlation between variables and the seasonal characteristics in actual weather data at the specified location. Coefficients for weather generation are available for up to 183 cities in the United States.

 

Moisture Retention and Hydraulic Conductivity Parameters

The HELP program requires values for the total porosity, field capacity, wilting point, and saturated hydraulic conductivity of each layer that is not a liner. Saturated hydraulic conductivity is required for all liners. Values for these parameters can be specified by the user or selected from a list of default values provided in the program. The values are used to compute moisture storage, unsaturated vertical drainage, head on liners and soil water evaporation.

 

Moisture Retention Parameters

The soil water content or storage used in the HELP model is on a per volume basis (q), volume of water per total volume. Engineers commonly express moisture content on a per mass basis. The two can be related to each other by knowing the dry bulk specific gravity (Gdb) of the soil (ratio of dry bulk density to water density), (q = w Gdb), or wet bulk specific gravity, Gwb (ratio of wet bulk density to water density), [q = w Gwb / (1+ w)].

 

Total porosity is an effective value, defined as the volumetric water content when the pores contributing to change in moisture storage are at saturation.

 

Field capacity is the volumetric water content at a soil water suction of 0.33 bars or remaining after a prolonged period of gravity drainage without additional water supply. Wilting point is the volumetric water content at a suction of 15 bars or the lowest volumetric water content that can be achieved by plant transpiration. These moisture retention parameters are used to define moisture storage and relative unsaturated hydraulic conductivity.

 

The HELP program requires that the wilting point be greater than zero but less than the field capacity. The field capacity must be greater than the wilting point and less than the porosity. Total porosity must be greater than the field capacity but less than 1 (one). The general relation among moisture parameters and soil texture class is shown below.

 

 

The HELP user can specify the initial volumetric water contents of all non-liner layers. Soil liners are assumed to remain saturated at all times. If initial water contents are not specified, the program assumes values near the steady-state values (allowing no long-term change in moisture storage) and runs a year of simulation to initialize the moisture contents closer to steady state.

 

Hydraulic Conductivity

The HELP program uses the saturated and unsaturated hydraulic conductivities of soil and waste layers to compute vertical drainage, lateral drainage and soil liner percolation. The vapor diffusivity for geomembranes is specified as a saturated hydraulic conductivity to compute leakage through geomembranes by vapor diffusion.

 

Saturated hydraulic conductivity is used to describe flow through porous media where the void spaces are filled with a wetting fluid (e.g. water). The saturated hydraulic conductivity of each layer is specified in the input.

 

Unsaturated hydraulic conductivity is used to describe flow through a layer when the void spaces are filled with both wetting and non-wetting fluid (e.g. water and air).

 

 

HELP Program Input Options

 

Enter / Edit Weather Data

Enter / Edit Soil and Design Data

Execute Simulation

 

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