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 Soil and Design Data