NAME
sharp - A quasi-three-dimensional, numerical finite-difference model
to simulate freshwater and saltwater flow separated by a
sharp interface in layered coastal aquifer systems
ABSTRACT
When the width of the freshwater-saltwater transition zone is small
relative to the thickness of the aquifer, it can be assumed that
freshwater and saltwater are separated by a sharp interface. The
sharp interface modeling approach, in conjunction with vertical
integration of the aquifer flow equations, facilitates regional
scale studies of coastal areas. This approach does not give
information concerning the nature of the transition zone but does
reproduce the regional flow dynamics of the system and the response
of the interface to applied stresses. SHARP is a quasi-three-
dimensional, numerical model that solves finite-difference
approximations of the equations for coupled freshwater and saltwater
flow separated by a sharp interface in layered coastal aquifer
systems. The model is quasi-three dimensional because each aquifer
is represented by a layer in which flow is assumed to be horizontal.
METHOD
An implicit finite-difference discretization scheme that is central
in space and backward in time is used to solve the freshwater and
saltwater flow equations for each model layer. Spatial
discretization is achieved using a block-centered finite-difference
grid that allows for variable grid spacing. In the central
difference approximations for the space derivatives, the thicknesses
at the grid block boundaries are linearly interpolated, and the
conductivity terms are estimated using the harmonic mean of nodal
values. At blocks containing pumped wells, the amount of freshwater
and saltwater extracted depends on the position of the interface
relative to the elevation of the screened interval of the well. The
rate of freshwater and (or) saltwater extraction from a block,
relative to the total fluid extraction rate, is determined linearly
on the basis of the proportion of screen penetrating the freshwater
and saltwater zones relative to the total open interval of the well.
The interface elevation in each finite-difference block is
calculated using the numerically-determined freshwater and saltwater
head distributions. The shape of the interface can be obtained by
connecting the discretized interface elevations. The position of
the interface tip (the intersection of the interface with the top of
the aquifer) and the interface toe (the intersection of the
interface with the bottom of the aquifer) are located by linearly
projecting a line defined by the interface elevations in adjacent
blocks until it intersects the top and bottom of the aquifer.
HISTORY
Version 1.1 1999/05/10 - Minor changes to bring code into closer
adherence to Fortran 77 standard and to improve code portability;
renamed subroutine "READ" as "READIN" to avoid name conflict with
Fortran READ statement.
Version 1.0 1990/09/12 - Initial release
DATA REQUIREMENTS
SHARP requires all of the input parameters typically required by a
finite-difference ground-water flow model (initial conditions,
boundary conditions, aquifer properties). However, because it
solves both freshwater and saltwater flow equations, it has
additional input requirements. The fresh and saltwater specific
gravities and dynamic viscosities must be specified. Freshwater
hydraulic conductivities are specified and saltwater hydraulic
conductivities are calculated in the model. Fresh and saltwater
specific storages, effective porosity and confining layer leakance
values must be specified. For interface tip and toe tracking, SHARP
requires elevations of the base of each layer and the thickness of
the layer. Offshore bathymetric elevations are required to
correctly represent offshore boundary conditions. See documentation
for details.
OUTPUT OPTIONS
See documentation.
SYSTEM REQUIREMENTS
SHARP is written in Fortran 77 with the following extension: use of
names longer than 6 characters. The code has been used on UNIX-
based computers and on DOS-based 386 or greater computers having a
math coprocessor.
DOCUMENTATION
Essaid, H.I., 1990, The computer model SHARP, a quasi-three-
dimensional finite-difference model to simulate freshwater and
saltwater flow in layered coastal aquifer systems: U.S.
Geological Survey Water-Resources Investigations Report 90-4130,
181 p.
REFERENCES
Essaid, H.I., 1990, A multilayered sharp interface model of coupled
freshwater and saltwater flow in coastal systems--model
development and application: Water Resources Research, v. 26, no.
7, p. 1431-1454.
Essaid, H.I., 1992, Simulation of freshwater and saltwater flow in
the coastal aquifer system of the Purisima Formation in the
Soquel-Aptos Basin, Santa Cruz County, California: U.S.
Geological Survey Water-Resources Investigations Report 91-4148,
35 p.
CONTACTS
Operation and Distribution:
U.S. Geological Survey
Hydrologic Analysis Software Support Program
437 National Center
Reston, VA 20192
h2osoft@usgs.gov
Official versions of U.S. Geological Survey water-resources analysis
software are available for electronic retrieval via the World Wide
Web (WWW) at:
http://water.usgs.gov/software/
and via anonymous File Transfer Protocol (FTP) from:
water.usgs.gov (path: /pub/software).
The WWW page and anonymous FTP directory from which the SHARP
software can be retrieved are, respectively:
http://water.usgs.gov/software/sharp.html
--and--
/pub/software/ground_water/sharp
If you would like to obtain the price of and (or) order paper copies
of USGS reports, contact the USGS Branch of Information Services at:
USGS Information Services
Box 25286
Denver Federal Center
Denver CO 80225
To inquire about Open-File Reports or Water-Resources Investigations
Reports:
Tel: 303-202-4200; Fax 303-202-4695
To inquire about other USGS reports:
Tel: 303-202-4700; Fax 303-202-4693
SEE ALSO
mocdense(1) - A two-constituent solute transport model for ground
water having variable density
sutra(1) - Saturated and (or) unsaturated, constant or
variable-density fluid flow, and solute or energy
transport (2-dimensional finite-element code)