NAMEsharp - A quasi-three-dimensional, numerical finite-difference model to simulate freshwater and saltwater flow separated by a sharp interface in layered coastal aquifer systemsABSTRACTWhen 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.METHODAn 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.HISTORYVersion 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 releaseDATA REQUIREMENTSSHARP 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 OPTIONSSee documentation.SYSTEM REQUIREMENTSSHARP 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.DOCUMENTATIONEssaid, 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.REFERENCESEssaid, 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.CONTACTSOperation 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 303-202-4693SEE ALSOmocdense(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)

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