FLOWTHRU - calculates ground-water flow regimes near shallow surface-water bodies

**FLOWTHRU DESCRIPTION**

Introduction to FLOWTHRU

FLOWTHRU Description

FLOWTHRU Assumptions

FLOWTHRU Input Requirements

Explanation of Graphical Display

Classification of Flow Regimes

Explanation of FLOWTHRU Output Files

FLOWTHRU Documentation

FLOWTHRU Requirements

FLOWTHRU for DOS and Mac is a totally new concept in ground-water modeling software. Rather than being a modeling package where users set up and run a numerical simulation model, FLOWTHRU superimposes precomputed solutions and then displays model results. FLOWTHRU is the result of five years of research. Although developed with shallow lakes in mind, the program can also be applied to wetlands, rivers, streams, canals, channels and drains. The main uses of the program are (1) for determining the depths of ground-water capture zones near shallow-water bodies (analogous to the problem of wellhead protection), and (2) as an educational tool, to allow users to visualize flow patterns near surface-water bodies.

FLOWTHRU is based on a two-dimensional anisotropic vertical section through water bodies which are long in the direction perpendicular to the direction of regional ground-water flow. It combines and displays a set of precomputed solutions obtained using the linear triangular finite-element model, AQUIFEM-N. Steady-state solutions are provided as a set of unformatted files for six water-body geometries and eleven values of bottom resistance. Additional solutions are available on request. FLOWTHRU is menu-driven, and no pre or postprocessors are required. Data requirements include lengths to characterize the geometry of a cross section, bulk hydraulic conductivities, and estimates of water-table slopes and average recharge or evapotranspiration.

FLOWTHRU provides a general framework for classifying ground-water flow patterns near elongated surface-water bodies. The program recognizes a large number of distinct flow regimes (seventeen flow-through, eleven discharge and eleven recharge flow regimes) which can occur under different regional flow and recharge conditions.

FLOWTHRU is the basis for and is supported by a recent journal paper: Nield, S.P., Townley, L.R., and Barr, A.D. (1994), __A framework for quantitative analysis of surface water-ground water interaction: flow geometry in a vertical section__, Water Resources Research, 30(8), 2461-2475.

FLOWTHRU displays heavy ground-water flow patterns in an aquifer near a surface-water body. Although written originally with shallow lakes in mind, the program also applies to wetlands, rivers, streams, canals, channels and drains. FLOWTHRU is based on a two-dimensional vertical section through water bodies which are long in the direction perpendicular to the direction of regional ground-water flow. Depending on the directions of flow at lateral boundaries, FLOWTHRU is relevant to flow - through lakes, to rivers or channels receiving water from both sides, or to channels or elongated pits which provide recharge to the regional aquifer.

FLOWTHRU is not a numerical ground-water model, but rather, combines and displays a set of precalculated solutions. These solutions were obtained using the linear triangular finite-element model, AQUIFEM-N. The solutions are provided as a set of unformatted files.

Numerous display options are possible. A user can choose to work in either nondimensional or physical variables, to view equipotentials, streamlines and/or dividing streamlines, and to view or hide spatial distributions or piezometric head on specified flux boundaries. The spatial distribution of seepage through the bottom of the surface water body can be displayed and written to an output file. Graphical output can also be saved as files containing Hewlett Packard Graphics Language (HPGL) or PostScript. Such files can be sent to a variety of plotters and laser printers.

FLOWTHRU displays steady ground-water flow patterns in a two-dimensional vertical section through a shallow surface-water body. The representation is only valid if the vertical section is aligned with the direction of regional ground-water flow and if the surface-water body is long in the direction perpendicular to that section. The model therefore applies to rivers or canals in valleys with negligible along-valley slope, and to elongated lakes and recharge basins.

FLOWTHRU is not intended for application to non-elongated water bodies. But preliminary results of three-dimensional analysis indicate that FLOWTHRU applies reasonably well to a vertical section through the diameter of large circular water bodies.

FLOWTHRU assumes that water bodies are shallow relative to the thickness of the aquifer. The model represents the water body as an infinitesimally thin layer of constant head lying at the top surface of the aquifer. FLOWTHRU's standard solutions should not be used to simulate water bodies which penetrate deeply into the aquifer, although additional solutions can be provided on request. FLOWTHRU fixes the lateral boundaries of the modeled domain at a distance of twice the aquifer thickness from the edge of the water body (in an equivalent isotropic coordinate system). Flow at the lateral boundaries is assumed to be uniform over the thickness of the aquifer. This assumption has been found to be valid for an isotropic aquifer when the distance from the water body to an upgradient ground-water divide is many times the aquifer thickness. FLOWTHRU assumes that the aquifer is homogeneous but possibly anisotropic with spatially-uniform horizontal and vertical hydraulic conductivities. The resistance of low-conductivity bottom sediments can also be taken into account. FLOWTHRU simulates flow in a rectangular domain with a horizontal upper surface. This is an approximation to real situations with a sloping water table as the upper boundary. But since water table slopes are generally small, the approximation is extremely good.

Aquifer geometry is defined by the length of the water body in the direction of regional ground-water flow (2a), the thickness of the aquifer (*B*), and a length used to represent the resistance of bottom sediments (*D*). The user provides three user-specified boundary flux parameters: the inflow into the aquifer domain at the left boundary (U+), the outflow from the aquifer domain at the right boundary (U-), and the net recharge to the water table on both sides of the water body (R). The user also supplies hydraulic conductivities Kx and Kz. FLOWTHRU is entirely interactive and requires only a few keystrokes for each new result. Standard solutions are provided for a number of values of 2a/*B*, varying by factors of 2, and for a number of values of *D*/*B*. If a user specifies values such that the ratios 2a/*B* or *D*/*B* are not exactly equal to those available, FLOWTHRU modifies the requested values to force a perfect match with those available and informs the user that an approximation has been made. Anisotropy is taken into account by shrinking horizontal grids by the square root of the anisotropy ratio.

**Explanation of Graphical Display**

The top half of the graphical display echoes the user-defined variables. The bottom half of the graphical display generally shows a large rectangle which represents the aquifer domain and contains the dividing streamline(s), and contours of streamfunction and equipotentials. The extent of the surface-water body is indicated above the middle of the rectangular domain. Arrows show the direction of fluxes through the sides and top of the aquifer.

At different times, users may wish to view different types of information such as: equipotentials only, streamlines only, equipotentials and streamlines (a classical flow net), equipotentials, streamlines and dividing streamlines, or dividing streamlines only.

Rectangular boxes at the ends of the aquifer show calculated vertical distributions of piezometric head along these prescribed flux boundaries. Ideally, the end head boxes should appear as empty rectangles, i.e., there should be no apparent deviation from a uniform vertical distribution of heads.

Two boxes along the top of the aquifer also show head distributions which can be interpreted as water-table elevations, i.e., the phreatic surface. The rectangular box directly above the water body shows the seepage flux distribution through the bed of the water body. It is possible to enlarge the spatial distribution of seepage flux to fill the bottom half of the display.

**Classification of Flow Regimes**

Nield et al. (1994) identify eleven types of recharge and discharge regimes (in which the surface-water body recharges water or receives water from the aquifer over the whole of its bottom) and seventeen types of flow-through regimes (in which the water body recharges water to and receives water from the aquifer in different parts of its bottom). FLOWTHRU identifies flow regimes on the basis of the occurrence of maxima and minima of potential f and streamfunction y on the boundary of the domain. Flow regimes can then be described in terms of the number and location of stagnation points and dividing streamlines.

Stagnation points can occur in the following locations:

- on the bottom of the water body in which case there must be a reversal in the direction of vertical flow (exhibited by a local maximum or minimum in y) and the regime must be a flow-through regime;
- on the top surface of the aquifer (exhibited by a local maximum or minimum in f) but this can occur when R = 0;
- on the bottom of the aquifer (exhibited by a local maximum or minimum in f); or
- in the interior of the domain (exhibited by a saddle point in both f and y).

Stagnation points are characterized by equipotentials meeting at 90^{o}, streamlines meeting at 90^{o} and equipotentials and streamlines meeting at 45^{o}.

**Explanation of FLOWTHRU Output Files**

FLOWTHRU produces a number of output files: FLOWTHRU.prt contains a formal description of each flow regime; FLOWTHRU.flx contains seepage values through the bottom of the water body; FLOWTHRU.rec is a record of all the cases displayed in each run of FLOWTHRU; and plotnn.hpg contains graphics command in Hewlett Packard Graphics Language (HPGL) which can be sent to an HPGL-compatible plotter or printer; and plotnn.ps contains PostScript graphics commands which can be sent to a PostScript printer.

The FLOWTHRU user's manual is 85 pages long, extremely detailed and fully indexed. It provides a detailed worked example of how FLOWTHRU can be used to answer typical questions.

**FLOWTHRU Requirements: **FLOWTHRU/PC - PC 486/Pentium, 2 MB RAM, and math coprocessor. FLOWTHRU/Mac - Macintosh with 2 MB RAM.

Προηγούμενη σελίδα