The following are source/sink attributes:
Specified head attributes are typically used with arcs but may be used with points, arcs, or polygons. The head may be assigned as either a constant or transient value. The object is represented in the grid model using the Time Variant Specified Head (CHD) package. This is done so the flux into (or out of) the group of cells can be computed.
When the specified head attribute is assigned to a polygon, the head is assigned uniformly over the entire polygon. When assigned to an arc, separate head values are applied to each of the nodes on the ends of the arc and the head is assumed to vary linearly between the nodes. When the attribute is assigned to a point, the head is assigned directly to the cell containing the point.
General head attributes may be defined using points, arcs, or polygons. General head attributes include elevation and conductance. The elevation may be assigned in either constant or time-varying format. Values for conductance also can vary with time.
When the general head attribute is assigned to a polygon, the head and conductance are applied uniformly over the entire polygon. When assigned to an arc, the conductance is applied uniformly over the arc, but separate head values are applied to each of the nodes on the ends of the arc and the head is assumed to vary linearly between the nodes. When the attribute is assigned to a point, the head and conductance values are assigned directly to the cell containing the point.
The variable head attribute is assigned to polygons. When a conceptual model is constructed using feature objects, the entire area of the model must be covered with some type of polygon. Any cells lying outside the defined polygons are assumed to be inactive. The variable head attribute is assigned to the polygon(s) covering the region of the model not covered by one of the other polygonal attribute types (specified head, general head or drain).
River attributes are typically used with arcs but may also be associated with polygons and points. The river parameters include elevation, stage, and conductance. Elevation is constant. The river stage and conductance may either be constant or vary with time.
When the river attribute is assigned to an arc, the conductance is applied uniformly over the arc, but separate elevations and stage values are applied to each of the nodes on the ends of the arc, and the elevation and stage are assumed to vary linearly between the nodes. When a river object is defined using a polygon or a point, all of the values are assigned directly to the cell(s) overlapped by the polygon or point.
Streams are used by the Stream/Aquifer Interaction package. Streams are always assigned to arcs, each arc representing one segment of the stream network. When creating the stream network, care should be taken to ensure that the arcs are defined in the proper direction. Specifically, the direction of the streams (upstream to downstream) must be consistent as shown in the following figure:
Proper Direction of Arcs on Stream Network.
When defining a stream network, each arc should be created from upstream to downstream. GMS assumes that when you create an arc, the first node you create on the arc is the upstream node and the last node is the downstream node. If you create an arc in the wrong direction, select the arc and choose the Feature Objects | Reverse Arc Direction command.
Once a stream network is created using a set of arcs, some of the stream attributes are assigned to the arc and some are assigned to the nodes of the arc. The attributes assigned to the arc are:
conductance
width
roughness coefficient
sinuosity factor (the sinuosity factor is multiplied by the arc length when GMS automatically computes the slope of the stream segment)
the incoming flow rate (this value must be specified for the arcs at the upper reaches of the stream network)
diversion flag (an arc can only be marked as a diversion if it has an upstream arc and there is another arc going downstream from the same junction)
The attributes assigned to the nodes are:
the elevation of the top of the streambed
the elevation of the bottom of the streambed
the initial stage
When the Map -> MODFLOW command is selected, GMS automatically does the following: classifies all of the cells beneath the streams as reaches, builds segments, numbers the reaches and segments, and assigns the appropriate values to the reaches and segments. These values defined at the nodes are linearly interpolated across the arcs when the model is converted. The slope assigned to reaches is computed by dividing the difference in the streambed top elevations at the ends of the arc by the arc length multiplied by the sinuosity factor.
Horizontal flow barriers representing slurry trenches, sheet pile walls, etc. are defined using arcs. Each arc is assigned a hydraulic characteristic. When the Map -> MODFLOW command is selected, the sequence of cell boundaries closest to the barrier arc are determined and classified as barriers for the HFB package.
The drain attribute may be associated with points, arcs, or polygons. There are two parameters that are associated with a drain: elevation and conductance. Elevation may be stored in constant format only. Conductance may be constant or vary with time.
When a polygon is defined as a drain, the elevation and conductance values are applied uniformly over the entire polygon. When an arc is assigned to be a drain, the conductance is applied uniformly over the arc but separate elevation values are applied to each of the nodes on the ends of the arc and the elevation is assumed to vary linearly between the nodes. When a point is classified as a drain, the elevation and conductance values are assigned directly to the cell containing the point.
The seepage face attribute may be associated to points, arcs, or polygons. There are two parameters that is associated with a seepage face: conductance and elevation offset. Conductance may be constant or vary with time. The elevation offset is used to offset the bottom elevation of the boundary condition from the top elevation of the 3D grid.
When this option is assigned to a polygon, all of the cells in the MODFLOW grid that are coincident with the polygon are assigned a drain object where the drain elevation is offset from the top elevation of the cell. The conductance for the drain cells is computed from a user-defined factor (typically a large value indicating minimal resistance). During the MODFLOW simulation, when the head is below the top elevation, the seepage face drains have no effect. If the head rises above the top elevation, the water begins to exit through the drains and the computed head is constrained to be approximately equivalent to the top elevation (assuming a large enough conductance).
The specified flow attribute may be associated to both arcs and polygons. In either case, the user specifies a total flow rate [L^3/T] for the object.
When this option is assigned to a polygon, all of the cells in the MODFLOW grid that are coincident with the polygon or arc are assigned a partition of the flow rate. For arcs, the portion of flow assigned to a cell is factored by the length of the arc overlapped by the cell divided by the total arc length. For polygons, the portion of flow assigned to the cell is factored by the area of the cell (or portion thereof assuming a partial overlap) divided by the total polygon area. The flow rates are assigned to the cells in the form of wells. This new option makes it possible to simulate well fields or lateral inflow on the model boundary.
The well attribute may only be associated with points. A well attribute requires two parameters: flow rate and concentration. Both parameters may either be constant or transient.
Refine attributes are assigned to points or nodes and are used to automatically increase the grid density around a point when the grid is constructed. Although refine attributes may be associated with any point or node, they are usually assigned in conjunction with wells.
Four of the object types listed above, general head, rivers, streams, and drains, include a conductance parameter. MODFLOW uses the conductance to determine the amount of water that flows in or out of the model due to general head, river, stream, and drain type stresses. The manner in which the conductance term should be computed and entered depends on whether the feature object is a polygon, arc or point. Before explaining this fully, a short review of the definition of conductance is appropriate. Darcy's law states:
where Q is the flow rate, k is the hydraulic conductivity, i represents the hydraulic gradient, and A represents the gross cross-sectional area of flow. Darcy's law can also be expressed as:
where DH represents the head loss and L represents the length of flow. Since the unknown on the right side is the head, it is convenient to group all of the other terms together and call them conductance:
This results in the following general definition for conductance:
This may be represented more specifically in the following form.
Where t represents the thickness of the material in the direction of flow, and lw represents the cross-sectional area perpendicular to the flow direction.
In the case of a river boundary condition, the conductance is defined in MODFLOW as the hydraulic conductivity of the river bed materials divided by the vertical thickness (length of travel based on vertical flow) of the river bed materials, multiplied by the area (width times the length) of the river in the cell. The last term, area, is the hardest parameter to determine by hand since it varies from cell to cell.
Fortunately, GMS can automatically calculate the lengths of arcs and areas of polygons. Therefore, when a conductance is entered for an arc, it should be entered in terms of conductance per unit length. For example, in the case of rivers, conductance should be entered as:
Where t is the thickness of the material and w is the width of the material along the length of the arc. When GMS applies the boundary condition from the arc to the grid cell, it automatically multiplies the entered value of conductance by the length of the arc that intersects the cell to create an accurate conductance value for the cell.
For polygons, conductance should be entered in a conductance per unit area form:
Where t is the thickness of the material. When GMS converts the stress from a polygon to a grid cell, it automatically multiplies the entered value of conductance by the area of the cell that is covered by the polygon to create an appropriate conductance value for the cell. This restores the dimensional accuracy to the expression for conductance.
When a general head, river, stream or drain attribute is assigned to an individual point, the conductance should be entered as a normal conductance value. This conductance is then directly assigned to the cell containing the point.
Nodes may be connected to multiple arcs of different types. Because of this, nodes may have more than one type of attribute. Nodes are limited though, to having only the same types of attributes that the attached arcs have.
Related Links:
MODFLOW Conceptual Model Approach
Steps in Defining a Conceptual Model