Solution to Exercise 2
2) Examine the scaled cross section of Figure Exercise 2 showing two surface-water bodies (solid blue: rivers, lakes or wetlands) in an isotropic and homogeneous setting. Equipotential lines (dashed) were constructed from a monitoring well network containing nests of wells (vertical black lines) open only at the bottom. The water table is indicated by a solid blue line. Equipotential lines represent intervals of 5 (any units can be used).
a) Construct flowlines.
b) Label local, intermediate and regional flow systems if they are present.
c) Identify where stagnation points form and circle the locations on the cross section if they are present.
SOLUTION:
a) Flow lines (blue arrows, local system; red arrows intermediate system; black arrows, regional system) are constructed at right angles to equipotential lines. A large number of flow lines group in the recharge area of the upper left side.
b) Flow systems are labeled. Local flow systems (blue arrows) originate in recharge areas (water table highs) and discharge at adjacent discharge areas (surface-water bodies in this case). The one intermediate system (red arrows) originates at a recharge area (left) and discharges to a discharge area not immediately adjacent to the recharge area. It incorporates at least one local flow system. It discharges to the surface-water feature with a stage of 90. The regional flow system (black arrows) starts at the left and flows underneath the local and intermediate flow systems discharging to the surface-water feature with a stage of 90.
c) Stagnation zones (red dots) occur at groundwater divides where flow systems converge (local and intermediate). Winter (1976) describes a stagnation point as “… a point on the divide (between flow systems) at which the head is a minimum compared to every other point along the divide.” … “It is a point in the flow field at which vectors of flow are equal in opposite directions and therefore cancel. … The stagnation point is a point of diversion of groundwater-flow paths.” In this system the stagnation points occur on the flow divide where two local and the intermediate system converge.