Environmental Engineering Reference
In-Depth Information
Fig. 3.38 Draw contour
lines based on the heads
displayed at the monitoring
wells and the wetland stage
of the flow net bounded by adjacent equipotential lines and stream tubes consist of
the area of the flow net bounded by adjacent flow lines.
The example in Fig.
3.37
contains seven equipotential head drops and six stream
tubes. The flow-net equation can be written as
MKbH
n
Q
ΒΌ
(3.58)
where
M
is the number of stream tubes,
n
is the number of equipotential head drops,
K
is the assumed hydraulic conductivity,
b
is the sediment thickness in the third
dimension, and
H
is the total head drop across the flow net.
M
is commonly presented
as m in most texts, but we use upper-case
M
here to distinguish it from
m
,the
shoreline length presented earlier in Fig.
3.21
.
Q
is in units of volume per time.
Some basic steps to follow are:
1. Determine boundaries and boundary conditions,
2. Draw equipotential lines by contouring head data from wells and wetland
stage,
3. Draw flow lines to create approximate squares (you should be able to draw a
circle bounded by the equipotential lines and flow lines),
4. Flow lines cross equipotential lines at right angles (assuming we have isotropic
conditions) and flow lines also intersect constant-head boundaries at right angles,
5. You can draw half-equipotential lines for areas with smaller gradients.
6. Five to ten flow lines usually are sufficient,
7. Count up stream tubes and equipotential drops to determine
M
and
n
,
8. Determine
H
, and estimate
b
and
K
.
9. Calculate
Q
for flow to and/or from the wetland.
Let's see how well this can work. The same wetland setting in Short Exercise 8 is
displayed in Fig.
3.38
. This is the same wetland shown in Fig.
3.21
but with head
values changed for three of the seven wells. Your task will be to determine the
extent to which changes in head will affect the interpretation of flow of groundwater
to and from the wetland. Draw contour lines based on the head data and then draw
flow lines based on the instructions provided above. After that, you will count up
flow tubes and head drops and calculate flow to the wetland and flow from the
wetland. Use
K
and
b
values from Short Exercise 8. You will then be able to answer
the following questions:
