RESEARCH· ·1970·

B220
RELATION· BETWEEN GROUND
erally prevail. In extended dry periods, however, the
water level drops below the confining layer of silt and
clay.·
Four observation wells, consisting of 2-inch pipe
with 60-gauze screen set approximately 9 feet below
land surface, were installed on line with the gage and
perpendicular to the river. Elevations referenced to
existing gage datum were determined for each well.
After the site was chosen and the observation ·wells
were installed, periodic measurements of water level
were· made to define the direction of subsurface flow.
Measurements were made during floods in the late winter
and spring of 1968. Analyses of the exchange
mechanism for three peaks which occurred are discussed.
The stream-stage hydrographs for one relatively
small peak, one medium peak, and one relatively
high peak (overbank) are shown in figure 2.
METHODS AND RESULTS ·OF STUDY
The method used in the analysis of these peaks will
be illustrated using peak 3 of figure 2. Potentiometric
profiles for stages A -E are shown in figure 3.
Under recession conditions (stage .A.), ground-water
movement is toward and into the stream. As the stream
level rises above the water level in the adjacent aquifer
(stage B) the hydraulic gradient at the interface is reversed
and water moves from the stream into the
banks. Immediately after the beginning of the rise,
therefore, the discharge of ground water into the
stream ceases. However, ground water continues to
move from the valley margins toward the stream, but
its progress is blocked by the opposing gradient. As
a result, the amount of ground water that would normally
have been discharged into the stream is tempo-
14
13
12
t;j
l.lJ 11
L&..
z
~ 10
:I:
(!)
ijj 9
:I:
w
(!) 8
<
(!)
6
Peak 1
----- Bankfull stage
Time .at which potentiometric{A 8
C
0
E
prof1les were made for stages 1 1 1 1 1
A-E. Profiles shown on figure
3
50 100 150 200 250 0 50 100 150 0 50 100 150
Began 1800 hours
January 27, 1968
Began 1530 hours
March 24, 1968
Began 1700 hours
April 3, 1968
TIME, IN HOURS
E'muRE 2.~Stream-stage hydrographs for three storm-peak flows
in winter and .spring of 1968.
WATER AND SURFACE WATER
13 Land surface
4 3
12 Well number
11
l.lJ
(!)
<
(!) 7
6
13
12
t;j 11
l.lJ
L&..
~ 10
~
a 9
ijj
J:
w 8
(!)
<
(!) 7
6
0 100 200 300 400 500 600 700 800 900 1000 1100 1200
HORIZONTAL DISTANCE, IN FEET
Land surface
I
4 3 2 Gage
Well number
Silt and clay
----------------~--~
Sand
'-
0 100 200 300 400 500 600 700 800 900 1000 1100 1200
HORIZONTAL DISTANCE, IN FEET
FIGURE 3.-Potentiometric profiles of storm peak 3 (fig. 2) at
stages and times shown on figure 2. Arrows indicate flow
direction.
rarily s'tored. This is indicated by corresponding rises
in the water level in the observation wells (stage 0).
Soon after the peak stream stage, the water in bank
storage (Langbein and Iseri, 1960) begins to move
back into the stream. At stage E, a gradient toward
the stream has been reestablished throughout the
section.
A convenient method for analyzing the volumetric
exchange of water between the stream and the ground
is by use of Darcy's law. It is stated mathematically
as the equation.
Q=TiL,
(1)
where
Q= dischar~e,
T~ transmissivity,
i =hydraulic gradient, and
L==width of section. ·
But
v
Q=-,
(2)
t
., '
""'
·~!
~' "·