Handbook of Civil Engineering Calculations

(10X290) (50)(250)

(^15) °-lo
glo(150C/0.5) ^
(^5) °°~ lo
glo(500C/0.5)
Solving for C and K we have
C=0.21 and JT- (5My*210) -0.093;
12,500
then
(20)(280)
e
~
0
'°
93
log 10 (0.210e/0.5)

2. Solve for the water flow by trial
   Solving by successive trial using the results in step 1, we find Q = 257 gal/min (16.2 L/s).
   Related Calculations. If it is assumed, for purposes of convenience in compu-
   tations, that the radius of the circle of influence, re, varies directly as Q for equilibrium
   conditions, then re = CQ. Then the Dupuit equation can be rewritten as

(he + hw)(he~hw)

U lo

glo (CS/O

From this rewritten equation it can be seen that where the drawdown (he - hw) is small
compared with (he + hw) the value of Q varies approximately as (he - hw). This straight-
line relationship between the rate of flow and drawdown leads to the definition of the spe-
cific capacity of a well as the rate of flow per unit of drawdown, usually expressed in gal-
lons per minute per foot of drawdown (liters per second per meter). Since the relationship
is not the same for all drawdowns, it should be determined for one special foot (meter),
often the first foot (meter) of drawdown. The relationship is shown graphically in Fig. 3
for both gravity, Fig. 1, and pressure wells, Fig. 5. Note also that since K in different

Ground surface

FIGURE 5. Hypothetical conditions for flow into a

pressure well. (Babbitt, Doland, and Cleasby.)

Aquifer

stratum

Non-water

bearing

strata

Static water Well pressure level.

Piezometric

surface during

pumping

Impervious