Page 408 - Civil Engineering Formulas

HYDRAULICS AND WATERWORKS FORMULAS 335

Permeability indicates the ease with which water moves through a soil and
determines whether a groundwater formation is an aquifer or aquiclude.
The rate of movement of groundwater is given by Darcy’s law:
Q KIA (12.134)

3
where Q flow rate, gal/day (m /day)
K hydraulic conductivity, ft/day (m/day)
I hydraulic gradient, ft/ft (m/m)
2
2
A cross-sectional area, perpendicular to direction of flow, ft (m )
WATER FLOW FOR FIREFIGHTING

The total quantity of water used for fighting fires is normally quite small, but the
demand rate is high. The fire demand as established by the American Insurance
Association is

G 1020 P(1 0.01 P) (12.135)
where G fire-demand rate, gal/min (liter/s); and P population, thousands.

FLOW FROM WELLS

The steady flow rate Q can be found for a gravity well by using the Dupuit
formula:
2
2
1.36K (H h )
Q (12.136)
log(D/d)
where Q flow, gal/day (liter/day)
K hydraulic conductivity, ft/day (m/day), under 1:1 hydraulic gradient
H total depth of water from bottom of well to free-water surface before
pumping, ft (m)
h H minus drawdown, ft (m)
D diameter of circle of influence, ft (m)
d diameter of well, ft (m)
The steady flow, gal/day (liter/day), from an artesian well is given by

2.73Kt (H h)
Q (12.137)
log(D/d)
where t is the thickness of confined aquifer, ft (m).