HYDRAULICS AND WATERWORKS FORMULAS 335Permeabilityindicates 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:
(12.134)whereQflow rate, gal/day (m^3 /day)
Khydraulic conductivity, ft/day (m/day)
Ihydraulic gradient, ft/ft (m/m)
Across-sectional area, perpendicular to direction of flow, ft^2 (m^2 )
WATER FLOW FOR FIREFIGHTING
The total quantity of water used for fighting fires is normallyquite small, but the
demand rate is high. The fire demand as established by the American Insurance
Association is
(12.135)
whereGfire-demand rate, gal/min (liter/s); and Ppopulation, thousands.
FLOW FROM WELLS
The steady flow rate Qcan be found for a gravity well by using the Dupuit
formula:
(12.136)
whereQflow, gal/day (liter/day)
Khydraulic conductivity, ft/day (m/day), under 1:1 hydraulic gradient
Htotal depth of water from bottom of well to free-water surface before
pumping, ft (m)
hHminus drawdown, ft (m)
Ddiameter of circle of influence, ft (m)
ddiameter of well, ft (m)
The steady flow, gal/day (liter/day), from an artesian well is given by
(12.137)
wheretis the thickness of confined aquifer, ft (m).
Q
2.73Kt (Hh)
log(D/d)Q
1.36K (H^2 h^2 )
log(D/d)