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Water Flow For Firefighting

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

G=1020P1/2(1 -0.01P1/2)

G =fire demand rate in gal/min (liter/s)
P= population in thousands.


Groundwater is subsurface water in porous strata within a zone of saturation. Aquifers are groundwater formations capable of furnishing an economical water supply. Those formations from which extractions cannot be made economically are called aquicludes.

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 =flow rate, gal/day (m3/day)

K= hydraulic conductivity, ft/day (m/day)

I =hydraulic gradient, ft/ft (m/m)

A =cross-sectional area, perpendicular to direction of flow, ft2 (m2)

Method For Determining Runoff


The most common means for determining runoff for minor hydraulic structures is the rational formula:


Q= peak discharge, ft3/s (m3/s)

C =runoff coefficient percentage of rain that appears as direct runoff

I= rainfall intensity, in/h (mm/h)

A= drainage area, acres (m2)


Chow lists 24 rainfall-intensity formulas of the form:
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Evaporation And Transpiration

The Meyer equation, developed from Dalton’s law, is one of many evaporation formulas and is popular for making evaporation-rate calculations:

E=C (ew-ea) K



E= evaporation rate, in 30-day month

C= empirical coefficient, equal to 15 for small, shallow pools and 11 for large, deep reservoirs

ew=saturation vapor pressure, in (mm), of mercury, corresponding to monthly mean air temperature observed at nearby stations for small bodies of shallow water or corresponding to water temperature instead of air temperature for large bodies of deep water.

ea=actual vapor pressure, in (mm), of mercury, in air based on monthly mean air temperature and relative humidity at nearby stations for small bodies of shallow water or based on information obtained about 30 ft (9.14 m) above the water surface for large bodies of deep water.

w=monthly mean wind velocity, mi/h (km/h), at about 30 ft (9.14 m) aboveground

K =wind factor

Prediction Of Sediment Delivery Rate

Rate of sediment accumulation in a reservoir can be found by two methods.
One approach depends on historical records of the silting rate for existing reservoirs and is purely empirical. The second general method of calculating the sediment delivery rate involves determining the rate of sediment transport as a function of stream discharge and density of suspended silt.

The quantity of bed load is considered a constant func- tion of the discharge because the sediment supply for the bed-load forces is always available in all but lined channels. An accepted formula for the quantity of sediment trans- ported as bed load is the Schoklitsch formula:

Gb=86.7*S3/2 (Qi – bqo)/ ? Dg
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