2006-12-19_REVISION - M2001107 - Laserfiche WebLink

Home Browse Search

OCCURRENCE AND MOYEMENT OF GROUNDWATER 75 hydraulic gradient. If the hydraulic gradient (head loss per unit length of travel) is doubled, the rate of flow in a given sand is also doubled. Conversely, doubling of the flow rate requires doubling of the hydraulic gradient. These ratios apply only to laminar flow, however. If turbulent flow is present, the flow rate does not change in direct proportion with the hydraulic gradient; doubling of the hydraulic gradient may increase the flow rate by only l.5 times. The information in this pazagraph is vital to understanding water-well hydraulics, which is presented in Chapter 9. The slope of the water table or potentiometric surface is the hydraulic gradient under which groundwater movement takes place. The total flow through any vertical section of an aquifer can be calculated if we know the thickness of the aquifer, its width, its average hydraulic conductivity, and the hydraulic gradient. The flow, q, through each foot of aquifer width is: q = Kb7 (5.12) where K is the hydraulic conductivity averaged over the height of the aquifer, b is the aquifer thickness in feet, and I is the hydraulic gradient. Hyarewb oonaucuvity (9vd/n~ fa 1w to fQ` 10• 10 1 1a-~ 10-= to-= fo-~ 10-• to-• fo-~ to-• Fine to coarse gravel Fme to coarse sane Sit, loess Glacial li9 UnweaNered marine clay Shale Unfracllretl igneous ono metamorplYC rocks Sanostone, we9 cemenleo, unjointeo Limestone. unjoimea crysta9me Tuff '' alone, made Fracturetl igneous ann mMamrpT~in nwea Veskvlar basaa Karst 9mestone 10• 10' 10= 10= 10 1 10-~ 10- 10-= 10-• 10-s 1p-• 10_r 10-• 10.9 10_,0 Hydaufic tonauctivhy (m/nay) Figtue 5.14. Typical B vdaea for consolidated and maasolidated agldlera. (After Davis, 1969; Dunn and Leopold, 1978; Freeze and Cherry, /979).