# Flow to a drainage slot

(1)Line drainage slots. Equations presented in figures 4-1 through 4-5 can be used to compute flow and head produced by pumping either a single or a double continuous slot of infinite length. These equa­tions assume that the source of seepage and the drain­age slot are infinite in length and parallel and that seepage enters the pervious stratum from a vertical line source. In actuality, the slot will be of finite length, the flow at the ends of the slot for a distance of about L/2 (where L equals distance between slot and source) will be greater, and the drawdown will be less than for the central portion of the slot. Flow to the ends of a fully penetrating slot can be estimated, if necessary, from flow-net analyses subsequently pre­sented.   Hydraulic head loss in a vell Hydraulic head loss in various wellpoints Equivalent length of straight pipe for various fittings Shape factors for wells of various penetrations centered inside a circular source

Flow and drawdown for slots from flow-net analyses Flow and drawdown for wells from flow-net analyses Diagrammatic layout of electrical analogy model

Note : A = artesian flow; G = gravity flow; c = combined artesian-gravity flow; F – fully

penetrating; P = partially penetrating.

LJ. S. Army Corps of Engineers

TOTAL HYDRAULIC HEAD LOSS IN A WELL (H ) IS    V He f Hsf Hr + HV 11021

WHERE H = ENTRANCE HEAD LOSS (SCREEN AND FILTER)
ESTIMATE FROM CURVE o.

H = HEAD LOSS IN SCREENED SECTION OF WELL:

ESTIMATE FROM CURVE b FOR A DISTANCE OF ONE – HALF THE SCREEN LENGTH.

Hfr HEAD LOSS WITHIN THE RISER AND CONNECTIONS. EStimateFROM CURVE b.(SEE fig. 4-26F0R tH e EQUIVALENT LENGTH OF STRAIGHT PIPE FOR VARI­OUS FITTI NGS.)

H = VELOCITY HEAD LOSS. ESTIMATE FROM CURVE c v v THE VALUE OF Hw MUST BE SUBTRACTED FROM THE COMPUTED VALUE OF h TO OBTAIN THE LIFT OR WATER LEVEL IN A WELL.

(Modified from “Foundation EngineeringG. A. Leonards, ed., 1962, McGraw-Hill Book Company. Used with permission of McGraw-Hill Book Company.)

Figure 4-24. Hydraulic head loss in a well

TOTAL HYDRAULIC HEAD LOSS IN A WELLPOINT (H )IS

w

H = H t H t H t H

w e s r

WHERE Ие – ENTRANCE HEAD LOSS (WELLPOINT ANO FILTER)

H = FRICTION MEAD LOSS WITHIN THE WELLPOINT

s

H = FRICTION HEAD LOSS IN RISER, SWING CONNECTION, AND VALVE r

H = VELOCITY HEAD LOSS IN RISER, SWING CONNECTION, AND VALVE

V

 HYDRAULIC HEAD LOSSES FOR TYPICAL WELLPOINTS AND RISERS BELOW.    0 20 40 60 80 Discharge, gpm

 0 20 40 60 80 Well discharge, gpm

 (b)

 (Modified from “Foundation Engineering," G. A. Leonards, ed., 1962, McGraw-Hill Book Company. Used with permission of McGraw-Hill Book Company.)

 Figure 4-25. Hydraulic head loss /«various wellpoints.

 I M      (2) Circular and rectangular slots. Equations for flow and head or drawdown produced by circular and rectangular slots supplied by a circular seepage source are given in figures 4-6 through 4-9. Equations for flow from a circular seepage source assume that the slot is located in the center of an island of radius R. For many dewatering projects, R is the radius of influ­ence rather than the radius of an island, and proce­dures for determining the value of R are discussed in a(3) above. Dewatering systems of relatively short length are considered to have a circular source where they are far removed from a line source such as a river or shoreline.

(3) Use of slots for designing well systems. Wells can be substituted for a slot; and the flow Qw, draw­down at the well (H—hw) neglecting hydraulic head losses at and in the well, and head midway between the wells above that in the wells Ahm can be computed from the equations given in figures 4-20, 4-21, and 4-22 for a (single) line source for artesian and gravity flow for both “fully” and “partially” penetrating wells where the well spacing a is substituted for the length of slot x.

(4) Partially penetrating slots, The equations for gravity flow topartially penetrating slots are only con­sidered valid for relatively high-percent penetrations.

c. Flow to wells.