Part 3 (1/2)

GAUGING FLOW IN SEWERS

A e is to fix a weir board with a single rectangular notch across the sewer in a convenient e; and then to ascertain the depth of water passing over the notch byfixed level with the bottom of the notch and at a distance of two or three feet away on the upstreae is so great, however, that if the notch is of a convenient width to take the maximum flow, the hourly variation at the tie on the notch to such a s the readings with sufficient accuracy to show such variations in the flow, and there will be great probability of incorrect results being obtained by reason of solid sewageon the notch When the depth on a l2 in notch is about 6 in, a variation of only 1-16th inch in the vertical measurement will represent a difference in the rate of the flow of approxiallons per day When the flow is about lin deep the saallons per hour, or 3,900 gallons per day Greater accuracy will be obtained if a properly-for pond is constructed independently of the13, or a triangular or V- shaped notch, as shown in Fig 14, used in lieu of the sie of weirs there are several forive different results, though co first a single rectangular notch and reducing the fore per foot in width of weir = C / H3

where H = depth from the surface of still water above the weir to the level of the bottom of the notch, the value of C will be as set out in the following table:--

TABLE No 5

RECTANGULAR NOTCHES

_____ Discharge per foot in width of notch = C / H3 ------------------------------------------------------------------ Values of C

--------------------------------------+--------------------------- H Measured in | Feet | Inches

---------------+-----------+----------+-----------+--------------- | Gallons | C ft | Gallons | C ft Discharge in | per hour | per min | per hour | per min ---------------+-----------+----------+-----------+--------------- Authority | | | | Box | 79,895 | 2136 | 1,922 | 513 Cotterill | 74,296 | 1986 | 1,787 | 478 Francis | 74,820 | 2000 | 1,800 | 481 Mo'esworth | 80,057 | 2140 | 1,926 | 515 Santo Crimp | 72,949 | 1950 | 1,755 | 469 ---------------+-----------+----------+-----------+---------------

In the foregoing table Francis' short formula is used, which does not take into account the end contractions and therefore gives a slightly higher result than would otherwise be the case, and in Cotterill's for half the width of the weir, or of the strea equal to 6-1/4 gallons instead of 6235 gallons, then, cubic feet per allons per day This table can be applied to ascertain the flow through the notch shown in Fig 13 in the folloay Suppose it is required to find the discharge in cubic feet per minute when the depth of waterSanto Crimp's formula the result will be

C/H3 = 469 /43 = 469 x 8 = 3752

cubic feet per foot in width of weir, but as the weir is only 6 in wide, we ure by 2, then

3752/2 = 1876 cubic feet, which is the discharge per minute

+------+ +------+ | | FIG 13 | | | | | | | | | | | +------+ +------+ | | | | | | | | | | | | | | +------+ | | | | | | | | | +----------------------------------+

Fig 13-ELEVATION OF DOUBLE RECTANGULAR NOTCHED GAUGING WEIR

+------+ +------+ | FIG 13 / | | / | | / | | / | | / | | / | | / | | / | | / | | / | | | | | | | | | | | +----------------------------------+

FIG 14-ELEVATION OF TRIANGULAR NOTCHED GAUGING WEIR

FIG 15-LONGITUDINAL SECTION, SHOWING WEIR, GAUGE-PEG, AND HOOK-GAUGE

If it is required to find the discharge in similar terms with a depth of water of 20 in, two sets of calculations are required

First 20 in depth on the notch 6 in wide, and then 4 in depth on the notch, 28 in minus 6 in, or 1 ft wide

____ _____ (1) C/ H3 = 469/2 / 103 = 2345 x 3162 = 7415 ____ ____ (2) C/ H3 = 10 x 469 / 43 = 10 x 469 x 8 = 3752

Total in c ft per htly in excess of this

In addition to the circuings taken by means of a weir fixed in a sewer there is also the fact that the sewage approaches the ith a velocity which varies considerably from time to time In order to make allowance for this, the head calculated to produce the velocity must be added to the actual head This can be embodied in the fore in cubic feet per minute, with H measured in feet, is written

__________________ 195/(113 + 035V - H2