112 GRAVEL AND PLACER MINING IN ALASKA. [bull. 268. 
At proper intervals waste gates should be arranged, so as to discharge the water 
when necessary without risk of damage to the ditch. In regions of heavy snowl 
these wasteways should be provided at intervals not greater than one-half mild 
* * * Ditches poorly built in the beginning subsequently require large and con- 
stant expenditure, and lose considerable amounts of water 
George H. Evans a says: 
At the different points along the line of race where fluming has to be resorted to, 
allowance should be made for an increase in grade, in order that the flume can be con- 
structed of much smaller dimensions than the ditch and yet carry all the water 
required. * * It will be well to remember that practical results have demon- 
strated that in ordinary ground the water should travel at the rate of from 180 to 200 
feet per minute. Then the grade will.be determined by the dimensions of the ditch 
and its intended carrying capacity. 
The above quotations may be considered as of genera) application, 
but many modifications in the hitherto accepted practice of ditch 
building have been found advisable in Alaska, and will be referred to, 
especially in the description of the practice in Seward Peninsula. 
Regarding the capacity of open conduits, Bowie makes the following 
statement: 
In the mining districts of California ditches are constructed badly, with steep 
grades and on irregular lines with numerous sharp curves. The cross sections, origi- 
nally uniform, became more or less varied. Absorption, percolation, evaporation, 
and leakage reduce the flow. A distinct, reliable factor for each of the sources of 
loss can not well be incorporated in the coefficient of discharge. * * * The sim- 
ple formula Q=ac\/rs expresses more fitly the result of experience in such casesj 
wherein: 
Q is the quantity of water which the ditch is capable of carrying in cubic feet per 
second. 
a, the effective area of cross section of ditch, as originally constructed, in square 
feet. 
r, the hydraulic mean depth in feet. 
s, the fall of surface in a unity of length. 
c, a coefficient covering all common losses. 
Statistics derived from experience on the Milton, La Granger, and Bloomfield 
ditches in California led to the adoption of values of the coefficient c varying from 
31 to 45, in estimating the capacity of ditches in heavy grades of 40 miles' length 
flowing from 60 to 80 cubic feet per second. 
The Texas Creek branch ditch (of the North Bloomfield ditch) is about seven- 
tenths of a mile long. Its sectional area is 13.5 feet and the grade is 20 feet per mile. 
The sides are rough and the curves sharp. With a flow of 32.8 cubic feet per second, 
the ditch runs about full. The value of c=33. In connection with this ditch there 
is a rectangular flume 2.67 feet wide by 2.83 feet deep, made of unplaned boards, set 
on a grade of 32 feet per mile. The flume has some sharp but regular curves, and 
the water from the ditch runs nearly full at these points. With the discharge 32.8 * 
cubic feet per second, c=59. 
Although the losses in leading water in Alaska have not been deter- 
mined by exact experiments, it is likely that they are less than in Cal- 
ifornia. In the interior the value of c for ditches would be slightly 
a Evans, G. H., Practical Notes on Hydraulic Mining, San Francisco, 1898. 
