CHAPTER X.

DITCHES AND FLUMES.

DITCHES.

The demand for water throughout the mining districts has caused the construction of thousands of miles of ditches. The cost of these has been immense, but the returns on legitimate enterprises have well repaid the capital invested. On account of the rugged character of the country traversed by the ditch lines, in order to lessen the cost and expedite the work, steep grades were used, high trestles were built (in some instances supporting large flumes at elevations of two hundred to two hundred and fifty feet), and wrought-iron pipes were introduced for conveying the water across the valleys and cañons. The boldness with which these works were undertaken was characteristic of their originators.

Location and Construction Principles. -In locating and constructing ditches the following rules should be observed:

(1) The source of supply should be at sufficient elevátion to cover the greatest range of mining ground at the smallest expense, great hydrostatic pressure being always desirable.

(2) An abundant and permanent supply of water during the summer months should be secured.

(3) The snow line, when possible, should be avoided, and the ditch, especially in snow regions, located so as to have a southern exposure.

(4) All water-courses on the line of the ditch should be secured; their supply partially counteracts the loss by evaporation, leakage, and absorption, and frequently fur

nishes an additional quantum of water during several months of the year.

(5) 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 snow these waste-ways should be provided at intervals not greater than one-half a mile.

(6) Ditches, when practicable and the cost not being excessive, should be preferred to flumes.

Surveying a Ditch Line.In the preliminary examination for the location of a long ditch, by means of careful comparative observations made with good aneroid barometers, the elevations not only of the termini, but also of intermediate points from which different surveying parties can start on the subsequent location of the line, can be approximately determined.

The various necessary points once established by survey, the line is staked. In levelling, all turning points should be made on grade. The stations should be properly numbered and staked, and pegs driven to grade. Every four or five stations the rodman should be required to call off the reading of the rod, which is checked by the notes of the surveyors. Stations may be from fifty to one hundred feet apart on ordinary ground, but a very irregular country demands shorter intervals, sometimes of a rod only. Bench marks should be placed every onefourth or one-half mile for convenient reference.

All details of tunnels, cuts, and depressions which require fluming or piping should be worked out in full. In this work the hand level can often be employed with advantage. Complete notes should be made of the character of the ground along the entire line, and also of any possible changes.

The size of a ditch is regulated by its requirements. Its form will be modified often by circumstances of which the engineer is the judge. The smallest section for any given discharge is when the hydraulic mean depth is one

half of the actual depth. As a general proposition, this is the most economical form of profile for water-channels with given side slopes. The amount of excavation is the least in that channel where the wetted perimeter for a given area is the smallest. In practice the forms commonly adopted for ditches and flumes are trapezoidal and rectangular.

With rectangular profiles the resistance due to friction is the smallest when the width is twice the height.

Of trapezoidal profiles, the half of a regular hexagon is generally used in canals and ditches.

Circular and square profiles are employed only in stone, wood, and iron constructions.

Narrow and Deep vs. Broad and Shallow Ditches. In a mountainous country narrow and deep ditches with steep grades will generally be found preferable to large conduits with gentler slopes. The first cost of excavation is much less, as is also the cost of repairs rendered necessary by snows and severe storms, the narrower aqueduct being more easily protected. The experience of the ditch-builders in this State has been uniformly favorable to these steep grades, but little trouble being caused by the washing of the banks due to high velocities. In the valleys with ashy soil such grades, of course, would not be practicable.

Ditches in California with carrying capacities as large as 80 cubic feet per second have been built, and are now in successful operation, with grades of sixteen to twenty feet per mile.

Excavating the Ditch.-Before the work of excavating is commenced the line is cleared of trees and underbrush for a sufficient width to render work afterwards easy and to prevent subsequent damage to the ditch. All trees which are liable to fall and injure the work should be removed before construction begins. On a flume line the brush for at least ten feet on each side is burned as a precaution against fire. So far as possible, and especially

along a side hill, the ditch should be dug so as to have walls of solid, untouched ground, and not made banks. The top of the solid bank on the lower side should be fully three feet wide. In such cases the top soil is first removed for the width of the ditch and bank; the material excavated to form the ditch is used to raise the lower bank, and in time consolidates to firm ground, thus increasing the capacity of the ditch.

The digging of ditches is usually let by contract at a given sum per rod, and heavy cuts per cubic yard. It is customary to excavate large ditches with a slope of 60° for the upper and 65° for the lower bank. These slopes, of course, the engineer will vary in accordance with the ground encountered. In practice they are changed eventually by erosion and denudation; but experience seems to warrant the above-mentioned slopes as the best to be adopted in laying out such works.

In large mining ditches constructed with high grades and running large amounts of water, the erosion and consequent enlargement of the ditch (when kept in order) is noticeable; moreover, the banks gradually become solidified, and thereby the loss by leakage and absorption is decreased. It is roughly estimated that the capacity of a weli-constructed ditch which is properly kept up is increased about 10 per cent. in eight years.

Ditches poorly built in the beginning subsequently require large and constant expenditures, and lose considerable amounts of water. The annual cost of running and maintaining large ditches, including all repairs and taxes, is estimated to be $400 per mile.

Examples of Ditches.—Among the principal ditches in the State are the North Bloomfield, the Milton, the Eureka Lake, the San Juan, the South Yuba Canal, the Excelsior or China ditch, the Bouyer, the Union, the El Dorado, the Spring Valley and Cherokee, the Hendricks and the La Grange.

North Bloomfield.-The North Bloomfield main

ditch, including distributers, is fifty-five miles long. Its size is 8.65 feet on top, 5 feet at bottom, and 31⁄2 feet deep. The ditch and distributers cost $466,707. Its grade is sixteen feet per mile, discharging 3,200 miner's inches.

the bottom, 7.6 feet FIG. 9. NORTH BLOOMFIELD MAIN DITCH. GRADE, 16 FT. PER MILE. SEC., 23.89 SQ. FT.

on top, and 32 feet

deep, discharging 3,000 miner's inches; cost, $462,998.

Eureka Lake.-The Eureka Lake main ditch is eighteen miles long and has a capacity of 2,500 miner's