Flood Control as Related to Highways

Flood Control as Related

ANY OF THE PROBLEMS met with in flood control work have been the natural result of progress. The inability of our predecessors to see into the future has caused great damage that must now be rectified. Through their lack of foresight our ranges, farms, highways and other investments have been endangered. Even today we are overlooking problems that will confront the future through our own indifference to the natural functions of the elements. Flood dangers are assuming proportions that threaten to overwhelm many of our communities over the state. Vivid examples of this are shown near the towns of Safford and St. David, and in the Salt River Valley.In and near the town of Safford this problem is becoming acute. Years ago the community, by placing a few furrows above the town, protected it from flood waters. Today these furrows have formed a canyon 200 to 300 feet wide and in places 20 to 30 feet deep. In order to protect the town from being muddy for a few days after a rain, the present damage is now greater than it was in the past. The town was protected, but the ranges and farmlands nearby have suffered untold damage from erosion.

As water seeks its level, so does the natural terraine. The bed of the largechannel is now the datum to which the surrounding country must be looked before stabilization is again reached.

A similar condition exists near the towns of Mesa, Chandler and Gilbert. Pioneer settlers of that community constructed an irrigation ditch from Queen Creek to supply water to their lands. Through their inability to control this water, many acres of land have been eroded away, the natural drainage of the water has been changed, and it isnow necessary for the forces of nature to re-establish its channel.

In this same valley the highways have suffered great damage, and will continue to suffer them until flood waters from the mountains are controlled.

St. David is another example of man's neglect of the future. The erosion of their homesteads has caused many of the people in the upper end of the valley to give up their homes and move to lower lands. In time, the greater part of the valley may be a monument to man's neglect of the future. A community of deserted homes and lost hopes is the reward. Other examples of similar conditions exist in the state; however, it is not necessary to recite these examples to impress upon us the need of flood control.

The problem of flood control must be attacked with greater forethought and execution. It is not a problem that may be solved through mathematics. Many of the functions entering into the evaluation of this problem must be determined through experimentation and continuous recording of the results of the past. While the process of erosion has been going on for countless ages, nature seems to have established a definite balance between the forces of erosion and flood run-off. It is only when man upsets this balance that the problem of flood control arises. When man built the first railroad through this country, he entered into the fight against the elements. And so far, it

to Highways Water Flow Damage Is Greatest Element of Destruction to Country's Roadways

By SWAN A. ERICKSON, Engineer of Dams and State Certification Board Might be said, he has come out on the short end of the deal.

Highways in some instances are con-structed in the same manner. Engineering of highways has reached the same high plane as the planning and constructing of railroads.

The greatest damage encountered by highways are due to floods. No other single element has caused as great a damage in as short a time as floods. Highways must be engineered and planned with this problem in mind. It might be stated at this time that many of the problems of flood control have been easier handled in the mountains than below in the valley. Seepage and underground water very seldom enter into the problem. Foundation conditions for bridges and culverts are easily taken care of. The cost of Construction is frequently the governing factor in the building and location of a highway. Consequently the road at the lower elevation is usually the cheapest. When a road runs through the lower part of the country the conditions reverse themselves in regard to flood control. Poorer foundation conditions are to be expect-ed on which to build bridges. The type of bridge opening is determined by the nature of the ground. Bridges built on piles are frequently used in this type of country. Culverts are paved to prevent erosion under them.

An ideal condition would be to provide a separate opening for each of these crossings. This would necessitate more structures and increase the expense to such an extent, that it would not be feasible to build the road. Some compromise must be made between an ideal solution and an economical one. Several smaller streams may be concentrated through side drainage ditches into a larger one, and the larger one allowed to pass through the roadbed. It is at this point that trouble may be started. Through concentrating the water into one opening, its velocity is often increased. The velocity is greater than its approach velocity, consequently its erosive power is increased as the cube of its velocity. In other words, a stream flowing ten feet a second will move approximately a ton and a half, a stream moving twenty feet a second will move sixty-four tons. Materials are shorn away by this higher velocity that were previously resistent to movement under lower velocities. caused by railroad and highway build ing.

Water is not an element to be treated with contempt. Previously cited examples have proved this. To concentrate water is to concentrate trouble. The less the established drainage of the terraine is displaced, the less the trouble incurred by the construction of a new highway. In many cases where farmlands and ranges have suffered damage, the highways also have suffered. This problem is a problem for all.

The problem in connection with high way building is to provide means by which flood waters may be handled in as short a time as possible, with the minimum amount of disturbance of the natural drainage of the country.

When a mountain stream leaves the mountains and enters a flat sloping land, its gradient is changed. The slope is not as steep as its mountainous por tion, consequently it must widen its courses to take care of the decrease in its velocity. This is also the cause of many streams breaking up into several courses and spreading over the lower lands. In crossing these streams with a highway, provision must be made to allow the water to pass through the fill or over the road.

Poorly designed openings may be responsible for a large damage to property along the right-of-way. Much study (Continued on Page 18) In high mountainous regions where water courses are deep and definitely established, the problem resolves itself into one of providing enough opening in the crossing of water courses to pass the maximum flood. Run-off from accumulated snows or heavy rains is