Earth dams, a study

by Burr Bassell

Catchment Area.

It is thought desirable to define a number of terms as we proceed, for the purpose of correcting erroneous usage and for a clearer understanding of the subject. The catchment area of a reservoir is that portion of the country naturally draining into it. The watershed is the boundary of the catchment area and may be correctly defined as the divide between adjacent drainage systems. In regard to the catchment area it is necessary to determine:

1. Its extent and area in square miles.

2. Its topography or the character of its surface.

3. Its hydrography or precipitation and run-off.

4. Its geology, or the character of its soils and subsoils, and the nature and dip of its rock strata.

5. Its flora, or the extent to which it is clothed with forest trees or other vegetation.

All of these elements affect the volumes of maximum run-off, which is the one important factor in the construction of earth dams that must not be underestimated.

If the proposed dam or reservoir is to be located upon a main drainage line; that is, upon a river or stream, it is necessary to know both the flood and low-water discharge of the stream. Frequently no reliable data on this subject are available, and the engineer must then make such a study of the whole situation as will enable him to estimate the minimum and maximum flow with considerable accuracy.

There are numerous factors entering into the solution of this first problem, such as the shape and length of the catchment area, its general elevation, the character of its surface, whether mountainous, hilly or flat, barren or timbered.

Good topographic maps, if available, furnish valuable data on these subjects and it is to be regretted that only a comparatively small portion of the United States has been thus mapped in detail.

The results of stream measurements, if any have been made in the catchment basin, are especially important: These are usually few in the high areas, on account of their inaccessibility. The year 1902 marked a notable beginning of such measurements in California. In many parts of the arid region of the United States, the best storage-sites are situated in the upper or higher portions of the drainage systems. This is especially true of the streams on the Pacific Slope having their source in the High Sierras. As regulators of stream-flow and for power purposes such storage is peculiarly valuable, while storage for irrigation and domestic uses may be located nearer the valleys and the centers of population.

Frequently the engineer is required to build his dam where no such data are available. In such instances he should endeavor to secure the establishment of rain gauges and make measurements of the flow of the main stream and its principal tributaries at various places to obtain the desired information. Even this may not suffice, owing to the limited time at his disposal, and he must resort to the use of certain empirical rules or formulas, and make such comparisons and deductions from known conditions and results as will best answer his purpose.

The engineer should know, approximately at least, the normal yield of the catchment area, the duration of the minimum and maximum seasonal flow, and the floods he may have to provide against during the construction of his dam. These data are absolutely necessary to enable him to provide ample wasteways for his reservoirs. Many of the failures of earth dams have been the result of over-topping the embankment, which would have been averted by an ample wasteway. The most notable example of this kind in recent years was that of the South Fork Dam, at Conemaugh, Pennsylvania, in 1889, resulting in what is generally known as the “Johnstown Disaster.”

There are several empirical rules and formulas for calculating the run off from catchment areas and for determining the size of spillways necessary to discharge this flow with safety to the dam. The proper formula to apply in any given case, with the varying coefficients of each, involves a thorough knowledge on the part of the designing engineer of the principles upon which the different factors are based.

It is unwise and often hazardous to make use of any important hydraulic formula without knowing the history of its derivation. Experiments are not always properly conducted, and often the deductions therefrom are unreliable. A presentation and discussion of these formulas would require more space than can be given in this study, and the technical reader must therefore consult for himself, as occasion may require, the various authorities cited. Formulas for the discharge or run-off from catchment areas, as determined by Messrs. Craig, Dickens, Ryves and others, are given by most writers on the subject of hydraulics.