Some Engineering Problems Of The Panama Canal In Their Relation To Geology And Topography

14 ENGINEERING PROBLEMS OF PANAMA CANAL. their flood plains and to their mouths has already been discussed. 1 he streams may, therefore, be looked upon as great gravel and silt manufacturing and sorting plants. They, of course, turn out and classify material vastly cheaper than the rock-crushing plants can. The Gamboa Bridge of the Panama Railroad crosses the Chagres River well above the normal flood plain, in. the gravel-dumping zone of the river, and hence is convenient to bars of good ballast and gravel for construction work. In this important item the streams con- tributed directly to the construction of the canal. On the other hand, because of the liability to floods in the streams, it has been necessary to have the bridges spanning them extra strong, and there- fore more expensive. VALLEYS. When water flows over a land surface it follows the line of least resistance, not only so far as the gradient is concerned, but also in respect to the ease with which, it may erode a channel for itself in the material over which it passes. It thus tonds to do minimum erosive work on the resistant areas of rock, so that in time these are loft as remnants or hills between tho streams. The softer rocks, being moro susceptible to stream erosion, become the sites of valleys. In this way, through geologic time, tho Obispo River and its tribu- taries kept wearing down tho drainage outlet from the Culebra Basin, thus lessoning tho excavation work necessary for the future Culebra Cut. The Chagres, the Gatun, tho Trinidad, and their tributaries kept excavating valleys that are now tho site of tho groat Gatun Lake. The Rio Grande River Valley and its tributary valleys were utilized, for these streams had centuries ago begun the excavation of what man has now elaborated into tho south end of Culebra Cut, the Miraflores Lake, the terminal port, and the dredged out approaches to tho locks on the Pacific side. Not only in the making of the canal, but in the building of the railway lines, the water and sowage systems, etc., tho valleys wore utilized as far as possible. BAYS, HARBORS, AND SHORE CONDITIONS. Several features of tho bays and harbors of tho Canal Zone have involved engineering problems. Of these the relative shallowness of Colon and Panama Bays and tho natural exposure of Colon Bay to northerly winds, and the Pacific entrance of tho canal to southerly winds are the most important. In order to make these bays more sheltered and safe for ships, great breakwaters have had to be built. Now come such practical questions as: Are Colon and Panama Bays likely to grow moro shallow so that much dredging will be necessary to keep them navigable, or will the ship channels in them gradually become deeper from tides and shore currents? It might also bo