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

RELATION OF DIFFERENT FACTORS TO ENGINEERING. 15 asked why Panama and Colon Bays are relatively shallow, whereas Porto Bello Harbor, not far distant, is about 60 feet deeper. The shallow bays are the slightly submerged margins of the adjoin- ing flat shore area. In relatively recent geologic time (Pleistocene) those bay areas were dry land. Approximately, the same gentle seaward slope of the land is maintained below the water level of Colon Bay out to the edge of the continental shelf as occurs above it, except for the marginal coral reefs and flats that form a wedge-shaped fringe around the present shore line. This fringe of limy rock and coral is about 35 feet thick at its outer edge and rests on the normal rock bottom. It locally presents an abrupt face to the sea and its outward margin is a few feet below high-tide level. There are also a few low headlands with steep slopes formed by the erosive action of sea waves on some of the points of higher land. Toro Point, 50 to 60 feet above sea level, is one of these. The land surrounding Porto Bello harbor is steep and mountainous, and its slopes continue relatively steep down to the flat bottom of the harbor about 90 feet below the surface of the water. In the upper end of this harbor, however, considerable silt has been dumped by streams. Two other factors have made both Panama and Colon Bays more shallow, while they have had relatively little effect on the deep harbor of Porto Bello. Theyare: (1) During the rainy season all streams are swollen to great size, and they come down red and mud-colored from their load of sediment. When they reach the ocean the sedi- ment is deposited, as already explained, and each year a thin layer of silt is left on the bottom of the bay into which they empty. In the time since this process began enough silt has been deposited to make the water considerably shallower. (2) In very recent geologic time the whole land mass has risen, as proven by old beach marks of fresh appearance, which, near Colon, are about 6 feet above sea level, and on the Panama side are a little higher. This rising has left Colon Bay about 1 fathom and Panama Bay more than a fathom shallower than they were previously. This recent rise of isthmian land, which reached a maximum of about 30 feet in Los Santos Province, probably took place within the last 1,000 years, according to evidence in hand. In fact, it may have been partly accomplished since Columbus’s ships first plowed the waters of Colon Bay in October, 1502. The Rio Grande River and its tributaries do not now enter Panama Bay except as overflow from the new Miraflores Ijakø. This lake acts as a settling tank and thus considerably reduces the sedimentation in the bay. Sedimentation, in the Colon Bay is not now nearly so active as formerly, because much of the land around the bay is nearly worn, to base level. On the other hand, the land is, very probably, still slowly rising, and if so the harbors and the approaches to both locks are still being slightly decreased in depth. The rise, if any,