The Pontchartrain route has been laid off, by engineers, beginning at the Canal, paralleling the south shore of the Lake Pontchartrain to the south draw of the Southern Railway bridge, thence to the Rigolets to Cat Island Pass, from there to Cat Island Channel and so to the deep water of the Gulf, a total distance of 75 miles.
Soundings and surface probings have been taken at frequent intervals over the entire route. These have shown the engineers the following:
Three-quarters of a mile from the south shore of the lake, and as far as the railroad drawbridge, a hard bottom is found. The material is principally packed sand, rather fine, with a small amount of clay, and occasionally some broken shells. Beyond this distance from the shore, the bottom is softer, consisting of mud mixed with sand. From the bridge over the remainder of the route, the bottom, with the exception of a few sand pockets, is soft—a blue mud with a large percentage of sand. This soft material has so much tenacity, however, that current and wave wash, which tend to fill up artificially dredged channels, would affect only the surface.
The government is conducting large dredging operations in Mobile Bay, Gulfport Channel, Atchafalaya Bay and the Houston Ship Channel. An outline of the results there will show how feasible the dredging of the Pontchartrain Channel would be, and how much cheaper in comparison.
The channel connecting Mobile Bay with the Gulf of Mexico has a bottom very soft for the most part, and with a small percentage of sand. Towards the outer end, the material is black mud, about equal in consistency to the softest material found in the Pontchartrain route. A sounding pole with a 4-inch disc on the end can be easily pushed three or four feet into the mud and pulled out again. Wave and current action cause the channel to shoal at the rate of 78,000 to 132,000 cubic yards per mile per year, depending on the softness of the bottom and the depth. Where the highest rate obtains, the surrounding material consists of soft mud, without a trace of sand. Experience shows that where there is a fair percentage of sand in the material adjacent to the channel bed, the shoaling is lessened. In general, the material along the Pontchartrain route contains a greater percentage of sand and is far more tenacious than that along the Mobile Bay Channel. Furthermore, the Pontchartrain route is not exposed to such strong cross currents.
The Gulfport Channel is dredged through very soft material, a grayish-blue mud of oozy consistency, into which the sounding pole penetrates six feet with very little exertion. On top, a small amount of sand is found, but practically none in the lower stratum. The material is considerably softer than any encountered on the Pontchartrain route, except for one small stretch. Yet the shoaling is not great. Where the shoaling is heaviest, between the end of the pier and Beacon 10, only about 700,000 cubic yards a mile has to be dredged out every year to maintain the channel. From Beacon 10 out, the average annual maintenance is less than 200,000 cubic yards a mile. Except for the four-mile stretch west of the inner entrance to the Cat Island Channel, the bottom, on the Pontchartrain route, is harder than that of the Gulfport Channel. Therefore, it is reasonable to conclude that the maintenance of the Pontchartrain Channel would not average as high as the outer portion of the Gulfport Channel.
The Atchafalaya Bay Ship Channel, extending from the mouth of the Atchafalaya River across the shoal waters of Atchafalaya Bay, to about the 20-foot contour of the Gulf, a distance of fifteen miles, is through a material of slushy mud, with occasional thin pockets of sand. The shoaling runs from 540,000 to 1,680,000 cubic yards a mile a year. The highest rate is obtained in shallow water. Except in the stretch mentioned, the material on the Pontchartrain route is not as soft as on the Atchafalaya, nor are the depths as shoal, nor is there the exposure to cross currents.
In the Houston Ship Channel, the material is composed of soft mud with a small amount of sand. A two-mile stretch through Red Fish Reef is practically self-maintaining. For the remainder of the channel, during the six years from 1915 to 1920, a total excavation of 13,574,000 cubic yards was necessary to maintain the depth. This is equivalent to 100,000 cubic yards a mile a year.
In summary, then:
1. The Lake Pontchartrain route is practically unexposed to cross currents, as is the case with the Mobile Bay, Gulfport, Atchafalaya, and, to a certain extent, the outer portion of the Houston Ship Channels.