With regard to the Pittsburgh, Fort Wayne and Chicago bridge of the Pennsylvania System, it is to be noted that this is a double-deck bridge, the upper tracks being used principally by passenger trains and the lower tracks by freight trains almost exclusively devoted to local freight business. The most serious consideration affecting this bridge is that any very considerable raising of the level of the lower tracks would throw them out of connection with the important local freight station to which those tracks run. Even if expense of reconstruction be wholly disregarded we believe no way can be devised by which the freight tracks of the Fort Wayne bridge, if raised as proposed by the local office of the United States Engineers can be connected with the freight station and industrial plants without involving greatly increased difficulty and delay in the handling of freight either on the tracks or in the station itself or in the teaming approaches to the station. When the large volume of local traffic handled at this station is considered, it is apparent that such a radical change is a serious matter for shippers and the great manufacturing and commercial industries of the city. Other than the expense of making changes in the bridge and its approaches no serious difficulty stands in the way of raising the clearance of the main span of the Fort Wayne bridge 2 or 3 feet to about 37 feet above pool level. To go above that figure involves the serious objections discussed above.
(b) Effect of Different Bridge Heights Upon River Traffic.—The effect upon river navigation of any standard that may be adopted for the heights of bridges depends upon the heights of the vessels using the river and the fluctuations of the river level itself. (See Diagrams 4 and 5.)
By means of Davis Island Dam in the Ohio River the water of Pittsburgh harbor is now kept practically at a minimum stage of six feet above the datum of zero at natural low water. This is the prevailing water level for the greater portion of the year. Floods come occasionally, produced by rains and melting snows, and, of course, with the floods come increased current velocities. These current velocities of each river depend upon the source of the flood. When the flood comes down the Allegheny River high velocities result. When the flood comes down the Monongahela the high water in the Allegheny is back-water without excessive currents. Under this condition the Allegheny becomes a harbor of refuge for Monongahela commerce; and the reverse is true that in an Allegheny River flood the Monongahela becomes a harbor of refuge for Allegheny commerce. The floods in the two rivers seldom come at the same time on account of the differences in the topography and climatic conditions along the two water-sheds. The most serious floods in the Allegheny generally come in the spring, when they are frequently accompanied with drift and ice to such an extent as to render navigation dangerous. At a stage of 15 feet in an Allegheny River flood the river current runs at rates of from 4 to 7 miles per hour. The record of fifty-five years shows that there is an average of 9 days each year when the river is above a 15-foot stage, and this is mostly in the winter and spring when navigation in the harbor is at its lowest ebb. There is presented herewith Diagram 4, showing graphically the average number of days each year during which the river has reached the various heights indicated.
DIAGRAM SHOWING WATER LEVEL IN ALLEGHENY RIVER—AVERAGE LAST 53 YEARS
There is also presented a hydrograph record of the river for four years past which indicates the conditions ordinarily met with as regards river stages at various times of year.
Towboat and barge passing under low bridge
In the balancing of interests between the traffic on the river and that across the bridges, it is believed to be fair and just that for boats of excessive size and height the navigation of the river above a 15-foot stage be eliminated from the problem; (1) because of the comparatively small number of these boats; (2) because of the questionable necessity of having such high boats at all; (3) because of the period of the year when these extreme stages are reached; (4) because these periods of time are so limited in length; (5) because of the generally accompanying swift currents, and (6) because of the oft-times accompanying dangerous floating drift and floating ice.