Last installment I lost readership offering the boring numbers from which the bridge was “engineered” – the physics that set the maximum amount of soldiers, troopers, and artillery pieces that could pass over the bridge based on weight. For someone like me, trained by the U.S. Army in the sciences of staff work, these read like great templates to plan a movement. But beyond the numbers, there were other factors which also governed the speed and efficiency of the crossing.
Picking up from the previous post, there were unique concerns for the passing of each type of traffic. Infantry could not march in step across the pontoon bridge. The issue was the force behind foot falls, and oscillation. With all that uniform “stomps”, the pontoons would start bobbing, with a rather obvious outcome. Thus soldiers would “route step” over the bridge. Well, the above mentioned Army training taught me that soldiers make better time when marching in step. So right off the bat we have a regulating factor that slows the column down. Not by much, but enough to notice if you have a stopwatch.
In the previous post, figures for both mounted and led horses were given. As seen from the raw numbers, mounted cavalry took up less space, and passed quicker. But the engineers preferred the troopers dismount and lead their horses. The animal was more likely to follow, and less likely to stall the pace of march. Horses were not allowed to trot, as this would affect oscillation of the pontoons and increase wear. Wagons and artillery, likewise, were moved across at a walking pace. Furthermore, a space of 20 yards was required between each, to ensure stability. These two considerations (dismounted cavalry and spacing between wagons) just added six miles to the “theoretical” column length discussed in the last post.
One note, looking back at the time line here. On June 25, Reynolds complained about the slow passage of XI Corps due to led horses. While this may be a reference to the convention encouraged by the engineers, I have a different take on this. From the response from Hooker, it appears the issue was a large number of extra horses. But I could be wrong.
General Cullum stated that infantry, cavalry, artillery and wagons should not be mixed for the bridge crossing. Mixing the types of traffic, again, might lead to oscillation and wear. So in practice, some distance was afforded between the infantry columns, their wagon trains, supporting artillery, and the cavalry.
Cullum called for care in regard to placement of abutments, the critical junction of bridge to the shore. An idea bridge site offered a low shoreline. But if needed, the banking was cut down to ensure the abutment spanned to the first pontoon at a shallow angle, if not flat. If it were too steep, the traffic would have difficulty passing. Even when level, abutments presented a challenge to the traffic. In the open bays, the pontoons are freely depressed and released, creating a small wave effect. But at the abutment, the shore line is static. This could wear and damage the bridge, and present a slope to climb for the traffic reaching the far shore. Thus a little bit of work to place the abutment could improve traffic flow, and reduce wear and damage.
Continuing with maintenance factors, another issue was the wear imposed by the traffic on the planking of the bridge. Brogans amd boots were not too hard on the wood, but horses hooves were. On June 24, (12:45 pm to be exact), Captain Turnbull mentioned the anticipated wear in a dispatch to General Benham. After all, if there was no decking on the bridge, there would be no crossing. So this was another time factor to consider. After the passage of each cavalry unit, battery of artillery, or wagon train, the engineers had to halt traffic to inspect and repair the decking.
Similarly maintenance was required on the anchor lines and the pontoons themselves. Also, the Potomac, particularly when high, collects a lot of debris. Clearing the debris, checking anchor lines, and bailing pontoons likely kept the engineers busy during the crossing.
Thus far the “factors” listed are still tied in some part to the nature of the bridge itself. But even before the first soldier stepped foot on the bridge, one important factor played into the bridge’s throughput – traffic control. Well out of scope for the engineer’s manuals, this was more so a provost marshal function. Trouble was the assent of the Corps toward the Leesburg area funneled the troops into a small area – constrained by the Catoctin Mountains to the west, and cut by the Goose Creek.
Standard military thinking calls for crossing sites at multiple locations to avoid congestion, and enemy interdiction. But before faulting Hooker, or his subordinates, the record of dispatches indicate he inquired about additional passage points. But with the river high, such options were limited. Regardless, the situation set up a rather large traffic jam outside Leesburg. I’ll discuss the road network and the traffic problems in more detail in another post.
Traffic control at river crossings is very much a planning factor even for the 21st century army. In FM 90-13, River-Crossing Operations, Chapter 1, the fundamentals of such operations are listed as: surprise, extensive preparation, flexible planning, traffic control, organization, and speed. The scope of the modern work covers opposed crossings. Without a direct ancestor of FM 90-13 dating to the mid-19th century, the closest match is Dennis Hart Mahan’s Outpost, with a section covering operations near rivers. While easy to relate, from a common sense standpoint, back to 1863, I don’t find a documented doctrinal linkage for these fundamentals. As such, I cannot say that the staff officers of the Army of the Potomac were consciously concerned about these fundamentals without more evidence. (And I’m not working a staff ride packet here….)
The main point to remember here is beyond the weights of traffic and time required for passage, the crossing was governed by the characteristics of the types of traffic (foot, horse, wheeled), maintenance required on the bridge, and what I would call factors external from the bridge itself – notably traffic control. Even under the “perfect” scenario scripted in the last post, I’d have to add six, if not ten, more miles to the length of the combat arms column described. And still further factor the “friction” of the traffic jam on the banks of the Potomac.