Crossing bridges

[Swampster]

I'm trying to work out how long an army might expect to take to cross a bridge. Lots of variables involved, obviously, but I'm sure I've seen 19th century military manuals where the time taken is estimated.
Any ideas?

I've come up with a figure based on:
6' depth per man.
bridge wide enough for eight files abreast
20000 men
March rate of about 2 mph

That would take about an hour and a half from the head to the tail of the column, but I'm sure it would actually take longer due to the concertina effect etc.

[Duncan Head]

"When Xerxes had done this, they proceeded to cross over, the whole army both the footmen and the horsemen going by one bridge, namely that which was on the side of the Pontus, while the baggage-animals and the attendants went over the other, which was towards the Aegean.  .... When Xerxes had crossed over into Europe, he gazed upon the army crossing under the lash; and his army crossed over in seven days and seven nights, going on continuously without any pause." (Herodotos VII.55-56)

That was a big army, though. And long bridges.

[Patrick Waterson]

Then again, if you are in a hurry ...

"And the multitude of the army which was assembled, as Ctesias of Cnidus has recorded, was three million foot-soldiers, two hundred thousand cavalry, and one hundred thousand chariots ... And river boats which could be taken apart she built to the number of two thousand, and she had collected camels to carry the vessels overland." - Diodorus II.17.1-2

These were to cross the Indus.  This is about two miles wide at its narrowest point, and presumably would have been about the same in Semiramis' time.  She set up her bridge of boats, fought a battle with the Indians - and it did not go well.

"But since all were fleeing to the pontoon bridge and so great a multitude was forcing its way into a single narrow space, some of the queen's soldiers perished by being trampled upon by one another and by cavalry and foot-soldiers being thrown together in unnatural confusion, and when the Indians pressed hard upon them a violent crowding took place on the bridge because their terror, so that many were pushed to either side of the bridge and fell into the river. 9 As for Semiramis, when the largest part of the survivors of the battle had found safety by putting the river behind them, she cut the fastenings which held the bridge together; and when these were loosened the pontoon bridge, having been broken apart at many points and bearing great numbers of pursuing Indians, was carried down in haphazard fashion by the violence of the current and caused the death of many of the Indians, but for Semiramis it was the means of complete safety, the enemy now being prevented from crossing over against her." - Dionysius II.19.8-9

"Semiramis, after exchanging prisoners, made her way back to Bactra with the loss of two-thirds of her force." - ibid.

This would bring the better part of a million or so men across a bridge of boats in a few hours - much better than Xerxes.  We doof course need to look at the arithmetic.

Semiramis has, say, 1,000 boats per mile.  If we assume a dismantleable riverboat is perhaps 30' long and (say) 10' wide, and they are secured side-to-side, 528 boats are required per mile.  If we are generous and allow them to be a bit larger, so that only 500 per mile are required, the she can have a bridge four boats wide, or about 120' wide.  If we use 120' as a working figure, this would allow 20 men abreast normally but at least double that in the circumstances of the rout.  Say we have 40 men abreast and they are barrelling through at an average rout speed of 4mph, with a spacing of 3' per man front-to-rear (any less and they jam), incidentally discarding additional people off the sides of the bridge as they go (and thus showing that the full width was used).

To calculate approximate throughput: 40 men each with a depth of a yard each are proceeding at an estimated 7,040 yards per hour; 7,040 x 40 = 281,600 men per hour, requiring 3.55 hours to get one million men across (this being slightly under a third of her stated initial force).

It looks efficient, but it seems to have been pretty wasteful and not a very good rule of thumb for normal use.  Then again, if the alternative is to be tusked, trampled or tossed by a raging elephant, it has its advantages. 

I've come up with a figure based on:
6' depth per man.
bridge wide enough for eight files abreast
20000 men
March rate of about 2 mph

That would take about an hour and a half from the head to the tail of the column, but I'm sure it would actually take longer due to the concertina effect etc.

For getting 20,000 men across a standard bridge, including regulars breaking step to do so, should we assume a 3' per man frontage (and hence a 24'+ wide bridge)?

If so, then leaving out animals, vehicles etc. and just considering the infantry, throughput will be (by my reckoning) 14,080 men per hour, so 1 1/2 hours looks about right.  (In theory 21,120 men could get through during this time.)

The big variable will be staff work, i.e. the efficiency with which contingents are handled.  If the column of march is already assembled and just carries on going, the men will be across a lot faster than if numbers of contingents are hanging around waiting for someone to tell them when to form up and start crossing.  An already marching column that does not have to change width can probably increase crossing pace to 3mph and get the job done in about an hour).  If the bridge is narrower than the marching column, it will impose a roughly proportionate delay which can be partly compensated for by doubling the men across, but there will still be a 'kink' in the column (concertina effect) for a while afterwards unless the van is halted for a short while once everyone has crossed.  Good staff work will produce one kink that works its way along the line; bad staff work will produce an effect looking like a snake that has swallowed a lot of footballs.

I think Peter's figures look good, at least from where I sit.

[Swampster]

To give a bit of context, I'm still thinking about Montaperti. I've just come back from holidaying there.

It could well have been easier for the infantry to wade across the river. If it was anything like today it only comes up to calf height. I know - I tried! I chose a couple of spots at random and managed to cross each very easily. The undergrowth next to the river is more of a  problem but that may have been grazed and browsed away in 1260. There are signs of flood defence so there might have been some boggy ground back then.

[valentinianvictor]

I won't be able to tell you where I got this from but here is some information you might find useful in determining the length of a column of troops of approximatley 20,000 men strong and how long the column would take to march.

'Sources that give details of how a Roman marching column was formed are rare. From the information we do have is that the column size would be determined by the width of any road the troops would have been travelling on. The width of an average Roman road was approximately 18 feet. If we presume there was a gap of 3 feet between each infantry file, which just about allows six men to march abreast along the average road. If we say there were 1000 infantry tasked with scouting at the head of the column, and a similar number protecting the rear of the column, that would have meant that there were 16,000 infantry remaining marching along the road. Each of the six files of infantry would have comprised of almost 2,666 men. If we say that the gap between each row of the file was six feet then that would have meant that the infantry marching long the road would have been in a column approximately 16,000 feet long. This is just over 3 miles. We then have to add in the two scouting infantry groups, who were probably much more dispersed, so say add another mile to the infantry total. We also have to figure in the cavalry who also would have been travelling with the column. If we say there were 5,000 cavalry present, who were divided to the front, right flank, rear and in the column itself then we could say that there were 500 cavalry scouting in the front, 500 protecting the rear, 500 protecting the right flank and 500 protecting the left flank then the remaining 2000 imbedding in the column itself. If we say that a maximum of four cavalrymen could ride along the same road, and they needed a depth a depth of 15 feet, and they were divided 500 to the front of the column, 500 to the rear 500 to the right flank , 500 to the left flank and the rest within the column itself, then we have an extra column depth of approximately 2 miles. If we add an extra mile for the cavalry who were scouting in front and those protecting the rear of the column this gives us a grand total for the length of the column of at least 7-8 miles. Increasing or decreasing the width of the column would of course have reduced or increased the depth, so if the column were 8 men wide then we were probably looking at it being approximately 4 miles long, a four man wide column nearer 10-12 miles long. If we use the rate of march given in Vegetius i.e. 20 miles in five summer hours then your looking at the head of a column 8 miles long crossing the bridge and then the tail crossing probably at least two and a half hours later.

[aligern]

Interesting numbers Adrian. We could add to your column at least a couple of miles of pack animals, probably more and, if it were a Persian army ten miles of engineers, engines in broken down form,sutlers, ostlers, vivandieres, concubines, wives, butchers, bakers, armour makers and sellers, vets, doctors and a mobile bordello or ten!
Once the column comes to include animals and wagons then we would also have to allow for the disruption factor of recacitrant mules and wagons that break an axle on the bridge and completely block it whilst they are dealt with. 
Roy 

[valentinianvictor]

I forgot I had left off the baggage train but as I copied this from something I had written about something you know about Roy, where there was no baggage train with the army in question, it does give a good example of how long marching columns of troops actually were.

Which raises another point. At Argentoratum Julian marched towards the Alamanni and then deployed. His column of march must have been at least 10 miles long, in which case the army would have taken between 5 to 7 hours just to reach the battlefield and deploy.

Few few historian's take that kind of information into account when looking at historical battles.

[Duncan Head]

Which raises another point. At Argentoratum Julian marched towards the Alamanni and then deployed. His column of march must have been at least 10 miles long, in which case the army would have taken between 5 to 7 hours just to reach the battlefield and deploy.

But does he have to cross a river at a single bridge, or go through any similar chokepoint?

If not, is it necessary to assume that the army marched in a single column?

[Patrick Waterson]

Good point: see Julian's marching order in Ammianus XXIV.1: there is an infantry column on each side of the baggage train, a cavalry column on one flank under Arintheus and an infantry column on the right under Nevitta.

On his march into Persia he had everyone marching at intervals wider than usual (to give an exaggerated idea of his numbers), "so that there were almost ten miles between the rear and the standard bearers in the van."  This was for an army of 65,000 (or 35,000 if one subtracts Procopius' contingent from the main body, which seems unnecessary).

Moving up to Argentoratum, a multi-column approach seems desirable because it speeds deployment and guards against ambush, and as everyone already knew how small his army was (c.13,000) there was no point in attempting deceptive measures such as extending the intervals between marchers.

We can thus surmise an infantry column on the left under Severus, a cavalry column on the right and perhaps two columns of infantry flanking the baggage, with at least one unit having been detailed for advance guard and another for rearguard purposes.  Hence we can divide the figure of 5-7 hours by a factor of 4 or 5.

[aligern]

Yes, doesn't Caesar deploy pretty quickly from camps and some f those camps are in close proximity to the enemy.  Presumably  the Romans have good drills worked out for deploying from all the gates into a battle line, otherwise the gate is going to act like a bridge/ defile and it would take hours to get out!  It is even more remarkable that Ariovistus manages to emerge from a camp and deploy.

Roy

[valentinianvictor]

Yes, doesn't Caesar deploy pretty quickly from camps and some f those camps are in close proximity to the enemy.  Presumably  the Romans have good drills worked out for deploying from all the gates into a battle line, otherwise the gate is going to act like a bridge/ defile and it would take hours to get out!  It is even more remarkable that Ariovistus manages to emerge from a camp and deploy.

Roy

I suspect deploying from a camp is going to be a damn sight easier and quicker than deploying from a column of march. My own research indicates that a typical Roman marching formation is similar to that which I described. Out in front of the column would be a detachment of 'light infantry' and cavalry scouting ahead. Behind these would be the column proper with the infantry marching in normally a single column. To the left and right flanks would be the bulk of the cavalry normally evening divided between the flanks. Behind the infantry would be the baggage train and then more 'light infantry' and following to the rear would be another body of cavalry. When the column reaches the battlefield the scouting cavalry and infantry screen the deployment. The right flanking cavalry wheel to the right to form the right wing the head of the column wheels to the right and then starts to form up, the rest of the column then marches forward and forms up from right to left. The left hand flanking cavalry during this stage wheels left to form the left wing and the rear guard light infantry and cavalry then move forward once the baggage is safely positioned. Of course this only applies to a marching army, not an encamped one.

[aligern]

Ilkke Syvanne thinks that before the Sambre battle Caesar is marching in a square formation that can fold out  to form a battle line (personally I think the terrain is too wooded for that).
Doesn't Crassus march in two lines that become a ?square to Carrhae?
Roy

[Patrick Waterson]

I suspect deploying from a camp is going to be a damn sight easier and quicker than deploying from a column of march.

I suspect that it would also be possible to march in a fashion that makes deployment for action a lot quicker, and some such system would be used when contact with the enemy was expected as opposed to just aiming to cover distance without being molested.  Besides, Julian was able to halt the march, gather the troops around him and give them a speech, all without significant loss of time.

"Already the beams of the sun were reddening the sky, and the blare of the trumpets was sounding in unison, when the infantry forces were led out at a moderate pace, and to their flank were joined the squadrons of cavalry, among whom were the cuirassiers [cataphractarii] and the archers, a formidable branch of the service. [8] And since from the place where the Roman standards had begun advancing, the distance to the enemy's camp was figured to be fourteen leagues—that is, twenty-one miles—Caesar had proper regard for both advantage and security, and having recalled his outposts [procursatores = advance guard], who had already gone ahead, and having proclaimed silence by the usual announcements, with his native calmness of speech he addressed the soldiers, who stood about him in companies [cuneatim = lit. 'in wedges'], as follows:"

I leave out the speech: it can be read in Ammianus XVI.9-12.  However the detail: "The day is already nearing noon; we are exhausted by the fatigue of the march" suggests that it was delivered after the majority of the distance to the Allemanni position had been covered (dawn to noon in August in Western Europe is about seven hours).

Note how the army is assembled to hear him speak in no more time than it takes to recall the advance guard.

When Julian finished his speech and the standard bearer cut in with the remark about his lucky star,

"On hearing this no delay was permitted, but the army moved forward and approached a hill of gentle slope, covered with grain already ripe, and not far distant from the banks of the Rhine."

This suggests that it took very little time to shake out from march formation to Hearing Caesar formation and thence to battle formation, in which the army seems to have advanced, or at least in something very close to it.

Doesn't Crassus march in two lines that become a square to Carrhae?

"All were greatly disturbed, of course, but Crassus was altogether frightened out of his senses, and began to draw up his forces in haste and with no great consistency. At first, as Cassius recommended, he extended the line of his men-at-arms as far as possible along the plain, with little depth, to prevent the enemy from surrounding them, and divided all his cavalry between the two wings. Then he changed his mind and concentrated his men, forming them in a hollow square of four fronts, with twelve cohorts on each side. [4] With each cohort he placed a squadron of horse, that no part of the line might lack cavalry support, but that the whole body might advance to the attack with equal protection everywhere. He gave one of the wings to Cassius, and one to the young Crassus, and took his own position in the centre." - Plutarch, Life of Crassus, 24.3-4

[andrew881runner]

I don't know if this is the right topic but playing rome 2 I noticed how easy is to keep control of a bridge putting a good unit, especially a pike phalanx, in it, or on its border... is this realistic? could they do the same?

[Mark G]

Most bridges were bypassed if defended, but a static defense is always vulnerable