Fortification Friday: Applying Mahan’s “relief” on a real fort

For the last several weeks, I’ve discussed Mahan’s guidance for modifying the plan of a fortification to take into account terrain relief.  Specifically, Mahan’s lessons focused on situations where high ground dominating the fortification was, by necessity, left open to the enemy.  We’ve looked at how terrain relief creates flaws in the defense and then how to address those issues by defining the height of parapets and traverses.  And we’ve considered Mahan’s guidance to avoid extreme solutions.  As alluded to at the close of last Friday’s post, the value of this knowledge is not that we might go out an build our own fortifications today (you see, the Wright Brother’s invention sorta throws this question of relief into a whole new discussion).  Rather that we can use Mahan’s teachings to assess, to a degree, the earthworks that have survived to our times.  In short, this is a key to unlocking a primary source – the actual ground over which the battles were fought.

A couple weeks back, after the Civil War Seminar, I made a side trip to visit the Staunton River Bridge battlefield.  This bridge was a key objective o the Wilson-Kautz Raid of June 1864.  A successful Confederate defense, using the works we will examine, on June 25, saved the bridge and ensured the Richmond & Danville Railroad remained open.  The Historic Staunton River Foundation has a short description of the battle, if you are unfamiliar with this action.  And I would also encourage you to visit this site, if you have not already… just a short drive from several other destinations that are likely on your bucket list.

Well before the Wilson-Kautz Raid, Confederate leaders recognized the need to defend the bridge.  They constructed a fine bastion fort atop bluffs on the right bank of the Staunton River next to the bridge.  This overlooked a broad flood plain on the opposite bank and directly covered access to the bridge.  The works still stand today, described in the National Historic Register application as among the most well preserved in Virginia (and that is saying a lot, given the number of earthworks in the state).  The state park includes the fort:

2016 Feb 7 Staunton River Bridge 026

Some supporting works:

2016 Feb 07 Staunton River bridge 047

The ground over which the Federals had to advance on the bridge:

2016 Feb 7 Staunton River Bridge 099

And the bridge itself:

2016 Feb 7 Staunton River Bridge 073

A magnificent structure if you are, like me, fascinated with things built in the days where iron and wood were state-of-the-art.

You see some shots of the battle-space that we need to consider.  But to go all “Mahan” on this battlefield, we need to look at this from the maps and use tools which are a bit more analytical than simple photos might offer.  The first step is to review a topographical map of the area.  Google Maps offers a handy, accessible version for our purposes:

TopoStauntonRBridge2

A “map recon” demonstrates some high ground which might be used by an attacker to defeat the fort at the bridge:

TopoStauntonRBridge2A

The fort is annotated with a red box in the center.  I’ve drawn a circle, approximately 1000 yards, to indicate the planning range cited by Mahan.  And there are five red stars, numbered 1 to 5 in order of danger, placed on prominent ground for our consideration.  The fort shows at 400 feet above sea level elevation, to be specific – 398 feet.  Four of the other five points are just above the 400 foot elevation line.

Oh… and if we want to unleash the power of Google Maps, we might even flip this into a 3D mode and consider that perspective…

StauntonRB_Google3D_A

Certainly more tools than were around in 1864, when they were instructed to use poles and cords to figure out these matters.  But then again, in 1864 the Confederates were working in a situation where they did not need worry about trespassing signs or where spades of dirt could be turned.  They were “hands on” while we are more “hypothetical”.

Let me describe the five points in a bit more detail:

  • Point 1 -About 1270 yards from the fort, at 411 feet above sea level.
  • Point 2 – 1880 yards, so just over a mile.  Elevation of 405 feet.
  • Point 3 – 2500 yards, or about a mile and a half.  Elevation at 437 feet.
  • Point 4 – 1230 yards and elevation of 396 feet.
  • Point 5 – 1581 yards and elevation of 418 feet.

Of these five, #4 and #5 are on the west side of the river.  I’ve included them here for some potential follow up analysis.  The only way these might have factored is in some “what if?” late-war scenario involving Sheridan or Stoneman.

Of the other three, all are outside the 1,000 yard range which concerned Mahan.  However, Mahan’s writings were before rifled artillery.  We know the most likely adversary for the Confederates in this case was the Federal cavalry.  And when those troopers went raiding, they were often accompanied by horse artillery armed with 3-inch Ordnance Rifles.  At ten degrees elevation, those could put a shell to 2788 yards distance.  So points #1, #2, and #3 are places the Federals could use to bombard the fort.  Point #1, being closest, presented the most risk.  So let us take a closer look at Point #1.

As it is on private property now days, our “look” is largely more of a map recon.  But we know the range and difference in elevation.  Using Pythagoras’s old trick, we apply the distance (1270 yards = 3810 feet) and the difference in elevation (13 feet) to find the straight line distance (the old “c” or hypotenuse) is only longer by some negligible fraction. So… ?  Well if you use the formulas from high school geometry to find the size of the angles, we find the angle off the horizon for an observer in the fort looking at point #1 is but 0.54°.

StauntonRB_triangle_Pt1

Aside from getting bonus points for some practical application of geometry, we have an important measure here.  That half degree would translate to the place on which the post would be marked by an observer calculating the needed height of the parapet.  Sure, we are only talking about inches here.  But that is a measure used in 1864 that we cannot gather using their “hands on” methods today, due to trees and the no trespassing signs.  And that measure tells us the parapets of the fort needed to provide about 6½ feet of height over the tread of the parapet:

2016 Feb 7 Staunton River Bridge 041

And here’s the resultant profile, given allowances for time and erosion:

2016 Feb 7 Staunton River Bridge 037

I say time and erosion, because despite the impressive height of the wall might be, from the interior that height is lacking…..

2016 Feb 7 Staunton River Bridge 055

That’s an artillery platform in the center of view, offering but a small two foot tall parapet today.  And maybe three to four feet tall on the sections away from the platform. I would not advocate moving a single shovel of dirt to “restore” these works. Instead, I would suggest that we must consider the specification height required here when viewing the works as standing today.  Likewise, we need to consider additional structures atop the parapet, such as headlogs and pickets.

Likewise, the measure we derived above answers another question the engineer might have considered in 1864:  Does this fort need a traverse?  Given the assessment for Point #1, the answer is “no.”  But we have to, as Mahan suggested, consider all possible points of threat.  There are traverses included at other places in these works, but not in the main bastion enclosure.

One more interesting aspect that I should mention with regard to Point #1 and analysis with our modern tool set is the line of sight profile.  There are several tools out there on the web. I’ll offer a result from Google Map Developers:

StauntonRB_LOSProfilePt1

Notice the intervening terrain here.  We see the river at the base of the bluff (on the left).  Then the low ground of the river bottom.  About halfway across that open field, we see a stream cutting through.  Then the rise of ground up to Point #1.  Elevation scale for the graph is in meters, so adjust values accordingly.

This is the sort of analysis I try to do for any Civil War earthwork visited.  While in most cases this does not produce any information that contradicts standing interpretation, such analysis does provide a level of detail I think is needed to fully understand these things at the tactical level.

Plus… I get to use neat tools!

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