Some might contend the study of fortifications is a dry subject. No so! Not at all! In fact, there is one form of obstacle which is all wet – the inundation. The basic idea for an inundation was to employ water as a barrier against enemy movement. Unless possessing some form of divine powers, the attacker could not charge through such a water barrier. And nobody wants to attack a parapet wearing wet wool uniforms!
Certainly, a river, lake, or other large body of water would make a significant obstacle. But let us consider those obstacles to maneuver or approach to the fortification. If you will, an obstacle at the “macro” level. And, yes these would prevent the enemy from getting near the fort from one or more directions. However, for purposes of constructing a field fortification, we are looking more to the “micro” level. Thus the need is an obstacle that would break up an attack directly on the fort. Rarely will nature provide the perfect water feature – a pond, swamp, creek, or other – to form such an obstacle. Often for that purpose, we would need to modify the natural water drainage in a manner to create the desired inundation. Mahan summarized this sort of obstacle as such:
Inundations. This obstacle is formed by damming back a shallow water-course, so as to make it overflow its valley. To be effective, an inundation should be six feet deep. When this depth cannot be procured, trous-de-loup, or else short ditches, placed in a quincunx order, are dug, and the whole is covered with a sheet of water, which, at the ditches, must be at least six feet in depth.
Twice we see the planning figure of six feet of depth. Obviously this derived from the average height of a man.
The dam, of course, was the key structure in this arrangement:
The dams used to form an inundation are made of good binding earth. They cannot, in general, be raised higher than ten feet; they need not be thicker than five feet at top, unless they are exposed to a fire of artillery, in which case they should be regulated in the same way as a parapet. The slope of the dam down-stream should be the natural slope of the earth; but up-stream the slope should have a base twice that of the natural slope.
In the post-war edition of his treatise, Mahan offered in addition to just earth, the dam could be created with a “crib-work of logs filled in with stone, gravel, and earth” or “successive layers of fascines and gravel.” The fine points of the dam construction lay more in the realm of civil engineering. And to those points, Mahan recognized the need for features to maintain the dam against its natural adversary – the impounded water:
Sluices are made in the dams in a similar manner to the sluices of a mill-dam, for the purpose of regulating the level of the water in the pool above, in case of heavy rains. Waste-wiers are also serviceable for the same purpose, but unless carefully made they may endanger the safety of the dam.
No fancy graphics for us to refer to here. But from the description, Mahan preferred sluices that channeled the top of the impounded water, and thus over the top of or to the side of the dam. And he warned against wiers that would require openings within the dam’s structure. Sort of makes sense from the military perspective. Wiers are more attractive for the civil engineer who need not worry about enemy artillery.
In most scenarios, more than one dam would be needed to build an inundation obstacle. So we must consider placement:
The distance of the dams apart will depend on the slope of the stream. The level of each pool should be at least eighteen inches below the top of the dam, and the depth of water below each dam should be at least six feet. These data will suffice to determine the center line, or axis of each dam.
So there you have it.. call upon the topographical engineers!
Mahan continued to offer advice on employment of inundations in the defense:
Artificial inundations seldom admit of being turned to an effective use, owing to the difficulties in forming them, and the ease with which they can be drained by the enemy. But when it is practicable to procure only a shallow sheet of water, it should not be neglected, as it will cause some apprehension to the enemy. In some cases, by damming back a brook, the water may be raised to a level sufficient to be conducted into the ditches of the work, and render some parts unassailable. The ditches in such cases should be made very wide, and to hold about a depth of six feet.
Yes, a lot of planning and work was needed to create an inundation. And that might be undone within a day by simply breaching the dam. Still, the inundation was attractive were water could be employed, as a by-product of the impoundment, to enhance the properties of the other defensive features of the fortification.
And… when the weather turned cold…
During freezing weather the ice should be broken in the middle of the ditch, and a channel of twelve feet at least be kept open, if practicable. The ice taken out should be piled up irregularly on each side of the channel; and, as a further precaution against a surprise, water should be thrown on the parapet to freeze.
Nothing worse than being wet and cold while looking up at some “frowning” defenses.
Turning back to the vulnerability of the dam, point offered in Mahan’s post-war edition impressed the need to defend those structures:
In a system of inundations the dams should, as far as practicable, be built at points the least exposed to the fire of the assailed. The head of each dam, on the side of the enemy, should be secured from surprise by a redan, stoccade, or other defense, and the dam itself and its approaches should be swept by musketry and artillery.
Overall, these artificial inundations were somewhat a luxury for the defender. In addition to all the work building parapets, ditches, traverses, revetments, and other obstacles, would the defender have time to play in the water and build dams? However, consider also that where running water was close by, the defender had more to worry about with erosion. A well placed dam might serve as a control measure against that “enemy” of the works.
Perhaps Junius Wheeler had the best assessment of the inundation as an obstacle:
If the depth of the water over the approaches is greater than five feet, the obstacle may be considered as practically insurmountable.
If the depth is less, the obstacle is still a serious one….
You see, soldiers just don’t like water.
(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 48-49; Mahan, An Elementary Course of Military Engineering: Part 1: Field Fortifications, Military Mining, and Siege Operations, New York: John Wiley & Son, 1870, pages 77-8; Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, page 181.)