Inundation

An obstacle created by damming a watercourse near the front of a fortification to produce a succession of pools of sufficient depth to either prevent a body of troops from wading across them or to conceal other obstacles, such as trous-de-loup, under a shallow sheet of water. Inundations were the single most difficult obstacle to use effectively due to the nature of the combination of landforms necessary for their adaptation to any particular fortification site. An inundation had to be within close musket range of the interior crest of a fortification for it to serve as an effective obstacle while the watercourse itself had to run through a relatively narrow valley or ravine capable of being dammed. The watercourse also had to be both relatively shallow and have a very moderate velocity so that the dams necessary for checking the water flow could be constructed with a minimum of labor and materials and still be strong enough to resist the pressure exerted by the flowing water and the weight of the water collected in the pools. In most cases sites near enough to damable streams to bring an inundation within close musket range were commanded by higher ground on both sides of a stream and were difficult to defilade or good fortification sites were on higher ground too far away from the banks of damable streams to cover the inundation with close musket fire.

Dams for backing water could be constructed from tenacious earth, log crib-work filled with stone, earth, and gravel, or using alternate courses of fascines and gravel. Earthen dams could be as tall as ten feet and as little as five feet wide at the top, with the specific thickness determined by the weight of shot that the structure would be required to resist if an enemy attempted to break it down with artillery fire. The forward talus facing the direction of the stream flow was given a base equal to twice the height of the dam while the down stream side was given the natural slope of the earth composing the dam. Since the purpose was to back water into pools and not to completely stop the flow sluices or waste-weirs were constructed at the center of the dam that allowed water to fall into the next downstream pool. Sluices generally required excessive time and labor and were inappropriate for temporary fortifications. Waste-weirs were constructed by cutting away a portion of the top of the dam and lining it with planks or fascines both at the top and on the downstream slope to prevent water flowing over the dam from carrying away the earth composition of the dam.

Both the number and specific height of dams required to complete an effective obstacle depended on the fall of the stream and the length of the required obstacle. Each dam had to hold a pool deep enough to present a good obstacle to the passage of troops; this required a minimum pool depth of five feet. Dams had to be positioned across a stream at whatever intervals were necessary to produce the required depth for each dam's pool. The illustration above shows the number of dams needed to produce an inundation 200 feet long in a stream that falls 1 foot in 50 feet; this is an extreme case since most streams do not fall this rapidly, but it does serve to exhibit the amount of labor that this type of obstacle could require compared to other types of obstacles that required much less time and labor to construct and were more difficult for an enemy to destroy from a distance. It should also be noted that the illustration also shows that this type of obstacle could be used to channel an attacking body of troops through several restricted defiles, that is, over the dams, which could themselves be obstructed with other obstacles such as small pickets or cheveaux-de-frise.

January, 2003