In Scharnhorst's Military Field Pocket Book, he mentions planks:
215. Fieldwork near a Bridge
An outguard of 100 infantry is ordered to defend a bridge. the army is cantonned near the river, on the banks of which there are posts at 2 or 3000 paces to the right and left. The bridge is to be maintained as long as possible, that the light troops on the opposite bank may be drawn in.

A pile of dry wood with straw should be placed under the bridge, and one immediately behind it; so that if the latter is set fire to, no person will be able to pass; and if the former, the flames will communicate to the bridge itself. A fleche should be thrown up in rear of the bridge, behind a garden paling, that the enemy may not be able to dislodge the party; the pales should be pierced with holes according to the direction necessary for the fire... sentries posted... on the bridge, the planks of which should be loosed for about 12 (? illegible) feet and the men directed to throw them into the river if it appears necessary; as the officer and his party may retire over the beams. This post will be drawn in during the night, and the planks, which have been loosened, removed from the bridge..."