 Fearing that Buell would not arrive in Louisville to prevent Bragg's army from capturing the city, Union General William "Bull" Nelson ordered the construction of a hasty defensive line around the city.He also ordered the placement of pontoon bridges across the Ohio to facilitate the evacuation of the city or to receive reinforcements from Indiana.Two pontoon bridges built of coal barges were erected, one at the location of the Big Four Bridge, and the other from Portland to New Albany.