Mormon does not mention where Moronihah was stationed, but logically he would have been at Manti, bypassed as Coriantumr drove toward Zarahemla. Having an army from Manti at his rear was always the real risk to Coriantumr, and this time his luck has run out. Moronihah cuts off his retreat up the valley. Between his army and Lehi’s, they have Coriantumr’s army surrounded (v. 31) and cut it to pieces (v. 30).
Mormon makes a point of saying Coriantumr’s army was surrounded on all sides and even names all four cardinal points. Lehi and Moronihah were on two of those sides. As I understand the topography, the Sidon would have been on a third side, and the land from the valley rising into the mountainous terrain would have hampered a retreat in that direction. In addition, smaller units of local Nephite militia would have been able to harass Coriantumr’s army and pick off stragglers during the retreat.