The first consideration when selecting movable bed limits should be which portion of the cross section the river will deposit or erode.  The movable bed limits delineate which cross-section, station-elevation points can move vertically.

However, placing the movable bed limits can also affect the total sediment transport capacity. Most transport equations compute transport capacity per unit width. HEC-RAS converts this transport-per-unit-width to total transport by multiplying the unit rate by the distance between the movable bed limits. Therefore, though there are complicating feedbacks between cross section shape, channel bank placement, and movable bed limits, movable bed limits that are farther apart, often generate more transport capacity.