Seasonal Variations of Manning's Roughness Coefficient in a Subtropical Marsh

Manning's roughness coefficient, N, ranged from 0.16 to 0.55 in a subtropical marsh over the flow depth varying between 65 to 40 cm. The N value increas-ed with a decrease in flow depth. The roughness coeffi-cient tripled over a six month period from June to November. The N value in a nonsprayed area was twice of that determined in a chemically defoliated area. The N value and vegetation density were parabolically related. The increase in N value was directly proportional to the 1/2 power of vegetation density and 2A power of flow depth in all types of vegetation.