A laboratory study was conducted to analyze the effects of a 3-D geosynthetic turf reinforced mat (TRF) and three different types of grasses (Centipede, Carpet and Bahia) on the Manning roughness coefficient (n) in channel flow. Sandy silt soil obtained from a local reservoir. The channel test conditions included each type of grass planted with and without TRF at a planting density of 25 g/m². Testing was conducted in three different stages of vegetation growth; one, two and four month(s). In total, there were eighteen test conditions tested using one flow condition of 90 HP pump capacity at 1 % slope. ASTM D6460 standard test method was used to conduct the storm-water induced runoff erosion performance test.

Bahia grass had the highest percentage of vegetation area coverage after four weeks with and without TRF. The vegetation coverage for the condition with TRF were slightly higher than without TRF after four weeks from planting. At eight weeks the grasses with TRF were taller than without TRF, and Bahia grass was the tallest in all the conditions followed by Carpet and lastly Centipede grass. The total soil loss rate for the three growth stages were more for the conditions without TRF in both methods of computing soil loss rate (depth and weight methods). Carpet, Centipede, and Bahia grass recorded 2.20 %, 2.04 %, and 2.49 % respectively without TRF. Installing TRF reduced the total soil loss rates by weight to 1.28 %, 1.17 %, and 1.31 % for Carpet, Centipede, and Bahia grass respectively. The Manning’s coefficient was directly proportional to vegetation growth and using TRF increased the coefficients after four weeks. At one month the Manning’s coefficients of Carpet, Centipede, and Bahia grass without TRF were 0.015, 0.015, and 0.016 while with TRF the values were 0.016, 0.015, and 0.016 respectively. At four months the coefficient values were 0.019, 0.020, and 0.021 without TRF while with TRF were 0.020, 0.021, and 0.024 for Carpet, Centipede, and Bahia grass respectively. The relationship between the Manning’s coefficient and the flow speed was found to be negative with and without TRF. On the other hand, the Manning’s coefficient, vegetation coverage and grass height were directly proportional.

The shear stress increased with the growth of vegetation and the use of TRF had a positive effect on the shear stress. Shear stress values for conditions without TRF were slightly lower than when TRF was used.