Erosion control effectiveness of straw, hydromulch, and polyacrylamide in a rainfall simulator

Abstract

Temporary ground covers are used on construction sites to help establish vegetation and reduce erosion on disturbed areas. This study was an evaluation of straw, with or without polyacrylamide (PAM), and wood fiber hydromulch, with or without PAM, for reducing erosion and improving runoff water quality. Soil was packed to a depth of 0.06 m (0.2 ft) into 1 by 2 m (3.3 by 6.6 ft) boxes, which were placed under a rainfall simulator at a slope of 18°. Straw was applied by hand to boxes at a single rate of 2,240 kg ha−1 (2,000 lb ac−1) with no PAM (control), with dry granular PAM at either 37 or 74 kg ha−1 (33 or 66 lb ac−1), or with dissolved PAM at 37 kg ha−1. A wood fiber hydromulch was applied using a commercial hydroseeder at either 1,970 or 2,940 kg ha−1 (1,760 or 2,620 lb ac−1) with or without 37 kg PAM ha−1 (33 lb ac−1) mixed in the tank with the mulch. The rainfall simulator was run for 40 minutes at a rate of 37 mm h−1 (1.5 in hr−1) followed the next day by 8 minutes at the same intensity, for a total of 29.6 mm (1.2 in) of rainfall. The runoff water was collected and analyzed for turbidity and total suspended solids (TSS), and the time to runoff initiation was recorded. Turbidity was highest for the straw alone treatment at 1,500 and 1,040 nephelometric turbidity units (NTU) for first and second events, respectively. The lowest turbidity was in the hydromulch plus PAM treatments, ranging from 62 to 151 NTU. Adding the granular PAM at 37 kg ha−1 (33 lb ac−1) to straw reduced turbidity but increased sediment loss, while the dissolved PAM and the higher rate of dry PAM did not have the latter effect. This was apparently due to the slow dissolution of the dry PAM for the first several minutes of the intense rainfall. Applying PAM with straw provided benefits in reducing turbidity, but the dry application could exacerbate erosion if heavy rain occurs soon after application. The best results were usually obtained with hydromulch plus PAM, but adding PAM to a less expensive straw ground cover produced similar or better results than the hydromulch application without PAM.