Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: March 10, 1996
Publication Date: N/A
Interpretive Summary: This paper discusses a conservation practice which has potential to be a valuable tool in our arsenal of erosion control technology: using narrow strips of stiff erect grass to slow runoff waters, thereby trapping transported sediment. Narrow rows of stiff erect grass are planted in parallel lines across the dominant slope of the cropped fields. As these rows mature into continuous hedges, they slow the flow of water through them during runoff events. Where runoff concentrates in rills and ephemeral gullies, these hedges pond water, causing a large part of the sediment load to deposit, thus filling the depression, and creating a series of benched terraces. These broad grass-edged bench terraces diffuse and spread the flow so that it trickles through the grassed backslopes with little or no further erosion. This paper summarizes research findings obtained during the last 5 years concerning how deep water will be ponded behind grass hedges and how much sediment they will trap. Also, several practical observations about applying grass hedges to real-world field situations area discussed. Grass hedges will benefit farmers, contractors, and landowners by providing an inexpensive, biological erosion control technology for agricultural lands and construction sites. They can improve water quality by trapping sediment. Also, if used as a leading edge to wider vegetated filters, hedges increase the efficiency and life of such filters by increasing sediment deposition above such filters and by spreading-out runoff to ensure uniform flow through such filters.
Technical Abstract: Grass hedges are narrow strips of stiff, erect, dense grass planted across concentrated flow zones that can retard and spread out surface runoff and cause deposition of eroded sediment. Strips from 0.15 to 0.5-m wide of switchgrass (Panicum virgatum), vetiver grass (Vetiveria zizanioides), eulalia (Miscanthus sinensis), and eastern gamagrass (Tripsacum dactyloides), were studied in flumes to develop a stage-discharge relationship based on vegetation properties and to evaluate their sediment-trapping effectiveness. For clear-water flows from 0.001 to 0.093 m**3sec-1m**-1, typical of those occurring in upland runoff channels, backwater depth was represented by a fractional (0.2) power function of hedge width and plant stem density and diameter. Response to discharge was approximately a linear function of Reynolds number up to 11700 and by a 0.5 power function at higher values. Backwater depths were increased by the introduction of sediment into the flow hedges became loaded with plant residues. Vetiver grass and switchgrass hedges (0.3-m wide) stood against backwater depths as great as 0.4 m. For slope of 0.05 and flow rates ranging from 0.005 to 0.04 m**3sec**-1m**-1, almost all sediment coarser than 0.125 mm was trapped in a delta formed downslope of a hydraulic jump, while 80% of sediment finer than 0.032 mm passed through the hedges. Trapping of intermediate sizes depended on the flow rate, ponded depth, and sediment density. Grass hedges are a promising new technology for erosion control in cultivated fields and construction sites. Where runoff concentrates, leading strips of tall stiff grasses can augment wider riparian filters by increasing sediment deposition above such filters and by spreading-out runoff to reduce preferential flow through those filters