Vegetation plays an important role in decreasing soil particle detachment and transport from sites following disturbance. Past rainfall simulator research (Caltrans 2004) using 2.5 m L x 0.6 m W soil test boxes with a 0.25 m sod strip (1 :10 slope proportion) at the slope toe found statistically significant reduction of total sediment loss to near zero. A principle limitation of the previous in house experiment is that a 1: 10 proportion of sod strip to slope length is not practical, nor equivalent with many industry specifications. Therefore, another rainfall simulator experiment was designed and conducted from October 2005 through June 2006 to compare effectiveness at reducing sediment loss of sod strips at 1:40, 1:20, and 1:10 proportions to slope length. Principal questions included 1) Does sod strip effectiveness vary directionally with sod strip length?; 2) Does sod strip effectiveness vary directionally with plant cultivar?; and 3) Are sod strips more or less effective than non-living EC blanket materials?

A sandy clay loam subsoil was collected from a California highway construction site. Test boxes were then filled with soil to 90% compaction and positioned at a 2H:1V slope. Bare soil (control), compost, mulch, and a straw mat erosion control blankets (ECB) were each used individually as a top treatment on vanous boxes. Toe treatments applied at lengths of 0.2 m (8 in), 0.1 m (4 in), or 0.05 m (2 in) included bare soil, mulch, straw mat erosion control blanket, jute netting, or a sod strip of a commonly used groundcover species: Carpobrotus edulis, Sea Fig; Lampranthus spectabilis, Trailing Ice Plant; Lantana montevidensis, Trailing Lantana, or Myoporum parvifolium, 'Pink Dwarf Myoporum. Storm water runoff was monttored for total water runoff, total sediment. sediment concentration. NTUs. pH. Electrical Conductivity (EC). nutrients, and selected metals over a series of simulated storm events.

As expected, toe treatments of mulch, ECB, jute, or vegetation performed significantly better than bare soil The 0.2 m (8 in) sod strips (1 :10 slope proportion) performed significantly better than the 0.1 m (4 in) sod strips (1:20 slope proportion), or the 0.05 m (2 in) sod strips (1 :40 slope proportion). Sod strips used in conjunction with jute netting or an ECB on the slope face above provided sediment concentration reductions to less than 2 g per liter of runoff. Although vegetation reduces sediment concentration in the water drastically when compared to bare soil as a toe treatment, effectiveness varies with species owing to inherent differences in plant grow form and architecture. Herbaceous leaf succulents, such as Sea Fig or Trailing Ice Plant, grow prostrate along the soil surface forming dense, continuous mats. Prostrate shrubs, such as Trailing Lantana or Pink Dwarf Myoporum, produce arching or recumbent branches, but the soil surface may remain vulnerable to overland flow.


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