The Design and Construction of a Rainfall Simulator

A reliable, accurate and portable rainfall simulator was needed for vegetative and erosion control research at California Polytechnic State University, San Luis Obispo (Cal Poly) for California Department of Transportation (Caltrans) and California State University Sacramento. This simulator was designed to be easily set up and maintained as well as able to create a variety of rainfall regimes. The nozzle performance tests and lateral spacing tests were performed at Cal Poly’s Erosion Research Facility. This simulator was designed and constructed based upon the principles of the Norton Ladder Type Rainfall Simulator. This simulator is the standard for research involving simulated rainfall. Construction took place at Cal Poly’s farm shop. The rainfall simulator is a pressurized no zzle type simulator with a cam-operated oscillating boom. It emits uniform rainfall on a plot 1 m (3 ft) wide by 3.56 m (12 ft) long. The nozzles at 47.6 kPa (7 psi), Spraying Systems Company’s Floodjet 3/8K SS45, emitted an average drop size of 1.7 mm (0.07 in) and a range of drop sizes of less than 1 mm to 7 mm (0.04 in to 0.3 in), correlating well to storms less than 50 mm·hr-1 (2 in ·hr-1) as is common on California’s Central Coast. The structure of the simulator was built from aluminum, supporting the four-nozzle boom. The nozzles are spaced 99 cm (39 inches) apart. A box with an opening of 15 cm by 11 cm (6 by 4.5 inches) was beneath each nozzle to create the proper spray angle, critical for lateral spray uniformity. An additional opening in the box is attached to a system which returns the unused water to the storage tank. Flow meters control the inflow of water from the storage tank, ensuring each nozzle has the same discharge rate, no matter the orientation of the simulator. A computerdriven motor and cam system controls the storm intensity. The number of oscillations per minute of the nozzle across the box opening determines the intensity. Design storms resemble a bell curve, typical of California storms. The support system is collapsible, easy to set up and maintain. The resulting simulator is economical (less than $7,000 to construct), made with commercially available parts, easy to set-up and maintain and highly accurate

A GIS to Select Plant Species for Erosion Control Along California Highways

Through road construction and maintenance activities, the California Department of Transportation (Caltrans) actively manages roadside rights-of-way that transect California\u27s 41 million hectares (101 million acres), spanning over 3,000 meters of elevational change from seashore to subalpine. State and federal highways are grouped into 12 districts, each encompassing from one to 11 of the state\u27s 58 counties. District personnel are typically responsible for implementing site-specific adaptations of general statewide guidelines for shortto long-term erosion control following construction or storm damage. Many erosion control projects involve reestablishing vegetation through seeding where the precarious life stages of germination and establishment are controlled by both unpredictable short-term weather events and often physically inappropriate seedbeds. Many revegetation failures result from improperly siting species such that individual plants are expected to germinate, grow, and persist in locations or on topographic aspects that present physical extremes in solar radiation, temperature, precipitation, surface water flow, or wind beyond their genetically-determined physical tolerances. Each district includes enough environmental heterogeneity that revegetation using a few, district-wide seed mixes will not adequately meet the need for erosion control among diverse project sites. Consequently, a geographic information system (GIS) is being developed which allows employees to rapidly access lists of plant species for revegetation that are both ecologically appropriate for the project site and potentially useful in minimizing erosion from roadcuts and roadsides. This GIS uses hydrologic units of CALWATER at 1:24000 as a means to link physiographic and climatalogical data together with presence or absence of selected plant species in each hydrologic unit. Plantclimate classifications follow the 19 general plantclimates depicted on the California Plantclimates map devised by the University of California Cooperative Extension Service in 1967, and revised in 1979. These 19 general plantclimates are being refined using elevation contours and topographic aspects derived from digital elevation models to allow assignment of different plantclimates to portions of hydrologic units that exhibit steep elevational gains or considerable landform diversity. Through the overlay of other data depicting county boundaries, roads, and places, users are able to locate project sites, query the plant species climate matrix, and export data tables to spreadsheets or reports. Guidebooks that index the same plant species climate matrix through a standard route+county+mile/km georeferencing system make these data available to district personnel in another format as well

Plant Establishment with Rainfall Simulators for Erosion Control

Hydroseeding failures on disturbed sites are usually attributable to combinations of improper species selection, seeding at inappropriate times, and/or improper seed mixes, fiber, and tackifier. To investigate these factors, California Polytechnic State University, San Luis Obispo, in conjunction with the California Department of Transportation (Caltrans) and California State University, Sacramento, conducted a study of these factors’ affect on vegetation establishment. The goal was to identify initially fast growing vegetation that demonstrates long-term erosion control effectiveness. Native plant species common to District 5, along the California Central Coast, were used. Treatments were conducted in 0.6 by 2 m by 30 cm soil test boxes set at a 2:1 (H:V) slope. Boxes were filled with a medium sandy loam soil (USDA), typical of District 5 fill slopes, compacted to 90 percent. Erosion control treatments included combinations of imprinted straw and hydroseeding of fiber, fertilizer, and tackifier. All boxes were planted with the same native seed mix that included shrubs, forbs, and grasses. Norton Ladder rainfall simulators were used to simulate natural rainfall patterns found in the area. The rainfall regimes applied were natural precipitation, 53.3 cm (21 in/yr during the study period) and uniform rainfall at the mean annual rate, 56 cm (22 in/yr), half the mean annual rate, 28 cm(11in/yr) and double the mean annual rate, 111 cm (44in/yr). The rainfall simulators mimicked rainfall characteristics for the California coast, such as drop size distribution, terminal velocity and a range of storm intensities. In all, 24 boxes were established and treated under rainfall simulators, eight additional boxes were subjected to natural rainfall, and two more boxes were untreated (bare soil). Percent cover and runoff quality (measured as Suspended Sediment Concentration) were measured for each box. The boxes treated with straw and fertilizer showed greater percent cover than those treated with tackifier and no fertilizer. The ANOVA results indicated that this effect statistically significant to a high degree (p=.001). The effect on runoff was marginally significant (p=.048). Runoff volume was greatest on the heavy rainfall treatments. Higher rainfall treatments showed an increase in the quantity of the native plants of yarrow (Achillea millefolium), lupine (Lupinus succulentus), and California brome (Bromus carinatus). Shrubs and deer lotus (Lotus scoparius) were the least common species under all rainfall regimes. This project demonstrates using hydroseeding that includes tack an

Vegetation Establishment For Erosion Control Under Simulated Rainfall

The California Department of Transportation (Caltrans) manages rights of ways that transect 41 m H (101m ac) and span over 3000 m (9000 ft) in elevation from seashore to sup alpine. There are approximately 4,900 native and 1,000 naturalized alien plant species in California. Only a few hundred are reliably useful in erosion and sediment control. Specifying native and naturalized vegetation mixes for use in hydroseeding or plug planting in conjunction with mechanical erosion control methods can have varying result for minimizing accelerated soil erosion. To investigate these factors, Cal Poly, San Luis Obispo, in conjunction with Caltrans and CSU, Sacramento, conducted a study establishing vegetation using hydroseeding and plug planting with erosion control practices of crimped straw, jute netting, gypsum, BFM, and guar tackifier. The vegetative treatments included native vegetation from Caltrans District 5, Bromus carinatus (California brome) seeds and plugs, a typical naturalized erosion control mix from Farm Supply, existing seed bank, mostly Lolium multiflorum (rye grass), and two control boxes left untreated. Percent cover and runoff quality were measured for each box. The goal was to identify initially fast growing vegetation that establishes within 70 days and demonstrates long-term erosion control. Treatments were conducted in 0.6 by 2 m soil test boxes set at a 2:1 (V:H) slope. Seeding rates were typical for District 5 and plugs were planted at 22 and 44/m2. Boxes were filled with a sandy clay loam (USDA) soil typical of District 5 fill slopes, compacted to 90 %. The rainfall simulators mimicked a 30-year storm along the California coast with 1.5” of rain in 1.5 hours. The highest percentage of vegetation was with the native seedings and plugs, with jute and straw consisting mostly of legumes and forbs. The EC mix and gypsum produced the least amount of grasses. The EC mix and BFM were very dense stands of legumes. Gypsum and tackifier treatments were relatively bare. Native plants were poorly established in all treatments. The plug plantings were well established. The lowest runoff sediment concentration was with both the native and EC mix seedings and jute, followed by BFM, plugs and jute and finally jute alone. The range was 7.8 to 1,0002.5 mg/L. The highest runoff sediment concentration was the existing vegetation and guar tackifier, crimpled straw, gypsum, and bare soil. The range was 6,921.4 to 46,894.2 mg/L

Analysis of Compost Treatments to Establish Shrubs and Improve Water Quality

Utilization of compost as an erosion control tool is gaining momentum for many reasons. Compost offers excellent surface protection for reducing topsoil loss while providing a favorable substrate for hydroseed mixes. Soil moisture is retained and nutrients for vegetation are provided, meanwhile inhibiting undesirable plant species. However, there is wide variation in available compost sources and cost. Possible interactions between compost composition, soil type and vegetation production may occur. Hence, an experiment aimed to determine whether there is a noticeable difference in erosion control and seedling germination performance between several common types of compost applied at varying rates and methods over two subsoils was established. The experiment was conducted at the Erosion Research Facility at California Polytechnic State University, San Luis Obispo, in conjunction with the California Department of Transportation, and the Office of Water Program at California State University, Sacramento. Fine sandy loam and silty clay subsoils were collected from two California highway construction sites. Test boxes were filled with one of the two soils, compacted, and positioned at a south-facing 2H:1V slope. After compost application, a hydroseed mix of four California native shrubs, Baccharis pilularis, Eriogonum fasciculatum, Eriophyllum confertiflorum, and Lotus scoparius, were seeded on the test boxes. Applications of the three composts included 1) topical 16 mm depth, 2) an admixture of soil and compost (25% by volume), as well as hydroseeding (finest textured compost only) at 3363 kg/ha with 1121 kg/ha fiber, 3) natural rainfall collected from boxes was analyzed for total water runoff, sediment load, sediment concentration, pH, total dissolved salts, and turbidity. In terms of water quality, all compost treatments performed significantly better than the control. Direct surface application consistently produced better water quality than mixed compost/soil application, yet mixing compost with the sandy clay loam produced more native shrubs

Overland Flow and Rainfall Simulation Studies on Ornamental Vegetation, Compost, and Jute Netting

The literature is replete with studies quantifying erosion control effectiveness from raindrop impact on various vegetation types and erosion control products. However, there is little published overland flow research documenting the effectiveness of ornamental vegetation and erosion control products in filtering sediment and nutrients from stormwater runoff. The California Department of Transportation and the Office of Water Programs, California State University, Sacramento, has conducted two studies at the Erosion Control Research Facility at Cal Poly State University, San Luis Obispo addressing the use of ornamental vegetation as an erosion control treatment. The first study addressed how well ornamental vegetation, jute netting, and a combination of jute netting and vegetation decreased soil erosion and runoff during rainfall simulation. The second study compared the performance of ornamental vegetation, 0.5 inches of compost, and jute netting treatments in decreasing sheet erosion due to overland flow. Both studies used sandy loam soil in test boxes set at a southwest aspect with 2:1 and 3:1 slopes, respectively. Treatments were evaluated by measuring the runoff quantity, sediment load, sediment concentration, pH, total dissolved solids (TDS), electrical conductivity (EC) and turbidity of the runoff. Ornamental plant species included Lonicera japonica, Lantana montevidenses, Carpobrotus edulis, Hedera helix L., Myoporum parvifolium, Rosmarinus officinalis L. and Vinca major. Rainfall simulation trials yielded significant reductions in total runoff and sediment by any treatment compared to bare soil, with 100 % vegetative cover yielding 98.6 % and 99 % reductions, respectively. Turbidity was significantly reduced by all treatments, while TDS and EC were not significantly different among trials. Average pH values for bare soil were significantly higher than those of jute netting and/or vegetation. In overland flow experiments, compost reduced runoff, sediment, and turbidity by greater than 96 % and increased EC by 430 % when compared to bare soil. Jute netting reduced runoff, sediment, turbidity, and EC by 43 %, 99 %, 97%, and 65 %, respectively, when compared to bare soil. Higher pH and salt concentrations were detected in runoff from boxes treated with compost; however, levels were not substantial enough (1673.9 µS) to be harmful to plants. Since no runoff was produced in overland flow trials, ornamental vegetation treatments were 100 % effective in controlling overland flow under test conditions. Differences among the plant species will be elucidated with future research involving steeper slopes and increased flow rates

Water Quality Relative to Slope Toe Strip Type and Length

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, \u27Pink 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