Browsing by Author "Lee, Dowon"

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    Long-term effectiveness and maintenance of vegetative filter strips
    Dillaha, Theo A. III; Sherrard, Joseph H.; Lee, Dowon (Virginia Water Resources Research Center, Virginia Polytechnic Institute and State University, 1986)
    Vegetative filter strips (VFS) on 33 Virginia farms were visited and observed over a 13-month period to evaluate their long-term effectiveness for water quality improvement. Operational problems observed during the site visits were documented and design or maintenance procedures to alleviate the problems were evaluated. Of the VFS observed, 36% were judged to be totally ineffective, were no longer in existence, or were simply extensions of pastures - although all were, or had been, part of the state cost-share program. Most of the sites visited had topographic limitations which severely limited VFS performance. Accumulation of surface runoff in natural drainageways within fields before it reached the VFS was the most common and critical problem. Runoff from the drainageways crossed the VFS in a few narrow areas, totally inundating the filters and rendering them ineffective for sediment and nutrient reduction. This situation is difficult to control and VFS are probably not appropriate for fields with extensive internal drainageways unless the VFS extend up into the fields and parallel the drainageways forming wide grassed waterways. Vegetative filter strips were judged to be beneficial even when they could not filter sediment and nutrients from runoff because they provided localized erosion protection in critical areas along streambanks. They did not act as filters, however, and should therefore be referred to as vegetative buffer strips or critical area plantings.
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    Simulation of phosphorus transport in vegetative filter strips
    Lee, Dowon (Virginia Polytechnic Institute and State University, 1987)
    This study investigated the effectiveness of vegetative filter strips (VFS) in removing phosphorus from surface runoff. Dissolved and particulate nutrients were treated separately due to differing transport and removal mechanisms. Nutrient transport in VFS appears to be a function of runoff rate, concentration and size distribution of suspended solids, and biological factors that influence hydrologic and chemical processes in filter strips. Three sets of experimental field plots were constructed to simulate VFS. Each set consisted of three plots containing sediment and nutrient source areas and 0.0, 4.6, or 9.1 m grass filter strips. Artificial rainfall was applied to the plots, and surface runoff, soil, and plant material samples were collected and physically and chemically analyzed. The VFS reduced surface runoff, suspended solids, and phosphorus losses. Most removal of sediment and phosphorus was accomplished in the first few meters of the VFS. The filter strips did not remove phosphorus as effectively as sediment, due to their ineffectiveness for filtering dissolved phosphorus and sediment-bound phosphorus associated with fine particles. The VFS often increased orthophosphorus losses in surface runoff. Laboratory batch experiments of phosph~rus desorption reaction suggested that plant residues, living plant canopy, and soil components of the strips could release dissolved phosphorus to surface runoff. A modified Elovich equation and a diffusion-control model were used to describe the phosphorus release from the plant and soil materials. A computer model, GRAPH, was developed to simulate phosphorus transport in VFS by incorporating phosphorus transport submodels into the VFS model in SEDIMOT II, a stormwater and sediment transport model. The model considers the effects of advection processes, infiltration, biological uptake, phosphorus desorption from the soil surface to runoff, the adsorption of dissolved phosphorus to suspended solids in runoff, and the effects of dynamic changes in the sediment size distribution on chemical transport. GRAPH was verified using the results of the physical plot simulations. The model's predictions and observed phosphorus transport compared favorably. Sensitivity analysis suggested that sediment and phosphorus removal was sensitive to the input parameters in the order: filter length and width, grass spacing, and filter slope and surface roughness. Increased filter width and length and aboveground biomass increased orthophosphorus loss from VFS.
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    Use of vegetative filter strips to minimize sediment and phosphorus losses from feedlots. Phase 1, Experimental plot studies
    Dillaha, Theo A. III; Sherrard, Joseph H.; Lee, Dowon; Shanholtz, Vernon O.; Mostaghimi, Saied; Magette, William L. (Virginia Water Resources Research Center, Virginia Polytechnic Institute and State University, 1986)
    A rainfall simulator was used to evaluate the effectiveness of vegetative filter strips for the removal of sediment and phosphorus from feedlot runoff. Simulated rainfall was applied to nine experimental field plots with a 5.5-m by 18.3-m bare source area (simulated feedlot) and either a 0, 4.6-m or 9.1-m filter located at the lower end of each plot. Fresh dairy manure was applied and compacted into the bare portions of the plots at rates of 7,500 kg/ha and 15,000 kg/ha. Water samples were collected from the base of each plot and analyzed for sediment and nutrient content. One set of plots was constructed so that flow through the filters was concentrated rather than shallow and uniform. The 9.1-m and 4.6-m vegetative filter strips with shallow uniform flow removed 91 percent and 81 percent of the incoming suspended solids, and 69 percent and 58 percent of the incoming phosphorus, respectively. Soluble phosphorus in the filter effluent was sometimes greater than the incoming soluble phosphorus load, presumably due to lower removal efficiencies for soluble phosphorus and the release of phosphorus previously trapped in the filters. Vegetative filters with concentrated flow were much less effective than the shallow uniform flow plots, removing 40 percent to 60 percent less sediment and 70 percent to 95 percent less phosphorus than plots characterized by shallow uniform flow. Observation of existing filter strips on cropland found that in-field filter strips were not likely to be as effective as the experimental field plots because of problems with flow concentration.