B. J. Hansen

PERFORMANCE OF EROSION CONTROL PRODUCTS ON A HIGHWAY EMBANKMENT

Unprotected soil at construction sites often results in large rates of erosion. Five different erosion control treatments were tested on the slopes of a highway sedimentation basin to determine their impact on vegetative growth, runoff, and erosion. The treatments were a bare (no treatment) condition, a disk–anchored straw mulch, a wood–fiber blanket, a straw/coconut blanket, and a bonded–fiber matrix product (hydraulically applied). A minimum of three replicates was used for each treatment. Straw mulch was selected as the standard treatment for statistical analyses. The site was planted with native prairie seeds, and the establishment of vegetation was monitored over the growing season. Above–ground biomasses for the bare and straw–mulch treatments were statistically greater than those of the bonded–fiber matrix treatment. Statistically significant differences in above–ground biomass for the other treatments were undetected at the 10% level. Weedy grasses and forbs were the dominant plant species. Runoff and erosion data were collected using a rotating–boom rainfall simulator for spring and fall sets of runs corresponding to little and good vegetative growth, respectively. Runoff depths were generally larger from straw–mulch and bare plots. There were no statistically significant differences in relative runoff depth between the blankets and the bonded–fiber matrix product. Under conditions with little vegetation, erosion from the straw–mulch plots was roughly one–tenth of that from the bare soil plots; erosion from the blanket and bonded–fiber matrix plots was roughly one–tenth of that from the straw–mulch plots. There were no statistically significant differences in relative sediment yield between the blankets and the bonded–fiber matrix. Erosion from bare and straw–mulch treatments was greatly reduced by vegetative growth that occurred between the spring and fall runs.

SHEAR STRESS PARTITIONING FOR IDEALIZED VEGETATED SURFACES

Vegetation and other surface roughness materials partition the shear force of flowing water into a portion acting on the vegetation (vegetal shear) and the remainder acting on the intervening soil surface (particle shear). The fraction acting on the soil surface is directly involved in subsequent particle detachment. The purpose of this study was to directly measure the components of shear stress and to quantify the shear partition for various densities of idealized elements representative of non-submerged rigid vegetation in overland flow. Insight into the magnitude of particle shear and vegetal shear is necessary for understanding the role of vegetation in reducing particle shear and, consequently, reducing potential erosion. Circular cylinders and idealized elements with differences in the rate of change in upstream frontal area with flow depth were used to model vegetation. Detailed spatial and temporal particle shear measurements were made using a unique hydraulic flume and hot-film anemometry. Drag force was measured on individual elements within test arrays. This combination of measurements allowed for direct determination of the shear partition. The tests were conducted on three uniform element densities at discharges of 0.005 and 0.01 m3/s. Element width-to-spacing ratios ranged from 0.04 to 0.20. Over the range of densities studied, particle shear accounted for 13% to 89% of the total shear, indicating that complete surface coverage is not required to significantly reduce the shear stress acting on soil particles. Existing shear partitioning theory, in which the partition is a function of the ratio of element to surface drag coefficients and the roughness density, was found to represent the observed partition reasonably well (mean squared error = 0.036). The results from this study are important for selecting appropriate plant species and densities for erosion control systems.

The Impact of Drainage Depth on Water Quality in a Cold Climate

The impact of drainage depth on hydrology and water quality in southern Minnesota was investigated through a field experiment. Subsurface drainage systems were installed on nine field-sized watersheds ranging in size from 0.8 to 2.5 ha. The nine systems comprised two drainage depths (90 and 120 cm) and conventional (13 mm/day design drainage rate) and narrow (one-half the conventional) drain spacings. Surface and subsurface drainage runoff and nitrate-nitrogen were monitored with automated equipment for 2001 and 2002. Results from the two years show that for the conventional drain spacing, annual drainage runoff and nitrate-nitrogen were reduced for the shallow drains by up to 40 and 47 percent, respectively. The results for the narrowly spaced drainage systems were more ambiguous, however. Reductions in nitratenitrogen were attributed primarily to reductions in drainage runoff volume because only minor differences in nitrate-nitrogen concentrations were observed among watersheds. It is hypothesized that the reduced drainage volume in the shallow systems was accompanied by an increase in deep seepage below the drainage systems. Data from 2003 are still be analyzed and modeling research is underway to predict the impacts of shallow drainage over long climatic records and for other soil types.

Water Quality Benefits of “Shallow” Subsurface Drainage Systems

The impact of drainage depth and intensity (design water removal rate) on water quality in southern Minnesota was investigated with a field measurements from 2001 through 2005. Subsurface drainage systems were installed on sub-field sized plots ranging in size from 0.8 to 2.5 ha. The nine systems comprised two drainage depths (90 and 120 cm) and conventional (13 mm/day) and narrow (one-half the conventional spacing: 51 mm/day) drainage intensities. Surface and subsurface drainage runoff and nitrate-nitrogen were monitored with automated equipment over the 5-year period. Reductions in annual drainage volume and nitrate-nitrogen loads were observed in every year of the study, but were not statistically significant due to variability among the nine plots. When ANOVA was performed on the aggregated 5-year dataset, reductions in drainage volume of and nitrate-nitrogen load were observed for the shallow and conventional drainage systems, and were found to be statistically significant. The 5-year average annual drainage volume and nitratenitrogen load were lower by 17 and 15% respectively, for the shallow drainage depth and 21 and 16%, respectively, for the conventional drainage intensity. Reductions in nitrate loads were attributed primarily to reductions in annual drainage volume.

OBSERVED AND SIMULATED WATER TABLE DEPTHS IN SUBSURFACE DRAINED SOILS IN NORTHWEST MINNESOTA

Farmers are increasingly considering the use of subsurface drainage in northwest Minnesota where annual precipitation averages 560 to 640 mm. Results of field observations and DRAINMOD simulation of water table depths in two soils of the Red River of the North Basin in Northwest Minnesota are presented. Water table reductions primarily occurred between April and June when crop ET is small and snowmelt and rainfall increase soil moisture. The effectiveness of drainage depended on drain spacing and soil properties. Narrow drain spacings were more effective at lowering seasonally high water tables than wider spacings. The DRAINMOD simulation showed that a simple calibration of the model by adjusting the monthly ET factors was sufficient to allow the model to simulate the high water tables associated with large summer rainfall events, but simulated water tables receded faster than those observed in the field. The model performed more poorly in the early spring when snowmelt and soil thaw processes occurred. Research continues to improve the simulation of drainage on these soils.

A LOW HEAD, LOW POWER SYSTEM FOR CONTINUOUS FLOW MEASUREMENT

A flow measurement system was designed and implemented for a subsurface drainage field experiment. Design constraints dictated that the measurement system operate on minimal elevation head difference and low power requirements, with a flow measurement range of 0 to 46 L/min (12 gal/min) from the drained area. A system utilizing a flume in series with a sump pump was implemented. A 12V battery–powered system was controlled by a datalogger that switched between the sump pump for low flow and the flume for high flow. The battery was recharged with a 100–W solar panel. The system performed reliably during its first year of use and the power supply remained stable. Diagrams and photographs of the system are provided as well as measured flow data from the initial year of operation.