Assessing the Effectiveness of Alternative Tile Intakes on Agricultural Hillslopes

Abstract

Existing surface inlets behind terraces and water and sediment control basins (WASCoBs) were replaced with alternative tile intakes (ATIs) in agricultural fields of southeast Iowa. These ATIs consisted of a buried column of gravel atop woodchips. Computational, experimental, and field methods were used to design and evaluate the ATIs’ capacity to reduce sediment and nutrient export. Single-storm simulations using the Watershed Erosion Prediction Project (WEPP) provided boundary conditions for permeameter experiments that yielded a hydraulic conductivity for the layered gravel–woodchip configuration of 4.59 cm/s ± 0.36 cm/s. Additionally, a proportional amount of sediment was retained in the permeameter (42%) compared to the amount that settled on the permeameter surface (58%). Event monitoring of field-installed ATIs during three growing seasons measured a sediment trapping efficiency of 86 ± 12% that led to deposition rates of 5.44 ± 3.77 cm/yr, quantified with 210Pb profiles. Percent reduction values were 43% for nitrate and 17% for ortho-phosphate. Finally, long-term continuous-storm modeling using the WEPP suggested that these ATIs could withstand at least 75 25-year events before clogging. Modeling using the Agricultural Conservation Planning Framework suggested watershed-scale load reductions of 1.6% for NO3 and 1.4% for total P for ATIs draining 6.8% of the modeled watershed. Using ATIs in conjunction with WASCOBs and terraces, or as standalone practices, can be a cost-effective means for keeping sediment and nutrients in the landscape.

Keywords

• alternative tile intakes
• agricultural drainage
• hydraulic conductivity
• trapping efficiency
• Iowa