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SWEEP Report #14

The Effect of Terraces on Phosphorus Movement

K.J. McKague, G.C. Watson, T.Lobb, and S.P. Mitchell, Ecologistics Ltd., Waterloo, Ont.

Executive Summary

Evaluation Summary (Tech. Transfer Report Summaries)

View / Download Final Report  [2638 KB pdf]

Associated SWEEP/LSP Research



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Completed: July, 1990

Key Words:

soil erosion, structural erosion control, phosphorus, sediment, monitoring, run-off, water quality, terraces

Executive Summary

A variety of structural erosion control techniques have been tested and implemented as a means of reducing the rate of soil loss from agricultural land. Terracing, in particular channel terracing, is one structural approach which is gaining popularity for use on Ontario farms. Unfortunately, data which quantifies the effect terraces have on the quality of runoff water is limited. A monitoring technique is needed to gather data required to assess terrace efficiency in reducing phosphorus and sediment loads from terrace-protected lands. In order to address this need, a number of tasks were identified and undertaken in this study. Tasks included the development, selection and laboratory testing of a terrace monitoring apparatus, the field installation of the apparatus, field data collection and analysis, and computer simulation of the monitored events.

The monitoring apparatus developed and installed can best be described when viewed as being two sub-systems which are integrated through data acquisition hardware and software. The purpose of the inflow monitoring sub-system is to enable determination of the quantity and quality of runoff water entering a terrace's ponding area. The outflow monitoring system measures the level of flow exiting the terrace through the tile system and facilitates collection of water quality samples from that same water.

Laboratory tests were conducted on the monitoring apparatus prior to its field installation. Field installation was undertaken on a terraced site receiving runoff water from a 2.4 hectares watershed planted to a white bean crop. Once installed, the system was checked weekly to download the continuously-collected data and ensure that power levels for the various electronic components were maintained.

The field site was monitored for a period of eight months (April through November inclusive). This period corresponded to a relatively dry period for the study site. As a consequence, only one (1) storm event occurred which was sufficient to generate runoff and ponding at the monitored terrace. While laboratory testing instilled confidence in the operation of the monitoring apparatus, the single storm event which was available to field test the equipment identified other technical problems which went undetected during the lab testing phase. Problems encountered in the initial field test, for the most part, could be overcome relatively easily and quickly. The most difficult problem to overcome centred around the sub-zero temperatures experienced just prior to the event which is believed to have damaged the submerged pressure transducer used to measure pond water level.

As a consequence of the problems encountered in the initial field testing opportunity, the data collected for the event was incomplete. A thorough analysis, along with some extrapolation of the data which were available however did enable formation of a complete picture of the event. For this rainstorm, which closely matched the characteristics of a 1 hour, 2 year return period storm, the terrace was found to be between 10 percent and 15 percent efficient in trapping sediment and associated phosphorus. This is much lower than the 97 percent sediment trapping efficiency measured in a previous U.S. study (Mielke, 1985). The reason for this difference is apparent when the inflow and outflow hydrographs for the event in this study are reviewed. Peak discharge rates were nearly identical for both the inflow and the outflow hydrograph suggesting little opportunity for the inflow water to pond and deposit sediment. Thus the terrace system in this situation was acting in a fashion more characteristic of a tile/catchbasin system. Further monitoring and analysis of a range of storm events is necessary before any general conclusions can be made concerning terrace trapping efficiency. Recommendations include techniques for avoiding the technical problems detected during the field test.

Three different soil erosion computer modules, AGNPS, CREAMS and SEDCAD, all of which include algorithms for assessing the effectiveness of terracing were applied to the study site using the monitored storm event as the input rainfall. AGNPS and CREAMS give results similar to the field observed data. SEDCAD algorithms overestimated terrace efficiency. For all models to generate reasonable results, calibration of the hydrology and sedimentology components was necessary. Results of both the monitoring and modelling aspects of the study have re-emphasized the need for designers of such systems to optimize the ponding time to facilitate the settling of sediment and yet avoid damage to the crop within the zone of temporary ponding.

Evaluation Summary

(From Technology Transfer Report Summaries - A. Hayes, L. Cruickshank, Co-Chairs)
Three sets of experiments were implemented to study the methods of monitoring the effectiveness of an erosion control structure for sediment ponding and run-off water quality. Laboratory tests examined a range of monitoring technologies. The most promising of these was field tested. The results of the only storm event measurable during an eight-month period were compared with the results of predictive erosion and run-off models (AGNPS, CREAMS, and SEDCAD).

The Water and Sediment Control Basin (WASCOB) monitored was found to be 10 to 15% efficient at trapping sediment and phosphorus from the one run-off event that occurred. The report emphasizes to erosion control designers the importance of the trade-off of allowing sufficient ponding time without causing excessive crop damage.


The laboratory and computer modelling aspects of the study should prove to be useful to researchers and those monitoring soil erosion and agricultural water quality. The single storm event renders the field testing portion of the study inconclusive and not particularly useful. It is anticipated that useful information regarding monitoring will be forthcoming from SWEEP Watershed studies.

Of possible interest to producers and conservationists is the reinforcement of the recommendation for careful planning and expert design in erosion control systems.

Associated SWEEP/LSP Research:

  • SWEEP Report #45 - Management of Farm Field Variability. III. Effect of Tillage Systems on Soil and Phosphorus Loss
  • SWEEP Report #53 - Phosphorus Movement in Soil as a Function of Phosphorus Solubility and Reactivity
  • SWEEP Report #60 - The Effect of Conservation Tillage Practices on the Losses of Phosphorus and Herbicides in Surface and Subsurface Drainage Waters

Future Research: ( ) indicates reviewers suggestion for priority, A - high, C - low.

(C) More data from several storm events are required to determine relative effectiveness of terraces as erosion control measures. Further methods research and development is required to determine the relative effectiveness of erosion control systems. This will be of particular concern as increasing emphasis is placed on conservation and environmental farm planning in the very near future.



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Created: 05-28-1996
Last Revised: Thursday, May 19, 2011 01:51:57 PM