- K.J. McKague, G.C. Watson, T.Lobb, and S.P. Mitchell, Ecologistics Ltd.,
Evaluation Summary (Tech. Transfer Report Summaries)
View / Download Final Report [2638 KB pdf]
Associated SWEEP/LSP Research
Completed: July, 1990
soil erosion, structural erosion control, phosphorus, sediment, monitoring,
run-off, water quality, terraces
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.
(From Technology Transfer Report Summaries - A. Hayes, L. Cruickshank, Co-Chairs)
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.
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
Of possible interest to producers and conservationists is the reinforcement
of the recommendation for careful planning and expert design in erosion
- 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
Thursday, May 19, 2011 01:51:57 PM