Evaluating flood risk and alterations to hydraulic patterns following installation of low-grade weirs in agricultural systems

Because low-grade weirs are designed to impede flow of water there is concern from producers that they will lead to decreased drainage and increased flooding in adjacent fields. The purpose of this study was to determine if weirs increased the risk of flooding and backflow. Analyses were conducted to evaluate the effect of low-grade weirs on hydrology. The first analysis targeted how weir-related modifications affected a single ditch by using pre and post weir-implementation data. Results of Wilcoxon–Mann–Whitney tests were mixed, with no significant differences for ditch W1 and significant differences at ditch W2 for both time to peak (p=0.009) and time to base (p<0.001) following weir installation. Differences in peak height were significant at W1 (p=0.004) and not significant at W2 (p=0.875). The second analysis determined if significant differences existed between ditches modified with weirs in general and ditches which were unmodified in general. Results of a MANOVA indicate no significant difference for time to peak (p=0.985) between ditches with weirs and those without weirs, and a significant increase in time to base in sites with weirs (p=0.003). The estimate of weir effect for time to base indicates that it takes approximately 23h 13min (s.e.=7h 30min) longer to return to initial water level when weirs are present. The third analysis considered differences between peak heights for ditches modified with weirs in general and ditches which were unmodified in general. Results of a Wilcoxon–Mann–Whitney test of normalized peak heights indicated that maximum peak heights during storm events were approximately 10% lower in ditches with weirs (18.4 vs. 28.5%, p=0.015). Results suggest weirs do not increase flooding potential but merely hold water in ditches longer by slowing return to pre-storm-event levels.