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Analysis of pond seepage for stream flow augmentation Farnworth, Melody Clare Armstead

Abstract

Many coastal streams in British Columbia have problems with low flow, which can have serious impacts on aquatic life. This research was focused on examining by experimentation and hydraulic modelling, the use of pond seepage as a novel method for stream flow augmentation. In the late summer dry period, four experiments were conducted to study the direction and magnitude of pond seepage flow under different conditions. An experimental pond (≈9 m³) was excavated to the hardpan in undisturbed soil. A ditch for capturing the seepage nearly surrounded the pond at a distance of approximately 2.4 m. The ditch was effective in collecting the seepage from both the pond bottom and the banks. Water was lost more slowly in the lined pond experiment than in the unlined experiment, and the liner also affected the direction of the seepage. From the collected data, it was determined that a combination of two hydraulic models, a wetland seepage model and an embankment seepage model, could be used to predict the experimental results. The good overall agreement between the experimental and theoretical results provided the rationale for using such models to design ponds to maintain low summer time flows in small, undisturbed streams, given that the site is similar to the experimental conditions regarding water table height. Coastal areas are the best location for using ponds as the climate conditions in these areas helps to reduce and compensate for evaporation losses. While significantly improved stream flow over a long term can be achieved with the use of multiple ponds, there are also two important benefits that can be realized from smaller projects: 1. to allow unrestricted fish movement over a short time period (less than one month) 2. to improve the quality of habitat available to fish over the summer. Even small increases in stream flow can improve the quality of habitat, especially in slower moving pooled areas, as well as allow the fish to migrate to other parts of the stream where better habitat might be found. Both of these benefits could be especially important to streams that current flow augmentation methods cannot help.

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