The nature and structure of a hanging dam in a gravel-bed river

The town of St. Raymond (QC, Canada) is subject to frequent ice-induced flooding of the St. Anne River. In addition to breakup ice jams, a major cause of winter flooding is associated with massive frazil deposition in the form of a hanging dam in the downtown reach of the river. Recent studies have identified a number of potential solutions to attenuate the flood risk. An important aspect of these solutions is associated with the reduction of frazil deposition, which amount had not been estimated before. The purpose of this study was to answer this need by quantifying the hanging dam’s dimensions (volume and mass) and structure. The 2014-2015 frazil dam’s length was 9.5 km, its volume was 620 000 m3 and its mass was 440 000 tons. Most of the hanging dam mass (74%) originated from the upstream part of the St. Anne River, where frazil transport in the water column was measured by manual sampling. In turn, most of the frazil was deposited in the downtown reach because of low velocities caused by the presence of the Chute-Panet dam located 3 km downstream of the town. The hanging dam core was also surveyed at multiple cross-sections. It presented frazil layers and accumulation zones of different densities. The hanging dam structure presented a greater complexity in town and was simpler near its head and its toe

The influence of system settings on positioning accuracy in acoustic telemetry, using the YAPS algorithm

Background: Acoustic positioning telemetry allows to collect large amounts of data on the movement of aquatic animals by use of autonomous receiver stations. Essential in this process is the conversion from raw signal detections to reliable positions. A new advancement in the domain is Yet Another Positioning Solver (YAPS), which combines the detection data on the receivers with a model of animal movement. This transparent, flexible and on-line available positioning algorithm overcomes problems related to traditional point-by-point positioning and filtering techniques. However, its performance has only been tested on data from one telemetry system, providing transmitters with stable burst interval. To investigate the performance of YAPS on different system parameters and settings, we conducted a simulation study. Results: This paper discusses the effect of varying burst types, burst intervals, number of observations, reflectivity levels of the environment, levels of out-of-array positioning and temporal receiver resolution on positioning accuracy. We found that a receiver resolution better than 1 ms is required for accurate fine-scale positioning. The positioning accuracy of YAPS increases with decreasing burst intervals, especially when the number of observations is low, when reflectivity is high or when information out-of-array is used. However, when the burst interval is stable, large burst intervals (in the order of 1 to 2 min) can be chosen without strongly hampering the accuracy (although this results in information loss). With random burst intervals, the accuracy can be much improved if the random sequence is known. Conclusions: As it turns out, the key to accurate positioning is the burst type. If a stable burst interval is not possible, the availability of the random sequence improves the positioning of random burst interval data significantly

European silver eel (Anguilla anguilla L.) migration behaviour in a highly regulated shipping canal

Among the many man-made structures that facilitate shipping, navigable canals take an important position. These canals may offer energetically favourable migration routes for diadromous fish, but they may also obstruct fish migration, for instance at shipping locks. Because the use of shipping canals by, and their effects on, migrating fish remain unknown, we assessed whether these canals can play a significant role in the migration of the critically endangered European eel. Only one third of 70 acoustically tagged silver eels completed migration through a shipping canal, and did so at a very low pace (average < 0.06 m s(-1)) due to delays at shipping locks and most likely also due to the disruption of water flow. These delays may come at an energetic cost, hampering the chances of successful migration. Knowledge on the impact of shipping canals on diadromous fish is crucial for proper management regulations. For instance, the observation that eels mostly migrated at night and during spring and autumn can support water managers to define adequate measures to improve eel migration in shipping canals

European silver eel (Anguilla anguilla L.) migration behaviour in a highly regulated shipping canal

Over the last 40 years, Anguilla species in the northern hemisphere have shown a strong decline in recruitment. Due to a 98% recruitment decline, the European eel is now classified as critically endangered according to the IUCN Red List. To aid conservation and recovery of European eel populations, the European Union recently adopted a Council Regulation which imposes a management system that ensures 40% escapement of the spawning stock biomass, defined as the best estimate of the theoretical escapement rate if the stock were completely free of anthropogenic influences. Various causes likely contribute to the eel decline (e.g. pollution, human-introduced parasites, changes in ocean climate, habitat deterioration…), but habitat fragmentation by migration barriers that prevent the movement of silver eels between freshwater and the sea is probably one of the most important bottlenecks. During the last decades, a substantial number of canals has been developed, creating new habitat for eels. However, eel migration and potential obstacles in these systems are still underexplored. In this study, we tracked 131 European eels (Anguilla anguilla L.) from October 2014 till March 2017 in the Belgian Albert Canal with acoustic telemetry. The 130-km long canal is on average 86 m wide, 5 m deep and functions as a shipping route between the rivers Schelde and Meuse. The canal has a highly regulated water flow and six shipping locks to overcome the 56-m fall, which may have a negative impact on silver eel escapement. Indeed, we found significant delays (i.e. periods with a significantly prolonged residence time) and a ca 50% lower swimming speed near shipping locks compared to riverine conditions. Depending on nothing but their accumulated fat for migration to their spawning grounds, delays can seriously impact eels by wasting precious energy resources needed for a successful trans-Atlantic migration

Quantifying frazil production, transport and deposition in a gravel-bed river : case study of the St. Raymond hanging dam

The study of frazil is often triggered by the problems that it causes. Frazil ice is known to block water intakes leading to shutdown of power and pumping stations and to form large accumulations in rivers, obstructing the flow and inducing flooding. In the downtown St. Raymond reach of the St. Anne River (Quebec, Canada), frazil accumulates yearly in the form of a grounded frazil jam or hanging dam. To adequately investigate ice-induced flood mitigation measures, knowledge is needed on the quantity of frazil produced by the river and deposited in town. Therefore, an intensive field campaign was conducted during the winter of 2014-2015 to evaluate the amount of frazil ice transported by the river and deposited in the hanging dam. This paper presents a method to quantify transported frazil ice, combining field samples and a complete heat budget. This method is compared to the theoretical ice production calculated from the heat budget only and to the spatial and vertical quantification of the hanging dam. Resulting from those methods, the 9.2 km-long hanging dam of 610,000 m(3) and 432 kt (kilotons), was estimated to contain about 237 kt of frazil ice, while it was estimated that the river had produced 350 kt (based on the heat budget alone) and transported 66 kt (based on field samples and heat budget) of frazil before mid-winter. Considering the uncertainties inherent to sampling moving frazil and evaluating the spatial river ice coverage, directly measuring frazil in the hanging dam appears to represent the most reliable quantification approach, although very labor-intensive