Passive Acoustic Monitoring of Bed Load for Fluvial Applications

Bed load is part of the total sediment load of a stream, comprising the coarsest fraction of mobile material that is frequently in contact with the stream bed. Sediment conveyed downstream may fill reservoirs and channels, impeding navigation, increasing the likelihood of flooding, and degrading water quality and aquatic habitats. Local erosion and deposition of bed material may also cause instability of channel banks. Any long-term program of channel stabilization or improvement must account for bed-load transport in order to avoid channel aggradation or degradation. The determination of bed load rates has been concentrated in four general methods (Hubbell 1964): (1) direct measurement by collecting physical samples, (2) using bed-load transport relations, (3) measuring the erosion or deposition of bed material sediment in a confined area, or (4) measuring bed-load transport indirectly using surrogate technologies. None of these techniques is suitable for a wide range of uses. No single bed-load transport relation has been shown to have general applicability (e.g., Gomez and Church 1989; Vanoni 2006, p. 221–222; Scheer et al. 2002). Many streams do not have a convenient area to carry out erosion or deposition measurements. Accurate determination of bed load using direct measurement methods is difficult because the rate and size of sediment in transport varies dramatically in time at a point and spatially at a given time over a channel cross section of a channel even when the flow is steady (Ehrenberger 1932; Hubbell et al. 1985; Whiting et al. 1988; Kuhnle and Southard 1988; Kuhnle et al. 1989; Gray et al. 1991; Bunte and Abt 2005). Matters are further complicated by unsteady flows. Designing a device that will sample with an equal efficiency over widely varying transport rates is itself a significant challenge. An additional challenge is to collect enough samples at a point and in a cross section to adequately define the mean rate of bed-load transport for a given flow strength. Recent improvements in technology have allowed the development of devices that do not collect physical samples of the sediment but collect indirect or surrogate information that is directly related to the transport of bed-load sediment. These surrogate methods have been divided into active and passive types by Gray et al. (2010), depending on whether the sensors emit signals and record properties of the reflected sound or passively record naturally generated sound emissions. Examples of active sensors consist of acoustic Doppler current profilers that track motion of the bed sediment (Rennie et al. 2002) and active tracking of tagged sediment particles (Emmett et al. 1996; Nichols 2004). Examples of passive sensors include impact pipes (Papanicolaou et al. 2009; Mizuyama et al. 2010), impact plates (Krein et al. 2008; Moen et al. 2010; Rickenmann et al. 2014), geophones (Tsakiris et al. 2014), and devices capable of sensing sediment-generated noise (SGN) associated with the transport of gravel and coarser sediment (Thorne 1986a; Barton et al. 2010). The following paragraphs will focus on passive acoustic techniques for measuring SGN using hydrophones deployed in the transporting flow. Bed load made up of gravel (diameter >2 mm) and larger particles has been shown in previous studies to generate sound through particle collisions and bed collisions, the amplitude of which is related to the particle transport rate (e.g., Thorne 1985, 1986a; Barton 2006). Passive acoustic instrumentation has several characteristics that make it attractive for use in measuring bed load. It is well suited for remote deployment because the hardware needed to sense and record sound is relatively inexpensive. Passive acoustic methods, which record sound that is generated at some distance from a hydrophone, is nonintrusive so the phenomenon being measured is minimally affected by the measurement process. Passive acoustic instrumentation integrates sound from a finite area, decreasing bias caused by spatial heterogeneity of bed-load transport. Continuous monitoring of bed-load transport in a minimally invasive manner is readily accomplished during infrequent high-magnitude events when coarse gravel and cobble sediments are in motion. These events are infrequent but have been shown to be responsible for transporting the majority of bed-load sediment in some streams (Kuhnle 1992). Challenges of using passive acoustics to measure bed load include the generally unknown size of the measurement volume and the acoustics of fluvial environments. This paper reviews the current state of knowledge supporting the use of passive acoustic technology for bed load monitoring using hydrophones deployed in the transporting flow to directly record interparticle collisions. It is shown that while prior SGN work has addressed the generation and characteristics of SGN signals and qualitative/empirical approaches to quantifying bed load using SGN in field settings, a lack of work to date on the propagation of SGN signals in the riverine environment limits SGN as a broadly applicable methodology. In the following section, the concept of using SGN to measure bed load is introduced, along with the development of the method to date. The acoustic characteristics of the riverine setting are then presented, including propagation, transmission losses, and boundary effects. Since sediment transport may not be the most dominant source of underwater sound in a river, the