Porosity problems: Comparing and reviewing methods for estimating porosity and volume of wood jams in the field

Abstract

Porosity, or void space, of large wood jams in stream systems has implications for estimating wood volumes and carbon storage, the impacts of jams on geomorphic and ecological processes, and instream habitat. Estimating porosity and jam dimensions (i.e. jam volume) in the field is a common method of measuring wood volume in jams. However, very few studies explicitly address the porosity values in jams, how porosity is calculated and assessed for accuracy, and the effect such estimates have on carbon and wood budgets in river corridors. We compare methods to estimate jam porosity and wood volume using field data from four different depositional environments in North America (jam types include small in-channel jams, large channel-margin jams, a large island apex jam, and a large coastal jam), and compare the results with previous studies. We find that visual estimates remain the most time-efficient method for porosity estimation in the field, although they appear to underpredict back-calculated porosity values; the accuracy of jam porosity, and thus wood volume, estimates are difficult to definitively measure. We also find that porosity appears to be scale invariant, dictated mostly by jam type, (which is influenced by depositional processes), rather than the size of the jam. Wood piece sorting and structural organization are likely the most influential properties on jam porosity, and these factors vary according to depositional environment. We provide a framework and conceptual model that uses these factors to demonstrate how modeled jam porosity values differ and give recommendations as a catalyst for future work on porosity of wood jams. We conclude that jam type and size and/or the study goals may dictate which porosity method is the most appropriate, and we call for greater transparency and reporting of porosity methods in future studies.

Floodplain Large Wood and Organic Matter Jam Formation After a Large Flood: Investigating the Influence of Floodplain Forest Stand Characteristics and River Corridor Morphology

“…Floodplain LW and CPOM also provide ecological functions, such as nutrient‐rich sites for seedling establishment (Pettit & Naiman, 2006), habitat for diverse biota (Benke, 2011; Mac Nally et al., 2001), a large reservoir of organic carbon (Lininger et al., 2017), and enhancement of floodplain soil nutrients (Zalamea et al., 2007). CPOM within jams can reduce jam porosity by filling in spaces between LW, increasing the impacts of jams on hydraulics (Livers et al., 2020; Manners et al., 2007).…”

Seasonal and diurnal fluctuations of coarse particulate organic matter transport in a snowmelt‐dominated stream

“…The fluvial transport of CPOM is one of the forms in which carbon is exported annually from a forested basin (Battin et al, 2008;Piégay, Thévenet, & Citterio, 1999;Worrall, Burt, & Howden, 2014). Accumulations of CPOM can influence the porosity and permeability of the stream bed and thus hyporheic exchange (Harvey & Gooseff, 2015), as well as the porosity and permeability of logjams within the channel and, therefore, the physical effects of such logjams on channel hydraulics and sediment transport (Livers, Lininger, Kramer, & Sendrowski, 2020).…”

Logjams as a driver of transient storage in a mountain stream

“…Channel-spanning jams are those that completely span the bankfull channel. For jams that included a substantial amount of finer organic material in the interstices between large wood pieces, we measured the hypothetical 3D rectangle that encompassed the jam and visually estimated the porosity of the jam (Livers et al, 2020).…”

“…Channel‐spanning jams are those that completely span the bankfull channel. For jams that included a substantial amount of finer organic material in the interstices between large wood pieces, we measured the hypothetical 3D rectangle that encompassed the jam and visually estimated the porosity of the jam (Livers et al, 2020). At each channel‐spanning jam, we also measured the residual volume of the backwater pool and the volume of fines (sediment finer than the average bed substrate and particulate organic matter) within the backwater pool by dividing the pool surface area into a grid and probing the depth of fines (Lisle & Hilton, 1992).…”