Steep landscapes evolve largely by debris flows, in addition to fluvial and hillslope processes. Abundant field observations document that debris flows incise valley bottoms and transport substantial sediment volumes, yet their contributions to steepland morphology remain uncertain. This has, in turn, limited the development of debris‐flow incision rate formulations that produce morphology consistent with natural landscapes. In many landscapes, including the San Gabriel Mountains (SGM), California, steady‐state fluvial channel longitudinal profiles are concave‐up and exhibit a power‐law relationship between channel slope and drainage area. At low drainage areas, however, valley slopes become nearly constant. These topographic forms result in a characteristically curved slope‐area signature in log‐log space. Here, we use a one‐dimensional landform evolution model that incorporates debris‐flow erosion to reproduce the relationship between this curved slope‐area signature and erosion rate in the SGM. Topographic analysis indicates that the drainage area at which steepland valleys transition to fluvial channels correlates with measured erosion rates in the SGM, and our model results reproduce these relationships. Further, the model only produces realistic valley profiles when parameters that dictate the relationship between debris‐flow erosion, valley‐bottom slope, and debris‐flow depth are within a narrow range. This result helps place constraints on the mathematical form of a debris‐flow incision law. Finally, modeled fluvial incision outpaces debris‐flow erosion at drainage areas less than those at which valleys morphologically transition from near‐invariant slopes to concave profiles. This result emphasizes the critical role of debris‐flow incision for setting steepland form, even as fluvial incision becomes the dominant incisional process.
- NSF-PAR ID:
- 10417853
- Editor(s):
- Van Stan, John Toland
- Date Published:
- Journal Name:
- PLOS ONE
- Volume:
- 18
- Issue:
- 3
- ISSN:
- 1932-6203
- Page Range / eLocation ID:
- e0281835
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
Abstract Biodiversity at larger spatial scales (γ) can be driven by within‐site partitions (α), with little variation in composition among locations, or can be driven by among‐site partitions (β) that signal the importance of spatial heterogeneity. For tropical elevational gradients, we determined the (a) extent to which variation in γ is driven by α‐ or β‐partitions; (b) elevational form of the relationship for each partition; and (c) extent to which elevational gradients are molded by zonation in vegetation or by gradual variation in climatic or abiotic characteristics. We sampled terrestrial gastropods along two transects in the Luquillo Mountains. One passed through multiple vegetation zones (tabonuco, palo colorado, and elfin forests), and one passed through only palm forest. We quantified variation in hierarchical partitions (α, β, and γ) of species richness, evenness, diversity, and dominance, as well as in the content and quality of litter. Total gastropod abundance linearly decreased with increasing elevation along both transects, but was consistently higher in palm than in other forest types. The gradual linear decline in γ‐richness was a consequence of opposing patterns with regard to α‐richness (monotonic decrease) and β‐richness (monotonic increase). For evenness, diversity, and dominance, α‐partitions and γ‐partitions evinced mid‐elevational peaks. The spatial organization of gastropod biodiversity did not mirror the zonation of vegetation. Rather, it was molded by: (a) elevational variation in productivity or nutrient characteristics, (b) the interspersion of palm forest within other forest types, and (c) the cloud condensation point acting as a transition between low and high elevation faunas.
Abstract in Spanish is available with online material.
-
null (Ed.)Abstract. Estimation of erosion rate is an important component of landscapeevolution studies, particularly in settings where transience or spatialvariability in uplift or erosion generates diverse landform morphologies.While bedrock rivers are often used to constrain the timing and magnitude of changes in baselevel lowering, hilltop curvature (or convexity), CHT, provides an additional opportunity to map variations in erosion rate given that average slope angle becomes insensitive to erosion rate owing to threshold slope processes. CHT measurement techniques applied in prior studies (e.g., polynomial functions), however, tend to be computationallyexpensive when they rely on high-resolution topographic data such as lidar,limiting the spatial extent of hillslope geomorphic studies to small studyregions. Alternative techniques such as spectral tools like continuouswavelet transforms present an opportunity to rapidly document trends inhilltop convexity across expansive areas. Here, we demonstrate howcontinuous wavelet transforms (CWTs) can be used to calculate the Laplacianof elevation, which we utilize to estimate erosion rate in three catchmentsof the Oregon Coast Range that exhibit varying slope angle, slope length,and hilltop convexity, implying differential erosion. We observe thatCHT values calculated with the CWT are similar to those obtained from2D polynomial functions. Consistent with recent studies, we find thaterosion rates estimated with CHT from both CWTs and 2D polynomialfunctions are consistent with erosion rates constrained with cosmogenicradionuclides from stream sediments. Importantly, our CWT approachcalculates curvature at least 103 times more quickly than 2Dpolynomials. This efficiency advantage of the CWT increases with domainsize. As such, continuous wavelet transforms provide a compelling approachto rapidly quantify regional variations in erosion rate as well aslithology, structure, and hillslope sediment transport processes, which areencoded in hillslope morphology. Finally, we test the accuracy of CWT and 2Dpolynomial techniques by constructing a series of synthetic hillslopesgenerated by a theoretical nonlinear transport model that exhibit a range oferosion rates and topographic noise characteristics. Notably, we find thatneither CWTs nor 2D polynomials reproduce the theoretically prescribedCHT value for hillslopes experiencing moderate to fast erosion rates,even when no topographic noise is added. Rather, CHT is systematicallyunderestimated, producing a power law relationship between erosion rate andCHT that can be attributed to the increasing prominence of planarhillslopes that narrow the zone of hilltop convexity as erosion rateincreases. As such, we recommend careful consideration of measurement lengthscale when applying CHT to estimate erosion rate in moderate tofast-eroding landscapes, where curvature measurement techniques may be prone to systematic underestimation.more » « less
-
more » « less
Abstract A number of studies have measured and numerically modelled near surface wind velocity over a range of aeolian landforms and made suppositions about topographic change and landform evolution. However, the precise measurement and correlation of flow dynamics and resulting topographic change have not yet been fully realized. Here, using repeated high‐resolution terrestrial laser scanning and numerical flow modelling within a bowl blowout, we statistically analyse the relationship between wind speed, vertical wind velocity, turbulent kinetic energy and topographic change over a 33‐day period.
Topographic results showed that erosion and deposition occurred in distinct regions within the blowout. Deposition occurred in the upwind third of the deflation basin, where wind flow became separated and velocity and turbulent kinetic energy decreased, and erosion occurred in the downwind third of the deflation basin, where wind flow reattached and aligned with incident wind direction.
Statistical analysis of wind flow and topographic change indicated that wind speed had a strong correlation with overall topographic change and that vertical wind velocity (including both positive and negative) displayed a strong correlation with negative topographic change (erosion). Only weak or very weak correlations exist for wind flow parameters and positive topographic change (accretion). This study demonstrates that wind flow modelling using average incident wind conditions can be utilized successfully to identify regions of overall change and erosion for a complex aeolian landform, but not to identify and predict regions where solely accretion will occur. © 2019 John Wiley & Sons, Ltd.
-
Within a forest, differences in landform spatial variation (i.e., geomorphic settings: valley, slope, and ridge) could affect the species richness and distribution present at a particular site. Previous studies have confirmed that plant species richness and biomass changes after a hurricane and such values can vary among geomorphic settings. Understory vegetation, including ferns, herbs, climbers, graminoids, and shrubs, accounts for more than two thirds of flora in tropical ecosystems, but there is limited information of the effect of hurricanes on these communities. We evaluated the structure and composition of understory vegetation in a post-hurricane forest in relation to geomorphic settings. This study was conducted in El Verde Research Area in the Luquillo Experimental Forest, Puerto Rico. We established 1-m2 plots within three geomorphic settings: riparian valley, slope, and ridge. Within each plot we identified species, estimated percent of cover, and collected biomass samples. Additionally, we estimated species accumulation curves and analyzed species composition among geomorphic settings using multivariate ordination. The relative species abundance of vegetation life-forms was similar among geomorphic settings, but graminoids and climbers exhibited differences in species composition. Higher forest understory biomass and percent vegetation cover was observed at this immediate post-hurricane period than what was reported pre-hurricane. The understory of valley areas had a more distinct species composition than what was observed among ridge and slope areas. The understory vegetation patterns observed would need to be followed through time and among the landforms to confirm the hurricane disturbances effects at these understory scale.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1371/journal.pone.0281835
