Athanasios N. Papanicolaou

One-dimensional hydrodynamic/sediment transport model applicable to steep mountain streams

A new one-dimensional (1-D) numerical model for calculating flow and sediment transport in steep mountain streams is developed. 3ST1D, which stands for Steep Stream Sediment Transport 1-D model, is applicable to unsteady flowconditions that occur over transcritical flowstream reaches such as flows over step-pool sequences. 3ST1D consists of two coupled components, the hydrodynamic and the sediment transport. The flow component is addressed here by solving the unsteady form of the Saint-Venant equations. The TotalVariation Diminishing Dissipation (TVD)-MacCormack scheme, which is a shock-capturing scheme capable of rendering the solution oscillation free, is employed here to approximate the hydrodynamic solution over transcritical flow stream reaches. The sediment component of the model accounts for multifractional sediment transport and incorporates a series of various incipient motion criteria and frictional formulas applicable to mountain streams. In addition, sediment entrainability is estimated based ...

Effects of a Fully Submerged Boulder within a Boulder Array on the Mean and Turbulent Flow Fields: Implications to Bedload Transport

The objective of this coupled experimental and numerical study is to provide insight into the mean and turbulent flow fields within an array of fully submerged, isolated, immobile boulders. Our study showed that the velocity defect law performed well for describing the mean flow around the boulder within the array. A prerequisite, however, was to accurately estimate the spatial variability of u* around the boulder, which was achieved via the boundary characteristics method. The u* exhibited considerable spatial variability within the array and form roughness was shown to be up to 2 times larger than the skin roughness in the boulder near-wake region. Because the boulders bear a significant amount of the flow shear, the available bed shear stress for entrainment of the mobile sediment, τols, near the boulders was roughly 50% lower than the ambient τols. The τols variability induced by the boulders could lead to a threefold overestimation of the sediment transport rate.

Signature of bedload particle transport mode in the acoustic signal of a geophone

ABSTRACT The purpose of this study is to develop a relationship for predicting the bedload rate from the acoustic signal of a geophone, under the rolling and saltating bedload transport modes. Controlled experiments corresponding to near-incipient, marginal, and general bedload motion were conducted. The geophone recorded impacts and resulting acoustic signal impulses at the lower “Frequency 1” (100–200 kHz) and the higher “Frequency 4” (380–480 kHz) ranges. Rolling particles produced stronger responses in Frequency 1 and weaker in Frequency 4 due to rotational vibrations. Instead, saltating particles generated stronger responses in Frequency 4 and weaker in Frequency 1 because of spheroid vibrations. The impulse in both Frequency 1 and 4 was related to bedload transport, as it accounts for the number and magnitude of the signal spikes due to the particle impacts. This study complements efforts for quantifying bedload transport using geophones by relating bedload to impulse for rolling and saltation.

Investigating the Role of Clasts on the Movement of Sand in Gravel Bed Rivers

The bed morphology of mountain rivers is characterized primarily by the presence of distinguishable isolated roughness elements, such boulders or clasts. The objective of this experimental study was to provide a unique insight into the role of an array of clasts in regulating sand movement over gravel beds for low relative submergence conditions, H/dc<1, and flow depth, H, to the diameter of the clast, dc, a process that has not been studied thoroughly. To assess the role of clasts in controlling incoming sand movement, detailed flume experiments were conducted by placing 40 equally spaced clasts atop a well-packed glass bead bed for replicating the isolated roughness flow regime. The experiments were performed for moderate (∼2.50τcr* where τcr* is the critical dimensionless bed shear stress) and high (∼5.50τcr*) applied bed shear stress conditions, representative of gravel bed rivers. For comparison purposes, experiments were also repeated for nearly identical flow conditions but without the presence of ...

Calibration and Verification of a 2D Hydrodynamic Model for Simulating Flow around Emergent Bendway Weir Structures

The predictive capability of a two-dimensional (2D)-hydrodynamic model, the finite-element surface water modeling system (FESWMS), to describe adequately the flow characteristics around emergent bendway weir structures was evaluated. To examine FESWMS predictive capability, a sensitivity analysis was performed to identify the flow conditions and locations within the modeled reach, where FESWMS inputs for Manning’s n and eddy viscosity must be spatially distributed for to better represent the river bed flow roughness characteristics and regions where the flow is highly turbulent in nature. The sensitivity analysis showed that high flow conditions masked the impact of Manning’s n and eddy viscosity on the model outputs. Therefore, the model was calibrated under low flow conditions when the structures were emergent and had the largest impact on the flow pattern and model inputs. Detailed field measurements were performed under low flow conditions at the Raccoon River, Iowa for model calibration and verificat...

Stability Analysis of Semicohesive Streambanks with CONCEPTS: Coupling Field and Laboratory Investigations to Quantify the Onset of Fluvial Erosion and Mass Failure

AbstractThe overarching goal of this study is to perform a comprehensive bank stability analysis that is phenomenologically sound by considering both mass failure and fluvial erosion. The nature of this study is twofold. First, field and experimental analyses are conducted to generate data for channel cross-section properties, soil index properties, and mechanical and erosional strengths at two sites in a representative, midsize, midwestern stream in southeastern Iowa that is subjected to frequent flash floods and characterized by active fluvial erosion and cantilever failure. Second, the channel surveys and data obtained from the field and laboratory analyses are used as input parameters for an established one-dimensional, channel evolution model, namely, the conservational channel evolution and pollutant transport system (CONCEPTS, version 2.0, Langendoen and Alonso 2008), to estimate the factor of safety for mass failure (FSm) and fluvial erosion (FSf) and simulate the bank retreat as a result of eithe...

From soilscapes to landscapes: A landscape‐oriented approach to simulate soil organic carbon dynamics in intensively managed landscapes

Most available biogeochemical models focus within a soil profile and cannot adequately resolve contributions of the lighter size fractions of organic rich soils for enrichment ratio (ER) estimates, thereby causing unintended errors in soil organic carbon (SOC) storage predictions. These models set ER as constant, usually equal to unity. The goal of this study is to provide spatiotemporal predictions of SOC stocks at the hillslope scale that account for the selective entrainment and deposition of lighter size fractions. It is hypothesized herein that ER values may vary depending on hillslope location, Land Use/Land Cover (LULC) conditions, and magnitude of the hydrologic event. An ER module interlinked with two established models, CENTURY and Watershed Erosion Prediction Project, is developed that considers the effects of changing runoff coefficients, bare soil coverage, tillage depth, fertilization, and soil roughness on SOC redistribution and storage. In this study, a representative hillslope is partitioned into two control volumes (CVs): a net erosional upslope zone and a net depositional downslope zone. We first estimate ER values for both CVs I and II for different hydrologic and LULC conditions. Second, using the improved ER estimates for the two CVs, we evaluate the effects that management practices have on SOC redistribution during different crop rotations. Overall, LULC promoting less runoff generally yielded higher ER values, which ranged between 0.97 and 3.25. Eroded soils in the upland CV were up to 4% more enriched in SOC than eroded soils in the downslope CV due to larger interrill contributions, which were found to be of equal importance to rill contributions. The chronosequence in SOC storage for the erosional zone revealed that conservation tillage and enhanced crop yields begun in the 1980s reversed the downward trend in SOC losses, causing nearly 26% of the lost SOC to be regained.

Evaluation of the reduction in the water storage capacity of Black Lake, AK

Abstract The paper provides a systematic evaluation of the causes that trigger reduction of a lakes water storage volume. This evaluation is demonstrated herein through combined numerical analysis and field work conducted on the south side of the Alaska Peninsula to address the reduction in the water storage volume of Black Lake. The contribution of this research was the development of an integrated approach that adequately quantifies the causes of reduction of water storage volume of lakes. For this purpose, a set of established routing, erosion, riverine, and lake models were integrated to identify quantitatively the causes of the Black Lake water storage reduction. Selection of the models was a compromise between their complexity, compatibility, capability, and available input data. The results of this study suggest that contrary to what has been long thought the lake receives sufficient water influx to maintain its maximum storage capacity. In addition, sedimentation was not found to the cause, because siltation since 1950 has reduced the storage capacity by only 1.0%. The reduction in the lake storage was mainly attributed to severe degradation at the lake outlet. The outlet elevation of the lake has decreased by at least 1.0 m over the past 55 years. This study recommended that a gated weir could be used as a control structure to maintain the elevation at the lakes outlet. In addition, an erosion model predicted that the lake may loose 80% of its current storage capacity within a 100‐year period causing a complete destruction of the ecosystem, if no action is taken.

Step-pool morphology in high-gradient countersunk culverts

Bed stability and morphology in countersunk culverts on steep slopes were examined to improve understanding of the parameters governing sediment flow characteristics in mountain streams. The knowledge gained was used to provide preliminary construction guidelines for the stream simulation approach of countersunk culverts, an approach that is under consideration in Washington State. Prototype conditions were evaluated in a flume for a 30 percent countersunk culvert with slopes ranging from 3 percent to 7 percent and particle relative submergence varying from 0.5 to 2.0 for three bed size distributions. The experiments were designed to satisfy the conditions of dynamic similarity, and they are preferred to field measurements because they allow a high degree of control over testing conditions. It was found that step-pool bedforms are the most ubiquitous features along the culvert gravel bed. A new formula was developed that correlates step height with the gravel-bed size distribution, relative submergence of the particles, and the Froude number. The step spacing was found to be related to step height and streambed longitudinal slope. This information was combined into a generic design method for streambed simulation of high-gradient countersunk culverts. An example case that illustrates the application of the newly derived formulas to the construction of a gravel bed for a countersunk culvert is provided.

Bedload Predictions by Using the Concept of Particle Velocity: Applications

Measurement of sediment transport is necessary to determine the amount of sediment load and to establish or check analytical or empirical sediment transport equations. These measurements differ in principle according to the mode of sediment transportation, i.e., bed load and suspended load, and the type of the transported sediment, i.e., clay, sand, and gravel. The focus of the proposed investigation is found on bed load transport (i.e., rolling, sliding, or bouncing) of gravel particles. Bed load is difficult to measure for several reasons. Any mechanical device placed in the vicinity of the bed will disturb the flow and hence the rate of bed-load movement. Moreover, it is very difficult to develop a device that accurately collects all the size fractions of the bed load. Finally, it is difficult to measure bed load rates in the field due to lack of access in some areas, especially at high flows, and lack of reliable measuring devices. The objective of this research is to employ an image analysis technique to monitor bedload motion in a laboratory flume under different flow and bed roughness conditions; and obtain unique information about the velocity of individual particles.