Modeling In-Sewer Deposit Erosion to Predict Sewer Flow Quality

High levels of suspended solids are typically observed during the initial part of storms. Field evidence suggests that these suspended solids derive from the erosion of in-sewer sediment beds accumulated during dry and previous wet weather periods. Suspended sediment transport rate models within existing sewer network modeling tools have utilized inappropriate transport rate relationships developed mainly in fluvial environments. A process model that can account for the erosion of fine-grained highly organic in-sewer sediment deposits has been formulated. Values of parameters describing the increase in deposit strength with depth are required. These values are obtained using a genetic algorithm based calibration routine that ensures model simulations of suspended sediment concentrations that correspond to field data collected in a discrete length of sewer in Paris under known hydraulic event conditions. These results demonstrate the applicability of this modeling approach in simulating the magnitude and temporal distribution of suspended in-sewer sediment eroded by time varying flow. Further work is developing techniques to enable the application of this type of model at the network level.     

European Research into Sewer Sediments and Associated Pollutants and Processes

Research investigating all aspects of solids in sewer systems has been underway in Europe for nearly two decades. Due to the early development of European sewer systems, originally as part of the industrialization process more than 100 years ago, urbanization has caused the original sewer networks to become overloaded and unable to function efficiently. Operational problems of interest include loss of ability to convey (designed) flows and the performance of “overflows” to relieve the high flows discharging directly into rivers and other watercourses. Research has characterized the nature of the solids getting into sewer systems, how they behave in terms of transport, and some of the main aspects of their effects. It has been possible to demonstrate that much of the pollutants found in suspension during storms, and likely to be discharged from overflows, originate from the predominantly organic “near bed solids” which accumulate in systems during dry weather. New ideas for the way in which the sediments are transported and the importance of the transformation processes, are leading toward the development of a unified and integrated understanding of the way in which sewer solids behave and the associated biochemical transformation processes.     

Erosion of Cohesive Sediments: Resuspension, Bed Load, and Erosion Patterns from Field Experiments

New field data on cohesive sediment erosion is presented and discussed, with particular focus on partitioning the total erosion into resuspension and bed load. The data were obtained using a recently developed in situ flume of the National Institute of Water and Atmospheric Research, New Zealand. The erosion rate is estimated from direct measurements of bed surface elevations by acoustic sensors, whereas resuspension rate is obtained using data on sediment concentrations measured by optical backscatter sensors. The bed- load contribution to the total erosion rate is evaluated from the conservation equation for sediments. To test repeatability, the data from the in situ flume are compared with those from a previous version of the flume. The results show that comparative studies of in situ flumes and standardized deployment procedures enable direct comparison of experimental data on cohesive sediment erosion. Overall, the data show that a commonly used assumption that the erosion rate is equal to the resuspension rate is not always valid as bed load plays a significant role in cohesive sediment erosion. The data also highlight the importance of clay content and other sediment physical characteristics in the sediment mixture.     

Upland Erosion Modeling in a Semihumid Environment via the Water Erosion Prediction Project Model

The major water quality impairment in the midwest United States is sediment eroded from agricultural lands. Yet, few understand the spatial and temporal variability of erosion, or soil erosion dynamics, in relation to precipitation, topography, land management, and severe events. The objectives of this paper are to (1) develop a methodology for estimating long-term spatial soil erosion and water runoff losses and (2) explore issues in applying an established physical-based process model, Water Erosion Prediction Project (WEPP), to a large area by establishing a prototype system for the state of Iowa. This study for the first time provides a comparison of the model predictions against long-term measurements of the sediment delivery ratio (SDR) in the South Amana Catchment of the Clear Creek Watershed (CCW), a heavily instrumented watershed that is roughly 10 times the maximum WEPP fold size. To further examine the performance of WEPP in a semihumid environment, such as the CCW, where runoff and raindrop impact to erosion may be significant, the SDR was plotted as a function of the runoff coefficient, defined as the runoff/rainfall ratio. In addition, the WEPP predictions are compared against the statistical relation of SDR vs. runoff coefficient developed by Piest et al. in 1975) for watersheds in Iowa. It is shown that WEPP follows the trend shown by Piest et al. quite closely and performs well for continuous simulations extended up to 300?years.     

Validation of Existing Bed Load Transport Formulas Using In-Sewer Sediment

Granular sediment in pipe inverts has been reported in a number of sewer systems in Europe. Given the range of flow conditions and particle characteristics of inorganic sewer sediments the mode of transport may normally be considered as bed load. Current commercial software for modeling the erosion and transport of sediments in sewer pipes still utilizes well-known, or modified versions of transport equations that were derived for transport of noncohesive sediment in alluvial streams. In this paper the performances of the equations of Ackers and White (originally developed for the transport of river sediments) and of May (derived from laboratory pipe experiments) are examined against two separate data sets. One set is from laboratory erosion experiments on sewer sediment obtained in Paris. A second data set has bed load transport rate measurements recorded in a sewer inlet pipe. The formulas were selected because of their widespread use in the prediction of in-sewer sediment transport both in commercial software and in the latest United Kingdom design guidance for new sewers. The results indicated that both the relationships performed poorly, even in such well-controlled conditions. These formulas have significant difficulties in predicting the erosion thresholds and fractional transport rates for non-uniformly sized in-sewer sediments. An empirical formula to adjust the threshold of motion for individual grain size fractions was developed which significantly improved predictions. Although such techniques have been used in gravel bed rivers, the threshold adjustment function for in-sewer deposits was significantly different from these previously published for fluvial gravels, indicating that a direct transfer of fluvial relationships to sewers may be inappropriate without further research.     

Fall Cone Test to Characterize Shear Strength of Organic Sediments

In this paper, the results of the standard percussion cup test, the penetration cone test, and the laboratory vane test performed on organic sediments and kaolin are discussed. First, the relationship between the water content and the cone penetration depth is explored. Second, the relationship between the shear strength, using the laboratory vane apparatus, and the water content is investigated. Finally, the relationship between the shear strength and the penetration depth of the cone, at different levels of water content, is explored.     

Modeling Uncoupled Solute Transport in Natural Organic Matter Size Fractionation by Ultrafiltration

Physicochemical separation of organic macrosolutes and colloidal particles is routinely required during the analysis of natural, waste, and process waters derived from aquatic and terrestrial environmental samples. This study was conducted to demonstrate the utility of a two-parameter nonlinear permeation coefficient model (PCM) in describing the uncoupled transport of solutes in dilute heterogeneous solutions subjected to batch ultrafiltration (UF). The PCM was used in conjunction with natural organic matter (NOM) permeate data for a natural water and six hydrophobic and hydrophilic subfractions to determine permeation coefficients p and NOM concentrations Cr0 with apparent molecular weight less than membrane specific cutoff values of moderately hydrophilic YC/YM series Amicon? UF membranes. Experimentally measured permeation coefficients p determined for the whole water were found to correlate well with composite permeation coefficients p? calculated using a mass-fraction weighted average of individual NOM subfraction permeation coefficient values. Correlation of experimentally measured and calculated permeation coefficient values (p and p?) indicated that the PCM can adequately describe uncoupled transport of chemically distinct solute fractions during batch UF of heterogeneous dilute solutions.     

Measurements of Sediment Erosion and Transport with the Adjustable Shear Stress Erosion and Transport Flume

Soil and sediments play an important role in water management and water quality. Issues such as water turbidity, associated contaminants, reservoir sedimentation, undesirable erosion and scour, and aquatic habitat are all linked to sediment properties and behaviors. In situ analysis is necessary to develop an understanding of the erosion and transport of sediments. Sandia National Laboratories has recently patented the Adjustable Shear Stress Erosion and Transport (ASSET) Flume that quantifies in situ erosion of a sediment core with depth while affording simultaneous examination of transport modes (bedload versus suspended load) of the eroded material. Core erosion rates and ratios of bedload to suspended load transport of quartz sediments were studied with the ASSET Flume. The erosion and transport of a fine-grained natural cohesive sediment were also observed. Experiments using quartz sands revealed that the ratio of suspended load to bedload sediment transport is a function of grain diameter and shear stress at the sediment surface. Data collected from the ASSET Flume were used to formulate a novel empirical relation for predicting the ratio of bedload to suspended load as a function of shear stress and grain diameter for noncohesive sediments.     

Erosion of the Upper Layer of Cohesive Sediments: Characterization of Some Properties

A recent companion paper reported an experimental protocol used to analyze sediment properties. This protocol identified for both freshwater and marine sediments a surface layer with specific dynamic properties (critical erosion shear stresses in the range 0.025–0.05?N?m?2) and a second layer with critical erosion shear stresses about ten times larger. The present study compares these former results with recent work which extended the applicability domain of the Shields diagram to very fine particles. The surface layer is shown to consist in fine and unconsolidated sediments that behave like noncohesive material whereas the second layer is characterized as being cohesive. The surface layer is mainly representative of recent deposits of suspended particles. This points out the existence of a fluffy layer of fine sized particles resting near the bed, with specific erosion characteristics, which has to be considered separately when studying sediment properties.     

Finite-Element Approach to the Erosion and Transport of Fine Particles in Granular Soils

The increase of the water table level presently occurring in the city of Milan led to some leakage of groundwater in underground facilities and subway tunnels. These structures, in fact, were completed about three decades ago when the water table was deeply located. To locally lower the ground water, and to eliminate or limit its flow towards the submerged openings, a series of pumping wells was placed in their vicinity. This provision, however, could lead to possible erosion of the fine fraction of the granular soil and to consequent settlements of nearby buildings. To investigate this phenomenon a finite-element approach has been developed for the analysis of the erosion and transport of the fine particles of granular soils subjected to a seepage flow. First, the continuity equation governing the problem and its finite-element formulation are discussed. Then, on the basis of the results of erosion tests presented in the literature, a law is derived that accounts for the nonlinear relationship between the total amount of eroded material, for time tending to infinity, and the velocity of seepage. Finally, this law is applied to the solution of one- and two-dimensional test examples, and some conclusions are drawn on the limits of the developed numerical model and on its possible improvement.     

Estimation of the Washout Depth of Fine Sediments from a Granular Bed

An exponential distribution of the bed-pressure fluctuations is used to estimate the depth within a porous gravel bed from which fine sediment of a given size can be removed. The coarsest grain size of the fine sediment that might be washed out is of O(10?1) in relation to both the gravel grain size and the equivalent grain roughness. A higher equivalent grain roughness results to a larger absolute cleaning depth, whereas the averaged gravel grain size is seen to be less important. The results are successfully tested for plausibility against the grain size distributions of an armored gravel bed and its underlaying bimodal layer as found in situ in the river Rhine. However, qualitative and quantitative experimental data for an in-depth validation remain to be performed.     

Sediment Erosion Characteristics in the Anacostia River

Four in situ experiments on sediment erosion characteristics were conducted at the Anacostia River that runs through Washington, D.C. Supplemental erosion rate data were also obtained by carrying out five laboratory experiments using sediment samples collected at the field. In laboratory experiments, the sediment samples were mixed with tap water and placed in the flume to form beds for finding the difference in terms of erosion characteristics caused by different sediment composition among the five samples. This approach enables the finding of erosion characteristics for the entire tidal Anacostia River with limited resources. The in situ measured critical bed-shear stresses τcr for erosion at the water-sediment interface z = 0 varies from 0.03 to 0.08?Pa. Field results indicated that τcr(z) increases with the depth z and becomes more than 0.6 to 0.7?Pa with an erosion thickness of less than 1?cm. Sediment beds prepared at a laboratory appear having an upper limit on how much τcr(z) can be developed.     

Aerial Assessment of Ephemeral Gully Erosion from Agricultural Regions in the Pacific Northwest

Soil erosion from agricultural areas continues to be problematic in terms of both financial and environmental measures. Ephemeral gullies contribute to the soil loss both by the volume of sediment eroded from the gullies and because they act as delivery channels for surface erosion. High resolution aerial imagery was used to quantify the amounts and locations of ephemeral gullies in the subbasins of the Potlatch River system. Areal ephemeral gully erosion rates varied from 33.6?mt/km2 (0.15 U.S. t/acre) in the Big Bear Creek Subbasin to 88.4?mt/km2 (0.39 U.S. t/acre) in the Middle Potlatch Creek Subbasin representing 2.3–7.7% of the total surface sediment load. An erosion potential index was proposed to assist resource managers predict gully locations at the watershed scale using readily available remote sensing and geographic information system layers.     

Numerical Modeling of Breach Erosion of River Embankments

The process of breach erosion of river embankments depends on the interaction among flow, sediment transport, and the corresponding morphological changes. Levees often consist of noncohesive material with a wide range of grain sizes. The dam material is mainly eroded due to the transport capacity of the overtopping water. Both bed load and suspended load are of importance. For breach formation, the lateral erosion due to slope instabilities has a significant impact. A depth averaged, two-dimensional numerical model was developed to account for these processes. The sensitivity of the discharge through the breach related to different processes and material parameters was investigated and compared to experimental and field data. The results show that the most sensitive parameter of an erosion-based dike-breach simulation is the breach side-slope angle which determines the lateral erosion. The application of the described Model 2dMb to different embankment failures at the Elbe River illustrates its capability in simulating overtopping breaching.     

Nonlinear Dynamic Properties of a Fibrous Organic Soil

The nonlinear dynamic properties of a fibrous peaty organic soil beneath a levee in the Sacramento–San Joaquin Delta in California are described herein. Thin-walled tube samples were obtained from four locations between the levee crest and the free field such that the in situ vertical effective stresses (σvo′) ranged from about 12 kPa in the free field to about 135 kPa beneath the levee crest. The peaty organic soil was very soft and highly compressible in the free field with initial water contents (wo) of 236–588% and shear wave velocities (Vs) of typically 22–27 m/s, and moderately firm beneath the levee crest with wo of 152–240% and Vs of typically 88–129 m/s. Stress–strain measurements in a cyclic triaxial device showed that the normalized secant shear modulus (G/Gmax) and equivalent damping ratio (ξ) versus cyclic shear strain amplitude (γc) relations were dependent on the consolidation stress (σvc′). Tests involving prior overstraining followed by reconsolidation showed that the effects of sample disturbance were likely small. Stress history, creep, and loading frequency effects were also examined. Tests on reconstituted specimens provided supplementary data on the functional relation between maximum shear modulus (Gmax) and consolidation stress conditions. Summary relations are provided for G/Gmax and ξ versus γc and for Gmax versus σvc′.     

Sorption of Neutral Organic Compounds in Mixtures to Mineral Surfaces and Humic Acid-Mineral Complexes

The sorption of 12 neutral organic compounds (NOCs) contained in mixtures to both hydrophilic mineral surfaces [uncoated, iron (hydr)oxide-coated, and aluminum (hydr)oxide-coated sands] and humic acid-mineral complexes with different fractions of organic carbon, foc ( = 0.051, 0.119, and 0.221%) was evaluated using batch equilibrium sorption tests. The 12 compounds included six nonpolar NOCs (1,2,4-trichlorobenzene; 1,4-dichlorobenzene; chlorobenzene; m-xylene; toluene; and benzene) and six polar NOCs (2,4-dimethyl phenol; p-cresol; phenol; 2-hexanone; 2-butanone; and acetone). For sorption of the nonpolar NOCs, mixture effects (e.g., competitive or cooperative effects) were not statistically significant (95% confidence level) regardless of changes in mixture composition or type of sorbent. In contrast, mixture effects for some of the polar NOCs (2,4-dimethyl phenol; p-cresol; phenol; and 2-hexanone) were statistically significant and depended on both the mixture compositions (concentration and polarity of the NOC) and the type of sorbent (availability of hydrophilic interaction sites). These results suggest that direct experimental measurement of sorption coefficients for polar NOCs contained in mixtures may be required to reduce errors in predicting the fate and transport of the polar NOCs and the risk posed to potential receptors.     

Complex Dielectric Permittivity of Soil–Organic Mixtures (20 MHz–1.3 GHz)

The dielectric permittivity of soils can be significantly modified by the presence of organic contaminant in the pore fluid. Thus, nondestructive techniques based on the propagation of electromagnetic waves may be used to detect contaminant plumes and to evaluate decontamination processes. Ground penetrating radar (GPR) and time domain reflectometry are two of the most relevant geophysical tools working in the frequency range of interest here. In this work, the complex dielectric permittivity of some soil–organic mixtures are measured in the frequency range of 20 MHz to 1.3 GHz. Tests are conducted in samples of silica sand, loess, and kaolinite mixed with varied amount of paraffin oil and lubricant oil. Additional tests are performed in soil–water samples for comparison. Mixtures formulas reported in the literature are extended from two to three and four phases in order to model the measured dielectric response of the contaminated soil samples. The results allow us to study the effect of the volumetric liquid content, organic type, mineral composition, and specific surface of soil particles on the dielectric permittivity of the mixtures. It is concluded in this work that the value of dielectric permittivity in soils is sensitive to the detection of contaminants when the organic concentration is high. On the other hand, the organic content can be determined providing that the total volume of fluid in the pores is known. The detection limits of organics in soils are discussed. Finally, a contamination process is monitored with GPR at the frequency of 1 GHz in a laboratory cell. The results show that organic contaminants are easily detected in dry sand, yet detection becomes very difficult in wet sand.     

Modeling Substrate Interactions during Aerobic Biodegradation of Mixtures of Vinyl Chloride and Ethene

Ethene (ETH) is often associated with vinyl chloride (VC) in contaminated groundwater, as it is formed along with vinyl chloride during reductive dechlorination of higher chloroethenes (e.g., perchloroethylene and trichloroethylene). In the present study the interaction between VC and ETH during their aerobic biodegradation by enrichment cultures was investigated. The cultures were able to use both compounds as growth substrates. In mixture experiments, the degradation rate of one substrate was affected by the presence of the other. A biokinetic model based on competitive inhibition described well the observed substrate interactions over a range of initial VC (0–144 μmol?L?1) and ETH (0–37.5 μmol?L?1) concentrations, using parameters estimated from single-substrate experiments. Notably, half-velocity coefficients could be used as competitive inhibition coefficients. This finding shows the importance of obtaining accurate measurements of half-velocity coefficients in order model competitive inhibition processes. Simulation results showed that when the initial ETH concentration was raised from 0 to 30 μmol?L?1, the apparent half-velocity coefficient for VC (KVCAPP) increased by nearly three times, from 12.9 to 35.4 μmol?L?1. This finding has strong environmental implications because a low half-velocity coefficient for VC is regarded as the major prerequisite for achieving efficient and complete VC degradation. Moreover, the effect of ETH on the efficiency of VC removal is strongly dependent on the KVC/KETH ratio, consequently determination of KETH for VC-degrading microbes is important when biodegradation (or bioaugmentation) is considered for clean up of VC-contaminated sites. Additional model simulations, using the ratio of KVC to KETH for previously characterized VC- and ETH-utilizing microorganisms (values ranged from 0.06 to 1.2) showed that their ability to degrade VC in the presence of ETH may differ significantly.     

Storm Water Detention Ponds: Modeling Heavy Metal Removal by Plant Species and Sediments

Laboratory and field studies were conducted to elucidate heavy metal removal by three wetland grasses and sediments in storm water detention pond. The removal of heavy metals including Cd, Cu, Pb, and Zn was mediated by fluid-flow intensity in the reactors. The growth of plants and the removal rates of contaminants were plant species dependent. All three wetland grasses removed contaminants from the spiked nutrient solutions. A first-order kinetic model adequately represented the removal of contaminants by plants. The analyses of undisturbed sediment cores in detention pond revealed strong stratification of heavy metal concentrations at the sediment–water interface. A simple model that integrates heavy metal removal by aquatic plants and sediments in storm water detention ponds is proposed. The model provides an estimate of contaminant residence time which can be related to hydraulic residence time in storm water detention ponds.     

Modeling Formation of Natural Organic Matter Fouling Layers on Ultrafiltration Membranes

Three simple mathematical models to describe fouling of an ultrafiltration membrane by natural organic matter (NOM) are developed and compared. These models attribute the fouling to: (1) an increase of the effective pore length by an amount equal to the thickness of the NOM gel layer that forms on the membrane surface; (2) formation of a uniform, microporous NOM gel layer on the membrane surface, made of primary particles comprising tens to hundreds of NOM molecules; or (3) narrowing of the membrane pores by sorption of a monolayer of NOM molecules along the full length of each pore. The key parameters characterizing each model are identified and estimated based on data for flux and film growth gathered in the same system. In each case, the estimated parameter values are plausible in light of the known physical properties of the membrane and NOM molecules.