John Dunbar

Texture, spatial distribution, and rate of reservoir sedimentation within a highly erosive, cultivated watershed: Grenada Lake, Mississippi

Water Resources Research, 2005

While more than 75,000 dams nationwide have served a multitude of purposes over the past 50 years... more While more than 75,000 dams nationwide have served a multitude of purposes over the past 50 years, current emphasis is being placed on decommissioning those aged infrastructures considered environmentally harmful. Grenada Lake, a relatively large flood control reservoir in Mississippi, is located in a highly erodible region of the United States and is fed by historically unstable stream channels. Through a variety of stratigraphic analyses, postimpoundment sediments are identified in the reservoir, showing that the lake has lost a nominal 3% of its flood storage capacity since its inception in 1954. A simple sediment budget constructed for the Yalobusha River's contribution of channel-derived sediments shows that about 76% of this sediment remains stored upstream, about 16% is stored in the reservoir, and about 8% has exited the lake. Sediment delivery ratios, known to be low for relatively large watersheds, appear to be unaffected by upstream channel instabilities and aggressive channelization projects.

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Hydrogeophysical Characterization of a Texas Expansive Heavy Clay Soil (Vertisol) Using Electrical Resistivity Measurements

AGU Spring Meeting Abstracts, May 1, 2006

We applied field and laboratory electrical resistivity measurements to characterize seasonal wett... more We applied field and laboratory electrical resistivity measurements to characterize seasonal wetting and drying of a Texas Vertisol. Vertisols are complex soils characterized by high clay content (&amp;amp;amp;gt;30%), high shrink/swell potential, microrelief expression of subsurface soil dynamics (gilgai), and low hydraulic conductivity. A total of thirty-three multi-electrode resistivity profiling lines were taken along the same profile between April 27, 2005

Geoelectric Studies of Seasonal Wetting and Drying of a Texas Vertisol

Vadose Zone Journal, Aug 1, 2007

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or b... more All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. ORIGINAL RESEARCH Vertisols are complex soils with high clay content (>30%), high shrink-swell potential, and microrelief features known as gilgai. We applied fi eld and laboratory electrical-resistivity measurements to characterize seasonal wetting and drying of a Texas Vertisol, and to quantify the effects of gilgai and cracks on seasonal hydrodynamics of the soil. Thirty-two multielectrode resistivity-profi ling lines were collected along the same profi le from 1 May 2005 to 22 Apr. 2006, using combined dipole-dipole and Schlumberger electrode confi gurations. The profi les were 17.5 m long and intersected two sets of microlows and microhighs of the gilgai. The resistivity data were inverted using the RES2DINV program and the inverted data were corrected for temperature. We measured variations of resistivity with soil moisture in the laboratory and the results were used to calibrate the fi eld data. To evaluate the resistivity results, in situ measurements of soil moisture were made using auger sampling. During the wetting cycle, three distinct soil moisture regimes were recognized in the upper 1.4 m of the Vertisol: an upper zone (0-0.5-m depth), which is the most dynamic with regard to wetting and drying; a middle zone (0.5-1.1 m), which is relatively saturated and less dynamic; and a lower zone (below 1.1 m), which is relatively less saturated compared with the middle layer. The saturation of the middle layer appears to be enhanced by preferential fl ow through cracks. Also, the microrelief topography exercises a control on spatiotemporal variations in soil moisture in that the microhighs dry faster than the microlows.

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A Geophysical Study of the Impact of Agricultural Management Practices on the Hydrologic Properties of a Texas Vertisol

Symposium on the Application of Geophysics to Engineering and Environmental Problems 2008, 2008

We integrate geoelectrical measurements with in-situ soil-moisture determination using core sampl... more We integrate geoelectrical measurements with in-situ soil-moisture determination using core sampling to assess changes in vadose zone properties induced by agricultural management practices in a Texas Vertisol. Two sites were established at the USDA research station, Riesel, Texas, where the underlying soils are cultivated soil for crop production and a soil in grass pasture. Center spread multielectrode resistivity data were collected on each site along the same profiles on February 22, May 7, and September 12, 2007. Data analysis involved inversion using a rapid least-square technique and temperature corrections. The results were calibrated against the in-situ soil-moisture values to generate apparent soil-moisture sections. Using the van Genuchten pedotransfer function, we used the apparent soil-moisture data to compute apparent distribution of pressure heads in the soil. The results for successive sections were then related to compute apparent soil-moisture flux over the intervals between each field survey. Significant variations in resistivity and soil-moisture dynamics between the two sites were observed in the upper 2 m of the soil, especially within the root zone. We show that, within the limitation posed by the problem of equivalence in data interpretation, management-induced changes in soil hydrologic properties can be characterized using the electrical resistivity method. Introduction Land management practices impact soils’ physical and chemical properties. Changes in physical properties (such as porosity and bulk density) manifest as changes in percolation rates and rooting depths of plants, and variations in depths to the water table (among others). Quantifying the effects of these practices is a major component of many ongoing conservation programs, e.g. Total Maximum Daily Load (TMDL) model (Torbert et al. 2005; Harmel et al. 2004). In the USDA Grassland Soil and Water Research Station (GSWRS), Riesel, Texas (Figure 1), hydrologic data have been collected since 1937. These data have been valuable in analyzing hydrologic properties of the soils for evaluating the effects of various management practices at the site (Torbert et al. 2005; Harmel et al. 2004, Harmel et al., 2006). For example, Harmel et al. (2006) used a comprehensive analysis of these data to establish a link between soil loss and erosion rate, and agricultural land use in the area. The authors also made quantitative comparisons of the earlier (pre-1964) and relatively recent (1982-2002) hydrologic data. The results show that conservation practices in recent times have helped minimize the impact of cultivated agriculture on the hydrologic properties of the soil. Geophysical surveying offer a potential alternative for quantifying the impact of agricultural land use on soils. Electrical-resistivity technique in particular can be useful in collecting spatial and temporal data to complement results of hydrological models. This method has the advantage that surveys can be carried out in 2D and 3D, and at various scales, which may be as small as centimetric scales (Samouelian et al. 2003). Besson et al. (2004) used this method to analyze structural heterogeneity of a tilled soil layer. The authors correlated resistivity distribution to variation in bulk density in the soil.

Acoustic Sub-Bottom Profiling Surveys of Flood Control Reservoirs

Environmental & Engineering Geoscience, May 1, 2013

Since the 1940s, the U.S. Department of Agriculture (USDA) has built over 11,000 flood-control re... more Since the 1940s, the U.S. Department of Agriculture (USDA) has built over 11,000 flood-control reservoirs (FCR) throughout the United States. Most were designed to hold 50 years' worth of sedimentation. Many have reached that age, but the amount of sediment they contain is unknown. Conventionally, reservoir sedimentation is monitored by measuring the change in reservoir capacity over time relative to an initial survey. However, the vast majority of FCR in the United States have never been surveyed. We evaluate a new method for surveying FCR in which an acoustic sub-bottom profiler is used to measure both water depth and sediment thickness in one survey. From these measurements both the current reservoir capacity and post-impoundment sediment volume are determined. We evaluate the method by conducting surveys of 21 FCR to find the frequency with which the base of postimpoundment sediment can be mapped throughout the reservoir. In 18 of the 21 FCR surveyed, both the water bottom and the base of post-impoundment sediment could be mapped throughout, allowing both the current and initial reservoir capacity to be determined. Comparing our estimates of initial reservoir capacities with as-built capacities estimated prior to construction we find that only seven of the 18 agree to within 10 percent. Some disagree by more than a factor of 2. We conclude that as-built capacities of USDA FCR should be used with caution for estimating post-impoundment sediment volumes and that in the majority of cases more accurate estimates can be made by direct measurement using sub-bottom acoustic profiling.

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A Study of Seasonal Salinity Variation in Lake Whitney, Texas Using Continuous Resistivity Profiling

Symposium on the Application of Geophysics to Engineering and Environmental Problems 2008, 2008

Lake Whitney, Texas is an example of many reservoirs whose water quality is controlled by highly ... more Lake Whitney, Texas is an example of many reservoirs whose water quality is controlled by highly variable sodium chloride loads originating from near surface evaporate deposits within the upper reaches of the contributing watershed. We investigate seasonal changes in salinity within Lake Whitney using the continuous resistivity profiling (CRP) method. From December, 2006 to October, 2007 we repeated a 30-km long profile along the axis of the reservoir six times. A 135-m long, 11-electrode, marine resistivity array was used to collect eight dipole-dipole readings at offsets from 5-120 m, at intervals of approximately 1 m along the profile. Each profile was collected in an 8 to 10 hour period to produce a “snapshot” of the salinity distribution. Starting from a well mixed and highly saline (3 ohmm/2,300 mg/L) state in early winter 2006, the reservoir became progressively fresher, due to the inflow of fresher water (4 ohm-m /1,700 mg/L) over the winter months. Spring rains, caused resistivity to rise and salinity to drop as layers of freshwater flowed out along the surface from north to south after each storm. The freshwater in these layers progressively mixed with deeper, more saline water as it moved south, raising the average resistivity of the reservoir. A second layer of cold freshwater flowed along the reservoir bottom in the north and out into mid-water depths as it moved south. The greatest contrast within the water column always occurred near the main tributary inlet and was least near the dam, suggesting progressive downstream mixing. We conclude that the CPR method can be used to delineate the depth and lateral extent of abnormally fresh and saline regions within water reservoirs and can resolve shallow and deep zones of contrasting salinity within the water column.

Physical and Stratigraphic Characteristics of Sediments Impounded Within a Flood Control Reservoir in Oklahoma

Transactions of the ASAE, 2003

Since 1948, the USDA-NRCS has constructed almost 11,000 upstream flood control dams across the U.... more Since 1948, the USDA-NRCS has constructed almost 11,000 upstream flood control dams across the U.S., and many of the reservoirs are filling with sediment as they near the end of their 50-year economic life. Removing the sediment remains a viable option for dam rehabilitation, yet little information exists on the physical and spatial characteristics of sediments impounded by these small reservoirs. To this end, the characteristics of sediment impounded in a flood control reservoir in Oklahoma were defined. Ten continuous, undisturbed sediment cores were obtained using vibracoring equipment, and the subsurface sediment distributions were imaged using an acoustic profiling system. Stratigraphic analyses showed that silt-and clay-sized sediments dominate the deposits within the reservoir pool, relatively high silt and clay contents were associated with relatively high total organic contents (about 2% by mass), and sand deposition was restricted to tributary and surface erosion sources. Stratigraphic correlation of sediment units across the reservoir was possible using the acoustic profiling results, pre-impoundment sediment was discriminated from post-construction deposition, and a map showing total thickness of deposited sediment was constructed. These results provide detailed information on sedimentation characteristics within a typical USDA-NRCS flood control reservoir. Moreover, the methods used herein were shown to effectively quantify reservoir sedimentation, which is critically important in reservoir management and rehabilitation programs nationwide.

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ASSESSING SEDIMENTATION ISSUES WITHIN AGING FLOOD CONTROL RESERVOIRS IN OKLAHOMA<sup>1</sup>

Journal of The American Water Resources Association, Oct 1, 2002

Since 1948, the USDA-NRCS has constructed nearly 11,000 flood control dams across the United Stat... more Since 1948, the USDA-NRCS has constructed nearly 11,000 flood control dams across the United States, and many of the reservoirs are rapidly filling with sediment. To rehabilitate these structures, the impounded sediment must be assessed to determine the volume of accumulated sediment and the potential hazard this sediment may pose if reintroduced to the environment. An assessment of sedimentation issues within two reservoirs, Sugar Creek No. 12, Hinton, Oklahoma, and Sergeant Major No. 4, Cheyenne, Oklahoma, is presented. Sediment cores obtained using a vibracoring system were composed of alternating layers of gravel, sand, silt, and clay. Stratigraphic analysis coupled with 137Cs dating techniques enabled the discrimination of pre-construction sediment from post-construction deposition. An acoustic profiling system was unencumbered by the relatively shallow water depth at Sugar Creek No. 12 and the seismic horizons agreed well with the sediment core data. Total sediment volume determined from the acoustic survey and the sediment core data for comparable areas differed by only 1.4 percent. The seismic profiling system worked well in the relatively deeper Jake of Sergeant Major No. 4 and showed good correspondence to the collected core data. Detailed chemical analyses showed that overall sediment quality was good at both locations and that chemical composition was spatially invariant. Implementation of these techniques will aid action agencies such as the USDA-NRCS in their assessment and effective management of aging flood control reservoirs.

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An evaluation of the electrical-resistivity method for water-reservoir salinity studies

Geophysics, Jul 1, 2008

An important potential application of the electrical-resistivity method occurs in salinity studie... more An important potential application of the electrical-resistivity method occurs in salinity studies of lakes and water reservoirs. However, uncertainty exists because there is a problem of equivalence in resistivity data interpretation and because the resistivity variation in the water layer in reservoirs is subtle (contrasts of a factor of two) and changes over short intervals of time (typically in hours or days). We carry out numerical modeling and inversion of synthetic data and field resistivity data from Lake Whitney, Texas, U.S.A., to examine how accurately resistivity in the water column in reservoirs can be determined using the electrical-resistivity method. Our objective is to advance the method as a tool in limnologic research for mapping freshwater zones in impacted lakes and water reservoirs. The simulated freshwater target in the synthetic data effectively is realized from the inversion with root-mean-square (rms) error less than 10%. However, the resolutionof the inverted sections decreases with increasing noise. Inversions of the field apparent-resistivity data from three profiles in the lake, computed using estimated optimization parameters from the synthetic study, reveal the possible pattern of salinity distributions in the reservoir. For unconstrained inversion schemes, comparisons of the inverted and independently measured in situ water electrical-conductivity data yield an average rms error of 10.8%. This error value is reduced to approximately 5% with inclusions of a priori information on water resistivity in the inversion scheme. We observe an inverse relationship between error level and number of constraints on water resistivity. In general, results show that the electrical-resistivity method is a viable tool for mapping salinity variations in reservoirs. However, good data quality and inclusion of measured water conductivity as constraints in the inversion schemes are important to enhance accuracy of the inversion results.

Stream Valley Resistivity In Central Texas: Laying The Foundation For Multi-Disciplinary Research

Increasing land-use and a changing climate can drastically modify floodplain habitats along lowor... more Increasing land-use and a changing climate can drastically modify floodplain habitats along loworder streams in central Texas. Investigating stream response to these factors on the site-scale can be costly. In an effort to streamline future site-specific investigations, a near-surface geophysical method was used along Williams Creek in Axtell, Texas. A 2-D dipole-dipole resistivity reconnaissance study was employed to estimate depth to bedrock and locate buried channels in a partial-valley cross-section. Auger borings provided ground-truth along the resistivity profile, which confirmed depth to bedrock and the significance of varying resistivities. The 276 meter resistivity profile was completed across a flowing Williams Creek and a portion of its floodplain, revealing a number of different fluvial landforms. Hand-augering confirmed the presence of a coarse-grained paleochannel fill that corresponds with a high-resistivity zone (50-100 ohm-m). At one location, auger refusal at bedrock occurred 3.15 meters below ground surface, consistent with the observed change to lower resistivity values. This decrease in resistivity at the bedrock contact is associated with groundwater flow along the fissile marl bedding planes. These data suggest that resistivity and ground-truthing combined, provide a cost-effective foundation to guide future research on the effects of climate and land-use on Williams Creek.

Watershed sediment measurement and sediment transport modeling techniques: Case study to quantify the impact of converting cropland to forested stream buffers on soil loss and water quality at the watershed scale

Assessing sedimentation issues within aging flood-control reservoirs

Geological Society of America eBooks, 2013

Since 1948, the USDA-NRCS has constructed nearly 11,000 flood control dams across the United Stat... more Since 1948, the USDA-NRCS has constructed nearly 11,000 flood control dams across the United States, and many of the reservoirs are rapidly filling with sediment. To rehabilitate these structures, the impounded sediment must be assessed to determine the volume of accumulated sediment and the potential hazard this sediment may pose if reintroduced to the environment. An assessment of sedimentation issues within two reservoirs, Sugar Creek No. 12, Hinton, Oklahoma, and Sergeant Major No. 4, Cheyenne, Oklahoma, is presented. Sediment cores obtained using a vibracoring system were composed of alternating layers of gravel, sand, silt, and clay. Stratigraphic analysis coupled with 137Cs dating techniques enabled the discrimination of pre-construction sediment from post-construction deposition. An acoustic profiling system was unencumbered by the relatively shallow water depth at Sugar Creek No. 12 and the seismic horizons agreed well with the sediment core data. Total sediment volume determined from the acoustic survey and the sediment core data for comparable areas differed by only 1.4 percent. The seismic profiling system worked well in the relatively deeper Jake of Sergeant Major No. 4 and showed good correspondence to the collected core data. Detailed chemical analyses showed that overall sediment quality was good at both locations and that chemical composition was spatially invariant. Implementation of these techniques will aid action agencies such as the USDA-NRCS in their assessment and effective management of aging flood control reservoirs.

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PFLOTRAN-SIP: A PFLOTRAN Module for Simulating Spectral-Induced Polarization of Electrical Impedance Data

Energies, 2020

Spectral induced polarization (SIP) is a non-intrusive geophysical method that collects chargeabi... more Spectral induced polarization (SIP) is a non-intrusive geophysical method that collects chargeability information (the ability of a material to retain charge) in the time domain or its phase shift in the frequency domain. Although SIP is a temporal method, it cannot measure the dynamics of flow and solute/species transport in the subsurface over long times (i.e., 10–100 s of years). Data collected with the SIP technique need to be coupled with fluid flow and reactive-transport models in order to capture long-term dynamics. To address this challenge, PFLOTRAN-SIP was built to couple SIP data to fluid flow and solute transport processes. Specifically, this framework couples the subsurface flow and transport simulator PFLOTRAN and geoelectrical simulator E4D without sacrificing computational performance. PFLOTRAN solves the coupled flow and solute-transport process models in order to estimate solute concentrations, which were used in Archie’s model to compute bulk electrical conductivi...

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Measured and modeled 3‐D far‐field radiation patterns for three large marine air gun arrays

SEG Technical Program Expanded Abstracts 1984, 1984

Stream Valley Resistivity in Central Texas: Laying the Foundation for Multi‐Disciplinary Research

Symposium on the Application of Geophysics to Engineering and Environmental Problems 2010, 2010

Increasing land-use and a changing climate can drastically modify floodplain habitats along lowor... more Increasing land-use and a changing climate can drastically modify floodplain habitats along loworder streams in central Texas. Investigating stream response to these factors on the site-scale can be costly. In an effort to streamline future site-specific investigations, a near-surface geophysical method was used along Williams Creek in Axtell, Texas. A 2-D dipole-dipole resistivity reconnaissance study was employed to estimate depth to bedrock and locate buried channels in a partial-valley cross-section. Auger borings provided ground-truth along the resistivity profile, which confirmed depth to bedrock and the significance of varying resistivities. The 276 meter resistivity profile was completed across a flowing Williams Creek and a portion of its floodplain, revealing a number of different fluvial landforms. Hand-augering confirmed the presence of a coarse-grained paleochannel fill that corresponds with a high-resistivity zone (50-100 ohm-m). At one location, auger refusal at bedrock occurred 3.15 meters below ground surface, consistent with the observed change to lower resistivity values. This decrease in resistivity at the bedrock contact is associated with groundwater flow along the fissile marl bedding planes. These data suggest that resistivity and ground-truthing combined, provide a cost-effective foundation to guide future research on the effects of climate and land-use on Williams Creek.

A Study of Seasonal Salinity Variation in Lake Whitney, Texas Using Continuous Resistivity Profiling

Symposium on the Application of Geophysics to Engineering and Environmental Problems 2008, 2008

Lake Whitney, Texas is an example of many reservoirs whose water quality is controlled by highly ... more Lake Whitney, Texas is an example of many reservoirs whose water quality is controlled by highly variable sodium chloride loads originating from near surface evaporate deposits within the upper reaches of the contributing watershed. We investigate seasonal changes in salinity within Lake Whitney using the continuous resistivity profiling (CRP) method. From December, 2006 to October, 2007 we repeated a 30-km long profile along the axis of the reservoir six times. A 135-m long, 11-electrode, marine resistivity array was used to collect eight dipole-dipole readings at offsets from 5-120 m, at intervals of approximately 1 m along the profile. Each profile was collected in an 8 to 10 hour period to produce a “snapshot” of the salinity distribution. Starting from a well mixed and highly saline (3 ohmm/2,300 mg/L) state in early winter 2006, the reservoir became progressively fresher, due to the inflow of fresher water (4 ohm-m /1,700 mg/L) over the winter months. Spring rains, caused resistivity to rise and salinity to drop as layers of freshwater flowed out along the surface from north to south after each storm. The freshwater in these layers progressively mixed with deeper, more saline water as it moved south, raising the average resistivity of the reservoir. A second layer of cold freshwater flowed along the reservoir bottom in the north and out into mid-water depths as it moved south. The greatest contrast within the water column always occurred near the main tributary inlet and was least near the dam, suggesting progressive downstream mixing. We conclude that the CPR method can be used to delineate the depth and lateral extent of abnormally fresh and saline regions within water reservoirs and can resolve shallow and deep zones of contrasting salinity within the water column.

Integrating continuous resistivity profiling (CRP) into water‐reservoir salinity studies — Numerical and field evaluation

SEG Technical Program Expanded Abstracts 2007, 2007

As demands for freshwater resources continue to rise, there are critical needs for improved techn... more As demands for freshwater resources continue to rise, there are critical needs for improved techniques for water quality assessment in lakes and water reservoirs. Because conventional methods of water reservoir studies largely involve point-based water and sediment sampling, obtaining spatial data for improved characterization of salinity variations in lakes and water reservoir remains a challenge. We explore the potential of using CRPmeasured resistivity data to determine accurately true resistivity distribution in water reservoirs. Our objective is to advance the method as a tool in limnological research for mapping freshwater zones within impacted lakes and water reservoirs.

Late Tertiary paleoclimatic interpretation from lacustrine rhythmites in the Gray Fossil Site, northeastern Tennessee, USA

Journal of Paleolimnology, 2008

Page 1. ORIGINAL PAPER Late Tertiary paleoclimatic interpretation from lacustrine rhythmites in t... more

Upland contribution of sediment and runoff during extreme drought: A study of the 1947–1956 drought in the Blackland Prairie, Texas

Journal of Hydrology, 2011

The 1950s drought severely impacted a 1.1 million km 2 area in the central US. This drought, alon... more The 1950s drought severely impacted a 1.1 million km 2 area in the central US. This drought, along with the famous 1930s drought, was among the most severe of the 20th century for large areas and is the drought of record for water supply planning in Texas. At the USDA-ARS Riesel Watersheds during the drought, average annual rainfall was reduced 27%, which produced 75% less runoff and 35% less sediment yield. Rainfall intensity during the drought was, however, typically greater than for the non-drought period. Based on long-term data from the Riesel Watersheds, the mean, 75th, 90th, and 95th percentile values of sediment yields on days with measureable soil loss were larger for the drought than non-drought periods. These results reflect the increase in rainfall intensity during the drought but more importantly the increased efficiency of drought rainfall to dislodge and transport sediment, which is attributed to the combined effects of reduced vegetative cover and increased soil erodibility. The potential for high sediment yields during drought periods illustrates the need to consider this landscape vulnerability in long-term planning and assessment and the importance of long-term monitoring to predict water supply impacts. This is especially evident in Texas, which is expected to experience a dramatic increase in population and water demand this century, with a corresponding decrease in reservoir storage capacity due to sedimentation.

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An evaluation of the electrical-resistivity method for water-reservoir salinity studies

GEOPHYSICS, 2008

An important potential application of the electrical-resistivity method occurs in salinity studie... more An important potential application of the electrical-resistivity method occurs in salinity studies of lakes and water reservoirs. However, uncertainty exists because there is a problem of equivalence in resistivity data interpretation and because the resistivity variation in the water layer in reservoirs is subtle (contrasts of a factor of two) and changes over short intervals of time (typically in hours or days). We carry out numerical modeling and inversion of synthetic data and field resistivity data from Lake Whitney, Texas, U.S.A., to examine how accurately resistivity in the water column in reservoirs can be determined using the electrical-resistivity method. Our objective is to advance the method as a tool in limnologic research for mapping freshwater zones in impacted lakes and water reservoirs. The simulated freshwater target in the synthetic data effectively is realized from the inversion with root-mean-square (rms) error less than 10%. However, the resolutionof the invert...

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