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Estimating Bulk Density in Vertically Exposed Stoney Alluvium Using a Modified Excavation Method

^a Department of Crop, Soil, and Environmental Sciences, University of Arkansas, 115 Plant Sciences Building, Fayetteville, AR 72701
^b Arkansas Department of Environmental Quality, Environmental Preservation Division, 8001 National Drive, Little Rock, AR 72219

^* Corresponding author (kbrye{at}uark.edu).

Received for publication January 24, 2004.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Despite many decades of education and refining land-use practices, accelerated stream bank erosion is still prevalent in the United States. Eroding stream banks produce a sediment load to the riverine system and can cause reduced water quality as a result of increased suspended sediment. As total maximum daily loads (TMDLs) for water bodies impaired by turbidity or suspended sediments become more numerous, a simple, in situ field technique will be needed to estimate the bulk density of readily erodible stream bank material so that reasonably accurate sediment loading rates can be estimated. In this study, the excavation/polyurethane-foam technique for estimating total bulk density was applied to vertically exposed alluvium with high coarse-fragment content. Though not previously attempted in vertically exposed alluvium with high coarse-fragment content, the excavation/polyurethane-foam technique appears to provide a reasonably accurate estimate of the total and soil (<2-mm size fraction) bulk density from vertically exposed, alluvial deposits with high coarse-fragment content (i.e., >70%) along eroding stream banks. Obtaining bulk density estimates using this method would facilitate calculation of sediment loading rates to riverine systems with actual field data.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
DESPITE SIGNIFICANT ADVANCEMENTS in the past 70 yr, beginning in 1933 with the Coon Creek Watershed Project near Coon Valley, Wisconsin, the nation's first watershed-scale project to investigate the relationships between soil erosion, sediment load to surface waters, and land-use practices (Leopold, 1935), accelerated stream bank erosion and sediment load to river systems continues to be a major environmental problem. Suspended sediment, particularly the <0.062-mm size fraction, contributes to turbidity, which in turn affects water quality and impairs the aquatic environment (Ravdkivi, 1976).

Section 303(d) of the Federal Pollution Control Act, known commonly as the Clean Water Act, is a statutory directive, regulated by the USEPA, that requires states to identify water bodies that are impaired by point and/or nonpoint sources for which effluent limitations required by Section 301 either do not exist or are not stringent enough to implement any applicable water quality standard (United States Congress, Senate, 1972). States are also required to prioritize waters on their 303(d) lists for total maximum daily load (TMDL) development (United States Congress, Senate, 1972). In many instances, the source of impairment associated with exceeding the suspended sediment standard could be linked to excessive amounts of sediment entering the stream from stream bank erosion. The goal of the TMDL is to identify the sources of the contaminant causing the impairment, the relative contribution of the various sources, and the load reduction that will result in the stream meeting water quality standards. Therefore, when preparing a TMDL for a stream impaired by excessive siltation, it is critical that accurate estimations of present sediment loading rates be obtained.

A method of estimating sediment loads from eroding stream banks has been developed that requires the collection of field data related to the erodibility of the stream banks (Rosgen, 2001). In particular, bulk density of stream bank materials is needed to determine the mass loading of sediment. This paper provides a relatively simple methodology for determining the total bulk density of vertically exposed stream bank deposits with high coarse-fragment content (Fig. 1). The excavation method, with use of water or expanding polyurethane foam to determine the sample volume, has been used for estimating bulk density in stoney soils when the traditional core method was not feasible (Muller and Hamilton, 1992; Page-Dumroese et al., 1999; Grossman and Reinsch, 2002). It has also been suggested that the excavation/polyurethane-foam technique may be used on vertical faces (Muller and Hamilton, 1992; Grossman and Reinsch, 2002). However, to our knowledge, no studies have attempted to apply the excavation/polyurethane-foam technique in a horizontal orientation into vertically exposed, highly stoney material; thus, the actual feasibility of the excavation/polyurethane-foam technique in these settings is unknown.

View larger version (147K): [in this window] [in a new window]   Fig. 1. Typical high stream banks with vertically exposed coarse- and fine-textured alluvium.

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Sampling Locations
The West Fork of the White River, located in Washington County, AR, is on Arkansas' 303(d) list as being impaired by siltation (i.e., excessive suspended sediment; USEPA, 2003a, 2003b) with the year 2005 set as the target date for TMDL development. The Environmental Preservation Division (EPD) of the Arkansas Department of Environmental Quality (ADEQ) has surveyed approximately 67.6 km (42 mi) of the West Fork branch of the White River and two of its main tributaries. During this survey, an inventory of eroding stream banks was developed, which included measurements of bank length and height as well as other characteristics that directly relate to bank stability. Of the banks included in the inventory, eight reaches were identified as representative of various degrees of erosion severity within the West Fork of the White River watershed. Each of the eight representative reaches have between 0.5 and 6 m of vertically exposed stream bank deposits (i.e., alluvium; Fig. 1). Several sites consist of multiple, alternating bands of fine- and coarse-textured, gravelly and cobblely material, while other sites consist solely of relatively uniform gravelly and cobblely material. At least two samples were collected at each reach representing the range of vertically exposed alluvium with high coarse-fragment content based on major visual differences in alluvial layers; thus, no true replicated samples were obtained. Figure 1 depicts the typical type of material sampled. A total of 18 samples were collected among the eight representative reaches.

Sampling Procedure
By definition, bulk density is the ratio of the mass of some material to the total volume that material occupies. Therefore, the field sampling procedure consisted of two phases: (i) material excavation for mass determination and (ii) volume estimation.

To begin the first phase, the outer 5 to 10 cm of the sampling location was cleared away with a shovel or by hand to expose a fresh area that had not had the finer particle-size fraction washed away during the last near-bank-full flow or flooding event. Coarse fragments were scraped and picked away to obtain as close to a flat vertical face as possible before beginning excavation into the vertically exposed alluvium. Once a reasonably flat face was achieved, a horizontal cavity was excavated using hand tools, making sure to collect all material removed from the cavity. Excavated material was collected into a plastic bag held beneath the excavated cavity. Excavated material was air-dried for several days, then oven-dried at 55°C for one week, and weighed.

The size of the cavities excavated in this study ranged from 676 to 2375 cm³ and averaged 1294 cm³, but were dictated by the thickness of the alluvial layers being sampled and their packing densities. Relatively thick alluvial layers allowed for a larger sample to be collected than from relatively thin layers. Similarly, the denser the material being excavated, the smaller the resulting cavity. The sizes of the cavities excavated in this study were within the range suggested by Grossman and Reinsch (2002) and similar to slightly larger than those of Muller and Hamilton (1992).

The volume-estimation phase of the sampling procedure consisted of filling the excavated cavity with waterproof, expanding polyurethane foam (Muller and Hamilton, 1992). Expanding polyurethane foam is available at most hardware stores and is typically used for sealing household gaps and cracks (e.g., Great Stuff; Dow Polyurethane Systems, Marietta, GA). The foam was sprayed into the cavity starting with the rear and moving toward the front of the cavity. Though it is usually recommended to fill a gap only 50% full to allow the foam to expand on its own to completely fill the gap without much excess to cut off and remove, the cavity was filled completely full to ensure complete expansion of the foam into all microtopographic variations in the outer surface of the excavated cavity (Muller and Hamilton, 1992). Expanding polyurethane foam typically expands to several times the volume of foam initially injected. The front of the cavity was usually left open to the atmosphere, but sometimes the semi-liquid foam tended to pour out of the cavity. To avoid this occurrence, a flat piece of Plexiglass was braced over the front of the cavity to keep the foam in the cavity (Muller and Hamilton, 1992). The foam then just expanded against and around the Plexiglass.

Once the cavity was filled, the foam was allowed to cure overnight to create a waterproof foam mold. After foam curing, a marker was used to trace the perimeter of the front of the cavity onto the foam mold. The entire foam mold was excavated from the surrounding alluvium and the excess foam that expanded and cured beyond the front of the cavity was cut away along the marker trace using a hacksaw blade. The foam mold represented the volume from which the alluvial material was originally excavated.

Several steps had to be performed before actual volume determination could be performed. Since the foam cures somewhat tacky, the mold had to be thoroughly cleaned by brushing and scraping away soil and picking out small pebbles and plant roots that stuck to the outside of the mold. In addition, though the foam cures to produce a waterproof outer shell, the surface of the mold that was exposed when the mold was cut to remove the excess is not waterproof and can imbibe some water. Muller and Hamilton (1992) indicated that the amount of water absorbed by the foam mold during volume determinations was consistently <1% of the sample volume. However, to minimize water absorption, the cut surface was sealed by spraying it with a thick coat of water-resistant, clear gloss urethane commonly used as an indoor–outdoor wood finisher (e.g., Helmsman Spar Urethane; Minwax Company, Upper Saddle River, NJ) and allowed to dry for at least 24 h.

Once the urethane seal was dry, the entire mold was submerged in a water-filled container of known dimensions by pushing the mold under water with dissecting needles. The volume of water displaced represented the volume of the foam mold, which represents the excavated cavity. The volume of the excavated cavity was determined using the measured difference between the final and initial heights of water in the container. The container used in this study had an average diameter of 27.1 cm, but was slightly tapered from top (28.7 cm in diameter) to bottom (25.5 cm in diameter). Total bulk density was calculated as the ratio of the total dry mass of material excavated to the volume of water displaced during submersion of the foam mold.

A soil bulk density, representing the <2-mm size fraction, was also calculated after subtracting the mass and volume of the >2-mm size fraction from the total mass and volume of the sample. The volume of the >2-mm size fraction was calculated from the mass and an assumed particle density of 2.8 g cm^–3 for the local lithology (J. Brahana, personal communication, 2003). Without the actual volume of the >2-mm size fraction, this is the recommended approach for estimating bulk density of the <2-mm size fraction (Grossman and Reinsch, 2002).

Particle-Size Analyses
In addition to bulk density determinations, the material excavated from the cavities was fractionated into various particle-size classes. The material was allowed to soak in distilled water and wet-sieved through a 2-mm mesh screen. Following initial wet-sieving, the <2-mm size fraction was oven-dried and fractionated into sand (0.02–2 mm), silt (0.002–0.02 mm), and clay (<0.002 mm) using the hydrometer method (Arshad et al., 1996). The >2-mm size fraction was also collected, oven-dried, and further separated gravimetrically by manually shaking the dry material through a nest of 4-, 8-, 16-, 31.5-, 53-mm mesh sieves simultaneously for 1 min.

	Results and Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Particle-Size Distributions
Particle-size distributions varied widely among the 18 samples collected at eight representative sites along the West Fork of the White River and one of its tributaries in Washington County, Arkansas (Fig. 2). The <2-mm fraction, averaged across all samples, represented 19.7%, ranging from 10.4 to 32.5%, of the total mass of material collected. All samples collected represented soil textural classes from fine-textured clay loam to coarse-textured loamy sand. The <0.02-mm fraction (i.e., silt and clay) averaged 6.5% by mass of the total amount of material collected and ranged from 1.4 to 20.2%. The >2-mm fraction, averaged across all samples, represented 80.3%, ranging from 67.5 to 89.6%, of the total mass of material collected. All samples collected represented the coarse-fragment textural modifiers of extremely gravelly or extremely cobblely (USDA Soil Survey Division Staff, 1983).

View larger version (91K): [in this window] [in a new window]   Fig. 2. Gravimetric particle-size range (mm) of samples collected in vertically exposed alluvium along the West Fork of the White River in Washington County, Arkansas, using the excavation/polyurethane-foam technique.

Bulk Densities
In theory, the bulk density, whether total or soil, of earthen material should not exceed the particle density of its individual constituents. This guideline can be used as a check on the accuracy of the resulting bulk densities calculated with volume measurements from the foam mold.

Based on the particle-density guideline, 5 of the 18 samples collected with this method resulted in total bulk densities greater than 2.8 g cm^–3, where the expected particle density range for the local lithology is 2.5 to 2.8 g cm^–3, but can be as high as 3.0 g cm^–3 (J. Brahana, personal communication, 2003); thus, these five samples were excluded from data analysis and summary. The large total bulk density values probably resulted from an underestimation of the total volume of the excavated cavity rather than from an overestimation of the mass of material excavated. In addition, there were four other samples that resulted in bulk densities of >2.5 g cm^–3, but less than the expected particle density maximum of 3.0 g cm^–3. Due to the lack of a valid reason to exclude these four samples, they were included in the data set for the following analysis and summary.

For the 13 of 18 samples collected that resulted in total bulk density values below the expected particle-density range, the total bulk density averaged across all samples was 2.20 (standard error [SE] = 0.11) g cm^–3 and ranged from 1.44 to 2.76 g cm^–3 (Fig. 3). The <2-mm size fraction comprised less than 20% by mass, on average, of each sample. The soil bulk density averaged 1.30 (SE = 0.2) g cm^–3 across all samples and ranged from 0.39 to 2.50 g cm^–3. These bulk density results are somewhat higher than those of Andraski (1991) who compared bulk densities from the balloon and core methods in alluvial desert soil, but with a much smaller volume of coarse fragments than what was sampled in the present study. However, Muller and Hamilton (1992) reported mean bulk densities of >2.0 g cm^–3 using the excavation/polyurethane-foam technique on freshly graded, stoney mine spoil.

View larger version (53K): [in this window] [in a new window]   Fig. 3. Total and soil (<2-mm size fraction) bulk densities from 13 of 18 sampling locations among eight representative sites along the West Fork of the White River and one of its tributaries in Washington County, Arkansas, using the excavation/polyurethane-foam technique.

Procedural Limitations
Though not compared with other methods in this study, the excavation/polyurethane-foam technique has been shown to yield similar bulk densities to other excavation methods, where both sand (Laundre, 1989; Muller and Hamilton, 1992) and water (Page-Dumroese et al., 1999) have been used to determine the sample volume, and to the small-diameter core method (Laundre, 1989; Page-Dumroese et al., 1999). However, since the excavation/polyurethane-foam technique has not actually been applied to vertical faces before, there were several procedural limitations encountered while applying the excavation/polyurethane-foam technique to vertical faces that need mention. First, it was challenging to collect all material while excavating a horizontal cavity into the vertically exposed stream bank deposits. Second, due to the highly stoney nature of the material, it was nearly impossible to achieve a perfectly flat outer face before excavating the cavity. Therefore, the resulting total bulk density value should be considered an approximation due to the likely combination of over- and underestimation of the sample volume near the outermost portion of the sample mold. However, the resulting field estimation of the total bulk density would be superior to any assumed value of the total bulk density of the material present at a particular location along a stream or river impaired by a high suspended sediment load.

Proposed Methods of Error Reduction
Since no other methods have actually been used to quantify bulk density from horizontal sampling into vertically exposed material with high coarse-fragment content, it was impossible to compare the excavation/polyurethane-foam technique with any other well-accepted methods. However, the excavation/polyurethane-foam technique has previously been reported to yield low standard errors when samples were collected vertically downward from the soil surface (Laundre, 1989; Page-Dumroese et al., 1999). In addition, Grossman and Reinsch (2002) indicated that the excavation/polyurethane-foam technique yields similar coefficients of variation to that of the sand-funnel method.

To reduce sources of potential error when applying the excavation/polyurethane-foam technique to vertical faces, several methodological suggestions can be made. First, prepare the outer face of the area to be excavated with as small an access-hole diameter as possible. This will reduce potential errors when cutting away excess foam after it has cured and minimize over- or underestimation error of the sample volume. Second, obtain the largest representative volume that is feasible. This can be accomplished by excavating the inside of the cavity as widely and as deeply as possible. Third, excavate the cavity with a subtle downward tilt, such that the cavity opening is at a slightly higher elevation than the back of the cavity, to facilitate complete expansion of the foam into the volume excavated and minimize the chance of the semi-liquid foam spilling out of the cavity under the force of gravity, leaving part of the cavity unfilled with foam. However, during this process, too much downward slope may make complete collection of the excavated material difficult.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Having accurate estimates of the total and soil bulk density of vertically exposed deposits along stream banks is a necessary component for determining sediment loading from stream bank erosion in a watershed where the stream is impaired from turbidity and siltation. Though not previously applied to vertical faces and despite a few procedural limitations that can be overcome with additional care, the excavation/polyurethane-foam technique appears to provide reasonably accurate estimates of the total and soil bulk densities of vertically exposed alluvial deposits with high coarse-fragment content such that the sediment load to a stream from an eroding stream bank can be estimated. More accurate estimates of total and soil bulk density can be obtained with this in situ field method compared with non-sample-based approximations or assumptions and will provide an important component of data needed to estimate sediment loads from eroding stream banks.

	ACKNOWLEDGMENTS

 
We would like to thank Sumit Sen and Laura Wilson for their laboratory assistance processing samples and H. Lee Silvey and Dave Rosgen for reviewing this manuscript before submission.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Trade names and companies are provided for the benefit of the reader and do not imply endorsement by the University of Arkansas.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 

• Andraski, B.J. 1991. Balloon and core sampling for determining bulk density of alluvial desert soil. Soil Sci. Soc. Am. J. 55:1188–1190.[Abstract/Free Full Text]
• Arshad, M.A., B. Lowery, and B. Grossman. 1996. Physical tests for monitoring soil quality. p. 123–141. In J.W. Doran and A.J. Jones (ed.) Methods for assessing soil quality. SSSA Spec. Publ. 49. SSSA, Madison, WI.
• Grossman, R.B., and T.G. Reinsch. 2002. Bulk density and linear extensibility. p. 201–254. In J.H. Dane and G.C. Topp (ed.) Methods of soil analysis. Part 4. SSSA Book Ser. 5. SSSA, Madison, WI.
• Laundre, J.W. 1989. Estimating soil bulk density with expanding polyurethane foam. Soil Sci. 147:223–224.
• Leopold, A. 1935. Coon Valley: An adventure in cooperative conservation. Am. For. 41(5):205–208.
• Muller, R.N., and M.E. Hamilton. 1992. A simple effective method for determining the bulk density of stoney soils. Commun. Soil Sci. Plant Anal. 23:313–319.
• Page-Dumroese, D.S., M.F. Jurgensen, R.E. Brown, and G.D. Mroz. 1999. Comparison of methods for determining bulk densities of rocky forest soils. Soil Sci. Soc. Am. J. 63:379–383.[Abstract/Free Full Text]
• Ravdkivi, A.J. 1976. Loose boundary hydraulics. Pergamon Press, New York.
• Rosgen, D. 2001. A practical method of computing streambank erosion rate. p. 9–15. In Proc. 7th Federal Interagency Sedimentation Conf., Reno, NV. 25–29 Mar. 2001. Vol. 2. USGS, Reston, VA.
• United States Congress, Senate. 1972. Federal Water Pollution Control Act Amendments of 1972. 33 United States Code 1251-1387. U.S. Gov. Print. Office, Washington, DC.
• USDA Soil Survey Division Staff. 1983. Soil survey manual. U.S. Gov. Print. Office, Washington, DC.
• USEPA. 2003a. Federal Register environmental documents: Clean Water Act Section 303(d): Final Agency Action Adding Waters to the Arkansas 2002 Section 303(d) List [Online]. Available at www.epa.gov/fedrgstr/EPA-WATER/2003/June/Day-18/w15254.htm (verified 4 June 2004). USEPA, Washington, DC.
• USEPA. 2003b. Water Quality, Region 6: South Central (WQR6-SC). Arkansas TMDL documents [Online]. Available at www.epa.gov/earth1r6/6wq/artmdl.htm (verified 4 June 2004). USEPA, Washington, DC.

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