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Geometry of inset channels and the sediment composition of fluvial benches in agricultural drainage systems in Ohio.

In Ohio, agricultural channel systems are the primary conveyors of discharges from fields to rivers. Many of these channels have been constructed or modified to transport surface runoff and subsurface drainage away from agricultural fields. The development of depositional bars in these agricultural channel systems has been a focus of maintenance activities commissioned by farmers. United States Department of Agriculture Natural Resource Conservation Service (NRCS) personnel, county engineers and other local organizations. Often, these depositional features are perceived to substantially reduce the conveyance capacity of the ditch and are periodically excavated. The effects of these maintenance practices on sediment transport, flow, and the ecological habitat of channelized systems have been documented by Rhoads and Herricks (1996). Formation of depositional features, such as adjustments of the channel to modification in its discharge and sediment regime, has been suggested as the mode of channel recovery from an unstable to a stable system (Landwehr and Rhoads 2003). If benches in agricultural channel systems act as mini-floodplains then, based on the Nanson and Croke (1992) classification method for floodplains, benches are valley confined Class C (low-energy cohesive floodplains) features whose stability is enhanced by the low-energy of the stream system and cohesiveness of floodplain sediments. In agricultural channel systems, streams, and rivers in Ohio, the discharges that spill onto the active floodplain or benches occur many times annually (Jayakaran et al. 2005; Powell et al. 2006). This high frequency of above-bench discharge events is an indicator that vertical accretion is a dominant process in floodplain construction (Knighton 1998). Landwehr and Rhoads (2003) suggest the adjustment of channel slope post channelization takes place, not by channel incision and bed erosion, but by the development of aggradational fluvial features. The authors attribute initial bench development to the deposition of coarse materials by bed load transport, upon which subsequent overbank deposition of suspended sediment, and the growth of grasses provide stability to the features. They also state that a key factor in bench development is an increased initial ditch width at the time of construction, which provides the requisite low-energy depositional environment for bench formation. This finding was corroborated in a study by Jayakaran et al. (2005) in agricultural channel systems in Northwest Ohio. In that study, the magnitudes of ditch width and drainage area at the study sites were found to be the most significant factors affecting bench stability. Frequency of bench inundating discharges and degree of inundation were considered significant factors in determining bench composition. Vegetation growing on the surface of benches further reduced discharge velocity and enhanced the sediment trapping capacity of the benches.

The goal of this study was to determine if the texture of bench materials was related to (1) depth from the surface of the benches, and (2) lateral position with respect to the fluvial channel (henceforth called an inset channel) that is located between the benches (figure 1). It was hypothesized that knowledge of the particle size composition of fluvial features could offer insight into the evolution of these features. Additionally, frequency of bench inundation and effects of inset channel geometry on bed material were also evaluated. Specific objectives were to (1) determine if there was a relationship between the inset channel geometry, as defined by its benches and geometry predicted by regional curves that related bankfull geometry to drainage area, (2) evaluate the effect of channel geometry on bed material particle size distribution, and (3) determine if particle size distribution of bench materials were related to depth from the surface and position on the bench.

Materials and Methods

There were 12 study sites in Mercer County, Ohio, and one study site in Fayette County, Ohio (figure 2). Drainage areas at the study sites ranged from 0.28 to 6.81 [km.sup.2] (0.11 to 2.63 [mi.sup.2]). The Maumee River drains the northern half of Mercer County and ultimately discharges into Lake Erie. The southern half of Mercer County drains into the Wabash River, which flows west from Ohio and ultimately discharges into the Mississippi River. The site in Fayette County (site 13) was located on Pone Creek, a headwater stream in the Ohio River Basin. Mercer and Fayette counties are both located in the Till Plains region of the Central Lowlands Physiographic Province. The landscape is characterized by low gradients that are regularly interrupted by glacial moraines and streams. Predominant soil parent materials in the region are poorly sorted clayey glacial till belonging to the Wisconsinan glacial era. More than 90% of the land use in both counties is row crop agriculture (Sugar 2004; Angle 2004).

Drainage areas at each site were derived using automated drainage area delineation tools available in a geographic information system (GIS-ArcGIS 9.0). Drainage areas to each site were derived using digital elevation models (DEMs; USGS National Elevation Database 30 m) with 30 m (98.4 ft) spatial resolution, and a streams shape layer (Shapefiles: National Hydrography Dataset 1:24,000). Bench features were identified on each of the reaches using a combination of 0.15 m per pixel (0.5 ft per pixel) resolution panchromatic digital orthophotoquads and visual reconnaissance surveys. The spatial extent of each panchromatic digital orthophotoquad was 610 m x 610 m (2000 ft X 2000 ft); 3566 such images provided a seamless coverage of the entire county. The digital files were obtained from the Mercer County Auditor's Office and proved a useful tool for the identification of bench formation in agricultural channels. Information was obtained from the Mercer County Engineer's Office pertaining to the year of ditch construction and the year of last maintenance carried out under the aegis of the Engineer's Office. Site 1 was the only site in Mercer County that was never officially maintained (table 1). Some ditch clean-out operations are performed by local farmers and generally tend to go unrecorded in official documents.

The extent of bench and inset channel formation in ditches throughout Mercer County was conducted on five areas of varying sizes, covering 263 [km.sup.2] (103 [mi.sup.2]) and representing 22.2% of the areal extent of Mercer County. The shortest distance between adjacent rectangular areas was about 5 km (3 mi). Upstream and down-stream photographs taken by the authors at 67 road crossings in Mercer County were used to train a visual identification of bench features from the digital orthophotos of the same locations. A visual evaluation was then made using just the orthophotos and each kilometer of ditch was classified as either having benches or not having benches.

Channel System Geomorphology. Channel geomorphology measurements were made during late autumn of 2005. Measurements were made along reaches that were about 20 times the bankfull width of the inset channel (Harrelson et al. 1994). Ditch morphology was characterized by measuring bed elevation, water depth, and the elevation of bankfull features at points of discernable change in geomorphic features within the channel system (Ward and Trimble 2003). Geomorphic characteristics determined from survey data for each reach were: bed slope, bankfull width, depth and cross-sectional area, and bench width (Ward and Trimble 2003). A Wolman Pebble Count (Wolman 1954) to characterize bed material was conducted at all the sites. The typical lower size limit for the characterization of bed material by the Wolman Pebble Count is 4 mm (0.16 in) (Kondolf et al. 2003). Buffington and Montgomery (1999) adopted a lower limit of 2 mm (0.08 in) in a study on gravel bed rivers in Washington and southeastern Alaska. Due to the generally fine nature of bed material contained within low gradient agricultural channels in Ohio, a lower size limit of 2 mm (0.08 in) was adopted in this study. (Particles smaller than 2 mm were assigned to a "less than 2 mm" size class).

Estimating Bankfull Dimensions. Empirical relationships between measures of channel geometry, discharge, and drainage area are useful tools in assessing stream morphology in the United States. Some of the earliest were developed by Leopold and Maddock (1953) who related flow width, mean depth, and flow velocity to the mean annual discharge. Regional curves are a derivation of such empirical relationships and are used to relate bankfull discharge and channel geometry (Dunne and Leopold 1978; Rosgen 1996). Regional curves are especially useful in stream restoration projects, where the identification of bankfull features is critical to the design of a stable system (Doll et al. 2002).

A regional curve for low gradient Midwestern agricultural channels with herbaceous riparian vegetation is reported in Jayakaran et al. (2005). That regional curve was based on 19 sites in Northwestern Ohio measured by the authors, and 25 sites in Minnesota measured by Magner and Steffen (2000). Bankfull dimensions from both regions were found to be statistically similar based on analysis of covariance using log transformed bankfull dimensions as dependent variables, log transformed drainage area as a concomitant variable, and region as an independent variable. This regional curve was used to assess channel cross sectional geometry at the 13 sites evaluated in this study. This regional curve will henceforth be referred to as the northwest Ohio-Minnesota regional curve. The position of the cored benches was evaluated against the position of an 'expected' bankfull feature predicted by the regional curve. At sites where the cored benches did not correspond to a regional bankfull elevation, an alternative bankfull feature was identified using site photos and topographical survey. The ratio of channel area at the predicted bankfull elevation to the cored bench elevation was used to estimate the likelihood that the cored bench was indicative of a regional bankfull feature. If the predicted channel cross-sectional area was within 50% to 150% of the calculated value, the cored bench was assumed to correspond to the regional bankfull elevation. Magnitude of discharge for flow depth at the elevation of the sampled bench was subsequently calculated.

Estimating Recurrence Intervals. Flow recurrence intervals were calculated based on United States Geological Survey (USGS) empirical equations for ungaged rural streams in Ohio (Koltun and Roberts 1990). The equations provide discharge estimates for recurrence intervals of 2, 5, 10, 25, 50, and 100 years. The approach was expanded by Ward and Mecklenburg (2005) to obtain discharge estimates for 0.25-, 0.5-, 1.0-, and 1.5-year recurrence intervals. These modified equations were used to estimate recurrence intervals of the calculated discharges at the top of the ditch (henceforth called ditchfull), at bankfull elevation of the inset channels, and if different, at the elevation of the sampled bench. No measured discharge data were available to calibrate the empirical equations; therefore, discharge versus recurrence interval relationships obtained from the USGS empirical equations should be considered only as an indicator of these relationships.

Bankfull discharge for each inset channel was calculated using Manning's equation together with the continuity equation. Manning's roughness coefficient values were based on a resistance factor and hydraulic radius (Thorne et al. 1997). The resistance factor term was in turn calculated by a variation of the Colebrook-White equation proposed by Hey (1979). The resistance factor is based on the hydraulic radius, maximum depth at the reference flow elevation (bankfull and ditchfull elevations), and the [D.sub.84]th size fraction of the bed material. The estimated Manning's value was verified at one site (site 13) that was instrumented with a stage-velocity measuring device (Teledyne-ISCO 750 area velocity flow module). The particle sized based estimate of Manning's roughness value (n = 0.023) at bankfull stage was consistent with a roughness value derived from measured flow velocities at bankfull stage (n = 0.022). Estimated Manning's values for the 13 study sites ranged from 0.018 to 0.026. This is consistent with Chow's (1959) lower limit for earthen, winding, and sluggish channels with no vegetation, which is 0.023. Calculated bankfull discharges at each site were then related to the recurrence interval relationship for that site.

Variability in Bench Elevations. At five sites, there were distinct bench features at more than one elevation, creating an element of uncertainty in the choice of bankfull elevation. Reasons for the occurrence of more than one bench at a site are unknown. The largest bench feature at a site was chosen for core extraction. Following sampling and preliminary analysis it was determined that the sampled bench at some sites did not correspond to bankfull discharge based on regional trends for agricultural watersheds. It was important to determine the position of the bench with respect to the bank-full elevation, as its relative elevation could significantly affect patterns of sediment entrainment and deposition within a channel (Rumsby 2000). Discharges above bankfull elevation in a stable channel would result in bench inundating discharges, increasing the probability of sediment-bench interactions. For a down-cutting channel, the same discharge and sediment discharge would be contained wholly within the channel, precluding sediment interaction with the bench surface. To account for this possible variation in the frequency of inundation of the sampled bench feature, a recurrence interval corresponding to the sampled bench was calculated at all the sites. Following an initial analysis with all the sites and all the cores, much of the further analysis was only based on sites that were consistent with geomorphological concepts.

Evaluating Shear Stresses on the Bed of the Inset Channel. Average boundary shear stress imposed on the channel bed was evaluated for the bankfull discharge of the inset channel and for ditchfull discharge, to see which flow most likely influences bed material texture. The average boundary shear was estimated as [[tau].sub.0] = [gamma]RS, where [gamma] is the specific weight of water, R is the hydraulic radius at the reference flow depth, and S is the channel slope expressed as a fraction. By assuming that the entire boundary shear stress was available to entrain bed material, a reference particle size was calculated based on the average boundary shear at a reference stage using Shields criterion (Knighton 1998; Simon and Castro 2003). A critical dimensionless shear stress of 0.06 for hydro-dynamically rough beds was adopted as per Simon and Castro (2003). This method of calculating a reference particle size is only an approximation of the complex processes that govern particle entrainment but has been found by the authors to be useful in evaluating some transport processes in Midwestern streams and agricultural channels (Powell et al. 2007). The heterogeneity of bed material measured at the sites, particle imbrication, and momentum losses due to hydraulic roughness are not accounted for with this approach.

Sample Collection and Analysis. Four cores were extracted from each bench, one from each of the following positions: upstream, downstream, near channel, and near bank (figure 3). Dimensions of the bench and location of the core samples were determined by measuring the cross-section of the ditch at three transects that ran laterally across the bench. One transect went through the upstream core location and another through the downstream core location. The third transect was across the middle of the bench, running through both the near-channel and near-bank core locations (figure 3). Cores were extracted using a 61 cm (24 in) long split-spoon sampler with a 7.6 cm (3 in) inner diameter. The sampler was driven into the bench using a hydraulic post driver. The length of the extracted soil core and the length of the resulting hole were measured in order to estimate any compression of the soil core. Soil cores were then wrapped in a plastic layer and transported to a laboratory for analysis.

Sample Preparation. All cores were photographed and measured in the laboratory, prior to division. Each core was divided into smaller samples at discernable changes in soil texture or 10 cm (3.9 in), whichever resulted in a smaller sample. A distinct color change only occurred in the bottom of some cores if the core extended into "C" horizon parent material. The section length and its position with respect to the top of the core were recorded. Sectioned cores were placed in brown paper bags and allowed to dry. The air-dried samples were crushed using a PVC mallet; large particles present in the sample were manually removed and added back after crushing and grinding. The crushed sample was further ground in a soil grinding machine and sieved through a 2 mm (0.08 in) sieve. The fraction retained on the 2 mm (0.08 in) sieve and the larger particles removed prior to grinding were combined to represent the [greater than or equal to]2 mm ([greater than or equal to] 0.08 in) fraction of the sample. The fraction passing the 2 mm (0.08 in) sieve was treated to remove organic matter and soluble salts. The size of particles and their distribution by weight were determined as per ASTM standards (ASTM D422-63: Standard Test Method for Particle-Size Analysis of Soils 2002). Particle size distributions for 308 samples were facilitated by the GRADISTAT program (Blott and Pye 2001).

Grain Size Analysis. Sedimentary samples tend to be poorly sorted and comprise samples with long tail distributions (Blott and Pye 2001). The arithmetic mean is influenced by long tail distributions and can lead to erroneous estimates of mean particle size of a sample. The geometric mean calculated by the Folk and Ward (1957) graphical method is a more robust measure of mean particle size and was therefore chosen. The percentage composition of a core sample comprising silt/clay (combined fraction) and sand and gravel was also calculated from the grain size analysis.

Normalizing Sample Depth. The ability to compare samples from the soil cores taken at all 13 sites was complicated by within site variation in bench micro-topography, unequal core lengths, and between site variation in inset channel and bench dimensions. In order to compare samples derived from benches of varying sizes, a non-dimensional normalized depth coordinate was derived for every sample. The normalized depth reflected a samples vertical location between the top of the bench and the elevation of the thalweg for the inset channel. Normalized depth was calculated as the ratio of a sample's vertical distance from the highest sampled bench elevation at a site: the vertical distance between the highest sampled bench elevation and the elevation of the thalweg. The normalized depth of the highest sampled surface at a site was 0, and that of the thalweg was 1. Samples obtained from elevations below the thalweg had normalized depths that exceeded a value of 1 (figure 4).

Statistical analysis of grain size distribution. Dependent variables tested were mean particle size in each sample and percentages of silt/clay, sand, and gravel in the sample. The position of core extraction from the bench and the normalized depth of that sample were the independent variables. As the data did not meet the assumptions of normality, the non-parametric Kruskal-Wallis test was used to test if bench composition varied significantly with sample position and depth from the surface. Non-parametric Mann-Whitney test was used to test differences between any two categories; a Bonferroni correction was applied to the p-value to adjust for multiple comparisons. The data were divided into six categories based on normalized depths of 0 to 0.25, 0.25 to 0.50, 0.5 to 0.75, 0.75 to 1.0, 1.0 to 1.2, and greater than 1.2 corresponding to the absolute depths shown in table 2. The SYSTAT software package was used to carry out all statistical tests (SYSTAT 2004).

Results and Discussion

The depth of the ditches ranged from 1.3 to 2.3 m (4.3 to 7.6 ft). The range in the top width of the ditches was 7.7 to 18.8 m (25.3 to 61.6 ft). Visual interpretation of digital orthophotos for the five rectangular areas resulted in an assessment of 253 km (157 mi) of agricultural channels to determine the extent of bench and inset channel formation. Some level of inset channel development and bench formation was exhibited by 59% of the ditches. Figure 5 shows the areas evaluated, an example of the extent of benches and inset channels for one area, and an example of the detail that was visually interpreted from the orthophotos.

Channel Characteristics. Average channel dimensions at the elevation of the sampled bench and characteristics of measured bed material are presented in table 3. Average width of the benches was 1.6 m (5.23 ft) (sd = 0.5 m [1.64 ft], n = 13), and average height of a bench above the channel bed was 0.53 m (1.74 ft) (sd = 0.24 m [0.79 ft], n = 13).

A power regression function was fitted to the data representing channel cross sectional area at the elevation of the sampled bench versus the drainage area (figure 6A). The regression line (not shown) only explained 7% of the variability in the data. The scatter in the data relating bench elevations to drainage area suggested that, at some sites, the sampled bench feature did not necessarily reflect current bankfull hydraulic geometry. This observation was supported by the fact that recurrence intervals for discharges corresponding to the elevation of the sampled benches showed considerable variability, ranging from 0.2 to 5.8 years (table 4).

A comparison of the bankfull inset channel cross sectional area predicted by the northwest Ohio-Minnesota regional curve with the inset channel cross sectional area measured at the sampled bench showed that at eight sites the inset channel cross sectional area was within 50% to 150% of the predicted bankfull cross sectional area (figure 6A). This range of variability is consistent with other Ohio regional curves with coefficients of determination exceeding 80%: for example, 53% to 381% (Sherwood and Huitger 2005) and 65% to 171% (Witter 2006). Benches sampled at these eight sites were then categorized as corresponding to the local bankfull feature. Channel cross sectional area associated with the benches at site 9 was less than 50% of the predicted bankfull channel area. These benches were poorly formed and the recurrence interval of the bankfull discharge for the inset channel was only 0.2 years (table 4). Visual observations in early spring showed mass wasting of the bench features. High frequency of bench inundation and evidence of mass wasting suggest an element of instability in bench formation at this site. Examination of the surveyed cross section at site 9 revealed a grade break at a higher elevation that was consistent with a bank-full channel cross sectional area predicted by the northwest Ohio-Minnesota regional curve. The recurrence interval associated with discharge at the elevation of this grade break was 0.5 years, which is consistent with reported bankfull recurrence intervals found in low gradient agricultural channel systems in Ohio (Jayakaran et al. 2005).

At sites 1, 2, 6, and 7, channel cross sectional area associated with the bench was greater than 150% of the predicted bankfull area for the inset channel. Sites 2, 6, and 7 had lower bench features that were less prominent but distinct. These lower and smaller benches were selected as likely bank-full features for that site. Figure 7 illustrates a cross-section for site 6 showing the bench sampled and the lower bench that was more consistent with the inset channel bankfull area predicted from a regional curve. Site 1 lacked an alternative discernable bankfull feature. At this site, channel cross sectional area at the elevation of the sampled bench was almost five times the area predicted by the regional curve suggesting that the bank-full elevation was probably well below the sampled bench elevation.

A new curve relating channel cross sectional area at the elevation of the bench feature with drainage area was developed based on the following: site 1 was not used because the bench feature was considered unrepresentative of the regional bankfull feature; site 9 was not used as the bench sampled at this site was unstable and therefore was not representative of a stable bankfull feature; at sites 2, 6, and 7 the lower unsampled bench was chosen. The analysis was based on the original sampled bench features for the remaining 8 sites. A scatter plot of the revised data is shown in figure 6B. Bankfull discharges and the associated recurrence intervals at the revised bench feature are tabulated in table 4.

A power function relating inset channel dimensions at the elevation of the bench to the drainage area explained 72%, 74%, and 77% of the variation in the log transformed data for bankfull depth, width, and area, respectively. Regression coefficients and exponents for the power functions at the 11 sites (sites 1 and 9 omitted) were 0.21 and 0.36, respectively, for bankfull channel depth and 1.33, and 0.30, respectively, for bankfull width. Regression coefficients and exponents are numerically similar to those derived from bankfull measurements made on 18 low gradient agricultural channel systems in Northwest Ohio (Jayakaran et al. 2005). Regression coefficients and exponents in that study were 0.21 and 0.3, respectively, for bankfull depth and 1.31 and 0.37, respectively, for bankfull width. The congruence between regression coefficients and exponents from the two studies suggests similar channel-forming discharge regimes.

Bed Material. The distribution of bed material, based on Wolman pebble counts at all the sites, showed poor sorting and positive skewness (i.e., a long tail in the coarser fractions). The median bed material ([D.sub.50]) was generally finer than the sampling limit of 2 mm (0.08 in), while the [D.sub.84] size was in the fine gravel range (mean = 8.9 mm [0.35 in], sd = 5.8 mm [0.23 in], n = 13).

Unit stream power, average boundary shear stress, and a reference particle diameter were calculated for ditchfull discharge and for discharge at the bench elevations (table 5). For ditchfull discharges, a linear regression function explained only 10% of the variation in [D.sub.84] with the average boundary shear stress. All of the ditchfull discharges at the sites in Mercer County had recurrence intervals larger than 100 years; the site in Fayette County had a recurrence interval of nearly 60 years. For bankfull discharges in the inset channels, a linear regression function explained 84% of the variation in [D.sub.84] with average boundary shear stress on the channel bottom (figure 8). For flow at bench elevation, the reference particle diameter derived from the average boundary shear stress was very similar to the [D.sub.84] particle size of bed material (figure 8). Bed material characteristics, unit stream power, and the reference particle size for flow at the elevation of the bench feature are tabulated in table 5.

Bench Materials. Average core length was 51 cm (20.08 in) (sd = 9 cm [3.54 in], n = 56), and average core compression was 10.7% (sd = 10%, n = 56). Most cores terminated just below the elevation of the channel bed where a hard pan layer with high clay content was encountered. Typically, at depths of 1.4 to 2.0 m (54 to 80 in), Blount soils in the region have a very firm "C" horizon of brown clay loam that also contains about 10% gravel. At depths of 1.3 m to 1.8 m (52 to 70 in), Pewamo soils have a "BC" horizon of yellowish brown silty clay loam. From 1.8 to 2.0 m (70 to 80 in) they have a "C" horizon of brown silty clay loam with both horizons containing about 5% rock fragments. In the NRCS County Soil Surveys, information is not reported for depths below 2.0 m (80 in) but from our cores, it appears these layers sometimes extend below 2.3 m (7.55 ft).

The mean particle size of all bench samples was 0.081 mm (0.003 in) (sd = 0.2 mm [0.008 in], n = 308), which classified as fine sand on the Udden-Wentworth scale (Udden 1914; Wentworth 1922). Average median particle size or [d.sub.50] for all bench samples was 0.103 mm (0.004 in) (sd = 0.32 mm [0.012 in], n = 308). (Note: Lowercase "d" as in [d.sub.50], used to describe bench material; upper case "D" as in [D.sub.50], used to describe bed material). Average silt/clay content in the benches was 66.0% (sd = 18.7%, n = 308), while sand and gravel content was 27.7% (sd = 13.9%, n = 308) and 6.3% (sd = 10.3%, n = 308), respectively.

Results of the Kruskal-Wallis test showed that the mean particle sizes of the bench samples were significantly different when grouped by normalized depth (Kruskal-Wallis test statistic H = 55.35, p < 0.01) and position on the bench (Kruskal-Wallis test statistic H = 21.23, p < 0.01). Examination of figure 9A shows that the smallest mean particle sizes are associated with the upper most samples obtained from the bench. The mean sample size of particles from this layer was in the coarse silt range of the Udden-Wentworth scale (Udden 1914; Wentworth 1922). There was a general coarsening of the samples in the lower half of the bench, normalized depths from 0.5 to 1.0, up to the elevation of the channel thalweg. Samples derived from below the channel thalweg appear to fine again reflecting the composition of hard pan soils encountered at this elevation at every site. Mean particle sizes associated with samples obtained from closest to the channel were significantly larger than those obtained from other positions on the bench (Mann-Whitney test statistic U = 10970.0, p < 0.005). Samples obtained from closest to the bank were significantly finer than samples obtained from other positions on the bench (Mann-Whitney test statistic U = 6399.5, p < 0.005).

Figures 9B and figure 10 show that the mean particle sizes are coarsest nearest the inset channel and finest near the bank of the ditch. As might be expected, there is little difference between the upstream and downstream sites, and the mean particle sizes for these sites fall between the values near the inset channel and bank of the ditch.

Summary and Conclusions

Elevation of the benches was associated with discharge events ranging from 0.3 to 0.5 years at 8 of the 13 sites. From the remaining 5 sites, site 1 appeared to have an anomalously large channel cross sectional area, and site 9 appeared to have a very small cross sectional area. A unique characteristic of site 1 was the presence of a small town in the upper section of this 1.86 [km.sup.2] watershed. Channel incision and widening due to increased peak discharges associated with land use changes could explain the enlarged channel area associated with the bench. This hypothesis is supported by the fact that the [D.sub.84] of the bed material at this site is the largest of all the study sites, indicating high shear stresses due to increased peak discharges (Booth 1990). At site 9, it was deduced that the benches were unstable and possibly still in the process of building. A distinct grade break was observed above the bench features, which was consistent with the elevation of a bankfull predicted by the northwest Ohio-Minnesota regional curve. Site 9 was also one of the two sites that were recently maintained under the authority of the County Engineer's Office. Site 9 was maintained in 1993. Alternative lower, less prominent benches were identified from inspection of the cross-section survey at the remaining three sites after it was determined that the original higher benches identified in the field did not reflect a regionally consistent bankfull feature. Channel dimensions of width and mean depth measured at the elevation of the bench were described fairly well by power functions that used drainage area as the sole independent variable.

The coarser [D.sub.84] bed material fraction had a better statistical relationship to the average boundary shear stress for flows at bankfull depth than at ditchfull depth. The statistical relationship between bed material and discharges at bench elevation indicate that the benches have some influence on bed materials found in ditches and that, in these ditches, channel-forming discharge concepts apply to the inset channel. Additionally, the reference particle size that was derived from the estimated average boundary shear stress at the bench elevation was numerically similar to the [D.sub.84] particle size of bed material.

An important aspect that was not quantified in this study is the effect of vegetation on the development of bench features. The percentage composition of organic matter in each sample could provide useful insight into the point at which the stabilizing effects of grasses come into play in the evolution of a bench feature. The banks and benches of all the ditches were vegetated with grasses. On the edges of the inset channel, there was often other vegetation such as cattails, bulrushes, lilies, and sedges.

The benches in our study are characterized by a distinct upper half comprised of fine material and a coarse layer just above the channel bottom. This finding is comparable to a top layer of finer material in benches along the Spoon River reported by Landwehr and Rhoads (2003). However, the bench materials were finer than materials studied by Landwehr and Rhoads (2003) in the Spoon River in East Central Illinois. Floodplain deposits analyzed in that study comprised 21% silt/clay and 67% sand fraction, compared to 66% silt/clay and 28% sand fractions in this study. The major parent soil map units (soil series and surface texture phase) in this study region were Pewamo silty clay loam and Blount silt loam, while the soil map unit near the Landwehr and Rhoads (2003) study site was Drummer silty clay loam. As per soil survey reports for these soil series, the silt/clay and sand contents for these parent soils are comparable so the formation of benches composed of finer materials in this study does not appear to be related to differences in the parent materials.

The upper half of the benches also comprised the most uniformly sized sediments suggesting vertical accretion of suspended sediments during overbank discharges. The highest sand content was in the region closest to the channel and closest to the elevation of the channel bed. A coarse layer at the elevation of the bed channel suggests that, at the inception of bench formation, a coarse layer formed the framework (Kondolf et al. 2003) upon which overbank vertical accretion promoted further bench building. This result is consistent with Landwehr and Rhoads' (2003) arguments that benches develop via vertical accretion on incipient bars comprised of bedload (i.e. bedload platforms). It is likely that this initial coarse layer may be derived from an initial slumping of the ditch bank just after construction or maintenance. In our study, the trend of upward fining of confined floodplains in a low-energy setting is in accordance with results reported by Nanson and Young (1981) for small low-energy streams in southeastern Australia. All benches showed fining with distance away from the channel. This conforms to published studies that show suspended sediment is transported away from the channel by diffusive turbulent discharges resulting in lowered transport capacity with distance from the channel (Bridge and Leeder 1979; Pizzuto 1987).

Grain size analysis of floodplain deposits have often been used to infer the principal form of sediment transport that formed the deposition feature. The three major forms of sediment transport in the inset channel of a river (suspended, saltating, and tractive transport) also contribute to the formation of floodplain features (Bridge 2003). They are generally associated with increasing orders of mean particle coarseness (Bravard and Peiry 1999). In typical floodplains, coarser particles tend to deposit closer to the inset channel; finer particles tend to be more uniformly distributed across the floodplain. Results for our study are consistent with published studies and suggest that depositional benches are associated with floodplain and channel forming processes that are found in more natural channel systems. Determining the effects of vegetation on bench development would be of great value in future stratigraphic analysis of benches.

Application to Soil and Water Conservation. The results provide useful insight on the development of inset channels and benches in agricultural ditches in Ohio. Knowledge that these features are associated with fluvial processes, such as the channel-forming discharge, is useful in developing alternative methods to maintain the drainage function of these systems. Such alternative maintenance methods could make agricultural ditches self-sustaining and enhance fluvial functioning, water quality and in stream biotic habitat as is the case with natural riparian systems. In a companion paper, a method for sizing two-stage geometry is described (Powell et al. 2007). In some cases, simply leaving these features intact might be the best solution as constructing a two-stage system or removing sediment and benches might have little influence on improving field drainage. Another new concept that is being adopted in Ohio is to excavate the ditch to an over-wide geometry that is related to bankfull width associated with a regional curve. It is expected that over time the ditch will build wide benches and a meandering inset channel.

Acknowledgements

The authors thank Dedra Woner. Phil Levison. Barry Allred, and Norm Fausey of the United States Department of Agriculture for making their expertise and laboratory facilities available. Assistance offered by Mike Podrosky. Janell Henry, Anora Bentley, and Trevor Ward with field work and laboratory analysis is greatly appreciated. Finally, advice and suggestions from Dan Mecklenburg of the Ohio Department of Natural Resources helped clarified aspects of this work. This study was made possible through funding provided by United States Department of Agriculture 406 Water Quality Initiative Competitive Grants Program. Support was also provided by the Ohio Agricultural Research and Development Center, the Ohio State University, and the Mercer County Engineer's Office.

References

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Anand D. Jayakaran is an assistant professor at Clemson University, Georgetown, South Carolina. Andrew D. Ward is a professor at the Ohio State University, Columbus, Ohio.
Table 1 Year of construction and year of last maintenance of the ditches
that comprise the 13 sites analyzed in this study.

Site  Year of construction  Year last maintained

 1    1937                  1937
 2    No records            No records
 3    1872                  1948
 4    No records            No records
 5    1934                  1958
 6    1881                  1947
 7    No records            No records
 8    No records            No records
 9    1954                  1993
10    1872                  1950
11    1934                  1957
12    1934                  1993
13    No records            No records

Table 2 Categorization of normalized depth for analysis by ANOVA.

Normalized depth  Absolute depth group
(dimensionless)   mean [+ or -] se (cm)

0 to 0.25          7.7 [+ or -] 1.8
0.25 to 0.5       17.1 [+ or -] 1.3
0.5 to 0.75       29.8 [+ or -] 1.3
0.75 to 1.0       35.8 [+ or -] 1.6
1.0 to 1.2        41.0 [+ or -] 2.0
>1.2              42.1 [+ or -] 1.9

Table 3 Characteristics of the inset channel as defined by the sampled
bench.

      Drainage area  Mean bankfull dimensions of the Inset channel
Site  ([km.sup.2])   Depth (m)    Width (m)    Area ([m.sup.2])

 1    1.86           0.56         3.48         1.91
 2    0.28           0.22 (0.15)  2.09 (0.96)  0.48 (0.14)
 3    5.34           0.41         2.96         1.21
 4    1.06           0.21         0.92         0.19
 5    1.99           0.28         1.67         0.47
 6    0.65           0.35 (0.24)  3.80 (1.64)  1.34 (0.38)
 7    4.97           0.61 (0.47)  3.47 (2.45)  2.13 (1.15)
 8    4.25           0.42         2.51         1.05
 9    5.40           0.17         1.72         0.28
10    4.79           0.33         1.95         0.65
11    3.50           0.24         0.99         0.24
12    1.01           0.13         1.22         0.16
13    6.81           0.43         2.50         1.04

Note: Values for the lower unsampled benches at sites 2, 6, and 7 are
tabulated in parentheses.

Table 4 Discharge magnitudes and recurrence intervals at the sampled
bench elevations for all 13 sites.

      Bankfull discharge*     Recurrence interval*
Site  ([m.sup.3] [s.sup.-1])  (years)

 1    3.24                    5.7
 2    0.77 (0.16)             5.8 (0.5)
 3    1.50                    0.4
 4    0.16                    0.3
 5    0.49                    0.4
 6    1.20 (0.25)             4.0 (0.4)
 7    3.36 (1.49)             1.6 (0.5)
 8    1.44                    0.5
 9    0.25                    0.2
10    0.56                    0.3
11    0.34                    0.3
12    0.17                    0.3
13    1.25                    0.3

Note: Values for the lower unsampled benches at sites 2, 6, and 7 are
tabulated in parentheses.
* Values for the inset channel associated with the revised bench.

Table 5 Bed material characteristics, average boundary shear stress on
the inset channel bed, and characteristic particle diameter when flow is
at the elevation of the bankfull bench feature.

                             Channel
Site        DA ([km.sup.2])  slope    [D.sub.50] (mm)  [D.sub.84] (mm)

 1[dagger]  1.9              0.51%    <2               19.0
 2          0.3              0.76%    <2                4.4
 3          5.3              0.29%     3.7             10.0
 4          1.1              0.36%    <2                7.0
 5          2.0              0.26%    <2                4.1
 6          0.6              0.13%    <2                4.4
 7          5.0              0.35%     3.8             16.0
 8          4.2              0.50%     6.4             19.0
 9          5.4              0.40%    <2                5.5
10          4.8              0.16%    <2                4.7
11          3.5              0.24%    <2                3.6
12          1.0              0.80%    <2                6.0
13          6.8              0.30%    <2               12.0

                                Average boundary
            Unit stream power   shear stress (N   Reference particle
Site        (watts [m.sup.-2])  [m.sup.-2])       diameter* (mm)

 1[dagger]  47.7                22                23
 2          12.0                 9                 9
 3          15.0                10                10
 4           6.1                 6                 6
 5           7.6                 6                 6
 6           2.0                 3                 3
 7          21.0                13                14
 8          27.8                17                17
 9           5.5                 6                 6
10           4.5                 4                 4
11           5.3                 5                 5
12          11.0                 9                 9
13          14.8                11                11

* Based on the average boundary shear using Shields criterion (Knighton
1998).
[dagger] At site 1, unit stream power and particle size at incipient
motion was calculated for flow at the sampled bench.
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Title Annotation:SPECIAL SECTION: DRAINAGE DITCHES
Author:Jayakaran, A.D.; Ward, A.D.
Publication:Journal of Soil and Water Conservation
Geographic Code:1U3OH
Date:Jul 1, 2007
Words:7968
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