Regional Analysis of Trends In Vertical Adjustment, Bank Erosion and Effectiveness of Erosion-Control Measures: Returning To The Scene In West Tennessee 40-Years Later

The channels of West Tennessee have undergone systematic adjustments to their geometry, including incision and widening, over much of the 20th century. As a result, bank erosion from these channels has become a significant source of sediment in the region. The principle aims of this study were to evaluate region-wide rates of bank erosion in the context of the effectiveness of potential erosion-control measures to reduce bank erosion along channels of West Tennessee. Regionalization of channel characteristics (extracted from LiDAR at 155 sites), boundary materials (from in situ field measurements at 80 sites) and a 13-year flow series (derived from records at 19 gaging stations) were used as input into the dynamic version of the Bank-Stability and Toe-Erosion Model (BSTEM-Dynamic). As documented in region-wide work from the 1980s and 1990s and verified here, rates of channel adjustment were determined to be far less today than they were in the second-half of the 20th century. Along main-stem channels, additional degradation and, consequent increases in bank heights are expected to be limited, with many reaches experiencing aggradation and slowly decreasing bank heights. Modeling runs were initially conducted for “existing” conditions, representing a “baseline” by which to later compare erosion rates under alternative mitigation strategies. In this regional approach, it was important to conduct the numerical analyses using bank-resistance values that represented not only the median value from each basin (50th percentile), but to include a broader range using the 25th and 75th percentiles from each basin. Median erosion rates for the 25th, 50th and 75th percentiles of bank resistance varied from 51.6 to 0.3 and to 0.0 m3/m/y, respectively. Considering peak values (the 99.99th percentile) as a loading rate (in t/m/y), bank erosion varied from about 3.8 t/m/y for the 75th percentile of bank resistance (stronger materials) to 265 t/m/y at the 25th percentile (weaker materials). Modeled results were interpolated and extrapolated to non-modeled reaches based on basin-specific regression equations (r2-values ~ 0.80) relating modeled bank-erosion rates to a metric defined as the area-gradient index (AGI) times excess boundary-shear stress at the “bankfull” discharge. Analysis of the potential for reductions in bank erosion (effectiveness) were limited to three broad measures: vegetation alone, rock at the bank toe and rock along the entire bank surface. Results of this study show that in general terms, the placement of rock or other resistant materials at the bank toe is the most effective means of achieving significant reductions in bank erosion along the stream channels of West Tennessee. In those reaches that produce high rates of erosion, the rock-toe option also represents the most cost-effective means of achieving water-quality improvements with regard to sediment concentrations and fine loads. Vegetation has also been shown to be not only effective, but cost effective as well under particular circumstances where bank heights are not overly high and erosion rates are, therefore, moderate. The AGI metric was also used to map and regionalize the effectiveness of erosion-control measures.