Laserfiche WebLink
change is the cutoff within the levees at river station 53.75. Between 1963 and <br />1998, the meanders in the upstream portion of the reach continued to laterally <br />migrate. Downstream, the channel began to avulse within the levees and in some <br />case developed small meanders. Lengthening of the channel can be attributed to <br />several factors. The aerial photographs and USGS quadrangle maps illustrate the <br />numerous abandoned oxbows within the reach. After the levees were completed <br />and the channel was shortened, the low-flow channel attempted to gain back some <br />of its lost length by meandering in the narrow zone between the levees. Another <br />potential factor is the general increase in annual peak discharge from 1968 to <br />present, as shown by the Alamosa Gaging Station data. When discharge increases, <br />the system attempts to maintain a balance by reducing slope (e.g., lengthening of <br />the channel). Channel lengthening is compounded by the slope reduction in the <br />channel due to aggradation, .....". <br />Channel Stability Index: <br />"3.3.5 Channel Stability Index <br />The river inventory and resulting GIS product documented characteristics of the <br />left and right riverbanks. For each homogenous bank stretch, the percentage of the <br />bank that was eroded and available for additional erosion due to a lack of surface <br />protections was approximated. This percentage was approximated as the amount <br />of bank area consisting of silts, sands, and gravels that were unprotected and <br />could be easily eroded during flood events. For example, if about 30% of the total <br />area of a bank was covered by riprap, willow, or river cobbles, but the remaining <br />bank was bare and consisted of erodible sandy gravel, then a rating of 70% was <br />noted. This degree of bank erosion could be considered the inverse of the percent <br />surface protection using Dave Rosgen's Bank Erosion Hazard Index (BEHI) <br />characterization scheme. <br />A Bank Erosion Potential Index was calculated for every homogenous stretch of <br />river by multiplying the percent of the bank available for erosion by the bank <br />height within the GIS. Therefore, this index is directly analogous to the height of <br />the bank available to erosion. The index was multiplied by several factors to <br />adjust for other bank characteristics. Flat and medium sloped banks were <br />multiplied by 0.8 as these banks were not considered as prone to erosion; banks <br />consisting of or protected by bars were multiplied by 0.5; the bank was multiplied <br />by 0.9 if it had a top covering of well established willows, and a 1.2 was <br />multiplied if it was noted that the river thalweg directly impacted the bank <br />(although this was not noted consistently in all stretches). <br />For the 1963 and 1998 imagery, centerlines were constructed in the GIS at the <br />exact midpoint between the right bank vertices and the nearest left bank edge. <br />Then, for each vertex of the 1998 centerline, the distance and direction was <br />calculated to the nearest edge of the 1963 centerline. Therefore, at approximately <br />one-tenth of a mile interval along the entire project reach, a distance was <br />calculated that represented the distance that the river moved, or meandered, <br />between 1963 and 1998. The 1941 imagery was not included in the meander <br />migration analysis because the 1941 imagery is of much lower accuracy. <br />Channel stability characterization should consider both the current erosion <br />potential of a bank and the distance that the bank has been moving horizontally in