FLOOD07356 - Laserfiche WebLink

Home Browse Search

RIO GRANDE LEVEE IMPROVEMENTS ALAMOSA, COLORADO MULLER ENGINEERING The Corps of Engineers agrees with the design concept. The following are comments and recommendations: 1. The HEC-2 water surface profile analyses should be performed for the peak discharge throughout the study reach for the frequency being analysed, i.e., 10,900 cfs for the 100-year analysis. 2. The riprap design and proposed thicknesses are satisfsctory. a. We recommend increasing the rock size and gradation, if available to that shown in COE ETL-lllO-2-l20, (copy furnished to Muller Engineering Co., Inc.) for a 12 inch thickness and specific weight of l65#/CF. It will provide additional protection against debris, and ice, and direct attack by meandering flows. b. We agree with the suggestion on page 2 of 2 of the design notes and computations, dated 2/21/86, of increasing the riprap thickness at the railroad bridge. The same should be done at the U. S. 160 bridge. We recommend extending the riprap upstream of this bridge on sheet 2 of 3 of the plans. 3. We encourage maintenance that will reduce forces against the levees aa mentioned dn page 5 of the report. 4. We recommend installation of slide gates on the storm sewer lines crossing the levees. 5. The maximum height of levee was not given and no mention was made of an inspection trench which would expose undesirable underground features such as buried logs, animal burrows, pockets of unsuitable material, or other debris. Side slopes should be no steeper than lV:lH and the trench depth should be a minimum of six feet (for embankments less than six feet in height, the minimum depth should equal to the embankment height). Inspection trenches should be excavated under all levees unless landside toe drains are constructed beneath the levees, to depths comparable to that required for inspection trenches. 6. Considering the permeability of a gravelly sand foundation material, the ability of 1.5-foot wide X 2.0 feet deep toe trench to intercept the seepage is questionable. Alao in the geotechnical report, fig. 3 shows filter sand surrounding the gravel drain material (which is surrounded by geotextile fabric). It is recommended that the entire toe drain be enclosed by filter fabric and only one drain material be utilized, to prevent the plugging of the drain. Also, the toe drain should be extended in depth so as to intercept foundation seepage effectively. Sufficient filter material is required between the perforated PVC pipe and the foundation material. It is recommended that a minimum trench width of two feet be utilized to achieve effective drainage.