HYDRO28913 - Laserfiche WebLink

Home Browse Search

• Presented in the Guidelines aze the minimum requirements proposed for gravel rr;ining operations located in or adjacent to 100-yeaz floodplains. The Guidelines are intended to help protect property owners adjacent to river and streams from unreasonable ha::ard resulting from river or stream instabilities. The Guidelines were originally designed to protect South Platte River and related properties and infrastructure from adverse impacts resulting from the extraction of gravel in or near the river by providing guideline:; for maintaining its stability and protecting overbank azeas from catastrophic failure. As a result the Guidelines aze directly applicable to operations along the South Platte River. However, they also provide initial guidance for operations along its tributaries, but may require modifications on a case-by-case basis to recognize differences in hydrology and site conditions. The Guidelines are not intended to supersede the Rules and Regulations for Mining Operations in the State of Colorado adopted by the Colorado Mined Land reclamation Board (34-32-108, CRS) but rather to supplement them. For the proposed Western Mobile, Inc. Lyons Pit the appropriate sections of the Guidelines have been used where applicable, and modified if it has been determined necessary in order to address site-specific conditions. III. Existin¢ St. Vrain Creek and Fooodplain CJ Existing_River Conditions The St. Vrain River drains a basin of over 219 square miles above the Lyons, wish 125 square miles on the North St. Vrain Creek and 94 square miles on the South St. `/rain Creek. The two tributaries have a confluence within the town of Lyons approxirr;ately 1.5 miles upstream from the proposed mine site. The basin topography changes from forested mountain terrain on the west, to rolling plains on the east, which are primarily p;~sture and cultivated fields. As shown on floodplain mapping (see Figure 1) the St. Vrain floodplain is const!~ained by topography upstream of U.S. 36 (Foothills Highway} into a relatively narrow width of 400 to 600 feet. After passing under the U.S. 36 bridge however, the floodplain spreads out to a width of almost ] mi]e near the eastern boundary of the proposed mine <.,ite. This floodplain pattern is similar to what is known as an "alluvial fan". During a flood event, as the heavily debris and sediment laden floodwaters transition from the steeper channel grades of the foothills onto the flatter grades of the plains, the decrease inflow velocities cause deposition of the debris and sediments. This deposition of materials creates a broad fan-shaped and relatively flat floodplain. The remnant meander scars that exist throughout the mining site are evidence of past alignments of the creek's main and split flow channels. • It appears that the main channel of the St. Vrain in the area of the proposed min: has