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,; DRAFT Discussion of Stability Issues For Proposed Gravel Extraction Activities Adjacent to the Big Thompson River Simons & Associates 6/9/00 The stability of the proposed gravel pits adjacent to the Big Thompson River has been evaluated with respect to erosion. The concern regazding gravel pits is the potential for water to overtop the riverbank and flow into the pit adjacent to the river. Most of the time, the flow of water in the river is contained within the banks of the river. Occasionally, the riverbanks aze overtopped such that water may flow over the banks and into the pit. As the water flows into the pit (when the pit is not full of water), it accelerates down the slope into the pit potentially causing erosion and possibly diverting the flow in the river duectly into the pit if erosion was lazge enough to breach the embankment between the river and the pit. Various factors need'to be considered in evaluating this possibility. These include the frequency, duration, and magnitude of riverbank overtopping flow, the size of the pit, distance from the riverbank to the pit, and the chazacteristics of the riverbank material between the river and the pit. Hydraulic analysis shows that the bankfull conveyance capacity of the Big Thompson River in the vicinity of the proposed gravel mining activity ranges from about 3000~cfs to about 9000 cfs. The magnitude of bankfi,ll flow varies from location to location along the river, but this range represents the variable nature and general magnitude of baticfull flow. Flows that aze lazger than bankfull cause water to spread out over the floodplain. In the vicinity of the pit, the overbank flow may spill into the pit. For comparison, the 100-yeaz peak flow that has been used in the floodplain analysis is 19,000 cfs. Flow data were obtained from the USGS gage on the Big Thomson River at Loveland, Colorado (station number 06741510). This gage is located in the immediate vicinity of the proposed gravel mine, downstream of St. Louis Avenue. A flow duration curve was developed based on these data (see Figure 1). Flows greater than 4000 cfs occur about 0.0142 percent of the time (for an average of about .052 days per yeaz or about 1.25 hours per year). For the period of record at this gage, 1979-1998, the maximum flow was 4240 cfs. Flow hydrographs were plotted for the lazgest several peak flow events to get an idea of how long the flow might exceed bankfull flow for a given flood event. The available data, however, do not provide information on this issue since the flow for the period of record has not exceeded bankfull flow except perhaps in the most constricted or lowest conveyance cross-sections along the river in this vicinity.