COLORADO FRONT RANGE STREAMFLOW, GEOMORPHOLOGICAL HAZARD AND RAINFALL RELATIONSHIPS, SEPTEMBER 11-13, 2013

The Colorado Front Range Flood of September 2013 was an uncommon meteorological and hydrological event. Studying the dynamics of this storm and the resultant flooding provides an opportunity to improve our understanding of the flooding process, the geomorphological hazard and the link between extreme precipitation, basin response, and runoff. Our investigation collected stratigraphic and radiocarbon data in six basins along the Front Range between Coal Creek Canyon and Estes Park to evaluate and compare the magnitude and return period of the flood.

In terms of rainfall intensity, the September 1938 and June 1965 storms produced similar spatial and temporal precipitation patterns with very large rainfall totals. A unique feature of the September 2013 storm was the occurrence of mesolows that aided in focusing intense precipitation over small spatial domains for several hours. In many locations, the precipitation amounts were estimated to be equal to or above a 1 in 1000 year event. However, the flooding response was not proportional (i.e., not AEP neutral) as many locations experienced floods estimated to be equal to or less than a 1 in 100 year event.

Topographic surveys of the channel geometry and elevation data for high water marks were collected to facilitate the accurate reconstruction of flood peak discharges which were computed using in most cases a single cross-section slope conveyance method and at one site a 1D-HEC-RAS model. These data were used to make flood frequency estimates for the September 2013 flood using historical records. Radiocarbon ages were collected and used for age determination of past flood deposits at most sites as well as the interpretation of non-exceedance bounds where stable surfaces were identified. The results of this study highlight the differences in extreme floods between adjacent basins that appear to correlate better to the physical characteristics of the basins rather than specific characteristics of the storm. Based on flood frequency and discharge results the 2013 flood can be associated to a return period as low as 25 and 50 years on the Middle Saint Vrain and Coal Creek drainages respectively, while on the North Fork of the Big Thompson, the South St Vrain and Left Hand drainages the return period varies respectively from 150 year, to 200 years and 200 to 350 years.