Abstract

Steady-state and falling-head infiltration tests in Gordon Gulch, a forested upland catchment in the Colorado Front Range, show that rates exceed 50 mm hr -1 in most areas. Soil infiltration rates, measured using a double-ring infiltrometer at 42 sites in upper Gordon Gulch, correlate with field measurements of soil resistance and with laboratory values of soil sorting, loss-on-ignition (LOI), and soil composition. These four soil properties are significantly correlated with infiltration rates (pvalue0.05). Rates are lower, generally 14 mm hr -1 on dirt roads, trails and recently logged areas. Peak discharges in channels reflect precipitation intensity and the distributed infiltration rate determined by soil physical factors. Because neither rainfall intensity nor peak discharge have been measured in the vicinity of Gordon Gulch, rates had to be estimated using syntheses of existing data and simple runoff models. Precipitation intensities for the 1-hr storm were estimated at 27 mm for the 10-year storm, 42 mm for the 100-year storm and 46 mm for the 200-year storm. Peak discharges for the 0.93 km ² upper Gordon Gulch were modeled under various test scenarios using the rational runoff formula and ArcHydro. Calculated peak discharge values from the two models show both similarities and differences, depending on the model scenario. Impervious areas and human alteration of the Gordon Gulch environment have decreased natural infiltration rates and the models suggest that the largest contribution to peak discharge comes from areas covered by roads and trails, fire preventative logging and possibly from rock outcrops. Calculated peak discharge, which ranges from 0.22 m 3 s ^-1 for the 5-year storm to 0.80 m 3 s ^-1 for the 200-year storm when roads and trails are included, is a small fraction of total peak discharges measured in the 804 km² Boulder Creek catchment. Despite the influence of altered land use, peak flows from upper Gordon Gulch alone do not present any immediate flood danger in Boulder Canyon. However, if there was already significant flow in Boulder Creek, thunderstorms covering a few tens of km² of watersheds, like upper Gordon Gulch, could represent a flash flood hazard for Boulder Creek similar to that experienced east of Estes Park in the Big Thompson Flood of 1976, which killed 139 people.

Citation

Buraas, E. M. (2009): Getting water into the ground and to the channel, Gordon Gulch, Colorado. Thesis, Degree of Bachelor of Arts, Geosciences, Williams College 2009 .