Flood Plain Maps and Alluvial Soils

 

The Great Colorado Flood 

Between 9 and 6 September 2013, heavy rainfall exceeding 450 mm, fell over large region of the Colorado Front Range foothills and adjacent plains. The rainfall was the most intense, widespread, and persistent rainfall along the Front Range occurred on 11 and 12 September. Flooding from rainfall in the mountain canyons is not an uncommon phenmenon. However, the 2013 floods were by far exceptional. Not only were flooding impacts felt in narrow mountain canyons, but flooding across the Front Range combined into a large-scale, multistate flood event as tributary waters swelled and flowed down the South Platte River onto the high plains across northeast Colorado and into Nebraska.

Several precipitation records were broken during the Great Colorado Flood. A USGS rain gauge at the ARMY base (Fort Carson), accumulated 301 mm of rain in a 24 hour period. The accumulation established a new daily rainfall extreme for the entire state of Colorado surpassing the long-standing gauge measurement of 281.4 mm, which occured on 17 June 1965. 

The city of Boulder set several records:

1 day -  230.6 mm 

2 days - 292.6 mm

3 days - 341.8 

7 days - 429.3 mm 

Month - 461.2 mm

Denver set a precipitation accumulation record for the month of September at 142.5 mm. Three of the top 10 largest 1 day rain events in state history will now be associated with the September 2013 storm. Additionally, according to analyses performed by the NOAA/Hydrometeorological Design Center, the annual exceedance probability for the worst case 24 hour precipitation was estimated to be less than 1/1,000 (NWS 2013a). This has led some to label the resulting flood as a “1,000-yr flood” event. The heavy rainfall resulted in over 1000 landslides and led to severe devastation throughout the state.

Alluvial Soils

The South Platte River and the underlying alluvial aquifer form an important hydrologic resource in northeastern Colorado that provides water to population centers along the Front Range and to agricultural communities across the rural plains. 

The South Platte River alluvial aquifer overlies the Denver Basin aquifer system along the valley of the South Platte River from Denver to Greeley. East of Greeley, alluvium along the South Platte River is held within an ancestral valley eroded into Cretaceous Pierre Shale. The underlying bedrock is generally much less permeable than surficial alluvium and forms a low permeability boundary to the alluvial aquifer. In the plains, which are areas east of the Rocky Mountain Front Range, sedimentary rocks underneath the alluvium often consist of shale and sandstone of Cretaceous age that form fairly broad, mildly sloping valleys. 

The South Platte alluvial is subject to a number of variable climate, hydrologic, geologic, and human factors interacting to create an extremely complicated set of conditions impacting the alluvial aquifer. In almost all years there is inadequate water in the river to meet all of the demands, and the excess flows that do occur occasionally are not always predictable in place and time.