4 Geologic Hazards of the Rocky Mountains and Great Plains

Wednesday, 21 April 2010
10:20 AM - 12:00 PM
Ponderosa Room

THE HERMOSA FLOOD OF AUGUST 17, 2007: EXTENT, EFFECTS, AND COMPARISON TO FEMA FLOOD INSURANCE RATE MAP

Paper number: 4-3
Program: Theme Sessions
Time: 11:00 AM – 11:20 AM

Authors

Abstract

In late afternoon on August 17, 2007, a strong storm cell stalled over the eastern edge of the Black Hills, centered four miles west of Hermosa. In about four hours, four to 12 inches of rain fell in ~23 square miles of Battle Creek drainage basin and four to ten inches fell in ~11 square miles along Grace Coolidge Creek. By 9 p.m. flood depths reached 16 ft along Battle Creek canyon west of Hermosa, 17 ft at the railroad bridge south of town, and six to 14 feet east of the village.

The FEMA FLOOD INSURANCE RATE MAP shows two categories for the flood plain: Zone A (100 yr flood): Zone B (100-500 yr flood). Along Grace Coolidge Creek, the flood occupied a portion of Zone A (terrace I: 4-6 ft above normal flow): Along Battle Creek, it covered all of Zone A and part of Zone B (the middle terrace {II: 11-12 ft} and portions of the upper terrace {III. 17-29 ft}in areas immediately downstream of abrupt meander bends). Flooding produced extensive damage to structures, but no loss of life, in predicted flooding areas shown on the FEMA map. Field-based mapping of bent grass and twigs, trapped debris, scoured surfaces, floated objects, e.g., cars, horses, etc. show that the earliest and latest flows paralleled the stream bed on terrace I, but not on II and III. Rupture of a railroad embankment and release of a 10-foot depth of water produced pressure sufficient to float and move houses up to one mile.