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I I I I I I I I I I I I I. I I I I I I INTRODUCTION The report, "Flood'Plain Information, Breckenridge, Colorado" presents data on flood hazards along the Blue River in the vicinity of Breckenridge, Colorado. The report defines the flood plain of the Blue River through the town of Breckenr idge and presents discharges for the la-year, 25-year, Intermediate Regional Flood, and Standard Project Flood at key points down- stream from the town on the river. The purpose of this addendum is to provide basic backup material used in the Breckenridge flood plain report. The appendix contains var ious sample computer printouts and other data relating to the hydrologic and hydraulic determinations. The secondary purpose of this addendum is to provide guidance to the town of Breckenridge and Summit County for use in their plan- ning efforts along the Blue River. Specific recommendations are contained in this addendum relating to the capacities of the ex- isting bridge and culverts crossing the Blue River, as well as recommendations for future crossing structure capacities and channel improvements. The basis for recommendations and other planning information is the Intermediate Regional Flood (IRF), or the lOa-year flood. This flood has a 1% probability of being equalled or exceeded during anyone year. The Intermediate Regional Flood is used by the Flood Insurance Administration and is generally accepted as the design flood for flood protection in the United States. It must be recognized, however, that floods of greater magnitude may occur. For this reason, the Standard Project Flood has also been delineated on the flood plain maps. The runoff from a Standard Project Flood is that runoff generated by a severe combination of meteorological conditions reasonably character- istic of the hydrologic region, excluding extremely rare combina- tions. The Standard Project Flood (SPF) as used in this report is generally considered as having a recurrence interval of approx- " imately once every 500 years. The Standard project Flood is not the largest flood that can occur in the study area. For example, the design flow used in the investigation for the safety of a high hazard dam is that flow derived from the Probable Maximum Storm. This storm is characteristic for any given region and is dependent on a combination of physical and meteorological ~ Leonard Rice Consulting Water Engineers. Inc.