Black Hawk/Central City Flood Response Plan (FRP) – Colorado Water Conservation Board
Black Hawk and Central City, Colorado

 In the 1990’s casino gaming in the towns of Black Hawk and Central City was instituted, which resulted in a daily “mobile” population of around 10,000 to 40,000 people combined within these two Cities. The Cities are located in a steep canyon with one two-lane road being the only access route. A majority of the structures are located in the FEMA defined 100-year floodplain.

A mudslide during the summer of 1998, after a thunderstorm rainfall of less than two inches, alerted Colorado Water Conservation Board (CWCB) to the flooding danger and a significant threat to life. CWCB selected HDR to develop an FRP to provide the towns with proactive actions to help reduce the threat for loss of life. HDR meteorologists developed a creative FRP that relies on active interaction of the local emergency response community, a community weather-watchers group, casino security forces, the National Weather Service (NWS) forecast office in Boulder, Colorado and a heavy rainfall potential tool to determine the flash flooding threat that can prompt the movement of people to safety.

Meetings were held throughout the project in order to keep County, City, State agencies, as well as the public informed of the status of the project. These meeting were also a forum for any questions or concerns to be addressed and to provide education of the flood threat in and around the area, and how the FRP would be utilized to realize this flood threat and initiate flood response activities. Levels of preparedness, identified in Figure 1, were developed that contained agency proactive flood related actions. NWS predictions and the specific advisory and warning products that they produce, and observations of heavy rainfall from trained weather spotters prompted these levels of preparedness. An innovative Heavy Rainfall Potential (HRP) tool was developed for use by response agencies to identify and assess heavy rainfall potential. The HRP tool uses temperature and dew point observations from the Pickle Gulch automated weather station, located about 8 miles north of the area. Temperature and dew point observations are compared to a matrix of temperature and dew point combinations that are color-coded based on the potential for heavy rainfall. The tool provides an awareness of a heavy rainfall and subsequent flooding threat. If the tool determines that heavy rainfall is a threat it can be used in concert with radar, and NWS predictions, to prompt levels of preparedness as heavy rain producing thunderstorms approach the area.

Due to the fast response basins (15-30 minutes) it was determined that there was not sufficient time to evacuate people out of the area. The decision was made to “move” people to the upper floors of the casinos using public address systems once an assessment was made that flooding was imminent.