Mitigation

Can the 1972 Flood Happen Again?

The answer is yes. The 1972 flood has an estimated recurrence interval of 500 years (Burr and Korkow, 1996), which means that a flood of this magnitude will occur on average once every 500 years. Every year there is a 0.2 percent chance (1 in 500) of experiencing a similar flood. Because of the short period of record for Rapid Creek gaging stations, the recurrence interval of 500 years may be revised substantially as additional data are collected. “Floods are natural and normal phenomena. They are catastrophic simply because man occupies the flood plain, the highway channel of a river (Bue, 1967).”

Today’s Flood Protection and Warning

In the aftermath of the 1972 flood, interim and long-range programs were initiated and millions of federal dollars were spent in Rapid City and the surrounding stricken communities (Rahn, 1984). Rapid City approved a flood-plain management program, known as the “greenway” concept, whereby most of the flood plain was converted into large parks. However, Rapid City and the surrounding communities are susceptible to extreme flooding because of their location in and around the Black Hills. Most floods in the Black Hills area are caused by intense rainfall over very steep watersheds, which allows little time for warning residents of flood threats.

In 1997, a flood-warning system was implemented by the USGS in cooperation with the Rapid City-Pennington County Emergency Management and the National Weather Service (NWS). Combination precipitation/streamflow-gaging stations monitor rainfall and subsequent rises in stream stage on a continuous basis during the flood season (April-October). This real-time information is relayed via a satellite network to the NWS, where forecasters can send warning to the public when streams rise to threatening levels. Rapid City-Pennington County Emergency Management can then coordinate emergency response, public safety services, and local governmental agencies as needed. Currently (2002), the USGS collects data at 20 flood-warning sites in the Black Hills area, which are located along Battle, Spring, Rapid, Victoria, Box Elder, Spearfish, and Bear Butte Creeks and their tributaries.

The USGS currently (2002) operates a network of about 120 continuous-record streamflow-gaging stations and 35 high-flow partial-record gages in South Dakota. Miscellaneous annual streamflow measurements also are made at several additional sites throughout the State. When flooding occurs, the USGS mobilizes personnel to collect streamflow data in affected areas. Streamflow data improves flood forecasting, provides additional data for USGS flood-frequency analysis that is used by the South Dakota Department of Transportation for bridge design, and provides information for use by emergency management agencies before, during, and after flooding.