Flood Preparedness Information
Emergency Phone Number: Dial 911
Additional Resource Numbers During A Flood Event
Emergency Operations Center: 285-8103
City of Rochester Emergency Management Coordinator: 328-2823
Record Rainfall Event of August 18-19, 2007
On August 18-19, 2007, Southeastern Minnesota received record rainfall amounts for Minnesota. The City of Hokah recorded an unfortunate State record rainfall of 17 inches. In the South Zumbro Watershed, which all flows into Rochester, we recorded a 24 hour record rainfall for the watershed in the range of 6 to 12 inches; greater than a 500 year rainfall event. The rainfall amounts recorded in that 24 hour event were likely the highest in history for the City of Rochester. The 24 hour rainfall amounts for the 1978 flood, Rochester’s flood of record, were much lower, but 5 inches of rain in three hours caused major damage. Even with these 2007 rains of record proportions, we were largely protected from surface water flooding in Rochester. However, we still had significant problems with sanitary sewer back-ups, localized storm system flooding, basement flooding from groundwater, and hillside failures. That record rainfall event was a significant test for our flood protection system and we believe it performed very well. Without the flood control system we believe areas along Cascade Creek, Bear Creek, and Silver Creek would have experienced a flood event equivalent to a 100 year flood. We are thankful that we were protected from that, but we remain concerned that our weather patterns are unpredictable. In order for the community to be better prepared in the event there are future historic high rainfall events we want to provide some flood preparedness information to our citizens.
Rochester Flood Control System – Capacity
With the assistance of the Corps of Engineers and the USDA Natural Resource Conservation Service (NRCS), the City of Rochester, Olmsted County, and the Olmsted Soil and Water Conservation District constructed a flood protection project from 1984 – 1995. The historic record flood of 1978 was used as a model to design the flood protection project, so that our flood protection is designed to handle any flood event that the community has experienced thus far. The Corps project deepened and widened the Zumbro River channel and Bear and Cascade Creeks to handle major flood flows. The NRCS project installed 7 flood storage reservoir structures to hold back flood events exceeding a 200 year storm from 35% of the watershed that flows into Rochester. These reservoirs provide the flood protection for Silver Creek, and reduce flood flows on Bear Creek, Cascade Creek, Willow Creek and the Zumbro River. If a rainfall event exceeding a 200 year storm event occurs in those watershed areas, the additional amount flows through an auxiliary spillway, which would increase flood heights downstream. To date no spillway flow has ever occurred, even in the August 18-19, 2007 rainfall event. In general, the City and areas between the reservoirs and the City are planned to be protected from flood events exceeding a 100 year flood event. Statistically, the chance of such an event occurring is less than 1% annually.
Our Flood Protection Capacity Is Very High, But It Is Not Limitless
The flood system’s protection capacity we have in Rochester is very high, but it is not limitless. If a State record rainfall, such as experienced in Hokah, were to hit Rochester, flooding would be greatly reduced, but there would be some flooding. We want our citizens to be at least mindful of that in the event we receive very extreme, record rainfall events. Our citizens should be monitoring and listening to Emergency Operations Center (EOC) and media reports during major rainfall events. At a minimum, the type of events that would our citizens should be attentive to would include:
- 24 hour rainfall events exceeding 5 inches. Our flood protection system has handled rainfall amounts greater than that without a problem, but citizen attention to rainfall amounts at that level is recommended.
- 12 hour rainfall amounts exceeding 4 inches. The flood protection project is designed to handle more than 6 inches of rain in 12 hours, but watchfulness at the 4 inch level is recommended.
- Specific reservoir structures are holding back 100 year or major storm flows. Advisory alerts will be issued in that situation. In this case there are two areas of concern:
- First, additional rainfall beyond a 200 year event could cause flow through the auxiliary spillway increasing flood heights downstream. Second, if there were damage to a reservoir structure, during a time that a major flood has been stored behind the reservoir, it could result in a release of stored waters and a sudden increase in flood heights downstream. The reservoir structures have been designed and constructed by the federal government (NRCS) to the highest standards, so we do not believe this will ever occur. However, in an absolute worst case scenario, if this were to happen, it could affect some areas downstream of the reservoir including some areas outside the former flood plain. That would be a serious situation with little warning time.
- Rapid spring snow-melt situations. High snow amounts and rapid spring run-off have the potential to cause flooding in the spring. The flood control system was designed with the capacity to handle any spring snow melt situation that has occurred thus far in the city’s history.
- Velocity of river and creek flows. The most significant surface water concern we had during the 2007 flood event was the high water level and the speed of the water in the river and the creeks in many neighborhoods. This poses a safety concern for persons, and particularly children, who are adjacent to those areas. Citizens need to be mindful that this is an unsafe situation and take precautions.
Reverse 911 Emergency Notification System
In 2007, two of the reservoirs to the east of Rochester on Silver Creek and Bear Creek stored flood waters equivalent to a 100 year flood event. Stored flood waters in the Bear Creek reservoir were within 2 feet of flowing through the auxiliary spillway. Pursuant to the warning plan that is in place we notified residents downstream that additional rainfall could cause spillway flow. Notices were provided by the EOC, the media, and through a new system known as “reverse 911”. The “reverse 911” system allows public safety personnel to notify hundreds of property-owners with a phone message in a short period of time of situations involving a public safety concern. We utilized and activated that system to provide telephone land-line notification to citizens in areas along Bear Creek and Cascade Creek in 2007 (at this time it would not provide information to properties with just wireless/cell phone service). This provides an additional informational and warning tool for public safety personnel during major storm events. However, citizens still need to monitor EOC and media reports as their primary source of information in major flood events.
Related Links: