The goal of extreme flood criteria is to maintain the boundaries of the pre-development 100-year floodplain, reduce risk to life and property from infrequent but very large floods and protect the physical integrity of a stormwater BMPs and downstream infrastructure.
The accepted design storm to manage extreme storms in most communities in Minnesota is the 100-year, 24-hour event. Designers are required to control the post-development 100-year, 24‑hour peak discharge rate to locally defined pre-development levels. Communities should carefully reassess extreme flood criteria since it requires the largest storage volume and greatest cost of any stormwater sizing criteria.
Communities may elect to waive 100-year peak discharge criteria in certain situations. The most common situation is when they have a buffer or floodplain ordinance that effectively excludes development from ultimate 100-year floodplain. Designers may also need to demonstrate that no downstream structures exist within the 100-year floodplain and that bridges and other infrastructure can safely pass the storm using an acceptable downstream analysis. This approach accomplishes the goal of extreme flood control by protecting the downstream ultimate 100-year floodplain rather than providing expensive upstream storage.
Hydrologists have often noted that extreme flood criteria may not always provide full downstream control from the out-of-bank events, due to differences in timing of individual peak discharges in the downstream portion of the watershed. Depending on the shape and land use of a watershed, it is possible that upstream peak discharge may arrive at the same time a downstream structure is releasing its peak discharge, thus increasing the total discharge. As a result of this “coincident peaks” problem, it is often necessary to evaluate conditions downstream from a site to ensure that effective out-of-bank control is being provided. Hydrologic and hydraulic models that can be used for analysis of downstream effects are provided in Appendix B.
Debo and Reese (1992) proposed the concept of the “10 percent rule” as the point to which a downstream analysis should extend. This is operationally defined as the downstream point where the development site represents 10 percent of the total contributing drainage area of a watershed. They contend that the hydrologic effects of flooding stabilize and remain constant further downstream. A typical downstream analysis will need a hydrologic investigation of the site area draining to a proposed detention facility and of the contributory watershed to the location of the 10 percent rule for the 10- and 100-year storms. As a minimum, the analysis should include the hydrologic and hydraulic effects of all culverts and/or obstructions within the downstream channel and assess whether an increase in water surface elevations will impact existing buildings or other structures. The analysis should compute flow rates and velocities for pre-developed conditions and proposed conditions both with and without the detention facility.
While the 10 percent rule is useful in establishing a limit for assessment, stormwater program managers still have some basic issues that need to be addressed. For example:
The following recommendations are provided to help answer these questions.
A local community may elect to waive the Vp100 criteria when a development project
Some Minnesota communities base their extreme storm design on a rain-on-snow scenario, rather than a specific design storm approach. Under this scenario, communities may define the effective 100-year event as having as much as 7.2 inches of equivalent rainfall that needs to be controlled to pre-development levels. There is little basis for this approach in Minnesota based on rainfall records or experience, and it clearly results in costly over-control, although some communities chose to continue its use for conservative design in land-locked basins.