Saturday, December 6, at Coralville Public Library
About 30 people in attendance.
Main speaker was George Hollins, University of Iowa business manager, speaking in his capacity as an experienced hydrology engineer (Iowa State graduate, worked with Iowa Natural Resources Council and as West Des Moins city engineer, specializing in groundwater and floodplain regulation, bridge and hydraulic structure design, and the like). Also present on behalf of the City of Iowa City was Rick Fosse, director of public works.
Hollins made these points:
Why do we develop on floodplains in the first place? People settle where transportation is convenient, e.g.,, on water; where land is flat; where water supplies and waste disposal methods are available.
“What we think we know sometimes isn’t reality.” We’re “brainwashed” into thinking about 100-year frequency floods, but even small rivers have big floods. Current stats are not necessarily based on a solid record (though Iowa’s is longer than most). With each flood, the calculations (e.g., the figure for 100-year flow in Iowa River, raised from 25,000 cfs pre-1993 to 29,000 as result of 2002 study) are increased, and will increase again; storms and climate change will keep pushing the figure up. Hydrology also doesn’t account for development in rural areas. He cautioned against having a “false sense of security,” and said the nature of future storms is an unknown, and the state of maintenance also cannot be predicted.
Definitions (with x-section drawing) of floodplain, floodway and river channel – the floodplain is the area from “bluff to bluff” on either side of the river; the floodway is the area covered with water if the river rises one foot above normal flow. the national flood insurance program allows building in the floodplain, premised on the idea that you can build on the “floodway fringe” to the point where what is called the “backwash” rises one foot. That’s why building permits in the 100-year floodplain require a one-foot elevation of the first finished floor. With a permit, you can still do lots of things in the floodplain. But the middle section, the floodway, has to be preserved with no construction. These demarcations also designate where mitigation works need to be situated.
Iowa rivers have interesting peculiarities. Mostly they are big and flat—so what’s called “subcritical” flow is most important – as opposed to “subcritical” flow (e.g., whitewater). Velocity changes don’t usually go subcritical except in spots like below the Burlington St. Dam. Contrary to what one might believe, silt is not that important in floods—in our case, the Coralville Dam releases what’s called “hungry water” without a “bedload” (i.e., silt is trapped behind the dam), and the water cuts a channel and picks up sediment as it goes along. (Sediment behind the dam is another question.)
[Fosse said in fact the river bottom in the Iowa City area is getting deeper; and IC monitors the beds around bridges to make sure the footings don’t get scoured.]
One advantage we have—or should have—is the luxury of warning time; we can anticipate flooding based on outflow from the Coralville Dam. So why, some people wonder, was there such sudden evacuation of Normandy Drive residents last summer? Sounds like people have to make their own calculations—we need to know the relationship of river outflow with depth downstream in our neighborhood, and monitor the river outflow at
We’re referred to the Association of State Floodplain Managers—which has a website, a lobby and some good white papers. For any flood mitigation structures, this organization advises designing to 500 years and a “freeboard” (additional height).
A bit of talk about the Army Corps of Engineers—seems unlikely they could have averted flooding, but could have let more water out earlier. In 1993, eleven times the volume of the reservoir passed through the dam, and it was even more in 2008. The Corps in the past has been “cooperative and willing to share information,” but they are in a no-win situation since they must answer to so many constituencies—recreational interests, farmers downstream, various communities, modulation with the Cedar River, public policy issues, etc.
Examples of various flood control structures may be found on the web—such as temporary portable coffer dams, which may not be so great; Hesco-barrier type structures, up to 12 feet high, which may be permanent or temporary; an “invisible wall,” or floodways that come out in sections on a permanent base, also to a height of 12 feet; and your basic earthen levee, with a floodwall and footing in the center. Costs vary a great deal and depend on material, depth of excavation required, and so on.
[Fosse said the PVT area has problems with porous soils and sands; the city is working with Stanley Consultants to develop a range of options. The city also is working with FEMA to get funding for long-term recovery, and is considering the raising of Dubuque St. and Park Road Bridge.]
Next PVT neighborhood meeting will be Saturday, December 20th, 3:30 pm, at the Iowa City Public Library; and then on January 10th, we’ll meet with Iowa City officials, place to be announced.
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