Thanks Judy!
Park View Terrace (Mosquito Flats) “Hydrology 101” session
Monday, August 4th, 6:30-8:30 p.m., Iowa City Public Library, meeting room A
Highlights:
Attendance at this meeting was perhaps 70 or 80 people—with half that many by the end
We first heard from Jude Moss of the Iowa City Water Division, who discussed procedures for reactivation/inspection/repair/shutoff of water/sewer accounts—more below.
The main portion of the meeting was devoted to a presentation by Frank Weirich, Iowa professor of hydraulics and civil & environmental engineering—detailed notes below. Among the main points—averting/mitigating future flooding requires a comprehensive solution for the entire drainage basin; operational plans for the Coralville dam are certain to be revised, but major changes to the dam and reservoir are very unlikely; focusing on the river water itself is key to a comprehensive solution, along with localized measures; local factors downstream such as the coffer dam and other dams and bridges exacerbated but did not cause the floods; flood probabilities for the future are more likely than previously thought.
Finally, those remaining in the room made a consensus decision that Steve McGuire, Jerry Anthony and Doug Jones—on the basis of their activism and expertise—will act as our neighborhood representatives in meetings, hearings, and so on related to flood aftermath, prevention, mitigation, etc. etc.
Water service:
City water/sewer accounts are being reactivated as of August 1st. Any homeowners who do not intend to reoccupy their homes need to inform the department. This is especially important because if a dwelling will remain vacant with the onset of cold weather, service needs to be shut off entirely.
Meanwhile, the department will hire a contractor to inspect the “stop boxes” which are built into the curb at each dwelling, and fix any in need of repair, and then water meter and radio read devices also will be replaced, with all costs born by the city and FEMA. All homeowners need to contact the department to schedule these activities.
The contact is: Jude Moss, Water Division customer service coordinator, 356-5163 or jude-moss@iowa-city.org.
Hydrology lesson:
[Caveat: these notes come through a double filter—they are the note-takers’ best possible interpretation of the speakers’ interpretation.]
The speaker introduces himself as a specialist in floods, landslides and fires, watershed and flood modeling, and inundation—“where it goes, why it goes there.”
Need to understand that floods take place in an “integrated system,” in which changes in one thing affect the entire system. In other words, change in our neighborhood will affect everybody else, and vice versa. This, all stakeholders need to discuss flood mitigation plans. “Solutions must be integrated and comprehensive.”
Starting point for understanding floods is a basic hydrology equation:
P (precipitation, i.e., rainfall, snow, ice melt) =
ET (evaporation) + R (surface runoff) + G (groundwater storage)
In other words, what doesn’t evaporate or go into the ground has to go somewhere else. Circumstances on the ground can change runoff, how much water goes in different directions and how fast; e.g., urbanization, with streets, asphalt, etc., makes the ground impervious and forces more runoff, in contrast to soil, in which 50% of water will go into the ground.
A hydrograph illustrates the output of a river system over time—expressed as:
Q (discharge, in cfs, or cubic feet per second)
and T (time, e.g., in days)
Here’s an example:
The key concern in flooding is not the volume of water, but that peak discharge—the highest level reached.
Also must understand basic river and flood plain hydraulics—how moving water behaves in and beyond the river channel. The basic equation for this is:
Q = AV
in which Q is discharge, A is area, taken as a cross-section of the river channel (width x depth), and V the velocity of the water, or its average speed.
Then comes the Manning equation, a calculation of velocity or water flow speed. The velocity can change depending on the channel configuration; it helps if the river is wider with increased surface area. The equation is:
V=1.49 x R x S
N
in which R is a function of the river basin’s area and perimeter, S is slope, and N is a coefficient expressing a “friction factor” based on the surface the water is going over – for instance, a rougher surface like a sandy bottom will slow water down, whereas a smooth surface such as concrete or glass means faster flow; and it also accounts for obstacles in the river or flood plain, such as those presented by trees or subdivisions, that will slow water down. FEMA models are based on this equation.
The drainage basin we’re dealing with extends to from 3,000 square miles above the Coralville Dam to a few hundred square miles below. The Coralville Reservoir holds some 410,000 acre feet of water. Excessive storms like we had last spring result in loss of storage capacity; during the floods, the water volume in the reservoir reached 500,000 acre feet and was still rising. Over time, the accumulation of sedimentation in the reservoir also is reducing storage capacity.
The Coralville Dam originally was designed to increase storage and minimize danger with controlled release of water, delaying its transit and buffering the flow, so that peak discharge in the river would be lower—in other words, the peak on the hydrograph would be lower. The dam was built as part of the Mississippi River flood system—with protection of Iowa City only a secondary benefit; and since them, other missions have been added, e.g., recreation, fishing, wildlife habitat, with plans changed to accommodate those additional functions.
Could we dredge the reservoir? Dredging is very expensive—and an additional problem is that the bottom of the reservoir is full of DDT from the days before it was banned, and dredging would stir it up. What about raising the dam? Raising the level of the reservoir would expand the surface and necessitate purchase of property rights around the shore, expensive and difficult; in addition, since the reservoir is very shallow, expanding it would have huge environmental consequences in destroying habitat.
The Army Corps of Engineers operational plan for the dam can be changed at any time, and definitely will be reevaluated after these floods. This will involve a cost-benefit analysis—for instance, if the pool is maintained at too high a level, more farmland will be flooded upstream and restitution will have to be paid, which is part of the tradeoff for protecting areas downstream. Every spring brings a “guessing game” about this balance.
Following the 1993 floods, the Corps changed the release plan to release more water earlier—and obviously this time it still was not early enough; release of more water in March or April would have avoided “a certain number of feet rise” downstream, but instead the key decision was made to maintain the normal release of 10,000 cfs to avoid flooding Dubuque Street. “In hindsight, you can always mitigate a flood.”
A lot of factors converged at PVT: Snow melt from the winter, early spring rain upriver, which worked its way down toward the reservoir, and meanwhile other storms to the west and the east dumped more water, less rain but more intense, and the ground became saturated so it could not drain any more, and then there were other exacerbating factors. The buildup of obstructions at the Park Road bridge, the University of Iowa construction coffer dam and the dam beyond the Burlington Street bridge all affected the system. The coffer dam “narrowed the flow” but did not cause the backup to Parkview Terrace; the bottom of the river remains mobile, and in fact higher velocity water cuts a deeper channel (called scouring—although later sand and silt will fill in the new holes). Construction, silting and so on all affect the river water level.
As our flood record grows, with increased data and also use of proxy records, and given climate change, river movement and other factors, estimates of flood probabilities for the future should become more pessimistic—e.g., the purportedly 100-year flood of 1993 may now be a 30-to-50-year flood, and the latest supposedly 500-year flood may actually be a 70-year flood. (Not for quotation!)
The best strategy for the future overall lies not in changes to local hydrology, but in focusing on what happens to the river water. For our neighborhood, the key question is: If PVT is not going to go away (to raze or raise?) how to protect the homes that remain? This could involve creation of grass levy systems and berms, culverts and overflow structures; also building homes so they are “more floodable,” e.g., putting utilities on a second floor. Raising the Park Road bridge to avoid creation of another “debris dam” would help.
The plans in any case need to be comprehensive and require coordination—perhaps through the Johnson County Council of Governments, and we definitely should make use of our congressional delegation. The Army Corps of Engineers has to approve anything; the Department of Natural Resources also has to approve. In the meantime, feedback at public hearings can have huge importance—the speakers’ wife used to hold public meetings for the Corps and she deems them “the most powerful agent for social change.” The plans ultimately will end up going to DC for the chief of the Corps to sign off.
