Stop the flooding

BY ED FELIEN

The Park Board can stop the flooding of basements in South Minneapolis and the flooding of the Hiawatha Golf Course, and it won’t cost taxpayers a dime.

The flooding is caused by the dam/weir at 27th Avenue at the outlet of Lake Hiawatha. It backs up about 4 to 5 feet of water, and that has raised the water table and groundwater around Lake Hiawatha. This means the ground can’t absorb water from a heavy rain, and the area floods. The Park Board should call up the U.S. Army Corps of Engineers and ask them to take back their dam/weir because it has become a public menace.

The Barr Engineering Study of Minnehaha Creek cites two other points that control the water level flowing out of Lake Hiawatha: “The existing control of water levels in Lake Hiawatha is either the high point in the channel upstream of 28th Avenue South or the rock weir under the pedestrian bridge at 30th Avenue South. The high point at 28th Avenue South appears to be caused by a gas main crossing the creek. The weir at 30th Avenue South appears to be manmade and its function is unknown. Both high point elevations are approximately 810.7 feet above sea level.”

The utility line is mentioned in the 6/21/2017 memo from Jennifer Koehler, PE & Kurt Leuthold, addressed to Michael Schroeder, Park Board planner, and Katrina Kessler, city engineer: “Hiawatha Golf Course Area—Water Management Alternatives”: “There is an abandoned CenterPoint gas main along 28th Avenue South and crosses on the downstream side of the 28th Avenue bridge. It is unclear if the abandoned main has been removed, but there are no other active lines crossing under Minnehaha Creek between Lake Hiawatha and Nokomis Avenue.” The Park Board should call up CenterPoint Energy and make sure they’ve removed their pipeline.

The rock dam/weir under the 30th Avenue footbridge was possibly an elaborate prank by someone who wanted to dam up the creek as proof of their mastery over nature. The Park Board should send a crew down there with a front-loader or a forklift and a truck and remove the boulders. The Boulder Dam joke has gone on long enough, and it ain’t funny anymore.

Removing these three obstacles will probably lower the level of Lake Hiawatha by 4 feet, from 812 to 808 feet above sea level.

The Barr Engineering Study says, “There is a concrete weir at the outlet of Lake Hiawatha with a 2-foot notch into which wooden planks or stop logs could be inserted to raise the lake elevation. However, this structure does not ultimately control the water levels in Lake Hiawatha. The elevation of the existing concrete weir notch is 809.9 feet,” and both the utility line and Boulder Dam are 810.7.

Two things argue against the Barr theory about what is holding back Lake Hiawatha:

When you stand on the bridge and look at the dam you don’t see water flowing through the notch; you see water flowing quite heavily over the top of the dam at 812. And, second, if the gas line or rock dam were holding back Lake Hiawatha, then why isn’t the level of Lake Hiawatha 810.7?

Michael Schroeder wrote to me: “If it were practical to remove ALL the obstructions to flow of the creek below Lake Hiawatha, it would reduce the elevation of the lake. However, as we’ve discussed previously, there are AT LEAST 19 obstructions in the first 2,000 feet of the creek below Lake Hiawatha, with the weir being one of those 19. Removal of some might be possible; removal of all would likely be impracticable.”

In spite of all my research I could not find 19 obstructions in the first 2,000 feet of the creek.
Enrique Velázquez, the surface water and sewers manager of Business Operations for the City of Minneapolis, Public Works, wrote to me:

“Here is a summary of pipes underground shown in Figure 9 from upstream to downstream:

1. City sanitary at 28th Ave: top of pipe appears to be at 808’

2. MCES interceptor sanitary sewer (city lines flow into this): top of pipe appears to be slightly below 808’

3. City sanitary at 29th Ave: top of pipe appears to be at 806.5’

4. City sanitary at 30th Ave: top of pipe appears to be at 805’

5. MCES interceptor sanitary sewer (same pipe as #2 above): top of pipe appears to be slightly above 807’

6. City sanitary just east of Nokomis Ave: top of pipe appears to be at 806.5’”

None of the pipes are above 808, so by removing the three obstructions we would lower the level of the creek and the water level of Lake Hiawatha by approximately 3 to 4 feet.

Would lowering the water level of Lake Hiawatha by 3 feet make a significant difference?

Let’s do some informal calculus. If the area of Lake Hiawatha was about four short blocks wide (4 X 330 = 1,320 feet) and two long blocks long (2 X 660 =1,320 feet), then it would be about 1,320 X 1,320 = 1,742,400 square feet. If the average depth of the lake were 10 feet, then the volume of the Lake would be 17,424,000 cubic feet. There are 7.48 gallons in a cubic foot, so, Lake Hiawatha contains approximately 130,331,520 gallons of water.

Park Board staff note that we are currently pumping 242 million gallons of water into Lake Hiawatha every year. But there are only 130 million gallons of water in Lake Hiawatha. How can that be? According to the Park Board 17% of the water pumped is storm-water runoff; 33% is seepage from the soggy surrounding soil; and 50% is groundwater streams. 83% of the water pumped out of the golf course comes right back into the golf course through seepage and underground streams.

If we lowered Lake Hiawatha by 3 feet, we would eliminate 39,102,041.4 gallons of water from this endless cycle. We would dry up the surrounding grounds, drain some of the underground streams and restore the golf course to being a useful floodplain.

We could probably stop pumping.

And wasn’t that the problem we were trying to solve in the first place?

PHOTO CAPTION: Boulder Dam under the 30th Avenue footbridge across Minnehaha Creek: Was this somebody’s idea of a joke?

Comments are closed