Conversations About Stormwater Solutions

After this particularly wet fall there has been a lot of conversation happening about the downtown flooding concentrated in the municipal parking lot. 

Downtown flood 1960

I wanted to take a moment and bring everyone up to speed on where we are with this and the conversation / planning that has been taking place in regard to the situation. 

*I want to make it clear that this is a summary. There is a great deal more information in the form of engineering reports and past project planning that I will mention, but don't have the space here or the training to be able to do much more than that... I will do my best though.

Historically the area where the Municipal Parking Lot is located was a pond. There are several photos at the historical society that show young people skating on what is now the parking lot. The photo below may give some idea of the historical condition of that area. These ponds were (and still are) the low points for the downtown and functioned as the water collection point for the downtown. There was also a canal that ran across town in order to assist the draining of the area. Eventually the pond was filled in and the resulting gravel lot was often used as an overflow parking place and a venue for large trucks to get dirty. The area still served the same purpose though: collecting stormwater.

Bally in center, parking lot right, City Hall left

In 2001 the City performed a comprehensive stormwater management plan that provided description/analysis of the hydrology in the area, the watersheds and their boundaries, locations of concern, and roughly 20 pages of proposed actions that could be taken to mitigate the problems that were happening with stormwater. 

These included actions that would improve the efficiency of the creek and stream system around town, that would direct the continued development of the City's stormwater sewer system, proposals to assist the City in not exasperating the problems when there is future development, the need for retention ponds throughout town (which transformed into rain gardens...), and a number of ordinance and best practices for the City Council to adopt to make the plan effective. 

As a result of this stormwater plan the City invested significant resources and sought grant funding to actualize some of these recommendations, which helped fulfill the goal of improving the water quality of Lake Superior around the City.

Also as a result of this plan the City did a complimentary drainage analysis in 2002 with the intent of:
-Reviewing the capacity of the existing storm drainage system that conveys water from the east downtown area to Lake Superior.

-Determine what improvements could be made to increase the drainage system capacity and reduce the periodic flooding that occurs within the downtown areas.

-Provide preliminary construction cost estimates for potential drainage improvements. 

This analysis showed that the first floor of City Hall is at 606' above sea level, the low point of the parking lot is at 604', and the storage pond (behind the Coop and Stone Harbor) is at 602 feet. In 2002 the average water level of Lake Superior was around between 600' and 601'. That means that when the pond fully drains out, it would have about 3-4 feet worth of water storage in the pond, or about 8,215 sq. ft. of storage... Now, that doesn't mean a lot to me, but it certainly doesn't seem like a lot to handle the draining of that area... It worked decently well though. With the paving complete and with an improved storm drainage system in place, that area was no longer the swampy, muddy, gravel lot that it was previously and suffered with only occasional flooding that would quickly drain. 

In 2007 the City performed another, not quite as complex Stormwater Management Plan based on identifying projects that the City could perform to continue improving the stormwater situation. This list of projects included a lot of culvert replacement, ditching, dredging, and berming that would slow down the stormwater before it hit the downtown area and the lake. Even though our rain events were becoming more severe even in 2007, the lake level in 2007 set the record for the lowest water level ever in February at 599.7'. This low lake level increased the storage volume of the downtown storage pond. 

Since 2007 the lake level has rebounded to near, or above the 602' level, the highest in a seventeen year period, which reduces the storage in that pond. Likewise, there has been more paving happening in the downtown area, which decreases infiltration and increases runoff. In addition, our rain events have become more extreme. All of these things correlate with and explain the increased occasion of flooding in the parking lot. 

In 2009 the City revisited and reassessed the Stormwater Management Plan, but mainly to check off things that had been done. 

At our last City Council meeting Cook County Soil and Water approached the City Council to encourage the City to weigh in on potentially getting some funding from the State to refresh our stormwater plan with the changed reality of higher lake levels and more severe weather events (we have had three "50+ year" weather events in the past 10 years...).  The Council identified this as a prudent action because our problems will likely only get worse unless we identify real solutions for the struggles that we are having right now. As a result, the City will be working with the County Soil and Water Department to pursue funding to do this study. 

Within the scope of Comprehensive Planning and Community Visioning, the Council feels pretty comfortable that it is a priority of the City's residents and business owners to manage our stormwater more effectively either to maintain our drinking water source, or to prevent property damage. (The Council has already heard from several residents and business owners that stormwater infrastructure is a major concern of theirs).

So, that is the history, where do we go RIGHT NOW with this problem?

I have been discussing this issue with several engineers across the state and have received several ideas both anecdotally and actually. This is a brief summary of the most viable of these ideas:

Hwy 61 Redesign-- The idea of using the Hwy 61 Redesign as a tool to effectively cut off all stormwater coming down the hill from ending up in the parking lot is a great idea and will definitely be included in the 2019 project. There are a few problems with this proposal though: a.) 2019 is a long time away and b.) where are we going to send that water? 

I believe that we cannot wait until 2019 to find a solution to this problem, but we can definitely still put elements of a solution into the redesign. When it gets included, how are we going to get all of that run-off into the lake without dumping a huge amount of sediment and other pollutants from our streets directly into our drinking water source? It would likely require a large (or larger than currently exists) culvert to run under the downtown area with some kind of filtration... As you can see, this is an element of the solution, but not the solution itself...

One of our local business owners, an engineer himself, proposed a solution that included connecting the storm drainage pipe that empties into the pond to a pump that would, when triggered by a flooding parking lot water level, would pump the water into the pond, thus artificially elevating the pond until infiltration brings the level back down. This would require some pretty significant engineering and equipment, as well as maintenance on said equipment, but I do definitely see that this would be a way to solve the situation... if we can pump enough water into that pond to not flood the back sides of the businesses there...

Another engineer advised me more on the theory of the situation than an actual plan. He stated that there are three things at work here:

1. Too much area draining to the parking lot

2. Too much water entering the drainfield too quickly

3. Infiltration happening too slowly

As for the area draining to the parking lot... well, I believe that we addressed that in the Hwy 61 Redesign idea. As far as the speed of the water is concerned, we could dig ditches... but we live on a hill. It can be hard to beat gravity... As for infiltration happening too slowly, well, we want slower infiltration because fast infiltration doesn't catch all of the pollutants... These are the ideas we have to work with though. It is good to have them identified.

Yet another engineer that I spoke with sought to find a more cost effective solution with minimal compromises. 

We talked about three concepts: 

1)      Dredge the pond in the hopes of improving its ability to release water

2)      Increase the amount of live storage by enlarging the pond, using the land to the northeast

3)      Create a spillway to release water at a certain elevation

We agreed that the most attractive approach seemed to be to do both 1 and 3. Increasing the size of the pond would likely not give us the increased storage we need and would change the nature of that area to a point that it may create other issues. 

I do not believe that the pond has been dredged in many years though, and it may very well be clogged up with silt and sediment, thus hampering its ability to drain properly. Removing the accumulated sediment would be a good idea anyway as that sediment probably contains most of the pollutants from the parking lot all of these years. Removing it wouldn't have a significant impact on the pond's ability to filter either as the pond was designed without the soil in there...

The spillway. An interesting idea and one that was included in the stormwater analysis in 2002. However the 2002 plan wanted to put a massive concrete structure on the beach and a Tideflex valve to drain the pond in cases of overflow. This wasn't a popular idea because it wasn't beautiful (as you can see from the photo) and it was extremely expensive. The Council at the time was not convinced that the lake wouldn't just pummel that thing and break it within a few years either, so nothing was built. 

This engineer believes that the City can, for not a lot of money, landscape a spillway (a low spot armored with an articulated revetment mattress like on the Gunflint Trail ditches) into the east wall of the pond at a height of about 605' so there would be maximum water retention in the pond for normal weather events and an overflow that would stem the flooding in the parking lot.

But what about the pollution in the overflow water? Well, that is the compromise on this plan, but it may be as bad as you may think. This is how it was explained to me:

When you get a rain event the first flush of the rain will succeed in washing off most of the junk from the roads. After that there is very little left on the roads to be problematic. The pond has already proven itself able to handle most of our rain events up here, but is increasingly unable to handle the massive weather that we have been seeing. In the cases of these floods in the parking lot there is either a tremendous amount of water in a short time, in which case most of that water is going to pour directly into the lake without being treated because of its velocity OR there is a long, continuous rain, in which case most of the pollutants will be washed out and filtered as the pond does its work. It would likely be relatively clean water that would be overflowing because of the pond's ability to allow the pollutants to settle...

I understand that this may be a rose-colored-glasses approach to this and there haven't been any formal plans written up for any of these suggestions (other than the Hwy 61 project... MN DOT knows we want to do stormwater there...) nor has the Council voted on any of these ideas, and there would likely need to be some kind of engineering done on this project so we can be sure that it will function like we want it to and so it won't be a waste of time and money. 

The proposal of the dredging and the spillway would be, by far, the cheapest option here, aside from doing nothing. 

We need to ask ourselves a few questions about this situation:

1. How much money is it worth to have the parking lot not  flood even though it was designed to be additional stormwater storage space?

2. How much water are we ok with in that parking lot?

The correlation between these two questions is important. If we are ok with the occasional 2 feet of water in the parking lot, then we don't have to spend any money and we leave it as it is. We already know that this isn't how a lot of people feel.

If we are ok with the occasional 1 foot of water in the center of the parking lot, then we can pursue mitigation efforts that are aimed at slowing down the water coming down the hill for a minimal investment and not focus on a redesign of the pond.

If we don't want any water in the parking lot at any time (this seems to be the leading perspective), then we are likely looking at a redesign of the pond of some sort and efforts to slow/divert the water from the downtown area at a larger expense. 

I think that is about it. I hope that wasn't too boring. If you made it all of the way through... wow. You are dedicated! I would love to hear your thoughts on potential solutions for the stormwater situation if you have any that differ greatly from the ones proposed here.