Estimating Flooding: Structures vs Properties

The Village of Wilmette has posted a Stormwater Action Plan, which describes the final three options the board is considering to reduce westside flooding.

There’s nothing new there (it’s the same stuff the village has debated and engineers have studied for years). But something caught my eye…

On the Potential Improvements page, the description for each option cites benefits for flooded structures, as well as flooded properties.

What’s the difference?

Structures vs Properties

Engineers use numbers to make estimates for sound project planning.

In our case, take a look at the next diagram, from the 2017 Value Study. Stantec consulting engineers created a 50-foot diameter circle, placed at the center of each lot, to serve as a rough “proxy” for the footprint of each building. Then they used elevation maps and data about rainfall and sewer surcharging to calculate how high up flood waters would rise, during a 10-year intensity storm.

The engineers created two classifications to evaluate flood impact:

  • Flooded Structure: Stormwater reaches (or fills) the “proxy” circle.
  • Flooded Property: Stormwater flows up onto the property, but not into the “proxy” circle.

This is an excellent tool for estimating the cost/benefit ratio for each storm sewer upgrade. But take a closer look at the lots at bottom left:

The fully flooded structure (center) is obviously in big trouble.

But what’s the difference between the flooded structure (bottom) and flooded property (top)? Almost nothing.

I would argue, for our purposes as decision makers, and when analyzing cost/benefit ratios, our focus should be on the approximately 1,300 vulnerable westside properties.

Our research backs this up…

Realistic 10-Year Storm Impact

Our AWSM team has conducted hundreds of conversations with westside residents, confirming that 10-year storms are impacting far more than a couple hundred residents. To make progress, we need to help not just owners of the estimated 311 flooding structures, but also of the 1,268 flooding properties.

Both Structures and Properties Are Damaged

It’s true that frequently flooded structures are experiencing the most extreme damage, since stormwater is all but guaranteed to flow over the doorsills, into garages, and through low-level windows and foundation cracks. There’s no way to stop that level of stormwater flow (or if you did, the building foundation might crack due to pressure from the surrounding water).

Nonetheless, frequently flooded properties, which experience varying flooding depths and coverage, are also significantly damaged. By the time a property is even partially flooded, stormwater has infiltrated the ground, creating significant hydrostatic (water) pressure on foundations.

Drainage is Required to Floodproof a Property

Owners of frequently flooding properties often rely on floodproofing systems for protection. However, outflows that become submerged under stormwater are useless. Hydrostatic pressure can overwhelm sump pumps. Power outages lead to loss of protection. Long-term flooding can outlast battery backups. Tanks can overfill. Equipment can wear out.

That’s why effective DIY floodproofing for the 1,300 vulnerable properties requires proper drainage.

Flooded Properties Become Flooded Structures

At a recent stormwater meeting, Trustee Kathy Dodd asked if we could solve our flooding problems by simply tearing down homes. Stantec engineer Joe Johnson explained that, in addition to the fact that it would cost more than simply installing proper drainage, it would not solve our flooding problems. Since there is very little elevation change throughout the westside, small changes in local conditions (like removing homes) will produce unpredictable stormwater flows, leading to random improvement–or increased flooding and damage–for nearby properties.

At another Wilmette storm sewer meeting, MWRD Commissioner Debra Shore explained that MWRD rules for detaining runoff apply only to large lots. When property owners develop small lots (prevalent in Wilmette), then small changes like adding a garage can create additional runoff, which obviously will flow somewhere. Here on the westside, we know that “somewhere” is likely to be the neighbor’s basement.

For example, westsiders have reported increased flooding when a neighbor cuts down a large tree, builds a larger home, or adds an impermeable driveway.

All too frequently, the owner of a vulnerable westside property becomes the owner of a damaged structure.

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