Kana: 3 post-flood tasks we need to complete before it’s too late

It was a wake-up call none of us wanted. The rains fell, the waters rose, and before we knew it, neighbors were being rescued from second-floor balconies.

Why did this happen? Why weren’t we warned? What can we do to prevent it from happening again?

Actually, we were warned, a full 72 hours in advance, that there would be historic rainfall. The warnings were based on computer models that involve some very difficult mathematics. Just 24 hours out, one weatherman was likening the coming event to a fire hose aimed right into our community. But we didn’t believe it; Hurricane Joaquin was a thousand miles away heading out to sea.

Of course, in hindsight, the weatherman was right because the models were accurate. Just like the model predictions were right in 2012 when they correctly predicted the unusual left turn that Hurricane Sandy would take into New Jersey.

Weather models are not perfect, but these days we ignore them at our peril. For models to get even better, they will need data from this storm. Here’s where we all can help.

While the memory of the flood in your section of town is fresh, everyone who was flooded needs to do these three things:

▪ Mark the position of the highest water level reached on your property.

▪ Note the approximate time and date the flood peaked at your location.

▪ Take photos of the high water mark with some sort of reference, like a window sill or porch deck.

Better still, if you can recall about how fast the water rose and fell to particular levels, record that. You’ll want to give all of this information to the field teams that will be making measurements in your neighborhood.

Meteorologists are doing a great job warning us of pending weather events. Now it’s time for hydrologists and engineers to refine the flood models. These are the professionals who will tell us how high the water will likely go and how much flow needs to be released over our dams to absorb the next flood with the least damage to property.

Many earthen dams along Gills Creek couldn’t handle the deluge and failed. The lakes in the system cascade down from dam to dam, so when one dam fails the next one downstream has to do the work of two dams. The sheer volume of water was too much for spillways, so the dams themselves were flooded over. Water seeped through the dams, taking soil and undermining the earthen core. The spillways had so much water flowing that in some cases their flanks were eroded on the back side, triggering breaches.

It will be a while before we know how much the specific flood level in someone’s home was caused by the volume of rainfall and how much by the dam breaks. But clearly, even one foot made a big difference in some neighborhoods. Those high-water marks on your property will help experts determine how much your home needs to be raised off the ground.

If dam breaks turn out to have played any role, there are things that can be done to avoid a repeat: Install bigger spillways; reinforce earthen cores around spillways with rock; inject grout cement into the core of the dam to reduce seepage; and install larger operable gates.

The lowest and largest dam in the Gills Creek system at Lake Katherine could have released more water ahead of time if the riser boards had been replaced by a mechanical gate that can be operated safely when the water is already up. We opened our flood gates all the way on Thursday before the storm at first warning, but the lake could only drop a couple of feet before the rains fell. We’ve done the other things on the list, and that probably saved our dam from breaching. But when the entire dam became covered with five feet of water, it was out of our hands.

The Gills Creek Watershed Association recognizes how all our lakes are connected and recognizes that water levels need to be better managed. I hope you will support this effort. A big need beyond recovery is to assemble the flood data and get it to the scientists and engineers who are anxious to develop practical and sustainable solutions for us.

This story was originally published October 15, 2015 at 4:29 AM.