Climate change and knowing your flood risks

2012-02-09T00:00:00Z Climate change and knowing your flood risksChris Malan Napa Valley Register
February 09, 2012 12:00 am  • 

In 1998, the voters of Napa passed a flood management tax commonly known as Measure A. The spirit and intent of Measure A was to restore the Napa River while managing, not preventing, floods. Napans opened up 400 acres of degraded wetlands south of the city of Napa by removing old agricultural levees along with several downtown structures, such as the Rough Rider building.

The removal of these structures will give the Napa River room to spread during storm events in what used to be the natural historic floodplain. We rebuilt five bridges with the intent of providing flood conveyance. Now our tax dollars are being used to build flood stage bypass structures for both the Napa River and Napa Creek.

Historically, the Napa Valley was rich in biodiversity that thrived in both fresh- and saltwater wetlands with interconnected waterways that seeped and flowed to the Napa River, from San Pablo Bay to Calistoga, proving to be one of the biological hot spots on the planet.

Explorers commented that the sky would darken as migrating birds passed through where they foraged in these abundant waters. Since the 1800s, settlers have gradually simplified and urbanized the landscape by draining the wetlands, filling in creeks and channelizing the Napa River. Now we are clear-cutting wildlands for vineyards. As urbanization takes over the watersheds, there will be more frequent flooding with increasing damage and erosion.

Flood projects are engineered to a theoretical 100-year storm event, which means that every year there is a 1 percent chance, and that the man-made structures could be overwhelmed by a 101-plus-year storm event.

The Army Corps of Engineers does not claim it can prevent flooding. It claims only that its flood structures could possibly prevent flooding up to the 100-year flood event. But the 100-year event is an arbitrary calculation.

Scientists studied storm events history for a very short period of time (1950-1960) and determined what a 100-year event could be, based on weather patterns in this narrow window of time. Not only are 100-year events based on only a sliver of historically documented flood events, but global climate change and the subsequent increase in severe storm events is now altering these marginal calculations.

Recently, the city of Napa approved several structures such as low-cost housing in the historic floodway of the Napa River. Floodways are not supposed to be built on, due to their susceptibility to high-velocity flooding.

However, the Federal Emergency Management Agency (FEMA) allowed these structures to be built in the historic floodway because the city applied for a FEMA flood map adjustment of the flood line. Keep in mind that insurance rates are based on the FEMA flood map delineations. The strategy of changing the FEMA flood map based on the recent construction of flood structures built to the 100-year flood event only theoretically removes the structures from the area that has historically flooded. Changing a line on a map does not change the reality of flooding despite engineered flood structures that fail. The danger with this process is our denial that rivers flood in response to their changing watershed dynamics and climate.

The insurance companies are aware of the accelerated climate change events and are expecting to recalibrate insurance rates. These businesses are scrambling to adjust insurance rates to fit with the perceived risks. Get ready, because we will soon be hit with these skyrocketing increases. Folks living adjacent to Salvador Creek will be slapped with increased insurance rates, as this area is now high risk for flooding.

People who live behind new flood structures may see insurance cost decreases, but this is a false security because we will continue to see severe weather changes and sea rise due to climate change. Ocean tides have crept through the city of Napa all the way to Hardman Lane, though a few decades ago, the tidal influence went only to Trancas Street.

The single best watershed protection is to immediately stop all clear-cutting of our wildlands and forests. In 2012, the Napa County Planning and Conservation Department is expected to approve more than 1,000 acres of wildland conversions to vineyards; 1,000 more acres of lost forest land.

Forests soak up water during storms and release it slowly into streams and rivers while recharging our groundwater aquifers. Wildlands sequester carbon and cool our landscape. It is up to 10 degrees cooler under an oak tree.

We cannot wait for laws and regulations enacted by politicians. There needs to be an immediate and significant change of values by individuals, and it will best happen by acting together to stop the destruction of our watersheds so we can ultimately survive climate change.

(Chris Malan is a longtime environmental activist in Napa County.)

Copyright 2015 Napa Valley Register. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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