Association of State Wetland Managers - Protecting the Nation's Wetlands.

The Wetland Wanderer: Flooding, Heavy Precipitation and Wetlands – It’s All Connected

by Brenda Zollitsch

News over the last couple of weeks has contained many stories from Texas, Oklahoma and other parts of the country where extreme precipitation events have led to flooding, in some cases at levels even older generations have not seen.  My brother lives just outside Austin, Texas and had his house flood unexpectedly with rainfall that his neighbors had not seen in more than 25 years.

water163152Experts bear out that these events and their frequency are changing, although flooding has always occurred at intervals over time.  Data indicate that some of these current major storm events remain within the normal variability of natural events, but the overall trend is towards a greater number of more intense storms and with greater impacts.  According to the U.S. Environmental Protection Agency, “the prevalence of extreme single-day precipitation events remained fairly steady between 1910 and the 1980s, but has risen substantially since then. Over the entire period from 1910 to 2013, the portion of the country experiencing extreme single-day precipitation events increased at a rate of about half a percentage point per decade.”

fig1water63152Figure 1 shows percent increases in the amount of precipitation falling in very heavy events (defined as the heaviest 1% of all daily events from 1958 to 2012 for each region of the continental United States.  The data shows trends larger than natural variations for the Northeast, Midwest, Puerto Rico, Southeast, Great Plains and Alaska with the largest percent increase in the Northeast. (National Climate Change Assessment, EPA, 2014)

Although we cannot control the amount of rain that falls, there are other actions that are within our control that affect where water goes, the speed with which it runs off and the infrastructure designed to manage it. First, the change in impervious cover, particularly in urban and urbanizing areas, in the last 50 years is substantial.  Lands that at one time had the ability to absorb water have been transformed into asphalt, cement and rooftops.  Next, wetlands that could capture, slow and filter waters have been lost. Wetlands serve as nature’s sponges, allowing precipitation to gather in wetlands and sink into the land and/or run off slowly.  Third, infrastructure that manages stormwater runoff was, in most places across the U.S., designed for smaller storm events than the larger events – generally 15-20 year storm events. We are seeing larger storms with increasing frequency.  Without the ability to absorb, store or channel stormwater effectively during major events, culverts blow out, pipes rupture, and combined sewer overflows channel sewage to our waterbodies.

It’s time that we looked towards alternative solutions.  One needs only look at the flooding in Vermont that resulted from Hurricane Irene to understand that we need to start thinking differently about how water is managed before, during and after major precipitation events.  At the same time, we need to think about the fact that California is in a severe drought and desperately needs to find ways to store water for public, private and ecosystem health.

water353152Wetlands need to be more commonly and broadly considered as part of adapting to extreme precipitation events.  The preservation and restoration of natural wetlands can lead to significant reductions in runoff quantities and velocity.  The creation of new wetlands or constructed wetlands (which are designed for the purpose of managing stormwater) are also part of the menu of wetland-related options for managing precipitation.

The concept that wetlands can mitigate storm events is not new conceptually.  ASWM’s own Jon Kusler developed and shared the graphic in Figure 2 that shows the positive impact of wetlands on peak stormwater flows.  According to Maine DEP, “along rivers, wetlands usually form natural pathways for flood waters from upstream to downstream points. If those pathways are altered or removed, flood waters can go elsewhere, potentially damaging property and threatening public safety. Without wetlands as a natural flood control mechanism, flooding can become more severe.”

Combining gray and green infrastructure can produce multiple benefits. It can reduce stormflows, treat stormwater runoff, and provide natural beauty and recreation opportunities, and even public spaces for things like art and music.  We also can no longer rely solely on weather data from the past to predict the future.  Future planning should consider the changes in weather patterns, extreme event frequency and the increases in impervious surface.

In May, 35 trillion gallons of water fell on the state of Texas in the form of precipitation (National Weather Service, 2015).  The National Weather Service warns that the saturated land, even as surface waters recede, provide dangerous conditions for the southern states, as they come into the hurricane season.  For those parts of the country where there is truly “nary a drop to drink” such as California, which is experiencing a drought not seen in more than a generation, wetlands once again have a role to play – with the ability to store water for slow release and provide essential resources to ecosystems and those who rely on them for their livelihoods.

As we watch yet another national tragedy unfold, we need to be thinking about both large-scale mindset changes and also small-scale incorporation of local actions that reduce runoff, slow flow and accommodate major rain events in ways that do not threaten public safety.  Instead, as part of the solution, we should consider leveraging the services that wetlands provide.

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