by Carrie Brown, Engineering Technician
It is a common misconception that a “100-year flood” happens once in 100 years. The term can be quite deceiving when taken at face value. In fact, it is possible for two 100-year storms to occur 50, 25, or even one year apart. Let’s break down the terminology a bit and discuss what “100-year” is actually referring to.
Flooding occurs when water overflows onto land that isn’t typically inundated. Floods can result from large rain events, waves from a large body of water coming onto land, rapid snow melt, or when an artificial barrier breaks (such as a dam).
A flash flood results when heavy rainfall falls at a faster rate than can be absorbed into the ground or stored in a reservoir or lake. Similar to a sponge, the ground eventually becomes saturated, and remaining water is left to flow on the surface. Large impervious areas, such as parking lots, roads, and building roofs, can cause water to accumulate even faster. Flash flood events can be particularly dangerous as there may be inadequate time to warn (and in some cases, evacuate) the public residing within the flooded area.
A floodplain is the portion of a valley that has historically been inundated by overflowing streams, creeks, and rivers. Consequently, these areas have a higher chance of experiencing flood conditions. The Federal Emergency Management Agency (FEMA) and other government agencies have mapped out these areas affected by past flooding events. These maps are often used to determine flood insurance requirements or to aid communities in regulating future development. Permits are needed for development within a FEMA regulated floodplain. Floodplain maps can be found at the FEMA Flood Map Service Center.
At Fairfield SWCD, we engineer grassed waterways and stabilization structures, such as the timber box seen above, to handle large rain events. Waterways help to filter water and prevent soil erosion.
As defined by the National Weather Service, “a 100-year rainfall event (or more accurately the 100-year rainfall amount for a specified duration and at a given location) is an amount that on average is exceeded every 100 years, so its average recurrence interval is 100 years.” This means that it has a 1% chance of occurring in any given year. The actual number of years between flooding events can vary dramatically; it can happen twice in one year, three times in 50 years, or any interval in between.
Similarly, a 50-year rainfall event has a 1 in 50 or 2% chance of occurring in a year.
Now you can see why it is incorrect to take these terms literally since the average recurrence interval is simply an average.
|Recurrence intervals in years||Probability of occurrence in any given year||Percent chance of occurrence in any given year|
|100||1 in 100||1%|
|50||1 in 50||2%|
|25||1 in 25||4%|
|10||1 in 10||10%|
|5||1 in 5||20%|
|2||1 in 2||50%|
The amount of rain that “qualifies” for a 100-year rainfall is determined by studying past floods. Scientists use statistics to observe the frequency of large rain events and the average number of years between these events. From this they determine the probability that a flood of any given size will take place within a year.
As you might expect, these rainfall amounts vary based on location. The document below, produced by the National Oceanic and Atmospheric Administration (NOAA), shows the average recurrence interval for rainfall durations, ranging from 5-minutes to 60-days for Lancaster, Ohio.
The Precipitation Frequency Estimate can be selected for any location within the United States.Precipitation-Frequency-Data-Server
Yes, many flood designations will change over time. When a river basin is altered, including dams or changes in upstream urban development (and increased impervious surfaces), scientists then reevaluate the frequency of flooding and the amount of damage it could produce. This new data may be used to alter a location’s precipitation frequency estimates.
Check out the following resources for more information on floods, floodplains, and rainfall events.
Fairfield County Regional Planning Commission: Floodplains
City of Lancaster Floodplain Information
City of Pickerington Floodplain Information
FEMA Map Service Center
National Weather Service’s Hydrometeorological Design Studies Center
USGS: The 100-Year Flood
USGS: Daily Streamflow Conditions
Community Collaborative Rain, Hail & Snow Network
This week’s 2021 Tree Sale highlight is the Red Osier Dogwood. Though it tends to be a bit of an underdog, this beautiful red-stemmed shrub has a boatload of functionality. As a Soil and Water Conservation District, we advocate for this species often, as it is at the top of its class at stabilizing soil and preventing erosion. This is especially true along streambanks.
Red Osier Dogwood is one tough cookie. This species can take a beating and keep on going. Once established, it can be inundated along a stream edge and live to tell its story. It can be pruned hard for live fascines (more on that later!) and easily make a comeback the following spring. Talk about resiliency!
Additionally, due to the striking red color of its stems, red osier dogwood is often used as an ornamental to beautify landscapes.
Let’s take a closer look at this shrubby native that has found the sweet spot between durability and charm.
The red osier dogwood doesn’t mind getting its feet wet…in fact it prefers it! They grow best in soils that are saturated for at least a portion of the growing season. Therefore, they are often seen growing on the edges of lakes, ponds, within wetlands, and on streambanks. Red osier dogwoods are perfect for sites that are nitrogen-rich and shallowly inundated in the spring, only to dry out by late summer.
On average, the red osier dogwood grows to a height and width of approximately 10’. It has a fast growth rate, gaining more than 2’ a year in height.
Red osier dogwood benefits from a type of pruning called coppicing. This management method involves cutting all stems to approximately 2-3 inches from the base in late fall, after the shrub has shed its leaves. Following pruning, apply mulch and fertilizer around the base. Coppicing will stimulate the shrub to send up new stems, often with especially vivid burgundy color.
The fleshy white berries that ripen in late summer are favored by many bird species, including eastern bluebirds, cardinals, woodpeckers, and grosbeaks. Gamebirds such as bobwhite quail, ring-necked pheasants, and wild turkeys also benefit from red osier dogwood fruit.
The fruit and foliage are enjoyed by mammals too, including black bear, beaver, squirrels, and deer.
Red osier dogwood typically begins leafing out in April. The bark, twigs, and leaves of the new growth are bright green in color.
White to cream-colored flower clusters appear from June to August, eventually developing to smooth, white berries that ripen in late summer.
Beginning in September, leaves turn from green to shades of red and purple and are eventually shed for the winter.
Bare, deep burgundy branches provide an interesting contrast with white snow and the drab browns of the off-season.
Because of its dense growth nature, red osier dogwood can be used as a secondary plant in windbreaks. It is also an ideal species to use for streambank stabilization as live fascines.
Live fascines are long bundles of live woody vegetation (6-8 inches in diameter) buried in a streambank in shallow trenches placed parallel to the flow of the stream. These branches are harvested from adult red osier dogwoods – a great use of stems leftover from a fall coppicing session!
The plant bundles sprout in the spring and develop a root mass that will hold the soil in place and protect the streambank from erosion. This method is often coupled with a row of stone placed at the toe (bottom of the slope that supports the weight of the bank) of an eroding bank.
Below are photos from a streambank stabilization project that Fairfield SWCD designed for Lancaster City School District to address streambank erosion issues along Fetters Run.
Bare-root Tree Planting Guide
National Tree Benefits Calculator
South Central Power: Plant the Right Tree in the Right Place
Fairfield SWCD Tree Planting Guide
I don’t know about you, but one of the keystone autumn memories I cherish from my childhood is raking leaves into an enormous pile…only to wreck my efforts by diving into them.
Of course, this would happen repeatedly until my father would declare the scattered mess off limits and hurry me inside to clean up.
As I got older, the task of raking lost its allure and became more of a chore, so as an adult, I’ve become a bit savvier when dealing with autumn’s leafy offerings.
Engulfed in leaves? Here are some options!
Option one, and likely the easiest route, is to simply leave the leaves where they fall!
If your yard is scattered with black cherry, locust, or other broadleaf species that have mostly smaller leaves, there may not be much action to take after they fall.
In the case of maple, oak, and other trees with larger foliage, however, a bit of mulching may be in order.
Use a lawnmower to shred the leaves into dime-sized pieces.
There are several benefits to gracing your grass with this mulchy mixture:
If your yard is rather arborous and fallen leaves are too thick to mulch with a lawn mower, it may be necessary to turn to plan B: Collect and compost leaves in a designated location.
Composting is a process in which microbes break down organic materials into a nutrient dense, soil-like material.
When done correctly, leaves and other fall garden debris can be composted and ready to use by late spring.
This is a great option for gardeners with the desire to build their own rich planting medium.
In the case of leaf piles, size and location matters!
A pile that is 3’ x 3’ x 3’ is manageable and large enough to maintain the heat needed for the composting process. Make as many piles as necessary, choosing a shaded site with good air flow.
“Feed” your compost pile throughout the fall season by adding freshly fallen leaves to a pile of older leaves.
You can also supplement by adding other natural materials you may have laying around such as grass clippings, garden debris, and kitchen scraps. (It’s important to note there are a number of materials that shouldn’t be added to backyard compost piles, such as animal products, that will attract pests and take too long to compost.)
Maintain your compost by keeping it moist and oxygenated.
Water is needed for the composting process, so it may be necessary to add water to your pile. Additionally, compost requires aeration. This can be accomplished by occasionally turning the pile with a spading fork or other garden tool.
Additional information on backyard composting can be found in OSU Extension’s Composting Series.
If you don’t have the space to compost your leaves and live in an area that offers leaf pickup, this service may be a good option. There are some important tips to remember when readying your leaves for collection however.
Keep yard waste, including leaves, grass clippings and garden debris, out of drainage ditches and storm drains.
If you caught our earlier article on stormwater, Only Rain Down the Drain, you likely remember that everything that enters a storm drain is ultimately outletted, untreated, into a local water body. Leaves, in particular, can be very problematic for storm sewer systems so never leave them in the path of stormwater. Even if leaves are left behind, water filtering through can become rich in nutrients. When this “leafy brew” makes its way into rivers and streams it can cause an overgrowth of algae and wreak havoc on water quality.
These same rules should also be followed in rural ditches and waterways.
Avoid piling leaves, grass clippings, etc. in drainage ditches and grassed waterways. Doing so blocks the natural path of water and can result in flooding and erosion.
When bagging leaves and yard waste, use biodegradable paper bags.
Paper is a better option, as plastic trash bags can take many years to break down. Better yet, designate a trash can for yard waste and mark it appropriately.
Specific leaf pickup guidelines for Fairfield County communities can be found below:
What comes to mind when you hear the word “storm water”? Perhaps you think of torrential rainfall deluging yards & overwhelming gutters in the springtime. Or maybe you imagine water screaming through large parking lots, making it impossible to avoid puddles as you tiptoe towards the store entrance. And it could be that you visualize the storm drains that line the curb outside of your home (that are often labeled “storm water”)!
These are all accurate representations of storm water – but we find there are many misconceptions as of where this water goes.
If you’re a resident of Lancaster, your storm water eventually spills into the Hocking River and travels southeast until it reaches the Ohio River.
If you’re from Pickerington or Lithopolis, the storm water surrounding you outlets into George, Walnut, Blacklick, or Sycamore Creek. It travels west and soon enters the Scioto River and makes its way down to the Ohio River.
Those of you in the Buckeye Lake watershed, the water entering your storm drains travels north to the Licking River. This travels east to the Muskingum River and then south to the Ohio River.
(If you wish to play the long game, the Ohio River later teams up with the Tennessee River to join the Mississippi and travels south to the Gulf of Mexico!)
Why is this all important? It’s proof that your actions matter. As storm water flows over parking lots, streets, and lawns it transports substances such as automotive waste, lawn chemicals, eroded soil, and just about any other small object in its way. And as we just discussed in our geography refresher, this water, and everything it is carrying, has a long path ahead of it!
* Keep chemicals, yard waste, and other materials out of storm drains.
* Check your car and lawn equipment for oil leaks and make repairs as soon as possible.
* Avoid washing your car in your driveway, as this provides detergents a direct path to the storm drain. Instead, use a commercial car wash or wash your vehicle in a grassy area so the ground can filter the water.
* Keep your soil and sediment from moving. Plant ground cover to protect and stabilize areas prone to erosion.
* Choose native plants for your landscapes, as these species need fewer chemical inputs and require less water.
* Cut down on your use of insecticides by providing habitat for pest-eating critters such as birds, bats, and beneficial insects.
* Is it time to drain your swimming pool? Make sure you know whether its contents can be discharged into the the storm sewer.
Fairfield SWCD Storm Water Pollution Reduction
Pickerington Storm Water Management
Lancaster Storm Water Management
Ohio EPA Storm Water Program
U.S. EPA Household Waste
Ohio EPA Office of Pollution Prevention
U.S. EPA Nonpoint Source Information