What is the meaning of a “100 Year Flood” or “100 Year Flood Plain?”

The term “100-year flood” or living in the 100-flood plain, does not mean that the chance of a flood comes only every hundred years, but rather it means that statistically there is a 1 percent chance of a flood occurring in any given year. For any given “X-year flood”, divide 100 by X and that is the percent chance of a flood for that year.

Here is some great information regarding Flooding and 100-year floods
Read original article HERE

Floods: Recurrence intervals and 100-year floods (USGS)

Possibly you can remember when a really big rain, be it from a hurricane or a large frontal system, hit your town. If flood conditions occurred because of the rain then you might have heard the radio or TV weatherman say something like “This storm has resulted in a 100-year flood on Soandso River, which crested at a stage of 20 feet.” Obviously, this means that the river reached a peak stage (height) that happens only once every 100 years, right? A hydrologist would answer “Well, not exactly.” Hydrologists don’t like to hear a term like “100-year flood” because, scientifically, it is a misinterpretation of terminology that leads to a misconception of what a 100-year flood really is.

Instead of the term “100-year flood” a hydrologist would rather describe this extreme hydrologic event as a flood having a 100-year recurrence interval. What this means is described in detail below, but a short explanation is that, according to historical data about rainfall and stream stage, the probability of Soandso River reaching a stage of 20 feet is once in 100 years. In other words, a flood of that magnitude has a 1 percent chance of happening in any year.

What is a recurrence interval?

“100-year floods can happen 2 years in a row”

Statistical techniques, through a process called frequency analysis, are used to estimate the probability of the occurrence of a given precipitation event. The recurrence interval is based on the probability that the given event will be equalled or exceeded in any given year. For example, assume there is a 1 in 50 chance that 6.60 inches of rain will fall in a certain area in a 24-hour period during any given year. Thus, a rainfall total of 6.60 inches in a consecutive 24-hour period is said to have a 50-year recurrence interval. Likewise, using a frequency analysis (Interagency Advisory Committee on Water Data, 1982) there is a 1 in 100 chance that a streamflow of 15,000 cubic feet per second (ft3/s) will occur during any year at a certain streamflow-measurement site. Thus, a peak flow of 15,000 ft3/s at the site is said to have a 100-year recurrence interval. Rainfall recurrence intervals are based on both the magnitude and the duration of a rainfall event, whereas streamflow recurrence intervals are based solely on the magnitude of the annual peak flow.

Ten or more years of data are required to perform a frequency analysis for the determination of recurrence intervals. Of course, the more years of historical data the better—a hydrologist will have more confidence on an analysis of a river with 30 years of record than one based on 10 years of record.

Recurrence intervals for the annual peak streamflow at a given location change if there are significant changes in the flow patterns at that location, possibly caused by an impoundment or diversion of flow. The effects of development (conversion of land from forested or agricultural uses to commercial, residential, or industrial uses) on peak flows is generally much greater for low-recurrence interval floods than for high-recurrence interval floods, such as 25- 50- or 100-year floods. During these larger floods, the soil is saturated and does not have the capacity to absorb additional rainfall. Under these conditions, essentially all of the rain that falls, whether on paved surfaces or on saturated soil, runs off and becomes streamflow.

How can we have two “100-year floods” in less than two years?

This question points out the importance of proper terminology. The term “100-year flood” is used in an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year. Likewise, the term “100-year storm” is used to define a rainfall event that statistically has this same 1-percent chance of occurring. In other words, over the course of 1 million years, these events would be expected to occur 10,000 times. But, just because it rained 10 inches in one day last year doesn’t mean it can’t rain 10 inches in one day again this year.

What is an Annual Exceedence Probability?

The USGS and other agencies often refer to the percent chance of occurrence as an Annual Exceedance Probability or AEP. An AEP is always a fraction of one. So a 0.2 AEP flood has a 20% chance of occurring in any given year, and this corresponds to a 5-year recurrence-interval flood. Recurrence-interval terminology tends to be more understandable for flood intensity comparisons. However, AEP terminology reminds the observer that a rare flood does not reduce the chances of another rare flood within a short time period.

Does a 100-year storm always cause a 100-year flood?

No. Several factors can independently influence the cause-and-effect relation between rainfall and streamflow.

Extent of rainfall in the watershed: When rainfall data are collected at a point within a stream basin, it is highly unlikely that this same amount of rainfall occurred uniformly throughout the entire basin. During intensely localized storms, rainfall amounts throughout the basin can differ greatly from the rainfall amount measured at the location of the rain gage. Some parts of the basin may even remain dry, supplying no additional runoff to the streamflow and lessening the impact of the storm.

Soil saturation before the storm: Existing conditions prior to the storm can influence the amount of stormwater runoff into the stream system. Dry soil allows greater infiltration of rainfall and reduces the amount of runoff entering the stream. Conversely, soil that is already wet from previous rains has a lower capacity for infiltration, allowing more runoff to enter the stream.

Relation between the size of the watershed and duration of the storm: Another factor to consider is the relation between the duration of the storm and the size of the stream basin in which the storm occurs. For example, a 100-year storm of 30-minutes duration in a 1-square-mile (mi2) basin will have a more significant effect on streamflow than the same storm in a 50-mi2 basin. Generally, streams with larger drainage areas require storms of longer duration for a significant increase in streamflow to occur. These and other factors determine whether or not a 100-year storm will produce a 100-year flood.

The 100-year flood level can change

Since the 100-year flood level is statistically computed using past, existing data, as more data comes in, the level of the 100-year flood will change (especially if a huge flood hits in the current year). As more data are collected, or when a river basin is altered in a way that affects the flow of water in the river, scientists re-evaluate the frequency of flooding. Dams and urban development are examples of some man-made changes in a basin that affect floods, as shown in the charts below.

chart from USGS article on flooding posted under whats new

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DPW website has been updated

The Department of Public Works website has been updated to include the most recent information on the
Indianapolis White River (North) Flood Damage Reduction Project

December 28, 2015

Canal West Bank Alignment Analysis

The City completed the Canal West Bank Alignment Analysis in 2015 to assist in decision making related to the final alignment of the Indianapolis North Flood Damage Reduction Project.  The analysis was completed by the City of Indianapolis as a result of stakeholder concerns regarding the protection of the water supply.  On November 9, 2015, Mayor Greg Ballard met with Colonel Beck, Commander, Louisville District, United States Army Corps of Engineers (Corps) to discuss the draft analysis and its findings.  On December 9, 2015, the Corps informed the City that READ MORE HERE

 

FEMA Updates Flood Maps

It’s actually somewhat old news that FEMA has updated their flood maps to include many more homes into the flood plain.  However many folks are still unaware that soon they will be required to pay flood insurance.  The maps below show the current homes in the flood plain and the homes that are newly considered in the flood plain.

flood dates

Flood Map Update Schedule

 

effective

Flood Plains (effective or “current”)

 

preliminary

Flood Plains (preliminary or “as of April 19, 2016”)

 

WNA Supports City’s Decision

December 21, 2015 

Dear Mayor Ballard and Mayor-Elect Hogsett,

On behalf of the Warfleigh Neighborhood Association (WNA), I am writing to support the Department of Public Works’ selection of the Westfield Blvd. alignment to complete the Indianapolis North Flood Damage Reduction Project. The completion of this project is essential to ensuring the safety and sustainability of our neighborhood.

For over 15 years, Warfleigh residents have anticipated the completion of this project. Many of our neighbors bought their homes after researching projected completion dates for this project that have been delayed multiple times by previous administrations. While we all knowingly chose to live in a flood zone and understood the associated risks at the time of purchase, we invested in this great neighborhood under the assurance from the city that the project would be completed by now.

As recently as 2012, the projected completion date was set for the fall of 2014. Each year the project is delayed, hundreds of thousands dollars in flood insurance premiums are funneled out of the local economy from the Warfleigh neighborhood alone. Even if the Westfield Blvd alignment construction is started on schedule, residents in the project zone would not realize full protection until late 2018. Cost savings would then follow approximately a year later, once FEMA revises its flood maps.

As a result of the repeated delays, the Warfleigh neighborhood was affected by the nationwide crisis caused by the implementation of the Biggert-Waters Act of 2012 (BW-12). In late 2013, homeowners were shocked to learn that the flood insurance premiums they had already been paying were highly subsidized. Due to the NFIP’s $23 billion of debt, Congress and FEMA were forced to begin phasing out these subsidies for all flood insurance policy holders nationwide. Unfortunately, BW-12 left Warfleigh residents facing unaffordable premium hikes and potential homebuyers were fleeing. Our current Vice President Barbara Moser experienced this first hand after moving into Warfleigh in November, 2013.  Their home’s previous owner’s flood insurance policy rate of just over $1000 per year ballooned to more than $8,000 for the Mosers under the rules of BW-12. Legislation passed in 2014 provided short-term relief to situations like Barbara’s by slowing the removal of subsidies, but residents in the flood zone will continue to face substantial increases in premiums every year until the wall is completed.

The WNA believes that the Westfield Blvd alignment is the correct path forward for the completion of this project. While it has been determined to be economically unfeasible to include Rocky Ripple in this federally subsidized project, we empathize with our Rocky Ripple neighbors’ concern for flood safety and rising flood insurance premiums. We will continue to call on the City of Indianapolis to determine a realistic alternative for flood protection for our neighbors in Rocky Ripple.

Sincerely,
Steve Brining,

President Warfleigh Neighborhood Association
president@warfleigh.net

Renderings of the Proposed Wall along Westfield

These are renderings of the proposed wall along that the Butler Tarkington Neighborhood Association (BTNA) opposes in part due to aesthetic reasons.  Another reason noted was there will be “limited access to the canal”

BTNA quote

In reality, this view shows that there will be an additional bridge across the Canal that currently is not there.  You can find the full document HERE

.fw2 fw1

Indianapolis Announces How It Will Complete Northside Levee

Find original article and photos HERE

By: Ryan Delaney

Updated, Dec. 17:

INDIANAPOLIS — Indianapolis public works officials have finally decided how the city will complete a flood wall along the White River on the city’s north side. The decision is being welcomed by some neighborhoods along the river, but denounced by others

The public works department said in a Monday afternoon announcement it will extend an earthen levee from Kessler Boulevard to Butler University along the east side of the White River and Central Canal and connect it to high ground near Butler’s athletic facilities.

That will require a flood gate to be built across the canal, north of Butler University’s campus. The canal is a major source of drinking water for the city.

Construction of the levee, which is intended to protect residents living south and east of the river from flooding began in 2002, but then stalled after the second phase over pushback from neighborhood groups down river.

The city will pay a quarter of the $40 million construct cost and the Army Corps. of Engineers will fund the rest. When completed, it will remove properties in the Broad Ripple, Warfleigh and Butler-Tarkington neighborhoods from needing expensive flood insurance.

But the current plan completely leaves the town of Rocky Ripple susceptible to rising river waters.

A neighborhood group called Save Warfleigh that has been lobbying for the levee to be completed called the announcement “a wonderful development.” Residents in Warfleigh have seen their flood insurance premiums increase as the city has spent two decades deciding how to complete the levee.

Save Warfleigh has been waging a public relations and petition campaign to pressure city officials to decide how they’ll complete the wall. It’s been delayed by studies, pushback by property owners down river and changing Army Corps. standards.

Right now, flood protection from the levee extends from Broad Ripple to about Kessler Boulevard, but since the levee isn’t connected to high ground, it’s not technically complete and would provide minimal protection in the event of a flood. This third phase is intended to complete the levee.

Butler-Tarkington and Rocky Ripple residents, meanwhile, are outraged, blaming the city of making an 11th hour decision that damages aesthetics and doesn’t protect all homes.

“The folks upstream only seem to want insurance protection. The folks downstream are looking for protection from floods,” said Dennis Faulkenberg, a Butler-Tarkington resident.

The neighborhood associated said this in part of a statement it put out.

BTNA has long opposed this option because of the threat it creates for catastrophic damage to the Central Canal and the Town of Rocky Ripple in the event of a major flood. Moreover, the Westfield Boulevard alignment requires the clearing of hundreds of mature trees and creates a 6 foot flood wall that runs along the canal and through historic Holcomb Gardens on the Butler University campus.

Butler University has expressed concerns about damage to Holcomb Gardens from levee construction.

The neighborhood association is asking the city to delay this decision in order for a new administration, that of Mayor-elect Joe Hogsett, to weigh in. Legal action would be premature, Faulkenberg said.

Contact Ryan: 317.489.4491 | rdelaney@wfyi.org | @rpatrickdelaney

Butler-Tarkington, Meridian-Kessler groups oppose city’s route for last leg of flood wall

John Tuohy, john.tuohy@indystar.com 6:18 a.m. EST December 16, 2015

Find the original article HERE

The city has agreed to an Army Corps of Engineers plan for construction of the last section of the Northside White River flood wall, a choice that placated some residents and angered others Tuesday.

The levee will be built mostly on the eastside of the Central Canal Towpath, along Westfield Boulevard from North Capitol Avenue southwest to the Holcomb Gardens at Butler University. But the 6-foot-tall concrete berm will start west of the canal on the east bank of the White River north of the Riviera Club at Illinois Street. The levee will cross over the canal at Capitol. A lock, or floodgate, on the canal will allow water to run through the crossover.

The $40 million Northside White River flood wall project (see project map), underway for 20 years, is intended to reduce flooding in the Northside neighborhoods of Broad Ripple, Meridian-Kessler, Butler Tarkington and Warfleigh. City officials said the flood protection will cut flood insurance costs and increase property values for homeowners

Residents of Warfleigh, where the flood wall already has been installed on the east side of the White River, applauded the route choice made by Indianapolis’ Department of Public Works.

“This is a wonderful development,” said Jim Polito, who organized a group called Save Warfleigh to encourage the city to complete the project. “We now must be very vocal in supporting the city’s decision and the Westfield Boulevard alignment so that the project remains on track.”

The city was considering two basic routes for the last leg. The other route was on the west bank of the canal, closer to the White River. But a $200,000 environmental study by the city concluded that the soil on that side wasn’t strong enough to support a wall. Most of the levee, running from the heart of Broad Ripple to Illinois Street, has been built.

Several neighborhood groups, including the Butler-Tarkington Neighborhood Association, the Meridian-Kessler Neighborhood Association and the Broad Ripple Village Neighborhood Association, have expressed opposition to the chosen route.

To build the nearly mile-long barrier, trees need to be cut down and vegetation cleared on Westfield. The opponents have said the levee would gather trash and destroy sight lines to the canal and its habitat, home to great blue herons, turtles and other wildlife.

Butler-Tarkington board member Dennis Faulkenberg said the flood wall’s crossing the canal and the building of floodgates could be troublesome for the water supply. He also questioned whether the wall would stop the canal area from flooding.

Butler and Citizens Energy, which owns the canal and uses it as a reservoir for most of the city’s drinking water, also had opposed the Westfield route.

Citizens had said the canal water supply could be threatened in a major flood if the wall was built on the east side. But the utility said Tuesday in a written statement that it now supports the route because the city had promised to build Citizens a new pumping station.

“The City has assured Citizens plans will include a new drinking water intake structure downstream of the proposed Central Canal flood gate, so over 60 percent of our community’s drinking water supply is not interrupted when the canal flood gate is closed,” a statement from the utility said. “Based on this assurance, Citizens fully supports Mayor Ballard’s decision to move forward with the Westfield (Boulevard).”

Department of Public Works spokeswoman Jennifer Hashem said in a written statement that “currently a pumping station has been considered, but the details need to be examined.”

“The station could create a secondary supply source,” she said, adding that the city had no cost estimate.

Butler spokesman Michael Kaltenmark said the university would likely continue to oppose the plan because the levee would cut through Holcomb Gardens.

Neither plan would do anything for Rocky Ripple, which is between the White River and the canal near 52nd Street. The town council voted in 1996 against the levee being built on the river because it would obstruct homeowners views to the water. Years later, town leaders decided they wanted the protection, but by that time the Army Corps said it would cost too much to change the route.

Hashem said a construction timeline has not been determined. But Polito said he understood from public meetings he attended it would take three to five years to complete.

Call Star reporter John Tuohy at 317 444-6418 and follow on Twitter @john_tuohy