Flood - NOLA Ready

Definition

A flood is the overflow of water onto land that is usually not inundated. The National Flood Insurance Program (NFIP) defines a flood as:

A general and temporary condition of partial or complete inundation of two or more acres of normally dry land area or of two or more properties from the overflow of inland or tidal waters, unusual and rapid accumulation or runoff of surface waters from any source, mudflow, or collapse or subsidence of land along the shore of a lake or a similar body of water as a result of erosion or undermining caused by waves or currents of water exceeding anticipated cyclical levels that result in a flood as defined above.

Factors influencing the type and severity of flooding include:

Natural variables
- Precipitation,
- Topography,
- Vegetation,
- Soil texture, and
- Seasonality
Anthropogenic factors (Originating from Human Activity)
- Urbanization (extent of impervious surfaces like asphalt, concrete, etc.),
- Land use (e.g., agricultural and forestry tend to remove native vegetation and accelerate soil erosion), and
- The presence of flood-control structures such as drainage pumps and levees

Flooding in New Orleans can be the result of weather events such as hurricanes, thunderstorms (convectional and frontal), storm surge, and winter storms. Excess precipitation, produced from thunderstorms or hurricanes, is often the major initiating condition for flooding. Flooding in New Orleans can also result from infrastructure failures such as levees, floodwalls, and water main breaks. It is also important to note the potential for combined hazards, as was the case of Tropical Storm Barry and the Mississippi being at flood stage in July 2019.

Louisiana can have high rainfall totals at any time of day or year, but the weather patterns that produce these rains vary seasonally. During the cooler months, slow-moving frontal weather systems produce heavy rainfalls, while the summer and autumn seasons produce major precipitation in isolated thunderstorm events (often on warm afternoons) that may lead to localized flooding. During the warmer seasons, heavy rains that produce flash flooding are common, while river floods caused by heavy streamflow generally occur during the cooler months.

In cooler months, particularly in the spring, New Orleans is in peak season for severe thunderstorms. The fronts that cause these thunderstorms often stall while passing over the state, occasionally producing rainfall totals exceeding ten inches within a period of a few days. Since soil tends to be nearly saturated at this time (due to relatively low overall evaporation rates), spring typically becomes the period of maximum streamflow across the state. Together, these characteristics increase the potential for high water, and low-lying, poorly drained areas are particularly prone to flooding during these months.

In Louisiana, six specific types of floods are of main concern: riverine, flash, ponding, backwater, urban, and coastal.

	Riverine flooding occurs along a river or smaller stream. It is the result of runoff from heavy rainfall or intensive snow or ice melt. This includes the Mississippi River; whose levels rise and fall slowly due to its large capacity.
	Flash flooding occurs when locally intense precipitation inundates an area in a short amount of time, resulting in local streamflow and drainage capacity being overwhelmed.
	Ponding occurs when concave areas (e.g., parking lots, roads, and clay-lined natural low areas) collect water and are unable to drain.
	Backwater flooding occurs when water slowly rises from a normally unexpected direction where protection has not been provided. A model example is the flooding that occurred in LaPlace during Hurricane Isaac in 2012. Although the town was protected by a levee on the side facing the Mississippi, floodwaters from Lake Maurepas and Lake Pontchartrain crept into the community on the side of the town opposite the Mississippi River.
	Urban flooding is similar to flash flooding but is specific to urbanized areas. It takes place when stormwater drainage systems cannot keep pace with heavy precipitation, and water accumulates on the surface. Most urban flooding is caused by slow-moving thunderstorms or torrential rainfall.
	Coastal flooding can appear similar to any of the other flood types, depending on its cause. It occurs when normally dry coastal land is flooded by seawater but may be caused by direct inundation (when the sea level exceeds the elevation of the land), overtopping of a natural or artificial barrier, or the breaching of a natural or artificial barrier (i.e., when the barrier is broken down by the seawater). Coastal flooding is typically caused by storm surge, tsunami, and gradual sea-level rise.

In New Orleans, all six types of flooding have historically been observed. For purposes of this assessment, ponding, flash flooding, and urban flooding are all considered to be flooding as a result of stormwater from heavy precipitation.

Location and Extent of Flooding

Across most of the Parish, elevation varies by only a few feet. Most of New Orleans is below sea level and/or surrounded by flood levees. As such, the entire City is at risk of flooding. Figure 10 displays the topography of New Orleans, showing elevations below and above sea level in cross-section looking west. The map shows that the highest areas of the City border portions of the natural levee of the Mississippi River, particularly the area near the Garden District and Central Business District. The topography of New Orleans has been particularly influenced by the natural levee of the Mississippi River. With each Mississippi River flood, water spilled out of the river, depositing its sediment to raise the natural levee to an original average of 10 to 15 feet above sea level, and one to two miles in width, sloping very gently into the back swamp. In the New Orleans area today, the Mississippi River flows 10 feet to 15 feet above sea level. The map also shows that the lowest elevations of the City are located in the areas of Lakeview, Gentilly, and New Orleans East.

Figure 10: Topography of New Orleans Showing SWBNO Drainage Basins and Drainage Pump Stations Source: City of New Orleans

As a result of this minimal elevation change, when heavy rainfall events occur, water tends to pool rather than run off rapidly. Elevations below sea level combined with little slope in topography and an extensive levee system mean that rainwater cannot flow out of the Parish and must be pumped out. The greater New Orleans metropolitan area is served by over 80 pumping stations in four Parishes (Orleans, Jefferson, St. Bernard, and Plaquemines) with a combined capacity of over 30 billion gallons per day. All stations are equipped with pumps that are either directly driven by diesel engines or by electric motors that receive their power from diesel-electric generators. New Orleans is drained by 24 pump stations with a total design capacity of 50,891 cubic feet per second. Because the river levees are higher than the lake levees, most rainwater is pumped into Lake Pontchartrain. Exceptions are the two (2) West Bank pumping stations and two (2) stations in Eastern New Orleans that pump rainwater into the Intracoastal Waterway or the Industrial Canal. Rainfall amounts of greater than 1-2 inches an hour cause flooding of 6-10 inches in some low-lying areas, particularly those shown as being below ground elevation in the above map.

Areas of highest flood risk are identified by zones AE and VE in the City’s Flood Insurance Rate Map (FIRM, commonly known as the flood map, Figure 11). These areas are vulnerable to flooding from the 100-year storm. Figure 12 shows the number of parcels in each flood zone, according to the FIRM.

Figure 11: Flood Insurance Rate Map

Source: City of New Orleans

Figure 12: Distribution of Properties by Flood Zone

Floods are commonly measured by the probability of occurrence.

A 10-year flood event, for example, is an event of small magnitude (in terms of streamflow or precipitation) but with a relatively high annual probability of recurrence (10%).

A 100-year flood event is larger in magnitude, but it has a smaller chance of recurrence (1%).

A 500-year flood event is significantly larger than both a 100-year event and a 10-year event. Still, it has a lower probability than both to occur in any given year (0.2%).

It is important to understand that an X-year flood event does not mean an event of that magnitude occurs only once in X years. Instead, it just means that on average, we can expect a flood event of that magnitude to occur once every X years. Given that such statistical probability terms are inherently difficult for most people to understand, the Association of State Floodplain Managers (ASFPM) promotes the use of more tangible expressions of flood probability. The ASFPM also expresses the 100-year flood event as having a 25% chance of occurring over the life of a 30-year mortgage.

The size of a specific flood event is defined through the statistical analysis of historical data on precipitation, flow, and discharge rates. The magnitude of an X-year flood event for a particular area depends on the source of flooding and the area’s location. Consequently, different 100-year flood events can have very different impacts. The 100-year flood events in two separate locations have the same likelihood to occur, but they do not necessarily have the same magnitude. For example, a 100-year event for the Mississippi River means something completely different in terms of discharge values (ft3/s) than for the Amite River. The definition of what constitutes a 100-year flood event is specific to each location, source of flooding, and time since floodplain and river characteristics change over time.

The 100-year event is of particular significance since it is the regulatory standard that determines the obligation or lack thereof to purchase flood insurance. Flood insurance premiums are set depending on the flood zone, as modeled by National Flood Insurance (NFIP) and shown in Flood Insurance Rate Maps (Figure 11). The NFIP and FEMA suggest insurance rates based on Special Flood Hazard Areas (SFHAs). A SFHA is the land area covered by the floodwaters of the base flood, which is the 100-year floodplain where the National Flood Insurance Program (NFIP)’s floodplain management regulations must be enforced and the area where the mandatory purchase of flood insurance applies.

Flooding Alerts

The National Weather Service (NWS) posts flood statements, watches, and warnings based on stream gauge levels and precipitation forecasts. The NWS issues the following weather statements concerning the severity of floods:

Table 14: Flood Category Definitions
Flood Category	Definition
Minor Flooding	Minimal or no property damage, but possibly some public threat.
Moderate Flooding	Some inundation of structures and roads near streams. Some evacuations of people and/or transfer of property to higher elevations.
Major Flooding	Extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.
Record Flooding	Flooding, which equals or exceeds the highest stage or discharge at a given site during the period of record keeping.
Flood Warning	Issued along larger streams when there is a serious threat to life or property.
Flood Watch	Issued when current and developing hydrometeorological conditions are such that there is a threat of flooding, but the occurrence is neither certain nor imminent.

The NOHSEP Public Information Officer also prepares and issues warnings to emergency managers and the threatened population through the City’s NOLA Ready public engagement campaign. Each of the potential sources of flooding, rainfall, riverine, and coastal storms is monitored differently. NWS, through the National Hurricane Center (NHC), provides tropical weather outlooks and warnings. USACE, through the River Forecast Center, tracks Mississippi River gauge height and provides forecasts and warnings. NOHSEP prepares pre-scripted warning messages tailored to escalating impact levels and distributes these messages via multiple media to supplement the alerts provided by NWS. Residents can sign up for emergency alerts by staying connected through NOLA Ready.

Previous Occurrences

Historically, there have been 69 flooding events that have created significant flooding in New Orleans between 1997 and 2019. Below is a brief synopsis of the 12 major flooding events that have occurred since 1996, including flooding events that have occurred since the parish’s last planning update. The following table represents data from the Storm Events Database from the National Centers for Environmental Information at NOAA. While this data provides a historical record of events over the last 70 years, some of the data is unreliable. For example, the NOAA data indicates that there has been no property damage associated with certain flooding events, which we know is not the case. This data is used to provide a baseline analysis and is supplemented, where appropriate, with other data sources. The blank cells in the table below are due to no data being available from the National Centers for Environmental Information, NOAA, Storm Events Database when this table was generated.

Table 15: Historical Floods in Orleans Parish with Locations from 1994 – 2019
Date	Event Type	Cause of Flood	Beginning Location	End Location
10/5/1996	Coastal Flood
5/19/1997	Flood		New Orleans	New Orleans
1/5/1998	Flood		New Orleans	New Orleans
2/15/1998	Storm Surge/Tide		Chef Menteur	Ft Pike
3/7/1998	Flood		New Orleans	New Orleans
4/29/1998	Flood		Countywide	Countywide
8/21/1998	Flood		New Orleans	New Orleans
9/11/1998	Flash Flood		Countywide	Countywide
9/12/1998	Storm Surge/Tide		Countywide	Countywide
8/9/1999	Flood		Countywide	Countywide
6/5/2001	Flood		New Orleans	New Orleans
6/7/2001	Flash Flood		Algiers	Algiers
6/11/2001	Flash Flood		New Orleans	New Orleans
6/21/2001	Flood		New Orleans	New Orleans
8/17/2002	Flood		New Orleans	New Orleans
8/22/2002	Flood		New Orleans	New Orleans
9/25/2002	Flash Flood		Countywide	Countywide
6/19/2003	Flash Flood		New Orleans	New Orleans
6/30/2003	Flash Flood		Countywide	Countywide
6/30/2003	Storm Surge/Tide
9/15/2004	Storm Surge/Tide
10/9/2004	Storm Surge/Tide
7/5/2005	Storm Surge/Tide
8/29/2005	Storm Surge/Tide
9/23/2005	Storm Surge/Tide
12/21/2006	Flash Flood	Heavy Rain	New Orleans	New Orleans
10/22/2007	Flash Flood	Heavy Rain	Vieux Carre	Gentilly
4/26/2008	Flash Flood	Heavy Rain	Algiers	Algiers
6/15/2008	Flash Flood	Heavy Rain	Algiers	Vieux Carre
6/29/2008	Flood	Heavy Rain	Vieux Carre	Vieux Carre
9/1/2008	Storm Surge/Tide
9/11/2008	Storm Surge/Tide
3/27/2009	Flash Flood	Heavy Rain	Algiers	Gentilly
9/13/2009	Flash Flood	Heavy Rain	Lee	Lee
12/12/2009	Flash Flood	Heavy Rain	Vieux Carre	Little Woods
4/23/2010	Flash Flood	Heavy Rain	New Orleans	New Orleans
5/16/2010	Flash Flood	Heavy Rain	New Orleans	New Orleans
9/2/2011	Storm Surge/Tide
4/3/2012	Flash Flood	Heavy Rain	New Orleans	Gentilly
4/4/2012	Flash Flood	Heavy Rain	New Orleans	Algiers
7/20/2012	Flash Flood	Heavy Rain	New Orleans	Vieux Carre
7/20/2012	Flash Flood	Heavy Rain	(New)Lkfrnt Arpt New	Vieux Carre
8/9/2012	Flash Flood	Heavy Rain	New Orleans	Algiers
8/28/2012	Storm Surge/Tide
8/29/2012	Flash Flood	Heavy Rain / Tropical System	New Orleans	New Orleans
5/2/2013	Flash Flood	Heavy Rain	New Orleans	New Orleans
11/26/2013	Coastal Flood
5/9/2014	Flash Flood	Heavy Rain	Mc Donoghville	Algiers
5/9/2014	Flash Flood	Heavy Rain	Mc Donoghville	Vieux Carre
4/14/2015	Flash Flood	Heavy Rain	Chef Menteur	South Pt
5/31/2015	Flash Flood	Heavy Rain	New Orleans	Mc Donoghville
10/25/2015	Coastal Flood
4/1/2016	Flash Flood	Heavy Rain	New Orleans Lakefront	Vieux Carre
6/21/2017	Storm Surge/Tide
8/5/2017	Flash Flood	Heavy Rain	Vieux Carre	New Orleans
8/8/2017	Flash Flood	Heavy Rain	Vieux Carre	Vieux Carre
10/2/2017	Flash Flood	Heavy Rain	Vieux Carre	Vieux Carre
10/2/2017	Flash Flood	Heavy Rain	Vieux Carre	Vieux Carre
10/7/2017	Storm Surge/Tide
5/18/2018	Flash Flood	Heavy Rain	Gentilly	Algiers
7/3/2018	Flash Flood	Heavy Rain	Cutoff	Mc Donoghville
7/3/2018	Flash Flood	Heavy Rain	Cutoff	New Orleans
8/18/2018	Flash Flood	Heavy Rain	Mc Donoghville	Vieux Carre
10/15/2018	Flood	Heavy Rain	New Orleans	New Orleans
5/12/2019	Flash Flood	Heavy Rain	Mc Donoghville	Gentilly
7/10/2019	Flash Flood	Heavy Rain	New Orleans	New Orleans
7/11/2019	Storm Surge/Tide
7/20/2019	Flash Flood	Heavy Rain	New Orleans	New Orleans
8/26/2019	Flash Flood	Heavy Rain	New Orleans	New Orleans

Source: National Centers for Environmental Information, NOAA, Storm Events Database. Accessed May 2020.

Hazard Impacts

Property Damage

The depth and velocity of floodwaters are the major variables in determining property damage. Flood velocity is important because the faster water moves, the more pressure it puts on a structure, and the more it will erode stream banks and scour the earth around a building’s foundation. In some situations, deep and fast-moving waters can push a building off its foundation. Structural damage can also be caused by the weight of standing water (hydrostatic pressure).

Another threat to property from a flood is called “soaking.” When soaked, many materials change their composition or shape. Wet wood will swell, and if dried too quickly, will crack, split, or warp. Plywood can come apart, and the gypsum wallboard can deteriorate if it is bumped before it has time to dry completely. The longer these materials are saturated, the more moisture, sediment, and pollutants they absorb.

Soaking can also cause extensive damage to household goods. Wooden furniture may become warped, making it unusable, while other furnishings such as books, carpeting, mattresses, and upholstery are usually not salvageable. Electrical appliances and gasoline engines will flood, making them worthless until they are professionally dried and cleaned.

Many buildings that have succumbed to floodwaters may look sound and unharmed after a flood. Still, water has the potential to cause severe property damage. Any structure that experiences a flood should be stripped, cleaned, and allowed to dry before being reconstructed. This can be an extremely expensive and time-consuming effort. The City of New Orleans provides guidance on cleaning up after a disaster for residents and businesses.[1]

Threat to People

Just as with property damage, depth and velocity are major factors in determining the threat posed to people by flooding. It takes very little depth or velocity for floodwaters to become dangerous. A car will float in less than two feet of moving water and can be swept downstream into deeper waters, trapping passengers within the vehicle.

Major health concerns are also associated with floods. Floodwaters can transport materials such as dirt, oil, animal waste, and chemicals (e.g., farm, lawn, and industrial) that may cause illnesses of various degrees when coming in contact with humans. Floodwaters can also infiltrate sewer lines and inundate wastewater treatment plants, causing sewage to backup and creating a breeding ground for dangerous bacteria. This infiltration may also cause water supplies to become contaminated and undrinkable.

Estimated Potential Losses

Using the HAZUS 4.2 Flood Model, along with the Parish DFIRM, the 100-year flood scenario was analyzed to determine potential losses. Table 21 shows the total economic losses that would result from this occurrence.

Table 16: Estimated Losses in Orleans Parish from a 100-Year Flood Event
Jurisdiction	Estimated Total Losses from 100-year Flood Event
Orleans Parish	$45,088,500,000

Source: HAZUS 4.2

The HAZUS 4.2 Flood Model also provides a breakdown of potential building damages for seven primary sectors (HAZUS occupancy) throughout the parish. The losses by sector are listed in the tables on the following pages. These sectors are comprised of privately-owned structures/facilities, as well as locally, state, and federally owned structures/facilities.

Table 17: Estimated 100-Year Flood Losses for Orleans Parish by Sector
Orleans Parish	Estimated Total Loss from 100-Year Flood Event
Agricultural	$40,971,000
Commercial	$7,422,191,000
Government	$560,202,000
Industrial	$1,088,619,000
Religious	$1,246,887,000
Residential	$33,606,098,000
Education	$1,123,532,000
Total	$45,088,500,000

Source: HAZUS 4.2

Vulnerability

Based on data over the past 25 years from 1994-2019, there is a 100% annual probability that there will be a flood in Orleans Parish. The vulnerability of a particular structure to flooding depends on its elevation and method of construction. The Hazard Mitigation Office uses data from several sources to understand property-level flood risk. See Appendix C for parish and municipality buildings that are susceptible to flooding due to proximity within the 100-year floodplain.

Flash Flooding and Urban Flooding

In New Orleans, heavy rains can occur at any time of the year. However, the rainiest months are June, July, and August, when tropical moisture is plentiful along the Gulf Coast. This section addresses flooding from rain events, including those that are the result of hurricanes, tropical storms, thunderstorms, and winter storms.

NOAA’s Atlas 14 is the current authoritative source for rainfall probability estimates. Figure 13 shows the probability assigned to various storms described by both volume and duration of rainfall. NOAA’s analysis makes some attempt to account for changing climate baselines, but the Administration is continuing to develop scientific methods of incorporated climate projections into the data and may not fully capture the probability for major rainfall events.[2] Recently published scientific studies of extreme rainfall in the US Southeast found a consistent, region-wide trend of increasing extreme rain intensity[3]. Across all levels of government, funding and coordinated effort to update and maintain precipitation forecast data have not been consistent, and these finding underscore the importance of continued support for scientific study to support planning and decision-making.

One challenge in measuring rainfall is a lack of official measurements, which means that the recorded values may not capture the actual rainfall in a given area. For example, on August 5, 2017, the rain gauge at Louis Armstrong International Airport recorded 2” total precipitation, while unofficial local government measurements were as high as 8-9 inches. The City of New Orleans, SWBNO, and private individuals in the City currently maintain rainfall gauges that help supplement NWS data; these local rainfall monitoring systems are discussed in the Capability Assessment and Mitigation Strategy sections of this plan.

Figure 13: NOAA Atlas 14 Point Precipitation Frequency Estimates, Station Name New Orleans WSO City Source: NOAA’s National Weather Service Hydrometeorological Design Studies Center

Recent Flash Flooding and Urban Flooding Events

New Orleans has experienced several major urban flooding events since the last Hazard Mitigation Plan update in 2015. As the definitions above note, urban flooding is similar to flash flooding but is specific to urbanized areas. It takes place when stormwater drainage systems cannot keep pace with heavy precipitation, and water accumulates on the surface.

Flash Flood Event August 5, 2017

A weakened frontal boundary along the Interstate 10 corridor in southeast Louisiana and south Mississippi, along with local land-lake/sea breeze interaction, aided in the development of thunderstorms producing heavy rain along this corridor on all Aug. 4th, 5th, and 6th. Reports of flash flooding were received from the New Orleans area on the 4th and 5th.

Thunderstorms repeatedly developed over the same area of New Orleans from mid-afternoon into early evening, producing 4 to 9 inches of rain during a 3 to 5-hour period. The rainfall overwhelmed the drainage system leading to widespread and deep street flooding. Many streets were impassable from the Mid-City area to just north of the French Quarter and Central Business District, and road and highway underpasses were deeply flooded. Numerous automobiles were also damaged by the flooding. Water entered at least several hundred homes and a number of businesses.,

During the August 5, 2017 flash flood event, there were a total of 685 flood claims from properties under the National Flood Insurance Program. FEMA paid out a total amount of $4,299,953 in damage costs to insureds. NFIP data only captures damage to structures that are covered by flood insurance; we can estimate that actual damages are much higher by looking at the rates of flood insurance coverage. Another significant impact of flash floods is damage to automobiles. Direct losses to vehicles from a city-wide 100-year flood occurring the daytime could be as high as $1.8 Billion (HAZUS 4.2). Non-NFIP losses are not tracked in a way that is accessible to us in developing this plan, but it is understood that actual damage numbers are relatively higher than initial estimates.

NWS analysis determined that this storm had approximately a one-in-one hundred annual chance of occurrence (roughly the 100-year storm). At the time of the storm, there were no direct measurements of flood depth and extent available. During floods, OPCD receives calls to report flood conditions. Using records of the number and density of these calls, we can estimate the general extent and severity of flooding that occurred (Figure 14).

Figure 14: Residential Flood Reports for August 5, 2017, Flash Flood Event

Source: City of New Orleans

Flash Flood Event May 18, 2017

An upper-level trough of low pressure over the central Gulf Coast combined with increasingly moist and unstable air to produce several severe thunderstorms during the afternoon hours of May 18, 2017. Extensive street flooding was reported in the Central City, Tremé, and Bywater neighborhoods of New Orleans. Radar rainfall estimates indicated that 3 to 5 inches of rain fell in a swath from Metairie through City Park and Mid-City, into the Bywater and Lower 9th Ward. Some vehicles were flooded at the height of the storm. Most of the rain fell between 3:30 pm and 6:00 pm.

Flash Flood Event July 10, 2019

On the morning of July 10, an outer band of the weather system that would produce Tropical Storm Barry caused heavy rain over large parts of New Orleans between 6:00 am and 9:00 pm. A SWBNO rain gauge in Tremé measured almost 8.4” of rain, and a gauge in the Lower 9^th Ward measured over 9”, most of which fell within about three hours. Almost six hundred flood insurance claims were made in the days following. This event led to road closures, damage to automobiles, and minor damage to structures.

Figure 15: Rainfall Totals July 10, 2019

Source: Sewerage & Water Board of New Orleans

Figure 16: Flood Report Density July 10, 2019

Source: City of New Orleans

Riverine Flooding

New Orleans’ location on the Mississippi River, and the risk of experiencing riverine flooding, is one of the defining features of the city. In 1719, a year after the colonial French established a settlement at the present site of New Orleans, a flood on the Mississippi River left the city underwater. The city is currently protected from flooding by the Mississippi River by a federal levee system managed by the United States Army Corps of Engineers (USACE). These levees are part of a larger project known as the Mississippi River and Tributaries Project (MRT), authorized by Congress after the devastating 1927 floods made clear that a coordinated approach to managing the river was needed. The city has not been flooded by the river since the system was completed.

The Mississippi River flood gauge at New Orleans has surpassed the 17-foot flood stage level 20 times since 2000, including six days in May of 2011, two days in January of 2016, one day in May 2019, one day in February of 2020, and 10 days in April 2020.[4] As the following figure demonstrates, river levels remained higher for longer periods of time in 2019 and 2020 in comparison to past years.

Figure 17: Flood Gauge Levels by Feet for Mississippi River at New Orleans – Carrollton

Source: US Army Corps of Engineers

River floods at New Orleans are most often controlled by the Bonnet Carré Spillway, which was constructed by USACE to provide an outlet for river water and to relieve pressure on the New Orleans-area MRT levees. Historically, the spillway was opened roughly once a decade. In recent years, the river stage has required opening the spillway more frequently and for longer periods than historically averaged. A number of factors influence the frequency and severity of Mississippi River flooding, including land use throughout the watershed, shifts in the water cycle associated with climate change, and management of the river for navigation and flood control.

Table 18: **Bonnet Carré Spillway Openings**
Year	Dates Open	Number of Bays Open	Maximum Flow in Cubic Feet Per Second (CFS)
1937	Jan. 28 to Mar. 16	285	211,000
1945	Mar. 23 to May 18	350	318,000
Year	Dates Open	Number of Bays Open	Maximum Flow in Cubic Feet Per Second (CFS)
1950	Feb. 10 to Mar. 19	350	223,000
1973	Apr. 8 to June 21	350	195,000
1975	Apr. 14 to Apr. 26	225	110,000
1979	Apr. 17 to May 31	350	191,000
1983	May 20 to June 23	350	268,000
1997	Mar. 17 to Apr. 17	350	243,000
2008	Apr. 11 to May 8	160	160,000
2011	May 9 to June 20	330	316,000
2016	Jan. 10 to Feb. 1	210	203,000
2018	Mar. 8 to Mar. 30	183	196,000
2019	Feb. 27 to April 11	206	210,000
2019	May 21 to July 27	168	161,000
2020	April 3 to May 1	90	90,000

Source: U.S. Army Corps of Engineers (USACE)

[1] City of New Orleans. (2020). NOLA Ready Guide What to do After a Flood.

[2] Perica, Sanja. 2017. Updates to NOAA Atlas 14 Precipitation Depth-Duration-Frequency (DDF) Curves. National Oceanic and Atmospheric Administration and National Weather Service. Available:

[3] Brown, Keim, and Black. 2020. Trend Analysis of Multiple Extreme Hourly Precipitation Time Series in the Southeastern United States. Journal of Applied Meteorology and Climatology. 59.3:427-422. Available:

[4] National Weather Service. 2019. Mississippi River Flood History 1543-Present. Available

Relevant Actions

Structural Projects

Action 2015.2.1

Improve drainage infrastructure through measures in high flood risk areas including, but not limited to, the upgrade and improvement of culvert design and construction and retention and detention areas.

Improve drainage infrastructure through measures including the upgrade and improvement of canals and drainage pipes and installation of retention and detention areas including green infrastructure.

Hazard: Flooding, Tropical Cyclones, Infrastructure Failure, Coastal Erosion, Subsidence
Goal: Goal 2: Reduce risk and vulnerability to the built environment including current and future structures; critical facilities; historic structures; and infrastructure, including communications infrastructure.
Status:	Timeframe: 5-10 years	Priority: High	2015 Priority: High
Lead Agencies: CNO (Department of Public Works), SWBNO	Support Agencies:	Cost: $2,000,000,000+	Funding source: FEMA, Army Corps of Engineers, City of New Orleans Capital Improvements	Emergency Support Function: ESF 3

Hazard: Flooding, Storm Surge, Infrastructure Failure, Coastal Erosion, Subsidence
Goal: Goal 2: Reduce risk and vulnerability to the built environment including current and future structures; critical facilities; historic structures; and infrastructure, including communications infrastructure.
Status:	Timeframe: 1-5 years	Priority: High	2015 Priority: Medium
Lead Agencies: CNO (Homeland Security & Emergency Preparedness)	Support Agencies:	Cost: Staff time	Funding source: Staff time	Emergency Support Function: ESF 5

Hazard: Flooding, Storm Surge, Tropical Cyclones, Coastal Erosion, Subsidence, Infrastructure Failure (levee failure)
Goal: Goal 3: Reduce risk and vulnerability to the natural environment including wetland restoration and recognition of New Orleans as a coastal city.
Status:	Timeframe: 1-5 years	Priority: High	2015 Priority: High
Lead Agencies: CNO (Office of Reslience & Sustainability), Tulane University Center for Environmental Law	Support Agencies:	Cost: City staff time	Funding source: CNO General Fund	Emergency Support Function: ESF 5

Hazard: Flooding, Tropical Cyclones, Coastal Erosion, Subsidence, Infrastructure Failure
Goal: Goal 4: Maximize the involvement of individuals, businesses, and groups in risk reduction measures through education/outreach on hazard mitigation appropriate to all groups, particularly vulnerable populations.
Status:	Timeframe: 1-5 years	Priority: Medium	2015 Priority: High
Lead Agencies: CNO (Homeland Security & Emergency Preparedness)	Support Agencies:	Cost: 225000	Funding source: CNO General Fund	Emergency Support Function: ESF 5

Hazard: Flooding, Storm Surge
Goal: Goal 5: To promote, implement, and sustain mitigation measures in Orleans Parish in order to reduce and manage risks to human life, the environment, and property.
Status:	Timeframe: 1-5 years	Priority: High	2015 Priority: Medium
Lead Agencies: CNO (Safety & Permits)	Support Agencies:	Cost: City staff time	Funding source: CNO General Fund, NFIP	Emergency Support Function: ESF 5

Hazard: Flooding, Tropical Cyclones, Infrastructure Failure, Coastal Erosion, Subsidence
Goal: Goal 6: To protect Orleans Parish and the surrounding region from the effects of natural and manmade hazards, ensuring community continuity in the event of such hazards.
Status:	Timeframe: 5-10 years	Priority: Med+G95:P98ium	2015 Priority: High
Lead Agencies: CNO (Homeland Security & Emergency Preparedness)	Support Agencies:	Cost: $2,000,000,000+	Funding source: FEMA, Army Corps of Engineers, City of New Orleans Capital Improvements	Emergency Support Function: ESF 3

Hazard: Flooding, Storm Surge, Extreme Heat, Subsidence, Coastal Erosion, Infectious Disease Outbreak
Goal: Goal 1: Reduce risk and vulnerability to the human environment including cultural resources, homeowners, renters, visitors, and transient populations.
Status:	Timeframe: 1-3 years	Priority: Medium	2015 Priority: N/A
Lead Agencies: CNO (Office of Resilience & Sustainability)	Support Agencies:	Cost: City Staff Time	Funding source: CNO General Fund, State and Federal Grants, Philanthropic Grants	Emergency Support Function: ESF 14

Hazard: Flooding, Storm Surge, Tornadoes, Tropical Cyclones, Coastal Erosion, Subsidence, Severe Thunderstorms, Extreme Heat, Winter Weather
Goal: Goal 1: Reduce risk and vulnerability to the human environment including cultural resources, homeowners, renters, visitors, and transient populations.
Status:	Timeframe: 1-3 years	Priority: Medium	2015 Priority: N/A
Lead Agencies: CNO (Office of Resilience & Sustainability)	Support Agencies:	Cost: City Staff Time	Funding source: CNO General Fund, State and Federal Grants, Philanthropic Grants	Emergency Support Function: ESF 14

Hazard: Flooding
Goal: Goal 1: Reduce risk and vulnerability to the human environment including cultural resources, homeowners, renters, visitors, and transient populations.
Status:	Timeframe: 1-3 years	Priority: Medium	2015 Priority: N/A
Lead Agencies: CNO (Homeland Security and Emergency Preparedness)	Support Agencies:	Cost: City Staff Time	Funding source: CNO General Fund, State and Federal Grants, Philanthropic Grants	Emergency Support Function: ESF 5

Hazard: Flooding, Tornadoes, Tropical Cyclones, Severe Thunderstorms, Winter Weather, Active Threats, Infrastructure Failure
Goal: Goal 2: Reduce risk and vulnerability to the built environment including current and future structures; critical facilities; historic structures; and infrastructure, including communications infrastructure.
Status:	Timeframe: 2-5 years	Priority: Medium	2015 Priority: N/A
Lead Agencies: CNO (Office of Resilience & Sustainability)	Support Agencies:	Cost: City Staff Time, Contract Labor	Funding source: Homeland Security, FEMA, Health and Human Services	Emergency Support Function: ESF 2, ESF 4, ESF 6, ESF 8, ESF 10

Hazard: Flooding, Extreme Heat
Goal: Goal 2: Reduce risk and vulnerability to the built environment including current and future structures; critical facilities; historic structures; and infrastructure, including communications infrastructure.
Status:	Timeframe: 1-5 years	Priority: High	2015 Priority: N/A
Lead Agencies: CNO (Department of Public Works, Parks and Parkways)	Support Agencies:	Cost: City Staff Time, Contract Labor, Community Partner Time	Funding source: CNO General Fund, State and Federal Grants, Philanthropic Grants	Emergency Support Function: ESF 11, ESF 14

Hazard: Flooding, Storm Surge, Coastal Erosion, Tropical Cyclones, Subsidence
Goal: Goal 4: Maximize the involvement of individuals, businesses, and groups in risk reduction measures through education/outreach on hazard mitigation appropriate to all groups, particularly vulnerable populations.
Status:	Timeframe: 3 to 5 years	Priority: Medium	2015 Priority: N/A
Lead Agencies: CNO (Office of Resilience & Sustainability)	Support Agencies:	Cost: City Staff Time	Funding source: Philanthropy, School Board	Emergency Support Function: ESF 15

Hazard: Flooding, Subsidence
Goal: Goal 5: To promote, implement, and sustain mitigation measures in Orleans Parish in order to reduce and manage risks to human life, the environment, and property.
Status:	Timeframe: 1 to 3 years	Priority: High	2015 Priority: N/A
Lead Agencies: CNO (Homeland Security & Emergency Preparedness)	Support Agencies:	Cost: City Staff Time	Funding source: CNO General Fund, HMGP	Emergency Support Function: ESF 5, ESF 14

Hazard: Flooding, Extreme Heat, Winter Weather, Tropical Cyclones
Goal: Goal 5: To promote, implement, and sustain mitigation measures in Orleans Parish in order to reduce and manage risks to human life, the environment, and property.
Status:	Timeframe: 1-3 years	Priority: Low	2015 Priority: N/A
Lead Agencies: CNO (Department of Public Works, Office of Resilience & Sustainability)	Support Agencies:	Cost: City Staff Time	Funding source: CNO General Fund	Emergency Support Function: ESF 5

Hazard: Flooding, Tropical Cyclone, Infrastructure Failure
Goal: Goal 2: Reduce risk and vulnerability to the built environment including current and future structures; critical facilities; historic structures; and infrastructure, including communications infrastructure.
Status:	Timeframe: TBD	Priority: High	2015 Priority: N/A
Lead Agencies: SWBNO Operations	Support Agencies:	Cost: 25000000	Funding source: PDM/SWBNO	Emergency Support Function: ESF 12

Hazard: All
Goal: Goal 2: Reduce risk and vulnerability to the built environment including current and future structures; critical facilities; historic structures; and infrastructure, including communications infrastructure.
Status:	Timeframe: TBD	Priority: Medium	2015 Priority: N/A
Lead Agencies: SWBNO Emergency Mgmt	Support Agencies:	Cost: 21000000	Funding source: PDM/SWBNO	Emergency Support Function: ESF 6

Hazard: Flooding,Storm Surge,Tropical Cyclones,Tornadoes,Subsidence,Winter Weather,Extreme Heat,Severe Thunderstorms
Goal: Goal 2: Reduce risk and vulnerability to the built environment including current and future structures; critical facilities; historic structures; and infrastructure, including communications infrastructure.
Status:	Timeframe: On Going	Priority: High	2015 Priority:
Lead Agencies: Tulane University (Office of Risk Management)	Support Agencies:	Cost: Cost will be project specific	Funding source: GOHSEP; FEMA	Emergency Support Function: ESF 3

Hazard: Flooding,Tropical Cyclones,Coastal Erosion,Tornadoes,Subsidence,Winter Weather,Extreme Heat,Severe Thunderstorms
Goal: Goal 2: Reduce risk and vulnerability to the built environment including current and future structures; critical facilities; historic structures; and infrastructure, including communications infrastructure.
Status:	Timeframe: 1 year	Priority: High	2015 Priority:
Lead Agencies: Tulane University (Office of Capital Projects and Real Estate)	Support Agencies:	Cost: Staff Time and Data Costs	Funding source: NOAA; USGS; NOAA; FEMA; TDEM; DOT; DHS; EPA; GOHSEP	Emergency Support Function:

Hazards

Flooding