Flooding & Drainage

Flooding is the nation’s number one natural disaster and it can happen anytime... anywhere. Floods occur in all 50 states and you can be a victim of flooding, even if you live miles away from water.

Page updated on Oct 20, 2020 at 12:28 PM

What's New?

CITY OF ALEXANDRIA ADVISES RESIDENTS ON CLEANUP FOLLOWING FLASH FLOODING

Following recent severe storms and flash flooding, the City of Alexandria reminds residents and businesses to exercise caution and be alert for indoor sewer backups, impassable roads, power outages, and other flood-related issues. Read more

Sept10_heatmap

 


Frequently Asked Questions

  • What types of flooding occur in the City?

There are different types of flooding that may occur. These include:

  1. A flash flood is caused by heavy or intense rainfall in a short period of time, generally less than six hours. Flash floods are usually characterized by raging torrents after heavy rains that rip through streams, streets, or alleys. They can occur within minutes or a few hours of intense rainfall.
  2. A river flood occurs when water levels rise over the top of river banks due to excessive rain from tropical systems making landfall, persistent thunderstorms over the river’s watershed for extended periods of time, combined rainfall and snowmelt,
  3. An inland flood occurs when moderate rain falls and accumulates over a number of days, saturating the ground.
  4. Sanitary sewer backups occur when rising ground water and inflow from stormwater get into the sanitary sewers, through cracks and joints in pipes, roof downspouts, and holes in manhole lids. Sanitary sewers are separate systems from the storm sewers (other than Combined Sewers in Old Town). However, they are subject to inflow and infiltration into the pipes from various sources and during large storm events, can become surcharged from these sources and flood basement drains and drains in walkout basement stairwells. See the City’s Infiltration & Inflow Program for more information.
  5. riverine flood (tidally influenced) is caused by higher than average high tide and worsened by heavy rainfall and generally occurs along the Potomac River waterfront. 
  •  What is FEMA floodplain and remapping process?

The City is participating in a Flood Insurance Study being conducted by the Federal Emergency Management Agency (FEMA) to update the City’s current floodplain maps. FEMA’s mapped floodplains focus on the regulatory 100-year floodplain that the agency maps to assign risk.  Flood risk associated with the 100-year floodplain changes over time due to weather patterns, land development, and erosion. The changes are likely to affect some residential and commercial property owners, who may need to obtain coverage under a new flood insurance policy or alter existing policies. FEMA began this process in 2014. FEMA sent the City draft revised maps in April 2020. Preliminary maps are due to be published by FEMA in September 2020. Learn more.

  • Why did I get water in my home during recent rain events?

The City experienced several events of flash flooding from severe thunderstorms in 2019 and 2020. The storm on July 23, 2020, dropped 2.5 to 3 inches of rain in 30 minutes, creating a brief, extremely high-intensity event that caused significant flash flooding in many areas, including some not normally impacted by flooding. Flash flooding is caused by heavy or excessive rainfall in a short period of time. Flooding can occur within minutes or hours of excessive rainfall. Flash floods may occur suddenly and may cause swift moving water. 

Heavy rains, which typically occur during hurricane season, sometimes cause sanitary sewer backups in some parts of Alexandria. With many hours of heavy rain, or an intense burst of rain, the sanitary sewer line may fill up with water and some sewage. Because of this, the sewer system may approach its carrying capacity and may overflow in some areas, such as streets. It may also back up into homes through floor or plumbing drains. The City offers financial relief to homeowners for backflow preventers. 

Using the Capital Improvement Program (CIP) process, the City is undertaking several infrastructure projects focused on spot improvements across the city to improve storm sewer capacity. Learn more.

  • What causes the Potomac River to flood in Old Town?

The area of the Potomac River that borders Alexandria, as well as parts of Four Mile Run, are tidally influenced meaning the levels rise and fall during the day at predicted intervals with the tide. Water levels may rise far greater than the natural tidal rise and fall due to excessive rain or snowmelt entering the Potomac River system. Smaller creeks and streams feed into the Potomac River far upstream of Alexandria which, in combination with the rising tide, may lead to a delay in river flooding. River flooding occurs when the water level rises and overflows the river’s banks onto adjacent land. This is a little different than coastal flooding which is caused by higher than average high tide and worsened by heavy rainfall , onshore winds and storm surge. It refers to sea flooding during a severe storm with strong winds.

  • How can you protect your property from flooding?

There are several different ways to protect your home from flooding water. First, evaluate and understand your flood risk. You can check to see if your property is in the federally-designated “floodplain” using FEMA's National Flood Hazard viewer. If your property is in or adjacent to the floodplain, understand what tools are available to you such as flood insurance. However, property outside of the floodplain may still be susceptible to flooding. No matter where your property is located, consider ways to help flood-proof your home. This may include installing a sump pump and/or backflow preventer if you have a basement or using sealant around windows and other surfaces. Assess the grading of your property, ensure downspouts are faced away from your building, and maintain distance between the building and mulch which can hold a lot of water and retain moisture. Other options may be to hire a contractor to install a French drain or a swale to divert the water flow from your structure. Keep an eye on inlets near your property to make sure they are not clogged with debris and use Alex311 to report clogged inlets and storm sewer concerns. Preventative measures should be taken such as keeping your home’s gutters and drains cleared from debris and maintaining your home and appliance pipe system – even small pipes like the line to the ice maker in your fridge can cause major damage when compromised. Click here for more ideas from the U.S. Army Corps of Engineers publication on Local Flood Proofing Programs. Finally, in advance of a storm seek ways to minimize water intrusion such as sand bags and other measures. Learn more.          


How Do You Protect Your Property?


Determine Your Flood Risk

The Federal Emergency Management Agency (FEMA) creates Flood Insurance Rate Maps (FIRMs) that show floodplain boundaries. As of 2011, approximately 20% of the City is mapped by FEMA in a floodplain. However, extremely high-intensity storms may cause flash flooding in areas that are not normally impacted, to include properties that may not be in a designated FEMA floodplain. Please visit the Flood Map page for more information on the FEMA flood maps.

Purchase Flood Insurance

Most homeowner’s insurance does not cover property damage from flooding but homeowners may purchase flood insurance through the National Flood Insurance Program (NFIP), even if your property is outside of a FEMA-designated floodplain. Alexandria participates in the NFIP’s Community Rating System (CRS) program, a voluntary incentive program that recognizes and encourages community floodplain management activities that exceed the minimum program requirements, and scores those activities to determine “classes” for those communities that participate. Recognized as a "Class 6" community, Alexandria homeowners are eligible to receive up to a 20% discount on flood insurance premiums. 

FLOOD MITIGATION TECHNIQUES AND PRACTICES

Homeowner’s interested in protecting their property from future floods have several options available. The FEMA Homeowner’s Guide to Retrofitting is a great resource for homeowners who want to know how to protect their homes from flooding, and many products and services can be found to protect family and property. Common practices include:
- Flood-proofing building foundations;
- Installing barriers around window wells and walkout basements;
- Installing flood shields over windows or doors; and
- Protecting utility and mechanical equipment such as by elevating an exterior fan unit or installing an interior flood wall around a mechanical ventilation system. 
Always check with the City's Department of Planning and Zoning and/or the City’s Permit Center before beginning any project on your property. Some flood protection measures may need a building permit and others may not be safe for your building.

CITY ASSISTANCE PROGRAM FOR SEWER BACKFLOW PREVENTION

The City of Alexandria offers financial assistance for sanitary sewer backflow prevention to  help prevent sanitary sewer flows from backing up into the drains of your home during a flood.  


Be Prepared

SIGN UP FOR EMERGENCY ALERTS

Alerts received at the right time can help keep you safe during an emergency and let you know about City services, like the availability of sand bags. The City of Alexandria offers notification services— eNews—that sends you a text or email alert in the event of a flood. Learn more about the City's social media accounts here.
Review your options for receiving Wireless Emergency Alerts from the National Weather Service.

Make a Flood Plan

Know evacuation routes and keep important papers in a safe, waterproof place. Please visit the Flooding Information page to learn more about to protect your property during flood events.

Be Flood Smart

Flooding is the nation’s #1 natural disaster and it can happen anytime and anywhere. Floods occur in all 50 states and you can be a victim of flooding, even if you live miles away from water. Learn more on what causes flooding and how to protect your property by visiting FEMA's Flood Smart web site.


Help Prevent Local Flooding

Not all storm drainage infrastructure is owned by the City. You can help prevent local flooding by keeping trash and other debris, like tree branches, away from storm drains and inlets. Utilize the Alex311 system to report storm sewer blockages or issues on public property. 


Proactive City Operations and Maintenance

The City has about 185 miles of storm sewer pipe, about 13,500 storm structures, and about 26 miles of streams. City crews and contractors perform proactive maintenance on storm sewer infrastructure and in response to  complaints to remove any blockages and to ensure that the storm sewer infrastructure is functioning as designed. Regular  street sweeping of roadways is performed to aide in the removal of nutrients and pollutants from our local waterways while removing road debris to prevent it from entering the storm sewer system.


City Flooding and Drainage Projects

The increasing frequency of more intense storm events has created more frequent flooding and drainage issues. The City identifies flooding and drainage projects through resident complaints, analyses, and field observations. These include small to medium ‘Spot Improvement’ capital improvement program (CIP) projects to mitigate drainage issues. After identification and initial investigations, these projects often require work to identify CIP resources, perform onsite survey, complete design, secure right of entry (if applicable), and procure a construction contractor to perform the work. The City completed the initial Storm Sewer Capacity Analysis that identified problem areas and prioritized potential locations for large, multi-year CIP projects to address capacity issues. Please visit the Waterfront Plan Implementation page for more information on flood mitigation projects specific to the waterfront.  

spot improvement projects

Since 2019, the City has completed five storm sewer spot improvements to make quantifiable repairs for many residents impacted by flooding. The City has $4.7 million in the current CIP for similar projects and uses a proactive, methodical approach to prioritizing these types of projects. Staff will continue to update its list of projects as they are completed, statues change, and as others are identified:

Project Description Status Completed
W. Alexandria Alley Improve alley drainage for nearby residents Completed FY 2019
Founders Park Repair infrastructure to improve drainage for park, adjacent property (parking garage) Completed FY 2019
901 N. Ashton Street Emergency repair of pipe, mitigate erosion, and restoration Completed FY 2019
Saylor Place Replace rusted ditch pipe with polyethylene plastic pipe Completed FY 2020
Angel Park Install infrastructure for street and park drainage Completed FY 2020
Hancock Ave at Alexandria Alley Install storm pipe at culvert Design nearly complete FY2021
708 W. Braddock New storm inlet Purchase Order FY2021
Lloyd’s Lane Drainage improvement, street onto private property Design complete FY2021
4320 Loyola Avenue Replace existing open channel with closed pipe to mitigate drainage issues Survey and Design near completion FY2021/2
Mt. Vernon Ave Cul-de-sac Replace inlets Preliminary analysis FY2021
Mt. Vernon Alley Improve alley drainage Preliminary analysis FY2021/2
Carlisle Drive Alley drainage improvements Preliminary analysis N/A
Key Drive Unnamed Tributary Channel Wall Channel wall is degrading in places; utilities exposed Preliminary analysis FY2021/2
Oakland Terrace Timber Branch Wall Degrading and eroding around it Preliminary, Easements FY2021
Walleston Ct. Stream Bank Stream bank erosion Maintenance N/A
636 W. Timber Branch Pkwy Retrofit catch basin to fix drainage issues Preliminary Design FY2021/2
Douglas Cemetery Undersized/condition Field analysis Planned

 

Capacity Projects 

In February 2016, the City finalized a  summary report of the City of Alexandria Storm Sewer Capacity Analysis (CASSCA) project, the purpose of which was to analyze the storm sewer system, identify problem flooding areas, and develop and prioritize solutions. As a high-level conceptual planning exercise, the storm sewer system was modeled to predict potential capacity issues for the City’ current design standard which is a 10-year storm. While the City’s design standard is consistent with or more protective than some of the City’s neighboring jurisdictions, the July 8, 2019, and the July 23, 2020, storms were more intense than this design standard, with the July 23 event about 30 times more intense. 

Out of the 83 "problem areas" in the City’s eight watersheds, the top two watersheds were Hooff’s Run and Four Mile Run, with 23 "problem areas" each. More detailed planning and analysis will take place to assess the overall implementation feasibility (including construction) prior to full design of these large-scale capital projects.

The City compares the modeled "problem areas" from the CASSCA project to (1) actual reports of flooding, drainage, and capacity issues during calendar year 2018 (one of the wettest years on record), (2) the July 8, 2019 regional flash flood, and (3) the July 23, 2020 localized flash flood to help determine the most appropriate areas for spot improvements, as shown in the table above. The CASSCA project recommended more than $40 million in storm sewer capacity investments (in 2016 dollars) in Hooff’s Run and Four Mile Run alone to bring the current storm sewers to the current design standard. The City's CIP has allocated $7.5 million towards storm sewer capacity improvements starting in fiscal year 2023.

Four Mile Run Levee Project

The City of Alexandria must maintain the structural integrity and the flow capacity of Four Mile Run channel through routine maintenance practices. Associated with the channel is a levee and floodwall system that was construction back in the 1970’s to mitigate flooding. City staff and in coordination with the U.S. Army Corps of Engineers to identify areas to address as ongoing maintenance in accordance with the City’s Operations and Maintenance program.

Transportation and Environmental Services routinely removes vegetation along the levee from the I-395 bridge to just downstream of the Mt. Vernon Avenue bridge at Four Mile Run Park. This maintenance work involves removing and cutting vegetation flush to the levee, which is part of the maintenance of the levee. Other maintenance includes inspection and maintenance of the infrastructure and removal of accumulated sediment that builds up in the channel over time. The City is currently working with Arlington County on the design and construction effort to remove sediment from the Four Mile Run channel. 

2020 Hooff’s Run Inspection and Cleaning Project 

In June of 2020, the city hired a contractor, Red Zone Robotics, to inspect and survey approximately 7,000 feet of the Hooff’s Run Culvert which conveys stormwater from a significant portion of Northridge, Del Ray, and Rosemont. The survey identified overall debris levels in the range of 5% with isolated sections of pipe having debris accumulation of approximately 15-20%. An overview of the report can be found here. T&ES staff is working with contractors to obtain cost estimates and to develop a plan to have the debris removed from the culvert.

Stream and Channel Maintenance

The City performs maintenance to streams and channels throughout the city to preserve their capacity to carry 100-year floodwaters, and for repairs to erosion damage, stream corridor degradation, stabilization/restoration in Holmes Run and Cameron Run watersheds, including smaller tributaries to these streams. The increasing frequency of intense storm events will require increasing funding for sediment and vegetation removal to ensure the conveyance capacity of these waterways as climate resiliency and adaption measures consistent with the City’s Climate Emergency Declaration. 



Top