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Flooding is an excess amount of water that covers land that is either flat or dry. For example, land that is made of less permeable pavement, such as concrete, would have a harder time getting water to pass through. This causes the water to stay on the flat land, also known as a flood.

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1. River Floods: when water levels surpass the top of river banks.
2. Coastal Floods: abundance/excess amount of water in dry land areas along a coast w/ seawater
3. Storm Surge: unusual rise in water levels in coastal areas
4. Flash Floods: flooding that occurs within 6 hours of heavy rainfall

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Believe it or not, flooding is the deadliest type of severe weather. There's probably a lot about floods and flooding you don't know.

In this guide, we'll go over everything there is to know about flooding; answering common questions like "What causes flooding?" and "Where does flooding occur?"

Use the buttons below to jump to the flood facts you care about the most, or read the entire Flooding 101 guide to become an expert!

Flooding Causes & Effects

Flood Safety & Prevention

Don't Have Time to Read the Entire Guide Now?

Download a PDF version of the guide you can reference later.

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What is a Flood? | Types of Floods

When discussing floods, it's important to understand what they are. Let's start with a flooding definition.

A flood is an overflow of water that submerges land that is usually dry. Floods are an area of study in the discipline of hydrology. They are the most common and widespread natural severe weather event.

Floods can look very different because flooding covers anything from a few inches of water to several feet. They can also come on quickly or build gradually. To better answer the question of "What is a flood?" we'll answer what each specific type of flooding event is.

According to our friends at the National Severe Storms Laboratory, there are five types of floods. They include:

River Flood

Coastal Flood

Storm Surge

Inland Flooding

Flash Flood

As you can infer from the list above, flooding can happen anywhere, including both coastal and inland locations. Let's take a detailed look at the different types of floods.

What is a River Flood?

The first type of flooding we will explore is river flooding.
What is River Flooding?
A river flood occurs when water levels rise over the top of river banks. This flooding can happen in all river and stream channels. This includes everything from small streams to the world's largest rivers.
Causes of River Flooding
River flooding typically happens for four reasons. They are:

Excessive rain from tropical storm systems making landfall

Persistent over the same area for extended periods

Combined rainfall and snowmelt
More on Riverine Floods
River floods can happen suddenly or slowly. Sudden river flooding events occur more often on smaller rivers, rivers with steep valleys, rivers that flow for much of their length over impermeable terrain, and normally dry channels.

On the other hand, low-rising river floods typically occur in large rivers with large catchment areas. In case you didn't know this already, a catchment area is any area of land where precipitation collects and runs off into a common outlet.

What is a Coastal Flood?

The next type of flood you should know about is a coastal flood.
What is Coastal Flooding?
A coastal flood is the inundation of normally dry land areas along the coast with seawater.
Causes of Coastal Flooding
Coastal flooding is typically a result of a combination of sea tidal surges, high winds, and barometric pressure.

These conditions typically come from storms at sea like:

Tropical cyclones

Tsunami

Higher-than-average tides

What is Storm Surge?

Another type of flooding that is often associated with coastal flooding is storm surge.
What is Storm Surge?
Storm surge is an abnormal rise in water level in coastal areas over and above the regular astronomical tide.
Causes of Storm Surge
Storm surge is always a result meteorological storms that cause higher than normal tides on the coast. There are three parts of a storm that create this surge. They are:

Wind

Waves

Low atmospheric pressure

Learn about storm surge from one of our expert meteorologists, Andrew Rosenthal in the below snippet from our weather safety warmup webinar on hurricanes:
More on Storm Surge
Storm surge is an extremely dangerous form of flooding. It can flood large coastal areas at the same time. It can also cause flooding very quickly.

Extreme flooding occurs when storm surge happens at the same time as high tide. This can cause storm tides to reach over 2o feet!

What is Inland Flooding?

The next type of flood on our list is an inland flood. Some organizations refer to inland flooding as urban flooding. A flash flood is also a type of inland flood.
What is an Inland Flood?
An inland flood is flooding that occurs inland or not in a coastal area. Therefore, coastal flooding and storm surge are not inland floods.
Causes of Inland Flooding
Rainfall is almost always to blame for inland floods. Rain causes inland flooding in two ways. It can happen with steady rainfall over several days or it can happen because of a short and intense period of rainfall.

Snowmelt also causes inland floods, although rainfall is a more common cause.

Another way inland flooding happens is when water ways get blocked by debris, ice, or dams.
More on Inland Floods
Inland floods are often worse in urban areas because there isn't anywhere for the water to go. The following urban features can create urban flooding or make inland floods worse:

Paved roads and streets

Low-capacity drainage equipment

Dense buildings

Low amounts of green space

What is a Flash Flood?

The most well-known and deadly type of flood is a flash flood.
What is a Flash Flood?
A flash flood is flooding that begins within 6 hours, and often within 3 hours, of heavy rainfall (or other cause).
What Causes Flash Floods?
Flash floods can happen for several reasons.

Most flash floods happen after extremely intense rainfall from severe thunderstorms over a short period of time (normally 6 hours or less). There are two key elements to determine is flash flooding is likely:

Rainfall rate

Rainfall duration

Flash floods also happen when damns break, when levees fail, or when an ice jam releases a large amount of water.
Dangers of Flash Floods
Flash flooding is the #1 severe weather killer in the United States.

Flash floods are extremely powerful. They have enough force to roll boulders, tear trees from the ground, destroy buildings and bridges, and scour out new channels. This type of flood is characterized by raging torrents that rip through river beds, urban streets, or canyons, wiping out everything in their paths. With heights reaching 30 feet, flash floods can completely cover communities.

Another reason why flash flooding is so dangerous is that it can happen with little to no warning. This is especially true when dams or levees break.

Flood Causes & Effects

No matter what type of flood you're dealing with, they are generally caused by the same key factors and there are always negative effects.

In this section, we'll cover the basic causes and effects of flooding to help you better understand this dangerous meteorological and hydrological phenomenon. If you read through the above section on types of floods, you might just want to skip down to flooding effects.

What Causes Flooding?

As we mentioned above, there are plenty of different causes of flooding. While different flood types typically have different causes, most floods are caused by one of the following activities.

Heavy rainfall is the simplest cause of flooding. When there is too much rain or it happens too fast, there just isn't a place for it to go. This can result in floods like flash flooding.

Overflowing rivers are another cause of floods. You don't necessarily need heavy rains though to experience river flooding. As we mentioned before, river flooding can happen when there is debris in the river or dams that block the flow of the water.

Speaking of dams, broken dams are another cause of flooding. Older infrastructure can fail when heavy rains come and water levels rise. When dams break, they unleash torrents of water on unsuspecting households. This is part of what happened when Hurricane Katrina hit New Orleans in 2005.

Storm surge and tsunamis also cause flooding. Storm surges from hurricanes and other tropical systems can cause sea levels to rise and cover normally dry coastal areas in several feet of water. Tsunamis on the other hand are giant waves caused by earthquakes or underwater volcanic eruptions. As these waves move inland, they build height and can push a lot of water inland in coastal areas.

Channels with steep banks are also to blame for flooding. Flooding often occurs when there is fast runoff into lakes, rivers, and other basins. This is often the case with rivers and other channels that feature steep sides.

A lack of vegetation can cause flooding. Vegetation can help slow runoff and prevent flooding. When there is a lack of vegetation, there is little to stop water from running off and overflowing river banks and streams.

Melting snow and ice is another common reason for flooding. When a large amount of snow and/or ice melts quickly, it often doesn't have somewhere to go except low-lying areas.

These aren't all the reasons that flooding can happen, but they are some of the most common.

Flooding Effects

No matter what causes of a flood, it can have devastating effects on your community. There are actually many dangerous flooding effects. Besides physical danger, floods also cause economic and social problems. In this section, we'll cover the most common flood effects.

Loss Of Lives

The gravest effect of flooding is death. In fact, flooding is the number one severe weather killer. Floods have claimed thousands of lives throughout history. But how does flooding kill?

Floods kill by carrying people away in fast-moving water or drowning them. It only takes six inches of water to wash a person away. Floods can also kill people by destroying buildings and creating unsafe environments. One often-overlooked deadly effect of flooding comes from waterborne illnesses.

From 2010 to 2018, the recorded hundreds of flooding deaths across the United States. Texas saw most of those deaths, with the 8-year total sitting at 212 fatalities.

Property Damage

Since it only takes two feet of flood water to wash a car away, flooding can also cause great loss of property. Surely you've seen images of cars floating away in flood waters. This is why it is so important to avoid flooded areas when driving. You don't want to be in your car when it gets washed away in the flood!

Flooding also causes property damage to buildings by blowing out windows, sweeping away doors, corroding walls and foundations, and sending debris into infrastructure at a fast pace. Not to mention the furniture and items inside a home or business that are damaged when flood waters make it inside.

Economic Impacts

The economic impact of flooding can be devastating to a community. This comes from damage and disruption to things like communication towers, power plants, roads, and bridges. This brings business activities in an area to a standstill. Oftentimes, major flooding results in dislocation and dysfunction of normal life long after flood waters recede.

Flooding hinders economic growth and development because of the high cost of relief and recovery associated with floods.

In frequently flooded areas, there is less likely to be any investment in infrastructure and other developed activities.

Psychosocial Flooding Effects

Flooding can also create lasting trauma for victims. The loss of loved ones or homes can take a steep emotional toll, especially on children. Displacement from one's home and loss of livelihood can cause continuing stress and produce lasting psychological impacts.

Flood Safety Tips

While floods have many causes, there are proven flood safety and flood prevention tips to help keep families, businesses, and communities safe.

Here are some steps you can take before, during, and after flooding to keep yourself and those you care about safe.

What Do I Do Before a Flood?

The first thing you should do before a flood is assemble an emergency kit. Your flood emergency kit should include food, water, and medicine. You should pack a supply of these items that will last each person three days. You should also put the following in your kit:

Phone Charger

Batteries

Blankets

Flashlights

First Aid Kit

Rubber Boots and Gloves

NOAA Weather Radio

You should also have a cell phone on hand. This way, you can communicate with your friends, family, and coworkers.

The next thing you can do to prepare for a flood is research your home or organization's location to assess flood risk. Are you located in a flood plain? What's the fastest way to get to higher ground? Knowing the answers to these questions can help you in the case of floods.

One last flood tip we recommend is to sign up for flooding notifications. The National Weather Service issues Flood Advisories, Flood Watches, and Flood Warnings to help people prepare for flooding dangers.

Familiarize yourself with your local emergency management agency and how they communicate with your community. Do they have a mobile app you can download? Should you follow them on social media? Will an announcement about flooding air on the TV or local radio stations?

Finally, for hyperlocal weather alerts that include NWS flooding notifications, current precipitation information, and rainfall totals in your neighborhood, you should look into getting a subscription to professional-grade weather tracking software.

Flood Watches And Warnings

The first level of flood watches and warnings is a flood advisory. A Flood Advisory means you should be mindful of possible flooding because a weather event is forecast to occur that could result in flooding. The National Weather Service issues these advisories when flooding isn't expected to be bad enough to issue a warning, but could still pose minor threats.

A Flood Watch means you should prepare for flooding because conditions are favorable for a specific hazardous weather event to occur.

A Flood Warning means a hazardous weather event with flooding is imminent or already happening.

Our parent company AEM sister company specializes in helping clients understand and manage their water-related missions, including flood and road weather safety. AEM's results help improve hydrologic decision making and design standards across the United States and the world.

AEM created a solutions guide for flood forecasting and early warning, where you can learn about flood resilience, planning, and how to manage flood risks.

You may be wondering, what is a Flood Warning System ? A complete, integrated flood early warning monitoring system solution includes all the remote site hydro-meteorological sensing instrumentation, communications equipment, central base station equipment, as well as data collection, archiving, processing, and management software designed for flood warning.

Contrail for Flood Warning is an effective software solution for flood warnings. The software continually collects and monitors rainfall, water level and flow data in streams, rivers, lakes, and dams in real time.

A useful tool for creating a more complete view of weather conditions- including image collection, hosting, and viewing platform- is Contrail® Camera, OneRain's centralized Web-based image storage and management tool that provides secure, 24×7 password-protected access to view and manage images.

Contrail® Camera has the following applications:

Flood Early Warning

Reservoir and Hydropower operations

High risk/remote Dams Safety Monitoring programs

Road Weather

Flooded Roadways

Post-wildfire burn areas for flood early warning

Other environmental monitoring

Contrail is an enterprise-level software solution that continually collects and monitors rainfall, water level and flow data in streams, rivers, lakes, and dams in real time. To learn more about AEM Flood Risk Management solutions, click here.

The National Weather Service provides on overview of the Preliminary US flood fatalities based on media sources and Storm Data / NWS input, showing only direct flood fatalities. It is critical to remain safe during flood watches and warnings so as to avoid flood fatalities.

A Flash Flood Warning means a flash flood is imminent or occurring. If you are in a flood prone area move immediately to high ground.

You should always pay attention to these notifications and understand what the different levels mean.

For businesses, access to real-time flood watches and warnings through a hyperlocal weather map like Sferic Maps can help keep employees safe and protect outdoor equipment from being damaged during a flooding event.
What Do I Do During a Flood?
If a flood event is happening, there are steps you can take to stay safe. The first and most important thing to remember is to steer clear of flood waters. This is true even if you are in a car.

Remember: Turn around, don't drown! Flood waters can rise or gain intensity in the blink of an eye. Stay away from flood water at all costs.

Another important tip that will help you survive a flood is to listen to evacuation orders from authorities. We also stress that you listen to evacuation orders the first time you hear them. Waiting even just a few minutes can be the difference between life and death. And staying behind can put you and others at risk if you need to be rescued from rising waters.

One last useful flood safety tip to pay attention to during a flood is to stay away from rooms where water covers electrical outlets. This is pretty self-explanatory; you don't want to be electrocuted walking through your flooded home or business.

What Do I Do After a Flood?

Once the flood waters recede, you might think you're safe. However, there are still plenty of flooding dangers that can threaten your health and property. There are also things you should in case a flood damages your property.

One of the most important safety precautions after a flood is making sure your water is safe to drink. Groundwater may have been contaminated during the flood so don't assume you can just turn on the faucet and the water is safe. Local authorities will let you know if water is safe to drink or if you should boil it before using.

After a flood, standing water is also a danger. While it might look tame compared to a raging flood, standing water left behind is a breeding ground for bacteria and can carry toxins or chemicals. If you have to touch standing water for cleanup or any other reason, protect yourself with rubber gloves, boots, and other protective gear.

Floodwaters can also hide sharp or dangerous objects, so it's best to avoid going into the water if you can.

One last flood safety tip to keep in mind for after waters recede is to wait for the all clear from local officials to return to buildings or areas compromised by the flood. While your home or office might look safe to you, there could be major structural or electrical dangers. Always wait for the all clear.

Common Flood Locations & Worst Flooding Events

While flooding can happen anywhere in the world, there are certain places that are more prone to flooding. These locations often see the worst flooding events in history.

In this section, we'll cover where floods occur most often in the U.S. and then take a look at some of the worst floods in history.

Where Do Floods Occur in the U.S.?

As we've mentioned above, floods can happen anywhere in the U.S., including coastal and inland locations. However, there are some places that see flooding more often than others.

FEMA compiled flooding data and produced a map of the U.S. showing where the most flooding occurred in the U.S. between 1996 and 2013.

As you can see from FEMA's map, the Northeast, desert Southwest, Midwest and Gulf Coast see some of the most flood activity in the U.S.

In the Northeast, snowmelt and urban flooding are to blame for the high frequency of flooding events. Tropical storms also contribute to flooding along the coast in the Northeast.

In the desert Southwest, a lot of flooding events happen during monsoon season. Since the ground is dry, it's tough for the ground to soak up water that falls at a high rate.

Along the Gulf Coast, flooding events are commonly caused by hurricanes and tropical systems. Hurricane Harvey is an example of a hurricane that caused a lot of flooding along the Gulf, especially in Houston, Texas.

What Were the Worst Floods in History?

Unfortunately, some of the worst flooding events in history didn't happen so long ago. Over the past few years, extreme flooding events have been common throughout the United States. Here's a list of the worst flooding events in U.S. history:

Central Valley, California, 1861-1862

Johnstown, Pennsylvania, 1889

Galveston, Texas, 1900

It's hard to fathom this, but when a Category 4 hurricane made landfall in Galveston, it came with storm surge that killed over 8,000 people. The "" made landfall at night and, even though residents could see the storm brewing offshore, they had no idea how bad it would be. This was a full 65 years before the National Hurricane Center was established.

Mississippi River Flood, 1927

Ohio River Flood, 1937

Hurricane Camille, 1969

Some readers might remember this last flooding event on our list. Hurricane Camille was a natural disaster that caused damage from the Gulf of Mexico to the Mid-Atlantic. It hit the hardest in Virginia, where it overflowed the James River and caused widespread flooding.

Rapid City, South Dakota, 1972

The 1970s were a bad decade for flooding. The Rapid City flood of 1972 killed 238 people and caused over $165 million in damages when flash flooding happened at night while most people were asleep.

Big Thompson Canyon, Colorado, 1976

Mississippi River, 1993

Another terrible flooding event happened on the Mississippi in 1993 when waters remained at flood stages for 81 consecutive days. This flood killed over 50 people, destroyed 50,000 homes, and covered 75 towns in flood waters. Damages totaled more than $15 billion.

Hurricane Katrina, 2005

Hurricane Harvey, 2017

Hurricane Harvey caused widespread devastation in the greater Houston area. The slow movement of Harvey from Aug. 26-30 contributed to the destructive and deadly floods in southeast Texas. The most rain fell in Nederland, Texas, which received more than 60 inches of rain from Aug. 24-Sept.1. The flooding in the region displaced 30,000 people and damaged or destroyed more than 200,000 homes and businesses.

Additional Resources

Still have questions about floods and flooding? Let us know by reaching out to us on Twitter and our meteorologists will answer them as soon as possible.

You can also learn more about other forms of severe weather on our Weather 101 Resources page.

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What is a Flood? | What Causes a Flood? | Flooding | Earth Networks

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Believe it or not, flooding is the deadliest type of severe weather. There's probably a lot about floods and flooding you don't know. In this guide, we'll go over everything there is to know about flooding; answering common questions like "What causes flooding?" and "Where does flooding occur?"

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Green infrastructures are different systems put in place to mitigate or reduce the amount of stormwater in an area. Some examples of green infrastructures are permeable pavement (easy to pass through), rain gardens, and green roofs. These green infrastructures are beneficial to both the human and environmental well-being by reducing the risk of flooding, filtering stormwater, and improving air quality. Additionally, these infrastructures create a space to restore and rehabilitate ecosystems that may have been impacted by these floods. As for human benefits, some infrastructures reduce noise and air pollution while providing a type of recreational area.

Image by David Mark from Pixabay

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Combined sewer systems are both wastewater and stormwater that get transported through the same set of pipes to filter out and decrease the amount of water in an area before re-entering the water.

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San Francisco's paved surfaces such as buildings, streets, and parking lots cover most of the City, preventing rainwater from infiltrating into the ground. Instead of slowly filtering into soils, stormwater runoff now travels quickly over these paved areas, picking up pollutants like oil, bacteria, and debris before it flows to the city's combined or separate sewer systems. Streets cover approximately 25% of the city's surface, contributing significantly to these stormwater flows.

San Francisco's Stormwater Infrastructure

The underground sewer system is an integral part of the city - carrying sewage and stormwater away from landscapes, businesses, and homes. There are two types of sewer systems in San Francisco: our combined sewer system and our separate sewer system.

Most of San Francisco is served by a combined sewer system which conveys wastewater and stormwater in the same set of sewer pipes. Typically, this combined effluent is sent to a treatment plant where it is treated to secondary treatment standards before being discharged into the Bay or Ocean. However, during heavy rain storms, the large influx of water can exceed the capacity of the city's treatment plants. During these events, our sewer system treats the combined effluent to the equivalent of primary treatment standards before discharging it to the bay or ocean. This is called a combined sewer discharge. During heavy rainfall, the sudden influx of stormwater can also lead to flooding in low lying areas of the city such as historic creek beds.

In combined sewer areas, one of the city's goals is to decrease the quantity of runoff traveling to our combined sewer system.

Some areas of San Francisco have separate storm sewer systems. In these areas, stormwater and sewage travel in separate sets of pipes. In San Francisco, about 10% of the city is served by separate storm sewer systems, including Mission Bay, Hunter's Point, the Presidio, and Lake Merced. In areas served by separate storm sewer systems, street runoff generally flows untreated directly to the bay or ocean.

In separate sewer areas, one of the city's goals is to improve the quality of stormwater runoff before it reaches the bay or ocean.

Combined Sewer System

Separated Sewer System

The City of San Francisco is developing multiple solutions to better manage stormwater in both the city's combined and separate sewer areas. Solutions include upgrades to our grey infrastructure (pipes and treatment plants), as well as low impact design-based solutions, also known as green infrastructure. Low impact design-based solutions are appropriate for integration into many streetscape projects, as they can provide additional benefits to the street, including greening and traffic calming.

Low Impact Design (LID) or Green Infrastructure

Low Impact Design (LID) is a stormwater management approach that mimics the natural hydrologic processes of the landscape by increasing retention, detention, and filtration of stormwater runoff at its source. LID methods use small decentralized stormwater treatment facilities to treat runoff before it reaches the sewer pipes.

Detention: Collection and holding (or slowing) of runoff in temporary storage or vegetated systems before it is slowly released.

Retention: Capturing stormwater on site and keeping it there - either through infiltration, rainwater harvesting, or evapo-transpiration from plants and soils - leading to an overall reduction in stormwater runoff to sewers.

Filtration: Removal of solid (particulate) matter from water by means of porous media such as sand, soil, or a man-made filter. Filtration is used to remove contaminants. (May include bio-filtration.)

Infiltration: Penetration of water into the soil from the ground surface.

LID Tools

Designers typically select from a toolkit of LID techniques that will best manage stormwater at a particular location.

Whereas some LID tools are appropriate for homeowners and small property owners to install in front of their property, others are more complex to design and are better implemented as part of a larger block-scale or neighborhood-scale project.

LID Tools that can be used in streets and are typically appropriate for both small and large projects include:

LID tools that can be used in streets and may be appropriate for large projects include:

LID is scalable to small sites and construction budgets. Unlike traditional wastewater treatment systems that necessitate large public investments, complex infrastructure, and centralized treatment facilities, LID represents a low-cost, distributed approach to stormwater management.

In addition to sustainably managing stormwater, LID offers several ancillary benefits, including:

Greening and landscaping

Beautifying the public realm

Providing traffic calming

Providing habitat for birds and insects

Functions of Stormwater Facilities

Detention

Retention

Infiltration

Conveyance

Water Quality

Permeable Paving

Flow-Through and Infiltration Planters

Swales

Rain Gardens

Channels and Runnels

Infiltration and Soakage Trench

Infiltration Boardwalks

Processs Overview

The permitting process for stormwater LID features varies depending on the particular tools selected. Typically, LID would require one or more permits from DPW, with potential review by other City departments. If you choose or are required to install stormwater LID features, you are responsible to obtain the relevant permits for the particular design feature and meet all applicable guidelines. See individual LID tools pages for specific process and permit information.

To learn more about the permitting process for street improvements, see Permit Process

Managing Stormwater on Individual Parcels

San Francisco encourages (and sometimes requires) stormwater management on individual parcels. In addition to the stormwater management tools described on this site, there are other LID techniques, such as rainwater harvesting and green roofs, which homeowners, businesses, and developers can implement on individuals parcels.

For more information on additional stormwater management techniques, see:
SFPUC Stormwater Design Guidelines
Requirements

New and redevelopment projects that disturb 5,000 square feet or more of ground surface, measured cumulatively for a site, are required to manage the stormwater that falls on their site. While these requirements can be met by managing the stormwater within the parcel, these requirements may also be partially or entirely met by providing stormwater treatment facilities in the right-of-way.

See Developer Requirements: Stormwater

General Design Guidelines for LID

See individual LID tools for more specific guidelines on LID tools appropriate for the streetscape.

Official Codes and Documents

Better Streets Plan Classifications

Street types: All

Best Fit for Stormwater Facilities by Street Type

PAVING

BIORETENTION

CONVEYANCE

OTHER

Permeable Paving*

Rain Garden

Flow Through & Infiltration Planter

Infiltration Boardwalks

Swales

Channels & Runnels

Infiltration & Soakage Trenches

Vegetated Buffer Strip

Vegetated Gutter

Commercial Streets

Commercial Throughway

Residential Streets

Industrial & Mixed-Use Streets

Special Streets

Ceremonial (Civic)

Small Streets

Alley

Shared Public Way

Sidewalk zones: Generally, all except travel lanes on collectors or arterials, bus stop concrete pads.

Stormwater Facilities by Location in the Right-of-Way

Site Constraints

Before implementing LID techniques, it is important to assess a site's existing conditions and choose stormwater treatment controls that will be successful in that location. Streetscape geometry, existing utilities, topography, soil conditions and climate determine the types of LID tools that can be implemented. The initial step in selecting a stormwater tool is determining the available open space and existing site constraints.

After identifying the appropriate stormwater facilities for a site, an integrated approach using several LID tools is encouraged. To increase the stormwater management benefits, multiple LID tools can be used in succession-called a treatment train approach. LID tools should be designed using available topography to take advantage of gravity for conveyance. Concentrating too much runoff in one area should be avoided.

Infiltration Considerations

Except in cases of very simple LID techniques, such as sidewalk landscaping, a complete geotechnical report considering infiltration rates, soil toxicity and stability, and other factors should be undertaken to determine whether or not it is acceptable to have stormwater infiltrate into underlying soils. Stormwater management benefits may be limited to runoff detention and filtration if lining is necessary to prevent stormwater from percolating into the soil below.

Stormwater management tools can still be incorporated into areas of low permeability or where infiltration of stormwater is not desirable if special measures are undertaken. Underdrains should be used in areas where ponding is a concern. The location of the underdrain is an important consideration: if placed higher in a facility, the stored water below the perforated pipe will be infiltrated; if placed at the bottom of a sealed system, the perforated pipe will release the stored water slowly over time. All stormwater management strategies must have an approved overflow location for discharge.

Minor details can translate into the ultimate success or potential failure of a system. For example, poor soils may cause ponding conditions in which some plants will not survive. Over-compaction or smearing (sealing) of subsurface soil during construction can lead to reduced infiltration capacity and cause ponding. The bottom surface of infiltration areas should be level and properly prepared to allow even stormwater distribution across the site and consistent soil permeability.

Landscaping

Landscaping should be chosen to fit the specific type of stormwater facility and should be appropriate for the local climate and soils. In general, all landscape-based stormwater facilities should be densely planted with drought-resistant and water-tolerant plantings that can survive periodic drought and inundation. Native, deep-rooted plants or Mediterranean plants have been proven most effective in San Francisco.

Landscape features in stormwater facilities should follow the placement and clearance guidelines for trees and understory plantings:
Street trees Sidewalk landscaping
Streetscape Considerations

Utilities

Sub-surface utility locations and building laterals should be considered in the design and placement of LID tools - access to utilities should be maintained. The presence of multiple driveways may also limit the ability to provide stormwater facilities. Where a high number of driveways or utilities reduce the ability to implement stormwater management tools on sidewalks, median locations for stormwater facilities should be considered.

See Utilities

Accessibility

Standard requirements for sidewalk accessibility and clearances apply to planting areas and other stormwater facilities incorporated into the sidewalk. Specifically, stormwater facilities should follow these guidelines:

Stormwater facilities that incorporate open planters, channels, or ditches should not be located in the through path of travel, at corners, or anywhere along the required accessible path.

Where stormwater facilities cross the path of travel, they should be covered by an Americans with Disabilities Act (ADA)-compliant cover.

Permeable paving systems in the path of travel should have no more than 1/4 inch gap between pavers.

Where there is a vertical drop between sidewalks and stormwater facilities (such as rain gardens or channels), a 4 inch high lip should be provided at the edge of the facility for tactile detection.

See Sidewalk Landscaping

Transit Stops

Appropriate strategies near transit stops include: building-adjacent planters, covered channels or runnels, and permeable paving. Any drainage feature within a transit waiting area should be covered by an ADA-compliant cover.

Certain stormwater facilities, such as those that use open planters or channels, are generally not appropriate within transit stops.

All stormwater facilities should maintain a clear path of travel to shelters and boarding transit vehicles:

See Transit Stops

Maintenance

Maintenance Responsibilities

Maintenance responsibilities for LID features vary depending on a number of factors.

In the case of sidewalk landscaping, it is the property owner's responsibility to maintain the improvement.

If stormwater treatment controls are installed in the right of way to meet the City's Stormwater Design Guidelines, it is the developer or owner of the development who is responsible for maintaining the improvement.

Maintenance responsibility for other LID features varies. It is important to meet with the permitting agency early on in the process to develop a long-term maintenance strategy for your project. This will help expedite the permitting process and ensure long term functionality and success of your LID improvement.

See Maintenance

Funding and Technical Resources

Funding

Technical Resources

Other Links

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San Francisco's paved surfaces such as buildings, streets, and parking lots cover most of the City, preventing rainwater from infiltrating into the ground. Instead of slowly filtering into soils, stormwater runoff now travels quickly over these paved areas, picking up pollutants like oil, bacteria, and debris before it flows to the city's combined or separate sewer systems. Streets cover approximately 25% of the city's surface, contributing significantly to these stormwater flows.

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Separate sewer systems includes both stormwater and sewage water and transports each type of water in a separate sets of pipes. Street runoff tends to gets little to no treatment and directly travels to bay, ocean, or closest body of water.

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San Francisco's paved surfaces such as buildings, streets, and parking lots cover most of the City, preventing rainwater from infiltrating into the ground. Instead of slowly filtering into soils, stormwater runoff now travels quickly over these paved areas, picking up pollutants like oil, bacteria, and debris before it flows to the city's combined or separate sewer systems. Streets cover approximately 25% of the city's surface, contributing significantly to these stormwater flows.

San Francisco's Stormwater Infrastructure

The underground sewer system is an integral part of the city - carrying sewage and stormwater away from landscapes, businesses, and homes. There are two types of sewer systems in San Francisco: our combined sewer system and our separate sewer system.

Most of San Francisco is served by a combined sewer system which conveys wastewater and stormwater in the same set of sewer pipes. Typically, this combined effluent is sent to a treatment plant where it is treated to secondary treatment standards before being discharged into the Bay or Ocean. However, during heavy rain storms, the large influx of water can exceed the capacity of the city's treatment plants. During these events, our sewer system treats the combined effluent to the equivalent of primary treatment standards before discharging it to the bay or ocean. This is called a combined sewer discharge. During heavy rainfall, the sudden influx of stormwater can also lead to flooding in low lying areas of the city such as historic creek beds.

In combined sewer areas, one of the city's goals is to decrease the quantity of runoff traveling to our combined sewer system.

Some areas of San Francisco have separate storm sewer systems. In these areas, stormwater and sewage travel in separate sets of pipes. In San Francisco, about 10% of the city is served by separate storm sewer systems, including Mission Bay, Hunter's Point, the Presidio, and Lake Merced. In areas served by separate storm sewer systems, street runoff generally flows untreated directly to the bay or ocean.

In separate sewer areas, one of the city's goals is to improve the quality of stormwater runoff before it reaches the bay or ocean.

Combined Sewer System

Separated Sewer System

The City of San Francisco is developing multiple solutions to better manage stormwater in both the city's combined and separate sewer areas. Solutions include upgrades to our grey infrastructure (pipes and treatment plants), as well as low impact design-based solutions, also known as green infrastructure. Low impact design-based solutions are appropriate for integration into many streetscape projects, as they can provide additional benefits to the street, including greening and traffic calming.

Low Impact Design (LID) or Green Infrastructure

Low Impact Design (LID) is a stormwater management approach that mimics the natural hydrologic processes of the landscape by increasing retention, detention, and filtration of stormwater runoff at its source. LID methods use small decentralized stormwater treatment facilities to treat runoff before it reaches the sewer pipes.

Detention: Collection and holding (or slowing) of runoff in temporary storage or vegetated systems before it is slowly released.

Retention: Capturing stormwater on site and keeping it there - either through infiltration, rainwater harvesting, or evapo-transpiration from plants and soils - leading to an overall reduction in stormwater runoff to sewers.

Filtration: Removal of solid (particulate) matter from water by means of porous media such as sand, soil, or a man-made filter. Filtration is used to remove contaminants. (May include bio-filtration.)

Infiltration: Penetration of water into the soil from the ground surface.

LID Tools

Designers typically select from a toolkit of LID techniques that will best manage stormwater at a particular location.

Whereas some LID tools are appropriate for homeowners and small property owners to install in front of their property, others are more complex to design and are better implemented as part of a larger block-scale or neighborhood-scale project.

LID Tools that can be used in streets and are typically appropriate for both small and large projects include:

LID tools that can be used in streets and may be appropriate for large projects include:

LID is scalable to small sites and construction budgets. Unlike traditional wastewater treatment systems that necessitate large public investments, complex infrastructure, and centralized treatment facilities, LID represents a low-cost, distributed approach to stormwater management.

In addition to sustainably managing stormwater, LID offers several ancillary benefits, including:

Greening and landscaping

Beautifying the public realm

Providing traffic calming

Providing habitat for birds and insects

Functions of Stormwater Facilities

Detention

Retention

Infiltration

Conveyance

Water Quality

Permeable Paving

Flow-Through and Infiltration Planters

Swales

Rain Gardens

Channels and Runnels

Infiltration and Soakage Trench

Infiltration Boardwalks

Processs Overview

The permitting process for stormwater LID features varies depending on the particular tools selected. Typically, LID would require one or more permits from DPW, with potential review by other City departments. If you choose or are required to install stormwater LID features, you are responsible to obtain the relevant permits for the particular design feature and meet all applicable guidelines. See individual LID tools pages for specific process and permit information.

To learn more about the permitting process for street improvements, see Permit Process

Managing Stormwater on Individual Parcels

San Francisco encourages (and sometimes requires) stormwater management on individual parcels. In addition to the stormwater management tools described on this site, there are other LID techniques, such as rainwater harvesting and green roofs, which homeowners, businesses, and developers can implement on individuals parcels.

For more information on additional stormwater management techniques, see:
SFPUC Stormwater Design Guidelines
Requirements

New and redevelopment projects that disturb 5,000 square feet or more of ground surface, measured cumulatively for a site, are required to manage the stormwater that falls on their site. While these requirements can be met by managing the stormwater within the parcel, these requirements may also be partially or entirely met by providing stormwater treatment facilities in the right-of-way.

See Developer Requirements: Stormwater

General Design Guidelines for LID

See individual LID tools for more specific guidelines on LID tools appropriate for the streetscape.

Official Codes and Documents

Better Streets Plan Classifications

Street types: All

Best Fit for Stormwater Facilities by Street Type

PAVING

BIORETENTION

CONVEYANCE

OTHER

Permeable Paving*

Rain Garden

Flow Through & Infiltration Planter

Infiltration Boardwalks

Swales

Channels & Runnels

Infiltration & Soakage Trenches

Vegetated Buffer Strip

Vegetated Gutter

Commercial Streets

Commercial Throughway

Residential Streets

Industrial & Mixed-Use Streets

Special Streets

Ceremonial (Civic)

Small Streets

Alley

Shared Public Way

Sidewalk zones: Generally, all except travel lanes on collectors or arterials, bus stop concrete pads.

Stormwater Facilities by Location in the Right-of-Way

Site Constraints

Before implementing LID techniques, it is important to assess a site's existing conditions and choose stormwater treatment controls that will be successful in that location. Streetscape geometry, existing utilities, topography, soil conditions and climate determine the types of LID tools that can be implemented. The initial step in selecting a stormwater tool is determining the available open space and existing site constraints.

After identifying the appropriate stormwater facilities for a site, an integrated approach using several LID tools is encouraged. To increase the stormwater management benefits, multiple LID tools can be used in succession-called a treatment train approach. LID tools should be designed using available topography to take advantage of gravity for conveyance. Concentrating too much runoff in one area should be avoided.

Infiltration Considerations

Except in cases of very simple LID techniques, such as sidewalk landscaping, a complete geotechnical report considering infiltration rates, soil toxicity and stability, and other factors should be undertaken to determine whether or not it is acceptable to have stormwater infiltrate into underlying soils. Stormwater management benefits may be limited to runoff detention and filtration if lining is necessary to prevent stormwater from percolating into the soil below.

Stormwater management tools can still be incorporated into areas of low permeability or where infiltration of stormwater is not desirable if special measures are undertaken. Underdrains should be used in areas where ponding is a concern. The location of the underdrain is an important consideration: if placed higher in a facility, the stored water below the perforated pipe will be infiltrated; if placed at the bottom of a sealed system, the perforated pipe will release the stored water slowly over time. All stormwater management strategies must have an approved overflow location for discharge.

Minor details can translate into the ultimate success or potential failure of a system. For example, poor soils may cause ponding conditions in which some plants will not survive. Over-compaction or smearing (sealing) of subsurface soil during construction can lead to reduced infiltration capacity and cause ponding. The bottom surface of infiltration areas should be level and properly prepared to allow even stormwater distribution across the site and consistent soil permeability.

Landscaping

Landscaping should be chosen to fit the specific type of stormwater facility and should be appropriate for the local climate and soils. In general, all landscape-based stormwater facilities should be densely planted with drought-resistant and water-tolerant plantings that can survive periodic drought and inundation. Native, deep-rooted plants or Mediterranean plants have been proven most effective in San Francisco.

Landscape features in stormwater facilities should follow the placement and clearance guidelines for trees and understory plantings:
Street trees Sidewalk landscaping
Streetscape Considerations

Utilities

Sub-surface utility locations and building laterals should be considered in the design and placement of LID tools - access to utilities should be maintained. The presence of multiple driveways may also limit the ability to provide stormwater facilities. Where a high number of driveways or utilities reduce the ability to implement stormwater management tools on sidewalks, median locations for stormwater facilities should be considered.

See Utilities

Accessibility

Standard requirements for sidewalk accessibility and clearances apply to planting areas and other stormwater facilities incorporated into the sidewalk. Specifically, stormwater facilities should follow these guidelines:

Stormwater facilities that incorporate open planters, channels, or ditches should not be located in the through path of travel, at corners, or anywhere along the required accessible path.

Where stormwater facilities cross the path of travel, they should be covered by an Americans with Disabilities Act (ADA)-compliant cover.

Permeable paving systems in the path of travel should have no more than 1/4 inch gap between pavers.

Where there is a vertical drop between sidewalks and stormwater facilities (such as rain gardens or channels), a 4 inch high lip should be provided at the edge of the facility for tactile detection.

See Sidewalk Landscaping

Transit Stops

Appropriate strategies near transit stops include: building-adjacent planters, covered channels or runnels, and permeable paving. Any drainage feature within a transit waiting area should be covered by an ADA-compliant cover.

Certain stormwater facilities, such as those that use open planters or channels, are generally not appropriate within transit stops.

All stormwater facilities should maintain a clear path of travel to shelters and boarding transit vehicles:

See Transit Stops

Maintenance

Maintenance Responsibilities

Maintenance responsibilities for LID features vary depending on a number of factors.

In the case of sidewalk landscaping, it is the property owner's responsibility to maintain the improvement.

If stormwater treatment controls are installed in the right of way to meet the City's Stormwater Design Guidelines, it is the developer or owner of the development who is responsible for maintaining the improvement.

Maintenance responsibility for other LID features varies. It is important to meet with the permitting agency early on in the process to develop a long-term maintenance strategy for your project. This will help expedite the permitting process and ensure long term functionality and success of your LID improvement.

See Maintenance

Funding and Technical Resources

Funding

Technical Resources

Other Links

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San Francisco's paved surfaces such as buildings, streets, and parking lots cover most of the City, preventing rainwater from infiltrating into the ground. Instead of slowly filtering into soils, stormwater runoff now travels quickly over these paved areas, picking up pollutants like oil, bacteria, and debris before it flows to the city's combined or separate sewer systems. Streets cover approximately 25% of the city's surface, contributing significantly to these stormwater flows.

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Planning on buying a house and you're not sure if it's a flood-risk zone? Use this link to help you out!

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Real Estate News and Advice | Realtor.com®

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"Is my property in a flood zone?" That's a question that concerns both homeowners and home shoppers -and not just those near large bodies of water.

In fact, there are a little over a dozen flood zone classifications. Those rankings, determined by the Federal Emergency Management Agency and represented on special maps, are based on the estimated frequency of storms that will cause flooding in an area. So while you may not live in the Gulf Coast (one of the U.S. regions most vulnerable to hurricanes), your property may still be part of an area that's been deemed at high risk of flooding.

So how do you find out if your home is in a flood zone, and what type of flood zone it is? The answer is on the map.

How to find out if your property is in a flood zone?

Most homes in high-risk flood zones are near a body of water. But regardless of your location, there are a few ways to determine your home's flood hazard status. "The best place to start is with FEMA's Map Service Center," says Adam D'Annunzio, a real estate agent with Keller Williams in New Jersey who sells real estate in flood zones.

Simply enter a property's address on the FEMA Flood Map Service Center website, and a map showing its flood zone hazard will pop up. Zones B, X, and C are at the lowest risk, while high-risk zones start with either an A or a V (V zones are coastal areas) on the map. Zone AE, for example, has a 1% probability of flooding every year, also known as a 100-year floodplain-a low flood hazard.

Keep in mind when looking at the FEMA map that it's possible that only part of your lot-and perhaps not your actual house-may be in the flood zone. Aside from FEMA and flood maps, other resources include tax record databases and title searches; both will indicate a property's flood risk.

"You can also ask your insurance agent," says real estate agent and attorney Bruce Ailion of Atlanta's Re/Max Town and Country.

Is a property's flood zone status disclosed to home shoppers?

While all states have laws regarding property disclosure, the specific laws regarding FEMA flood zone status vary from state to state, says Johnny Harper, owner of J and J Home Inspections in Nashville, TN. Federal laws don't require disclosure of previous flooding.

"However, full disclosure is certainly the ethical responsibility of the seller and the listing agent," Harper says.

As a buyer, you should research the property and flood maps-or have your real estate agent check-to see if it's in a flood hazard area. If you're obtaining financing, the mortgage company will investigate the most updated flood zoning as well.

"If the property is found to be in a flood zone, the mortgage company will require flood insurance," says D'Annunzio.

Flood maps are updated periodically, based on new developments and FEMA-funded flood-control projects within a given floodplain, so be sure you're working with FEMA's most current information.

How much is flood insurance?

The federal government offers subsidized flood insurance to property owners through the National Flood Insurance Program. The amount you pay for your policy is based on such factors as year of construction, building occupancy, number of floors, location of contents, flood risk, location of the lowest floor (in relation to the Base Flood Elevation on a flood map), and your deductible.

The average cost of flood insurance through the NFIP is $700 per year. Of course, the amount you pay is all based on your property's flood risk and where you are in the floodplain, as specified on the flood insurance rate map. If you are in a high flood-hazard zone, Harper says flood insurance can be significantly more expensive-as high as $6,000 per year.

Does being in a flood zone affect a home's price?

Whether you're a seller or a buyer, keep in mind that a home's price is likely to be affected if it is in a flood plain.

"Sellers know the cost of yearly flood insurance could affect what buyers can afford," says D'Annunzio.

So when the home is priced to sell, they will probably take those extra expenses into account. The property could also be seen by buyers as less desirable if the risk of damage from future flooding is high. So a highly motivated seller might list the home lower than the market value.

Do you need flood insurance?

Did you know that most home insurance policies don't cover damage or other losses caused by flooding? Homes in high-risk areas that participate in the NFIP are required to purchase flood insurance. But if you live in a moderate- to low-risk area within or just outside a floodplain, should you buy flood insurance?

Flood zones are based on an estimation of the number of storms that will cause flooding in a year, but there's no guarantee that your area will flood. Still, you want to be covered in the event that something does happen.

If you're wavering between buying and not buying flood insurance, ask yourself this question: Do you think the possibility of flooding-and the damage associated with it-is worth the insurance premium? In some cases, having flood insurance is less expensive than the losses from a flood.

How to minimize your flood losses

If your home is in a flood zone, Harper advises taking these steps:

Elevate your furnace, water heater, and electric panels to protect them from possible floodwaters.

Keep storm drains and gutters free of debris, and install check valves (or one-way valves) to keep floodwater from backing into your drains.

Seal your basement walls with waterproofing materials.

Store your valuables and legal documents on your upper floors.

Margaret Heidenry is a writer living in Brooklyn, NY. Her work has appeared in the New York Times Magazine, Vanity Fair, and Boston Magazine.

The realtor.com® editorial team highlights a curated selection of product recommendations for your consideration; clicking a link to the retailer that sells the product may earn us a commission.

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'Is My Property in a Flood Zone?' The Easiest Way to Determine If You're at Risk

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"Is my property in a flood zone?" That's a question that concerns both homeowners and home shoppers -and not just those near large bodies of water. In fact, there are a little over a dozen flood zone classifications.

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This link is for California residents to stay informed of how to prepare for a flood in your area.

Photo Credit: Linda Russ from Pixabay

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The risk of catastrophic floods exists every year, and heavy downpours often lead to dangerous flooding conditions. Wildfires add another layer of risk during the rainy season because dangerous landslides and debris flows can rush down fire-scarred hillsides. Because flooding can occur anywhere throughout the state, it is important for Californians to assess their flood risk, prepare and practice response plans, and learn from past floods. We assist with flood activities, and through collaboration with federal agencies are improving our understanding of atmospheric rivers and their effects on flooding and water supply.

DWR's role in flood preparedness includes:

Working with communities to improve community safety and lower insurance premiums for residents

Holding preseason briefings for emergency responders to learn about resources and coordinate with other local agencies to prepare for flood season

Conducting meetings with flood emergency response partners and county emergency operation officials throughout the state

Providing training in effective flood fighting methods for mitigating a flood

Activating our Flood Operations Center when flooding is imminent or occurring to coordinate response between local agencies and provide technical assistance as requested

Providing grant funding for flood management projects, emergency preparedness, feasibility studies, Delta-specific and other state projects, and forecast-coordinated operations in the Central Valley

Flood emergency preparedness and response requires coordinated, well-planned actions, primarily by local government agencies and special districts. Because each community has a unique vulnerability to flooding, we created a program to work with local emergency responders to develop flood emergency preparedness and response plans that address these vulnerabilities and accommodate local needs. This program aims to improve emergency preparedness and flood response capabilities at the local level and within our department. Pilot studies have demonstrated the types of approaches that will help communities throughout California work effectively with state and federal agencies.

In addition, flood preparedness and response activities are effective tools in protecting life and property vulnerable to flooding. Coordination has improved among key response agencies as a result of extensive effort since 2007. The lead agencies involved are highly motivated to continue improving partnerships among State, federal, and local agencies.

Our Flood Operations Center (FOC) is the central point for preparing and responding to floods, making operational decisions, gathering intelligence, and sharing information with the media and public. We work cooperatively with local responders and county operational areas to improve local preparedness.

Partners include:

Each fall before the flood season begins, we conduct meetings with flood emergency response partners and county emergency operation officials throughout the state. Staff from emergency response agencies provide regional and local updates on annual flood preparedness activities, as well as an opportunity to coordinate.

Topics include:

Flood fight materials and training

Latest DWR grant news

Local flood concerns

Regional updates

Roles and responsibilities under the Standardized Emergency Management System

Web resources

Winter weather outlook

We prepare for floods throughout the year and provide training in effective flood fighting methods for mitigating a flood. State agencies, counties, reclamation districts, tribes, and nonprofit agencies request this free day-long training to learn how they can best protect their properties and communities from high water and flooding incidents.

The training consists of an in-class presentation of the flood fighting methods applicable to different types of flooding, followed by outdoor hands-on practice. View Emergency Flood Fighting Methods (PDF) for more information.

Training, exercises, and preparing for flood events are rudimentary but fundamental elements of emergency preparedness. Improving staff training needs requires establishing a comprehensive program to improve flood emergency readiness. We build and maintain relationships with the incident command teams, local maintaining agencies, and local emergency response officers to effectively manage floods in coordination with local agencies.

How You Can Prepare for Floods

Visit Flood Preparedness Week to know your risk and "Flood Prepare California!"

Use the information and tools to:

Figure out their risk of flooding at home and while traveling

Learn what food and supplies are needed to shelter at home during a flood

Determine what you should take with you if evacuating

Maps

Best Available Maps (BAM) represent the 100- and 200-year composite floodplains located within the Sacramento-San Joaquin Valley watershed.

Contact Us

Find general contact information in the directory, or select the button below for Division of Flood Management details, including how to report flooding through the Flood Operations Center .
Contact Division
Mailing List

Join the Flood News mailing list to receive flood management announcements and flood news, including information about our grant opportunities, new reports, and comment periods.

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The risk of catastrophic floods exists every year, and heavy downpours often lead to dangerous flooding conditions. Wildfires add another layer of risk during the rainy season because dangerous landslides and debris flows can rush down fire-scarred hillsides.

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This map illustrates the current drainage system set in place for San Francisco, CA. Useful for citizens to know where their water is transferred.

Photo Credit: Creek & Watershed Map of San Francisco

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Christopher Richard, Oakland Museum of California

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If you don't live in San Francisco or you're curious to see which areas are more at-risk, check out this map!