Flood - Water Level, River Level, FEMA Flood Zones and Forecast

The Flood kit shows you what floods before it happens. Draw an area (or pick one you already drew) and the kit can:

• Water level: raise the water over the real terrain with a slider and watch the flooded area, depth and volume change as you move it.
• River level: raise every river and stream in the area above its normal level, or show its 2 to 50-year flood, and see what floods, on NOAA's river model (US).
• FEMA flood zones: show FEMA's flood zones for the area: floodways, the 100-year and 500-year floodplains, coastal high hazard zones, levee areas and base flood elevations (US).
• Flood forecast: show where NOAA maps water right now, or the most flooding it expects over the next 5 days, from its National Water Model (US).

Each run goes on your map as a layer, with a report card of its numbers next to the area.

Who can use it

Flood is a Pro Business kit. Every signed-in account also gets 3 free flood analyses to try it. Loading the terrain for a water level, loading a river level, reading the FEMA zones and reading a flood forecast each use one. A run that fails does not use one, and moving the water level or saving the map never uses one. The kit shows how many free analyses you have left.

An area can be up to 25,000 acres (about 100 square kilometres), large enough for a whole neighbourhood or a stretch of river.

Add the kit

1. Open Maps & Data and go to the Kits tab.
2. Find Flood and click Use Kit.

You can also right-click a polygon, rectangle or circle on your map and choose Simulate flood. The kit opens with that shape as the area.

Choose the area

• Use selected shape: opens a list of the areas on your map. Click one, or click Select On Map and then click the shape on the map.
• Draw area: click the map to outline the area, then click the first point to finish.

The chosen area shows under the buttons with its icon, name and size. If the area is over 25,000 acres, draw a smaller one.

The first time you click Create layer on your account, the kit shows This is a simulation and asks you to confirm that you will not use it to decide whether to evacuate. Click I understand to go on. It asks once per account.

Water level

1. Open the Water level tab and click Create layer. The kit loads the terrain for the area once.
2. Choose where the water comes from in Water comes:
   • Across the edge (the default): water stands at the same level all around the area and floods in wherever the ground is lower, as a river in flood or the sea would.
   • Lowest ground: water starts at the single lowest point in the area, for a lake or an enclosed basin.
   • A marker you pick: water starts at a marker where it enters, such as a breach, a burst water main, a drain or the shore. Pick the marker from the list, or click Select On Map and click it.
   • Every low spot: every place below the level fills, connected or not, like hollows filling with rain.
3. Choose how it floods in Flood by:
   • Raising the level: one still water level over all the ground the water can reach. Use it for a river or sea level, or a base flood elevation.
   • Spreading out: you set how much water is released, and it spreads outward from the source over lower ground, nearest first. A marker uses this unless you change it.
4. Move the slider, or type the rise (raising) or the water released (spreading). The water on the map and the numbers change as you move it. The report card on the map shows the same numbers and has its own slider.
5. The flood is added to your map as soon as the terrain loads, in a Flood folder: the water as an image layer and a report card with the numbers. The report card has its own slider, so you can change the water from the map even with the Flood kit closed. Save your map to keep the water and the numbers as they are.

Except with Every low spot, only ground the water can reach floods: a levee or a raised road in the terrain holds the water back until the water rises over it.

The numbers:

• Water surface: the elevation of the water.
• Rise: how far that is above the ground where the water comes from.
• Flooded: the area under water, and its share of your area.
• Water volume (or Water released when spreading): how much water that is.
• Deepest and Average depth: how deep the water is.

The colours run from light blue (shallow) to dark blue (8 m or deeper).

This is still water on the terrain, not a flood forecast. It does not model how fast water flows, rainfall, storm drains, or flood walls and pumps the terrain does not show.

River level

The River level tab asks a different question from the water level: what floods if the rivers rise? Open it and click Create layer. The kit loads, for every spot in your area, how high it sits above its nearest river or stream. This comes from NOAA's river model, the same method NOAA uses for its own flood maps, at 10 m.

Move the slider, or type the River rise above its normal level. Every place that sits less than that height above its own stream floods, so the main river and its creeks rise together. The water and the numbers change as you move it, and the flood goes on your map in the Flood folder with its report card and slider, like a water level.

The numbers:

• River rise: how far the water stands above the normal river channel.
• Flooded: the area under water, and its share of your area.
• Water volume, Deepest and Average depth: how much water that is and how deep.
• Not mapped by NOAA: the share of your area NOAA's model leaves out: lakes, reservoir pools and land behind levees. That ground is never shown flooding.

Flood sizes

Instead of choosing a rise, you can show a flood of a given size. In the Rivers list beside the slider, pick 2-year flood, 5-year flood, 10-year flood, 25-year flood or 50-year flood. A 10-year flood is the flood NOAA's National Water Model expects about once every 10 years on each stream, a 10% chance in any year.

Each stream gets its own flood height. NOAA's river flow for that flood size is turned into a water height with the stream's own rating curve, the way NOAA maps its flood extents, so a big river rises more than a small creek. The water shows how deep it is, and the slider now steps through the five sizes, so you can watch the flood grow. Pick Raised above normal to go back to your rise. Switching does not use another analysis.

The report shows the Flood size in place of the river rise. Streams with no NOAA flow is the share of the mapped ground on streams NOAA gives no flow for, including lake and reservoir reaches. That ground is never shown flooding.

NOAA's National Water Model has no 100-year flood. For the 100-year floodplain, use the FEMA flood zones tab.

River level covers the contiguous United States. It is still water at one height above every stream at once, not a flood forecast: it does not model how a flood travels down a river, rainfall or storm drains. For flooding happening or expected now, use the Flood forecast tab.

FEMA flood zones

Open the FEMA flood zones tab and click Create layer. The zones are added to your map, coloured by type:

• Regulatory floodway: the river channel and nearby land that must be kept clear so the 100-year flood can pass.
• 1% annual chance (100-year): zones A, AE, AH and AO.
• Coastal high hazard (V): coastal zones with wave action.
• 0.2% annual chance (500-year): shaded zone X.
• Reduced risk due to levee: zone X behind an accredited levee.
• Minimal flood hazard: unshaded zone X.
• Possible but undetermined (D): zone D.
• Open water.

The report shows each type's share of the area, how much of it is in the 100-year floodplain, and the base flood elevations (BFE), the height the 100-year flood is expected to reach. Buildings in the 100-year floodplain with a federally backed mortgage must carry flood insurance.

If you read the FEMA zones before you create a water level, and they give a base flood elevation in NAVD88 that is within the slider's range, the water level controls show a Base flood elevation button. It sets the water to that height, so you can see the 100-year flood on the terrain.

Some older flood maps give elevations in the NGVD29 datum. Our terrain uses NAVD88, so those can differ by up to about half a metre; the report says when that applies.

FEMA flood zones are for planning only. They are not an official flood zone determination.

Flood forecast

Open the Flood forecast tab, choose what to show and click Create layer:

• Flooding now: the ground NOAA maps as under water right now, from river flows measured at gauges and the rain that has fallen. Updated every hour.
• Next 5 days: the most flooding NOAA expects at any time over the next 5 days, from its rainfall forecast. Updated every few hours. In Hawaii and Puerto Rico NOAA looks 2 days ahead, and the report says so.
• Next 5 days (River Forecast Centers): the 5-day flooding downstream of the official River Forecast Center forecast points that reach action stage or worse.

The flooded ground is added to your map in blue, one polygon per river reach. Each polygon is named by its National Water Model reach and shows the river flow it was mapped for, so you can look the reach up at water.noaa.gov. The report shows:

• Flooded: the ground under water inside your area, and its share of the area.
• River reaches: how many river reaches put water in the area.
• Peak flow: the highest river flow among them.
• Model run and Updated: when NOAA's model ran and when the map was last rebuilt, in your local time.

If NOAA maps no water in the area, the report says so. That is a real answer: nothing is flooding there now, or nothing is expected over the period.

NOAA covers the contiguous United States, Alaska, Hawaii, Puerto Rico and the US Virgin Islands. Next 5 days (River Forecast Centers) covers the contiguous United States and Alaska only. Outside NOAA's coverage, the report says there is no flood forecast.

NOAA calls this an experimental service. The maps are made automatically from the National Water Model with the HAND method and are not checked by a forecaster. Use them to plan and to watch a flood develop, not to decide whether to evacuate. In an emergency, follow your local emergency services.

The four tools answer different questions. FEMA flood zones are the regulatory floodplain: where a 100-year flood is expected over the long run. The flood forecast is what is happening or expected this week. The water level is one water height you choose on the terrain, and the river level is how far you choose to raise the rivers, or the 2 to 50-year flood on every stream.

Where the results go

Everything goes in a Flood folder on your map, with one folder per area and one per run inside it. The water and the FEMA zones are ordinary layers: they save, share and undo like anything you draw. The water level run is listed in Layers & Data as Flood simulation, and a river level run as River level simulation. Its report card keeps its slider while you work, even with the kit closed. When you save the map, the water is saved as it is at that moment. Delete the water image to end the run; its report card stays as a record.

Where the data comes from

• Terrain: Scribble Maps elevation: USGS 3DEP 1 m lidar across most of the US, and 10 to 30 m elevation elsewhere in the world. Each report card names its source and resolution.
• Flood zones: the FEMA National Flood Hazard Layer. When the zones come from Scribble Maps' own copy, the report names the date of that copy.
• Flood forecast: NOAA National Water Center flood inundation maps (experimental), read live from NOAA each time you run them. Scribble Maps keeps a copy of every run's extents, so past floods can be studied later.
• River level: NOAA Office of Water Prediction's height above nearest drainage (HAND) model at 10 m, from Scribble Maps' own copy. The report card names the version.
• Flood sizes: NOAA's National Water Model 3.0 flood flows for every stream (2, 5, 10, 25 and 50 years), from Scribble Maps' own copy, turned into flood heights with the HAND model's rating curves.

Ask Scribble

You can ask Scribble to do it for you, for example:

• "Raise the water 3 metres over this area."
• "What would a 2 metre sea level rise flood here?"
• "Show the FEMA flood zones for this neighbourhood."
• "Show the 100-year flood on the terrain here."
• "Is anything flooding here right now?"
• "What floods if the rivers here rise 3 metres?"
• "Show the 10-year river flood for this area."
• "Show the NOAA flood forecast for the next 5 days."