Flood - Water Level Simulation and FEMA Flood Zones

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.
• 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).

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 and reading the FEMA zones 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.

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.

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.

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.

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.

Once you have read the zones, the Water level tab offers a Base flood elevation button that 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.

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. 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.

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."