The item page for the Boulder County Geologic Hazard Zones map, owned by Learn_ArcGIS, appears.

The map loads in Map Viewer Classic.

Now you'll make a copy of the map so you can save changes to it.

The new map is saved to My Content and the new title is shown at the top of the page.

The layer has four classifications: Major Hazard, Moderate Hazard, Moderate Constraint, and Minor Constraint. These classifications represent the likelihood of major geotechnical problems—primarily landslide potential—determined by slope (steepness of terrain), aspect (direction of slope), and soil type. Most of the Major Hazard zone in is on the western side of the county, and a thin corridor of the zone extends up the central part of the county. There are also additional Major Hazard zones scattered in the southeast.

A table opens that shows the attributes of the layer. For this data, there is only one visible attribute: the Description field, which describes the type of hazard zone. You'll use this field to filter the layer.

The Filter window opens, which allows you to create an expression that will limit the features a layer displays.

The expression is set to Description is Major Hazard, which means to show only features that are major hazards.

The map is drawn again, now with only the major hazards (in red) showing.

A layer of points is displayed. Each point indicates the centroid of a census block, a small geographic unit used by the United States Census Bureau to track population. Larger points indicate higher populations.

A visual comparison of the census blocks to the geologic hazard zones brings some good news: the majority of the census blocks lie outside of the Major Hazard zones. This means most people (and property) will be safe when disaster strikes, right?

Unfortunately, although landslides and other geologic hazards may begin in high geologic hazard zones, that's not necessarily where they end. Floods can carry mud and debris from the narrow canyons of the Rocky Mountains as far as a kilometer downstream and cause subsequent flooding upstream, impacting areas outside of the hazard zones.

To expand your analysis to include the possibility of geologic hazard flow from one area to another, you'll examine the Floodways layer.

A layer of what looks like rivers is drawn on the map. A floodway is the channel of a stream and the areas adjoining the channel that carry floodwater downstream. They comprise part of the floodplain, which is the area that may be underwater during a major flood event.

The floodways flow from west to northeast, draining from the high elevation of the Rocky Mountains into the relatively low elevation of the eastern plains. Unlike the Geologic Hazard Zones layer, many of the floodways intersect large population centers. During a flood, large numbers of people may be at risk.

Two hundred meters is an estimation of the flood fringe, or the area around a floodway that may become inundated with water during a major flood. The floodway and flood fringe combined make up the floodplain.

When the analysis is complete, a new layer is added to your map.

At this point, you no longer need the Floodways layer.

Now that you have estimated the extent of flooding, you'll investigate a few places around Boulder County where flooding may pose a particular problem.

The map zooms into the city of Boulder.

Boulder is the most populous city in Boulder County as well as the seat of the county government. Several estimated floodplains run through the city, with many census block points (and thus large numbers of people) at risk. The damage here during a major flood in both property and life could be catastrophic if authorities are not properly prepared.

The map zooms into the city of Lyons.

Lyons is a small town nestled in the Rocky Mountains where two floodway buffers converge. Compared to Boulder, few people live here. However, the floodway buffers cover all major roads into and out of the town, including the Ute Highway and the South St. Vrain Highway. While fewer people may be at risk in this area, those who are at risk may be potentially stranded if flooding renders the roads unusable.

The map zooms into the city of Louisville.

Louisville is a suburban community in southern Boulder County, near the Denver metropolitan area. A major floodway buffer runs along the southern fringe of the city. However, although many more people live in Louisville than in Lyons, a relatively small portion of that population lives in an area at risk of flooding. Furthermore, while some major roads are in danger of being blocked by floodwater, the floodplain mostly runs through non-vital infrastructure, such as the Coal Creek Golf Course and the Dutch Creek Park. While it is an area that may be a flood risk, at the present moment, Louisville does not appear to be a particularly high-risk area.

The Data Browser is a directory of all of the attributes with which you can enrich your layer.

A new layer is added to your map. It looks like the original Floodway Buffers layer, but this new layer contains attribute information about population. You can find this information in the layer's table or the layer's default pop-up.

The 2022 Total Population attribute is in the pop-up, but it's not displayed in the most intuitive way. The pop-up could also use some information explaining what the floodway buffer represents.

This will display the name of the creek used as the basis of the floodway buffer.

A window opens, listing the layer's attributes. Every attribute has two names: the field name, which is the layer's internal name for the attribute, and the field alias, which is the name displayed in the layer's table and pop-up.

A few options appear to the right of the fields.

Now when your pop-up displays the population field, it will not include any decimal places. It doesn't make sense to have a fraction of a person living somewhere.

The Custom Attribute Display window opens. You'll create a custom attribute display using the {TOTPOP_CY} field.

A total of {TOTPOP_CY} people live in this 200-meter floodway buffer meant to estimate the area underwater during a major flood event.

You'll need to build an expression to find floodway buffers that intersect hazard zones.

Based on this expression, the tool will create a layer that includes only floodplains that touch the geologic hazard zones. The operation only considers filtered features, so only Major Hazard zones will be used for the analysis.

A new layer is added to the map, depicting only floodway buffers that intersect Major Hazard zones. (You can check and uncheck the box next to the new layer to turn it off and on so you can see the new buffers.)

While these floodway buffers have the potential of ferrying landslide debris downstream, it is unlikely that the entire length of the floodplain constitutes a landslide risk zone. Landslide debris can only travel so far along the relatively flat eastern plains before it comes to a stop.

You want to find the exact places where floodplains come in contact with hazard zones.

A new layer is added to the map, although it may be difficult to see at first because its features are relatively small.

The new layer represents the floodplains that intersect zones classified as a major hazard. These are areas where rock and mud from the hazard zones may funnel into the floodplains and cause a landslide.

As when you found the potential landslide risk areas earlier, you'll construct an expression to determine the areas you want.

One kilometer is the distance you'll use to estimate how far a landslide may travel from the initiation area. Normally, the distance will be affected by the slope and terrain of the area over which the landslide travels, but one kilometer will suffice for the purposes of your analysis.

This expression will derive a new layer that includes all locations in the Potential Landslide Risk Areas layer within one kilometer of the Initiation Areas layer—in other words, all the areas one kilometer upstream and downstream of where a landslide enters a floodplain.

A new layer is added to the map. The new layer depicts areas potentially affected by a landslide.

Now that you've mapped the areas prone to landslide activity, you'll take some time to explore the map a little more closely to see how population and infrastructure may be affected by a landslide event.

As with flood risk, the number of people in danger of landslides is relatively high in Boulder, because it is a major population center at a confluence of many floodways and geologic hazard zones. The landslide risk here is primarily downstream, meaning that people and property are under threat of being buried by landslide debris itself.

Only one relatively small landslide risk area threatens Lyons. However, unlike Boulder, the landslide threat here is primarily upstream rather than downstream. The people and property of Lyons aren't in danger of being buried under landslide debris as much as they are in danger of the extreme upstream flooding that might occur if a landslide dams up a floodplain. A landslide event in Lyons may exacerbate the already-severe flooding and compound problems for the people of Lyons.

Only one landslide risk area travels through Louisville, but it is by far the largest risk area in Boulder County, due to the presence of multiple scattered geologic hazard zones throughout the city. As you discovered before, relatively few people in Louisville live in the southern fringe of the city where flooding and landslides are most likely to take place. However, the sheer size of the landslide risk area in Louisville may pose a particular difficulty for disaster preparations.

The Data Browser window opens. This time you are adding several variables in addition to total population, so you'll use the Data Browser search function to facilitate the process.

This statistic will indicate at-risk property.

This statistic will give you insight into the characteristics of the population at risk.

This statistic will inform you of the number of elderly people who live in an area, who may require special assistance in preparing for a flood event.

The new layer adds to the Contents pane. Next, you will add data on road density.

The Data Browser window appears.

This statistic indicates the average total road length in an area per square kilometer, which will reveal the potential danger to road infrastructure in the landslide risk areas.

The new layer is added to the map.

It looks like the original Landslide Risk Areas and Enriched Landslide Risk Areas layers, but this new layer contains the additional demographic and landscape attribute information. You can view this information in the layer's table or the layer's default pop-up.

Buffer distance in Meters measures the size of the buffer for each feature in the Landslide Risk Areas layer. You could use any field to symbolize the layer, but this field is convenient because it only contains one value—200, the size you set your buffer distance to be in the previous tutorial. This results in one symbol type.

Normally, only point features can be symbolized with an icon. However, you can symbolize polygon features as if they were point features by using a graduated classification scheme such as size or color.

A new window opens. You can use this window to change the background of your symbol's icon.

Another window opens, allowing you to change the graduated circles symbol (as opposed to the symbol for the polygon features). Instead of using a default symbol, you'll add a custom symbol.

A text box appears.

https://learngis.maps.arcgis.com/sharing/rest/content/items/53a098f903b64daa8cf235a428b73d42/data

The URL belongs to a symbol specifically designed to communicate landslide risk. The default size of 12 pixels is rather small, but you currently cannot change the shape size because you are symbolizing by an attribute Buffer distance in Meters. Although Buffer distance in Meters is always the same value, by default, it is treated as though it has multiple values. You can change this by using the Classify Data option, which will allow you to set the size of the customized symbology.

The symbol size is set to a larger size. Classify Data sets the number of classes to 1, because Buffer distance in Meters has only one value.

The symbols on the map change to reflect the new symbology.

For the Floodway Buffers pop-up, you created a custom attribute display to explain what the layer displayed. However, Landslide Risk Area is more self-explanatory, so you will configure the pop-up using a list of field attributes.

• 2022 Total Population
• 2021 Total Housing Units (Esri 2022)
• 2022 Average Household Income
• 2022 Seniors (Age 65+)
• Relative Road Density