Name: Flood Hazard

Display Field: FLHHazardUnit

Type: Feature Layer

Geometry Type: esriGeometryPolygon

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Sources

Description: Flooding is the overflow of water onto lands that are normally dry and is the most commonly experienced natural hazard. Damage from flooding includes inundation of land and property, erosion, deposition of sediment and debris, and the force of the water itself, which can damage property and take lives. Historically, flooding is the most prevalent, costly, and destructive (on an annual basis) hazard in Utah.Debris flows and related sediment flows are fast-moving flow-type landslides composed of a slurry of rock, mud, organic matter, and water that move down drainage-basin channels onto alluvial fans. Debris flows generally initiate on steep slopes or in channels by the addition of water from intense rainfall or rapid snowmelt and often occur after rangeland or forest fires. Flows typically incorporate additional sediment and vegetation as they travel down-channel. When flows reach an alluvial fan and lose channel confinement, they spread laterally and deposit the entrained sediment. In addition to being debris-flow-deposition sites, alluvial fans are also favored sites for urban development; therefore, a debris-flow-hazard investigation is necessary when developing on alluvial fans. The hazard investigation may indicate that risk reduction is necessary for sustainable development on the alluvial fan. A debris-flow-hazard investigation requires an understanding of the debris-flow processes that govern sediment supply, sediment bulking, flow volume, flow frequency, and deposition. However, a uniform level of acceptable risk for debris flows based on recurrence or frequency/volume relations, such as the 100-year flood or the 2% in 50-year exceedance probability for earthquake ground shaking, has not been established in Utah. Historical records of sedimentation events in Utah indicate that debris flows are highly variable in terms of size, material properties, travel distance, and depositional behavior; therefore, an elevated level of precision for debris-flow design parameters is not yet possible, and conservative engineering parameters and designs must be used where risk reduction is necessary.Large-volume debris flows are low-frequency events, and the interval between large flows is typically deceptively tranquil. The debris-flow hazard on alluvial fans can be difficult to recognize, particularly on alluvial fans that are subject to high-magnitude, low-frequency events. Debris flows pose a hazard very different from other types of landslides and floods due to their rapid movement and destructive power. Debris flows can occur with little warning. Fifteen people have been killed by debris flows in Utah. Thirteen of the victims died in two different night events when fast-moving debris flows allowed little chance of escape. In addition to threatening lives, debris flows can damage buildings and infrastructure by sediment burial, erosion, direct impact, and associated water flooding. The 1983 Rudd Canyon debris flow in Farmington deposited approximately 90,000 cubic yards of sediment on the alluvial fan, damaged 35 houses, and caused an estimated $3 million in property damage.The data in this feature class shows where flood hazards may exist and the mapped relative severity, based on geologic assessment and are not a replacement for Federal Emergency Management Agency (FEMA) Flood Insurance Risk Maps (FIRM). However, this data can be used in combination with the FIRM maps to more accurately depict flood hazard.See the geodatabase metadata and the UGS website (https://geology.utah.gov) for more information.

Copyright Text: Utah Geological Survey, Geologic Hazards Program (Steve D. Bowman, Jessica J. Castleton, Gordon E. Douglass, Ashley A. Elliott [resigned], Ben A. Erickson, Adam I. Hiscock, Emily J. Kleber, Tyler R. Knudsen, William R. Lund [retired], Greg N. McDonald, Adam P. McKean, Garrett S. Vice [resigned]) and others depending upon the mapped area. Program Manager: Steve D. Bowman, Ph.D., P.E., P.G. Project Manager: Jessica J. Castleton, M.S., P.G. Project Reviewer: Greg N. McDonald, P.G. GIS Reviewer: Gordon E. Douglass

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Supported query Formats: JSON, geoJSON, PBF

Use Standardized Queries: True

Extent:

XMin: 258701.5331
YMin: 4074685.8042
XMax: 630741.7
YMax: 4511702.431
Spatial Reference: 26912 (26912)

Drawing Info:

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Object ID Field: OBJECTID

Unique ID Field:

Name : OBJECTID

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Global ID Field: GlobalID

Type ID Field: FLHHazardUnit

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• OBJECTID (type: esriFieldTypeOID, alias: OBJECTID, SQL Type: sqlTypeOther, length: 0, nullable: false, editable: false)
• FLHHazardUnit (type: esriFieldTypeString, alias: FLHHazardUnit, SQL Type: sqlTypeOther, length: 50, nullable: true, editable: true)
• FLHUpdateDate (type: esriFieldTypeDate, alias: FLHUpdateDate, SQL Type: sqlTypeOther, length: 8, nullable: true, editable: true)
• FLHHazardLabel (type: esriFieldTypeString, alias: FLHHazardLabel, SQL Type: sqlTypeOther, length: 50, nullable: true, editable: true)
• FLHMappedScale (type: esriFieldTypeInteger, alias: FLHMappedScale, SQL Type: sqlTypeOther, nullable: true, editable: true)
• Hazard_Symbology_Text (type: esriFieldTypeString, alias: Hazard_Symbology_Text, SQL Type: sqlTypeOther, length: 50, nullable: true, editable: true)
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Types:

ID: VHflh
Name: Very High Flood Hazard
Domains:

Templates:

Name: VHflh
Description:
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Prototype:

Attributes:

• FLHHazardUnit: VHflh

---

ID: Hflh
Name: High Flood Hazard
Domains:

Templates:

Name: Hflh
Description:
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Attributes:

• FLHHazardUnit: Hflh

---

ID: Mflh
Name: Moderate Flood Hazard
Domains:

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Name: Mflh
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Attributes:

• FLHHazardUnit: Mflh

---

ID: Lflh
Name: Low Flood Hazard
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Name: Lflh
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Prototype:

Attributes:

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---

ID: VLflh
Name: Very Low Flood Hazard
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Name: VLflh
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---

ID: Uflh
Name: Undefined Flood Hazard
Domains:

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Name: Uflh
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Attributes:

• FLHHazardUnit: Uflh

---

Relationships:

• Unit_Descriptions (0) -- Related To: Unit_Descriptions (20)

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Has Contingent Values: false

Supports Rollback On Failure Parameter: true

Last Edit Date: 3/4/2025 11:55:01 PM

Schema Last Edit Date: 3/4/2025 11:55:01 PM

Data Last Edit Date: 3/4/2025 11:55:01 PM

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