08 09

06

10

18

19

19

30

07

18

08

0405 03

30

13

11

14

15

31

22

30

18

19

05

09

11

11

11

30

19

16

18

25

07

07

12

13

12

06

01

0102

09

04

15

10

14

27

22

15

24

24

26

10

13

26

03

12

21

29

22

12

15

25

23

22

27

01

23

13

23

12

20

12

10

13

24

24

29

24

25

36

13

36

1314

09

25

29

14

21

28

20

08

03

28

13

28

21

17

21

17

24

36

24

23

29

22

09

25

20

16

02

16

14

09

04

21

28

16

21

1716

15

07

36

08

3332

23

10

3131

36353534 3436 3233 3332

36

343332

26

06

06

31

30

18

19

27

07

18

31

11

30

19

11

25

11

31

18

19

11

17

11

11

0206

35

11

26

31

30

11

31

22

06

07

18

05

06

30

19

07

30

18

09

19

18

31

06

04

18

07

30

19

03

20

07

01

10

36

08

01

07

34

19

31

1715

04

06

05

03

24

29

0503

27

24

24

17

13 14

08

03

12

12

05

26

22 23

30

22

25

14

12

16

13

13

28

13

25

12

10

21

05 02

36

01

23

10

26

01

26

02

10

03

04

23

35

15

15

34

10

32

22

01

14

03

10

01

35

36

12

33

26

21

27

15

35

27

24

20

22

33 34

24

15

04

20

10

22

13

32

16

25

28

22

23

17

04

23

25

20

08

08

29

17

09

36

27

24

27

24

02

25

09

17

1613

32 34

21

01

09

36

33

12

29

12

08

25 28

33

27

21

20

29

27

28

32

05

12

14

27

16

28

15

34

29

34

24

26

35

32

28

26

35

26

32

21

25

26 25

05

23

20

2928

35

20

35

14

23

34

29

16

33

33

36

27

09

36

20

16

32

28

34

35

22

17

33

23

14

28

02

21

26

03

33

16

09

01

34

14

04

06

15

04

02

27

02

08

0102

21

17

31 32 3336 3435

35

Quail

reviR nigriV

atnaS

aralC

revi

R

nigriV

tro

F

ecra

eP

hsa

W

Bloomington

Bloomington HillsSouth

.dR sdleiF notgnihsa

W

Telegraph St.

wonS

yawk

raP

noyn

aC

tsa

E 00

2

®q

FLOOD-HAZARD MAP FOR THE ST. GEORGE–HURRICANE METROPOLITAN AREA

Basemap consists of National Agricultural Imagery Programnatural color aerial photography.

Universal Transverse Mercator Projection, zone 12.North American Datum 1983.

OldUS-91

Bloomington HillsNorth

LittleValley

§̈¦15

§̈¦15

§̈¦15

§̈¦15

Middleton

.dR r

evi

R

Sun River

St. George Blvd.

.tS f

fulB

reviR

EXPLANATION

GreenValley

Sunbrook

!(8

!(18

37 N113 15' W

!(18

eixi

D

.rD

tniuqanoT.r

D.r

DBloomington

Man O War Rd.

.dR mahgirB

§̈¦15

reviR

.dR

nigriVreviR

Stone Cliff

Knolls

Ledges

Diamond Valley

Riverside.rD

0541

1580 South

WashingtonFields

tsaE 0074

htuoS

§̈¦15

!(9

Creek

Reservoir

Sand

Hollow

Reservoir

!(9

!(59

!(9

!(17

R 16 W R 15 W

R 17 W R 16 W

T 42 ST 43 S

113 22' 30" W113 30' W113 37' 30" W R 15 W R 14 W

T 42 ST 41 S

T 41 ST 40 S

R 14 W R 13 W

R 13 W R 12 W

37 7' 30" N

37 15' N

R 17 W R 16 W

R 16 W R 15 W

R 15 W R 14 W

R 14 W R 13 W

T 41 ST 40 S

T 42 ST 41 S

T 42 ST 43 S

37 15' N

37 7' 30" N

37 N

113 22' 30" W

113 30' W

113 37' 30" W

by

FEDERAL EMERGENCY MANAGEMENT AGENCY (FEMA)FLOOD INSURANCE RATE MAP (FIRM) ZONES

0.2-percent-annual-chance (or 500-year) flood plain

DISCUSSION

USING THIS MAP

MAP LIMITATIONS

SYMBOLS

Municipality boundary

Interstate Highway

State Highway

Major local surface street

Other road

troF

ecraeP hsaW

1 0 1 20.5Miles

1 0 1 20.5Kilometers

Scale 1:24,000

Approximate meandeclination, 2007

14o

HTR

ON

EU

RT

HTR

ON

CITEN

GAM

UTAH

Map Location

AE

A

AO

DER

SNIA

TNU

OM

wonS

noyn

aC

Santa ClaraHeights

dnaS

woll

oH

hsa

W

HTUOS SLLIH

tseW

kcalB

egdiR

lliH bbeW

slliH ytiC ecirP

note

lddi

M

kcal

B

egdi

R

bonihSebiK

emoD notgnihsaW

RE

NR

AW

EGD

IR

RENRA

W

YELLAV

DNAS

NIATNUOM

notgnihsaW

kcalB

egdiR

lliM

keer

C

emoD grubsirraH

keer

C

doownottoC

liau

Q

keer

C

sdeeL

keerC

eniveparG

hsaW

hsA

keer

C

SFFILC E

NACI

RR

UH

nikreV aL

keerC

nigriV

reviR

ENA

CIR

RU

H

SFFILC

dluoG

hsa

W

ellivnik

tA

hsa

W

ENA

CIR

RU

H

SFFI

LC

NIAT

NU

OM

KE

ER

C ELTTI L

Zone AE: Areas with a 1% annual chance of flooding and a 26% chance of flooding over the life of a 30-year mortgage. Flood eventdetermined by detailed methods. Mandatory flood insurance purchase requirements and flood-plain management standards apply.

Zone A: Areas with a 1% annual chance of flooding and a 26% chance of flooding over the life of a 30-year mortgage. Flood eventgenerally determined using approximate methodologies. Mandatory flood insurance purchase requirements and flood-plain managementstandards apply.

Zone AO: Areas subject to inundation by 1-percent-annual-chance shallow flooding (usually sheet flow on sloping terrain) where averagedepths are between one and three feet. Some Zone AO have been designated in areas with high flood velocities such as alluvial fans andwashes.

Flooding is the overflow of water onto lands that are normally dry, and is the most prevalent and destructive (on anannual basis) geologic hazard affecting the St. George – Hurricane metropolitan area. Damage from floodingincludes inundation of land and property, erosion, deposition of sediment and debris, and the force of the water itself,which can damage property. The high flood hazard in the study area results from the complex interaction of thearea’s rugged topography and seasonal weather patterns that deliver moisture to southwestern Utah. Three types offloods typically occur in the study area: riverine (stream) floods, flash floods/debris flows, and sheetfloods. All threetypes of floods are associated with natural climatic fluctuations and may, under certain circumstances, occur incombination with each other. The risk from flooding can be significantly increased by human activities such asplacing structures

The Flood-Hazard Map is based on limited geological, geotechnical, and hydrological data; site-specific studies are required to produce more detailed flood-hazardinformation. The map also depends on the quality of those data, which varies throughout the study area. The mapped boundaries of the flood-hazard categories areapproximate and subject to change with additional information. The flood hazard at any particular site may be different than shown because of geological andhydrological variations within a map unit, gradational and approximate map-unit boundaries, and the generalized map scale. Small, localized areas of higher or lowerflood hazard may exist within any given hazard area, but their identification is precluded because of limitations of map scale. The map is not intended for use at scalesother than the published scale, and is designed for use in general planning to indicate the need for site-specific studies.

William R. Lund, Tyler R. Knudsen, Garrett S. Vice, and Lucas M. Shaw2008

PLATE 3

EROSION HAZARD ZONESErosion-hazard zones: these zones are independent of FIRMs 100-year flood zones, and are intended to prevent damage from erosionduring flooding, “whether or not the property is located in a FIRMs 100-year flood zone” (CH2MHILL, 1997). Erosion-hazard zones weredelineated by JE Fuller/Hydrology & Geomorphology, Inc, (2005, 2007a, 2007b) and are based chiefly on a geomorphic analysis of riverbehavior over time, and are determined through a combination of air photo interpretation, field observations, geology and soils mapping, andconsideration of the location and design of structures in active stream channels including bridges, water diversion dams, and channelstabilization structures. Alternating yellow and black bar indicates the erosion-hazard zone study limit.

HAZARD REDUCTIONEarly recognition and avoidance of areas subject to flooding are the most effective means of flood-hazard reduction. However, avoidance may not always be a viableor cost-effective hazard-reduction option, especially for existing developments, and other techniques are available to reduce potential flood damage. These mayinclude, but are not limited to, source-area stabilization, engineered protective structures, flood and debris-flow warning systems, and floodproofing. Some of thesetechniques can be expensive and their cost versus benefit ratio should be carefully evaluated. With regard to sheetflooding, a properly sized and integrated system ofstreet and storm drains is usually adequate to mitigate this hazard. If hazard-reduction techniques are not implemented, risk may be accepted, but an informeddecision is only possible if the flood potential and consequences are clearly understood and disclosed. If the risk is significant but acceptable, the individual housesmay be insured, either through NFIP, if eligible, or by a private insurance provider so damaged items covered by insurance can be repaired if flood damage occurs.

Flooding studies are recommended in all hazard categories reported in the adjacent table. The first consideration in stream-flow-flooding- and debris-flow-hazardreduction is proper identification of hazard areas through detailed mapping, and qualitative assessment of the hazard (Giraud, 2004, 2005). The stream-flow-flooding-hazard assessment should determine the active flooding area, the frequency of past events, and the potential inundation and flow depths. A debris-flow-hazardassessment should determine active depositional areas, the frequency and volume of past events, and sediment burial depths. The level of detail for a hazardassessment depends on several factors including the type, nature, and location of the proposed development; the geology and physical characteristics of the drainagebasin, channel, and alluvial fan; the history of previous flooding and debris-flow events; the level of risk acceptable to property owners and land-use regulators; andproposed risk-reduction measures.

Where development is proposed in areas identified on the Flood-Hazard Map as having a potential flood hazard, a site-specific study should be performed early in theproject design phase. A site-specific investigation can establish whether a flood and/or debris flow hazard is present at a site and provide appropriate designrecommendations.

placing structures and constrictions in flood plains or erosion-hazard zones, developing urban areas withoutadequate flood and erosion control, and poor watershed management practices, such as overgrazing or allowingindiscriminate off-road vehicle traffic. Additionally, portions of the study area are subject to inundation in the event ofan unintentional release of water from an engineered water-retention or conveyance structure.

For additional information about the flood hazard in the St. George – Hurricane metropolitan area, refer to the Flood-Hazard text document in this report.

effectiveness is beyond the scope of this study.

Through the National Flood Insurance Program (NFIP), FEMA makes federally subsidized flood insurance availableto qualified individuals residing in participating communities. Many mortgage lenders require NFIP insurance beforeloaning money for home purchase or construction within a FIRM 100-year flood zone. Flood insurance rate mapsare legal documents that govern the administration of the NFIP. Homeowners who suspect that they live within aFIRM 100-year flood zone, or individuals contemplating purchasing a home within a flood-prone area, should consultthe corresponding FIRM directly and strongly consider purchasing NFIP insurance. Individuals who own their homesmay elect to purchase NFIP insurance on a voluntary basis.

Although this product represents the work of professional scientists, the Utah Department of Natural Resources,Utah Geological Survey, makes no warranty, expressed or implied, regarding its suitability for a particular use.The Utah Department of Natural Resources, Utah Geological Survey, shall not be liable under any circumstancesfor any direct, indirect, special, incidental, or consequential damages with respect to claims by users of thisproduct.

For use at 1:24,000 scale only. The Utah Geological Survey does not guarantee accuracy or completeness ofdata.

FLOOD-HAZARD CATEGORIES BASED ON GEOLOGIC DEPOSITSMAPPED BY THE UTAH GEOLOGICAL SURVEY

Very High(VH)

HazardCategory

GeologicUnits1 Description Hazard Type Comments

High(H)

Medium(M)

Low(L)

Qal , Qaf ,Qafy, Qa,Qa/Qafo

Qal , Qac,Qaec

Qap , Qc,Qca, Qat

Qae, Qaes,Qea

Active flood plains and lowterraces along perennial streams,and active alluvial fans.

Stream channels, flood plains, and low terraces along ephemeral streams.

Active pediment surfaces, higher stream terrace surfaces, and sloping depositional surfaces flanking upland areas.

Valley bottoms receiving active deposition and minor ephemeral drainages.

Riverine flood, flash flood, debris flow

Flash flood, debrisflow

Chiefly sheetflood, possible flash flood and debris flow

Sheetflood, minor flash flood

Chiefly the Virgin and Santa Clara Rivers, Fort Pearce Wash, and active alluvial fans at the base of the Hurricane Cliffs.Normally dry streams with comparatively small drainage basins subject to flooding during infrequent cloudburst storms.

Active depositional surfaces on the flanks and at the base of upland areas subject to flooding during infrequent cloudburst storms.

Valley bottoms subject to infrequent flooding from adjacent upland areas during cloudburst storms.

1

2

1

1

1Refer to UGS 1:24,000-scale geologic maps (see SOURCES OF DATA and REFERENCES in accompanying text) for a description of map units.

The Flood-Hazard Map shows drainages covered by FIRMs, other flood-prone areas identified using geologic data,and erosion-hazard zones along the Virgin and Santa Clara Rivers and Fort Pearce Wash. The map provides abasis for requiring site-specific studies and identifies areas where FIRMs can be consulted to determine theavailability of federally subsidized flood insurance. Site-specific studies can resolve uncertainties inherent ingeneralized hazard mapping and help ensure safety by identifying the need for flood-resistant design. However,because intense cloudburst storms may create a potential for flash floods and possible debris flows, and sheetfloodsanywhere in the study area, even locations outside of identified flood-prone areas could be subject to periodicflooding. This map also shows where existing developments lie in flood-prone areas where flood-resistant-designmeasures should be considered. An evaluation of flood-mitigation measures already in place and their likelyeffectiveness

UTAH GEOLOGICAL SURVEYa division of

Utah Department of Natural Resources

Plate 3

Utah Geological Survey Special Study 127

Geologic Hazards and Adverse Construction Conditions,

St. George−Hurricane Metropolitan Area, Washington County, Utah