POOR LEGIBILITY

ONE OR MORE PAGES IN THIS DOCUMENT ARE DIFFICULT TO READDUE TO THE QUALITY OF THE ORIGINAL

-

Gale Wm. Fraser, II. P.E.General Manager) Chiel Engineer

BOARD OF DIRECTORS

Lance M. MaloneCnairman

Clark County

Cresent HardyVice-Chairman

City of Mesqutie

Larry BrownCity ol Las Vegas

Jan Laverty JonesCity ol Las Vegas

Bryan A. NixCity ol Boulder City

John K. RhodesCity ol North Las Vegas

David Woodj. Cily of Henderson

Bruce L. WoodburyClark County

REGIONAL FLOOD COMTROL DISTRICT

SFUND RECORDS CTR

2095986TO: Distribution

FROM: Timothy E. Sutko, Senior Hydrologist

SUBJECT: September 11, 1998 Flooding

DATE: October 2, 1998

Attached please find a copy of the Rainfall Event Report for the September 11, 1998 event.Please don't hesitate to contact me if you have any questions or require additionalinformation.

Distribution: RFCD Board membersRFCD TAG membersRFCD CAC MembersLarry Jensen, NWSRon McQueen. NWSJohn James, State ClimatologistMike Tumipseed, State EngineerBob Cullins, Emergency Management Coordinator, City of Las VegasJack Finney, Emergency Management Coordinator, City of HendersonBob Andrews, Emergency Management Coordinator, Clark CountyVicki Taylor, Public Information Officer, City of HendersonTom Warden, Public Information Officer, Clark CountyPhu Simpson, Clark County Community DevelopmentCurt Chandler, City of HendersonDennis Moyer, City of Las VegasLenny Badger, City of North Las Vegas

ffl

301 E. Clark Avenue, Suite 301 • Las Vegas. Nevada 89101(702) 455-3139 • FAX: (702) 455-3870

RAINFALL EVENT REPORTSeptember 11,1998

prepared byTimothy E. SutkoSenior Hydrologist

Clark County Regional Flood Control DistrictClark County, Nevada

Severe weather moved through the Las Vegas Valley and northeast Clark County beginning latein the morning on September 11,1998 causing wide-spread drainage problems and other damages.While the rainfall intensities and depths recorded in many parts of the County were not atypicalof warm season convective storms in the region, perhaps the most notable feature of this stormsystem was the size of the area which was impacted. Based on weather radar images it isestimated that 40% of the county may have received up to one inch of rainfall or more. Therainfall was accompanied by hail throughout much of the Las Vegas Valley as well as reports oftornadoes in the Henderson area. In the Las Vegas Valley the largest rainfall depths appear tohave occurred in the Green Valley area of Henderson and the east pan of the Valley extendingnorth to the Nellis AFB area. The Green Valley rainfall caused significant flow in Pittman Washand Duck Creek, perhaps the largest flows recorded in those drainages in recent years. On theeastside of the Valley, the Sloan Channel, Flamingo Wash, and Las Vegas Wash all experiencedsignificant flows. Flood flows from the entire Valley combined above Lake Las Vegas where thepreliminary peak flow is estimated to have been in the 7000-9000 cfs range.

The rainfall depths recorded in the California Wash and Muddy River drainages were far moreimpressive than those recorded in the Las Vegas Valley. For the twelve hour period ending atmidnight on September 11, four RFCD rain gages hi that area recorded in excess of two inchesof rainfall, and one of those recorded more than three inches of rain. The resulting runoff in theCalifornia Wash at the Hidden Valley Road may have been up to 10,000 cfs. This flow combinedwith flow in the Muddy River above Glendale and overtopped SR 168 at Glendale, washed outthe Gubler Avenue crossing of the Muddy River in Logandale, and caused some residentialflooding in Overton. There was one swift-water rescue which took place on Hidden Valley Roadas flood waters from the California Wash overwhelmed a delivery van which attempted to crossthe flooded dip-section.

This storm system moved generally from the southwest to the northeast across the southern andeastern portions of the Las Vegas Valley before moving into the California Wash and MuddyRiver drainage areas. In the Las Vegas Valley, rainfall began to move into the south end of theValley between 11:30 am and noon. At approximately 11:30 am, the National Weather Service(NWS) office issued a Severe Thunderstorm Warning for Northeast Clark County, including theMoapa Valley, Mesquite and Bunkerville. Shortly thereafter (11:50 am), a Severe ThunderstormWarning was issued for the Las Vegas Valley as weather radar indicated increasing rainfallintensities moving into the area and marble-sized hail was reported at the NWS offices. The firstFlash Flood Warning was issued by the NWS at approximately 12:15 pm. These warnings were

repeatedly extended and expanded by the NWS throughout the remainder of the day. District staffprovided updates to the local public works departments as the NWS issued or extended theweather statements. District staff also advised both Clark County Public Works and the ClarkCounty Sanitation District of the possibility of significant flows in lower Duck Creek as therainfall began entering the Las Vegas Valley.

Owing to the location and movement of this storm, most of the runoff occurred in streets andwashes and did not, with the exception of the Range Wash/Confluence Detention Basin andCarey/Lake Mead Detention Basin, enter any of the regional detention basins. Flow in the LasVegas Wash at Vegas Valley Drive was observed to have been within one foot of the maximumchannel capacity. The following listing presents estimates of the peak flows at various locationsin the Las Vegas Valley as determined by the US Geological Survey (USGS) and/or RegionalFlood Control District. These values are preliminary estimates and are subject to change as betterinformation is developed.

Location Peak Discharge

Pittman Wash at Wigwam Ave 1430 cfsDuck Creek at Eastern Avenue 650 cfsDuck Creek near Hollywood Blvd 3000 cfsFlamingo Wash at Decatur Blvd 130 cfsFlamingo Wash at Eastern Avenue 920 cfsFlamingo Wash at Nellis Blvd 1500-2000 cfsRange Wash Tributary at Las Vegas Blvd 920 cfsSloan Channel at Charleston Blvd 1230 cfsLas Vegas Wash near Sahara Ave 2240 cfsLas Vegas Wash below confluence with Flamingo

Wash 5000-8000 cfsLas Vegas Wash at Lake Las Vegas 7000-9000 cfs

The peak flow adopted for Flood Insurance Study purposes in the Las Vegas Wash at VegasValley Drive is 7100 cfs. Based upon these flow estimates, it appears that in some reaches ofthe Las Vegas Wash this event may have approximated a 100-year flood.

The lower end of Duck Creek experienced high lows for at least the third time this summer. Inthe vicinity of the Stadium Mobile Home Park, approximately 100 feet of block wall collapsed,an 18-inch water line and a sanitary sewer line ruptured, and almost 300 feet of a 28-inch naturalgas line was exposed by the flooding. At Lake Las Vegas, the rate of flow in the Wash farexceeded the capacity of the two by-pass pipelines and a large fraction of the Wash flow spilledinto the Lake. The two low level bladder dams were deflated and the lake drain was opened inorder to allow the storm flows to discharge from the Lake.

There have been numerous reports in the media regarding erosion in the Sunrise MountainLandfill area. Operators of the landfill have informed District staff that rainfall depths of 2.2-,2.25-, and 2.5-inches were collected by the three NWS-type rain gages located at the landfill site.While this type of gage does not record rainfall intensities, the operators stated that the rainfalloccurred in less than a one hour period.

The rainfall cells appeared to have intensified as they left the Las Vegas Valley to the northeast.Rainfall, flow rates and damages were higher in the Moapa Valley area than they were in the LasVegas Valley. While the rainfall in the Las Vegas Valley generally took place within a two-hourperiod, in the drainages impacting the Moapa Valley intense cells appear to have built and re-builta few times throughout the afternoon and evening. Four RFCD rain gages recorded more than2-inches of rainfall, and one of those recorded over 3-inches. Each of these gages recordedrainfall rates of 1.15 inches per hour or more. This rainfall generated flows in the CaliforniaWash which washed out 150-feet of the paved road east of the Byron exit from 1-15. At sometimeduring this storm, runoff was flowing over 1-15 in the vicinity of the rest area south of theGlendale/Moapa exit. Flows in the California Wash at Hidden Valley Road may have been in the7000-10,000 cfs range (the 100-year flow rate for the California Wash in this area is 14,600 cfs).A swift water rescue was successfully performed at this location by the volunteer Fire Departmentshortly before the driver's van was completed submerged by the flow. Flows in the Muddy Riveralso over-topped Hidden Valley Road on the northside of the Hidden Valley Dairy. The flowsin these two watercourses effectively isolated the dairy for over 12 hours.

Flows in the Muddy River overtopped the SR-168 bridge in Glendale and washed out the lowlevel crossing at Gubler Avenue in Logandale. According to the damage assessment preparedby the American Red Cross, thirteen homes in the Overton area suffered major damages and twomobile homes in the Glendale/Moapa area were destroyed by flooding. The preliminary USGSestimate of the Muddy River flows two miles below Glendale is 6500 cfs. The Clark CountyPublic Works Department has estimated that the area suffered approximately $400,000 in damagesto roadways.

While a significant amount of rainfall was not recorded by any of the District's rain gages locatednorth of Mesquite, this same storm system which impacted the Las Vegas Valley and MoapaValley must have moved through that area, as the water level behind the Jim Wilson (aka TownWash) Detention Basin rose to over 10 feet before receding. The Red Cross damage assessmentindicates that 5 homes in Bunkerville suffered major flood damages.

Attached to this report are summaries of the rainfall data collected by the District.

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4374 4394 4084 4174 4284 4074orecip precip precip precip orecip precip

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09/11/98143014151400134513301315130012451230121512001 145

TOTALS :

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1 . 38

Bu sk inear i nteNe 1onon

1 i s BlDe se r tSi oan

near 1 nteatLasCon

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0.79 0.94 0.91 0.94

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. 20

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Rose Golf Course 1/4 m i l e South of Sahara AvenueChannel at Charleston B l v drsection of Washinciton Avenue and Lamb Bl vd

inter section of Lake Mead B l v d and Pecos RdVeqas B l v d North 1/4 East of Lamb B l v d

fluence Detetnion Basin

Clark County Re g i o n a l Flood Control DistrictSeptember 11. 1998 (California Wash)

De v i c e l D 3144DataTvpe precipUni t s

09/112300224522302215220021452130211521002045203020152000194519301915190018451830181518001745173017 151700164516301615160015451 530151515001445143014151 400134513301315130012451 23(31215120011451 130

TOTAL

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i n

/980 . 000 . 040 . 000 . 000. 200.000.000.000. 120 . 000.000.000.000.080. 390. 120 . 000 . 000. 160 .000 . 000 . 000.000.000.000.000 . 000 . 000.000 . 000 . 000. 160 . 350.040 .000.000. 350. 390. 550.000 .000.000 . 000. 240 . 000.000 . 00

S: 3.19

Cal i forniaC a l i f o r n i aC a l i f o r n i aMuddy R i v eMeadow Val7 m i l e s N

3164 3184 3264 3384precip precip precip precip

in

0.000.000.000 . 000 . 040 . 000 . 000 . 000 .000 . 000.000 . 000 .00

0.080.000 . 000.000 .080.040 .000.000.000.000.080.000.000.000 . 000 .000.0S0.080. 160 . 000.000.000. 200.470 . 280. 200.000.000 . 000 . 000.000 . 000.000.00

1.77

Wash. 13 mWash. 7 miWash. 4 mi

in

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2 . 32

i les SSWles SSI/ oles SW of

r at Warm So r inns Rdlev Wash, 1of Glendale

15 m i l e s NW of Gl ends3 m i l e s N EOv/e rton A i

of Bowmanroc r t

1n

0.000 . 000 .000 .000.000 . 000.000.000.000.000.000 . 040. 120 . 000 .000.000.000.000 .040.040 .000.000.040.280.000.000.000.000 .000.000.000 .000 . 040.000.0B0.080.790.200.000.000 . 000.040. 310. 000 . 000.000 . 00

2.09

of Gl endaf GlendalGl enda 1 e; 6 m i l e s

in

0.000.000.000.000 .000 .000 .000.000.000.000.000. 000.080.000.000 .040 .000.000.000.000 . 000.000.040 .000.000.000.000.000. 160. 000. 390 . 200.000.000. 160 . 590. 390.000.080.000.040.000.080.000.000.000.00

2.24

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.00

.00

.28

.00

.00

.00

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. 12

.00

.00

.04

.00

.00

. 20

. 24

. 08

.00

.00

.00

.00

.00

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.00

.00

.00

.00

. 30

3234precip

in

0.000 . 000.000.000.000 . 000.000.000.000 . 000 .000.000 . 000.000 .000.000.000 .000 . 000.000.000.000 . 000.000.080.080.000. 160. 160.000 . 000 . 000.000 . 000.000.000 . 040.000 . 000 . 000.000.040.000 . 000 . 040.080 . 00

0.67

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in

. 08

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. 16

.00

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. 00

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. 55

. 00

.00

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. 00

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.00

. 00

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.00

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. 22

3294prec i p

i n

0.160 .080.000 . 000.000 . 000.000.000 . 000.000 . 000.000 . 000 . 000 . 000.000.000 .000.000.000.000.000 . 000.000 . 000.000 . 040 .000.000 . 000 . 000.040.000.000.000 .000 . 000 . 000.000.000 . 000 .000 . 000 . 040.000.300.00

0.35

Gl enda 1 e1 m i l e s NNW of Glendale

1-5Re s e v/o i r

Figure la (above) and Ib (below). Las Vegas Wash looking north and south from Vegas ValleyDrive. The channel is approximately 100-feet wide and 8-feet deep; discharge is approximately 9000

cfs. (September 11, 1998)

I"V . > • • • •

flit i m^im/^^^

Figure 2a (above) and 2b (below). Muddy River Bridge at SR-168 in Glendale. High Hows in theMuddy River were 2-3 feet deep across the roadway on September 11, 1998.

Figure 3. Wash out of paved road east of I-15 at rhe Ute exit resulting from flows in the CaliforniaWash September 11, 1998.

Figure 4. Debris washed up 18-inches on fence at rest area on 1-15 south of Moapa/Glendale exit fromflow in California Wash drainage across the Interstate on September 11, .998.

Figure 5. Muddy River below Gubler Avenue crossing. Note shed perched on bank in left center ofphoto after flows of September 11, 1998.

U.S. DEPARTMENT OF COMMERCENational Oceanic and Atmospheric AdministrationNATIONAL WEATHER SERVICE785! Industrial RoadLas Vegas. Nevada 89128

October 21, 1998

Harding Lawson Associates5145 South Arville, Suite ALas Vegas, NV 89118

TO: Joe RumannSUBJECT: Radar Images and Event Summary for Heavy Rainfall at Sunrise Mountain on Septmeber 11, 199S

Dear Joe,

Please find enclosed a series of radar images for the September 11, I99S rain event as per your request Explanation follows

Summary: Based on analysis of KESX WSR-88 Doppler Radar data, rain began in the vicinity of Sunnse Mountain between 11 :45a.m. and noon, but was very light until about 12:20 p.m. PDT Periods of heavy rain occurred from nvo successive thunderstormsbetween 12:20-1:40 p.m., with additional light-to-moderate rain until about 2:15 p m.

During the heaviest rainfall, radar estimated rain rates of 2.5-3.0 inches per hour Rainfall totals for the entire storm were maximizedin the Sunnse Mountain area at 4.4" total accumulation While this may be slightly overestimated due to hail contamination, a stormtotal of 3-4" is reasonable, most of which fell in a time period of less than an hour and a half

Imagery: The radar images I've provided are of two types: reflectivity and rainfall estimates The first type is a measure of reflectedradar energy back to the antenna in decibels of reflectivity (dBZ). Echoes shaded in reds and purples represent the strongest returns(>50 dBZ) and typically depict moderate to heavy rainfall. The second image type is rainfall estimate, an algorithm based onconverting reflectivity to hourly ram rate and storm total precipitation. The equation used during this event was Z=300R'', whereZ is reflectivity in dBZ, and R is rain rate.

The date/time group for each image is listed in the upper right, four rows down from the top. The date/time stamp in the top rowis the date which the image was printed, so ignore that. The second line is labeled "CMP REF", which means "CompositeReflectivity". This simply means that each pixel represents the highest three-dimensional reflectivity for that location at that timeThe third line down is labeled "124 NM" and ".54 NM RES", defining the range and individual pixel resolution for that scan Thefourth line, "09/11/98 19:21" represents the dale and time in UTC During the summer, our local time is UTC - 7 hours, so a timestamp of 19:21 represents 12:21 PDT.

The five reflectivity images indicate the time, duration, and intensity of rainfall over the Sunnse Mountain area on 11 SEP 9S The19:21 (12:21 PDT) image depicts an area of heavy rain approaching the western slopes of Sunnse Mountain from the direction ofBoulder Highway. At 12:34, heavy rain was clearly overspreading the mountain The heavy rain associated w' l th the firstthunderstorm crested the mountain and appeared to be moving downwind at 12:57 p m This is not to suggest rain had stopped atthis time, but had briefly diminished in intensity By 1 :15 p.m., rainfall intensified again with the onset of the next thunderstormThis persisted for at least 15-20 minutes, then began to taper off into light to moderate showers for another ha l f -hour or so The lastreflectivity image indicates the trailing edge of showers cleanng the area of interest at 2 :13 p m.

The image labeled "1-HR PRECIP"depicts the radar-estimated hour ly precipitation rate for the \o lumc scan ending at 20.03 L'TC(13 03 PDT) It shows a large area exceeding two inches-hour extednmg from the McCollough Range across a narrow swath of theeastern side of the valley to the Sunrise/Frenchman Mountain region Maximum rainfall was estimated on the western slopes ofSunnse/Frenchman peaks at 26 inches/hour

The final image, labeled "STM PRECIP" represents an accumulated precipitation estimate for the entire storm event Once again,the maximum (4.4 inches) occurred near Sunrise Mountain.

If you have any further questions, feel free to call or e-mail me

Regards,

Kim Runk, Science & Operations Officer Phone 2(>3-9744 c\t .22-1National Weather Service - Las Vegas E-mai l Kim R u n k - a noaa cov

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Prepared in cooperation -with the -;:

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northWorksheet for Irregular Channel

Project DescriptionProject File p:\fmw\sunnse.fm2Worksheet northFlow Element Irregular ChannelMethod Manning's FormulaSolve For Discharge

Input DataChannel SlopeWater Surface ElevationElevation range: 96.00 ft to

0100,

100.00ft.Station (ft) Elevation (ft)

0.0012.0014.0032.0042.0051.0066.00

100.98.99.9897,96

100

00000000,00.00.00

.020000 ft/ft

.00 ft

Start Station0 .00

ResultsWtd. Mannings CoefficientDischargeFlow AreaWetted PerimeterTop WidthDepthCritical Water Elev.Critical SlopeVelocityVelocity HeadSpecific EnergyFroude NumberFull Flow CapacityFlow is supercritical.

0.0351190.26

128.5067.0666.00

400100.22

ft'/sft2

ftftftft

0.014221 ft/ft9.261.33

101.331.17

1190.26

fu'sftft

ft'/s

End Station Roughness66.00 0.035

Ocl 15 1998 Your Corrpany Flow.Masler vd 1135214 Haeslad Methods. Inc 37 Brocksice Road Waterbury CT 06708 (203)755-1666 Page 1 of 1

24+47Worksheet for Trapezoidal Channel

Project DescriptionProject File p:\fmw\sunrise.fm2Worksheet dropFlow Element Trapezoidal ChannelMethod Manning's FormulaSolve For_ Discharge

Input DataMannings CoefficientChannel SlopeDepthLeft Side SlopeRight Side SlopeBottom Width

0.0340.029000 ft/ft5.00 ft3.30 H : V2.50 H:V

11.00 ft

ResultsDischargeFlow AreaWetted PerimeterTop WidthCritical DepthCritical SlopeVelocityVelocity HeadSpecific EnergyFroude NumberFlow is supercritical.

1998.86127.5041.7040.00

6.210.011417

15.683.828.821.55

ft3/sft2

ftftftft/ftft/sftft

Oc: 15. 1993 Your Company FiowMas'.er v4 i135240 Haestad Methods. Inc 37 Brookside Road Waterbury. CT 06708 (203)755-1666 Page 1 of 1

47+85Worksheet for Trapezoidal Channel

Project DescriptionProject FileWorksheetFlow ElementMethodSolve For

p:\fmw\sunrise.fm2ditchTrapezoidal ChannelManning's FormulaDischarge

Input DataMannings CoefficientChannel SlopeDepthLeft Side SlopeRight Side SlopeBottom Width

0.0400.027000 ft/ft5.50 ft1.30 H:V1.30 H:V

16.00 ft

ResultsDischargeFlow AreaWetted PerimeterTop WidthCritical DepthCritical SlopeVelocityVelocity HeadSpecific EnergyFroude NumberFlow is supercritical.

1872.70127.33

34.0430.30

6.290.016423

14.713.368.861.26

ft3/sft'ftftftft/ftft/sftft

Ocl 16 1998

135302

Your CompanyHaeslad Methods. Inc 37 Brookside Road Walerbury CT 06708 (203)755-1666

FlowMasIer v4 iPage 1 of 1

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