Storm Event Summary ReportAugust 12th-13th, 2014

Weather Summary

The city experienced an intense storm during the night hours of August 12th, 2014 and continuing into theearly morning of August 13th. The rainfall totals and intensities varied considerably across Bellevueresulting in differing responses from our built and natural drainage systems.

The storm occurred from 8pm on Tuesday the 12th to 6am on Wednesday the 13th, a duration of 10 hours.The storm deposited an average of 1.55 inches of rain however it had some very intense localized zones.The I-405/Coal Creek rain gage recorded the most accumulation as well as greatest intensities. Itreceived 2.75 inches during the 10 hour storm with intensities of rainfall well exceeding our designstandards. Not surprising it is the rain gage nearest to the most extensive reported flooding. . Floodingwas reported at multiple locations in the early morning of August 13th with the most extensive in theFactoria Mall vicinity.

The least amounts of rain were recorded at NE 40th (1.14 inches) and the Meydenbauer Tank (1.16inches) rain gages. As a comparison, the Bellevue Service Center (BSC) rain gage measured 1.42 inchesin this one event alone. The average at this location for the entire month of August is 1.0 inches. Thehighest monthly accumulation at the BSC is 4.55 inches which occurred in 2004.

Individual rainfall amounts are listed in Appendix A. Rain gages and flooding are displayed on the mapin Appendix A and a chart of cumulative rainfall totals can be found in Appendix B.

The city’s storm system is designed to convey a 24 hour storm that occurs in the winter months. TheAugust 12th-13th storm was a short duration, high intensity summer storm with 6-hour and 1-hour peakrainfall that produced rainfall intensities that far exceeded typical storms in the Puget Sound at our CoalCreek/I-405 rain gage.

Analysis of storm statistics:

Because the storm duration was only 10 hours, the average 24 hour rainfall totals are not relevant.If the evaluation were to include a comparison with known 24-hour rainfall totals, only the I-405/Coal Creek gage would have an accumulation sufficient to compare with the IntensityDuration Frequency (IDF) curves for the Seattle region. That total comparison would reveal thestorm to be a 5 year storm recurrence interval (Appendix C). Again, this is only for comparisonpurposes and has no real relevance to describing the storm event.

In terms of 6 hour peak rainfall, this storm exceeded the criteria for a 1,000 year recurrenceinterval storm at the I-405/Coal Creek gage where 2.65 inches of rain were recorded between11pm and 5am. Six hour peak rainfall can be found in Appendix D1, and the 6 hour IDF curvefor the Seattle region is located in Appendix D2.

In terms of one hour peak intensity, the Coal Creek/I-405 rain gage recorded 1.33 inches duringthe 3 am hour of August 13th, which exceeds the 500 year recurrence interval storm based onstatistical analysis. One hour peak intensities are shown in Appendix E1 and the 60 minuteSeattle IDF curve is in Appendix E2.

The recorded rainfall at the Coal Creek/I-405 rain gage represents a peak-15 minute intensity atan estimated 200 year recurrence interval.

The Coal Creek/I-405 rain gage was checked after this storm event and was found to be operating withinthe manufacturer’s specifications. This indicates an extremely localized storm occurred in the Factoriaarea.

Systems and Community Impacts

Regional detention pond levels spiked during and after the storm. The Lower West Tributary Tank nearlyexperienced an overflow (96.2% full). A comparison of pond elevations can be found in Appendix F.

The storm caused flooding at multiple locations throughout Bellevue, including:

Formula 1 Fast Lube – 3625 Factoria Blvd SE (Estimated at 4 feet deep at Factoria Boulevardbetween SE 36th St and SE 38th St and backed up into the underground oil changing bay releasingan unknown amount of oil into the street and drainage system)

Newport Villa Condominiums at 12510 SE 42nd St (Seven units impacted) 16461 SE 34th St (Yard impacted) 3419 164th Ave SE (Yard impacted) 2436 W Lake Sammamish Pkwy (Water and mud in driveway and porch area, erosion in yard) 3432 167th Ave SE (Basement and garage impacted, culvert in front of property was half plugged

with sediment) Sunset Auto Glass 11660 NE 8th St (CBs overflowed into warehouse area)

APPENDIX A: Total Rainfall by Gage for August 12th & 13th, 2

Circles indicate

relative rainfall

amounts. The larger

the circle, more

rainfall occurred.

10 Hour Storm Totals (Precipitation in inches)

RG#1 RG#2 RG#3 RG#4 RG#5 RG#6 RG#7 RG#8 RG#9 RG#10 RG#11

NE40thTank

CherryCrest

LakemontPark

LocalBSC

MeydenTank

I405/Coal

Creek

ForestHills

CrossroadsTank

CougarTank

ParksiteTank

PhantomLake

STORM Inches Inches Inches Inches Inches Inches Inches Inches Inches Inches Inches

8/12 to8/13

1.14 1.57 1.69 1.42 1.16 2.75 1.45 1.28 1.20 1.78 1.56

APPENDIX B: Cumulative Rainfall Totals

0.00

0.50

1.00

1.50

2.00

2.50

3.00

8/12/14 12:00 AM 8/12/14 10:00 AM 8/12/14 8:00 PM 8/13/14 6:00 AM

Inch

es

Cumulative Rainfall TotalsBellevue, WA

August 12th-13th, 2014

NE40thTank

Cherry Crest

LakemontPark

BSC

Meydenbauer

Coal Creek

Forest Hills

Crossroads

Cougar Mtn

Parksite

PhantomLake

APPENDIX C: Isopluvial Map

Average of 1.55 inches1.14-2.75 inches /24 hour stormAugust 12th-13th, 2014

APPENDIX D1: 6 Hour Peak Rainfall

1.08

1.501.60

1.36

1.09

2.65

1.35

1.181.11

1.69

1.48

0.00

0.50

1.00

1.50

2.00

2.50

3.00

NE40thTank

CherryCrest

LakemontPark

LocalBSC

MeydenTank

CoalCreek

ForestHills

CrossroadsTank

CougarTank

ParksiteTank

PhantomLake

Inch

es

Peak 6-hour Rainfall Totals August 12-13, 2014

Peak 6 hr 8-12 to 8-13

1000 year- 6 hour storm = 2.50 inches

200 year- 6 hour storm = 2 inches

20 year- 6 hour storm = 1.50 inches

APPENDIX D2: 6 Hour IDF Curve Seattle

*The above graph (red line) plots the expected rate of recurrence for 6-hour precipitation events in the Seattle, WA region. The y-axis shows theamount of precipitation in inches that corresponds with the storms expected recurrence (x-axis). Blue boxes represent documented 6-hourstorms in the area. The reading at I-405 represents a storm off the extents of this chart.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

2.20

2.40

2.60

6-H

OU

RP

RE

CIP

ITA

TIO

N(i

n)

RECURRENCE INTERVAL (Years)

1.01 100 500 1000

Extreme Value Type 1 Plotting Paper

1.25 3.32 200

Regional Curve

505 10 20

Seattle WA

APPENDIX E1: One Hour Peak Rainfall

0.32

0.45

0.61

0.53

0.30

1.33

0.36

0.25 0.26

0.540.57

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

NE40thTank

CherryCrest

LakemontPark

LocalBSC

MeydenTank

CoalCreek

ForestHills

CrossroadsTank

CougarTank

ParksiteTank

PhantomLake

Inch

es

Peak 1-hour Rainfall Totals August 12th-13th, 2014

Peak 1 hr 8-12 to 8-13

50 year- 1 hour storm = 0.80 inches

2 year- 1 hour storm = 0.40 inches

500 year- 1 hour storm = 1.12 inches

APPENDIX E2: IDF Curve for Seattle 60 Minute/1 Hour Precipitation

*The above graph (red line) plots the expected rate of recurrence for 1-hour precipitation events in the Seattle, WA region. The y-axis shows theamount of precipitation in inches that corresponds with the storms expected recurrence (x-axis). Blue dots represent documented 1-hour stormsin the area. The reading at I-405 represents a storm off the extents of this chart.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.206

0-M

INP

RE

CIP

ITA

TIO

N(i

n)

RECURRENCE INTERVAL (Years)

1.01 100 5001.25 32 200

Regional Curve

505 10 20

Extended Time-Series158-Year Period

APPENDIX F: DMP Percent Full

16.3%

69.8%

39.6%

53.3% 54.9%

96.2%

30.7%

64.0% 64.6%

36.3%

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

Per

cen

tFu

ll

DMP Percent Full at Peak ElevationAugust 12-13, 2014

APPENDIX G: Bellevue Flooding

APPENDIX G: Bellevue Flooding

APPENDIX G: Bellevue Flooding