pw/engineering june 20, 2013 - vbgov.com · 6/18/2013 · microsoft powerpoint - eastern shore dr...

June 20, 2013

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PW/Engineering

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Three Questions

• Why is the Eastern Shore Drive Drainage Area Experiencing Recurrent Flooding?

• What can be done about it?

• What is being done?– Comprehensively– On an Interim basis

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Why is the Eastern Shore Drive Drainage Area Experiencing

Recurrent Flooding?

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Recurrent Flooding & Sea Level Rise

• Storm water & flooding: A major citizen concern

• Flooding is increasing• Sea level is rising

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Sea Level RiseInformation

• Sewell’s Point: + 0.18 in/yr • CBBT: + 0.24 in/yr • Rise last 50 years: ~ 1 ft • Projected rise next 50 years: 1 – 3 ft• Result:

Sea Level Rise

CoastalStorms+ =

Increased and Recurrent Flooding

in Many Areas

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Ocean Park

Cape Henry Shores

Lynnhaven Colony

Back Bay

Eastern ParkKings Grant

Cape Story

Windsor Woods

Princess Anne Plaza

Thalia

Glenwood

Old Beach

Oceanfront

Sandbridge

Green RunOcean Lakes

730th Street

8Goodspeed Road

9Dockside Vicinity

10Bubba’s

11Eastern Shore Drive Neighborhoods

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A Particularly Vulnerable Area

-High Tides -Storm Surge -Rainfall

Eastern Shore Drive Area

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Close-up of Eastern Shore Drive Area

Flooding & Sea Level Rise

What can be done about it?

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Recurrent Flooding Adaptation Strategies

• Management/Retreat– Prevent development in high risk areas– Discourage rebuilding in high risk areas

• Accommodation– Elevating buildings/Flood proofing homes– Evacuation routes/warning systems

• Protection Measures– Focus of Today’s Discussion

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Recurrent Flooding & Sea Level RiseAdaptation Strategy: Protection Measures

• Engineering Flood Control Measures:– Complex– Costly– O&M Intensive– Limit to Effectiveness (LOS)

• LOS Exceedence = Major Flooding

What is being done?

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Comprehensive Flood Control Measures

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Comprehensive Flood Control Measures

Major CIP: 7-151 Eastern Shore Drive Drainage ImprovementProgrammed Funding: $53.13 MFinal Cost: TBDProject Development Study Underway

• Identify Improvements: Pump Stations, Outfalls, Flood Control Gates, Flood Walls

• Lowest Lying Areas May Be Problematic• Develop Implementation Plan (Phasing)• Results By End of This Year

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• Some have suggested installing backflow preventers to keep tides out

Interim Flood Control Measures

Lynnhaven Colony – Cape Henry Ditch

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Detailed analysis:• Determine where backflow prevention devices

will help reduce flooding from high tides• Determine where backflow prevention devices

will not increase flooding from rainfall• Determine type of backflow prevention device

appropriate for location (flexible check valve or automated sluice gate)


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For Each Location Evaluated:• Determined tidal elevation at which flooding will

occur, even with backflow preventer in place• Determined elevations of streets and homes• Calculated available storage for rainfall in each

drainage basin• Hydraulically modeled numerous rainfall / tidal

elevation combinations to determine benefits and impacts of installing backflow prevention devices

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Interim Flood Control Measures39 Locations Considered

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Lynnhaven Colony - Tidally Adjacent Area


• Range of Effectiveness– Up to overtopping elevations of 3.3 to 4.7 Feet (NAVD88)

depending on location– Up to 4 to 6 inches of rainfall depending on location

• Cost: $244,000 (5 Locations- 4, 5, 8, 10, 11)• Houses Protected: 0• Garages Protected: 4• Linear Feet of Lynnhaven Drive Protected: 320 L.F.

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Lynnhaven Colony - Tidally Adjacent AreaResults


Lynnhaven Colony Cape Henry Ditch

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Lynnhaven Colony Cape Henry Ditch

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• Range of effectiveness:– Up to Overtopping Elevation: 5.4 Feet (NAVD88)– Up to 1 inch of rainfall with tide gates closed

• Total Cost: $672,000 (2 Locations- 18 & 26)• Houses protected: 4• Garages protected: 9• Length of Cape Henry Drive protected: 150 L.F.

Results


Cape Henry Area

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Cape Henry Area

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Results• Range of Effectiveness

– Up to Overtopping Elevation: 5.2 Feet (NAVD88)– Up to 1 inch of rainfall with tide gate closed

• Cost: $290,000 (Location 39)• Houses Protected: 1• Garages Protected: 2• Linear Feet of Street Protected: 0 L.F.


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• In eight locations backflow prevention devices can provide limited benefit by reducing the frequency of flooding from certain moderately high tides

• Backflow prevention devices cannot eliminate all flooding

• There has been at least three storm since 2009 for which the backflow preventers would have had absolutely no benefit:– Nor’Ida (November 2009) Elev. 6.9’– Irene (August 2011) Elev. 5.5’– Sandy (October 2013) Elev. 5.7’

Summary of Findings

All elevations are in NAVD 88 datum.

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Sewell’s Point Tidal Surge InfoBenchmarks and Events MLLW (FT)

Category 4 ‐ (13‐19 Feet Surge at High Tide) 21.56Category 3 ‐ (9‐12 Feet Surge at High Tide) 14.56Category 2 ‐ (6‐8 Feet Surge at High Tide) 10.56

1933 Hurricane 8.02' Above MLLW 8.028.0 FT OCCURS ONCE EVERY 80 YEARS (EXTREME) 8.00Hurricane Isabel 9/18/2003 7.89' Above MLLW 7.89

November Nor'easter 11/12/2009 7.75' Above MLLW 7.75Category 1 ‐ (3‐5 Feet Surge at High Tide) 7.56

Hurricane Irene 8/27/2011 7.55 Above MLLW 7.55Ash Wednesday Storm 3/7/1962 7.22 ' Above MLLW 7.227.0 FT OCCURS ONCE EVERY 20 YEARS (SEVERE) 7.00Hurricane Sandy 10/29/2012 6.81' Above MLLW 6.81

Hurricane 1936 6.2' Above MLLW 6.72Thanksgiving Nor'easter 2006 6.63' Above MLLW 6.63

Twin Nor'easter #2 1998 6.58' Above MLLW 6.58Columbus Day Nor'easter 6.52' Above MLLW 6.52

Nor'easter 1956 6.12' Above MLLW 6.126.0 FT OCCURS ONCE EVERY 7 YEARS (MODERATE) 5.00

Twin Nor'easter 1998 6.04' Above MLLW 6.04Hurricane Floyd 1999 5.97' Above MLLW 5.97Hurricane Flossy 1956 5.92' Above MLLW 5.92Hurricane Donna 1960 5.92' Above MLLS 5.92

4.5 FT OCCURS 3 TIMES PER YEAR (MINOR) 4.502.56 OCCURS 3 TIMES PER YEAR 2.56

MEAN HIGH HIGH WATER‐MHW (Average all High Tides) 2.56MEAN SEA LEVEL 1.35

MEAN LOWER LOW WATER‐MLLW (Average all Low Tides) 0.00

Lowest Tide of Record 1/31/1996 ‐3.58

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• Total Cost of Interim Improvements: $1.2 M• Benefit is some reduction in flooding frequency for:

– 5 house– 15 Garages– 570 L.F. of Street

• Comprehensive improvements are budgeted and initial phases could begin in 2016

• A decision on whether to fund the Interim Improvements is needed

Return On Investment

If Authorized

• Procure / Install Check Valves at 5 locations in the Lynnhaven Colony Tidally Adjacent Area (Fall 2013, $244,000)

• Detailed Engineering / Permitting / Construction for 1 Check Valve and 2 Sluice Gates in Cape Henry Ditch (1 gate at West Great Neck and 1 gate at First Landing Lane, Check Valve at Sunstates Court; Construct Fall 2014, $962,000)

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Two Step Process

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• Brief the City Management Leadership Team √• Brief the Bayfront Advisory Committee √ • Brief the District Council Member• Brief the City Council

Decision Process

QUESTIONS

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FOLLOW UP NEXT DOOR

Report Located on VBGOV.COM under Public Works Stormwater http://www.vbgov.com/government/departments/public-works/storm-water/Pages/east-shore-dr-drainage-impvts.aspx

pw/engineering june 20, 2013 - vbgov.com · 6/18/2013 · microsoft powerpoint - eastern shore dr...

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