Sea Level Rise and Recurrent Flooding Update

Informational BriefingCity Council Meeting – September 18, 2018

Thomas M. Leahy, Deputy City Manager

The Sea, Inland Waters, & Groundwater are Rising

VB SLR Planning Range

Projected change in water elevation recurrence

Scenario Recurrence Intervals, yr

Today 10 50 100 500

1.5 ft SLR 3 8 14 100

3 ft SLR 1 2 3 14March 2018 VIMS 2050 Projection

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NOAA Atlas 14 Rain Intensity & Frequency

Virginia Beach Latitude: 36.7379°Longitude: -76.0767°Elevation: 10.93 ft

Stormwater Design Standards

2016 Storms

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July 31, 2016 Storm

Tropical Storm Julia Sep 19-21, 2016

Hurricane MatthewOctober 8-9, 2016

VA Beach Experienced Three Storms in 2016 With 100-200 yr or Greater Return Periods

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Rainfall Intensity & Frequencies are Rising

Upward trend of Annual Maximum Precipitation Series: 3 to 7% per decade5

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Analysis of Historical and Future Heavy PrecipitationNOAA Atlas 14 does not Represent Current Reality

Rainfall NOAA Atlas14 In 204510-yr Storm 5.4 inches 6.6 inches100-yr Storm 9.4 inches 11.7 inches

NOAA Atlas 14 2045 Projected200-yr storm 70-yr storm100-yr storm 40-yr storm

10-yr storm 5-yr storm

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Coastal + Stormwater Flooding = Combined Flooding

• Stormwater Conveyance

• Combined Flooding

• Coastal Flooding

• Higher coastal water levels diminish stormwater system performance

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Conversion of Pervious Area to Impervious Area Creates More Runoff

With Cumulative Impacts Theoretical 30 Acre Residential Rezone

10-year Storm 100-year Storm

Pre-DevelopmentPeak Flow Rate

33 CFS 82 CFS

Post-DevelopmentPeak Flow Rate

32 CFS 146 CFS

Change -3% +78%VSMP Requirements The VSMP mandates lower peak

flow rates for the 10-year storm if the system is subject to localized flooding. Development still creates more runoff volume. Stormwater will flow for a longer period of time at a slightly lower rate.

Neither the volume of runoff nor the rate at which it flows is addressed by the VSMP

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Sea Level Rise and Stormwater StudiesSea Level Rise and Recurrent Flooding StudyI. Define Impacts of 1.5

and 3.0 feet of Sea Level Rise: Complete

II. Identify Alternatives and Adaptation Strategies: Underway

III. Implementation: TBD

Stormwater Modeling and Master Planning StudyI. Develop Computer

Models for 31 sub-watersheds: Underway

II. Identify and Prioritize Projects (with 1.5 and 3.0 feet of Sea Level Rise): Begin in FY19

III. Implementation: TBD

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Sea Level Rise Phase I: Total Annualized Losses

Watershed

Total Building InventoryBillions Today 1.5 ft SLR 3 ft SLR

Estimated Value % of Total Combined

Loss Loss Ratio Combined Loss Loss Ratio Combined

Loss Loss Ratio

Elizabeth River $10.3 13% $2.70 0.03% $8.57 0.08% $24.25 0.24%Lynnhaven $42.7 53% $17.29 0.04% $55.94 0.13% $164.08 0.38%Southern $24.5 30% $5.14 0.02% $19.58 0.08% $170.84 0.70%Oceanfront $3.6 4% $0.55 0.02% $2.50 0.07% $16.20 0.44%

City-wide $81.1 100% $25.69 0.03% $86.58 0.11% $375.37 0.46%

• Structural, Content, Business Disruption LossesLosses, Millions

$0$50

$100$150$200$250$300$350$400

Today 1.5 ft SLR 3 ft SLR

Loss

es, M

illion

s

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Central Resort District – Current and Future Floodplain Map For 100-year Flood

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Transition Area – Current and Future Floodplain Map For 100-year Flood

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Rural Area – Current and Future Floodplain Map For 100-year Flood

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Sea Level Rise Study Phase II: Adaptation StrategiesEnvironmental & Policy Engineering & StructuralLand Use and Development

Policies/Regulations

Natural Solutions – Dunes,Marshes, Wetlands

Floodplain Management

Relocation, Migration

Tax and Market-Based Incentives

Public Awareness and Education

Flood Warning/Preparedness

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Stormwater Modeling and Master

Planning Stormwater Models for

31 sub-Watersheds Models will provide

drainage design parameters such as downstream tailwaters to developers for projects

Models will identify deficiencies and gaps in existing stormwater infrastructure

Preliminary Analysis to identify future Capital Drainage Projects in conjunction with results of Sea Level Rise Study

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Stormwater Modeling and Master PlanningPhase I - Computer Model Development

WATERSHED SUB-WATERSHED SCHEDULED COMPLETION

LYNNHAVEN 3 thru 8, 30 Completed

LITTLE CREEK 1, 31 December 2018

ATLANTIC 29 December 2018

ELIZABETH RIVER 2, 17 thru 22 December 2018 (2), August 2019 (17-22)

SOUTHERN RIVERS

Upper West Neck 10 Completed

Stumpy Lake 12 Completed

Canal #1, South 9 October 2018

Southern 11, 16, 23 thru 28 December 2018

Upper North Landing 13 thru 15 August 2019

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Sea Level Rise/Recurrent Flooding Study Schedule

Activity Date

Define Impacts of 1.5 and 3.0 feet of Sea Level Rise (Phase I) Complete Fall 2017

Public Outreach of Phase I: Share Impact Analysis, Capture Perceptions, Increase Awareness

Fall 2017 – Spring 2018 (ongoing)

Engineering, Analysis, and Evaluation of Alternatives and Adaptation Strategies

Winter 2018 – Oct2018

Develop Draft Alternative and Adaptation Strategies Document Oct – Dec 2018

Staff and MLT Review of Alternatives and Strategies Nov 2018 – Jan 2019

Begin Discussion with City Council of Alternatives and Adaptation Strategies February 2019

Begin Public Outreach Spring 2019

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Questions?

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