Sea Level Rise and Recurrent Flooding Update

Transcription

1 Sea Level Rise and Recurrent Flooding Update Informational Briefing City Council Meeting September 18, 2018 Thomas M. Leahy, Deputy City Manager

2 The Sea, Inland Waters, & Groundwater are Rising Projected change in water elevation recurrence Scenario Recurrence Intervals, yr Today ft SLR ft SLR March 2018 VIMS 2050 Projection VB SLR Planning Range 2

3 NOAA Atlas 14 Rain Intensity & Frequency Virginia Beach Latitude: Longitude: Elevation: ft Stormwater Design Standards 2016 Storms 3

4 VA Beach Experienced Three Storms in 2016 With yr or Greater Return Periods July 31, 2016 Storm Tropical Storm Julia Sep 19-21, 2016 Hurricane Matthew October 8-9,

5 Rainfall Intensity & Frequencies are Rising 5 Upward trend of Annual Maximum Precipitation Series: 3 to 7% per decade

6 6

7 Analysis of Historical and Future Heavy Precipitation NOAA Atlas 14 does not Represent Current Reality NOAA Atlas Projected 200-yr storm 70-yr storm 100-yr storm 40-yr storm 10-yr storm 5-yr storm Rainfall NOAA Atlas14 In yr Storm 5.4 inches 6.6 inches 100-yr Storm 9.4 inches 11.7 inches 7

8 Conversion of Pervious Area to Impervious Area Creates More Runoff With Cumulative Impacts Theoretical 30 Acre Residential Rezone Pre-Development Peak Flow Rate Post-Development Peak Flow Rate 10-year Storm 100-year Storm 33 CFS 82 CFS 32 CFS 146 CFS Change -3% +78% 8 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

9 Coastal + Stormwater Flooding = Combined Flooding Higher coastal water levels diminish stormwater system performance Coastal Flooding Stormwater Conveyance Combined Flooding 9

10 Sea Level Rise and Stormwater Studies Sea Level Rise and Recurrent Flooding Study I. Define Impacts of 1.5 and 3.0 feet of Sea Level Rise: Complete II. III. Identify Alternatives and Adaptation Strategies: Underway Implementation: TBD Stormwater Modeling and Master Planning Study I. Develop Computer Models for 31 subwatersheds: Underway II. III. Identify and Prioritize Projects (with 1.5 and 3.0 feet of Sea Level Rise): Begin in FY19 Implementation: TBD 10

11 Sea Level Rise Phase I: Total Annualized Losses Structural, Content, Business Disruption Losses Losses, Millions Watershed Total Building Inventory Today 1.5 ft SLR 3 ft SLR Billions Estimated Combined Combined Combined % of Total Loss Ratio Loss Ratio Loss Ratio Value Loss Loss Loss Elizabeth River $ % $ % $ % $ % Lynnhaven $ % $ % $ % $ % Southern $ % $ % $ % $ % Oceanfront $3.6 4% $ % $ % $ % City-wide $ % $ % $ % $ % 11 Losses, Millions $400 $350 $300 $250 $200 $150 $100 $50 $0 Today 1.5 ft SLR 3 ft SLR

12 12 Central Resort District Current and Future Floodplain Map For 100-year Flood

13 13 Transition Area Current and Future Floodplain Map For 100-year Flood

14 14 Rural Area Current and Future Floodplain Map For 100-year Flood

15 Sea Level Rise Study Phase II: Adaptation Strategies Environmental & Policy Engineering & Structural Land 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 15

16 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 16

17 Stormwater Modeling and Master Planning Phase 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

18 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 Engineering, Analysis, and Evaluation of Alternatives and Adaptation Strategies Fall 2017 Spring 2018 (ongoing) Winter 2018 Oct 2018 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

19 19 Questions?