Gwinnett County Capital Improvement Planning Utilizing Floodplain Models

Transcription

1 Gwinnett County Capital Improvement Planning Utilizing Floodplain Models Jonathan Semerjian, PE Dept. of Water Resources Stormwater Management Clay Daniel, CFM Dewberry Seyoum Asamenaw, PE Dewberry

2 Gwinnett s Capital Improvement Planning Processes Developing Solutions Conceptual Designs Scoring and Ranking Risk Map Products Other Design Considerations Benefits Capital Budget Planning Stormwater Utility Development Sharing Information Emergency Response 2

3 Gwinnett s DFIRM Program Background Comprehensive Countywide Study Began Program in 1999 Final DFIRMs Issued September 29, 2006 PMR Map Revision to Chattahoochee River and 4 other watersheds went effective March 4,

4 Gwinnett s DFIRM Program Background ~ 400 stream miles of Detailed Study ~ 400 miles Limited Detail and Approximate Multiple frequencies were added to the LD models for flood risk assessment and CIP considerations. 4

5 CIP Development Initial Flood Study Apalachee River Watershed Recognized Leverage that Flood Study Affords Models help identify overtopping bridges/culverts Implemented Capital Improvement Project Began with Alcovy Watershed Completed 219 CIPs in 2009 (Detailed Study Models) Added 368 CIPs in 2012 (Limited Detailed Study Models) 5

6 Initial Phase CIP Development 6

7 Final Phase CIP Development 7

8 CIP Development - Ranking The following weighting and ranking criteria were used: Criteria Road Class Depth of Flooding Structure Condition Alternative Routes Weighting Factor 2x 2x 1x 1x 8

9 CIP Development - Ranking Road Class Description Score Major Arterial 10 Minor Arterial 8 Major Collector 7 Minor Collector 7 Collector Road 7 Local Road 4 Non County Maintained Roads and Private Drives 0 9

10 CIP Development - Ranking Frequency of Flooding Criteria Score 2 Year Storm 10 5 Year Storm 9 10 Year Storm 8 25 Year Storm 7 50 Year Storm Year Storm 5 10

11 CIP Development Ranking Leveraging Risk Map Products Determine the depth of flooding at specific road crossings. Allows for the visualization of the frequency of flooding. 11

12 CIP Development - Ranking Alternate Routes - Flooded roadways that provide the only access score the most points - Multiple routes that are not flooded simultaneously score the middle points Alternate Route Description Provides only access to subdivision or other developed area Alternate routes are available but also flooded Alternate routes are available that are not flooded Score

13 CIP Development - Design Concept Level Design Identified overtopped Bridges and Culverts Effective and Limited Detail Models 100-year Level of Service Design Developed solutions to provide 100-year LOS with freeboard Flood Models had detailed structure invert, low chord, high chord, and road elevations Leverage SW Inventory in Limited Detail Areas Contains XY coordinates, structure size, type, and length Utilized the most up-to-date DEM to assign invert and road elevations in Limited Detailed and Zone A areas 13

14 CIP Development - Design CIP design considerations: - Maintain the existing road profiles to minimize design and construction costs - Provide at least 1.5 freeboard - Raise the road profile if the road crossing is submerged by tail water - Maintain invert and rise of culverts to avoid relocating utilities - Cause no adverse impacts to avoid the need for purchasing FP easements 14

15 CIP Development - Design Proposed Solutions include: Add additional barrels Raise road elevations Adding flanking barrels in overbanks where existing structure is concrete Replace existing structures with Bridges Concrete Arch Culverts (Conspan) Reinforced Concrete Box Culverts Pipe Culverts 15

16 Other Considerations CIP is a Planning and Budgetary Guideline USACOE Regional Conditions for RCBs 20% Embedment for Fish Passage Main Span no greater that natural channel width Road Profile Driveways Side Roads Impacts to adjacent properties Utilities Water and sewer conflicts 100-year LOS not always attainable 16

17 Emergency Responses During Sept 2009 flood event 25 Extended Road Closures 15 Major Culvert Failures Required Replacement 4 Shelved CIP Designs Completed 3 CIP Designs Underway 17

18 Emergency Responses Limited Detail Models Leveraged Allowed for Quick Design Outer Bank Dr. River Valley Dr. 18

19 Benefits of CIP Development Share CIPs DOTs Other Jurisdictions Emergency Response Identified Problem Areas Coordinate with Emergency Services Targeted Responses Flooding Events / Tropical Systems Emergency Repairs Road Failure During Flooding Events Can pull solution with budget off the shelf 19

20 Final CIP Products Final deliverables for this project include following: HEC-RAS models with separate plan for each CIP RiskMAP products (water surface elevation and depth grids for each frequency) Supporting GIS Files Updated CIP database,and CIP Report. 20

21 Summary Effective Planning Tool Leverage Models Concept Level Solution and Cost Estimate Benefits Capital Budget Planning Stormwater Utility Development Share Information with Other Agencies Emergency Response Only a Concept Final Design required Permits Emergency Response has leeway 21

22 Questions Jonathan Semerjian, P.E. Chief Engineer Gwinnett Co. Dept. of Water Resources 684 Winder Highway Lawrenceville, GA (678) Seyoum Asamenaw, PE Dewberry Clay Daniel,CFM Dewberry 22