Stream Buffers Sharpest Tool in the SWM Toolbox Tom Hegemier, PE, D.WRE, CFM

Transcription

1 Stream Buffers Sharpest Tool in the SWM Toolbox 1 Tom Hegemier, PE, D.WRE, CFM

2 Stream Buffers - Overview 2 Why stream buffers? Floodplain management and other benefits Existing regulatory approaches Comparison with standard SWMM Storm drain outfall/creek protection Stormwater program approach

3 Importance of Headwater Streams 3 Headwater streams 80% of stream network Rivers originate from headwater streams Primary source of our nation s waters Generate local recharge Mitigate flooding Filter stormwater Habitat

4 Consequences of Headwater Stream Loss Increase runoff rate and volume Stream widening and down-cutting Damage roads, bridges, utilities, buildings, property Extensive habitat and floodplain storage loss Increased sediment volume Impair quality, habitat, lakes Increase treatment costs Reduced water supplies 4

5 Stream Buffer Standards 5 Buffer width based on drainage area or the floodplain No development, construction, alterations Allow utility crossings perpendicular to stream Allow low impact development parks, trails Minimize the number of road crossings

6 Stream Buffer 6

7 LCRA Watershed Ordinance (1,200 square miles, 3 counties, 18 cities, 5 lakes) 7 Found - 50 buffer achieves nearly irreducible pollutant concentrations

8 Central Texas Stream Buffers 8 LCRA HLWO* City of Austin TCEQ Edwards Aquifer Classification DA (ac) Set back DA (ac) Set back DA (ac) Set back Headwater Minor Intermediate Major > Watershed > > *Alternative 100-year floodplain plus 25 feet Austin Erosion Hazard Zone in addition to buffers

9 TCEQ Edwards Aquifer Enhanced Measures Buffers 9

10 TCEQ Edwards Aquifer Buffers 10

11 Why buffer zones matter 11 Accelerated runoff and loss of floodplain storage Loss of recharge, habitat, and natural appearance 11

12 Floodplain Loss 12

13 Buffers Floodplain Management Tool 13 Wimberley Blanco River May 2015

14 Upper San Marcos River Buffers Begin at 120 Acres 14

15 Upper San Marcos River Buffers Begin at 32 Acres 15 18% of land area

16 Stormwater Detention and Water Quality Basins 16 Sedimentation-Filtration Wet ponds 200 in Austin Flood, WQ, erosion Detention Basins 3,800 in Austin Flood management Recreation options 2,700 in Austin Water quality, erosion

17 Comparison of Structural Measures to Stream Buffers 17 Management Component Stormwater Management Stream Buffers Measure Design plan requirements High Minimal Design engineer experience High Minimal Plan reviewer experience High Minimal Construction inspection High Minimal Contractor experience Moderate to High N/A Construction quality High N/A Maintenance requirements High Minimal Service life Years to Decades In perpetuity Sustainability Minimal to moderate High Floodplain management Moderate High Resiliency Minimal to moderate High

18 Buffer Zone Benefits 18 Easy to design and administer Flexible, can be defined by each community Floodplain management/non-mapped areas Preserve natural creek corridors Parkland, hike & bike trail opportunities Erosion control insurance policy Groundwater well setback to protect base flow

19 Buffer Zone Challenges 19 Developers may claim loss of developable land (economic impact) Cannot serve as a stand-alone measure in a developing watershed (hydrologic change) Could increase wastewater line costs as no longer installed in the creek bed Annexation by adjacent landowners

20 Storm Drain Outfalls to Buffer Zones 20 Disconnect conveyance Multi-use buffer (filtering) Avoid outfall erosion issues Facilitate maintenance Ease permitting

21 Stream/Buffer Protection in Developing Watersheds 21 Manage hydrologic change Impervious cover reduction incentives Development design Street/drainage network design Disconnect runoff building sites and outfalls Runoff volume control (flood detention is not enough) Conventional basins Low impact development techniques Simplified design approach TFMA Fall 2017

22 Stream Buffer Summary 22 Local government administration ease Simple design approach/no construction oversight Minimal maintenance - Resilient Every day ecosystem benefits Recreational opportunities Protect with runoff volume control Stormwater program foundation

23 Contact Information 23 Tom Hegemier