Urban Water and Drainage Seminar Series

Waller Creek Center

Austin, TX

May 14, 2013

Roger Glick City of Austin, Watershed Protection

Study Watershed: Tributary to Gilleland Creek

4.99 km2

1233 acres

Elevation Data

Min Elev = 435 ft

Max Elev = 609 ft

Model Sub-basins

Site Slopes

Site Soils

City of Austin Ordinances: Land Use & Controls Undeveloped [UND]

Pre-Waterways Ordinances [Pre-ORD], <1974

• No controls • Limited creek easements, >320 ac.

Waterway Ordinance [WO], 1974-1986 • Detention only • Wider easements, >320 ac

Comprehensive Watershed Ordinance [CWO], 1986-present* • Detention and ½”+ sed-fil • Creek buffer and water quality transition zone, >320 ac

Watershed Protection Ordinance [WPO], proposed • Detention and ½”+ sed-fil • Creek buffer, >64 ac (no WQTZ)

*Major watershed specific ordinances for Barton, Williamson & Lake Austin were passed in 1980; the CWO extended to remaining suburban watersheds.

Undeveloped Land Use

Pre-Ord Land Use (<1974)

WO Land Use (~1974-86)

CWO Land Use (1986-present)

WPO Land Use (proposed)

HRU Distribution

Model Scenarios Developed Conditions

Irrigation and fertilizer on lawns and commercial; except high slopes

Increased roughness & conductivity in channels

100% of developed residential & commercial land treated by BMPs; some land uses excluded.

One large detention basin mid-basin (reach 9)

Detention Pond Location

Model Studies Impacts of different regulatory scenarios:

WO, CWO and WPO and assess flood, erosion and aquatic life impacts.

Impact of water quality controls with respect to detention sizing

Impact of changing capture volume and drawdown time with respect to erosion

Effects of Ordinances

Impacts on Flooding

Impacts on Erosion Potential

Impacts on Erosion Potential

Impacts on Erosion Potential

Impacts on Erosion Potential

Impacts on Aquatic Life

Impacts on Aquatic Life (cont.)

Impacts on Aquatic Life (cont.)

Impacts on Aquatic Life (cont.)

Impacts on Aquatic Life (cont.)

Impacts on Aquatic Life (cont.)

Impacts of Water Quality Controls on Detention Study used pre-ordinance land use only

Flow at detention basin (reach 9)

Impacts of Water Quality Controls on Detention

Impacts of Water Quality Controls on Detention

Impacts of Water Quality Controls on Detention

Impacts of Water Quality Controls on Detention

Impacts of Water Quality Controls on Detention

Erosion Study 3 impervious cover scenarios: 34.9, 51.2 and 64.4 %

No detention

4 basin sizes: ½”, CWO, LCRA and SOS

3 drawdown times: 24, 48 and 72 hours

4 median particle sizes: 12.5, 19, 24.5 and 38 mm

Channel shear

Conclusions Austin regulations since CWO implementation have

been beneficial with respect to flooding, erosion and aquatic life potential.

Flood detention alone will not address issues of erosion and aquatic life (and my be detrimental).

Water quality controls may have an impact on flood detention sizing for small storms (<2-yr) but have negligible impacts for larger storms (>10-yr)

Conclusions (cont.) Larger water quality capture volumes may be

detrimental if the stream bed and bank has small particle diameters.

Extending drawdown times may reduce excess shear.

Optimal capture volume and drawdown rates need to be sized based on stream geomorphology when assessing erosion.

Impact of Buffers Only - old

Impact of Detention

Impact of Sed-Fil

Effects of Ordinances (small storms)

Land Use Scenarios:

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

UND Pre-Ord WO CWO WPO

Pasture

Open Space

WQ Transition Zone

Parks

SFR Med. Density

SFR High Density

Multi-Family

Mobile Home

Civic

Office

Commercial

Industrial

Neighborhood Street

Residential Collector

Arterial Road

Model Scenarios

Undeveloped

Modeled as pasture

No irrigation

Higher roughness & conductivity in channels

0%

20%

40%

60%

80%

100%

UND Pre-Ord WO CWO WPO

0.3%

52.5%

51.6%

49.7%

46.3%

Pervious Area

Impervious Area

Developed