10/5/2015
City of Austin Little Walnut Creek Flood Improvements Case Study
Project Type & Location
• City of Austin Flood Hazard Reduction Project • Project located along Little Walnut Creek in northern part of Austin • Goal: Reduce Interior Inundations and Bridge Overtopping
CITY OF AUSTIN WATERSHED PROTECTION
Watershed Characteristics
• 1,247 acres • Fully-developed with mix of industrial, multi-family, and single-family • Residential built in 1970s before flood ordinance • Industrial built in 1980s after flood ordinance - with on- site detention • 8,500 linear feet of open channel drains to the Mearns Meadow Detention Pond • 100-yr peak flow of 5,800 cubic feet per second at pond LWC Flood Hazard Reduction Improvements (LW1-LW5)
Streams CITY OF AUSTIN Subwatersheds WATERSHED PROTECTION
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Existing Channel / Drainage Easement
• Undersized channel causing flooding • Creek within 60’ drainage easement (typical) • Lined with residential single-family • 24” wastewater line down easement and frequently exposed
CITY OF AUSTIN WATERSHED PROTECTION
Existing Flood Hazard
• No. 5 on our Regional Top 15 Flooding Priorities • Interior Inundations ‒ 16 in 10-year ‒ 45 in 25-year ‒ 82 in 100-year • Bridge Inundations ‒ All bridges inundated in 10-year ‒ 2 bridges inundated in 2- year
LWC Flood Hazard Reduction Improvements (LW1-LW5)
Ex 10-Yr Inundated Structures Ex 25-Yr Inundated Structures Ex 100-Yr Inundated Structures Ex 25-Yr Floodplain Ex 100-Yr Floodplain CITY OF AUSTIN Streams WATERSHED PROTECTION
Preliminary Investigations
Channel vs. Flow Diversion Improvements 90 82 80 70 58 • Buyout and Regional Detention investigated 60 ‒ Both options not economically effective 50 40 31 30
‒ Buyout does not help reduce flooding to Inundations yr - bridges and culverts 20
100 10 • 2008 PER outlined channel improvement 0 options Existing PER Channel Flow Condition Improvements Diversion ‒ Limited flood hazard reduction due to lack Improvements of flow area within channel ‒ Ability to increase flow area within channel limited due to constraints of existing easement, the preservation of the channel invert, and potential tree impacts • 2011 PER Review ‒ Recommended investigation of flow diversion concept
CITY OF AUSTIN WATERSHED PROTECTION
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Flow Diversion
• Flow Diversion Concept ‒ Diversion · Divert flow just upstream of inundations · Divert between 1500 cfs to 2000 cfs of 3,600 cfs in 100-yr · Maintain creek base flow ‒ Diversion Routing · Route diversion through 3,500 lf of large box culvert along Mearns Meadow Blvd to regional detention pond ‒ Downstream Mitigation · Enlarge regional detention pond from 90 af to 130 af ‒ Overall best option to meet project flood hazard goals
CITY OF AUSTIN WATERSHED PROTECTION
Recent Project Events
Oct 2013 2013 Dec 2014 April 2015 2015 60% Design Halloween Public USACE Memorial Submitted Flood Meeting Meeting Day Flood
Recent Project Events Result in Major Questions to Address Before Progressing Forward in Design 1. USACE – Pursue Alternative or Obtain Individual Permit? 2. Existing Flood Hazard Accurate? 3. Are private ponds playing a major role?
CITY OF AUSTIN WATERSHED PROTECTION
USACE and ‘Street Diversion’ Concept
• Street Diversion Concept ‒ Developed as a result of USACE meeting ‒ Divert flood flow from street and stay out of creek ‒ Place multiple curb inlets along street ROW in areas of high street inundation ‒ Connect new inlets to large diameter culvert to convey flows to regional detention pond ‒ ICM allows concept to be dynamically modeled ‒ Similar 100-yr flood depth reductions as creek diversion ‒ Bottom Line – Worth further investigating and consideration Street Diversion Concept Preliminary 100-yr Flood Depth Reductions in ICM
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Historic Flood Event
• 1981 Memorial Day Flood ‒ Little Walnut Creek was among the creeks that flooded ‒ Estimated 6” of rain over a 3 hour period for study area ‒ 50- to 100-yr event ‒ Reports of 8” in 90 min ‒ Widespread flooding and property damage reported in Central Austin
CITY OF AUSTIN WATERSHED PROTECTION
2013 Halloween Flood Event
• Widespread property Study damage in the Onion Creek watershed Area • Little Walnut Creek Study Area ‒ 4.2” of rain over a 12 hour period ‒ Between 5-yr and 10-yr event ‒ Reported Flooding · 2 bridge crossing overtopped · 1 home FFE inundated · Storm debris a factor
CITY OF AUSTIN WATERSHED PROTECTION
2015 Memorial Day Flood Event
• Shoal Creek flooded parts of Study western downtown Area • Historic Flood on Blanco River occurred on May 23rd with over 10” of rain recorded • Wettest May on Record w/ +17” prior to event • Little Walnut Creek Study Area ‒ 3.3” of rain over 3 hour period ‒ Just above a 5-year event ‒ Reported Flooding · 3 bridges overtopped · Storm Debris a Factor
CITY OF AUSTIN WATERSHED PROTECTION
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Are the Existing Condition Models Accurate? ICM Model of Study Area
‒ H&H Model Accuracy Questions 1. How well does hydrodynamic 2D model (ICM) match up with established HEC models 2. How well does ICM match up with actual observed flood data? 3. Are privately maintained ponds playing a major role in reducing the flood hazard?
CITY OF AUSTIN WATERSHED PROTECTION
ICM vs HEC models HEC-HMS vs ICM Peak Flows 7000 -3.4% 1.0% 15.8% 6224 5770 5841 5899 6000 5576 5460
5000 • Project uses ICM and HEC 4.8% 4000 3576 3737 15.9% • Hydrodynamic 2D Model 3000 2254 1947 Updated 2000
‒ InfoWorks SD to InfoWorks 1000
ICM 0 ‒ Improved modeling of D/S of Det Pond U/S of Det Pond Parkfield Quail Valley Metric HEC-HMS ICM riverine reaches Q (cfs) Q (cfs) • Validate against established HEC-RAS vs ICM Peak WSELs methods 725 · ICM Peak flows within 16% 720 of HEC-HMS at major 715
hydrologic junctions 710
Mountain Mountain Quail
· ICM WSELs general 0.5’ 705 Parkfield
higher than HEC-RAS 700 Golden Meadow Golden
695 Spillway Pond Quail Valley Quail 690
685 CITY OF AUSTIN 37000 38000 39000 40000 41000 42000 43000 44000 45000 WATERSHED PROTECTION HEC-RAS ICM Bridge Elev (ft) Elev (ft) Deck (ft)
ICM vs Observed Flood Data
• ICM vs. Recent Flood Observations 5.75” 3.34” ‒ COA FEWS provided Gauge- adjusted radar data · Memorial Day 2015 · Halloween 2013 4.90” 4.85” 4.80” 4.14” 2.82” 3.34” 3.47” 3.74” ‒ Field Observations gathered ‒ ICM run for storm events and results compared to field observations 4.81” 4.22” 3.94” 3.59” 3.02” 3.40” 3.53” 3.80”
RED=Halloween 2013 BLUE=Memorial Day 2015 3.92” 3.43” 2.97” 3.05” 3.61” 3.56”
CITY OF AUSTIN WATERSHED PROTECTION
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Quail Valley Bridge Observed • ±6” over bridge • WSE: ±710.4 ICM: • Not Overtopped • WSE: 708.6 • Halloween 2013 ‒ Just above a 5- yr event Mountain Quail Bridge Observed: ‒ 4.2” in 12hrs • ±12” over bridge ‒ ICM correlates • 40% Culverts Blocked fairly well • 2”-3” of water in nearby home • WSE: ±702.9 ICM: • Decent Correlation Detention Pond • WSE: 703.4 Observed: • WSE: ±683.6 ICM: • Good Correlation • WSEL: 683.7 Mearns Meadow Det. Pond
CITY OF AUSTIN WATERSHED PROTECTION
Quail Valley Trib at Quail Ridge Observed: • 6” to 12” over bridge • WSE: 712.4 to 712.9 ICM: • Good Correlation • • Memorial Day WSE: 712.5 2015 Mountain Quail Bridge ‒ Between a 5-yr Observed: and 10-yr • ±6” over bridge event • 1 Culvert 50% blocked • WSE: ±702.5 ‒ 3.3” in 3hrs ICM: ‒ ICM correlates • Good Correlation well • WSE: 702.7
Neans Trib Bridge Observed: Detention Pond • ±6” over bridge Observed: • WSE: ±697.5 • WSE: ±686.6 ICM: ICM: Mearns • Good Correlation Meadow Good Correlation • WSE: 697.3 • WSEL: ±686.5 Det. Pond
CITY OF AUSTIN WATERSHED PROTECTION
Private Ponds Role of Private Ponds ‒ ±108 private ponds providing ±56 af ‒ Conservatively incorporated only the larger ponds as not all reliable long term ‒ 17 larger ponds were identified providing 39 af ‒ 75% of total private volume ‒ Regional Det. Pond provides an additional 89 af in 100-yr
CITY OF AUSTIN WATERSHED PROTECTION
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ICM Results w/ Metric ΔQ = -264 cfs Ponds Δtime = +0:51 min Quail Valley ΔQ = -437 cfs Δtime = +0:59 min 100-Yr Peak Flow Comparison ICM No Ponds vs w/ Ponds Models 7000 -9.8% -6.9% 6000 -5.7% 5000 -11.7% 4000 3000 -11.7% 2000 1000 0 Metric Quail Valley Parkfield U/S of Det PondD/S of Det Pond
ICM ICM No Ponds w/ Ponds Parkfield ΔQ = -620 cfs Hydrologic ICM ICM Δtime = +0:35 min Junction No Ponds w/ Ponds Delta
Metric 12:22:10 12:23:01 0:00:51 U/S of Pond ΔQ = -407 cfs Quail Valley 12:29:02 12:30:01 0:00:59 Δtime = +2:05 min Mearns Meadow Parkfield 12:30:00 12:30:35 0:00:35 D/S of Pond Det. Pond U/S of Det Pond 12:35:40 12:37:45 0:02:05 ΔQ = -315 cfs D/S of Det Pond 12:42:02 12:47:46 0:05:44 Δtime = +5:44 min
CITY OF AUSTIN WATERSHED PROTECTION
Modeling Private Ponds in HEC-HMS
• HEC Update ‒ Not feasible to route every small pond ‒ Modified HEC to reflect 17 largest private ponds ‒ Adjusted · Impervious Cover · Lag Time
CITY OF AUSTIN WATERSHED PROTECTION
HEC Results with HEC-HMS Private Ponds 100-Yr Peak Flow Comparison No Ponds vs w/ Ponds 7000 -6.4% -3.8% -5.4% -372 cfs -217 cfs 6000 -294 cfs • HEC-HMS Results 5000 -7.2% 4000 -257 cfs ‒ HEC Peak Q decreased 4% to -7.6% 3000 8% -148 cfs 2000 ‒ ICM Peak decreased 6% to 1000 12% 0 • HEC-RAS Results Metric Quail Valley Parkfield U/S of Det Pond D/S of Det Pond HEC HEC ‒ Reduced existing flood hazard No Ponds w/ Ponds HEC-RAS Existing Flood Hazard FFE Inundations
90 82 80 74 70 60
50 42 Effective (no ponds) 40 31 30 Corrected
# of FFE Inundations FFE of# 20 16 Effective 7 10 (w/ ponds) 0 0 CITY OF AUSTIN 0 2-yr 10-yr 25-yr 100-yr WATERSHED PROTECTION Storm Event
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Conclusions and Project Next Steps
• Conclusions ‒ Study area dodges the brunt of recent floods ‒ Storm debris a factor in local flooding ‒ ICM model proves to be very accurate ‒ Private ponds have a moderate impact on watershed • Next Steps ‒ Revision of effective City Models to include private ponds under review ‒ Revisit flow diversion concept in consideration of USACE and revised existing conditions · Proceed with ‘creek diversion’ and USACE Individual Permit · Proceed with ‘street diversion’ concept and avoid USACE jurisdiction, working in creek, storm debris risk, and reduced long term maintenance
CITY OF AUSTIN WATERSHED PROTECTION
Questions?
• Thuan Nguyen (Project Sponsor) ‒ (512) 974-3513 ‒ [email protected] • Bryan Martin (Design Team Project Manager) ‒ (512) 879-0405 ‒ [email protected]
CITY OF AUSTIN WATERSHED PROTECTION
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