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Upper Brushy Creek Water Control & Improvement District
Ruth Haberman, General Manager, UBCWCIDMarch 21, 2013
DISTRICT OVERVIEW Original District was
formed by the Texas Legislature in 1956 for flood and erosion control within the Brushy Creek watershed
Primary focus has been operation and maintenance of 23 dams constructed by the SCS (now NRCS) in the 1950s and 1960s
DAM MODERNIZATION
DAM 14
DAM 11 Since 2003, over $15M in tax revenue have been spent to modernize 21 dams.
Two dams remain to be modernized; Dams 7 and 8.
MISSION STATEMENT
The mission of the Upper Brushy Creek Water Control and
Improvement District is to maintain and improve flood control
structures and take appropriate measures to protect public safety
as well as economic infrastructure of the District, in consultation and
cooperation with other governmental entities. The District will actively
foster a regional perspective and will encourage cooperation among
governmental entities. We will accomplish these tasks utilizing cost-
effective methods, minimizing the impact to the environment,
considering the community values of our stakeholders, and
conducting our business with openness, honesty and integrity.
JURISDICTIONAL BOUNDARIES
DRAINAGE AREAS
MAJOR CHANGES IN THE COUNTY
1960 1970 1980 1990 2000 2010
Williamson County 35,044 37,305 76,521 139,551 249,967 422,679
http://www.census.gov/population/cencounts/tx190090.txt
http://www.txcip.org/tac/census/profile.php?FIPS=48491
1960 1970 1980 1990 2000 2010 -
50,000
100,000
150,000
200,000
250,000
300,000
350,000
400,000
450,000
Williamson County, Texas
Popula
tion
TS HERMINE FLOODING – September 8-9, 2010
FLOODING ISSUES IN THE DISTRICT Tropical Storm Hermine
District Infrastructure:
The dams functioned as designed, constructed and maintained.
The dams experienced only minor damage from the flooding.
The web-based Flood Monitoring System allowed District engineers, elected officials, and both City and County emergency managers to monitor the rainfall in the area.
Even with the dams operating as expected, there were still threats to public safety and risk of property damage.
UPPER BRUSHY CREEK WATERSHED STUDY
Purpose:
• Analyze watershed hydrology and hydraulics.
• Identify potential flood hazard areas.
• Propose alternative solutions for regional flood hazard mitigation.
• Provide final report, models and documentation to all participants.
• Coordinate with FEMA and local government entities to incorporate study results into new regulatory FEMA Risk Maps (floodplain maps) for the entire watershed.
Upper Brushy Creek Hydrologic Model
Jeff Irvin, URS
Study Area
Getting on Same Sheet of Music
City of Austin City of Round Rock Risk Map Models Selected
Topo 2006 LiDAR 2006 LiDAR 2006 LIDAR 2006 LiDAR
Watershed sizeLarger upstream, smaller adjacent to study reach
< 1 sq mi
Roads/ RailroadsPer LiDAR, aerial photo search for cross-drainage structure
Per LiDAR, aerial photo search for cross-drainage structure
Storm Drains Considered?
No Stakeholder input, otherwise no
24 Hour/1 day 100-Year Depth
10.2 inches 1-day depths not provided.Varies (9.0-9.9)
USGS SIR 2004-5041
Create surface from (9.0-9.9)- USGS SIR 2004-5041 and select point at center of drainage area
for each major tributary.
Temporal Distribution
SCS 24-hour Type III SCS 24-hour Type III SCS 24-hour Type III SCS 24-hour Type III
Impervious AreaPer TR55 Tabulation for
existing, per max allowable in ultimate
Per TR55 TabulationPer TR55 Tabulation, other
(Table 3, TSDN)Per TR55 Tabulation, other
(Table 3, TSDN)
Antecedent Moisture
Average (II) Average (II) Average (II) Average (II)
Initial abstraction Per NEH-4 Per NEH-4 Per NEH-4 Per NEH-4
Losses after initial abstraction
SCS Curve Number (TR-55)
SCS Curve Number(TR-55)
SCS Curve Number(TR-55)
SCS Curve Number(TR-55)
Reach Routing Method Muskingum-Cunge Acceptable Muskingum-Cunge Muskingum-Cunge
MethodSCS Method: Overland flow + Shallow Concentrated flow +
channel flow
SCS Method: Overland flow + Shallow Concentrated flow +
channel flow
SCS Method: Overland flow + Shallow Concentrated flow +
channel flow
SCS Method: Overland flow + Shallow Concentrated flow +
channel flowmax overland flow
lengthurban: 150 feet; undeveloped
300 feeturban: 150 feet; undeveloped
300 feet100 feet
100 feet, adjusted during calibration
Channel velocityBankfull condition, normal
depthBankfull condition, normal
depth
If mapped channel: HECRAS velocity; if unmapped: bankfull
and normal depth
If mapped channel: HECRAS velocity; if unmapped: bankfull
and normal depth
Parameters
Drainage Area
Rainfall
Infiltration and Loss
Time of Concentration/Lag
Method
449 Watersheds
Hydrology: the Mindset
Hydrology = Data (Rainfall, Runoff, Land Use)Data bad = Hydrology badData good = Hydrology good
How do you test data?
Hydrology: the Mindset
The data test:Representative?
in place (where data taken)in time period (same as application time period)in amount of data?
Homogeneous? – applied on any data to be aggregated/ averaged
in placein timein collection method and accuracy
Hydrology: the Mindset
The most representative and homogeneous data set is the best data set
Hydrologic Model Calibration Data
• Rainfall– Sources of rainfall data?– Which storms?
• Runoff (flow or stage plus hydraulics)– Sources?
Choice of Calibration storms The runoff hydrograph has two main parameters that define shape:• A parameter that defines
how much rain runs off (runoff volume)
• A parameter that defines time of peak (runoff temporal shape)
Choice of Calibration Storms
Runoff volume (for a given rainfall) is a function of:
Choice of Calibration Storms
Runoff volume (for a given rainfall) is a function of:• Rainfall• Land Use• Soil Type• % Impervious• Antecedent Runoff Condition
Choice of Calibration Storms
What are data validity tests for a calibration storm used to calibrate a model rainfall/runoff relationship?• Rainfall
Choice of Calibration Storms
What are data validity tests for a calibration storm for a model rainfall/runoff relationship?• Rainfall
– Representative• In location and time• In temporal shape• In size• Are there enough data?
– Spatially vs storm shape– Temporally versus storm shape
Upper Brushy WCID Dams
Choice of Calibration StormsRepresentative in location and time?Are there enough data?
– Spatially vs storm shape
Choice of Calibration Storms
Are there enough data?– Spatially vs storm shape
Choice of Calibration Storms
Are there enough data?– Temporally versus storm shape
Choice of Calibration Storms
Representative?– In temporal shape
Choice of Calibration Storms
Point of Comparison June 2007 Storm TS Hermine Comment
Spatial Variation High intensity rainfall in northern County and Leander
Extreme rainfall throughout west and central watershed, to include Leander, Cedar park, and Austin
Both storms much less intense in eastern watershed
Antecedent Runoff Condition
Moderate: substantial rains within two weeks of main event
Very dry: very little rain over previous two months
Should expect a calibrated CN for 2007 to be on our about a ARCII condition, and for 2010 on or about a ARCI condition
Return Period, Rainfalll durations <= 1 hour
On the order of 20-50 year return period storm in main area of storm
On the order of 2-5 year storm in main area of storm
A storm similar to the 2007 storm would be expected to stress small to medium watershed (approx 1 hour lag time) local drainage: storm drains, road conveyance. The 2010 storm would have less sever effect.
Return Period, Rainfall durations 2 hour
On the order of 20-50 year return period storm in main area of storm
varies within main storm area: part has 5 to ten year return period, part has 20-175 year return period Both storms similar for this duration
Return Period, Rainfall durations 3 hour to 24-hour
Return period diminishing with increase in duration
Return period increasinging with increase in duration, up to 300+ years for 24-hours
Downstream main stem (with large watershed with longer lag time) expected to have much worse flooding in 2010 than 2007
Return Period, Rainfall durations 24-hour
5 to ten year return period, much less than design storm for flood pools of dams
90 to 320 year return period, equal to to much greater than design storm for flood pools of dams
District dams provided designed flood protection in 2007, capacity (to contain regulatory flood) exceeded in 2010
Results of Rainfall/Runoff Calibration
2007 Event
Tropical Storm
HermineDam 1 82.52 72.2 43.0Dam 2 80.19 64.9 *Dam 3 79.83 66.7 40.0Dam 5 79.48 60.0 40.0Dam 6 80 * **Dam 11 78.2 60.7 51.0Dam 12 80.25 61.2 40.0Dam 13A 80 61.6 46.0Dam 14 80.53 84.8 23.0Dam 16 80.08 - 47.0Dam 19 77.58 - 49.0Average(exluding dam 14) 79.88 63.9 44.5
- Gage not installed* Bad stage data from gage** Bad precip data from gage
Curve Numbers Derived Per Calibration Using District Gage Precip
Watershed
Computed Values Per
TM2
Why are results inconsistent?
Can we compare 2007 storm runoff results to 2012 storm runoff results?
Are the conditions that affect runoff homogeneous between the two storms?
RainfallLand UseSoil Type% ImperviousAntecedent Runoff Condition
Antecedent Runoff
2007 Rainfall
2010 Rainfall
Are the two storms homogeneous in terms of antecedent conditions?
Results of Rainfall/Runoff Calibration
2007 EventTropical Storm
HermineDam 1 82.52 72.2 63.4Dam 2 80.19 64.9 *Dam 3 79.83 66.7 60.3Dam 5 79.48 60.0 60.1Dam 6 80 * **Dam 11 78.2 60.7 70.6Dam 12 80.25 61.2 59.8Dam 13A 80 61.6 66.4Dam 14 80.53 84.8 41.4Dam 16 80.08 - 66.6Dam 19 77.58 - 69.3Average(exluding dam 14) 79.88 63.9 64.6
- Gage not installed* Bad stage data from gage** Bad precip data from gage
Watershed
Curve Numbers Derived Per Calibration Using District Gage Precip Computed
Values Per TM2
Results of Rainfall/Runoff Calibration
No Adjust- ment
Adjusted Per TXDOT, 2011 2007 Event
Tropical Storm Hermine
Dam 1 82.52 67.52 72.2 63.4Dam 2 80.19 65.19 64.9 *Dam 3 79.83 64.83 66.7 60.3Dam 5 79.48 64.48 60.0 60.1Dam 6 80 65 * **Dam 11 78.2 63.2 60.7 70.6Dam 12 80.25 65.25 61.2 59.8Dam 13A 80 65 61.6 66.4Dam 14 80.53 65.53 84.8 41.4Dam 16 80.08 65.08 - 66.6Dam 19 77.58 62.58 - 69.3Average(exluding dam 14) 79.88 64.88 63.9 64.6
- Gage not installed* Bad stage data from gage** Bad precip data from gage
Watershed
Computed Values Per TM2
Curve Numbers Derived Per Calibration Using District Gage Precip
Upper Brushy Creek Water Control and Improvement District
Dustin Mortensen, Freese and NicholsMarch 21, 2013
DAM 7 MODERNIZATIONCE 374K HYDROLOGY
FNI OVERVIEW
• Multi-service engineering,architecture and environmental science firm
• 118-year history
• 520+ employees across14 offices throughout Texas
• Client satisfaction is ourtop priority
• We offer cost-effectivesolutions
Dam 7 OVERVIEW
• Intermediate Sized, High Hazard• Completed in 1965• Hydraulic Capacity - Passes 46% PMF • Drawdown Time (Auxiliary Spillway-Normal Pool)
Normal Level
Flood Level
Flooded Park
Spillway HydraulicsSpillway discharge is calculated using weir equation
Where: Q= discharge (cfs or m3/s)C=Discharge coefficientL=Length of the “lip” over which the water flowsH=Head above the weir
Increase Capacity
Existing dam
Increase H
Raising the dam
Increase L
Widening the spillway
Increase L and H
Range of options…
Increase L
Labyrinth Weir• Increases L without increases spillway footprint width• Dam 7 alternative has 1,800 feet of weir in 300 foot wide footprint• C is dependent on wall angles, wall height, head and crest shape
Model Studies
Model Studies
Dam 7 Upstream
Dam 7 Upstream
Dam 7 Downstream
Dam 7 Downstream
Downstream
Wall Shape and Layout Options
Labyrinth Weir Construction
Platform Slab Construction
Crest Shape
Labyrinth Wall Construction
Labyrinth Wall Construction
Discussion / Q&A
THANK YOU