upper brushy creek water control & improvement district

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

Popu

latio

n

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 size Larger upstream, smaller adjacent to study reach

< 1 sq mi

Roads/ Railroads Per 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 al lowable 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)



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


channel flow


channel flowmax overland flow

lengthurban: 150 feet; undeveloped

300 feeturban: 150 feet; undeveloped

300 feet100 feet 100 feet, adjusted during

cal ibration

Channel velocityBankfull condition, normal

depthBankfull condition, normal

depth

If mapped channel: HECRAS velocity; i f unmapped: bankfull

and normal depth

If mapped channel: HECRAS velocity; i f 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?


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


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:


Runoff volume (for a given rainfall) is a function of:• Rainfall• Land Use• Soil Type• % Impervious• Antecedent Runoff Condition


What are data validity tests for a calibration storm used to calibrate a model rainfall/runoff relationship?• Rainfall


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 StormsAre there enough data?

– Spatially vs storm shape

Choice of Calibration StormsAre there enough data?

– Temporally versus storm shape

Choice of Calibration StormsRepresentative?

– In temporal shape


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?


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


Watershed

Curve Numbers Derived Per Calibration Using District Gage Precip Computed

Values Per TM2


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


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

Dam 7 Upstream

Dam 7 Downstream

Downstream

Wall Shape and Layout Options

Labyrinth Weir Construction

Platform Slab Construction

Crest Shape

Labyrinth Wall Construction

Discussion / Q&A

THANK YOU

upper brushy creek water control & improvement district

Documents

data rainfall

watershed hydrology

mindsetthe data test

hydrology good

homogeneous data set

district engineers

mindset hydrology

watersheds hydrology