The Effects of Impervious Cover on a Hydrologic System

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The Effects of Impervious Cover on a Hydrologic
System

• BRUSHY CREEK WATERSHED
• By Ruth Haberman

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OUTLINE

• Introduction to Brushy Creek Watershed
• HEC-HMS Model
• Effects of Impervious Cover
• Conclusions
• Future Work

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Getting Oriented
Texas
Williamson County
Brushy Creek Watershed
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• WILLIAMSON COUNTY
• 27th most rapidly growing county in the US
  between 2000 and 2005
• 30.4 Growth rate (U.S. Census Bureau)
• Round Rock 41.2
• Cedar Park 84.8
• Leander 135.0

Source http//www.upperbrushycreekwcid.com/Dams/m
aps/ETJ.pdf
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HEC-HMS Model Parameters

• ArcHydro/NHDPlus
• Subbasin Areas and Centroids
• Longest Stream Paths
• Stream Slopes
• Developed Area (percent)
• Assumptions/Methods
• Synthetic Storm - SCS Type III Rainfall
  Distribution
• Snyder Unit Hydrograph
• Muskingum-Cunge Routing Method
• Type C Soils
• Calculations/Derivations
• Curve Numbers
• Impervious Area (percent)
• Lag Time
• Initial Abstraction (based on Curve Numbers)

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Parameter Calculations
Areas to improve model Calibration Better
terrain data - LIDAR More accurate slope
calculations Actual impervious cover Include
soil types in CN derivation Incorporate actual
precipitation data



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Effects of Impervious Cover
INCREASED IMPERVIOUS COVER OVER 14 OF AREA
DECREASED IMPERVIOUS COVER OVER 13 OF AREA
Peak 8,500 CFS at 650 PM
Peak 8,500 CFS at 650 PM
Existing Conditions
Existing Conditions
Peak 8,300 CFS at 655 PM
Peak 8,800 CFS at 1035 AM
Decreased Impervious Cover in Subbasins 2, 3. 4
and 6
Increased Impervious Cover in Subbasins 1 and 5
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Effects of Impervious Cover
DECREASED IMPERVIOUS COVER OVER 13 OF AREA
INCREASED IMPERVIOUS COVER OVER 14 OF AREA
Peak 4,900 CFS at 530 AM
Existing Conditions
Peak 3,250 CFS at 755 AM
Peak 6,400 CFS at 435 AM
Decreased Impervious Cover in Subbasins 2, 3. 4
and 6
Increased Impervious Cover in Subbasins 1 and 5
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New Subdivision Improvements
New Educational Facility
New Development

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Conclusions

• Impervious cover affects runoff
• Quantity
• Timing
• Hydrologic models can help predict the effect of
  a development site
• Model should cover an entire watershed

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Future Work

• Refine and calibrate model
• Develop hydraulic model HEC-RAS
• Model water quality

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• QUESTIONS?

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