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Roadway Detention San Antonio District
Guidelines for Detention
Nellie Gómez Shannon, PE, CFM San Antonio District Hydraulic Engineer
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BACKGROUND
• Addition of District Hydraulic Engineer 2005 – Improved coordination with local governments – Improved understanding of local government’s
drainage issues
• Address concerns for roadway project impacts • Proactive position in regards to drainage
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Acknowledge • TxDOT roadway projects impact
– Add impervious cover-small part of watershed – Pass development storm water runoff – Found in high urban growth areas – Impacts small watersheds and outfalls
• Lack of adequate outfalls in urban areas Proactive approach -Mitigate TxDOT project’s adverse impacts
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POSSIBLE IMPACTS
• Increase of discharge • Rise of water surface • Increase of velocities • Change of time of concentration • Perception of impact
“We have never flooded until you changed the roadway.”
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Detention?
• Do we really need to detain TxDOT’s project’s increase of runoff?
• Is the outfall adequate? • Are there any adverse impacts with increased
discharges contributed by TxDOT’s project?
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Possible Resources • Flooding History
– Maintenance Foreman – Local floodplain administrator – Various county, city, and river authority watershed
committees and staff
• Mapping Tools • FEMA floodplain maps • Aerials
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Mapping
City of San Antonio GIS Mapping of Flood and Water Resources Data
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Our Goal
• No significant adverse impact • No additional runoff into undersized
outfalls • Detain within existing right of way
– Detention within the storm sewer – Small ponds within right of way – Pipe arrays
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Determination of Storage
• Calculate the existing and proposed discharges contributed from TxDOT right of way
• Analyze the outfall • Use FHWA HEC 2, Highway Hydrology, Chapter
8 Detention Pond Analysis and Design • Use FHWA HEC 22, Urban Drainage Design
Manual, Chapter 8 Detention and Retention Facilities
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Triangular Hydrograph
From HDS-2, Chapter 8
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Ponds IH 410 at San Pedro
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IH 35 and SP 422
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Small Pond within ROW
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In-Line Detention
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Pipe Array
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DOCUMENTATION
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Stage-Storage
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Questions?
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