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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