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TDOT Roadway Design Division

Module 4:

Outfalls

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Outfalls - Definition• Stormwater runoff, snow melt runoff, and surface runoff and drainage• Stormwater must be in a discernable/discrete/confined conveyance

– pipes and culverts– ditches and channels– curb and gutter– catch basins or curb inlets (outfall or sub-outfall)

• May include the discharge of– sediment filter bags– dewatering structures

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• Leaves the project:– ROW– Project termini– Easement (i.e. temporary construction or permanent drainage)

• Directly enters TDEC jurisdictional features (streams, springs, wetlands and sinkholes) – Does not include ephemeral streams

Outfalls - Locations

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Outfall Examples

ENHANCED ROCK CHECK DAMS

EXISTING DRAINAGE SWALE

ROWClearing and Grubbing

Natural drainage features need to be protected

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DIVERSIONS

SEDIMENT TRAP

EXISTING DRAINAGE SWALE

Outfall Examples

Intermediate Grading

Natural drainage features need to be protected

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Outfall Examples

Final Stabilization

Rip rap channel to stream. ROW

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Outfall Examples

Multiple culverts discharging into a wet weather conveyance

ROW

TWO SEPARATE DISCHARGES

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Outfall Examples

Multiple discharges entering stream.

TWO SEPARATE DISCHARGES TO A STREAM

ROW

STORM DRAINAGE SYSTEM

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Outfall Examples

Natural drainage swales or toe ditches that discharge to streams

ROW

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ROW

Outfall Examples

Relocated stream channel discharging to another stream

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Outfall Examples

Pipe/culvert/ditch discharging at ROW or easement before entering offsite stream

ROW OR EASEMENT

STREAM

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Outfall Examples

Discharge from a sediment filter bag

STREAM

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Outfall Examples

Low area discharge in silt fence from sediment filter bag

ROW OR EASEMENT

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Sub-Outfalls - Definition• A sub-division of a main outfall to accomplish the

following:‒ To reduce the drainage area (eliminate need for

sediment basins and/or traps)‒ To account for drainage in a closed storm system

that is collected in area drains or curb inlets• Closed drainage system directly discharges to a

jurisdictional feature or outfall within the project

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Closed Drainage System Inlets - Last Place to Capture Sediment

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Outfall or Sub-Outfall?• Identify the Outfall(s)

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Outfall or Sub-Outfall?• Identify the Outfall(s)

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Outfall or Sub-Outfall?• Identify the Outfall(s)

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Outfall or Sub-Outfall?• Identify the Outfall(s)

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Outfall or Sub-Outfall?• Identify the Outfall(s)

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PROJECT TERMINI

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Sub-Outfall Examples

EPSC Stage 2 of curb inlets

SUB-OUTFALLS

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Sub-Outfall Examples

EPSC Stage 3 of curb inlets SUB-OUTFALLS

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Sub-Outfall Examples

Curb inlet with no inlet protection

SUB-OUTFALL

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Sub-Outfall Examples

Area drain/catch basin protection

SUB-OUTFALL

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Sub-Outfall Examples

Stormwater manholesSUB-OUTFALL

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Sub-Outfall Examples

Stage 2/3 EPSC

Area drain during construction

SUB-OUTFALL

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Sub-Outfall Examples

Stage 3 EPSC

Area drain after construction

SUB-OUTFALL

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Outfalls• Outfalls may change in the EPSC

stages– Existing– Intermediate– Final

• Roadway designer to provide for each EPSC stage:– Outfalls– Respective drainage area– Average slope of drainage area

(Percent slopes)

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Outfall Location(s) - Present (existing) Conditions – Stage 1 EPSC

NATURAL DRAINAGE FEATURE

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PROPOSED SPECIAL & TOE DITCHES

NATURAL DRAINAGE FEATURE FILLED IN

Outfall Location(s) – Proposed Conditions – Stage 2 EPSC

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OUT-#10

Outfall Location(s) – Proposed Conditions – Stage 2 EPSC

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• Toe Ditch or Special Ditch– Formed where fill meets existing

ground– Becomes a “v-ditch”– Steep slope– Concentrated flow– High velocities– Protection needed

• During construction • Post construction

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Outfall Locations

Present (existing)Conditions

Stage 1 EPSC

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Outfall Locations

Proposed Conditions

Stage 2/3 EPSC

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Outfall Locations

Proposed Conditions

Stage 2/3 EPSC

OUT-#13

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Outfall Locations – Stage 1 EPSC (Clearing & Grubbing)

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Outfall Locations – Stage 2 EPSC (Intermediate - Mass Grading)

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Outfall Locations – Stage 3 EPSC (Final Grading)

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Outfall Locations – Stage 1/2

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Questions?