Proposed Hydraulic Design Manual Updates

Short Course Hydraulics Session October 14, 2020

Proposed Hydraulic Design Manual Updates

Hydrology and Hydraulics Section

October 14, 2020

Ab Maamar-Tayeb, P.E.


Agenda

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Hydraulic Design Manual Updates Chapter 4 Hydrology (Updates)

– Update to NOAA Atlas 14 rainfall data guidelines.– Update on Rainfall Temporal Distribution based on

NRCS guidance.– Peak Rate Factor (PRF) guidance based RTI Research. – Statistical Analysis of Stream Gage Data based on RTI

Research.

Chapter 2 and Chapter 13 (Updates)– Drainage Policy and Storm Water Management

• Developers Outfall Policy and Detention

Chapter 16 (New Chapter)– 1D and 2D Unsteady Analysis


Updates: Chapter 4 - Hydrology

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Chapter 4 Updates:– Four Updates Topics

• NOAA Atlas 14• Rainfall Distribution Guidance• Peak Rate Factor• Statistical Gage Analysis Generalized

Skew Update


Updates: Chapter 4 - Hydrology

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Chapter 4 Updates: NOAA Atlas 14

• Rainfall data updated with the publication of NOAA Atlas 14 in 2018• Considered best available data. • All projects must use NOAA Atlas 14 Data.

Two Sources for NOAA Atlas 14 Data

NOAA’s National Weather Service Website – “NOAA Atlas 14 Point Precipitation Frequency Estimates” :

https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=pa

EBD Lookup Spreadsheet (EBDLKUP2019) completed by RTI project 0-6980• Provides intensities for Rational Method input (not depths)

https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=pa


Updates: Chapter 4 – Hydrology – Atlas 14 Rainfall Data

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Chapter 4 Updates: NOAA’s National Weather Service Website –“NOAA Atlas 14 Point Precipitation Frequency Estimates” :


Updates: Chapter 4 – Hydrology – Atlas 14 Rainfall Data

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Created e,b,d spreadsheet– EBDLKUP-2019-vC6.2.10.xslm (replaces

EBDLKUP-2015)– New e, b, d coefficients determined for

intensity, duration, frequency (IDF) curves – Allows for partial duration or annual

maximum series. – Developed sub-county rainfall zones due to

rainfall variation across some counties. 500 zones total based on spatial error of 20, 15 and 10 percent

– Includes KMZ google earth map file

Compiled new IDF curves in library– Library integrated with Bentley OpenRoads

Designer


Updates: Chapter 4 – Hydrology – Rainfall Distribution

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– Rainfall temporal distribution• NRCS states Type II-III storms out of date• NRCS Type II-III storms not included in

HEC-HMS 4.3• New recommended method: Balanced

Storm Methods (Frequency storm – HEC-HMS)

• Storm Peak at 67 percent of Duration


Updates: Chapter 4 – Hydrology – Peak Rate Factor

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Peak Rate Factor (PRF) guidance• Peak Rate Factor is parameter that defines the

shape of the hydrograph• Suggests consideration of PRF values other than

the default value of 484– Possible range: 100-600

• PRF selection will be based on slope and basin area.

• RTI Research Project 7010


Updates: Chapter 4 – Hydrology – Gauge Data

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Statistical Analysis of Gauge Data (Generalized Skew Update)• Flow frequency relation by statistical analysis of series of recorded annual

Maximum flows. • USGS recently update Gauge Analysis Guidelines, Bulletin #17C • HDM currently recommends use of Bulletin 17C procedures, • General Skew Coefficients are outdated. • RTI 0-6977 “Generalized Skew Update” – Ongoing Research to develop

updated GenSkew values with Map, Regional Values or statewide constant.


Updates: Chapter 2 – Policy and Chapter 13 – Storm Water Management

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Policy and Guideline Development of Impact Mitigation: Impact mitigation downstream of TxDOT ROW due to TxDOT Activities and Developers Impact

Mitigation on TxDOT ROW– Separate Policies but similar and related

Chapter 2 Policy - Update– Impact Definition and Developers Outfall Policy

Chapter 13 – Update and Expansion– Current chapter is very brief and missing topics. Specifically guidance on impact mitigation

(detention) due to TxDOT Activities



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Chapter 2 Policy Updates

First update to Chapter 2

Impact Definition- Changes in WSE, Peakflow, Volume, Velocity, Stream Stability

• Downstream vs Upstream

• Insignificant vs adverse (can be difficult to define and quantify)

• Insignificant impact can occur if impact remains in ROW with no flooding to adjacent property. 100-year storm rise that is contained in the main channel banks with no flooding impact to structures, storm sewers and channel erosion

• Adverse Impact can be defined and measured by flooding to offsite property, roadways, driveways due to WSE increase or a change in drainage pattern; erosion due to volume and velocity increases etc.



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Update to Chapter 2 Policy Update Developers Outfall Policy Currently TxDOT’s Developer's Outfall Policy is described in a Memorandum separate from the

Hydraulic Design Manual. – Policy was signed and effective on May 25, 2017

Proposed Policy to be included in HDM is under development– Work-Group discussions with District will continue through the fall. – TxDOT’s Developers Outfall Policy Memorandum the current guideline.

Developer’s Outfall Policy is tied to the Driveway Permit.



13



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Chapter 13 UpdatesBulk of Revisions/Updates contained in Section 3 Quantity Management Section 5 Storm water Impact AnalysisSection 3 Quantity Management– Land Development activities designed to concentrate and remove flow from site, potentially resulting in

increase in runoff, velocity, WSE, erosion– Designers need to identify risks and may need to mitigate by managing quantity leaving the site through – Benefits of Quantity Management include

• Reducing peak flow• Reduction of WSE• Reduction in velocities

• Reduction in erosion



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Chapter 13 Updates

Section 5 Stormwater Impact Analysis

– Is a procedure and process to determine and assess risk.– Provides a process to determine when detention is necessary to

mitigate potential adverse impacts– Early evaluation starting during planning phase is important.

• Detention may require ROW and easement acquisition



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Chapter 13 Updates

Impact Analysis Steps

1. Preliminary assessment that is qualitative2. Detailed Drainage Impact Analysis3. Downstream Risk Assessment



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Chapter 13 UpdatesPreliminary AssessmentSeries of qualitative questions –Flowchart to assist Designer

– Type of Project (Rehab vs added capacity)

– Urban vs Rural– Increase of Impervious – Increase in size of cross

drainage structure opening

No

Yes

No

Yes

Yes

No

Yes Yes

No

Maintenance (SC, OV, RES)

2 Project Type

4 Is the POI

in an “Urbanized Area”?

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Is there a risk of a measurable change to downstream drainage

conditions?

9 Are there

existing downstream flooding, erosion, or other

community concerns?

Low Risk for Hydrologic Impact from TxDOT

Project

Perform Detailed Drainage Impact

Analysis

Stormwater Management

Preliminary Evaluation

3 Identify each project outfall

and/or Point of Interest (POI) Complete Drainage Data

(Section 1)

All Other Projects

No

Yes

1 Complete Stormwater

Management (Summary Worksheet)

6 Complete Basic (POI)

Impervious Cover Sub Basin Data

(Section 2)

7 Are there Increases to

Impervious Cover or Sub Basin Area at the

POI?

8 Complete (POI)

Downstream Evaluation (Section 3)

5 Are there any

existing downstream flooding or erosion

concerns?



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Chapter 13 Updates

If a Preliminary Assessment determines need for further detail and assessment, a full detailed Drainage Impact Analysis is needed:

Delineation of existing and proposed conditions drainage basins and runoff parameters.

Computations of existing and proposed conditions peak flows and hydrographs

Hydraulic Computations of existing and proposed conditions (WSE and velocities)



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Chapter 13 UpdatesDownstream Risk Assessment:

– Compare project existing and proposed condition WSE elevations to structure elevations.• What is the change/magnitude in flood risk?

– Guidelines do not provide a specific standard such as WSE change to define an allowable change. • Consult the DHE



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Chapter 13 UpdatesConsequences of not performing a Impact Analysis:– Example design of proposed Detention Ponds at every outlet based on a volume increase when

no real adverse impact due to WSE increase and flooding would occur, potentially costing the state millions of dollars in additional construction costs.



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Chapter 13 UpdatesConsequences of not performing a Impact Analysis:Example of post construction impacts due to concentration of flow and diversion of flow directed at downstream development.


New Chapter 16 : 1D Unsteady and 2D Analysis

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New chapter 16 to provide guidance on 1D and 2D Unsteady Analysis:

1D Unsteady Analysis– 1D steady vs 1D unsteady– When to use 1D unsteady

2D – 1D vs 2D Analysis– When to Use 2D vs 1D– 2D Design Levels– 2D Examples– Suggestions on presenting 2D Outputs



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Chapter 16 1D Unsteady:– 1D Steady Analysis – Discharge at a given cross-section remains constant, and depth and

velocity do not change over time.

– 1D Unsteady Analysis – Discharge at a given cross-section is not constant. Depth and velocity vary with time.



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Chapter 16 1D Unsteady:When to use 1D unsteady:

• When you have impact to storage(Considerable Ponding upstream ofcrossing)

• When project requires informationon timing (when and how long willa roadway overtopped)



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Chapter 16 1D vs 2D :



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Chapter 16 1D vs 2D :


New Chapter 16 : 1D / 2D

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Application of 1D and 2D Analysis

1D Flow is dominantly in 1 direction (1 dimensional)

2D Flows paths are not well defined. Braided, Split/Diverted Flow Conditions Multiple Openings Wide Floodplains Depressions and storage

Chapter 16 recommends District Hydraulic Engineer approval


New Chapter 16 : 1D and 2D Analysis Model Selection

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Figure 16-1 Chapter 16• Model Selection Flow Chart

• Tool to assist designer in model selection.

– Does the project include the following challenges?• Ponding, impact on upstream

storage• Split flow• Complex high risk Urban area.• Does the project have an

adequate terrain dataset(Lidar)


TxDOT 2D Example Project – US 67

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Project Summary: • Preliminary engineering study

• Assess impact of US 67 on localized flooding in

city

• Identify alternatives for local flood reduction



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– Flat topography– Sheet flow primary

conveyance– Numerous low

points/overland storage– Multi-direction/intra-basin

flow– Difficult to analyze and

represent in 1D



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– Flat topography– Multi-direction/intra-basin

flow


Chapter 16 – Types of 2D Models

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Level 1 Concept

• Quick to setup• High level of

assumptions• Limited Data input• Used to inform

concepts for assumptions for other models (1D)

Level 2 Preliminary

• More detail than Level 1• Better available data• Used in Schematic or

Feasibility Studies• Should Not be used in

PSE as only model

Level 3 - Design

• Detailed data inputs• High level pre-

processing data • Field survey required at

infrastructure• Detailed Hydrology • Validated/Calibrated


Presenting Results

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Hydraulic results– 1D Models:

• Results presented in comparison table at cross-sections.

– 2D Models:• Guidelines for presenting 2D results not developed

yet for– Hydraulic Data Sheet– Schematic and PS&E documentation– What Water Surface Elevation is reported Highest, Channel, Average?


Presenting Results

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Looking Forward…

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TxDOT encourages designers to get exposure to 2D software– 2D models can be a tool to inform 1D models

• Can help determine:– Flowpaths– Flow directions– Drainage areas– Cross-section placement

– Most of the state now has LiDAR• Available from TNRIS and USGS

Proposed Hydraulic Design Manual Updates

Documents

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