Chapter 4 Hydrology · Jorge A. Millan, P.E., CFM TxDOT Design Division/Hydraulics . 2012 Transportation Short Course . October 17, 2012 . Chapter 4 Sections 1 – Hydrology’s Role

Chapter 4 "Hydrology" Revised TxDOT Hydraulics Manual

Jorge A. Millan, P.E., CFM TxDOT Design Division/Hydraulics

2012 Transportation Short Course

October 17, 2012

Chapter 4 Sections 1 – Hydrology’s Role in Hydraulic Design 2 – Probability of Exceedance 3 – Hydrology Policies and Standards 4 – Hydrology Study Requirements 5 – Hydrology Study Data Requirements 6 – Design Flood and Check Flood Standards 7 – Selection of the Appropriate Method for Calculating

Runoff 8 – Validation of Results from the Chosen Method 9 – Statistical Analysis of Stream Gauge Data 10 – Regional Regression Method 11 – Time of Concentration 12 – Rational Method Equation 13 – Hydrograph Method 14 - References

Technical guidance

Procedural guidance

Procedural Guidance Sections

1 – Hydrology’s Role in Hydraulic Design

Intuitive Definitions Basic Considerations

2 - Probability of Exceedance Annual Exceedance Probability (AEP)

AEP (%) AEP (probability)

Annual recurrence

interval 50% 0.50 2-year 20% 0.20 5-year 10% 0.10 10-year 4% 0.04 25-year 2% 0.02 50-year 1% 0.01 100-year

3 – Hydrology Policies and Standards

Guidelines, not hard and fast rules. Minimum standards. Higher standards

may be required by TxDOT District. Effort should match hazards and

concerns.

4 - Hydrology Study Requirements

Explain and justify assumptions. (analysis method, choice of parameters and other inputs.)

Provide enough to enable a reviewer to understand and to reproduce the results. (program used, relevant input values, specify options)

5 - Hydrology Study Data Requirements

Intuitive Explains the data required Definitions, descriptions, and

examples

Recommended Design Standards Table

6 – Design Flood and Check Flood Standards

CHANGE

50% 20% 10% 4% 2% Principal arterials: Culverts X [X] X

Small bridges X [X] X

Major river crossings [X]

……. Off system projects Culverts FHWA policy is “hydraulically

same or slightly better” Bridges

6 – Design Flood and Check Flood Standards

7 – Selection of the Appropriate Method for Calculating Runoff

TxDOT has no standard method

You are the Engineer

8 – Validation of Results from the Chosen Method

“Design flows estimated with any method should be confirmed and validated”

VALIDATE YOUR RESULTS

Technical Guidance Sections

9 – Statistical Analysis of Stream Gauge Data

Improved, not new, discussion Focus on Bulletin #17B, with some Texas-

specific refinements: Data Requirements Log-Pearson type III distribution fitting

procedure Weighted skew value Accommodating outliers Transposing gauge analysis results

10 – Regional Regression Method

Old vs. New

96 equations replaced by 9 equations More recent gage data and statistical

methods used Expected to be superior and have less

potential bias and error Results should still be compared against

other methods Valid for small watersheds – down to 1

square mile

10 – Regional Regression Method

10 – Regional Regression Method

OmegaEM and Annual Precipitation Maps

10 – Regional Regression Method

These equations were developed for natural basins.

These equations are therefore not

applicable to urban watersheds.

11 – Time of Concentration

Kerby-Kirpich Method

tc = tov + tch

where: tov = overland flow time (Kerby equation) tch = channel flow time (Kirpich equation)

Kerby-Kirpich Method

The Kerby Method (overland flow)

tov = K(LxN)0.467S-0.235

The Kirpich Method (channel flow)

tch = KL0.770S-0.385

12 – Rational Method Equation

Q = CIA

I = Pd/tc where: Pd = Depth of rainfall (in) tc = time of concentration (hr)

New source of Pd ….

New Precipitation Maps

13 – Hydrograph Method

Storm depth Areal depth adjustment Texas storm hyetograph development

procedure Texas initial and constant-rate loss model Climactic adjustment to CN Other new discussion

Other New Discussion Green and Ampt loss model Unit Hydrograph (UH) model

Snyder’s NRCS

Kinematic wave model Hydrograph Routing

Modified Puls Muskingum Muskingum-Cunge

Storage Routing

Questions?