Chapter 4 "Hydrology"

Transcription

1 Chapter 4 "Hydrology" Revised TxDOT Hydraulics Manual Jorge A. Millan, P.E., CFM TxDOT Design Division/Hydraulics 2012 Transportation Short Course October 17, 2012

2 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 Procedural guidance Technical guidance

3 Procedural Guidance Sections

4 1 Hydrology s Role in Hydraulic Design Intuitive Definitions Basic Considerations

5 2 - Probability of Exceedance Annual Exceedance Probability (AEP) AEP (%) AEP (probability) Annual recurrence interval 50% year 20% year 10% year 4% year 2% year 1% year

6 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.

7 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)

8 5 - Hydrology Study Data Requirements Intuitive Explains the data required Definitions, descriptions, and examples

9 6 Design Flood and Check Flood Standards CHANGE Recommended Design Standards Table

10 6 Design Flood and Check Flood Principal arterials: 50% 20% 10% 4% 2% Culverts X [X] X Small bridges X [X] X Major river crossings. Off system projects Culverts Bridges Standards [X] FHWA policy is hydraulically same or slightly better

11 7 Selection of the Appropriate Method for Calculating Runoff TxDOT has no standard method You are the Engineer

12 8 Validation of Results from the Chosen Method Design flows estimated with any method should be confirmed and validated VALIDATE YOUR RESULTS

13 Technical Guidance Sections

14 9 Statistical Analysis of Stream Gauge Data Improved, not new, discussion Focus on Bulletin #17B, with some Texasspecific refinements: Data Requirements Log-Pearson type III distribution fitting procedure Weighted skew value Accommodating outliers Transposing gauge analysis results

15 10 Regional Regression Method Old vs. New

16 10 Regional Regression Method 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

17 10 Regional Regression Method

18 OmegaEM and Annual Precipitation Maps

19 10 Regional Regression Method These equations were developed for natural basins. These equations are therefore not applicable to urban watersheds.

20 11 Time of Concentration Kerby-Kirpich Method t c = t ov + t ch where: t ov = overland flow time (Kerby equation) t ch = channel flow time (Kirpich equation)

21 Kerby-Kirpich Method The Kerby Method (overland flow) t ov = K(LxN) S The Kirpich Method (channel flow) t ch = KL0.770S-0.385

22 12 Rational Method Equation Q = CIA I = P d /t c where: P d = Depth of rainfall (in) t c = time of concentration (hr) New source of P d.

23 New Precipitation Maps

24 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

25 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

26 Questions?