Hydrologic%20Design%20and%20Design%20Storms - PowerPoint PPT Presentation

View by Category
About This Presentation
Title:

Hydrologic%20Design%20and%20Design%20Storms

Description:

Readings: Applied Hydrology Sections 13.1-13.2 ... costs, benefits and does the risk analysis Planning Methodology Identify potential project components ... – PowerPoint PPT presentation

Number of Views:701
Avg rating:3.0/5.0
Slides: 51
Provided by: maid8
Learn more at: http://www.ce.utexas.edu
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Hydrologic%20Design%20and%20Design%20Storms


1
Hydrologic Design and Design Storms
04/18/2005
  • Readings Applied Hydrology Sections 13.1-13.2

2
Hydrologic extremes

















  • Extreme events
  • Floods
  • Droughts
  • Magnitude of extreme events is related to their
    frequency of occurrence
  • The objective of frequency analysis is to relate
    the magnitude of events to their frequency of
    occurrence through probability distribution
  • It is assumed the events (data) are independent
    and come from identical distribution

3
Hydrologic design
  • Water control
  • Peak flows, erosion, pollution, etc.
  • Water management
  • Domestic and industrial use, irrigation, instream
    flows, etc
  • Tasks
  • Determine design inflow
  • Route the design inflow
  • Find the output
  • check if it is sufficient to meet the demands
    (for management)
  • Check if the outflow is at safe level (for
    control)

4
Hydrologic design scale
  • Hydrologic design scale range in magnitude of
    the design variable within which a value must be
    selected
  • Design considerations
  • Safety
  • Cost
  • Do not design small structures for large peak
    values (not cost effective)
  • Do not design large structures for small peak
    values (unsafe)
  • Balance between safety and cost.

5
Estimated Limiting Value (ELV)
  • Lower limit on design value 0
  • Upper limit on design value ELV
  • ELV largest magnitude possible for a hydrologic
    event at a given location, based on the best
    available hydrologic information.
  • Length of record
  • Reliability of information
  • Accuracy of analysis
  • Probable Maximum Precipitation (PMP) / Probable
    Maximum Flood (PMF)

6
(No Transcript)
7
TxDOT Recommendations
8
Hydrologic design level
  • Hydrologic design level magnitude of the
    hydrologic event to be considered for the design
    or a structure or project.
  • Three approaches for determining design level
  • Empirical/probabilistic
  • Risk analysis
  • Hydroeconomic analysis

9
Empirical/Probabilitic
  • P(most extreme event of last N years will be
    exceeded once in next n years)
  • P(largest flood of last N years will be exceeded
    in nN years) 0.5
  • Drought lasting m years is worst in N year
    record. What is the probability that a worse
    drought will occur in next n years?
  • sequences of length m in N years N-m1
  • sequences of length m in n years n-m1

10
Example 13.2.1
  • If the critical drought of the record, as
    determined from 40 yrs of data, lasted 5 yrs,
    what is the chance that a more severe drought
    will occur during the next 20 yrs?
  • Solution
  • N 40, m 5 and n 20

11
Risk Analysis
  • Uncertainty in hydrology
  • Inherent - stochastic nature of hydrologic
    phenomena
  • Model approximations in equations
  • Parameter estimation of coefficients in
    equations
  • Consideration of Risk
  • Structure may fail if event exceeds Tyear design
    magnitude
  • R P(event occurs at least once in n years)
  • Natural inherent risk of failure

12
Example 13.2.2
  • Expected life of culvert 10 yrs
  • Acceptable risk of 10 for the culvert capacity
  • Find the design return period
  • What is the chance that the culvert designed for
    an event of 95 yr return period will not have its
    capacity exceeded for 50 yrs?

The risk associated with failure of culvert when
the flow exceed 95 yr flood in the next 95 years
is
The chance that the capacity will not be exceeded
during the next 50 yrs is 1-0.41 0.59
13
Hydroeconomic Analysis
  • Probability distribution of hydrologic event and
    damage associated with its occurrence are known
  • As the design period increases, capital cost
    increases, but the cost associated with expected
    damages decreases.
  • In hydroeconomic analysis, find return period
    that has minimum total (capital damage) cost.

14
(No Transcript)
15
Beargrass Creek Case Study
  • Description of the Study Area
  • Hydrology Hydraulics
  • Economic Analysis
  • Project Planning
  • Assessment of the Risk Based Analysis Methodology

From Risk Analysis and Uncertainty in Flood
Damage Reduction Studies, NRC Report
http//www.nap.edu/catalog.php?record_id9971
16
Beargrass Creek Study Area
Ohio River
North Fork
Middle Fork
South Fork
61 mi2 Drainage Area
Buechel Br
17
Levee on the Ohio River
18
Pump Station at the Levee(Capacity 7800 cfs!)
19
Concrete-Lined Channel
20
Detention Pond
Inlet Weir
21
Beargrass Creek at the Detention Pond
Pond Outlet Pipe
22
Damage Reaches
South Fork Beargrass Creek (12 miles)
1
15
14
13
Example Reach SF-9
12
11
2
10
Buechel Branch (2.2 miles)
5
9
3
4
7
6
4
8
3
2
5
1
23
Beargrass Creek Case Study
  • Description of the Study Area
  • Hydrology Hydraulics
  • Economic Analysis
  • Project Planning
  • Assessment of the Risk Based Analysis Methodology

24
Flood Frequency Curve (SF-9)Separate curve for
each reach and each plan
25
Uncertainty in Frequency CurveReach SF-9,
Without Plan Conditions

Prob Mean (cfs) Mean 2 SD Mean -2 SD Log10 (SD)
0.01 4310 3008 6176 0.0781
0.5 1220 1098 1356 0.0229
26
Water Surface Profiles
South Fork Beargrass Creek (202 cross-sects)
1
15
14
13
12
11
2
Buechel Branch (61 cross-sects)
10
5
9
3
4
7
6
4
8
3
2
5
1
27
Water Surface Profiles
28
Uncertainty in Stage-Discharge
Constant
Reduces prop. to depth
SD 0.5 ft at 100 yr flow
29
Beargrass Creek Case Study
  • Description of the Study Area
  • Hydrology Hydraulics
  • Economic Analysis
  • Project Planning
  • Assessment of the Risk Based Analysis Methodology

30
Computation of Expected Annual Damage (EAD)
Discharge (Q)
Discharge (Q)
Exceedance Probability (p)
Stage (H)
Damage (D)
Damage (D)
Stage (H)
Exceedance Probability (p)
31
Damage Categories
  • Single-family residential
  • Multi-family residential
  • Commercial buildings
  • Public buildings
  • Automobiles
  • Cemeteries
  • Traffic disruption
  • Utilities

32
Structures
p0.002
p0.01
p0.1
p0.999
33
Index Location
  • Each damage reach has an index location
  • All structures are assumed to exist there
  • First floor elevation adjusted to reflect the
    change in location within the reach

p0.01
Index for SF-9
p0.1
p0.5
Invert
Rm 10.363
Rm 10.124
Rm 9.960
34
Building Damage
  • Value of the structure, V
  • Value of the contents, C kV
  • kV/C, contents to value ratio (40)
  • Damage is a function of depth of flooding,
    expressed as ratio,r(h), of value

Depth, h r1(h) r2(h)
3ft 27 35
6ft 40 45
35
Uncertainty in Building Damage
  • Value of structure,
  • SD10 of V for residential
  • Commercial distribution described by
  • Value of contents (SD of k in CkV)
  • Uncertainty in first floor elevation, SD0.2ft
  • Uncertainty in damage ratios, r(h)

36
Stage-Damage Curve
Multi-family Residential, Reach SF-9
37
Stage-Damage Curves
  • Each structure is treated individually
  • Stage-damage curve with uncertainty is produced
    for each damage category for each reach
  • Added together to give the total stage-damage
    curve for the reach(?)

38
Beargrass Creek Case Study
  • Description of the Study Area
  • Hydrology Hydraulics
  • Economic Analysis
  • Project Planning
  • Assessment of the Risk Based Analysis Methodology

39
Planning Team
  • Three key people
  • Planner formulates project alternatives, works
    with local sponsor
  • Hydraulic Engineer determines discharge and
    stage data
  • Economist estimates damage, costs, benefits and
    does the risk analysis

40
Planning Methodology
  • Identify potential project components (detention
    ponds, levees, )
  • 22 initially proposed, 11 on Beargrass Creek, and
    11 on Buechel Branch
  • Evaluate them all individually to see if net
    benefits are positive
  • 8 components on Buechel Branch eliminated
  • Combine components into plans, incrementally
  • 10 components in NED plan 8 detention ponds,
    1 floodwall, 1 channel improvement

41
Three Plan Development Reaches
1
1
15
14
13
2
12
11
2
10
3
5
9
3
Buechel Branch
4
7
6
4
8
3
2
5
1
42
Risk of Flooding
  • Establish a target stage at each damage reach
    index point
  • Find annual probability of exceeding that stage
  • Find reliability of passing design floods

Target Stage
43
Assessment of Engineering Risk
F(h)
  • Conditional probability
  • Assumes a particular flood severity
  • Annual probability
  • Integrates over all flood severities
  • Risk measures actually used
  • Annual exceedance probability
  • Conditional nonexceedance probability

1
Exceedance probability
Nonexceedance probability
0
H
Target Stage
44
Computation of Engineering Risk Measuresfrom
the Stage-Frequency Curve
Q
H
H
Q
Target Stage
H
f1(Qp)
H
f2(HQ)
f3(Hp)
p
p
Q
pe
p
p
Q
  • Annual exceedance probability
  • Find pe for target stage at each Monte Carlo
    replicate
  • Get expected value and median of pe values over
    all simulations
  • Get long term risk as 1-(1-pe)n
  • Conditional nonexceedance probability
  • Find H for given p at each replicate
  • Find of replicates for which H lt Target stage

45
(No Transcript)
46
Beargrass Creek Case Study
  • Description of the Study Area
  • Hydrology Hydraulics
  • Economic Analysis
  • Project Planning
  • Assessment of the Risk Based Analysis Methodology

47
Overall Assessment
  • The core methodology is solid and is an advance
    in engineering practice of flood risk assessment
  • Focus is completely on damage reaches considered
    as statistically independent entities
  • Whole project risk and 25,50,75 damage values
    cannot be built up this way
  • Can specification of standard deviations of
    analysis variables be improved?

48
Beargrass Creek 100 year Flood Plain Map
Middle Fork
South Fork
49
Spatial Subdivision of the Region
Spatial Unit Used for
Whole River Expected Annual Damage (EAD), Benefit-Cost analysis
3 Main River Reaches Incremental analysis to get NED plan
22 Damage Reaches Basic unit for analysis using HEC-FDA
263 Hydraulic Cross-sections Water surface elevation profile computation
2150 Structures Structure inventory
50
Whole Project Risk Assessment
  • Take a flood of severity, p, and integrate the
    damage along the reach
  • Without any plan (o)
  • With a plan (w)
  • Benefit of plan is B Do - Dw
  • Randomize the flood discharge and stage for the
    whole project rather than for each reach
  • Compute project-based damage values for each
    randomization and use them to get B25, B75 values
About PowerShow.com