Benefit of Gathering Site-Specific Evidence In Developing Engineering Solutions to Climate Change Events
March 2016
Adeniyi Aje Ahmad Khattab
β’ The Context
β’ Case Study β Somerset 2013/14
β’ Typical Assessment Approach
β’ New Assessment Approach
β’ Validation of Approach
β’ Future Use & Benefits
β’ Limitations
β’ Questions & Answers
β’ Our Climate is changing with time
β’ Our Environment is also changing with time
β’ EA updates β Feb 2016
a) Increase in intensity/frequency of rainfall β increased flow rate.
Key Parameter (affecting structures along walkways)
b) Discharge Flow Rate, Q = Velocity, V x Area, A
Constant (Channel Section)
c) Climate Change β Scour in waterways around existing structures d) Factors that affect velocity and may consequently lead to scour:
β’ Upstream and downstream ground conditions β’ Laminar or turbulent flow β’ Discharge flow rate β’ Catchment run-off vs. Percolation β’ Channel alignment β’ Bed roughness β’ Fluid Viscosity β’ Obstructions β’ Bed slope
(Somerset) Catchment Area River Parrett
King Sedgemoor Drain
Dunball New Bridge* Dunball Old Bridge
Sluice Gate
Sluice Gate
Dunball New Bridge* Dunball Old Bridge
Open Sluice Gate
Old Abutment
Flow around Structures (Piers)
Residual Structure
Flow Direction
Displaced Rock = Hydraulic Jump
Concrete Invert Slab
Narrow Channel = Increased Velocity
Bed Conditions Slope
Approach Formulae Design Velocity
Continuity Equation for Rectangular Open Channels based on approximate channel geometry
πΈπΈ = π½π½π½π½ = (ππ.ππππ ) Γ π½π½ Γ (πΉπΉππ/ππ)(πΊπΊππ/ππ) 8.0 m/s
Critical Velocity for Live bed scour (HEC 18 β Evaluation of Scour at
Bridges) π½π½ππ = (π²π²ππ) Γ (ππππ/ππ) Γ (π π ) 4.6 m/s
Supercritical Flow back-analysis for rectangular channels at critical
bed velocity ππππ =
π½π½ππππππππ
> ππ 5.6 m/s
Single Pier Scour Hole Back- Analysis (HEC 18)
ππππππππ
= ππ.ππ Γ π²π²ππ Γ π²π²ππ Γ π²π²ππ Γππππππ
ππ.ππππ
Γ ππππππ.ππππ 10.2 m/s
a) Info Received β 1D Hydraulic Model (V=2.1m/s) β Scour Hole Depth β Upstream Flow Rate, Qβ70m3/s
b) Due Diligence/Check β Existing Formulae β Engineering Judgement
β Observations (Site Conditions)
Tools
a) Hydraulic Model Results:
b) Further Due Diligence/Checks β New Approach Required.
Approach Estimated Design Velocity 1D Hydraulic Modelling 2.1 m/s
2D Hydraulic Modelling 3 to 4.5 m/s
Change in Bed Conditions
Angle of Current Attack
Influence of Bridge Piers
Effect of Residual Structure/ Abutment
Mismatch in results β model velocities relatively low. Effects listed above not considered.
β’ Diving Inspection Report
β’ Flood Event Videos - YouTube
β’ Aftermath Photos
β’ As-Built Drawings
β’ Bathymetric Surveys
β’ Observed Displaced Rocks (size)
a) What is actually happening? What have we observed?
b) Non-conservation of Energy
Vertical Translation Velocity Horizontal Translation Velocity
π£π£β =(ππππππππππ β πππ€π€π€π€π€π€π€π€ππ) Γ ππ Γ πππ£π£
(3 Γ πΆπΆπ·π· Γ ππβ4 Γ ππππππππππ
+ πͺπͺππ) Γ πππ€π€π€π€π€π€π€π€ππ
12 π£π£π£π£ =
8 Γ (ππππππππππ β πππ€π€π€π€π€π€π€π€ππ) Γ ππππππππππ Γ ππ3 Γ πͺπͺπ³π³ Γ πππ€π€π€π€π€π€π€π€ππ
12
πππππππππππ€π€πππ€π€ππ + πΎπΎπππππππππ€π€πππ€π€ππ + πΎπΎπΎπΎπππΎπΎ π«π«πΎπΎπππ«π« = πππππππππππ€π€ππ + πΎπΎπππππππππ€π€ππ
Scour Deposition
FD FI FB
FL
FG
(vv+vh)
β’ FG = Gravity (weight) Force β’ FL = Lift Force β’ FB = Buoyant Force β’ FD = Drag Force β’ FI = Inertia Force
a) Stone Γ = 0.2m b) CL = 0.25 β 0.35? c) CD = 0.47
Trends correlate with those on graph which are based on intensive experimentation
Approach Design Velocity 1D Model 2.1 m/s
2D Model 3 to 4.5 m/s
New Approach 5.4 m/s to 6.6 m/s
Velocity Forces on Submerged Rocks Design Chart
CL = 0.35
CL = 0.25
CL = 1.29
π£π£β =(ππππππππππ β πππ€π€π€π€π€π€π€π€ππ) Γ ππ Γ
(3 Γ πΆπΆπ·π· Γ4 Γ + πΆπΆππ) Γ πππ€π€π€π€π€π€π€π€ππ
12
π£π£π£π£ =8 Γ (ππππππππππ β πππ€π€π€π€π€π€π€π€ππ) Γ Γ ππ
3 Γ Γ πππ€π€π€π€π€π€π€π€ππ
12
a) Removes the complexity of upstream effects (~7m/s) b) Based on a limited number of variables c) Uses site-specific information and engineering judgement d) Can be used in preliminary design as a simple hand calculation e) Can we use this equation to size rock (i.e. Riprap) protection?
i.e. V = known CD = known
CL = ? Displacement β 0
Stone Size =