Benefit of Gathering Site-Specific Evidence In Developing Engineering Solutions to Climate Change Events

March 2016

Adeniyi Aje Ahmad Khattab

• The Context

• Case Study – Somerset 2013/14

• Typical Assessment Approach

• New Assessment Approach

• Validation of Approach

• Future Use & Benefits

• Limitations

• Questions & Answers

• Our Climate is changing with time

• Our Environment is also changing with time

• EA updates – Feb 2016

a) Increase in intensity/frequency of rainfall → 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 → Scour in waterways around existing structures d) Factors that affect velocity and may consequently lead to scour:

• Upstream and downstream ground conditions • Laminar or turbulent flow • Discharge flow rate • Catchment run-off vs. Percolation • Channel alignment • Bed roughness • Fluid Viscosity • Obstructions • 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

𝑸𝑸 = 𝑽𝑽𝑽𝑽 = (𝟏𝟏.𝟎𝟎𝒏𝒏 ) × 𝑽𝑽 × (𝑹𝑹𝟐𝟐/𝟑𝟑)(𝑺𝑺𝟏𝟏/𝟐𝟐) 8.0 m/s

Critical Velocity for Live bed scour (HEC 18 – Evaluation of Scour at

Bridges) 𝑽𝑽𝒄𝒄 = (𝑲𝑲𝒖𝒖) × (𝒚𝒚𝟏𝟏/𝟔𝟔) × (𝒅𝒅) 4.6 m/s

Supercritical Flow back-analysis for rectangular channels at critical

bed velocity 𝑭𝑭𝑭𝑭 =

𝑽𝑽𝟐𝟐𝒈𝒈𝒚𝒚𝟐𝟐

> 𝟏𝟏 5.6 m/s

Single Pier Scour Hole Back- Analysis (HEC 18)

𝒚𝒚𝒔𝒔𝒚𝒚𝟏𝟏

= 𝟐𝟐.𝟎𝟎 × 𝑲𝑲𝟏𝟏 × 𝑲𝑲𝟐𝟐 × 𝑲𝑲𝟑𝟑 ×𝒂𝒂𝒚𝒚𝟏𝟏

𝟎𝟎.𝟔𝟔𝟔𝟔

× 𝑭𝑭𝑭𝑭𝟎𝟎.𝟒𝟒𝟑𝟑 10.2 m/s

a) Info Received → 1D Hydraulic Model (V=2.1m/s) → Scour Hole Depth → Upstream Flow Rate, Q≈70m3/s

b) Due Diligence/Check → Existing Formulae → Engineering Judgement

→ Observations (Site Conditions)

Tools

a) Hydraulic Model Results:

b) Further Due Diligence/Checks → 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 → model velocities relatively low. Effects listed above not considered.

• Diving Inspection Report

• Flood Event Videos - YouTube

• Aftermath Photos

• As-Built Drawings

• Bathymetric Surveys

• Observed Displaced Rocks (size)

a) What is actually happening? What have we observed?

b) Non-conservation of Energy

Vertical Translation Velocity Horizontal Translation Velocity

𝑣𝑣ℎ =(𝜌𝜌𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 − 𝜌𝜌𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑟𝑟) × 𝑔𝑔 × 𝑑𝑑𝑣𝑣

(3 × 𝐶𝐶𝐷𝐷 × 𝑑𝑑ℎ4 × 𝑑𝑑𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟

+ 𝑪𝑪𝒎𝒎) × 𝜌𝜌𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑟𝑟

12 𝑣𝑣𝑣𝑣 =

8 × (𝜌𝜌𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 − 𝜌𝜌𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑟𝑟) × 𝑑𝑑𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 × 𝑔𝑔3 × 𝑪𝑪𝑳𝑳 × 𝜌𝜌𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑟𝑟

12

𝑃𝑃𝑃𝑃𝑖𝑖𝑖𝑖𝑖𝑖𝑤𝑤𝑖𝑖𝑤𝑤𝑖𝑖 + 𝐾𝐾𝑃𝑃𝑖𝑖𝑖𝑖𝑖𝑖𝑤𝑤𝑖𝑖𝑤𝑤𝑖𝑖 + 𝑾𝑾𝑾𝑾𝑭𝑭𝑾𝑾 𝑫𝑫𝑾𝑾𝒏𝒏𝑫𝑫 = 𝑃𝑃𝑃𝑃𝑓𝑓𝑖𝑖𝑖𝑖𝑤𝑤𝑖𝑖 + 𝐾𝐾𝑃𝑃𝑓𝑓𝑖𝑖𝑖𝑖𝑤𝑤𝑖𝑖

Scour Deposition

FD FI FB

FL

FG

(vv+vh)

• FG = Gravity (weight) Force • FL = Lift Force • FB = Buoyant Force • FD = Drag Force • FI = Inertia Force

a) Stone Ø = 0.2m b) CL = 0.25 – 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

𝑣𝑣ℎ =(𝜌𝜌𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 − 𝜌𝜌𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑟𝑟) × 𝑔𝑔 ×

(3 × 𝐶𝐶𝐷𝐷 ×4 × + 𝐶𝐶𝑚𝑚) × 𝜌𝜌𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑟𝑟

12

𝑣𝑣𝑣𝑣 =8 × (𝜌𝜌𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 − 𝜌𝜌𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑟𝑟) × × 𝑔𝑔

3 × × 𝜌𝜌𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑟𝑟

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 ≈ 0

Stone Size =