An experimental analysis of the influence of screen geometry and position on the performance of trash

screens at culvert inlets.

Janice Blanc, Scott Arthur & Grant Wright

• Material delivered into the river system includes natural

(organic) debris

The problem

• Culverts represent pinch-points in the river system which often

have trash screens installed to prevent internal blockage.

• However these can be a flood hazard in themselves if not

cleared and maintained at an appropriate inspection interval.

and also anthropogenic trash.

Braid Burn – Edinburgh, Scotland, UK.

[5]

Debris composition – Belfast, UK.

Small

Vegetation

Domestic

Refuse

Large Household

Waste Large Non-Domestic Refuse

Large

Veg.

misc

Culvert Design and Operation

Guide

(CDOG) (CIRIA, 2010)

<commercial publication>

Current guidance

Trash and Security Screen Guide

(TSSG) (EA, 2009)

<available EA website>

Estimating blockage extent

Current best-practice methodology for estimating

potential screen blockage (CDOG and TSSG).

Field Work - Belfast

• 495 Trash Screens – Filtered to 140 with reliable data.

• Inspected on average twice per month.

• Data available: April 2002 - October 2008.

• Screen % area blocked and blockage material type

recorded.

Data-set obtained from Northern Ireland Rivers Agency for Belfast Region

Probability of Blockage at a Screen (Pb) ≈ Frequency of blockage events / # Observations

Annual frequency of

blockage varies

between <2% and

>70%

Therefore some sites

are virtually never

blocked; some are

more than likely to be

blocked

Average Annual Frequency of Blockage by Screen

Probability Equations from Empirical Analysis

Scheme for decision support

calculate SAb using

appropriate equation

compute increase in afflux

determine change in

inundation extent risk not acceptable

reduce debris load

increase

screen size

alter screen

properties A / S

calculate clear screen

area:

SAc = SAt - SAb

select screen

equation from

statistical analysis

risk not acceptable

modify screen

risk not acceptable

reduce debris load

source control risk acceptable

no action

Field Work - Edinburgh

Deonie Allen

Experiment design and methodology

Experiment design and methodology

0

10

20

30

40

50

60

1 2 3

Nu

mb

er

of

succ

ess

fully

tra

nsp

ort

ed d

eb

ris

Monitoring locations (increasing in travel distance)

Dimensions of sucessfully transported debris

150, 10

150, 15

150, 25

350, 10

350, 25

350. 15

750, 10

750, 15

750, 25

Length, Diameter

Learn more – Room 602, 8:30am on Friday

Laboratory Work

Culvert inlet trash screens

Aims of the project

• To develop a robust physical model that would allow

investigation of trash screen performance.

• To identify which structural elements have an

influence on blockage potential.

• To define blockage potential in terms of influencing

components.

• To assess the impact of the findings on current

screen design guidelines.

Physical model

• Froude scaled model of

circular culvert

• Variable trash screen

arrangement

Key variables

screen angle

bar spacing

debris

length

flow discharge

and velocity

Key variables

distance of screen from culvert inlet

Test methodology

Main findings – bar spacing

Screen angle 60°

Discharge 0.035 m3/s

Main findings – screen angle

Discharge of

0.035 m3/s

Screen angles 60°

45°

30°

60°

30°

45°

Key variables

screen angle

bar spacing

debris

length

flow discharge

and velocity

Main findings – screen position

1

3

2

4

Main findings – screen position

Discharge 0.035 m3/s

Screen angle 60°

Distance of screen

from culvert inlet

0.10m (1)

0.15m (2)

0.25m (3)

0.35m (4)

1

3

2

4

Main findings Screen position

1

3

2

4

Discharge 0.035 m3/s, Screen angle 60°

Distance of screen from culvert inlet

0.10m (1), 0.15m (2)

0.25m (3), 0.35m (4)

Empirical relationship

Significant driving variables: bar spacing,

debris length,

screen position,

screen angle

Full empirical model

Simplified linear models

Offer an acceptable predictive capability

With debris length

Without debris length

End-user focused tools Nomograph Interactive spreadsheet

An analysis of the performance of trash screens at culvert inlets

The first major experimental assessment of trash screen performance

in hydraulic conditions specific to a culvert inlet has resulted in:

• The development of a novel methodology that can be used to assess

trash screen performance

• The identification of key elements of screen design that influence

blockage by debris

• The development of empirical relationships that define blockage in terms

of contributing factors

• The development of end-user focused tools to aid estimation of blockage

For further information please contact:

Janice Blanc jb201@hw.ac.uk

Thank You