Dudbridge Locks Hydro Scheme

Lesson Report

By Keith Tibbitts

Photo: Illustrative image for the 'Dudbridge Locks Hydro Scheme' page

Marling School Task Description 13.3.12

Flowrate Measurement Technique

Mark Seward from SDC/Halcrow and Sam Jeddere-Fisher, RenewablesFirst previously proposed an alternative to using the round drainage pipes and timing a float.

They suggested that we measure the height upstream of the existing broad-crested weir at Dudbridge Upper Lock using the formula:



q=1.5 x b x (H2)



q = flow rate (m3/sec)

b= width of weir(m)

H2 = height of water above weir crest(fix measuring staff at least 3 x H2 upstream).

eg) if depth of water over weir is 250mm fix level gauge 750mm upstream.

An alternative flow measurement technique could be considered in the intake duct by measuring average velocity using the SHS propeller instrument and multiply by cross sectional area. This could be checked by the students by making and fixing a graduated staff to the intake wall (near where the inlet screen will be fixed).

The students have been divided into 2 sub-groups as follows:

Group A

Produce a detailed fabrication drawing of the “Bellmouth” inlet based on the 3D cardboard/styrene models & CAD designs and compare alternative cost estimates for marine plywood/GRP, stainless steel or concrete.(or a hybrid).Choose most durable and cost-effective solution. Take dimensions from Halcrow drawings and Segen Final Report. Materials? Thickness? Concave or convex? If possible carry out laboratory scale model tests measuring pressure loss with different shapes. Contact steel fabricators & find local GRP fabricators.

Marling Technician to make full scale Bellmouth to previous 3D drawings in plywood.

Ask GT to quote for bellmouth cast in concrete using plywood version as shutter. Extend existing cardboard model to include bypass channel/fishpass inlet.


Group B

Flow measurement using either a propeller meter(borrowed from SHS). GT have already fixed a level staff to the existing canal wall upstream of the concrete “broad crested” weir at Dudbridge Upper Lock. Richard Scott is taking daily H2 measurements until work begins on the lock chamber refurbishment which can be checked weekly by the students each Tuesday.

These measurements could be checked by timing a float in the bund near Dudbridge Upper Lock or under the DudbridgeRoadBridge. This will need to take account of Ruscombe Brook.


Groups A & B can then work jointly to find the best permanent flow measurement technique to be installed in the inlet duct with perhaps a remote signal for continuous monitoring.

The same technique could be used to also check the 70l/s flowrate is achieved in the fishpass once it is constructed.

An important aspect of the turbine control system design requires an approximate measurement of the drop in water level when Dudbridge lock gates are operated. It is suggested that students go to Newtown Lock under CCT guidance to measure the change in level when paddle gear is operated & lock gates are opened to simulate boats passing through over say 15-20 minute periods and plotting the results.


Next Group B site visit at 3.45pm 13/3 sign in at Galliford Try site hut with suitable footwear!!


Keith Tibbitts

Project Manager

Dudbridge Locks Hydro Scheme

Cotswold Canals Trust

Tel 01453 860403


This page was added by Keith Tibbitts on 16/03/2012.

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