Local Problems – Local Solutions Using Attenuation Ponds, Plate

Weirs and Leaky Dams to Manage Flood Risk

By Stuart Bradshaw

BSc(Hons) MSc DIC CEng MIStructE MASCE FGS. Consulting Engineer Terrain Geotechnical

Email: info@terraingeotech.com Web: www.terraingeotech.com

What can we do to prevent this?

Case Study - Oldroyd, Todmorden

Oldroyd - The Problem

• Oldroyd sits at the base of a steeply sloping hillside the catchment above it is around 4.5 ha, about 1.0 ha is steeply sloping with poor infiltration rates

• In heavy rainfall water runs off the slope and is funnelled towards the terrace cottages flooding them and races down the access road in a torrent

• Flow rates of 250 litres/second estimated on Boxing Day on the access road

Calderdale Repair and Renew Grant Scheme

• Under the above scheme each affected property can apply for a grant of up to £5000 incl. VAT

• With five properties eligible to claim the budget is £25000 incl. VAT

• Initial (householder) solution was to pipe all the water away to a brook around 300 metres away

• Flowrates suggest that 500 mm diameter pipes were required to do this, cost £20000

Oldroyd - Proposed Solution

• Excavate a ditch to intercept water before it reaches the terrace

• Run the water through a concrete channel designed to pick up water running down the access road

• Run the channel into an attenuation pond to store water during the storm

• Water drains from the pond through a 150 mm diameter pipe to run in a channel down the access road, the same route it would have otherwise taken

What are the economics of attenuation ponds?

• First lesson in civil engineering, earthworks = cheap, concrete and steel = expensive

• Small ponds near brooks or culverts can be created in a day with a man and a mini excavator

• Land use, ponds are normally dry therefore the land can still be used for grazing

• Embankment sizes can be kept to a minimum by excavating soil and creating a depression below existing ground levels

Downstream Benefits – “Slow the Flow”

• The pond allows water to slowly disperse downhill so that the water arrives at the main river slower than it otherwise would have done

• If attenuation ponds were installed across the catchment the benefits would accrue rapidly, significantly reducing the flood risk to the main valley

Re-useable mill ponds - ready made attenuation ponds at Crimsworth

Three ponds in total linked by leats. Weir, overflow structures, aqueduct, penstocks, all intact

Infilled aqueduct Penstock on inlet culvert

Squashing the curve

Pickering Main Bund

Pickering Main Bund

Plate weirs, what are they?

• A plate or board installed across a water channel that interrupts the steady flow of water

• Common feature, they contain an opening for water to pass through

• Allow water to build up behind forming a small attenuation pond, thus slowing the flow

Several types are available

• V notch

• Rectangular notch

• Inverted V notch

• Letterbox

• Orifice

16

“Moors for the Future” – modelling different weirs (gully block designs), to test effect on reducing and delaying peak flows …

Simple, full width board

Slot weir

Inverted V-notch weir

Letterbox weir

Porthole weir

V-notch weir

Almost no

effect!

Weak to moderate

effect

Moderate to strong

effect

Strong effect

Not modelled but likely to have a strong effect v.

similar to letterbox design

Where can they be installed?

• On brooks or ditches, larger versions possible on small streams

• Ideally look for watercourses with deep channels, preferably with sloping soil sides

• Ideally look for a stretch where the water course follows the contours, or crosses them obliquely, a shallower gradient results in a larger attenuation pond

• Ideally look for a stretch where several can be installed to form a cascade

What kind of slot is best?

• Some research already undertaken suggests the letterbox slot works the best, this is simply a rectangular slot cut in the weir plate

• A series of circular holes is also a possibility, these can be cut in-situ with a cordless drill fitted with a hole boring tool

What size slot do I need?

• It all depends on the flow rates, too small a slot will result in the weir pond filling up in low normal flows, this is not desirable

• Too large a slot will allow all the water even in high flows to pass through

•Experiment to find the correct slot size by trial and error

Practical tips

• Try a 20 or 40 mm deep slot about half the width of the channel as your workshop installed slot, if this is too large you can always make it smaller easily by bolting a short plate onto the weir face, you can then adjust the plate left or right to alter the width of the slot

• If your weir slot is too small, boring holes is easier than trying to cut in-situ rectangular holes with a hand saw, use a cordless drill with a hole boring tool

A 20 mm deep x 50 mm wide slot

Pond to rear

Slot at bottom of plate

Weir plates are they feasible throughout the catchment?

• Better and more permanent weir plates than these experimental ones are still constructible at low cost, piping in these could be minimised

• Occasional maintenance to clear the slots of debris and silting up could be carried out by landowners via an enhanced wayleave payment system

• Weir plates can be clad with natural stone to blend them into the environment, although in many places they are small enough not to be noticeable, they could even be flagstones

Type 1 – “Fence Panel”

Results, 20 mm deep x 50 mm wide slot weirs

Date / Depth of Flow

Inflow (l/s) Outflow Weir 4 (l/s)

Outflow Weir 5 (l/s)

Weir 4 % Reduction

Weir 5 % Reduction

28/01/16 1.31 0.20 0.43 84.5 67.4

DOWOC (mm) 355 325

29/01/16 1.37 0.81 0.61 40.6 55.2

DOWOC (mm) 320 315

01/02/16 1.38 0.84 1.23 39.4 10.9

DOWOC (mm) 330 260

Weir cascade in low flow

Weir 3 over-topping

Leaky Dams

• Similar to plate weirs the intent is to slow the flow by creating ponds in the river channel

• Over bank flow, slower flow than channel flow is also a feature possible with leaky dams

• Leaky dams can be constructed from timber logs or using large rocks

Are they possible on the Calder catchment?

Crimsworth Dean

Calder Catchment Flood Studies Network

Core Objectives

• To identify suitable sites on the Calder catchment that would benefit from intervention with a view to slowing the flow and hence reducing flood risk downstream.

• To investigate property locations on the higher catchment that have been affected by flood, attempt to understand the cause with a view to proposing possible intervention solutions.

• To attempt to quantify by either calculation or by experimental means the perceived benefits of any intervention measures for each potential site identified at 1) and 2) above.

• To cost intervention solutions at chosen sites to develop cost/benefit ratio against the cost of repairing real or perceived damage caused by flooding.

• To investigate via effective flood modelling the effects of piecemeal interventions to determine their effects to ensure that they do not increase flood risk at the catchment scale.

• To apply for funding to implement intervention measures at selected sites.

• To promote the ethos of Natural Flood Management and Slowing the Flow and their roles within the catchment with the public at large.

• To lobby government departments, public bodies, landowners and other interested parties to bolster support for Natural Flood Management adopting an evidence based approach.

• To garner support from landowners and stakeholders with a view to obtaining permissions to undertake interventions.

• To assist The Source Partnership with technical aspects of NFM.

Links

• https://www.linkedin.com/pulse/pound-could-simple-weir-save-our-uplands-from-future-stuart-bradshaw?trk=prof-post

• https://www.linkedin.com/pulse/flood-compensation-time-re-think-how-community-paid-off-bradshaw?trk=prof-post

• https://www.linkedin.com/pulse/flood-alleviation-scheme-upper-calder-valley-other-pennine-bradshaw?trk=mp-reader-card