Floods & Dredging: a reality check

Dredging can play an important role in flood risk management in some cases, but is not a standalone
solution. It should be considered in the context of a range of tools and the origins of different sources of flood water, and comes with significant risks that must be understood at a local and catchment scale.



The term dredging is routinely used to refer to the systematic removal of accumulated material from river or other watercourse channels. In its most extreme form dredging may be used to re-align river channels creating linear, canalised watercourses.

• It would be infeasible to dredge channels with the capacity to carry flood flows of the kind witnessed this winter (2013/14). However, there is significant evidence that dredging can increase channel conveyance, reducing water levels and small floods.
• This is borne out by studies of the Somerset Level and Moors system which suggest that the proposed dredge would have not prevented flooding but could significantly reduce the length of time water stands on the land.
• Numerous studies have pointed to significant unintended consequences of dredging, namely:
o Increasing flood risk for communities downstream by speeding up the movement of flood water through the river and drainage network.
o Destabilising river banks, causing erosion and so risking damage to infrastructure.
o Loss of wildlife and habitats both within the river and across the wider floodplain. These impacts can be significant and permanent.

• It is also important to note that dredging can be a conservation tool, particularly in heavily modified environments where natural processes that maintain ecosystem function are constrained.
• Flood risk management strategies should look to a range of interventions, and include action to reduce runoff by working with natural practices to slow water, and increase infiltration and storage throughout the catchment.
• Strategies will also need to manage the use of naturally flood prone land through a combination of behavioural and engineering options, including flood zoning, warning, changes in land use practices, as well as flood defence structures and operations.
• Land management lies at the heart of these strategies, so the design of farm subsidies and engagement with stakeholders, especially landowners, land managers and farmers is critical to flood risk management. The Catchment Based Approach provides a platform for this engagement.



Water Abstraction Data (UK overview)

Presentation on water abstraction and discharge data (begins at 58mins) and how it impacts on river flows

UK have point data on abstraction – but it’s size of licenced abstraction (not how much water is actually abstracted), if it’s from surface or groundwater abstraction, and what that abstracted water is used for (by sector, eg agricultural irrigation)

The UK data links the catchments to the abstractions

Don’t look at just the data of water taken out, but also need to look at ‘consumption’ – what’s discharged (put back in at same location)

UK data is online and easy to access

Ecosystem Accounting

The SEEA Ecosystem Accounting (SEEA EA) constitutes an integrated and comprehensive statistical framework for organizing data about habitats and landscapes, measuring the ecosystem services, tracking changes in ecosystem assets, and linking this information to economic and other human activity.

The United Nations Statistical Commission adopted the SEEA Ecosystem Accounting at its 52nd session in March 2021. The report of the Commission can be found here.

This adoption follows a comprehensive and inclusive process of detailed testing, consultation and revision. Today, ecosystem accounts have already been used to inform policy development in more than 34 countries.


Drivers of and pressures arising from selected key water management challenges: a European overview

EEA Report No 12/2021

This report aims to give a European overview of the main drivers and pressures that are at the core of key water management challenges and which put European water bodies most at risk of not achieving key environmental objectives.

Identifying the pressures from and drivers of key water management challenges at the European level can help in prioritising the main issues that should be tackled with measures.

Catchment management of metaldehyde (UK Pilot Project)

Anglian Water is coming to the end of five years of the Slug it Out trial. The Slug it Out trial is a payment for ecosystem services based approach trialled in key natural catchments from June 2015 until June 2020.

The Slug it Out trial focused solely on agricultural sources of metaldehyde and was designed based on the core tenet of both the “Appraisal of Policy options to Manage Pesticides” report (DEFRA WT0963) and the Metaldehyde Stewardship Groups “high risk fields” approach that regulatory compliant water (individual pesticide level below 0.1µg/l) can be achieved with less than complete exclusion of metaldehyde within a catchment.

The Slug it Out scheme was implemented in seven natural catchments around Anglian Water reservoirs. These were: Grafham, Pitsford, Hollowell, Ravensthorpe, Ardleigh, Alton, Rutland (from 2016/17 onward). A one year trial of the SiO scheme was carried out in the pumped Covenham 1 catchment in 2017/18. In Slug it Out catchments, farmers were paid to not use metaldehyde to control slugs. The amount the farmers were paid was based on the amount of arable land (hectares) that the farm had in the catchment plus a standard hosting fee. An additional water quality bonus was paid if the watercourse had no metaldehyde exceedances from catchment monitoring sampling points. Each catchment had a dedicated Anglian Water Catchment Advisor to engage with the farmers in that catchment and be the link between the Agricultural sector and the Water sector.

The implementation of SiO was successful in many ways and led to a reduction in metaldehyde levels in all natural catchments. In particular, no metaldehyde exceedances were recorded in the five year SiO period in the Grafham, Hollowell and Ravensthorpe Catchments or during the one year trial in the pumped Conveham 1 catchment. Other Catchments such as Alton took a year before not exceeding the pesticide limit while more complicated catchments (numerous additional sources) such as Ardleigh took two years. The implementation of the SiO scheme lead to an average of ~70% reduction in the number of metaldehyde exceedances seen in these catchments compared to the average number of metaldedye exceedances  before the SiO scheme was introduced.

As well as the reduction in the level of metaldehyde in natural catchments, SiO also had other benefits, for example farmers deciding to use cultural controls or ferric phosphate and not metaldehyde to control slugs on land outside of the Slug it Out scheme, thus reducing the overall metaldehyde burden from agriculture entering our raw waters.

The Slug it Out trial also demonstrated that positive and consistent communication between Catchment Advisor and farmers within the catchment – supported by local data is vitally important to success. Another key factor is engagement with local agronomists and other agricultural stakeholders who help disseminate and validate messages while strengthening the building trust of farmers. This itself has led to good two way communication channels benefiting both sectors.  Another important takeaway from the Slug it Out scheme is that providing farmers the financial backing to try something new can lead to positive changes after the payment ends or in areas where the payment does not apply – for example some changes and new ways of thinking benefit the farm business resilience anyway.

As part of the Slug Out scheme drip trays and cab stickers were handed out to all Slug it Out farmers across the seven Slug it Out catchments.


How Can Investment in Nature Close the Infrastructure Gap?

This report provides an estimate of how much nature-based infrastructure can save costs and create value relative to traditional grey infrastructure.

How does nature-based infrastructure (NBI) perform compared to grey infrastructure? How can NBI help to bridge the infrastructure gap? And what outcomes would emerge from shifting investments from built infrastructure to nature?

Building on this overarching question, this working paper aims to answer these questions by bringing together two elements of research: a literature review on infrastructure investment needs and the results from IISD’s Sustainable Asset Valuations (SAVi) of NBI projects.

This research shows that NBI provides the same services but is up to 50% cheaper than traditional infrastructure. In addition, NBI provides 28% better value for money.

It also shows that an infrastructure swap could create additional benefits worth up to USD 489 billion every year—a figure that rivals the annual GDP of countries such as Austria, Ireland, or Nigeria.

Quantifying the hydrological implications of pre- and post-installation willowed engineered log jams (Pennine Uplands, NW England)

Nature Based Solutions (NBS), including Natural Flood Management (NFM) schemes are becoming an important component of many governmental and organisation responses to increases in flood and aridity risk.

NFM structures may take multiple forms to slow, store, disconnect and filter distributed overland flowpathways within a catchment that coalesce to generate a flood-wave downstream and runoff rather than infiltrate groundwaters.

To date few studies have conducted observations pre- and post-installation monitoring at river reach-scales, despite widespread and frequent installation, to investigate the efficacy of willowed engineered log jams (WELJs) interventions used in abating flood-flows, through backing-up flood-pulses with consequent reductions in downstream discharges.

This paper examines the efficiency, before and after installation of five 1 m high WELJs incorporating 1,000 Bay willow (Salix pentandra) saplings supporting the dead horizontal timber, across a total of 130 linear metres spanning the floodplain of a decommissioned reservoir. One rain gauge, two fixed point time-lapse wildlife cameras and three water level stations were installed: upstream-of, within, and downstream-of all WELJs.

The findings demonstrate a substantial reduction is achieved for most events, with an average of 27.3% reduction in peak discharge being achieved post-installation. The time to peak is little impacted, however there is demonstrable evidence of a longer and higher recessional limb to the events. These findings quantify for the first time the role that WELJs can play in a move towards re-naturalisation of water level regimes, with lower peak water flows achieved, and waters released from the river-reach more slowly.

Furthermore, baseflow during dry periods is also elevated by 27.1%, offering greater resilience to dry periods and droughts. Consequently, over the river-reach scale (0–130 m), WELJs play an important role in alleviating flood and drought risk through suppressing flood peaks and increasing baseflow during low flows; steps towards improved hydro-morphological quality overall.


Willowed log jams (∼dams) have been installed frequently to reduce flood risk

Few studies have assessed pre- and post- installation changes to watercourse flows

Discharge data shows an average 27.3% reduction on peaks, following installation

River-reach (0–130 m) wildlife camera photos and levels confirm attenuation

Willowed log jams re-naturalise flows, locally alleviating floods and droughts.

Click to access pdfft