Catchment News

Significant Pressures: Hydromorphology

Hydromorphology is a relatively new discipline which is described in the Water Framework Directive. Hydromorphology is the study of physical form, condition and processes within a surface water body, that create and maintain habitat

It stems from the term ‘fluvial geomorphology’, a discipline that focuses on the processes that operate in, for example, a river system (e.g. both water and sediment production and movement, erosion, deposition), and the features that these processes create (e.g. pools, riffles, sediment bars). As these processes create and maintain such features, this in turn will create and maintain habitats for invertebrates, fish and plants.

Modification of the hydromorphological characteristics of surface waters is estimated to be a significant pressure in 345 (24%) of the 1,460 waterbodies that are At Risk of not meeting their water quality objectives. This includes 329 river water bodies, 10 lakes and six estuaries nationally (Table 1). It is the 2nd most prevalent significant pressure within surface water bodies.

Table 1: Number of At Risk water bodies with hydromorphological modification as a significant pressure.

Waterbody (WB)	Number of WBs	Number of At Risk WBs	Number of WBs with Hymo as a significant pressure	% of WBs with Hymo as a significant pressure	% of At Risk WBs with Hymo as a significant pressure
River	3192	1178	329	10.3%	27.9%
Lake	818	132	10	1.2%	7.6%
Transitional	195	56	6	3.1%	10.7%
Coastal	111	13	0	0%	0%
Groundwater	513	73	0	0%	0%
	4829	1452	345	7.1%	23.8%

What is hydromorphological modification?

Hydromorphological modification means change to the physical habitat, and/or a water bodies’ natural functioning caused by, for example, channelisation which is the dredging and straightening of rivers, land drainage, or hard infrastructure such as dams, weirs, barriers, locks, embankments, culverts, piers, ports and sea walls. A variety of issues cause hydromorphological modification to be a significant pressure (Table 2).

Status of water bodies impacted by hydromorphological modification

Half of water bodies impacted by hydromorphological modification are at Moderate status (Figure 2) and may not be too far away from the Good status target, but it is difficult to know at present what measures to take as the relationships between hydromorphology and ecological status are not clear cut. Further work is needed to achieve a greater understanding of the impacts from hydromorphological modification. It is anticipated that as our knowledge and understanding of hydromorphological pressures improves, the extent of the impacts identified across the country will change.

Improving hydromorphological assessment methods and tools

The EPA is currently developing the evidence base to identify the physical conditions necessary to support ecological status. This evidence base will facilitate the design of improved mitigation measures. A number of new assessment tools are also under development, as noted by the River Basin Management Plan 2018-2021.

The Morphological Quality Index (MQI), an Italian fluvial geomorphological assessment, was recommended as a best practice method by the EU funded FP7 REFORM project (www.reformrivers.eu) and is being adapted for use in Irish conditions. The assessment considers multiple scales, and assesses a river’s morphological condition and processes, and its response to physical pressures, allowing for the identification of significant pressures.

The MQI assessment has been trialled in the Suir catchment and is being rolled out nationally during 2018 and 2019. The output of this assessment will provide both an understanding of the hydromorphological condition of our river water bodies, the identification of significant hydromorphological pressures and support the identification of measures. The EPA has also enhanced the use of GIS for assessing the hydromorphology of lakes, transitional waters and coastal waters bodies, with the output contributing to our understanding of hydromorphology and supporting the identification of both significant hydromorphological pressures and measures.

Inland Fisheries Ireland (IFI) are identifying the location and extent of barriers (e.g. dams and weirs) along rivers which may be impacting on a range of migratory species. Barrier assessment tools are being developed, and a national barrier inventory is being created with barriers ranked according to the risk they pose to fish migration.

OPW and IFI have developed best practice guidance for the maintenance of channels within arterial drainage schemes to minimise the impact on water quality.

The Department of Housing Planning and Local Government is developing best practice technical planning guidance for managing physical modifications in rivers.

Find out more:

You can view the individual water bodies where hydromorphology is a significant pressure on www.catchments.ie/maps

For planning and policy, including measures, see www.bit.ly/rbmp20182021

Hydromorphological pressure	Example of impacts
Channelisation (e.g. widening, deepening, straightening, removal of in-channel obstructions (i.e. trees), removal of in-channel and riparian vegetation)	• Modification of the river bed, bank and riparian area. • Modification of the stream network. • Alteration of the natural flow and sediment regime. • Increasing the level of fine sediment (silt/clay) entering a river. • Degradation of physical habitat. • Disconnection to the floodplain.
Bank protection (e.g. walls, gabions, rip rap	• Impede the lateral movement (i.e. erosion) of a river.
Flood protection (e.g. walls, embankments)	• Impede the lateral movement (i.e. erosion) of the river. • Disconnection to the floodplain.
Dams, barriers, locks and weirs	• Impede the movement of water, sediment and aquatic species (notably fish). • Alteration of the natural flow and sediment regime.
Culverts	• Impede the movement of water, sediment and aquatic species (notably fish) along the river.
Land drainage	• Alteration of the natural flow and sediment regime due to the increase in connectivity of drains to the river network.

Table 2: Hydromorphological pressures affecting rivers and examples of associated impacts