National Id
Site name
River Tolka
The River Tolka runs through Dublin, Ireland. A series of measures was applied to the urban sections of the river to slow flood flows, reduce pollution and support wildlife. These included the establishment of retention ponds to manage runoff storage; bank engineering to slow flows and prevent erosion; and planting trees along the river to slow runoff. Two phases of retention pond construction have been carried out, the latter as part of a wider ‘Greenway’ project to develop a green corridor with cycling route.
Light or indepth?
The in-depth description of the case study
Location description
The River Tolka is located in eastern Ireland. It flows eastwards through Dublin and in to the Irish sea. The improvement sites are located along the urban reaches of the Tolka, particularly at the Tolka Valley Park.
Southern and Eastern
RBD code
Data provider
Heather Williams, AMEC
Climate zone
cool temperate moist
Mean rainfall
Mean rainfall unit
Average temperature
Mean runoff
Mean runoff unit
450 - 600 mm
Average runoff coefficient
Case Study Info
Monitoring impacts effects
Monitoring parameters
There has been varied monitoring at the Tolka. Monitoring relevant to the NWRMs include ecology surveys (incl. macrophytes, amphibians, invertebrates), and water quality monitoring.
Performance impact estimation method
Catchment outlet
Performance impact estimation information
Performance of the retention ponds/ wetlands is monitored by measurements of water quality and observation of fauna and flora
Application scale
Installation date
Performance timescale
< 1 year
Area (ha)
18.2 ha
Area specifications
The total size of the Tolka Valley Park is 18.2 ha.
Constructed wetlands depend on suitable hydrological regime that allows permanent presence of water in them.
Favourable preconditions
Availability of parkland space prior to the project
Design contractual arrangement
Arrangement type Responsibility Role Comments Name
Design consultation activity
Activity stage Key issues Name Comments
Design land use change
Land use change type
Design authority
Authority type Role Responsibility Name Comments
Initiation of the measure
Dublin City Council
City Council
National Transport Authority
Funded Greenway
Tolka Trout Anglers
Volunteer work
Atkins Global
Key lessons
- Parks in urban areas can serve as biodiversity reserves and offer opportunities to contribute to achieving good ecological status of waterbodies under the WFD.
- Soft engineering techniques can be cost-effective and enhance the biodiversity potential of urban cactchments.
- Local community involvement is key to project success.
Success factor(s)
Success factor type Success factor role Comments
Attitude of relevant stakeholders
main factor
Attitude of the public
secondary factor
Financing type Comments
Barrier type Barrier role Comments
Driver type Driver role Comments
Balancing different objectives
main driver
There were multiple objectives to the improvements to the Tolka, relating to the WFD, Floods Directive, EU LIFE Project, "Greenway" development, recreational and access objectives in Dublin
Financing share
Financing share type Share Comments
Policy description
The Tolka has poor ecological status (WFD) due to diffuse pollution (urban runoff) and point pollution (waste disposal sites)
Part of wider plan
Policy target
Target purpose
Pollutants Removal
Peak-flow reduction
Improved Biodiversity
Oher Societal Benefits
Policy pressure
Pressure directive Relevant pressure
Policy area
Policy area type Policy area focus Name Comments
Policy impact
Impact directive Relevant impact
Policy wider plan
Wider plan type Wider plan focus Name Comments
Environment & Biodiversity
Renew4GPP (Ireland pilot project)
Control of invasive species
Policy requirement directive
Requirement directive Specification
Costs total
Costs total information
Full cost of the &#039;Greenway&#039; programme of which the ponds and wetlands were part
Compensations annual information
Assume none (land was already public parkland prior to implementation)
Information on Economic costs - income loss
Assume none (land was already public parkland prior to implementation)
Ecosystem improved biodiversity
Information on Ecosystem improved biodiversity
The ecology of the river has improved as a result of these measures and other improvements in the catchment. Otter and salmon have now returned to the Tolka catchment (altough NWRMs are only some of the actions that have contributed to this).
Ecosystem provisioning services
Information on Ecosystem provisioning services
Angling quality has improved as a result of these and other actions in the Tolka catchment.
Ecosystem impact climate regulation
No specific impact
Ecosystem erosion control
Water quality overall improvements
Positive impact-WQ improvement
Information on Water quality overall improvements
The wetland influences water quality on its own. In addition, barley straw bales were added to ponds in 2012 to prevent algal blooms (and can be assumed to have been successful, since there has been no evidence of the algae since).
Information on Ecosystem erosion control impact
Before and after photos of sediment capture measures reveal less exposed soil on river banks.
Wq Improvements n unit
% reduction pf pollutant
Information on Water quality Improvements (N)
The constructed integrated wetland achieved:
- 91% reduction in ammonia concentrations
- 16% reduction in nitrate concentrations
- 6.5% reduction in nitrite concentrations.
The addition of barley straw provided further benefits, reducing ammonia by 55% and nitrate by 38%
Information on Water quality Improvements (e.coli)
The addition of barley straw to the pond/weltand achieved a 99% reduction in e-coli concentrations
Soil quality overall soil improvements
N/A info
Information on Soil quality overall soil improvements
Some erosion control measures were implemented on the floodplain of the River Tolka. These measures may improve soil quality by retaining soils, but no specific information was available.