Flood Risk
Tidal surges and high spring tides.
As stated in the pre-draft consultation document, flood defences are in place in Waterford city and other flood prone areas. While these may need to be expanded, flood defences are only part of the solution when dealing with flood risk. It is vital that assessment be done on a whole-catchment level, since decisions made in upstream areas can have an impact on people living downstream. In Ireland, we have traditionally relied on hard engineering solutions, but there is a need to incorporate building with nature principles to engineering design. When developing flood defence schemes, nature based solutions should be considered first, while still acknowledging that more traditional concrete defences may ultimately be the preferred solution.
The document refers to planting of marram grass on sand dunes, and this is a welcome move in an effort to stabilise sand dunes. However, more radical features may be required.
Examples:
- Investigate the possibility of beach nourishment and measures similar to the Dutch Sand Engine. This is a large-scale beach nourishment project, constructed in 2011. Instead of replenishing sand on the South Holland coast every few years, twenty years’ worth of sand was mined from the North Sea and deposited on the vulnerable coast. This allowed flora and fauna to colonise the area, promoting biodiversity, and it also created a recreational space for humans.
The Rijkswaterstaat in the Netherlands has been collecting data on the Sand Motor, available here: https://dezandmotor.nl/en/ . There may be some learnings for Waterford in this project.
2. A solution similar to the Sand Engine is one of the solutions being considered for the Burrow beach in Portrane. Fingal County Council placed hexagonal concrete defences known as “sea bees” on the beach, but local residents claim they are not protecting sufficient protection. One house has already been lost to the sea.
3. The estimation of a 1 metre sea level rise by 2150 may prove to be too low. Unpalatable as it is, we may need to start considering managed retreat, since there could be some parts of the Waterford coastline that just cannot be protected. Plans are already in place in parts of Wales for managed retreat from the sea. Achieving this in a just manner will be an enormous undertaking. The sooner we start planning, the less difficult it will be for everyone.
Fluvial flooding
Flood alleviation should be planned on a catchment-wide basis, not in a piecemeal reactive manner.
In general, there is much that we can learn from the Netherlands when it comes to flood protection. The Room for the River is another very interesting series of projects. The Slow the Flow project in Pickering, Yorkshire is also a very interesting scheme, from which we can learn. We have to accept that it is natural for rivers to flood. We can build flood walls, but there is only so much protection that these walls can give. Moreover, moving flood risk from one area to another is not an ideal solution. Storage capacity throughout a river catchment needs to be identified.
One solution: leave space for the river to flood in an area that can temporarily store the water.
Examples:
- River Poddle Flood Alleviation Scheme – South Dublin County Council and Dublin City Council.
The River Poddle is heavily culverted as it flows through suburban and urban areas in South Dublin. It has overflowed its banks on several occasions, most recently in October 2011, when 90mm of rain within a 6 hour period, causing severe flooding in Crumlin and Harold’s Cross. The flood alleviation scheme includes an integrated constructed wetland area in Tymon Park, which can store water during storm events. This wetland will also improve water quality and serve as an amenity in the park.
- Nieuwe Driemanspolder N3MP
The Nieuwe Driemanspolder, a former agricultural area between the cities of Zoetermeer, The Hague Leidschenveen, and Leidschendam-Voorburg, is being redeveloped into a peak water storage area combined with nature and recreational functions. The Midden Holland region is in need of peak water storage facilities. Extreme precipitation causes a rapid water level rise in the polder ditches and the surrounding drainage pools. This puts pressure on the local water system, while the probability of local waterlogging increases. The construction of the peak water storage area in the Nieuwe Driemanspolder will reduce this risk and improve flood risk management throughout the Rijnland region.
Source: https://ruimtelijkeadaptatie.nl/english/examples/projects/@199733/driemanspolder/
Pluvial flooding / urban flooding
The United States Federal Emergency Management Agency (FEMA) defines of urban flooding as:
“…the inundation of property in a built environment, particularly in more densely populated areas, caused by rain falling on increased amounts of impervious surfaces and overwhelming the capacity of drainage systems”
Causes:
- Rainfall
- Rain falling on impervious surfaces – a result of urbanisation
- Local drainage systems are overwhelmed by the volume of water flowing in them.
Is this a problem?
Due to the dense population patterns of towns and cities, flooding in an urban area can affect large numbers of people.
Many sewer pipes in Ireland are old and in poor condition. Newer developments separate foul sewage from rainwater, but in older areas, the two types of wastewater are combined. When sewers become overwhelmed, they can back up as a result. If foul and rainwater are carried together in one pipe, there are public health and pollution risks associated with this back up.
If rainwater is going into sewers, it is not filtering through the soil and recharging the groundwater. Local aquifers can become depleted, even during periods of heavy rain.
The Future
Climate change is likely to lead to an increase in the frequency and intensity of storms, resulting in more rainwater runoff.
As towns and cities expand, the area of impervious surfaces will increase. Infill sites in urban areas also reduce the permeable area available. As gardens are converted to driveways, the proportion of impermeable surfaces increases.
Increasing populations in our urban areas require greater sewer capacity.
Recommendations
Reducing impermeable surfaces – use permeable pavement instead of concrete/asphalt for paved areas, such as car parks and pedestrianised areas.
Incorporate rainwater harvesting into new building designs, and retrofit them to existing buildings. Even a simple rainwater butt or rainbox planter can reduce the amount of water entering the sewers.
All new developments should include Sustainable Urban Drainage Systems (SuDS). These systems can also be retrofitted in built-up areas.
What are SuDS?
SuDS (Sustainable Urban Drainage Systems) are systems of dealing with rainwater in a more sustainable way than traditional systems.
- Aim to mimic the natural flow of water.
- Attenuate rainwater flow, delaying it from entering sewers, and preventing them from becoming overwhelmed during storm events.
- Allow water to infiltrate into the ground, preventing the need for water to enter the sewer system.
- Provide some treatment, removing pollution from water before it enters the ground or the sewer.
Elements of SuDS include:
- permeable surfaces
- filter and infiltration trenches
- green roofs
- detention basins
- underground storage
- wetlands and/or ponds
There has been a requirement for SuDS in new developments in Waterford last two development plans. In spite of this requirement, there are very few good examples of this technology in Waterford City or County. I would recommend that it be included in all new developments (public and private), with nature based SuDS the preferred option rather than attenuation tanks. It should also be included in Section 38 road projects, where possible. A 10,000 raingardens project in Waterford could also be an interesting trial.
References and Resources
Article on urban flooding in USA
https://www.nrdc.org/experts/anna-weber/what-urban-flooding
Susdrain: the community for sustainable drainage
Dublin City Council Sustainable Drainage Systems and Integrated Constructed Wetlands
San Francisco Better Streets
Melbourne 10,000 Raingardens
Include Nature Based Solutions in all projects involving rainwater or possible flooding
Improve biodiversity while also dealing with excess water