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If stream diversions are to achieve good aquatic ecological outcomes, a new set of paradigms is required when considering maintenance, flooding, erosion and visual amenity.
According to Statistics NZ, in June 2018 New Zealand’s population was growing by one person every 8 minutes and 7 seconds. This expansion can create conflicting issues: the need to supply housing, roads and services to meet the demands of the population; and the need to mitigate the impact that people have on the environment.
These concerns were reflected in the recent general elections, where both housing and the health of New Zealand’s freshwater systems were prominent discussion points.
How do we continue developing and growing, while protecting (ideally, enhancing) our waterways? As land is developed for housing, we see increasing pressure on small- and medium-sized waterways; typically in lowland settings. Not (at least initially) from contaminants, but because they are perceived to be "in the way".
Piping the waterway used to be the answer. Today, there’s a move, through good planning, to include waterways in landscape designs of residential developments. Diverting waterways into new and improved stream channels is also becoming an acceptable alternative to past practices of piping freshwater habitats under houses, car parks, and roads. The trick is to manage stormwater and flood conveyance issues, but still recreate aquatic ecological condition and values.
Three key challenges appear forefront:
- why are we doing it and is it necessary;
- what barriers do we need to overcome;
- and what benefits can we achieve?
Why are we doing it? The simple answer: "We need the space"
When unmodified by humans, streams meander through the landscape; they erode soft organic banks while being pushed by hard surfaces. This meandering pattern can leave awkward spaces that are difficult for developers to build within.
It is often perceived as more practical to move (i.e. permanently divert) the stream into a newly constructed, “anthropocentric” channel than to “give up” usable land. This also allows developers to maximise allotment numbers, and often reduce the number of stream crossings needed within the development.
A stream diversion is frequently a central component of consent-required offset / mitigation. As such, there is often a requirement to achieve measurable ecological gain as a (perhaps debatable) priority over the other requirements. However, the ecological integrity of the diversion can be impacted by the demands of other stakeholders. This mix of agendas leads to difficult decisions, concessions, and compromises being made.
Examples of such compromises include extensive armouring of the stream diversion; the desire to rapidly convey flood waters; and a requirement to meet visual amenity objectives or allow increased public access.
Stormwater engineers are likely to propose armouring the bed, banks, and initial riparian zone of the newly constructed stream channel with rock rip-rap. The intention is to maximise the longevity of the diversion channel, while minimising erosion and potential maintenance.
However, excessive armouring restricts the proximity of riparian planting and reduces lateral surface water filtration. Armouring the banks and bed also limits the ability to establish sufficient shade cover (as few plants are able to grow in amongst the rock rip-rap). Shading is an important component of waterway function: it regulates water temperatures and limits aquatic weed growth. Armouring of the bed also minimises wetted bank heterogeneity - the ability for holes, pools, undercut banks, and overhangs to form - all of which are essential habitat for some aquatic fauna. In some of the worst cases, excessive armouring results in an absence of surface water in the stream; rather, the water flows amongst the depths of the rock rip-rap.
In addition to armouring seen by stormwater engineers as essential for longevity of the channel, hydrologists strive to build waterways that rapidly convey water under flood conditions. Flood control primarily revolves around a stream's ability to quickly flush flood waters out of the system.
To ensure this, hydrological modellers recommend limiting the ‘roughness’ of the bed, banks and flood plain by limiting or excluding groundcover plants and bed obstructions, such as logs or boulders; and reducing meander.
However, just like stream shading, undercut banks, and overhangs, riparian plants and in-stream habitat features are essential for a healthy, functional ecosystem. Riparian vegetation - of a variety of height tiers and diversity of species - will shade the stream, help filter water entering the stream, slow stream velocity in places, provide organic inputs (food) for the aquatic fauna, improve the sustainability and representativeness of the feature, and ultimately reduce the maintenance of aquatic weeds.
Often, when waterway diversions are planned, there is a need to meet a certain level of visual amenity, and perhaps to increase public access. This might be through installation of a boardwalk, which can increase the impermeable surface area around the stream; or by restrictions on vegetation density, height or cover in the riparian zone so lines-of-sight are not obstructed; or specimen or exotic trees might be planted rather than eco-sourced native plants.
A sustainable and healthy stream ecosystem requires riparian planting that enhances the ecological function of the stream. A well-functioning riparian zone would usually consist of closely planted native species, which will form a dense canopy and provide shade over the stream. This would include a shrub layer to assist in filtration, and slow overland flows into the stream; as well as planting along the banks to provide erosion control and improve aquatic habitat complexity.
Conflicts between these agendas are not infrequent.
Where the purpose is primarily ecological mitigation, then hydrological flood considerations, erosion restrictions, and structure maintenance have to take a back seat to ecology.