Too much focus on imperviousness as the cause of urban stream syndrome

Usually, one of the first things people think of when thinking about the effects of urbanization on degraded aquatic habitats (when they think about these sorts of things), is impervious surface area. Impervious surface seals off subsurface storage in underlying soils, so that when it rains, more surface runoff occurs. This surface runoff, often carrying with it pollutants litter, sediments and grease, is “flashier” than what normally occurs in un-urbanized catchments. This large value, high peak flow response to rainfall results in flows that cut into stream banks and cause erosion.

For the past twenty years, this understanding, with impervious surface cover as the main causal factor for the flashy response, has been the major focus of water-land planning. The10% Rule emerged in the mid-1990s: when the percent of a watershed that is covered in impervious surface cover exceeds 10%, measurable negative impacts on aquatic habitat can be observed. Others have focused on distinguishing types of imperviousness, recognizing that connectivity of impervious cover is an important physical processes that explains how equal levels of development might result in different impacts to hydrological regime. Generally, impervious surface that is more connected will result in flashier response than impervious surface that is “disconnected.” In fact, the concept of “disconnected” impervious surface area is the principal behind green stormwater infrastructure. Sometimes called “source control” best management practices (BMPs), facilities such as bioswales and rain gardens aim to intercept surface runoff from impervious surfaces close to where the rain falls, thus “disconnecting” it from the response. Bulk lot coverages – or the percent of a property that can be developed as impervious is used to control aesthetics and intensity of development, but also for stormwater runoff management purposes.

This conceptual model of runoff generation has worked well for conventional infrastructure planning because it allows infrastructure to be sized accordingly. However, there is increasing evidence that when looking more broadly at ecological systems, increased imperviousness is just one of many changes that accompanies urbanization. There are changes in vegetation that could result in less water evaporated into the air than pre-development forests. There is water imported to the area for consumption and lawn watering/irrigation. There may be septic systems are designed to discharge water imported from elsewhere into local soils. Soils may be more compacted through site re-grading, construction and settling. Seemingly “pervious” grassy areas may actually be served by drainage infrastructures that discharge to nearby streams. Perhaps most importantly, in their focus on impervious surface cover, many have ignored the role of differential soil saturation and subsurface dynamics in capacity recovery. In a recent publication in fact, I show that among urbanized watersheds in the US, the proportion of overall development, not the proportion of impervious surface in the watershed is the strongest predictor of “flashy” hydrologic response. Developed open space, such as suburban lawns and golf courses actually function more similarly to impervious roofs and roads in their effect on flashiness than they do to undeveloped areas.

Focus on imperviousness as cause of flashiness is not wrong, per se, it may just miss the larger point: that it is overall development–the infrastructure, the changes in vegetation, and the compaction of soils, the habits that we have for keeping nice, neat lawns mowed and watered– that should be limited to prevent negative changes to hydrological regimes. Imperviousness has served well as a convenient measurement of development. But it has also resulted in a false logic: that increased imperviousness is correlated with negative ecological impacts does not necessarily mean that removal of imperviousness or disconnection of imperviousness would correct them.

Recognition of the difference between “imperviousness” and “development” moves the concept of “green infrastructure” from a hyper local scale to the regional scale. If development, rather than impervious surface has been the best predictor of flashy hydrological flows, then we need to think about roles for conservation as potentially the most effective type of “green infrastructure”. One might be tempted to argue that funding for regional conservation planning be prioritized over urban impervious surface retrofit programs (construction of rain gardens and bioswales). Such an argument would be reductionary however, since green infrastructure has been shown to increase livability and community amenity in existing urban areas, which might in turn attract more residents back to urban cores rather than to greenfield suburban development. Widespread use of site-scale green infrastructure to mitigate runoff from urban development is also relatively new practice, and the effect of developments that have deployed significant levels of engineered green infrastructure (such as rain gardens and bioswales) may still be overshadowed of the effects of conventional development in urbanized catchments. What is clear however is that better understanding of hydrologic alteration is needed, these changes and potential interventions occur at different scales and involve different stakeholders. A truly comprehensive “green infrastructure” plan should actually be multiple plans that reference each others’ goals at different scales and make conceptual connections where possible to strengthen transformation of the culture of integrated land-water-infrastructure management. These plans should not merely focus on limiting imperviousness, but on the general tenets of discouraging greenfield development, natural vegetation and soil conservation and rehabilitation, encouraging urban revitalization and livability in existing urbanized areas, and broader cultural change to facilitate stakeholder communication and participation.

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