Part 2 – Urban Stream Syndrome

In Part 1 of this series, we abstractly discussed the need for stormwater management as a logical necessity for directing water away from our streets and buildings to help prevent flooding and property damage. In this post, we will dive further into the reasons why this management is so important for both our built and natural environment, in terms of public and environmental health.

Why is Rain an Issue in Cities?

The problem of stormwater in urban settings isn’t as much a volume issue as it is a flow issue. There is a big difference between an inch of rain over the course of a day and an inch of rain over the course of an hour. While “normal” rain conditions will vary from city to city, infrastructure is no doubt designed to handle this “normal” amount of rain without issue. However, with extreme precipitation events that are becoming more and more common under climate change, almost all systems in America experience events that exceed their maximum capacity (Pyke et al., 2011). This is true in urban and rural settings alike, however cities face some particular challenges that exacerbate the problems associated with significant rainfall. Impermeable surfaces such as asphalt and concrete do not allow stormwater to infiltrate the soil as it does in more rural/natural systems, and instead creates instant runoff that travels at relatively high speeds down slope.

The speed at which this stormwater runoff races through our sewer systems and urban streams gets worse and worse as the amount of impervious surfaces, and consequently runoff, increases and causes “flashy” systems which are bombarded with periods of high peak flows after rain events and low flows after the water is drained (Walsh et al., 2012). In other words, the system does not retain water and instead cascades like rapids through our pipes to either our water treatment plants or to urban streams. Streams subject to these conditions are said to suffer “urban stream syndrome” and are associated with a number of environmental and public health concerns.

Impacts to Public Health and the Environment

When water washes over our buildings, lawns, roads, and sidewalks, it is no longer “just water.” It is carrying a countless number of substances pervasive in our built environment including motor oil, debris, heavy metals, fertilizers, pesticides, pet waste, bacteria, etc., visible and invisible (Lee & Bang, 2000). This is a pretty nasty list of materials being flushed straight into out waterways every time it rains, and that does not even include the cases where significant rainfall leads to sewage overflow, in some cases causing raw sewage to flow through the streets which we will discuss in the next post. The results to waterways can be biologically devastating, either from toxicity poisoning, nutrient loading leading to algal blooms and subsequent fish kills, suffocation via crude oil or debris (Magaud et al., 1997), or any other number of unintended consequences.

Obviously, the impacts of these substances reaching our surface waters are not great for public health. Swim advisories or full beach closures following rain events are common in LA due to the spike in contamination (CLAPH, 2019). For communities harvesting food such as shellfish from their local water bodies, exposure to heavy metals and microbial pathogens via runoff can lead to these resources being unsuitable for human consumption (Campbell, 1994; Conte & Ahmadi, 2011). For communities using surface water to supply drinking water, all these added pollutants increase the costs associated with treatment, and may push certain waterbodies beyond acceptable levels, forcing communities to either invest in new treatment infrastructure or find a new drinking water source altogether.

For communities dependent on groundwater sources, flashy systems pose a long-term problem. Groundwater-dependent communities do not have lakes or reservoirs to draw water from, and instead pump water to the surface from an underground source called the aquifer. Aquifer’s water supply is recharged by infiltration of water through the earths surface and downward through the soil column, typically at a very slow rate. The rate at which groundwater can recharge its reservoir is hampered by impervious surfaces and runoff, as water cannot infiltrate the soil and rapidly leaves the system. This can lead to communities taking out water from these sources faster than it is replenishing, which eventually means they will tap the source dry and, in many cases, will have no alternatives to abandoning the area, as finding new groundwater sources is much more difficult than finding new surface water sources (Shah, 2008).

In addition to these impacts on water quality, flashy systems are stressful to the physical composition of our rivers and streams. High peak flow rates cause rivers and streams to flow at higher speeds during rain events, which leads to streambank erosion and deterioration of the riverside habitat or riparian zone. This alters stream form and function, increases sediment loading from the river, and reduces wildlife habitat (Booth et al., 2016;Walsh et al., 2005).

Moving Forward

In this section we have covered that impervious surfaces in cities lead to excessive amounts of runoff, which causes large amounts of water to inundate our water bodies and sewers at high speeds. As the water washed through our cities, it picks up a number of dangerous pollutants, contaminating our surface waters. These high peak flows and low water retention cause what is known as urban stream syndrome, as well as reduces the rate at which groundwater sources recharge. In the next post, we will dive into the systems used in America to direct stormwater and sewage out of our cities, and the consequences of when these systems overflow and release untreated water and sewage into the environment.

References:

Booth, D. Roy, A., Smith, B., & Capps, K. 2016. Global perspectives on the urban stream syndrome. Freshwater Science, 35(1), 412-420.

Campbell, K. 1994. Concentrations of heavy metals associated with urban runoff in fish living in stormwater treatment ponds. Archives of Environmental Contamination and Toxicology, 27(3), 352-356.

Conte, F. & Ahmadi, A. A computerized model for evaluating new rainfall closure rules for conditionally approved shellfish growing areas. Transactions of the American Society of Agricultural and Biological Engineers, 54(3), 909-914.

County of Los Angeles Public Health. 2019. http://publichealth.lacounty.gov/phcommon/public/eh/water_quality/beach_grades.cfm Retrieved on 04/15/2019.

Lee, J. & Bang, K. 2000. Characterization of urban runoff. Water Research, 34(6), 1773-1780.

Magaud, H., Migeon, B, Morfin, P., Garric, J., & Vindimian, E. Modelling fish mortality due to urban storm run-off: Interacting effects of hypoxia and un-ionized ammonia. Water Research, 31(2), 211-218.

Pyke, C., Warren, M., Johnson, T., LaGro, J., Schafenberg, J., Groth, P., Freed, R., Schroeer, W., & Main, E. 2011. Assessment of low impact development for managing stormwater with changing precipitation due to climate change. Landscape and Urban Planning, 103(2), 166-173.

Shah, T. 2008. Groundwater management and ownership: Rejoinder. Economic and Political Weekly, 43(17), 116-119.

Walsh, C., Roy, A., Feminella, J., Cottingham, P., Groffman, P., & Morgan, R. 2005. The urban stream syndrome: current knowledge and the search for a cure. Journal of the North American Benthological Society, 24(3), 706-723.