Problems Associated with Urbanization and Stream Function

Anyone walking along a degraded urban stream may see signs of the effects of urban development: heavily eroded stream banks, trash in overhanging tree branches, discarded tires, or remnants of stormwater conveyance infrastructure (Figure 1.1).  It is obvious that intense alterations to the landscape and water network occur when land is developed.   The most immediate consequences include an increase in impervious surface area with resultant increased runoff to receiving streams, higher peak discharges, greater water export and higher sediment loads during construction.[i]

Figure 1.1 Debris and blown out concrete armouring in an urban stream

In terms of stream hydrology, an altered flow regime with high peak flows and reduced baseflow is the prevalent effect of urbanization (Figure 1.2).

Figure 1.2 Hydrographs before/after urbanization (Stream Corridor Restoration, 1998)

As a result of increased impervious surface, the natural hydrologic cycle is compromised with a reduction in evapotranspiration and soil infiltration (Figure 1.3).  Higher peak discharges, flashier stream flows, and a reduction in groundwater recharge/discharge are common results.  The reduction in groundwater storage causes many urban streams to experience reduced baseflow.[ii]  In most urban areas, conventional stormwater drainage systems tend to further the problem of high peak flows in connecting areas throughout the catchment directly to receiving streams by pipes.[iii]  Piped drainage systems effectively bypass the streams floodplain, conveying water and associated pollutants to streams more frequently than under natural conditions because surface runoff enters streams even during small rain events.[iv]  In addition, high loads of suspended sediment and other non-point source pollutants become part of stream flow and result in poor water quality.[v]  Common sources of pollution to urban streams include fertilizer and pesticides, animal waste, leakage from sewage lines, erosion from construction sites, automobile fluid, and vehicle and industrial emissions.[vi]

Figure 1.3 Changes to natural hydrologic processes (Stream Corridor Restoration, 1998)

As new construction slows and the watershed is built out, changes to the water network remain.  However, the transport of sediment to streams decreases substantially leading to channel erosion and increases in channel depth and width (i.e. incision).[vii]  These channel adjustments disconnect the stream from its floodplain, induce straightening, and simplify stream profiles and habitat.[viii]  Such impacts are generally sharpened by piped drainage systems and channelization, which has often been utilized to manage flooding and maintain physical channel stability.  A major impact of urbanization on stream geomorphology is a change in drainage density.  Natural channel densities decrease in urban catchments as small streams are filled in, paved over, or placed in culverts.  The widening, deepening, and/or straightening of a stream channel is known as channelization.[ix]

In the past, the desire to maintain physical channel stability has led to a furtherance of simplified urban stream channels.  In some cases urban streams are confined in concrete channels and underground pipes.  More commonly the banks of urban streams are armored using gabions, large boulders, or rip rap to prevent bank erosion and lateral channel movement.  Although these armored channels may temporarily stabilize banks, they are often far from physically stable as streams may continue to adjust, enlarge, and erode their channels.  An urban stream channel often has increased slope and thus high in-stream flow volumes as a consequence of lateral constraints to channel migration.  As a result, there is a minimal variation in depth and particle size of bed material.[x]  Furthermore, high volume scouring flow causes downcutting or channel incision which is a common feature of urban stream channels.

Addressing stream function in highly urban settings with high percentage impervious cover is a complex task.  Broadly speaking, urban stream restoration may more suitably fall in to the categories of enhancement and rehabilitation; however, it cannot be over emphasized that all of these efforts start with recovering the natural flow regimes in the catchment basin (i.e. watershed) surrounding a given stream.  To do so, communities need to prioritize and implement pre and post construction stormwater control measures (SCMs) and best management practices (BMPs). Having said this, assisted enhancement of individual stream reaches in urban locations should not be overlooked.  Connecting the community with urban streams is integral to the success of watershed-wide restoration projects.  In observing an aesthetically pleasing stream with greater ecological function, a community can acknowledge the successful design of stormwater management planning and the stream rehabilitation project as a mutually beneficial endeavor to recover a valuable resource.