Robert J. Hawley

Addressing the urban stream disturbance regime

Thresholds for particle entrainment and natural disturbance frequency vary across hydrogeomorphic settings, but urbanization increases the rate and extent of channel erosion and sediment transport in alluvial channels. The urban disturbance regime is a change in the frequency, magnitude, and duration of hydrologically induced disturbance on the stream channel and ecosystem that can lead to geomorphic and ecological degradation. To preserve stream stability and ecological function, stormwater management systems should be optimized to maintain the natural disturbance regime of streambed material within the context of societal and environmental goals. Our proposed framework, based on data from 195 sites across 2 continents, shows that the tools of river mechanics can be used to develop relatively simple, regionally appropriate, streambed-specific guidance for stormwater designers, so that engineers can calibrate stormwater facilities that address the urban stream disturbance regime. In the absence of detailed hydrogeomorphic data, practitioners can use our model to predict an order-of-magnitude approximation of the critical discharge for bed particle entrainment (Qc) based solely on bed material class (e.g., cobble vs sand) and the respective 2-y peak discharge (Q2). The estimate also can guide practitioners toward the types of stormwater management strategies that are likely to be most effective at protecting stream stability in a given setting. For example, duration controls for large events (≥Q2) may be very important for preserving stability in gravel/boulder streams where Qc is expected to be >∼0.1 to 1 × Q2, but could have relatively little effect on the overall stability of sand-dominated systems where Qc is likely to be <0.01 × Q2 and controls of much more frequent events (<<Q2) may have a greater influence.

Ecological resistance in urban streams: the role of natural and legacy attributes

Urbanization substantially changes the physicochemical and biological characteristics of streams. The trajectory of negative effect is broadly similar around the world, but the nature and magnitude of ecological responses to urban growth differ among locations. Some heterogeneity in response arises from differences in the level of urban development and attributes of urban water management. However, the heterogeneity also may arise from variation in hydrologic, biological, and physicochemical templates that shaped stream ecosystems before urban development. We present a framework to develop hypotheses that predict how natural watershed and channel attributes in the pre-urban-development state may confer ecological resistance to urbanization. We present 6 testable hypotheses that explore the expression of such attributes under our framework: 1) greater water storage capacity mitigates hydrologic regime shifts, 2) coarse substrates and a balance between erosive forces and sediment supply buffer morphological changes, 3) naturally high ionic concentrations and pH pre-adapt biota to water-quality stress, 4) metapopulation connectivity results in retention of species richness, 5) high functional redundancy buffers trophic function from species loss, and 6) landuse history mutes or reverses the expected trajectory of eutrophication. Data from past comparative analyses support these hypotheses, but rigorous testing will require targeted investigations that account for confounding or interacting factors, such as diversity in urban infrastructure attributes. Improved understanding of the susceptibility or resistance of stream ecosystems could substantially strengthen conservation, management, and monitoring efforts in urban streams. We hope that these preliminary, conceptual hypotheses will encourage others to explore these ideas further and generate additional explanations for the heterogeneity observed in urban streams.

Urban stream renovation: incorporating societal objectives to achieve ecological improvements

Pervasive human impacts on urban streams make restoration to predisturbance conditions unlikely. The effectiveness of ecologically focused restoration approaches typically is limited in urban settings because of the use of a reference-condition approach, mismatches between the temporal and spatial scales of impacts and restoration activities, and lack of an integrative approach that incorporates ecological and societal objectives. Developers of new frameworks are recognizing the opportunities for and benefits from incorporating societal outcomes into urban stream restoration projects. Social, economic, cultural, or other benefits to local communities are often opportunistic or arise indirectly from actions intended to achieve ecological outcomes. We propose urban stream renovation as a flexible stream improvement framework in which short-term ecological and societal outcomes are leveraged to achieve long-term ecological objectives. The framework is designed to provide additional opportunities for beneficial outcomes that are often unattainable from ecologically focused restoration approaches. Urban stream renovation uses an iterative process whereby short-term ecological and societal outcomes generate public support for future actions, which may provide opportunities to address catchment-level causes of impairment that often exist across broad temporal scales. Adaptive management, education, and outreach are needed to maintain long-term public engagement. Thus, future work should focus on understanding how ecological and societal contexts interact, how to assess societal outcomes to maintain stewardship, developing new methods for effective education and outreach, and multidisciplinary collaborations. We discuss potential abuses and the importance of linking societal outcomes to long-term ecological objectives.

When do macroinvertebrate communities of reference streams resemble urban streams? The biological relevance of Qcritical

The threshold discharge for streambed mobilization (Qcritical) has been proposed as a mechanistically relevant management target for geomorphic stability and biological integrity. The geomorphic relevance of Qcritical (Qc)is well established, but its influence on biological communities is less documented. In urban watersheds, where increased frequency of Qc exceedance is nearly ubiquitous (i.e., the urban disturbance regime), excessive streambed disturbance typically co-occurs with other well-established drivers of the urban stream syndrome, such as habitat degradation and poor water quality. Thus, the specific influence of excess streambed disturbance on aquatic communities is not clear. We used a 7-y study of biotic integrity, geomorphic stability, and Qc exceedance at a reference site (Middle Creek [MDC 5.5]) with excellent habitat and water quality to isolate the effects of streambed disturbance. At MDC 5.5, streambed disturbance was the dominant driver of biotic variability through time, and biological indices and geomorphic stability were significantly correlated with time since a Qc event. During 2011, a year with particularly high frequency and magnitude of Qc events, the Macroinvertebrate Biotic Index (MBI) at MDC 5.5 fell to its lowest score on record. In the context of 73 monitoring sites across a gradient of urbanization, the 2011 MDC 5.5 MBI of 30 (poor) was more similar to that of communities in streams draining watersheds with ∼30% total impervious area than to reference-stream scores (∼60) during more typical sampling years. Our study underscores the contribution of excess Qc exceedance to poor biological communities. We suggest calibrating stormwater management to maintain the natural streambed disturbance regime in addition to the more common management objectives of water quality and flood control.

How Poor Stormwater Practices Are Shortening the Life of Our Nation's Infrastructure--Recalibrating Stormwater Management for Stream Channel Stability and Infrastructure Sustainability

Civil engineers provide services aimed at meeting society’s primary infrastructure needs in a safe and cost-effective manner. In most cases, work in one discipline does not adversely affect the value or sustainability of the resources and services in other disciplines. However, in the field of stormwater management, a case can be made that the opposite is true. Conventional stormwater management is widely documented as a primary driver of stream channel instability, which, in turn, causes adverse impacts to adjacent infrastructure such as roads, bridges, and utility lines. The fact that design decisions in one field result in a shortened design life of public/private assets from other disciplines is by definition unsustainable. Moreover, what’s bad for our nation’s infrastructure is also detrimental to the quality and biotic integrity of our water resources. By compiling available cost data of infrastructure damages attributable to channel instability in a Northern Kentucky case study, this paper underscores the business case for a recalibration of stormwater management for stream channel stability and infrastructure sustainability.

Detention Outlet Retrofit Improves the Functionality of Existing Detention Basins by Reducing Erosive Flows in Receiving Channels

By discharging excess stormwater at rates that more frequently exceed the critical flow for stream erosion, conventional detention basins often contribute to increased channel instability in urban and suburban systems that can be detrimental to aquatic habitat and water quality, as well as adjacent property and infrastructure. However, these ubiquitous assets, valued at approximately

Predicting Hydromodification in Streams Using Nonlinear Memory-Based Algorithms in Southern California Streams

600,000 per km2 in a representative suburban watershed, are ideal candidates to aid in reversing such cycles of channel degradation because improving their functionality would not necessarily require property acquisition or heavy construction. The objective of this research was to develop a simple, cost-effective device that could be installed in detention basin outlets to reduce the erosive power of the relatively frequent storm events (~ < two-year recurrence) and provide a passive bypass to maintain flood control performance during infrequent storms (such as the 100-year recurrence). Results from a pilot installation show that the Detain H2O device reduced the cumulative sediment transport capacity of the preretrofit condition by greater than 40%, and contributed to reduced flashiness and prolonged baseflows in receiving streams. When scaling the strategy across a watershed, these results suggest that potential gains in water quality and stream channel stability could be achieved at costs that are orders of magnitude less than comparable benefits from newly constructed stormwater control measures.

How do flow peaks and durations change in suburbanizing semi-arid watersheds? A southern California case study

AbstractHydromodification is a serious management concern in semiarid regions and is expected to become worse with land use and climate change. Potential stream channel responses range from increas...