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 $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.     

Reach‐Scale Geomorphic and Biological Effects of Localized Streambank Armoring

Armoring of streambanks is a common management response to perceived threats to adjacent infrastructure from flooding or erosion. Despite their pervasiveness, effects of reach‐scale bank armoring have received less attention than those of channelization or watershed‐scale hydromodification. In this study, we explored mechanistic ecosystem responses to armoring by comparing conditions upstream, within, and downstream of six stream reaches with bank armoring in Southern California. Assessments were based on four common stream‐channel assessment methods: (1) traditional geomorphic measures, (2) the California Rapid Assessment Method for wetlands, (3) bioassessment with benthic macroinvertebrates, and (4) bioassessment with stream algae. Although physical responses varied among stream types (mountain, transitional, and lowland), armored segments generally had lower slopes, more and deeper pools and fewer riffles, and increased sediment deposition. Several armored segments exhibited channel incision and bank toe failure. All classes of biological indicators showed subtle, mechanistic responses to physical changes. However, extreme heterogeneity among sites, the presence of catchment‐scale disturbances, and low sample size made it difficult to ascribe observed patterns solely to channel armoring. The data suggest that species‐level or functional group‐level metrics may be more sensitive tools than integrative indices of biotic integrity to local‐scale effects.     

Channel Evolution Model of Semiarid Stream Response to Urban‐Induced Hydromodification1

Hawley, Robert J., Brian P. Bledsoe, Eric D. Stein, and Brian E. Haines, 2012. Channel Evolution Model of Semiarid Stream Response to Urban‐Induced Hydromodification. Journal of the American Water Resources Association (JAWRA) 48(4): 722‐744. DOI: 10.1111/j.1752‐1688.2012.00645.x Abstract: We present a novel channel evolution model (CEM) that qualitatively describes morphologic responses of semiarid channels to altered hydrologic and sediment regimes associated with urbanization (hydromodification). The CEM is based on southern California data from 83 detailed channel surveys, hundreds of synoptic surveys, and historical analyses of aerial photographs along 14 reaches. Channel evolution sometimes follows the well‐known sequence described by Schumm et al. (Incised Channels: Morphology, Dynamics, and Control, Water Resources Publications, Littleton, Colorado, 1984) for incising, single‐thread channels; however, departures from this sequence are common and include transitions of single thread to braided evolutionary endpoints, as opposed to a return to quasi‐equilibrium single‐thread planform. Thresholds and risk factors associated with observed channel response are also presented. In particular, distance to grade control and network position emerged as key controls on channel response trajectory. The CEM and quantitative extensions provide managers with a framework for understanding channel responses and rehabilitation alternatives, and may be transferable to other semiarid settings. It also offers insights regarding channel susceptibility to hydromodification, highlights key boundary conditions for high‐risk channels, and underscores critical knowledge gaps in predicting the complex, discontinuous response trajectories that are highly prevalent in urbanized watersheds.