Geology topics

Christopher J. Walsh

Publications and source records attributed to Christopher J. Walsh.

4 recordsLinked to original sources

USGS2025

Quantifying the success of stormwater control measure networks using effective imperviousness

The deleterious effects of directly-connected impervious surfaces on urban streams have been widely recognized. To deal with these effects, the use of stormwater control measures that aim to disconnect impervious surfaces and prevent stormwater from reaching the stream has surged. However, we lack widespread use of consistent metrics that describe how effective these stormwater control measures are for mitigating the effects of untreated stormwater. Using total impervious area neglects the effect of stormwater control measures whereas directly-connected impervious area assumes that stormwater control measures perform perfectly. Comparing the success of stormwater control measures across many watersheds and cities will require use of consistent metrics of effective imperviousness, describing actual performance of stormwater control measures in reducing impervious areas hydraulically connected to the stream. This work applies two published approaches to quantify effective imperviousness, one that measures the frequency of downstream flow disturbances and another that computes parameters from a paired rainfall-runoff regression analysis. We apply these approaches in two settings: 1) two watersheds with new low impact development in Clarksburg, Maryland, USA and 2) five watersheds with stormwater retrofits in Melbourne, Australia. These methods gave largely similar results, with differences in effective imperviousness ranging from 1%-9%. Using these approaches in Clarksburg, the effective imperviousness for the treatment watersheds was 6–12%, whereas the total imperviousness was 33–44% and the directly-connected imperviousness was 0%. In Clarksburg, effective imperviousness better described stream hydrologic and biotic outcomes compared to either total imperviousness or directly-connected imperviousness. In Melbourne, effective imperviousness was a better metric for hydrologic and water quality changes that are likely to provide ecological benefits. In both cases, new development and retrofits, we demonstrate the utility of effective imperviousness metrics for predicting stream outcomes and how these metrics may be used to understand the success of stormwater control measure using a consistent metric.

PLOS Water↗

USGS2017

Restoration of contaminated ecosystems: adaptive management in a changing climate

Three case studies illustrate how adaptive management (AM) has been used in ecological restorations that involve contaminants. Contaminants addressed include mercury, selenium, and contaminants and physical disturbances delivered to streams by urban stormwater runoff. All three cases emphasize the importance of broad stakeholder input early and consistently throughout decision analysis for AM. Risk of contaminant exposure provided input to the decision analyses (e.g. selenium exposure to endangered razorback suckers, Stewart Lake; multiple contaminants in urban stormwater runoff, Melbourne) and was balanced with the protection of resources critical for a desired future state (e.g. preservation old growth trees, South River). Monitoring also played a critical role in the ability to conduct the decision analyses necessary for AM plans. For example, newer technologies in the Melbourne case provided a testable situation where contaminant concentrations and flow disturbance were reduced to support a return to good ecological condition. In at least one case (Stewart Lake), long-term monitoring data are being used to document the potential effects of climate change on a restoration trajectory. Decision analysis formalized the process by which stakeholders arrived at the priorities for the sites, which together constituted the desired future condition towards which each restoration is aimed. Alternative models were developed that described in mechanistic terms how restoration can influence the system towards the desired future condition. Including known and anticipated effects of future climate scenarios in these models will make them robust to the long-term exposure and effects of contaminants in restored ecosystems.

Restoration Ecology↗

USGS2016

Urbanization and stream ecology: Diverse mechanisms of change

The field of urban stream ecology has evolved rapidly in the last 3 decades, and it now includes natural scientists from numerous disciplines working with social scientists, landscape planners and designers, and land and water managers to address complex, socioecological problems that have manifested in urban landscapes. Over the last decade, stream ecologists have met 3 times at the Symposium on Urbanization and Stream Ecology (SUSE) to discuss current research, identify knowledge gaps, and promote future research collaborations. The papers in this special series on urbanization and stream ecology include both primary research studies and conceptual synthesis papers spurred from discussions at SUSE in May 2014. The themes of the meeting are reflected in the papers in this series emphasizing global differences in mechanisms and responses of stream ecosystems to urbanization and management solutions in diverse urban streams. Our hope is that this series will encourage continued interdisciplinary and collaborative research to increase the global understanding of urban stream ecology toward stream protection and restoration in urban landscapes.

Freshwater Science↗

USGS2016

Principles for urban stormwater management to protect stream ecosystems

Urban stormwater runoff is a critical source of degradation to stream ecosystems globally. Despite broad appreciation by stream ecologists of negative effects of stormwater runoff, stormwater management objectives still typically center on flood and pollution mitigation without an explicit focus on altered hydrology. Resulting management approaches are unlikely to protect the ecological structure and function of streams adequately. We present critical elements of stormwater management necessary for protecting stream ecosystems through 5 principles intended to be broadly applicable to all urban landscapes that drain to a receiving stream: 1) the ecosystems to be protected and a target ecological state should be explicitly identified; 2) the postdevelopment balance of evapotranspiration, stream flow, and infiltration should mimic the predevelopment balance, which typically requires keeping significant runoff volume from reaching the stream; 3) stormwater control measures (SCMs) should deliver flow regimes that mimic the predevelopment regime in quality and quantity; 4) SCMs should have capacity to store rain events for all storms that would not have produced widespread surface runoff in a predevelopment state, thereby avoiding increased frequency of disturbance to biota; and 5) SCMs should be applied to all impervious surfaces in the catchment of the target stream. These principles present a range of technical and social challenges. Existing infrastructural, institutional, or governance contexts often prevent application of the principles to the degree necessary to achieve effective protection or restoration, but significant potential exists for multiple co-benefits from SCM technologies (e.g., water supply and climate-change adaptation) that may remove barriers to implementation. Our set of ideal principles for stream protection is intended as a guide for innovators who seek to develop new approaches to stormwater management rather than accept seemingly insurmountable historical constraints, which guarantee future, ongoing degradation.

Freshwater Science↗