Category: News

NEW PUBLICATION: HYDROLOGICAL CONNECTIVITY OF IMPERVIOUS SURFACES

We are excited to share a new Riverlab article, linked here, published this month in the Journal of Hydrology. This work advances current understanding of the controls on hydrological connectivity of impervious surfaces to downstream channels and storm-sewer networks and presents new methods of their estimation.

Connected impervious areas – those impervious surfaces that contribute directly to runoff in a storm network or stream – are a better indicator of hydrologic response, stream alteration, and water quality than total impervious area. Most methods for quantifying connected impervious areas require major assumptions regarding the definition of ‘connection’, potentially over-simplifying the role of variable climates, slope gradients, soils conditions, and heterogeneous flow paths on impervious surface connectivity.

In this study, we present a new metric, hydrologically connected impervious areas (HCIA), to refer to spatially explicit (mapped) estimates of the proportion of impervious surfaces that are hydrologically connected to the storm sewer system or stream network. HCIA is comprised of impervious surfaces that contribute directly to the storm-sewer network and are physically connected, Aphys, or those that contribute indirectly and are therefore variably connected (Avar) (see Figure 1). The degree to which Avar is “hydrologically connected” is represented with a coefficient, ϕvar, that ranges between 0 and 1, with 0 representing full connectivity (i.e. all runoff infiltrates downslope), and 1 representing no connectivity (i.e. no runoff infiltrates downslope).

Figure 1. Conceptual model of impervious surface categories: directly or physically connected (Aphys) and variably connected (Avar) (impervious that drains to pervious). The hydrological connectivity Aphys and Avar are given by ϕphys and ϕvar respectively.

Using a combination of hydrologic modeling in the PySWMM, a python interface for the EPA’s Stormwater Management Model, and machine-learning regression tree analysis, we evaluate the controls on ϕvar across varing soil types, slopes, rainfall scenarios, antecedent soil moisture conditions, as well as amounts of impervious and pervious areas. Figure 6 shows that of the factors tested, soil texture (panel A), fraction of downslope pervious area ϕperv (panel B), soil moisture (panel C), and precipitation (panel D) are sensitive, while total area (panel D), width of impervious area (panel F), and slope (panel G) are insensitive parameters.

Figure 6. Sensitivity of ϕvar to saturated soil texture and saturated hydraulic conductivity Ks (panel A), pervious fraction ϕperv (B), antecedent soil moisture conditions ASM (SAT = saturated, FC = field capacity, WP = wilting point) (C), total area A (D), precipitation depth P (E), width W (F), and slope S (%) (G).

To assist with dissemination of these methods in practice, we apply the regression tree in a geospatial tool for estimation of HCIA in ungauged urban catchments. We test the tool in a case study to an urban sewershed in Colorado, and find that the contribution of Avar to HCIA (compared to the contribution of Aphys) varied across the precipitation and soil moisture conditions. Avar contribution to HCIA was low at low precipitation depths and increased rapidly with increasing precipitation and initial soil moisture conditions (see Figure 9).

Overall, our results suggest that, for catchments consisting of highly impermeable soils, Avar contributes to HCIA such that HCIA approaches the total impervious area, but for catchments with highly permeable soils, Avar does not contribute significantly to HCIA, and thus the physically connected impervious area ( Aphys) could be used as a suitable surrogate for HCIA. In between these two extremes, however, lies a wide range of conditions that call for detailed and spatially explicit estimates of Avar connectivity.

References Cited

Sytsma, A., Bell, C., Eisenstein, W., Hogue, T., & Kondolf, G. M. (2020). A geospatial approach for estimating hydrological connectivity of impervious surfaces. Journal of Hydrology, 591, 125545. https://doi.org/10.1016/j.jhydrol.2020.125545 >>link to paper

GEOMORPHIC & ECOLOGICAL FUNDAMENTALS FOR RIVER & STREAM RESTORATION CANCELLED FOR 2020 DUE TO COVID-19, BUT ON FOR 16-20 AUG 2021

With great regret, we cancel the 2020 shortcourse at Sagehen Creek Field Station, due to the many complications arising from the CVID-19 pandemic and the challenges in avoiding problems in holding the shortcourse at the station.  Those already registered are entitled to a full refund or may defer their participation to next year’s course offering, 16-20 August 2021.   We apologize for this very disappointing news, but look forward to better conditions under which we can once again hold the course next year.  We thank you for your understanding.

 

Matt Kondolf and the Sagehen Teaching Team

Geomorphic and Ecological Fundamentals for River and Stream Restoration

HYDROLOGIC IMPACTS OF UPSTREAM DAMS ON THE MEKONG DELTA

You may be interested in a new analysis of the effects of recently completed Xayaboury Dam (on the Mekong mainstem near Luang Prabang) and a cascade of dams upstream in China on flow patterns in the Lower Mekong River. The paper, Mekong River, Xayaboury Dam, and Mekong Delta in the first half dry season 2019-2020 by Nguyen Ngoc Tran was published in Vietnamese in TIA SANG, a scientific journal published by the Ministry of Science and Technology. The English version is now available here. As illustrated in excerpts of Figures 5 and 6 from the paper, the hydrologic analysis shows that flows this dry season have been significantly lower than in prior years’ dry seasons.

Riverlab members have contributed scientific papers on the cumulative effects of upstream dams on the sediment budget of the Mekong Delta and other threats to the sustainability of the Delta (Kondolf et al 2014, Kondolf et al 2018) and the potential for strategic dam planning to minimize impacts of dams on downstream sediment budgets and fish migration (Schmitt et al 2019).


Water levels in the Mekong River at Nakhon Phanom reflecting severe drought conditions in the current dry season of the 2019-2020 water year. (Source: Nguyen Ngoc Tran. 2020, Mekong River, Xayaboury Dam, and Mekong Delta in the first half dry season 2019-2020, Figure 5.)


View of the exposed bed of the Mekong River at Nakhon Phanom in late October 2019, reflecting severe drought conditions in this year’s dry season. (Source: Nguyen Ngoc Tran. 2020, Mekong River, Xayaboury Dam, and Mekong Delta in the first half dry season 2019-2020, Figure 6.)

References Cited

Kondolf, G.M., Z.K. Rubin, J.T. Minear. 2014. Dams on the Mekong: Cumulative sediment starvation.  Water Resources Research 50, doi:10.1002/2013WR014651. >>link to paper

Kondolf, GM, RJP Schmitt, P Carling, S Darby, M Arias, S Bizzi, A Castelletti, T Cochrane, S Gibson, M Kummu, C Oeurng, Z Rubin, and T Wild. 2018. Changing sediment budget of the Mekong: Cumulative threats and management strategies for a large river basin. Science of the Total Environment 625: 114-134. https://doi.org/10.1016/j.scitotenv.2017.11.361 >>link to paper

Schmitt, R, S Bizzi, AF Castelletti, J Opperman, GM Kondolf. 2019. Planning dam portfolios for low sediment trapping shows limits on sustainable hydropower in the Mekong. Science Advances 5: eaaw2175 >>link to paper

CAMBODIA ADOPTS MORATORIUM ON DAMS ON THE MEKONG

The government of Cambodia announced on 16 March that it would postpone development of any of new dams on the mainstem Mekong River for 10 years, citing the need to develop alternative sources of energy for the country’s future development. While Cambodia has built a large dam on the SeSan-SrePok (important downstream tributaries), and left open the possibility it might build other tributary dams, the mainstem dams long-planned for Sambor and Stung Trang are on hold for the next decade. See story in the Guardian here.

Restoring river habitats below dams through gravel augmentation and bank erosion

Building on recent research on the Rhine River between France and Germany, a research team based in Strasbourg has published a review of scientific literature on projects to restore channel complexity downstream of dams.  While dam removal has attracted enormous attention in recent years, with notable successes on the Elwha River, the reality is that most dams are here to stay and most river reaches in the developed world are downstream of dams.  As these dams capture sediment, they create conditions of sediment deficit in many river reaches downstream. This review found relatively few studies documenting projects to restore sediment supply via gravel augmentation and fewer still via restoration of channel erosion processes below dams (mostly examples from northern Europe).  Biological monitoring shows benefits from these projects, whose increasing popularity reflects growing interest in restoration of fluvial process, and an evolving perspective towards adaptive or coupling management approaches to promote the recovery of natural processes in rivers below many dams and thus to improve ecological response.  

 

The paper, Restoring fluvial forms and processes by gravel augmentation or bank erosion below dams: A systematic review of ecological responses, by Cybil Staentzel et al. is available for free download here until 01 February 2020.

Reservoir sedimentation and what to do about it.

In June 2019, Reservoir Sediment Management: Building a Legacy of Sustainable Water Storage Reservoirs was released by the National Reservoir Sedimentation and Sustainability Team (NRSST), a consortium of engineers and scientists from federal agencies, consulting firms and universities, including UC-Berkeley’s RiverLab, studying the impacts of sediment on the nation’s water supply.

 

This paper outlines the origins and legacy of reservoir sedimentation, where sediment being transported by a river begins collecting behind a dam. While sediment transport is of great benefit to riverine ecologies, the trapping of sediment means decreased water storage capacity in dams, greater flood risk, and reduction in hydropower functions.

Additionally, the paper proposes the following management strategies for mitigating further sedimentation and dealing with existing sediment:

  • Reduce sediment yield entering the reservoir by trapping more upstream;
  • Move sediments away or through reservoirs;
  • Flush or dredge existing sediment deposits;
  • Adapt to and plan for reduced storage volume in the future. (Randle, 2019)

 

Read the full paper by clicking here

 

Tim Randle of the NRSST and Manager of the Sedimentation and River Hydraulics Group at the Bureau of Reclamation was featured in H2O Radio’s recent story “Damned from the Start” discussing the reservoir sedimentation as it applies to the flooding of the Niobara River behind the Gavins Point Dam in Nebraska.

 

Listen to the full story here

ReNUWIt Research Spotlight

For the past year, RiverLab’s Anneliese Sytsma has been working with Engineering Research Center ReNUWIt – Re-inventing the Nation’s Urban Water Infrastructure – on her PhD dissertation research. This month, ReNUWIt highlighted her research on ‘connected impervious surfaces’ on the ReNUWIt website- see here. Anneliese is working on the manuscript now and hopes to publish before the end of the year.

Connected or unconnected? A new method for estimating connectivity of impervious surfaces across variable soils, slopes, and rainfall scenarios

Accurate methods to predict impervious surface connectivity are needed to improve hydrologic modeling and efficient siting of distributed stormwater technologies. ReNUWIt researchers are developing a new method for estimating impervious surface connectivity across different soil types, slopes, rainfall scenarios and landcover parameters. The outcomes of this research may be used as hydrologic model inputs and to inform more efficient distributed stormwater control siting across heterogeneous urban landscapes.

AGU Session: Managing multifunctional watersheds for the 21st century

Join us at American Geophysical Union (AGU) fall meeting in the session “Managing multifunctional watersheds for the 21st century‘ (Session # GC052). Organized by Rafael Schmidt (RiverLab alum), P. James Dennedy-Frank, and Dr. Kondolf, this session will tackle the increasing demand for watershed services and capacity of green-grey solutions to meet this demand. We invite submissions to this session that showcase both exemplary case studies and systematic cross-site analyses addressing key questions for an integrated and strategic  management of multifunctional watersheds:  (1) at what scales and contexts do green solutions provide tangible benefits to society; (2) and how can  combined  grey and green infrastructure portfolios be designed to maximize benefits for both nature and people?

The deadline for abstract submission is 31 July 2019 23:59 EDT/03:59 +1 GMT.

2019 AGU Fall Meeting

The American Geophysical Union (AGU) annual fall meeting will be held 9 – 13 December 2019 in Moscone Center, San Francisco. The Fall Meeting is the largest international Earth and space science meeting in the world, with speakers from around the globe presenting and facilitating discussion on cross-disciplinary geophysical topics, including atmospheric and ocean sciences; solid-Earth sciences; hydrologic sciences; and space sciences.

You might find us at one of these sessions:

Managing Multifunctional Watersheds for the 21st Century (GC052)

A changing climate and growing population will lead people to demand more and potentially different watershed services, including water resource regulation, energy generation, and geomorphic hazard reduction. Green solutions such as watershed restoration and improved agricultural practices have been shown to have important benefits for local livelihoods and biodiversity. These solutions are a corrective to grey infrastructure such as dams and levees that provide valuable services but may also produce major environmental externalities. However, alone these green solutions may not provide the magnitude of services required. We invite submissions showcasing both exemplary case studies and systematic cross-site analyses addressing key questions for an integrated and strategic  management of multifunctional watersheds:  (1) at what scales and contexts do green solutions provide tangible benefits to society; (2) and how can  combined  grey and green infrastructure portfolios be designed to maximize benefits for both nature and people?

Biophysical Processes of Rivers Under Extreme and Changing Conditions (EP006)

Rivers are naturally dynamic systems, characterized by a suite of biophysical processes that are regularly subject to exogenous factors. Under ranges of natural variability, the physics and biota of rivers are resilient to external changes. However, river basins globally are undergoing landscape-scale changes. These changes, which are associated with widespread land use, water management, and climate change, can fundamentally alter biophysical processes. This session focuses on the science and management of integrated biophysical processes in river systems undergoing changing variability, including greater and/or more frequent extremes. Topics may address questions such as: How do changing and extreme events (e.g. streamflow magnitude, frequency, timing, temperature) influence river processes or form and resultant ecosystem structure and function (e.g. habitat quality and availability, egg survival, food webs, algal blooms)? How should management and restoration of rivers be designed and prioritized to mitigate and/or be resilient to these large-scale changes?

Managing and Modeling Tradeoffs and Challenges of Environmental and Low Flows in the 21st Century. (H094)

Rivers are the main source of water, food and energy for billions of people, but the (mis-)management of this critical resource has deteriorated aquatic ecosystems globally. Quantifying how much flow is needed to maintain the ecological integrity of rivers, especially during low flow periods, has become a point of conflict and convergence, particularly in arid regions where most large rivers are regulated. Better managing tradeoffs between environmental flows and consumptive demands requires an improved understanding of watershed hydrology and the low flow characteristics of riverine systems, along with cascading effects on fluvial geomorphology, aquatic ecology, and social systems. This session invites contributions demonstrating recent advances in understanding and resolving competing water demands together with methodological advances on novel ways to define and simulate low flows. We invite contributions that bridge across scientific disciplines and that represent a diversity of regions around the world where water management conflicts are emerging.

Reservoir Sedimentation in Disturbed Landscapes: A Real Look at Lost Water Storage and Fish Passage Opportunities (EP033)

Aging infrastructure and loss of water storage capacity due to sedimentation will cause the social, economic, environmental, and political importance of reservoirs to increase progressively. Reservoirs provide flood control, water supply, and power generation but may hinder survival of anadromous fish. Sediment regimes in disturbed and contaminated landscapes, including the hydraulic mining-impacted Sierra Nevada, complicate efforts to restore storage capacity due to concern about contaminant mobilization. The best available science on mercury fate and transport can stimulate new discussion about sediment removal and maintenance activities. Measures to address sedimentation at reservoirs nearing total storage loss need to be identified and solutions evaluated, including installation of upstream traps, sediment pass-through, flushing or mechanical removal. Site-specific reservoir sedimentation surveys that account for unique sediment regimes of disturbed landscapes are needed to inform cost-benefit analysis of maintaining aging infrastructure at the expense of restoring volitional fish passage.

 

You are invited to submit an abstract for a presentation or poster to any of these sessions;  the abstract submission deadline is 31 July 2019 23:59 EDT/03:59 +1 GMT.