Many cities worldwide now have available waterfront land, often in the city center, thanks to de-industrialization and concentration of navigation elsewhere. As cities seek to take advantage of this remarkable real estate, they may be prone to some classic ‘traps’ such as copying projects successful in another city but which fail in the new location due to differences in scale, topography, urban form, etc. In this recent publication, Pedro Pinto and Matt Kondolf present an idiosyncratic list of ‘wrongs’ that are evident in many such projects. The paper is freely available, via open access here.
Pinto, JP, and GM Kondolf. 2020. The fit of urban waterfront interventions: matters of size, money and function. Sustainability 12: 4079; doi:10.3390/su12104079
Figure 1. Viana do Castelo, Portugal: three recently built, out-of-scale public buildings cut off the old town from the Lima riverbanks. (source Google)
River restoration projects in North America that involve reconfiguration of stream channels are dominated by symmetrical, single-thread meandering channels. Although the meander dimensions are commonly justified by relations between channel width and meander wavelength, the universal preference for single-thread meandering channels in restoration projects is rarely questioned. The aesthetic appeal of s-shaped curves in art and landscape design may help explain the prevalence of this form in river restoration projects. Riverlab alumna Kristen Wilson (Nature Conservancy) gave 300 freshwater scientists attending her keynote talk 5 minutes to draw a restored stream. She compiled the results to see what mental images these scientists had for restored channels. Most depicted single-thread meanders for their restored channel, although there were interesting variants. See Kristen’s just-published paper here.
Figure 1. The ideal meander with its repeating, regular, and symmetrical bends as drawn by select respondents, isolated to the channel pattern, and overlaid to emphasize the similarity in the form. A.—One wavelength. B.—1.5 wavelength. C.—Two or more wavelengths drawings.
Throughout the humid tropics, increased land disturbance and concomitant road construction increases erosion and sediment delivery to rivers. Building road networks in developing countries is commonly a priority for international development funding based on anticipated socio-economic benefits. Yet the resulting erosion from roads, which recent studies have shown result in at least ten-fold increases in erosion rates, is not fully accounted for. While effects of road-derived sediment on aquatic ecosystems have been documented in temperate climates, little has been published on the effects of road-induced sediment on aquatic ecosystems in developing countries of the tropics. Along the south bank of the Rio San Juan (Nicaragua and Costa Rica), attempts to build a road without engineering or plans resulted in massive failures and erosion in areas where steep slopes impinge upon the river bank. Pre-existing tributary streams received elevated sediment loads, creating new deposits on pre-existing tributary deltas. In some reaches with rapidly eroding sites, completely new deltas of freshly deposited sediment were formed, prograding into the river channel.
Riverlab alumni Blanca Rios and Scott Walls joined with Matt Kondolf to study periphyton biomass and macroinvertebrate communities on the deltas of Río San Juan tributaries, comparing north-bank tributaries draining undisturbed rain forest with south-bank tributaries receiving runoff from the partially-built road experiencing rapid erosion. Periphyton biomass, richness and abundance of macroinvertebrates overall, and richness and abundance of Ephemeroptera, Plecoptera and Trichoptera were higher on the north-bank tributary deltas than the south-bank tributary deltas. These findings were consistent with prior studies in temperate climates showing detrimental effects of road-derived fine sediment on aquatic organisms. A Non-Metric Multidimensional Scaling (NMDS) analysis showed the impacted community on the south-bank deltas was influenced by poorly-sorted substrate with greater proportions of fine sediment and higher water temperatures. The paper is freely available (open-access) here.
Rios-Touma, B, GM Kondolf, and SP Walls. 2020. Impacts of sediment derived from erosion of partially-constructed road on aquatic organisms in a tropical river: the Río San Juan, Nicaragua and Costa Rica. PLoSONE 15(11):e0242356. https://doi.org/10.1371/journal.pone.0242356
Figure 1. Reach of Rio San Juan from approximately River Km 83.3 to 84.3 (downstream of the outlet of Lake Nicaragua) Figure 2. Macroinvertebrate richness of south-bank versus north-bank tributary deltas of the Rio San Juan, MarchMay 2014.
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 ϕphysand ϕvarrespectively.
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 ϕvarto 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).
Figure 9. Maps showing spatial variability in contribution of Avar to HCIA for three antecedent soil moisture conditions (rows) and five rainfall scenarios (columns) .
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
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
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
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.
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.
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)
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.
