COUPLED ENGINEERED AND NATURAL DRAINAGE NETWORKS: DATA-MODEL SYNTHESIS IN URBANIZED RIVER BASINS

Abstract

In\nurbanized river basins, sanitary wastewater and urban runoff (non-sanitary\nwater) from urban agglomerations drain to complex engineered networks, are\ntreated at centralized wastewater treatment plants (WWTPs) and discharged to\nriver networks. Discharge from multiple WWTPs distributed in urbanized river\nbasins contributes to impairments of river water-quality and aquatic ecosystem\nintegrity. The size and location of WWTPs are determined by spatial patterns of\npopulation in urban agglomerations within a river basin. Economic and\nengineering constraints determine the combination of wastewater treatment\ntechnologies used to meet required environmental regulatory standards for\ntreated wastewater discharged to river networks. Thus, it is necessary to\nunderstand the natural-human-engineered networks as coupled systems, to\ncharacterize their interrelations, and to understand emergent spatiotemporal\npatterns and scaling of geochemical and ecological responses. My\nPhD research involved data-model synthesis, using publicly available data and\napplication of well-established network analysis/modeling synthesis approaches.\nI present the scope and specific subjects of my PhD project\nby employing the Drivers-Pressures-Status-Impacts-Responses\n(DPSIR) framework. The defined\nresearch scope is organized as three main themes: (1) River network and urban\ndrainage networks (Foundation-Pathway of Pressures); (2) River\nnetwork, human population, and WWTPs (Foundation-Drivers-Pathway of Pressures); and (3) Nutrient loads and their impacts at\nreach- and basin-scales (Pressures-Impacts).Three\ninter-related research topics are: (1) the similarities and differences in\nscaling and topology of engineered urban drainage networks (UDNs) in two\ncities, and UDN evolution over decades; (2) the scaling and spatial\norganization of three attributes: human population (POP), population\nequivalents (PE; the aggregated population served by each WWTP), and the\nnumber/sizes of WWTPs using geo-referenced data for WWTPs in three large\nurbanized basins in Germany; and (3) the scaling of nutrient loads (P and N) discharged\nfrom ~845 WWTPs (five class-sizes) in urbanized Weser River basin in Germany,\nand likely water-quality impacts from point- and diffuse- nutrient sources. I investigate the UDN scaling using\ntwo power-law scaling characteristics widely employed for river networks: (1)\nHack’s law (length-area power-law relationship), and (2) exceedance probability\ndistribution of upstream contributing area. For the smallest UDNs, length-area\nscales linearly, but power-law scaling emerges as the UDNs grow. While\narea-exceedance plots for river networks are abruptly truncated, those for UDNs\ndisplay exponential tempering. The tempering parameter decreases as the UDNs\ngrow, implying that the distribution evolves in time to resemble those for\nriver networks. However, the power-law exponent for mature UDNs tends to be larger than the range\nreported for river networks. Differences in generative processes and\nengineering design constraints contribute to observed differences in the\nevolution of UDNs and river networks, including subnet heterogeneity and\nnon-random branching.In\nthis study, I also examine the spatial patterns of POP, PE, and WWTPs from two\nperspectives by employing fractal river networks as structural platforms:\nspatial hierarchy (stream order) and patterns along longitudinal flow paths\n(width function). I propose three dimensionless scaling indices to quantify:\n(1) human settlement preferences by stream order, (2) non-sanitary flow\ncontribution to total wastewater treated at WWTPs, and (3) degree of\ncentralization in WWTPs locations. I select as case studies three large\nurbanized river basins (Weser, Elbe, and Rhine), home to about 70% of the\npopulation in Germany. Across the three river basins, the study shows\nscale-invariant distributions for each of the three attributes with stream\norder, quantified using extended Horton scaling ratios; a weak downstream\nclustering of POP in the three basins. Variations in PE clustering among\ndifferent class-sizes of WWTPs reflect the size, number, and locations of urban\nagglomerations in these catchments. WWTP\neffluents have impacts on hydrologic attributes and water quality of receiving\nriver bodies at the reach- and basin-scales. I analyze the adverse impacts of\nWWTP discharges for the Weser River basin (Germany), at two steady river discharge\nconditions (median flow; low-flow). This study shows that significant\nvariability in treated wastewater discharge within and among different five\nclass-sizes WWTPs, and variability of river discharge within the stream order\n<3, contribute to large variations in capacity to dilute WWTP nutrient\nloads. For the median flow, reach-scale water quality impairment assessed by\nnutrient concentration is likely at 136 (~16%) locations for P and 15 locations\n(~2%) for N. About 90% of the impaired locations are the stream order < 3. At\nbasin-scale analysis, considering in stream uptake resulted 225 (~27%) P-impaired\nstreams, which was ~5% reduction from considering only dilution. This result\nsuggests the dominant role of dilution in the Weser River basin. Under the low\nflow conditions, water quality impaired locations are likely double than the median\nflow status for the analyses. This study for the Weser River basin reveals that\nthe role of in-stream uptake diminishes along the flow paths, while dilution in\nlarger streams (4≤ stream order ≤7) minimizes the impact of WWTP loads. Furthermore,\nI investigate eutrophication risk from spatially heterogeneous diffuse- and\npoint-source P loads in the Weser River basin, using the basin-scale network\nmodel with in-stream losses (nutrient uptake).Considering long-term shifts in P\nloads for three representative periods, my analysis shows that P loads from\ndiffuse-sources, mainly from agricultural areas, played a dominant role in contributing\nto eutrophication risk since 2000s, because of ~87% reduction of point-source P\nloads compared to 1980s through the implementation of the EU WFD. Nevertheless,\npoint-sources discharged to smaller streams (stream order < 3) pose\namplification effects on water quality impairment, consistent with the\nreach-scale analyses only for WWTPs effluents. Comparing to the long-term water\nquality monitoring data, I demonstrate that point-sources loads are the primary\ncontributors for eutrophication in smaller streams, whereas diffuse-source\nloads mainly from agricultural areas address eutrophication in larger streams.\nThe results are reflective of spatial patterns of WWTPs and land cover in the\nWeser River basin.Through\ndata-model synthesis, I identify the\ncharacteristics of the coupled natural (rivers) – humans – engineered (urban\ndrainage infrastructure) systems (CNHES), inspired by analogy, coexistence, and\ncausality across the coupled networks in urbanized river basins. The\nquantitative measures and the basin-scale network model presented in my PhD\nproject could extend to other large urbanized basins for better understanding\nthe spatial distribution patterns of the CNHES and the resultant impacts on\nriver water-quality impairment.