<p></p><p></p><p></p><p>In
urbanized river basins, sanitary wastewater and urban runoff (non-sanitary
water) from urban agglomerations drain to complex engineered networks, are
treated at centralized wastewater treatment plants (WWTPs) and discharged to
river networks. Discharge from multiple WWTPs distributed in urbanized river
basins contributes to impairments of river water-quality and aquatic ecosystem
integrity. The size and location of WWTPs are determined by spatial patterns of
population in urban agglomerations within a river basin. Economic and
engineering constraints determine the combination of wastewater treatment
technologies used to meet required environmental regulatory standards for
treated wastewater discharged to river networks. Thus, it is necessary to
understand the natural-human-engineered networks as coupled systems, to
characterize their interrelations, and to understand emergent spatiotemporal
patterns and scaling of geochemical and ecological responses. </p><br><p></p><p></p><p>My
PhD research involved data-model synthesis, using publicly available data and
application of well-established network analysis/modeling synthesis approaches.
I present the scope and specific subjects of my PhD project
by employing the <i>Drivers-Pressures-Status-Impacts-Responses</i>
(<i>DPSIR</i>) framework. The defined
research scope is organized as three main themes: (1) River network and urban
drainage networks (<i>Foundation</i>-<i>Pathway of Pressures</i>); (2) River
network, human population, and WWTPs (<i>Foundation</i>-<i>Drivers</i>-<i>Pathway of Pressures</i>); and (3) Nutrient loads and their impacts at
reach- and basin-scales (<i>Pressures</i>-<i>Impacts</i>).</p><br><p></p><p></p><p>Three
inter-related research topics are: (1) the similarities and differences in
scaling and topology of engineered urban drainage networks (UDNs) in two
cities, and UDN evolution over decades; (2) the scaling and spatial
organization of three attributes: human population (POP), population
equivalents (PE; the aggregated population served by each WWTP), and the
number/sizes of WWTPs using geo-referenced data for WWTPs in three large
urbanized basins in Germany; and (3) the scaling of nutrient loads (P and N) discharged
from ~845 WWTPs (five class-sizes) in urbanized Weser River basin in Germany,
and likely water-quality impacts from point- and diffuse- nutrient sources. </p><br><p></p><p></p><p>I investigate the UDN scaling using
two power-law scaling characteristics widely employed for river networks: (1)
Hack’s law (length-area power-law relationship), and (2) exceedance probability
distribution of upstream contributing area. For the smallest UDNs, length-area
scales linearly, but power-law scaling emerges as the UDNs grow. While
area-exceedance plots for river networks are abruptly truncated, those for UDNs
display exponential tempering. The tempering parameter decreases as the UDNs
grow, implying that the distribution evolves in time to resemble those for
river networks. However, the power-law exponent for mature UDNs tends to be larger than the range
reported for river networks. Differences in generative processes and
engineering design constraints contribute to observed differences in the
evolution of UDNs and river networks, including subnet heterogeneity and
non-random branching.</p><br><p></p><p></p><p>In
this study, I also examine the spatial patterns of POP, PE, and WWTPs from two
perspectives by employing fractal river networks as structural platforms:
spatial hierarchy (stream order) and patterns along longitudinal flow paths
(width function). I propose three dimensionless scaling indices to quantify:
(1) human settlement preferences by stream order, (2) non-sanitary flow
contribution to total wastewater treated at WWTPs, and (3) degree of
centralization in WWTPs locations. I select as case studies three large
urbanized river basins (Weser, Elbe, and Rhine), home to about 70% of the
population in Germany. Across the three river basins, the study shows
scale-invariant distributions for each of the three attributes with stream
order, quantified using extended Horton scaling ratios; a weak downstream
clustering of POP in the three basins. Variations in PE clustering among
different class-sizes of WWTPs reflect the size, number, and locations of urban
agglomerations in these catchments. <b></b></p><br><p></p><p></p><p>WWTP
effluents have impacts on hydrologic attributes and water quality of receiving
river bodies at the reach- and basin-scales. I analyze the adverse impacts of
WWTP discharges for the Weser River basin (Germany), at two steady river discharge
conditions (median flow; low-flow). This study shows that significant
variability in treated wastewater discharge within and among different five
class-sizes WWTPs, and variability of river discharge within the stream order
<3, contribute to large variations in capacity to dilute WWTP nutrient
loads. For the median flow, reach-scale water quality impairment assessed by
nutrient concentration is likely at 136 (~16%) locations for P and 15 locations
(~2%) for N. About 90% of the impaired locations are the stream order < 3. At
basin-scale analysis, considering in stream uptake resulted 225 (~27%) P-impaired
streams, which was ~5% reduction from considering only dilution. This result
suggests the dominant role of dilution in the Weser River basin. Under the low
flow conditions, water quality impaired locations are likely double than the median
flow status for the analyses. This study for the Weser River basin reveals that
the role of in-stream uptake diminishes along the flow paths, while dilution in
larger streams (4≤ stream order ≤7) minimizes the impact of WWTP loads. </p><br><p></p><p></p><p>Furthermore,
I investigate eutrophication risk from spatially heterogeneous diffuse- and
point-source P loads in the Weser River basin, using the basin-scale network
model with in-stream losses (nutrient uptake).Considering long-term shifts in P
loads for three representative periods, my analysis shows that P loads from
diffuse-sources, mainly from agricultural areas, played a dominant role in contributing
to eutrophication risk since 2000s, because of ~87% reduction of point-source P
loads compared to 1980s through the implementation of the EU WFD. Nevertheless,
point-sources discharged to smaller streams (stream order < 3) pose
amplification effects on water quality impairment, consistent with the
reach-scale analyses only for WWTPs effluents. Comparing to the long-term water
quality monitoring data, I demonstrate that point-sources loads are the primary
contributors for eutrophication in smaller streams, whereas diffuse-source
loads mainly from agricultural areas address eutrophication in larger streams.
The results are reflective of spatial patterns of WWTPs and land cover in the
Weser River basin.</p><br><p></p><p></p><p>Through
data-model synthesis, I identify the
characteristics of the coupled natural (rivers) – humans – engineered (urban
drainage infrastructure) systems (CNHES), inspired by analogy, coexistence, and
causality across the coupled networks in urbanized river basins. The
quantitative measures and the basin-scale network model presented in my PhD
project could extend to other large urbanized basins for better understanding
the spatial distribution patterns of the CNHES and the resultant impacts on
river water-quality impairment.</p><p><br></p><p></p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/9964232 |
| Date | 17 October 2019 |
| Creators | Soohyun Yang (7484483) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/COUPLED_ENGINEERED_AND_NATURAL_DRAINAGE_NETWORKS_DATA-MODEL_SYNTHESIS_IN_URBANIZED_RIVER_BASINS/9964232 |
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