Assessing the impact of urbanization on streams using ecohydraulics

© 2019 Dr. Desmond Ofosu AnimUrban stormwater runoff is a primary degrader of stream ecosystems. Excess stormwater runoff causes altered flow regimes, reduced in-stream water quality, and modified channel form. Recognition of such impacts has renewed interest in the protection or restoration of the hydrologic cycle in urban stormwater management. One of the greatest unknowns, however, is how the urban stormwater impacts on the flow regime translates to hydraulic conditions in the channel. This understanding is critical to identifying how the hydraulic environment supports complex and dynamic ecosystem functioning. My thesis investigates how the flow regime and channel form, and their interactions, deliver in-stream hydraulic outcomes for urban streams. I use hydrodynamic modelling (TUFLOW) to predict hydraulic outcomes, with the models developed using hydrographic and topographic data from two streams in the Melbourne region. I firstly predicted a range of ecologically relevant in-stream hydraulic metrics for a natural reach and compared the metrics to those predicted for a downstream urban reach of the same stream. I found that compared to the natural reach, the urban reach experienced much greater channel bed disturbance (~4 times higher), refuge habitat loss (~2 times smaller) and limited floodplain connectivity. This work thus supports with evidence casual physical mechanisms which likely drive urban stream degradation and habitat quality. I disentangled the relative role that the flow regime and channel form play in influencing in-stream hydraulic conditions. I did this through modelling different combinations of flow regime and channel form, - e.g. altered flow regime in a natural channel versus natural flow regime in a modified channel. I found that both flow regime and channel form play key roles in setting the hydraulic conditions. The work revealed that the hydraulic regime is sensitive to the channel morphology which controlled key aspects of the hydraulic regime (e.g. magnitude, frequency and duration). After establishing that both flow regime and channel form play important roles in regulating in-stream hydraulics, I investigated potential management actions which could be applied in both developed- and developing landscapes. I tested the benefit of alternative channel rehabilitation design configurations applied to a (synthetic) degraded urban stream. This work showed that channels designed to increase morphological complexity yield ecohydraulic benefits, but not to levels which approach those predicted under natural flow- and channel conditions. With altered flow regime limiting the effectiveness of such channel designs, I tested if this stressor could be mitigated in developing landscapes using alternative approaches towards urban stormwater management. It demonstrates that widespread application of Stormwater Control Measures (SCMs) could influence the flow regime in ways which translate to ecohydraulic conditions remaining at natural levels. This work suggests that for urbanized catchments, management of in-stream hydraulics requires attention to both the flow regime (using SCMs) and channel form (e.g. through sediment regime management to allow appropriate levels of sediment supply and transport). For developing catchments, SCMs should be implemented as part of the construction phase in order to minimize downstream hydrologic impact and thus geomorphic degradation. My thesis provides an improved mechanistic understanding of why streams draining urban landscapes are commonly physically and ecologically degraded. Consideration of ecohydraulic indicators in urban stream management could provide targets, and help inform efforts towards stream protection or restoration, particularly when ecological objectives are central

Urbanization and stream ecosystems: the role of flow hydraulics towards an improved understanding in addressing urban stream degradation

Catchment urbanization is widely recognised as a primary driver of stream degradation by increasing stormwater runoff causing major changes to key ecosystem processes. Reinstating the ‘natural’ hydrogeomorphic conditions is central in designing successful, self-sustaining restoration actions. However, addressing urban stream degradation by re-establishing the hydrogeomorphic conditions remains a challenge and comparatively limited measurable progress has been observed particularly achieving ecological objectives. This paper articulates that stream restoration goals might be better achieved when management measures take a broader approach that considers anticipated hydraulic conditions effects that liaise relationships between flow and ecology. The study argues that fluvial systems are characterised by complex and dynamic ecosystem processes primarily governed by the hydraulic conditions (e.g. velocity, depth, shear stress), thus, as the practice of addressing urban stream restoration becomes increasingly common, it is critical to explore and understand the anticipated response of the hydraulic conditions. This paper describes how hydraulic regime consideration provides further opportunity for a holistic approach to urban stream management given their capacity to account for multiple ecological and geomorphic objectives. The paper suggests that developing suitable flow-biota-ecosystem processes nexus is critical towards addressing urban stream degradation and hydraulic consideration in restoration actions provide an important step towards that. The paper discusses opportunities to evolve management actions to achieve restoration goals by highlighting how the management of the two key levers (addressing altered flow regime and morphology) to improve the hydraulic conditions can help to address the urban stream disturbance.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Evaluation of NDVI Using SPOT-5 Satellite Data for Northern Ghana

Monitoring environment changes has become a necessity as a result of current environment deteriorating due to human activities and climate change. However, in most developing countries like Ghana, acquiring information concerning the current condition and the dynamic changes of the environment for a rapid monitoring is difficult. A method to monitor the environmental condition in Northern Ghana by the way of the vegetation cover using the Normalized Difference Vegetation Index (NDVI) is proposed. The method involves the use of remotely sensed data based on the absorption, transmittance and reflectance of energy by the vegetation which is significantly correlated with the amount of green leaf biomass on ground. The accuracy of this indicator is assessed with other in-situ geographic data. The main aim was to assess whether the NDVI-time series extracted from SPOT-5 images may give reliable information to assess the environment herein vegetation or land cover in Northern Ghana where there is a current rapid environmental change. It is shown that the NDVI data gives some valuable information about the vegetation hence land cover in these areas. The measurement of the amount of greenness in different areas in Northern Ghana would be simple but an objective method to assist in regular evaluation of the environmental situation of an area. The study demonstrated that NDVI technique can be employed to evaluate the vegetation cover and hence monitor the environment. Consequently, this method can be applied for other areas in the country and will be useful as an effective tool for regularly monitoring to support and create environmental awareness about the vegetative changes

Laboratory simulations of wave attenuation by an emergent vegetation of artificial Phragmites australis: an experimental study of an open-channel wave flume

This paper presents a well-controlled laboratory experimental study to evaluate wave attenuation by artificial emergent plants (Phragmites australis) under different wave conditions and plant stem densities. Results showed substantial wave damping under investigated regular and irregular wave conditions and also the different rates of wave height and within canopy wave-induced flows as they travelled through the vegetated field under all tested conditions. The wave height decreased by 6%–25% at the insertion of the vegetation field and towards the downstream at a mean of 0.2 cm and 0.32 cm for regular and irregular waves respectively. The significant wave height along the vegetation field ranged from 0.89–1.76 cm and 0.8–1.28 cm with time mean height of 1.38 cm and 1.11 cm respectively for regular and irregular waves. This patterns as affected by plant density and also location from the leading edge of vegetation is investigated in the study. The wave energy attenuated by plant induced friction was predicted in terms of energy dissipation factor (fe) by Nielsen’s (1992) empirical model. Shear stress as a driving force of particle resuspension and the implication of the wave attenuation on near shore protection from erosion and sedimentation was discussed. The results and findings in this study will advance our understanding of wave attenuation by an emergent vegetation of Phragmites australis, in water system engineering like near shore and bank protection and restoration projects and also be employed for management purposes to reduce resuspension and erosion in shallow lakes. First published online: 21 Oct 201