It is widely accepted that urban development results in larger flood peak discharges with faster catchment response times, larger total runoff volumes and lower base flow volumes. However, these effects have not previously been studied in the context of the specific characteristics of fast-growing urban areas in developing countries, which are generally unregulated. This study quantifies the effects of urban development on runoff, including: a) runoff volumes, b) base flow volumes and c) flood peaks from eight South African catchments using the Mann–Kendall test and Kendall’s τ. Both total runoff and base flow volumes are found to increase with increased development levels and possible reasons for this are discussed. The flood peak analysis finds statistically insignificant trends in most catchments. However, there is an increasing trend in the catchment with the highest proportion of informal development. Recommendations are made for further investigation into reasons for the findings.

The imperviousness of urban surfaces is an important parameter in simulating urban hydrological responses, but quantifying imperviousness can be challenging and time-consuming. In response, this study presents a new framework to efficiently estimate the imperviousness of urban surfaces, using satellite images with Red, Green and Blue bands and a land cover dataset with multiple built-up urban classes through remote sensing, machine learning and field verification. The methodology is adaptable to other regions with similar datasets. For a case study in Pretoria, South Africa, major differences in median total impervious area percentages (mTIA%) were identified when compared between land cover groups: residential areas had a lower imperviousness median (mTIA% = 38%) than commercial (mTIA% = 81%) and industrial (mTIA% = 89%) land cover. The mTIA% also varies between 17 and 61% for a range of different formally developed residential classes and between 14 and 43% for a range of different informally developed residential classes. These mTIA% are recommended for any urban area within the South African National Land Cover dataset. These values can be incorporated into hydraulic and hydrological models, which improve the efficiency of parameter estimation for modelling. The methodology successfully quantified temporal imperviousness changes in the study area.

Culverts often fail to handle increased flood peaks due to urbanisation and climate change. Modifying culvert inlets to increase discharge capacity can negate the need for additional culvert barrels or to rebuild the entire structure. Although some previous studies investigated hydraulic culvert improvements, this study is the first to test different combinations of headwall and wingwall angles, and the effect of aeration vents, to improve capacity of inlet-controlled culverts. This physical modelling study evaluates various modified box and circular culvert inlets, quantifies their impact on capacity, develops a coefficient for use in standard equations, and verifies the alignment of results with established references and guidelines. A 15° headwall with a 30° wingwall added to a box culvert or a rounded inlet edge for a circular culvert improved the flow by up to 34% at a headwater depth of twice the culvert height (2D), or up to 18% at 1.2D for box culverts and 24% at 1.2D for circular culverts. An air vent after the inlet had an insignificant influence on the capacity. A novel flow improvement coefficient was developed to calculate improved capacity with existing design equations. Culvert inlet improvements will reduce flood risks and contribute to sustainable drainage infrastructure.

Side-by-side comparison of a Pressurised (PRS) and Drop-Forming Rainfall Simulator (DRS), with icons indicating intensity, uniformity, drop size, and velocity. PRS suits high-intensity studies; DRS offers broader versatility. Rainfall simulators are typically categorised as either pressurised or drop-forming systems, each offering distinct advantages depending on the research application. This study presents a comparative experimental analysis of a pressurised nozzle-based rainfall simulator and a drop-forming needle-based rainfall simulator, assessing their suitability for urban stormwater management research. Both setups were constructed indoors with a 1.5 × 1.5 m rainfall area and a 2.3 m raindrop fall height. The simulators were evaluated based on rainfall intensity, spatial uniformity, drop size distribution and drop velocity. The pressurised system produced intensities from 23.9 to 109.7 mm/h, while the drop-forming system achieved 4.5–21.0 mm/h. Uniformity coefficients ranged from 81.2 to 93.3% (cups) and 86.6 to 96.4% (trays) for the pressurised system, and 85.7% (cups) and 94.8% (trays) for the drop-forming system. D50 raindrop sizes ranged from 0.8 to 1.2 mm for the pressurised system and 1.7 mm for the drop-forming system. Estimated median drop velocities ranged from 82.5 to 98.5% of terminal velocity. While both systems replicated key rainfall characteristics, the pressurised simulator was limited to high-intensity events. The drop-forming simulator offered finer control and broader applicability, supporting its use in urban hydrology and stormwater management research.