Small Urban Watershed Research Articles

Anthropogenic Macroscale and Microscale Debris (Including Plastics) Have Differing Spatial Distributions Across a Small Urban Watershed

Anthropogenic Macroscale and Microscale Debris (Including Plastics) Have Differing Spatial Distributions Across a Small Urban Watershed

Assessing wet season water quality and health risks of Chaobai River: impacts of natural weathering and anthropogenic activities in a small urban watershed

ABSTRACT Small urban watersheds are susceptible to external influences, facing increased solute inputs during high-flow periods without sufficient dilution. This study determined the sources of major ions in the Chaobai River, which is the essential surface water source of North China. The dominant river water type was HCO3-Ca·Mg based on the Piper diagram. PCA and ion ratios revealed that natural weathering (evaporate dissolution, carbonate, and silicate weathering by carbonic and sulfuric acid) and anthropogenic activities (urban domestic sewage and industrial effluent) were the dominant factors affecting the chemical composition. Sodium adsorption ratio (SAR), soluble sodium percentage (Na%), and residual sodium carbonate (RSC) assessments indicated potential sodium and moderate salinity hazards. Meanwhile, the non-carcinogenic health risks of riverine solutes evaluated by health risk assessment need to be noticed, particularly for children. The limitation of urban sewage discharge is crucial for improving water quality in urban watershed and needs further studies.

Use of neural network as a support tool in water level forecasting and issuing flash floods early warnings to three small Brazilian urban watersheds

Use of neural network as a support tool in water level forecasting and issuing flash floods early warnings to three small Brazilian urban watersheds

Calculation of ecological water replenishment in small coastal watersheds based on environmental capacity constraints: A case study of eastern Xiamen, China

In response to the issues of severe coastal pollution and exacerbated eutrophication, it is crucial to consider characteristics such as small flow in small coastal urban watersheds and water of low quality when implementing water replenishment measures. The selection of appropriate constraints and methods for calculating the ecological water replenishment (EWR) is the top priority. Here we present a calculation model appropriate for pin down the range of EWR in small coastal watersheds. Taking four of the eastern regions of Xiamen City as examples, the minimum and maximum EWR are independently computed based on the envelope curve and the restriction of the estuarine area's environmental capacity. Rational results are achieved with the approaches of Tennant as well as landscape water requirement, and it shows that the range of EWR for the four watersheds is 1.77×106 – 12.41×106 m3. The calculation model proposed in this paper has greater environmental feasibility from the perspective of the river channel demand and coastal environmental protection. Additionally, it also can provide a reference for EWR plans for urban rivers in coastal regions.

Rainfall-Runoff Modelling Using HEC-HMS Model, Remote Sensing and GIS in Middle Gujarat, India

Hydrological modeling is a widely used approach for estimating the hydrological response of a basin to precipitation. Floods are among the most catastrophic natural disasters in small urban watersheds, inflicting loss of life, massive property destruction, and a severe danger to the economy. As a result, appropriate modeling can be a useful tool in preventing and mitigating such flood hazards. Despite this, flash flood prediction remains one of the challenges of hydrological modeling in ungauged basins due to a lack of runoff observations. This study aims to calibrate and validate the rainfall-runoff transformation model for Hathmati river sub watershed in the Sabarmati River basin using HEC-HMS (Hydrologic Engineering Centre Hydrology Modeling System). For the loss rate, SCS Curve Number method was selected while Clark Unit Hydrograph and SCS unit hydrograph was used for the transform method. The model is calibrated and verified using two rainfall-runoff events from 2006 and 2007 year The model calibration and validation efficiency were verified for both methods using the Nash-Sutcliffe efficiency (NSE), The coefficient of determination (R2), and the Percent Bias (PBIAS) As a result, the model calibration and validation were found to be satisfactory with the acceptable value of NSE between 0.869 to 0.914, with R2 0.901 to 0.947 and PBIAS from 9.76 to 14.8. it is observed that the model shows a very good correlation between simulated flow and observed flow. As a result, the model can be used to forecast river flow and aid in flood mitigation efforts to lessen their effects and associated costs. Additionally, the findings of this study can serve as guidelines for future assessments of the flood risk in the study area.

Assessing the Effects of Urbanization on Water Flow and Flood Events Using the HEC-HMS Model in the Wirynka River Catchment, Poland

Floods are among the most devastating natural disasters in small urban watersheds resulting in loss of life and enormous damage to property and posing a serious threat to the economy. Appropriate modeling can be a useful tool to prevent and reduce such flood risks. It is currently important to use hydrological models to assess and predict the water availability of river basins due to climate change to develop a strategy to cope with the changing environment. The study used an integrated approach combining a geographic information system (GIS) and hydrological modeling to assess the impact of urbanization on flash floods in peri-urban developing areas. The modeling was carried out for two spatial aspects relating to 1990 and 2018. The model allowed simulation of the river water flow that can occur under different rainfall probabilities. The study showed that rapid urbanization adversely affects hydrological processes. Reduced infiltration capacity increases water runoff, thereby increasing the risk of flooding or waterlogging. Therefore, it is extremely important to properly manage rainwater in the area.

The nitrogen budget of an urban watershed: Zeekoevlei, Cape Town

Urban watersheds receive excessively high Nitrogen (N) loads predominantly via N deposition, fertilizer use, stormwater runoff, treated effluent discharge and sewage leaks. Beyond engineered solutions, the capacity of these systems to remove this excess N is largely dependent upon the functioning of riparian and wetland ecosystems, which in urban areas are highly modified, fragmented and increasingly under pressure. Appropriate management of urban watersheds needs to consider N flux and process rates that occur within the urban landscape. This research provides a preliminary quantification of the relative magnitudes of N flux in the context of a small urban watershed comprising key features of cities in the Global South: a mosaic of informal and formal residential, peri-urban agriculture and stressed service delivery. For a watershed system of 88 km2, total N inputs amount to 2.82 Gg N yr−1, 78 % of which is in the form of household food. N outputs amount to 2.35 Gg N yr−1 with 86 % of that coming out of wastewater streams either as treated effluent or during the denitrification process. An estimated 0.17 Gg N yr−1 accumulates in the system as plant biomass, in groundwater, soils and landfill. The Zeekoevlei wetland system is hypothesised to remove 76 % of total N which drains off the urban landscape, indicating an important service provided to the City, in addition to other well documented ecosystem services. In this urban system, our estimates indicate N flux rates almost 3 times higher than what would be here naturally in a nutrient-deplete Fynbos system. This work daylights important monitoring and research needs in order to better understand the role of the urban landscape and wetland ecosystems in a highly human-mediated N cycle.

Multiscale and multi event evaluation of short-range real-time flood forecasting in large metropolitan areas

Many large metropolitan areas are especially susceptible to floods generated by heavy, short-duration rainfall. The high density of people, buildings and infrastructure in these areas underline the importance of developing flood resilient cities and communities. An accurate real-time flood forecasting system can support decision making for launching preparedness and response actions in short-range (hours to days), and assist mitigating the disturbances caused by floods. This study explores the short-range predictive capability of a real-time flood forecast system by coupling the High-Resolution Rapid Refresh (HRRR) meteorological forecasted variables with a fully distributed hydrological model (WRF-Hydro). We provide a comprehensive analysis of short-range (36 h) forecasts for 19flood events generated by heavy rainfall in small urban and suburban watersheds (<200 km2) in the National Capital Region of the United States. Flood forecast performance are then assessed using different metrics based on observed data from U.S. Geological Survey stream gauges and reanalysis simulations using the StageIV quantitative precipitation estimates. Results show that despite the high variability of the HRRR cycle-to-cycle precipitation forecasts, the real-time flood forecast system can produce skillful forecasts with similar overall performance as the reanalysis simulations, achieving 65 % of flood detection rate. This variability had more impact on forecast skill in smaller subbasins, and flood forecasts were more consistent for longer duration and larger spatial extent rainfall. Hourly flood forecasts performed well even in smaller watersheds, correctly detecting flood occurrence with up to 34 h lead time and resulting in low peak flow magnitude and timing errors.

Predictive models of phosphorus concentration and load in stormwater runoff from small urban residential watersheds in fall season

Urban street trees are a key part of public green infrastructure in many cities, however, leaf litter on streets is a critical biogenic source of phosphorus (P) in urban stormwater runoff during Fall. This study identified mass of street leaf litter (Mleaf) and antecedent dry days (ADD) as the top two explanatory parameters that have significant predictive power of event end-of-pipe P concentrations through multiple linear regression (MLR) analysis. Mleaf and volume of runoff (Vol) were the top two key explanatory parameters of event end-of-pipe P loads. Two-predictor MLR models were developed with these explanatory parameters using a 40-storm dataset derived from six small urban residential watersheds in Wisconsin, USA, and evaluated using storms specific to each study basin. The MLR model validation results indicated sensitivity to storm composition in the datasets. Our analysis shows selected parameters can be used by environmental managers to facilitate end-of-pipe P prediction in urban areas. This information can be used to reduce the amount of P in stormwater runoff by adjusting the timing and frequency of municipal leaf collection and street cleaning programs in urban areas.

Water sensitive spatial planning in terms of sustainable stormwater management: The case of Bornova Stream Catchment (Izmir), Turkey

ABSTRACT Sustainable stormwater management in developing countries requires more research, and budget compared to developed countries. Runoffmeasurement in small urban watersheds is often not carried out; city plans aremade without much consideration of the effect of runoff. In this study, the effect of land use decisions on the surface runoff was assessed using land use change scenarios in an urban catchment located in Izmir, Turkey. Answers to the following two questions were sought: (1) how the runoff will be affected if the city plan decisions currently in use are implemented and (2) whether it ispossible to control the runoff in an urban catchment by developing rational, sustainable planning suggestions based on water. The main novelty of this study is to show the importance of the maquis cover, as well as the permeability of the built-up environment and the existence of open spaces in lower catchments are vital in meeting the runoff.

Impacts of Rainfall Data Aggregation Time on Pluvial Flood Hazard in Urban Watersheds

Pluvial floods occur when heavy rainstorms cause the surcharge of the sewer network drainage, representing one of the most impacting natural hazard in urban watersheds. Pluvial flood hazard is usually assessed considering the effect of annual maxima rainfall of short duration, comparable with the typical concentration times of small urban watersheds. However, short duration annual maxima can be affected by an error of underestimation due to the time resolution as well as the aggregation time of the rainfall time series. This study aims at determining the impact of rainfall data aggregation on pluvial flood hazard assessment. Tuscany region (Central Italy) is selected as a case study to perform the assessment of the annual maxima rainfall underestimation error, since the entire region has the same temporal aggregation of rainfall data. Pluvial flood hazard is then evaluated for an urban watershed in the city of Florence (Tuscany) comparing the results obtained using observed (uncorrected) and corrected annual maxima rainfall as meteorological forcing. The results show how the design of rainfall events with a duration of 30 min or shorter is significantly affected by the temporal aggregation, highlighting the importance of correcting annual maxima rainfall for a proper pluvial flood hazard evaluation.

Comparison between NARX-NN and HEC-HMS models to simulate Wadi Seghir catchment runoff events in Algerian northern

ABSTRACT This paper presents a comparison between the black box Nonlinear Auto-Regressive with eXogenous inputs-Neural Network (NARX-NN) and the conceptual Hydrologic Engineering Centre-Hydrologic Modelling System (HEC-HMS) Rainfall-Runoff models. The two models were applied on a small urban watershed to assess its response to fourteen hourly real storm events. The differences between the steps engaged in each model to reach the hydrograph were presented in detail. The estimation of the best parameters is carried out using a weighted average function during the calibration phase. A statistical evaluation was conducted to assess the model’s performance thereafter; a critical comparison was made to illustrate the differences and discuss the steps involved. The results indicate that both models successfully reflect the urban basin runoff. However, the NARX-NN outperforms in the testing phase owing to their strength generalization feature. The NARX-NN model has more strength to produce the shape bending of the hydrograph. Consequently, this model is better to highlight the curvatures resulting from the local peaks of rainfall.

Vegetated Buffer Zone Restoration Planning in Small Urban Watersheds

Vegetated buffer zones (VBZ) are accepted worldwide as a low impact method to avoid non-point source pollution and restore the balance of river ecosystems. Strongly influenced by industrialization and urbanization, urban river ecology is seriously damaged, and restoration is tricky. This study established a complete buffer zone construction framework suitable for the small urban watershed, and its feasibility is verified in a small watershed in Northern China. First, common plants in the study area were selected to test their ability to purify pollutants, and plant combinations were optimized. Secondly, according to the field investigation, the reference buffer zone was determined, and its sewage interception capacity was tested through a runoff simulation experiment. Then, based on GIS and Phillips time and hydraulic models, the normal buffer width of the study area was obtained; 60 m for mainstream and 40 m for tributaries. By optimizing the vegetation scheme and delimiting an efficient buffer zone, the land occupation can be reduced by 17%. Finally, combined with the characteristics of different river sections, an elaborate VBZ restoration scheme is designed from the aspects of vegetation, planning, and zoning. Generally, this research will provide government and land managers scientific and practical ideas and technologies to formulate a land management policy for urban river buffer zones in order to find a balance between aquatic ecological protection and urban land use planning and optimize the allocation of construction funds.

The impact of the spatiotemporal structure of rainfall on flood frequency over a small urban watershed: an approach coupling stochastic storm transposition and hydrologic modeling

Abstract. The role of rainfall space–time structure, as well as its complex interactions with land surface properties, in flood response remains an open research issue. This study contributes to this understanding, specifically for small (&lt;15 km2) urban watersheds. Using a flood frequency analysis framework that combines stochastic storm transposition (SST)-based rainfall scenarios with the physically based distributed Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model, we examine the role of rainfall spatial and temporal variability in flood frequency across drainage basin scales in the highly urbanized Dead Run watershed (14.3 km2), Maryland, USA. The results show the complexities of flood response within several subwatersheds for both short (&lt;50 years) and long (&gt;100 years) rainfall return periods. The impact of impervious area on flood response decreases with increasing rainfall return period. For extreme storms, the maximum discharge is closely linked to the spatial structure of rainfall, especially storm core spatial coverage. The spatial heterogeneity of rainfall increases flood peak magnitudes by 50 % on average at the watershed outlet and its subwatersheds for both small and large return periods. The framework of SST–GSSHA-coupled frequency analysis also highlights the fact that spatially distributed rainfall scenarios are needed in quick-response flood frequency, even for relatively small basin scales.

Estimation of rainfall threshold for flood warning for small urban watersheds based on the 1D–2D drainage model simulation

Flood warning systems can provide an effective and low-cost solution to protect urban residents especially in developing countries suffering from severe flood damages caused by weak capacity of flood defense structures. Rainfall Threshold for Flood Warning (RTFW) is an essential key of accurate flood warning system. This study proposed an easy-to-use mathematical equation for RTFW for small urban watersheds based on computer simulations. First, a coupled 1D–2D dual-drainage model was used to simulate the flooded area in nine watersheds of Seoul, Korea corresponding to 540 scenarios of various synthetic rainfall events and watershed imperviousness. Then, the results of the 101 simulations that caused the critical flooded depth (0.25 m–0.35 m) were used to develop the equation that relates the value of RTFW to the rainfall event temporal variability (represented as coefficient of variation or CV) and the watershed imperviousness (represented as NRCS Curve Number or CN). The results suggest that (1) RTFW exponentially decreases as the rainfall CV increases; (2) RTFW linearly decreases as the watershed CN increases; and that (3) RTFW is dominated by CV when the rainfall has low temporal variability (e.g., CV < 0.2) while RTFW is dominated by CN when the rainfall has high temporal variability (e.g., CV > 0.4). For validation, the proposed equation was applied for the flood warning system with two storm events occurred in 2010 and 2011 over 239 watersheds in Seoul. The system showed the hit, false and missed alarm rates at 69% (48%), 31% (52%) and 6.7% (4.5%), respectively for the 2010 (2011) event.

Impacts of Highway Runoff on Metal Contamination Including Rare Earth Elements in a Small Urban Watershed: Case Study of Bordeaux Metropole (SW France).

High temporal resolution sampling of runoff (15 samples/4h) and river water (24 samples/24h) was performed during a major rainstorm (41mm/4h) in the Bordeaux Metropole, after a dry and high vehicle-density period. Runoff was sampled at the outlet of one collector draining Northern Bordeaux Highway (NBH; 80,000-93,000 vehicles/day) and river water in the downstream Jalle River. The studied metals, including priority and emergent (Rare Earth Elements [REEs]) contaminants, showed major temporal and spatial variations in the dissolved and particulate concentrations. Hierarchical cluster analyses distinguished metal groups, reflecting different: (i) sources (i.e., automotive traffic: Zn-Cu-Ce and wastewater treatment plant: Cd-Ag-Gd) and/or (ii) processes (i.e., groundwater dilution by rainwater and sorption processes). The contribution of the particulate fraction to total metal fluxes was predominant in the NBH collector (except for Sr and Mo) and highly variable in the Jalle River, where the highest particulate metal loads were due to the export of road dusts exported by the NBH collector. Metal fluxes from the NBH collector represented highly variable fractions of daily fluxes into the Gironde Estuary at the outlet of the Jalle River, depending on elements and partitioning. The resulting relative contributions ranged from: 5% (Sr) to 40% (Cu) for dissolved phases and 30% (As) to 88% (Cu) for particulate phases. The first 40min of the event accounted for 65% of the suspended particulate matter flux (and associated particulate metals) exported by the NBH collector, whereas the respective water flux contribution was 35%. This finding clearly demonstrates the importance of monitoring the first minutes of rainy events when establishing mass balances in urban systems.

Analysis of rainfall extremes in the Ngong River Basin of Kenya: Towards integrated urban flood risk management

Extreme rainfall events are a major cause of highly disruptive flooding in small urban watersheds with limited flood risk management systems. In the Ngong River Basin of Kenya, such floods affect more than 0.5 Million residents within the Kibera informal settlements of Nairobi. However, there is paucity of information about the characteristics of the extreme rainfalls to support flood risk management. This study investigated the best-fit probability distribution models for the extreme rainfalls of the Ngong Basin using Block Maxima approach as a basis for anticipatory flood risk management. Daily rainfall data for the period between 1968 and 2017 were acquired from the existing two rainfall stations to support the analysis at monthly, seasonal and annual timescales. The Gamma, Pearson Type III, Gumbel and Generalized Extreme Value distributions were selected and applied to each timescale. Parameters of the distributions were determined using the Maximum-Likelihood estimator. The validity of the fitted probability models was tested using the Kolomogorov- Smirnov, Anderson-Darling and Cramer von Misses measures for Goodness of Fit. The best-fit probability distributions were subsequently used to establish the rainfall frequencies and return levels at annual timescales. The results show that Pearson Type III provided the best fit at monthly timescales during the dry spell months, while the Generalized Extreme Value distribution provided best results during the wet periods. At seasonal timescales, the Gamma distribution was noted to be the best-fit model. The return levels developed could essentially support the design of urban flood control structures for appropriate flood risk management.

Assessment of Stormwater Drainage System for Small Urban Watershed: Case of Shambu Town, Oromia Region, Ethiopia

Stormwater runoff in a small urban watershed is one of the major challenging issues facing many countries where financial capacity is very weak to provide sufficient drainage systems. This study aims to assess the stormwater drainage system in the small urban watershed as a case study of Shambu town, Oromia Region, Ethiopia. The main objective of this study was to assess whether this excess stormwater runoff is due to hydrological change or insufficient hydraulic design of the existing drainage system in the town. The existing drainage system in this small urban watershed was analyzed by statistical analysis and the main purpose of this was to know whether the designed drainage systems were sufficient hydraulically. The existing drainage system in this small urban watershed was analyzed by statistical analysis and the main purpose of this was to know whether the designed drainage systems were sufficient hydraulically. The existing design document was taken in this analysis and field analysis also made to check whether the drainage was constructed according to the specification. The area also analyzed hydrologically by delineating the watershed using ArcSWAT and the peak discharge was computed the outlet of the watershed using Rational method. Some data such runoff coefficient and manning’s values were also generated from the watershed based on the reality on the ground. The two results from hydrological and hydraulic models were compared and that of peak discharge obtained from rational method became greater than of the Peak discharge obtained from existing drainage measurement. Therefore, in this study it was concluded that since the watershed is steep in slope, it is contributing additional runoff to this small urban watershed, additional new 50% of the existing drainage system and rehabilitation works should be implemented to modify the draining capacity of the existing scheme. Additionally, managing and improper construction alignment problem in the existing system were identified, so to control this challenge periodic cleaning and modification of slope is recommended. Keywords: Hydraulic Model, Hydrological Model, Runoff, Small Urban Watershed, Stormwater DOI: 10.7176/CER/12-9-01 Publication date: September 30 th 2020

Assessment of Stormwater Drainage System for Small Urban Watershed: A Case of Shambu Town, Oromia Region, Ethiopia

Stormwater runoff in a small urban watershed is one of the major challenging issues facing many countries where financial capacity is very weak to provide sufficient drainage systems. This study aims to assess the stormwater drainage system in the small urban watershed as a case study of Shambu town, Oromia Region, Ethiopia. The main objective of this study was to assess whether this excess stormwater runoff is due to hydrological change or insufficient hydraulic design of the existing drainage system in the town. The existing drainage system in this small urban watershed was analyzed by statistical analysis and the main purpose of this was to know whether the designed drainage systems were sufficient hydraulically. The existing drainage system in this small urban watershed was analyzed by statistical analysis and the main purpose of this was to know whether the designed drainage systems were sufficient hydraulically. The existing design document was taken in this analysis and field analysis also made to check whether the drainage was constructed according to the specification. The area also analyzed hydrologically by delineating the watershed using ArcSWAT and the peak discharge was computed the outlet of the watershed using Rational method. Some data such runoff coefficient and manning’s values were also generated from the watershed based on the reality on the ground. The two results from hydrological and hydraulic models were compared and that of peak discharge obtained from rational method became greater than of the Peak discharge obtained from existing drainage measurement. Therefore, in this study it was concluded that since the watershed is steep in slope, it is contributing additional runoff to this small urban watershed, additional new 50% of the existing drainage system and rehabilitation works should be implemented to modify the draining capacity of the existing scheme. Additionally, managing and improper construction alignment problem in the existing system were identified, so to control this challenge periodic cleaning and modification of slope is recommended.

The Impact of Pavement Permeability on Time of Concentration in a Small Urban Watershed with a Semi-Arid Climate

Despite the popularity of the permeable pavements in urban areas in recent years, a few empirical equations have been developed to estimate the time of concentration (Tc) on pervious surfaces, and almost none of them is evaluated over the permeable pavements. In this paper, we aim to investigate the effect of low impact development (LID) on Tc and develop a new equation for estimating overland flow Tc compatible with both permeable and impermeable pavements in small urban watersheds with a semi-arid climate. An experimental model was devised to determine the new equation. Synthetic rainfall (22–40 mm.h−1) was applied to a rectangular shaped (6 m × 1 m) test watershed with an adjustable longitudinal slope (0.5–8%) and zero lateral slope. Porous asphalt and permeable interlocking concrete paver (PICP) were employed as the permeable pavements. Then, to validate the obtained equation from experimental data, a field study was conducted over a 1-ha watershed during a Low intensity (<4 mm.h−1) rain event. The comparison between 13 empirical and semi-empirical equations (including the one developed in this study) for estimating Tc in the test condition indicates that Chen and Wong’s equation with −0.03 bias and the Nash-Sutcliffe efficiency (NSE) equal to 0.9 and 0.7 for impervious and pervious surfaces respectively is the most precise equation. On the other hand, for the field study, the newly developed formula outperformed other equations and estimated the Tc with a 3.7% error. Moreover, the results of experiments on the PICP surface showed that replacing 10 to 15% of an urban watershed surface by PICP in a region with the rainfall intensity less than 40 mm.h−1 and storm duration less than 1 h causes a considerable reduction (almost 100%) in the surface runoff.