Probabilistic model for the annual number of storm overflow discharges in a stormwater drainage system

The paper presents an attempt to develop a probabilistic model for predicting an annual number of storm overflow discharges. Forecasting the occurrence of an overflow discharge event involved the application of the logistic regression, which does not require the development of complex hydrodynamic catchment models. The performed calculations showed that the logistic regression model can be successfully used to evaluate the performance of the emergency overflow weir. The resultant logit model eliminates the necessity to develop hydrodynamic models, to conduct continuous measurements of the flow intensity in the stormwater drainage system and to collect detailed information on the characteristics of the subcatchments within the analyzed catchment. The hydrodynamic model was used to simulate the annual number of discharges. The analysis of the results demonstrated that they are in the range of stochastic values, which indicates an application-related character of the method.     

武汉市汉阳区的暴雨径流污染特征

为研究合流制城区的暴雨径流污染特征，在武汉市汉阳区的4个合流制集水区布置了监测断面，对暴雨径流过程中的水质、水量进行了连续监测。结果表明：污染物浓度的峰值一般提前或同步于径流量的峰值，初期径流污染较重；采用污染物浓度变异指数可以反映不同集水区暴雨径流污染的差异性；汉阳城区暴雨径流的SS、COD、TP和TN平均浓度分别为601．1、299．2、0．88和12．26mg／L，同珠海市相比，其暴雨径流污染水平明显要高。     

Calibration of urban stormwater drainage models using hydrograph modelling

Use of mathematical models requires the estimation of model parameters, which is usually known as the calibration of the model. In general, parameter optimization is preferred in model calibration to the trial-and-error visual comparison of observed and modelled output responses, due to subjectivity and the time-consuming nature of the latter approach. An optimization procedure, called two-stage inner/outer optimization, is described in this paper, which can be used to estimate the model parameters of any urban stormwater drainage catchment modelled with any urban drainage computer modelling software. However, the ILSAX computer software was used in this study. The method is designed to provide the ‘best’ set of model parameters that consider several storm events simultaneously. Impervious area parameters are obtained from frequent ‘small’ storm events, while the pervious area parameters are obtained from less-frequent ‘large’ events. The Giralang catchment in Canberra (Australia) was used to demonstrate the method. Several ‘small’ and ‘large’ storm events of the catchment were considered in parameter optimization. Few other storm events, which were not used in model calibration, were used to validate the model parameters obtained from calibration. Results from both calibration and validation showed that the ‘best’ set of model parameters obtained for the catchment was able to produce hydrographs similar to the observed hydrographs. Pervious and impervious area parameters obtained from calibration agreed well with the information gathered from other sources such as aerial photographs and published literature.     

基于IHACRES和GLUE的降雨径流过程模拟

根据降雨径流过程的特征,修正了IHACRES水文模型,并将其与GLUE方法结合,提出了资料缺乏区域降雨径流的分析方法,该方法包括模型参数范围的确定、参数组的筛选与权重的计算、验证与不确定性评价、预测等。利用该方法对深圳石岩河流域两场降雨径流过程的分析表明:模型校准和验证的Nash效率系数R2分别为0.917和0.59,验证时实测径流量基本位于90%置信度预测区间内。该方法对研究流域降雨径流过程的模拟效果较好,径流预测及其不确定性分析可靠,其结果可为资料缺乏区域初期雨水截留规模的设计和方案优化提供依据。     

Regionalisation of impervious area parameters of urban drainage models

Stormwater drainage models are increasingly being used in design and analysis of urban drainage systems. If these models are to be used for ungauged drainage systems for which no storm and corresponding flow data are available, then the model parameters have to be estimated through regional equations. These regional equations define the model parameters via measurable catchment properties. In this study, regional equations of two impervious area parameters, namely directly connected impervious area percentage (DCIA) and its depression storage (DS_i ), were developed for use in urban drainage models, using 15 gauged urban catchments in Melbourne Metropolitan area (Australia). The ‘small’ storm events of these gauged catchments were first used to calibrate the impervious area parameters, since the ‘small’ storm events produce runoff only from the impervious areas. These calibrated impervious area model parameters were then analysed with several measurable catchment properties to derive the regional equations. The results of the study revealed that DCIA was depended only on household density of the catchment, while DS_i was not correlated with any of the catchment properties analysed.     

利用DEM划分雨水汇水面积的方法研究

针对传统的雨水汇水面积划分方法对地形考虑粗略、自动化程度低和人为因素影响大的特点,以城市路网和雨水管线共同组成的排水路线为研究对象,结合数字高程模型(DEM)数据,通过填洼、流向分析及修正、汇水面积划分等方法获得各段排水路线的汇水面积.分析过程突出了地形对汇流的影响,划分准确且自动化程度高.在此基础上可进一步获取各段排水路线上游的汇水面积以及汇水面积特征宽度等重要参数.研究结果可为雨水管网的规划设计及优化分析提供有价值的参考.     

Towards the determination of an optimal scale for stormwater quality management: Micropollutants in a small residential catchment

Stormwater and atmospheric deposits were collected on a small residential urban catchment (0.8 ha) near Paris in order to determine the levels of certain micropollutants (using a preliminary scan of 69 contaminants, followed by a more detailed quantification of PAHs, PCBs, alkylphenols and metals). Atmospheric inputs accounted for only 10%–38% of the stormwater contamination (except for PCBs), thus indicating substantial release within the catchment. On this small upstream catchment however, stormwater contamination is significantly lower than that observed downstream in storm sewers on larger adjacent urban catchments with similar land uses. These results likely stem from cross-contamination activity during transfers inside the sewer system and underscore the advantages of runoff management strategies at the source for controlling stormwater pollutant loads. Moreover, it has been shown that both contamination levels and contaminant speciation evolve with the scale of the catchment, in correlation with a large fraction of dissolved contaminants in upstream runoff, which differs from what has been traditionally assumed for stormwater. Consequently, the choice of treatment device/protocol must be adapted to the management scale as well as to the targeted type of contaminant.     

Adaptation of the Natural Resources Conservation Service (NRCS) curve number (CN) model in estimating direct run‐off from humid tropical forest catchments

Suitability of the Natural Resources Conservation Service (NRCS) curve number (CN) model of run‐off prediction was evaluated on three humid tropical forested catchments in Kimakia, Kenya. The catchments were dominated by Pinus patula (catchment A), Arundinaria alpina (catchment C) and Pennisetum clandestinum (catchment M). The study used discharge and rainfall data collected between 1958 and 1986. Seventy‐three (73) isolated storms were graphically separated into baseflow, interflow and surface run‐off. Forest cover types significantly improved catchments characteristics that influence baseflow and interflow generation in catchment C but not those that influence surface run‐off production. In its original form, the NRCS CN model resulted in direct run‐off estimates that deviated from observed ones by between 43.8% and 55.3%. These discrepancies were minimized through modification of the β and CN parameters. CN generated empirically using storm rainfall predicted the direct run‐off satisfactorily. Therefore, the modified NRCS CN model adequately estimates direct run‐off from humid tropical forested catchments.     

图解法用于雨水渗透下凹式绿地的设计

下凹式绿地是一种生态型的雨水渗透设施,目前还没有相关设计规范。综合绿地服务汇水面面积、土壤渗透系数、设计暴雨重现期、周边设施的布置情况等多种影响因素,提出了下凹式绿地的设计方法,合理确定了下凹式绿地的设计控制参数——绿地下凹深度及绿地面积。为了简化设计计算过程,建议用下凹式绿地设计计算图取代数值计算,并以邯郸为例介绍下凹式绿地设计计算图的绘制方法及应用。各地区均可依据当地的雨水排放设计标准、地质条件等因素绘制设计计算图,以便于下凹式绿地的推广应用。     

Modelling on-site detention on a catchment-wide basis

Many countries are either currently using or considering the use of on-site detention (OSD) as a structural flood mitigation method. On-site detention is a structural element of a property drainage system that limits the site discharge of stormwater using outlet restriction devices. The excess stormwater runoff is temporarily stored on site in underground tanks or above-ground storage areas until the storm event recedes.

Sydney, Australia, has a stormwater system that is completely separate from the sewerage system. The stormwater systems are mainly managed by the 43 local councils and most of these use OSD as a means of reducing site discharge of stormwater, thereby relieving catchment flooding. Sheas Creek is an urbanized catchment that lies within the South Sydney City Council Local Government Area. Since 1984 an OSD policy has been implemented in the Sheas Creek catchment. The associated design code, like most others in Sydney, is based on discharge from individual properties without consideration of runoff from other parts of the catchment. This design code is therefore not based on a total catchment approach.

The objective of this study is to assess the effectiveness of OSD in a catchment-wide manner. More specifically, to see how existing and planned OSD affects flooding in the Sheas Creek catchment, and to test the effectiveness of the current rate of permissible site discharge (PSD). The study also looks at the effectiveness of OSD when different spatial distributions are used across a catchment. The methodology involved using a computer rainfall–runoff model, OSDSAX, which is based on the ILLUDAS model. This was configured for the Sheas Creek catchment and was calibrated to match flood level data recorded during four large storm events. The model was then adopted to investigate the impact of OSD. Various scenarios were tested to satisfy the objectives of the study. Some of the results challenge conventional wisdom and these arise from catchment-specific characteristics. The analysis shows that the perceived beneficial effects of OSD are in some cases unrealistic and that catchment modelling is required to test and quantify the actual impacts for specific catchments.     

An investigation into the inputs controlling predictions from a diffuse phosphorus loss model for the UK; the Phosphorus Indicators Tool (PIT)

A simple catchment scale model simulating diffuse phosphorus (P) loss from agricultural land to water, the Phosphorus Indicators Tool (PIT), has been developed. Previous research has shown that this model worked well in simulating the average annual P lost from two catchments: Windermere and Windrush, but it was not known which drivers in the model had the greatest control on predicted P delivery to water from agricultural land. In order to simulate the P export from each catchment source via each hydrological pathway specified individually, 108 coefficients are used in the model code. A univariate sensitivity analysis was conducted to evaluate which coefficient exerted the greatest control on the model output. Results from the univariate analysis suggest that the model is sensitive to a number of coefficients, but importantly, not all of the coefficients that were varied in the sensitivity analysis, altered the model output. The PIT model has been calibrated by optimizing results from the univariate analysis against observed data in the Windermere catchment. The simulated results from model calibration fit the observed data well, at the 95% level. This paper describes the methodology developed for the univariate analysis and evaluates the model calibration procedure against observed data from the Windermere catchment.     

Source characterisation and loads of metals and pesticides in urban wet weather discharges

Contaminants in urban wet weather discharges originate from a number of sources such as materials from wet and dry atmospheric deposition, wastewaters, urban surface erosion, traffic-related activities, in-sewer deposits, etc. In the current study, four contributions (rainwater, dry atmospheric deposition, dry weather discharge and catchment surface + possible erosion of in-sewer deposits) to the total concentrations of priority substances have been assessed at the outlet of two urban catchments (one residential catchment with a combined system and one industrial area with a separate stormwater system) for 12 storm events (six for each catchment). Mass balances were calculated for seven metals and four pesticides, as well as for total suspended solids and chemical oxygen demand. The respective contributions of dry and wet atmospheric deposition, wastewater and catchment surface differ for each pollutant type, corresponding to different land use, activities, environments and sewer systems. For most of the pollutants, the catchment surface appears to be the main contribution, with significant storm event variability, excepted for atrazine in one catchment.     

Hydrological controls of pollutant removal from highway surfaces

Sediment and metal loadings to roadside gully-pots (weight/area of catchment/runoff depth) are calculated for two defined sites within a highway catchment and compared with data from other urban studies. The higher metal loadings associated with the more active site are particularly noticeable for Cu but at both sites the relative sizes of the metal loadings reflect the expected metal availabilities.The measured total surface runoff volumes represent between 34 and 83% of the total incoming rainfall volume for the catchment. The possible reasons for these water losses are assessed in terms of surface characteristics, seasonal variations and instrument limitations.Stepwise linear regression analysis of pollutant loadings against five selected hydrological parameters indicates that total runoff volume and storm duration together explain over 90% of the observed variance in Pb, Cd, Mn and sediment loadings. The results demonstrate the lack of importance of antecedent dry period and rainfall intensity in controlling the removal of particulate associated pollutants from this catchment. The derived model is used to predict temporal pollutant removal rates and these are shown to be consistent with those reported in other urban catchment studies.     

Finite-Element Modeling of Storm Water Runoff Using GRASS GIS

Abstract: This paper presents methodologies used in the model r.water.fea for simulating storm water runoff using the finite-element method and a raster geographic information system (GIS). The internally integrated model r.water.fea simulates storm water runoff by interfacing the raster data structure with the finite-element solution of the kinematic wave equations. The integrated system offers many advantages because the GIS and simulation model exchange input and output data internally. Distributed simulation of the spatially and temporally variable hydrologic process is possible with r.water.fea. Storm water runoff simulated with r.water.fea compares favorably with analytic solutions for a simple domain. An example simulation of a catchment demonstrates the capabilities of r.water.fea.     

Runoff generation in small catchments under a native rain forest and a rubber plantation in Xishuangbanna,southwestern China

For the purpose of assessing the potential influence of tropical rain forest conversion to rubber plantation on catchment hydrology, a stable oxygen isotope was used as a tracer to identify the event water and prestorm water components of storm runoff during low‐ and high‐flow conditions in a native tropical rain forest catchment (TRFC) and an artificial rubber plantation catchment (ARPC) in SW China. Isotopic hydrograph separations for the storm of April and September 2004 reveal that the event water made up only a small proportion of discharge (29 and 31%, respectively) at peak flow in the TRFC, while it made up the dominant component at peak flow (62 and 69%, respectively) in the ARPC. The analyses suggest that in the ARPC, the universal presence of compacted terraced tracks associated with land management would encourage infiltration‐excess overland flow. This isotopic investigation demonstrates that tropical rain forest conversion to rubber plantation would markedly change the runoff generation processes in this area.     

137Cs loss via soil erosion from a mountainous headwater catchment in central Japan

Eroded sediment and coarse organic material were sampled for a year at the outlet of the study catchment in central Japan to investigate characteristics of 137Cs loss and to quantify erosional 137Cs loss in a forested headwater catchment. Results showed that loss of both eroded sediment and 137Cs varied markedly according to the magnitude of rainfall events. About 90% of the total sediment delivery and the total 137Cs loss was associated with only two storm rainfall events. The significant contribution of high-magnitude-low-frequency storm events to 137Cs loss from the catchment was emphasized. To quantify the contribution of erosional 137Cs loss to 137Cs cycling, the current mean 137Cs inventory of the study catchment was estimated from direct measurements of 137Cs in soil cores collected from 27 points. The input of 137Cs through litterfall to the soil was estimated from monitoring the annual litterfall of the study catchment. The present 137Cs fallout input was estimated from published 137Cs fallout data. Consequently, the loss of 137Cs associated with eroded sediment was estimated to represent 0.013% of the 137Cs inventory of the study catchment, which is comparable to the input through litterfall. The equivalent value for 137Cs runoff with coarse organic material was estimated as 0.0003%. Therefore, the effect of 137Cs loss in association with coarse organic material on estimates of erosion rate obtained by measuring 137Cs inventories in soil core samples was inferred to be negligible in Japanese cypress plantations. Results confirm the applicability of 137Cs measurements to estimate erosion rates in other relatively stable forested areas.     

Quantifying costs and lengths of urban drainage systems with a simple static sewer infrastructure model

A generic model is introduced that is capable of quantitatively representing the combined sewer infrastructure of a settlement. It consists of a catchment area module, which calculates the length and size distribution of the needed sewer pipes on the basis of rain, housing densities and area size. These results are fed into the sewer construction costs module in order to estimate the combined sewer costs of the entire catchment area.

The model could be successfully fitted to existing Swiss sewer systems, indicating that it can emulate their principal characteristics. It could also identify fundamental differences in sewer designs in cities with historic roots. The results confirm that there are economies of scale for combined sewer systems in Switzerland. The modelling approach proved to be an effective tool for understanding the factors underlying the cost structure for water network infrastructures.     

A model for predicting chloride concentrations in river water in a relatively unpolluted catchment in north-east Scotland

The River Dee is an oligotrophic soft water system, in the NE of Scotland, with a catchment area of approximately 2100 km2. The river rises in the Cairngorm Mountains and enters the North Sea at Aberdeen, approximately 140 km from its source. Water chemical quality data was collected every 2 weeks over 12 months for 59 sites distributed throughout the catchment. River water chloride concentrations increased significantly from west to east. In depth investigation of the relationship with distance from the coast revealed the significant difference in spatial distribution of river water chloride concentrations between upland and lowland/agricultural areas, suggesting the possible importance of agricultural practices to streamwater chloride concentrations. Thirty of the sample sites are independent and have been used to develop a simple model for prediction of streamwater Cl- concentration throughout the catchment. The model has been validated using data from the remaining sub-catchments. The model shows that mean Cl- concentration may be reliably predicted from distance from the coast and the percentage of improved grassland and arable land cover in each sub-catchment (r2 = 0.98). It is postulated that the land use effects may be partly due to the evolved link between landuse and catchment altitude characteristics, rather than just the direct effect of applied potassium chloride fertiliser on agricultural land. It was noted that there was insufficient forestry within the River Dee Catchment to reliably include % forest cover in the model.     

Spatial variations of storm runoff pollution and their correlation with land-use in a rapidly urbanizing catchment in China

The composition of land use for a rapidly urbanizing catchment is usually heterogeneous, and this may result in significant spatial variations of storm runoff pollution and increase the difficulties of water quality management. The Shiyan Reservoir catchment, a typical rapidly urbanizing area in China, is chosen as a study area, and temporary monitoring sites were set at the downstream of its 6 sub-catchments to synchronously measure rainfall, runoff and water quality during 4 storm events in 2007 and 2009. Due to relatively low frequency monitoring, the IHACRES and exponential pollutant wash-off simulation models are used to interpolate the measured data to compensate for data insufficiency. Three indicators, event pollutant loads per unit area (EPL), event mean concentration (EMC) and pollutant loads transported by the first 50% of runoff volume (FF50), were used to describe the runoff pollution for different pollutants in each sub-catchment during the storm events, and the correlations between runoff pollution spatial variations and land-use patterns were tested by Spearman's rank correlation analysis. The results indicated that similar spatial variation trends were found for different pollutants (EPL or EMC) in light storm events, which strongly correlate with the proportion of residential land use; however, they have different trends in heavy storm events, which correlate with not only the residential land use, but also agricultural and bare land use. And some pairs of pollutants (such as COD/BOD, NH₃-N/TN) might have the similar source because they have strong or moderate positive spatial correlation. Moreover, the first flush intensity (FF50) varies with impervious land areas and different interception ratio of initial storm runoff volume should be adopted in different sub-catchments.     

Investigation of mechanical behaviour of a quasi-brittle material using Karagozian and Case concrete (KCC) model

The mechanical behaviour of a quasi-brittle material,i.e.Pietra Serena sandstone,was investigated both numerically and experimentally in order to build a reliable numerical modelling system applicable to more complex cases.The Karagozian and Case concrete(KCC) model was exploited as the material constitutive law and a new method to utilise this model for efficient and accurate simulation of quasibrittle materials is discussed.The capability of this model is evaluated by comparing the results of the numerical simulations with the corresponding experimental results,and the method itself is critically assessed.