Urban Impervious Area Research Articles

Floods triggered by natural conditions and by human activities in a mediterranean coastal environment

ABSTRACT The occurrence of high intensity storm rainfall in the western Mediterranean results in severe and often catastrophic flooding. In the Catalan coastal catchments alone, located in the northeaste of the Iberian peninsula, 296 zones have been estimated to be at risk from flooding. The change in catchment response to rainfall induced by urbanisation is one of the most dramatic of human impacts on the hydrological cycle. The basic effect of the increment of urban impervious areas on the rainfall–runoff processes is to increase storm runoff, thus increasing flood potential. Major trends of land use changes in the Mediterranean coastal streams show a marked progressive increment of the urbanised areas, basically related to tourist development of the coast. This study presents the characteristics of intense storm rainfall and flash flooding, which occur in the Ridaura catchment, located near the northeastern Spanish coast. The maximum daily rainfall values associated with significant runoff events are in the order of 200 mm over a 24 hours period and a recurrence interval of 7 to 10 years. The discharges resulting from the rainfall events are approximately 200 to 250 m3/s and produce flooding in the downstream summer resort of Platja d'Aro.

Application of a water quality model in the White Cart water catchment, Glasgow, UK.

Water quality models of urban systems have previously focused on point source (sewerage system) inputs. Little attention has been given to diffuse inputs and research into diffuse pollution has been largely confined to agriculture sources. This paper reports on new research that is aimed at integrating diffuse inputs into an urban water quality model. An integrated model is introduced that is made up of four modules: hydrology, contaminant point sources, nutrient cycling and leaching. The hydrology module, T&T consists of a TOPMODEL (a TOPography-based hydrological MODEL), which simulates runoff from pervious areas and a two-tank model, which simulates runoff from impervious urban areas. Linked into the two-tank model, the contaminant point source module simulates the overflow from the sewerage system in heavy rain. The widely known SOILN (SOIL Nitrate model) is the basis of nitrogen cycle module. Finally, the leaching module consists of two functions: the production function and the transfer function. The production function is based on SLIM (Solute Leaching Intermediate Model) while the transfer function is based on the 'flushing hypothesis' which postulates a relationship between contaminant concentrations in the receiving water course and the extent to which the catchment is saturated. This paper outlines the modelling methodology and the model structures that have been developed. An application of this model in the White Cart catchment (Glasgow) is also included.

A preliminary model for predicting heavy metal contaminant loading from an urban catchment

The toxicity of heavy metals to biota in urban catchments has been regarded as a very important non-point source pollution issue. Numerous studies on heavy metal pollution in urban receiving waters have found that metal transport by surface runoff is closely correlated to the partitioning of the metal forms between dissolved and particulate phases, where sediment plays an important role in the transport process. Sediment cycling on urban streets, metal binding form, and rainfall character in the catchment area are considered to be the key factors for metal transport. A preliminary model is developed based on these considerations. Starting from classical build-up and wash-off processes for the suspended sediment (SS) on the urban impervious surface, the model links the transport of suspended sediment to the transport of metal species. Monitoring data from a small highway catchment were used in the model development. A total of 47 rain events over 1 year were monitored intensively at short time intervals (5–10 min) for hydrological data, rainfall intensity, and stormwater quality. In developing the model, lead was used for the metal load prediction, as it has been a common fuel additive for urban transportation. Agreement between model results and monitoring data indicates that the model can be used in predicting metal load from impervious urban areas, such as streets and roadways, on a long-term basis.

Variably Saturated Flow in Storm-Water Partial Exfiltration Trench

Storm water from impervious urban areas can adversely impact water quality and quantity. The partial exfiltration trench (PET) is a control device designed to moderate both the quality and the quantity of urban runoff. This paper uses a 2D numerical model to evaluate variably saturated flow profiles and residence time distributions for a PET subject to storm water loading. Parameters estimated from laboratory experiments and hydrographs measured at a prototype PET are used to calibrate the numerical model. Simulation experiments show that flow through the PET is influenced strongly by the rate and duration of the hydraulic loading and by the type and properties of the surrounding soil. Unless the surrounding soil is nearly saturated or highly impermeable, propagation of the wetting front through an unlined PET occurs as 2D variably saturated flow. Variably saturated 2D flow through the PET is characterized by skewed residence time distributions, long mean travel times (relative to plug flow), high exfiltration losses to the surrounding soil, and low tracer mass recovery at the underdrain. These features of the PET performance are beneficial for storm-water treatment because the first flush of runoff often contains significantly higher pollutant concentrations than those present in later phases of the storm.

Estimation for urban runoff quality modeling

Many models have been developed which simulate the quality of runoff from urban impervious areas. Common to these models is the use of exponential accumulation and washoff equations. The two equations have often been calibrated separately by trial and error. However, field data of nonpoint source investigations are usually collected at the watershed outlet or at a few points on impervious surfaces, both of which reflect the combining effects of accumulation in dry weathers and washoff in wet weathers. It is therefore preferable to estimate these parameters simultaneously to obtain the global optimal values. In this study, the accumulation and washoff equations have been formulated into one model. And optimization techniques for estimating the best fit values of the parameters in the model have been developed using adaptive step size random search and Marquardt method. Bounds of these parameters have been set during optimal search to make sure that the calibrated values are physically meaningful. The method is demonstrated in Kyoto catchment in Japan. Results show that the simulated and measured data match quite well and the calibration approach can generate accurate estimates efficiently.

Estimation for urban runoff quality modeling

Many models have been developed which simulate the quality of runoff from urban impervious areas. Common to these models is the use of exponential accumulation and washoff equations. The two equations have often been calibrated separately by trial and error. However, field data of nonpoint source investigations are usually collected at the watershed outlet or at a few points on impervious surfaces, both of which reflect the combining effects of accumulation in dry weathers and washoff in wet weathers. It is therefore preferable to estimate these parameters simultaneously to obtain the global optimal values. In this study, the accumulation and washoff equations have been formulated into one model. And optimization techniques for estimating the best fit values of the parameters in the model have been developed using adaptive step size random search and Marquardt method. Bounds of these parameters have been set during optimal search to make sure that the calibrated values are physically meaningful. The method is demonstrated in Kyoto catchment in Japan. Results show that the simulated and measured data match quite well and the calibration approach can generate accurate estimates efficiently.

Derived Frequency Distribution for Storm Runoff Pollution

An analytically derived frequency distribution is reported to determine the probability of occurrence of solids washoff in different amounts from impervious urban areas. Also an expression is derived for the expectation of annual solids washoff. The resulting expressions contain several basin parameters related to land‐use practices. These parameters can be obtained from local data on storm runoff quality associated with only a few storms. Detailed historical rainfall data is needed since the joint probability density function of rainfall depth and time period between two storms should be known to apply the proposed approach. The use of the derived frequency is illustrated by a sample application.

Estimation of accumulation parameters for urban runoff quality modeling

Many recently developed watershed models utilize accumulation and washoff equations to simulate the quality of runofffrom urban impervious areas. These models often have been calibrated by trial and error and with little understanding of model sensitivity to the various parameters. Methodologies for estimating best fit values of the washoff parameters commonly used in these models have been presented previously. In this paper, parameter identification techniques for estimating the accumulation parameters from measured runoff quality data are presented along with a sensitivity analysis of the parameters. Results from application of the techniques and the sensitivity analysis suggest a need for data quantifying the magnitude and identifying the shape of constituent accumulation curves. An exponential accumulation curve is shown to be more general than the linear accumulation curves used in most urban runoff quality models. When determining accumulation rates, attention needs to be given to the effects of residual amounts of constituents remaining after the previous period of storm runoff or street sweeping.