Peter Steen Mikkelsen

SUDS, LID, BMPs, WSUD and more - The evolution and application of terminology surrounding urban drainage

The management of urban stormwater has become increasingly complex over recent decades. Consequently, terminology describing the principles and practices of urban drainage has become increasingly diverse, increasing the potential for confusion and miscommunication. This paper documents the history, scope, application and underlying principles of terms used in urban drainage and provides recommendations for clear communication of these principles. Terminology evolves locally and thus has an important role in establishing awareness and credibility of new approaches and contains nuanced understandings of the principles that are applied locally to address specific problems. Despite the understandable desire to have a ‘uniform set of terminology’, such a concept is flawed, ignoring the fact that terms reflect locally shared understanding. The local development of terminology thus has an important role in advancing the profession, but authors should facilitate communication between disciplines and between regions of the world, by being explicit and accurate in their application.

Pollution of soil and groundwater from infiltration of highly contaminated stormwater - a case study

A surface and a sub-surface infiltration system that received runoff water from trafficked roads for several decades was dug up and the contamination with heavy metals, PAH and AOX was investigated. Most measured solid phase concentrations exceeded background concentrations in nearby surface soils and sub-surface sediments and some even exceeded guidelines fixed to preserve the fertility of soil. However, the contamination decreased rapidly with depth. None of the measured metal concentrations in simulated soil solutions exceeded defined drinking water quality standards. Surprisingly, the surface and the sub-surface infiltration system seemed to be equally good at retaining pollution. This indicates that the runoff sludge found in such infiltration systems plays an important role both as a source and a sorbent for stormwater contaminants. The study does not point at a considerable risk for groundwater contamination due to stormwater infiltration, but highlights that well absorbable contaminants readily available in urban stormwater runoff eventually build up in surface soils and sub-surface sediments to environmentally critical concentration levels. Thus, on the one hand stormwater infiltration systems may act as effective pollution traps and on the other, they may pose a potential solid waste disposal problem that future stormwater management based on local infiltration will have to face.

Potential future increase in extreme one-hour precipitation events over Europe due to climate change.

In this study the potential increase of extreme precipitation in a future warmer European climate has been examined. Output from the regional climate model (RCM) HIRHAM4 covering Europe has been analysed for two periods, a control period 1961-1990 and a scenario 2071-2100, the latter following the IPCC scenario A2. The model has a resolution of about 12 km, which is unique compared with existing RCM studies that typically operate at 25-50 km scale, and make the results relevant to hydrological phenomena occurring at the spatial scale of the infrastructure designed to drain off rainfall in large urban areas. Extreme events with one- and 24-hour duration were extracted using the Partial Duration Series approach, a Generalized Pareto Distribution was fitted to the data and T-year events for return periods from 2 to 100 years were calculated for the control and scenario period in model cells across Europe. The analysis shows that there will be an increase of the intensity of extreme events generally in Europe; Scandinavia will experience the highest increase and southern Europe the lowest. A 20 year 1-hour precipitation event will for example become a 4 year event in Sweden and a 10 year event in Spain. Intensities for short durations and high return periods will increase the most, which implies that European urban drainage systems will be challenged in the future.

Three Points Approach (3PA) for urban flood risk management: a tool to support climate change adaptation through transdisciplinarity and multifunctionality.

Urban flood risk is increasing as a consequence of climate change and growing impervious surfaces. Increasing complexity of the urban context, gradual loss of tacit knowledge and decreasing social awareness are at the same time leading to inadequate choices with respect to urban flood risk management (UFRM). The European Flood Risk Directive emphasises the need for non-structural measures aimed at urban resilience and social preparedness. The Three Points Approach (3PA) provides a structure facilitating the decision making processes dealing with UFRM. It helps to accept the complexity of the urban context and promotes transdisciplinarity and multifunctionality. The 3PA introduces three domains wherein water professionals may act and where aspects valued by different stakeholders come into play: (1) technical optimisation, dealing with standards and guidelines for urban drainage systems; (2) spatial planning, making the urban area more resilient to future changing conditions; and (3) day-to-day values, enhancing awareness, acceptance and participation among stakeholders. Based on in-depth interviews conducted in The Netherlands and Denmark, we describe the complexity of decision making in practical UFRM and explain how the 3PA can be used when organising participatory processes. We introduce a theoretical framework characterising the large range of aspects involved in decision making related to UFRM and evaluate the usefulness of the 3PA in dealing with it. We conclude that the 3PA offers water managers and operators an efficient communication tool and thinking system, which helps to reduce complexity to a level suitable when organising strategy plans for UFRM and urban adaptation to climate change.

Application of global sensitivity analysis and uncertainty quantification in dynamic modelling of micropollutants in stormwater runoff

The need for estimating micropollutants fluxes in stormwater systems increases the role of stormwater quality models as support for urban water managers, although the application of such models is affected by high uncertainty. This study presents a procedure for identifying the major sources of uncertainty in a conceptual lumped dynamic stormwater runoff quality model that is used in a study catchment to estimate (i) copper loads, (ii) compliance with dissolved Cu concentration limits on stormwater discharge and (iii) the fraction of Cu loads potentially intercepted by a planned treatment facility. The analysis is based on the combination of variance-decomposition Global Sensitivity Analysis (GSA) with the Generalized Likelihood Uncertainty Estimation (GLUE) technique. The GSA-GLUE approach highlights the correlation between the model factors defining the mass of pollutant in the system and the importance of considering hydrological parameters as source of uncertainty when calculating Cu loads and concentrations due to their influence. The influence of hydrological parameters on simulated concentrations changes during rain events. Four informal likelihood measures are used to quantify model prediction bounds. The width of the uncertainty bounds depends on the likelihood measure, with the inverse variance based likelihood more suitable for covering measured pollutographs. Uncertainty for simulated concentration is higher than for Cu loads, which again shows lower uncertainty compared to studies neglecting the hydrological submodel as source of uncertainty. A combined likelihood measure ensuring both good predictions in flow and concentration is used to identify the parameter sets used for long time simulations. These results provide a basis for reliable application of models as support in the development of strategies aiming to reduce discharge of stormwater micropollutants to the aquatic environment.

Experimental assessment of soil and groundwater contamination from two old infiltration systems for road run-off in Switzerland

Surface and sub-surface infiltration systems for road run-off at two sites in Switzerland were investigated by sampling and analysing the run-off sludge and soil found in the systems. The infiltration systems were between 12 and 45 years old. Measured pollutant parameters included the heavy metals Cr, Co, Ni, Cu, Zn, Cd and Pb, a number of polycyclic aromatic hydrocarbons (PAH) and adsorbed organically bound halogens (AOX). Total concentrations were measured to assess the extent of soil contamination. Further, metal concentrations were measured in equilibrated aqueous extracts to simulate naturally occurring concentrations in percolating water. In the infiltration systems the pollutant concentrations decreased rapidly to background levels within depths less than 1.5 m. The potential for groundwater contamination from infiltration of road run-off appears to be limited, but soil and run-off sludge found in infiltration systems can be heavily contaminated. Noteworthy, the run-off sludge itself plays an important role as pollutant source and sorbent.

Estimation of regional intensity-duration-frequency curves for extreme precipitation

Regional estimation of extreme precipitation from a high resolution rain gauge network in Denmark is considered. The applied extreme value model is based on the partial duration series (PDS) approach in which all events above a certain threshold level are modelled. For a preliminary assessment of regional homogeneity and identification of a proper regional distribution L-moment analysis is applied. To analyse the regional variability in more detail, a generalised least squares regression analysis is carried out that relates the PDS model parameters to climatic and physiographic characteristics. The analysis reveals that the mean annual number of extreme events varies significantly within the region, and a large part of this variability can be explained by the mean annual rainfall. The mean value of the exceedance magnitudes can be assumed constant for intensities with durations less than one hour. For larger durations a pronounced metropolitan effect is evident, the mean intensities in the Copenhagen area being significantly larger than found in the rest of the country. With respect to second and higher order moments the region can be considered homogeneous for intensities with durations less than 24 hours. A regional parent distribution is identified as the generalised Pareto distribution.

Hydrologic behaviour of stormwater infiltration trenches in a central urban area during 2¾ years of operation

Two infiltration trenches were constructed in a densely built-up area in central Copenhagen and equipped with on-line sensors measuring rain, runoff flow from the connected surfaces and water level in the trenches. The paper describes the field site, the measuring system and the results from an initial soil survey. Although the two trenches are placed close to each other they function rather differently, corresponding to effective soil permeabilities of 2·10−6 m/s in one trench and a factor 10 smaller in the other. During 2¾ years of measuring 89 events were recorded, of which 7 caused overflow. Analyses of falling water tables after rain indicated slight clogging, but this effect is less important than the general lack of knowledge about soil permeability for normal design situations. The results indicate that the stormwater infiltration in central urban areas with compressed soils and backfill is more feasible than previously anticipated.

Infiltration practice for control of urban stormwater

Infiltrating stormwater locally into the ground instead of discharging to conventional pipe sewers is increasingly considered as a means of controlling urban stormwater runoff. This paper reviews t...

Passive Dosing to Determine the Speciation of Hydrophobic Organic Chemicals in Aqueous Samples

A new analytical approach to determine the speciation of hydrophobic organic analytes is presented. The freely dissolved concentration in a sample is controlled by passive dosing from silicone (poly(dimethylsiloxane)), and the total sample concentration at equilibrium is measured. The free fraction is determined as the ratio between measured concentrations in pure water and sample. (14)C-labeled fluoranthene served as model analyte, and total sample concentrations were easily measured by liquid scintillation counting. The method was applied to surface water, stormwater runoff, and wastewater. In the untreated wastewater, 61% of the fluoranthene was bound to suspended solids, 28% was associated to dissolved organic matter, and 11% was freely dissolved, while in treated wastewater, the speciation was 16% bound to suspended solids, 4% bound to dissolved organic matter, and 80% freely dissolved. The free fraction in roof runoff (85%) and surface water (91%) was markedly higher than in runoff from paved areas, which ranged from 27 to 36%. A log K(DOC) value of 5.26 was determined for Aldrich humic acid, which agrees well with reported values obtained by fluorescence quenching and solid phase microextraction (SPME). This analytical approach combines simplicity with high precision, and it does not require any phase separation steps.