Clogging-up of a stormwater infiltration basin

Clogging-up of a stormwater infiltration basin: a laboratory approach using image analysis Anaïs Coulon, Patrice Cannavo, Sylvain Charpentier & Laure Vidal-Beaudet

Journal of Soils and Sediments ISSN 1439-0108 J Soils Sediments DOI 10.1007/s11368-014-0951-z

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Author's personal copy J Soils Sediments DOI 10.1007/s11368-014-0951-z

SOILS AND SEDIMENTS IN URBAN AND MINING AREAS

Clogging-up of a stormwater infiltration basin: a laboratory approach using image analysis Anaïs Coulon & Patrice Cannavo & Sylvain Charpentier & Laure Vidal-Beaudet

Received: 29 October 2013 / Accepted: 23 July 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract Purpose The main function of stormwater infiltration basins is to favour stormwater drainage. However, the gradual clogging-up process caused by sediment accumulation inside these basins raises questions about their hydrodynamic functioning. Therefore, the objective of this work is to study the evolution of sediment pore distribution and its relationships with water retention and infiltration characteristics. Materials and methods Cheviré basin (Nantes, France) ageing was simulated in the laboratory, using PVC columns (10 cm diameter, 32 cm height). Seven columns were first filled in with 20 cm of thick sandy Loire river alluvia, i.e. the same material as in the basin over which sediment accumulates. The objective of the experiment was to simulate 36 months of basin ageing within 9 weeks in the laboratory. Every day, the columns were submitted to 2 cycles composed of 4 h of rain separated by 8 h without rain. During the experiment, (1) the water flow at the bottom of the column, (2) the sediment layer thickness, (3) sediment water retention and hydraulic conductivity at saturation (Ks), and (4) pore space distribution by image analysis were measured. Results and discussion After 36 months of experimental simulation, 3 cm of sediment had accumulated (i.e. 1 cm year−1); this rate was representative of in situ observations. This progressive accumulation generated the formation of a water

layer above the sediment, revealing early clogging-up by the sediment. Using HYDRUS 1D inverse resolution, a decreased Ks values from 25 to 6×10−6 m s−1 was observed after 6 and 36 months, respectively. The mean equivalent pore radius decreased 1.6-fold, from 606 to 380 μm after 6 and 36 months, respectively. These observations were confirmed by an image analysis study, whereby internal organisational changes were clearly evidenced in the sediment. Sediment particles, at first well individualised, progressively bound to one another, leaving hardly any voids. Conclusions While the clogging-up process in stormwater infiltration basins has often been studied, very little has been done about sediment hydrodynamic properties. Pore space characterisation by image analysis is a major scientific progress and showed that the presence of high levels of organic matter did not favour sediment aggregation. On the contrary, sediment gradually constituted a barrier to water flow, leading to clogging-up.

Responsible editor: Gerd Wessolek

Rainfalls that turn into runoff on waterproof zones in urban areas can generate important water flows likely to cause flooding. Such rainfalls can also transport high quantities of anthropogenic organic and inorganic pollutants that are deposited on washed surfaces (Thorpe and Harrison 2008). To limit these impacts, retention–infiltration stormwater basins have been built along road axes and in urban areas for almost 30 years (Chocat et al. 2007). Thanks to these basins, it is possible to decrease instantaneous flows towards rivers and favour groundwater recharge. They also act as filters

A. Coulon : P. Cannavo (*) : S. Charpentier : L. Vidal-Beaudet Agrocampus Ouest—Centre d’Angers, IRSTV, UP EPHOR, 49042 Angers, France e-mail: [email protected] Present Address: A. Coulon Departement for Geotechnics, Environment, Natural Hazards and Earth sciences, French Institute of Science and Technology for Transport, Development and Networks (IFSTTAR), 44341 Bouguenais, France

Keywords Clogging . Hydraulic conductivity . Image analysis . Sediment . Soil column . Soil pore distribution . Stormwater infiltration basin

1 Introduction

Author's personal copy J Soils Sediments

that use the soil as a micro-pollutant trap (Clozel et al. 2006; Ruban 2009). The solid particles present in runoff waters accumulate at the soil surface inside these basins and constitute a sediment layer. The sediment is composed of materials of different natures and origins: on one hand, a natural origin with organic (plant debris, insect fragments) and inorganic (aluminosilicates, siliceous particles, metallic oxides) materials, and on the other hand, an anthropogenic origin (polystyrene, glass, vehicle carcasses, bituminous coats, break pads or tyre residues, etc.) from cars and infrastructures (Legret and Pagotto 1999; Roger et al. 1998). Sediments range within the loamy soil texture category (Badin 2009; Delmas-Gadras 2000). They have a fragmentary structure: colloids and organic matter act as a cement leading to a relatively compact aggregated soil (Larmet 2007). Many authors have observed the unequal spatial distribution of suspended matter, rich in organic matter, from the basin inlet all the way to the opposite end (Barraud et al. 2005; Cannavo et al. 2010; Legret et al. 2005; Winiarski et al. 2006). Organic particles are not very mobile in the soil; most of them generally remain concentrated in the first 30–40 cm of the soil. On a long-term scale, the progressive influx of suspended matter on an initially highly permeable soil results in the progressive filling of the soil pores by the finest elements (Siriwardene et al. 2007). This phenomenon leads to pore clogging-up, a decreased infiltration capacity of the basin, overflowing and a drop in treatment efficiency (Ishizaki et al. 1996; Schuh 1990). This decrease in infiltration capacity highly depends on the hydric status of the basins (Lassabatere et al. 2010). Indeed, they can stay dry for several weeks and then suddenly receive several metres of water within a few hours. These important soil water content variations are expected to act on the soil aggregation degree and on its structural stability. Very few studies about soil aggregation levels in these basins exist, and particularly about the life cycle of aggregates. Badin et al. (2009b) observed that macro-aggregates (>160 μm) in two basins represented more than 30 % of the sediment volume. Microbial processes are also involved, depending on the sediment water content (Nogaro et al. 2006; Zhao et al. 2009). There is no consensus about sediment organic matter biodegradability and aggregation. Microbial biomass can be as high as in a natural context, yet without displaying an important activity (Coulon et al. 2013). Water supplies should therefore favour aggregate disruption, with a consequent degradation of the soil hydrodynamic properties. The hydraulic conductivity at saturation is a hydrodynamic parameter currently used to describe basin performance. Several weeks after putting a basin into service, it can decrease by 70 % (Le Coustumer et al. 2009). This rapid decrease raises questions about the evolution of intrinsic sediment permeability, and particularly about pore size evolution.

Therefore, the objective of this work is to study the timecourse of sediment clogging at the millimetre scale in order to characterise the evolution of its hydrodynamic properties. A precise characterisation is required to predict sediment evolution at the basin scale. To do this, a laboratory experiment using soil columns was carried out. This study reproduces 3 years ageing of a retention–infiltration stormwater basin within 9 weeks. Clogging-up mechanisms were investigated at different dates by image analysis, after encapsulation in polyester resin.

2 Materials and methods 2.1 Presentation of the Cheviré stormwater basin The study site is presented in detail in Cannavo et al. (2010) and Coulon et al. (2013). It is a retention/infiltration basin located in Nantes at the bottom of the bridge known as the “Pont de Cheviré” (northwestern France, 47°11′30″N, 1°36’ 50″W). Commissioned in 1991, this bridge currently supports a daily traffic of 91,000 vehicles. The climate is oceanic with a long-term annual average rainfall of 820 mm, characterised by long, frequent but low-intensity rainy periods, with maximum rainfall in autumn. From a geological point of view, the Cheviré basin was dug in modern alluvia of the Loire River; it is composed of sands, clays and silts. These alluvial deposits have been observed down to approximately 30 m deep. Below, red sands of the Pliocene were identified down to approximately 60 m deep. Highway runoff waters from the south part of the bridge are collected by drains and gutters, and discharged into the basin by a 0.8-m diameter collector. The active part of the catchment contributing to runoff is around 19,000 m2 and the basin surface area is 780 m2, with a depth of −1.25 to −1.75 m. In case of overflow, excess water is discharged into the Loire River by an overflow located downstream the basin. The bottom of the basin was not equipped with an impermeable membrane. An important part of the water can drain quickly by infiltrating into the sandy subsoil. The basin has not been cleaned since 1991, except for a small area close to the main water inlet. The basin is currently composed of two soil layers: a sediment layer and an underlying sandy layer (alluvia of the Loire River). The sediment layer of the topsoil has a various thickness depending on the location from 5 cm at the main water inlet to 30 cm at the overflow outlet (Cannavo et al. 2010). A topographic study of the Cheviré basin was performed by Cannavo et al. (2010) and a sampling network was designed in which each rectangular mesh was 14 m long and 4 m wide (Fig. 1). Then, 11 sampling points were defined, according to the NF ISO 10381–5 (AFNOR 2005). Chemical analysis of the two soil layers is presented in Table 1. Loire

Author's personal copy J Soils Sediments Fig. 1 Topography of the retention–infiltration basin of Cheviré. Localisation of the sampling points. Gradients are expressed in metres (from Coulon et al. (2013))

River alluvia consisted of 85 % sand (0.05–2 mm size fraction) and 15 % coarse elements (2–5 mm size fraction). 2.2 Experimental design The study was performed in the laboratory, using transparent PVC columns (10 cm diameter, i.e. 7.85×10−3 m2 section area). This diameter was chosen to study media containing large-size particles (Dano et al. 2006; Lassabatere 2002, 2007a, b). Each column was composed of four 8-cm high identical cylinders (Fig. 2). Cylinders were screwed two by two and waterproofness was ensured by O-rings. The bottom of the column consisted of (1) a filter (25 μm mesh) to retain fines particles, lying on (2) a plate to support the soil (10 cm diameter, 1 cm thick), with 1 mm-diameter holes drilled every 5 mm; and (3) a polypropylene funnel, 12 cm-diameter and 60° slope, to favour water flow-through out of the column. A supply plate was installed at the top of the column, to distribute water homogeneously on the soil surface (Larmet 2007). The plate was drilled every 10 mm by 17 holes (7-mm diameter). A 10-mm height tube was introduced in each hole

to allow water accumulation on the plate at the beginning of water delivery and to supply all the holes by a simultaneous flood. Columns were supplied with water using a peristaltic pump (Masterflex® L/S), ensuring a steady flow. At the bottom of the column, free water drainage (without any suction) was quantified throughout the experiment, using weighing scales (METTLER®, ±1 g accuracy). Water mass acquisition made it possible to calculate the outflow Qs (mL min−1), considering that 1 g of water was equivalent to 1 mL, and neglecting the possible presence of particles in collected water. 2.3 Column filling and sediment supply In April 2011, sediments and Loire river alluvia were collected separately. For the sediment, a composite sample representative of the basin was made up from the two diagonals of the basin (points 1, 3, 5, 8 and 10) (Fig. 1) by collecting all the sediment over a 0.3×0.3 m2 area at each sampling point. Thereafter, sediments were sieved through a 5-mm mesh. Sieving was realised to eliminate important size wastes that

Table 1 Main chemical characterisitics of the sediment and alluvia of the Cheviré infiltration basin. OM, organic matter; TOC, total organic carbon. Means and standard errors are presented (n=5) pH H2O

Sediment Alluvia

7.1 (0.1) 6.6 (0.4)

pH KCl

6.2 (0.3) 6.2 (0.6)

CEC

CaCO3

meq kg−1

g kg−1

98.6 (23.3) 32.7 (7)

8.3 (4.5) 6 (10)

OM

TOC

Total N

117.3 (20) 6.8 (0.3)

82.8 (14.8) 1.5 (0.1)

3.7 (0.5) 0.1 (0.0)

Author's personal copy J Soils Sediments Fig. 2 Detailed drawing of the soil column (unit, mm)


represented 10 % of the total sedimen and that would be bulky for the columns. Eliminated wasted corresponded to plant fibres, glass, plastic materials, expanded polystyrene, etc. According to Paute et al. (1994), the ratio between particle size and column size was adequate. Loire river alluvia were sampled several metres from the basin and were not sieved. In the 32-cm-high columns, Loire river alluvia were first laid in order to reach 20 cm height with the same physical properties as in situ conditions (bulk density of 1.55 g cm−3, total porosity of 0.41 m3 m−3). Then, the columns were progressively saturated with deionised water, via the bottom of the columns and using an ascending flow to eliminate the air present in the pores thanks to a higher saturation front. The top of the columns was at the atmospheric pressure. Sediment quantity supply was based on the suspended matter content observed in rainfall water runoff in the basin. The mean concentration observed was 400 mg L−1 with a standard error of 91 mg L−1 (Bechet et al. 2007; Durin et al. 2007). Suspended matter content was measured in 2004– 2005, 15 times along the year: seven times in autumn, three times in winter, two times in spring and three times in summer. The important variability was not explained by the season effect, but mainly by (1) the intensity and rainfall duration and (2) the duration between two rainfall events. The supplied water quantity was based on the rainfall regime of the study site. Mean annual rainfall and mean rainfall intensity over the past 50 years were 820 mm and 1.9 mm h−1, respectively (Meteo-France 2011). Water flow was calculated so as to be as representative as possible of in situ characteristics, and the methodology proposed by Larmet (2007) was followed. The drainage area, the basin area, the imperviousness coefficient (equal to 1), annual rainfall and the column section area (7.85×10−3 m2) were required parameters. Rainfall entering directly into the basin was taken into account; water runoff around the basin was negligible. An annual rainfall of 820 mm corresponded to a water volume per column of 138.5 L of water. Moreover, the mean rainfall duration on the study site was 4 h (Meteo-France 2011). Thus, the water flow duration for each column was of 4 h. Infiltration basin soils are frequently submitted to drying/ wetting cycles due to meteorological changes. Therefore, we decided to alternate rainy and dry cycles. Several tests in the laboratory showed that water drainage at the base of the column stopped 1–2 h after water supply stopped. From these results, it was decided to apply dry cycles of 8 h between two water supplies. Finally, to simulate 3 years of basin functioning within 9 weeks in the laboratory, we applied a flow of 20 mL min−1 (equivalent to a rainfall intensity of 7.5 mm h−1). Every day (from Monday to Saturday) we applied 2×4 h of water flows every 8 h. It was difficult to maintain sediment in suspension throughout the experiment and to control the exact quantity added to the column. To prevent the holes of the supplying plate being clogged up, the sediments were added

to the column surface before each rainfall by hand. The amount corresponded to 1.92 g of sediment, calculated from the suspended matter content of 400 mg L−1. A total of 11 columns were prepared. Three columns (i.e. three replicates) were used to study water outflow at their bottom, recorded every 10 min thanks to the weighing scales connected to a datalogger (Deltalogger DL2E, Delta Device, UK). Four columns were prepared to analyse pore space filling over time, and each of them was stopped at different periods corresponding to 6, 12, 24 and 36 months of simulation (one replicate per period). Four columns were prepared to study carbon migration across the column profile over time (6, 12, 24 and 36 months of simulation). 2.4 Data analysis Material hydrodynamic properties Sediment and alluvium water retention curves were measured following the NF ISO 11274 standard (AFNOR 1998). First, the two materials were saturated with distilled water for 48 h. Then they were gradually dried using sand suction tables (De Boodt et al. 1974) with potentials equivalent to 0.1, 0.32, 0.5 and 1 m. For higher suction values (3.3 and 10 m), a ceramic pressure plate was used (Richards 1941). After equilibration (2–3 days), the samples were dried in an oven at 105 °C for 48 h and then weighed. The water retention for a given pressure was calculated and expressed in water content per volume θ (m3 m−3). The results thus obtained allowed us to draw up a soil water retention curve. Each measurement was repeated three times. These retention curves were modelled using the van Genuchten model (1980): Se ¼

1 θ−θr 1 ; with Se ¼ ; et mi ¼ 1− ni ð1 þ ðαψÞni Þmi θs −θr

ð1Þ

where Se is effective saturation, ψ is soil water suction (m), α and n are fitting parameters, θ is the material volumetric water content (m3 m−3), and θr and θs are residual and saturated water contents, respectively (m3 m−3). Alluvium and sediment hydraulic conductivity at saturation (Ks) values were determined using the HYDRUS 2.02 model (Simunek et al. 2005), which describes water movement in two-dimensional transport domains under an axisymmetrical vertical flow in variably saturated porous media. Ks and alpha were the two parameters which were determined by the inverse method using Levenberg–Marquardt algorithm (Marquardt 1963). Parameter optimisation consisted in minimising differences between calculated and measured water outflow values at the bottom of the column. The optimisation process gave a unique solution. Characterisation of material organisation in the columns Every 2 days, the thickness of the sediment layer formed above the


2.5 Statistical analysis The modelling of the van Genuchten curves was analysed by calculating the root mean squared error (RMSE). Statistical analyses were performed using version 2.11.1 of software R (R Development Core Team 2011). Mean differences and correlation significance were obtained using the Tukey HSD test after testing the normality of data dispersion. A p value threshold