2144
© IWA Publishing 2013 Water Science & Technology
|
68.10
|
2013
Modelling of percolation rate of stormwater from underground infiltration systems Ewa Burszta-Adamiak and Janusz Łomotowski
ABSTRACT Underground or surface stormwater storage tank systems that enable the infiltration of water into the ground are basic elements used in Sustainable Urban Drainage Systems (SUDS). So far, the design methods for such facilities have not taken into account the phenomenon of ground clogging during stormwater infiltration. Top layer sealing of the filter bed influences the infiltration rate of water into the ground. This study presents an original mathematical model describing changes in the infiltration rate variability in the phases of filling and emptying the storage and infiltration tank systems, which
Ewa Burszta-Adamiak (corresponding author) Janusz Łomotowski Wroclaw University of Environmental and Life Sciences, Institute of Environmental Engineering, pl. Grunwaldzki 24, 50-365 Wroclaw, Poland E-mail: [email protected]
enables the determination of the degree of top ground layer clogging. The input data for modelling were obtained from studies conducted on experimental sites on objects constructed on a semitechnological scale. The experiment conducted has proven that the application of the model developed for the phase of water infiltration enables us to estimate the degree of module clogging. However, this method is more suitable for reservoirs embedded in more permeable soils than for those located in cohesive soils. Key words
| clogging, hydraulic resistance, modeling, storm water management, underground infiltration system
NOMENCLATURE
INTRODUCTION
F
bottom surface of infiltration facility (cm2)
H0
water level in the infiltration module at the end of the filling phase (cm)
H(t)
water level in the infiltration module at time t (cm)
Hs
water level above ground surface (cm)
hf
negative pressure head at wetting front (cm)
I(t)
accumulated water infiltration into the ground (cm)
Kf
wetting zone hydraulic conductivity (cm min1)
Q
infiltration flow (cm3 min1)
QF
variable
parameter
used
in
model
(cm min1) q(t)
infiltration rate at time t (cm min1)
Rsubstituted hydraulic resistance (min) t
time (min)
tf
the filling phase time (min)
Vz
suspension volume (dm3)
Z(t)
depth of wetting front (cm)
doi: 10.2166/wst.2013.467
(10)
Local infiltration and retention are matters of immense importance for stormwater management. Such tasks can be realised with the use of underground or surface storage and infiltration systems. In spite of a growing interest in such facilities’ application, they are still designed and operated without taking into consideration the ongoing process of clogging. Assumption of the lowest value of the filtration coefficient from the collection of results obtained during geotechnical studies – for calculation purposes – does not correspond to the dynamic changes in the filtration coefficient caused by the clogging process. An additional assumption, that the filtration coefficient remains constant throughout the operation period is not only incorrect, but also harmful, as with time the need to modernize existing elements of the system and to invest additional funds occurs. Numerous studies (Burszta-Adamiak ; Marla & Lee-Hyung ; Zhuanxi et al. ) have shown that water flowing into facilities contains a significant amount of pollutants, which deteriorate the infiltration parameters of the infiltration modules. Infiltration modules are becoming blocked over time.
2145
E. Burszta-Adamiak & J. Łomotowski
|
Modelling of percolation rate of stormwater in underground infiltration systems
This process is known as ground clogging. We distinguish between physical, biological and chemical clogging. In fact, the process is a very complex one, being a result of specific individual processes. Physical clogging during the infiltration of stormwater is caused by additives that remain in a suspended state. Physical clogging can also be caused by bubbles of gas exuded from water or from the soil. The development of biofilm in the sediment zone and in the layer adjacent to soil contributes to biological clogging. Chemical clogging takes place when suspensions or insoluble minerals are deposited on grains of soil (Nivala et al. ). The phenomenon of clogging occurs on infiltration water intakes, during the filtration of water through rapid and slow filters, in the course of exploitation of underground water intake facilities (clogging of well filters and drains), trickling filters, sand filters and subsurface wastewater infiltration systems (Rinck-Pfeiffer et al. ; Pedretti et al. ). Regardless of the type of the given infiltration system, clogging is an undesirable phenomenon (Gautier et al. ; Bouwer ; Lloyd et al. ). The phenomenon of clogging develops at various rates. Usually, during the first year of infiltration facilities operation, no significant decrease in water infiltration to the ground is observed, although in subsequent years such a decrease can reach up to even 50% of the initial permeability (Balades et al. ). On the other hand, Geiger & Dreiseitl () disagree, claiming that the phenomenon of clogging of stormwater infiltration facilities is most intense during the initial phase. The infiltration rate is the basic technological parameter that is taken into account during the process of artificial infiltration systems design. The first model of water infiltration into the ground was developed by Green and Ampt in 1911 (Williams et al. ; Ying et al. ). This simple model is based on the assumption that the water penetrates into the ground according to Darcy’s law, whereas the infiltration rate is determined by head loss in the saturated and wetted zones Z: q(t) ¼
Hs þ Z(t) hf Hs hf dI(t) þ Kf ¼ Kf ¼ dt Z(t) Z(t)=Kf
(1)
Assuming that: Z(t) ¼ Zconst
(2)
and introducing R parameter, defined by the ratio: R ¼ Zconst =Kf
(3)
Water Science & Technology
|
68.10
|
2013
Equation (1) could be rewritten as: q(t) ¼
Hs hf þ Kf R
(4)
The Green-Ampt model offers a good description of measurement results in cases characterised by stable inflow conditions, i.e. when a fixed depth of water remains on the ground surface. However, for variable inflow conditions (which are typical for stormwater infiltration facilities due to the random nature of rainfall) both the error in transient infiltration rate prognosis and accumulated infiltration of water into the ground increase. The Iwasaki equation (Iwasaki ) is a well-known model that describes the mechanical clogging of filter beds taking into account the changes in the concentration of suspended solids into liquid during the flow through porous media. Other examples of models describing chemical and biological clogging can be found in the works of Vandevivere et al. (), Taylor et al. () and PérezParicio (). In spite of the fact that numerous models that allow for a better understanding of the nature of phenomena occurring in porous media have been developed, the process of clogging in stormwater infiltration facilities is still typically analysed by means of the change evaluation in infiltration intensity during a given test period (Raimbault et al. ). The intensity of infiltration decreases gradually, until over time, a layer of low-permeable soil is created that does not meet design requirements (Balades et al. ). This can be illustrated by the equation used for the hydraulic evaluation of clogged infiltration systems performance, which was developed by Bouwer (). Numerous variations of the Bouwer model can be found. In the works of Dechesne et al. (), the model was applied for the purposes of infiltration basins evaluation with a clogged sediment layer on the bottom. Gautier et al. () tested and then described the process of water infiltration through absorptive surfaces, dividing the flow into infiltration through the clogged bottom and banks of the basin. The models used to evaluate the phenomenon of clogging in stormwater infiltration facilities presented in the literature have been developed based on the test results performed for surface infiltration systems. Due to the fact that the area designed for the construction of infiltration systems is often limited, it is quite often necessary to use underground infiltration systems. Underground storage facilities first store the collected inflow of stormwater and then
E. Burszta-Adamiak & J. Łomotowski
2146
|
Modelling of percolation rate of stormwater in underground infiltration systems
enable free infiltration of water into the ground. The evaluation of the clogging phenomenon in underground storage facilities progress is difficult due to the fact that the layer of suspended solids and/or biomass is deposited on the infiltration surface located below the land surface. Thus, in this type of facility, it is difficult or even impossible to perform any declogging activities that are traditionally performed in surface infiltration systems. Insufficient information concerning the hydraulic fundamentals of designing underground stormwater infiltration facilities lead to these experiments. The aim of the analysis was to develop simple models describing the changes in the infiltration rate in the phase of filling and emptying the retention and infiltration reservoirs and to test their applicability in clogging progress evaluation.
Figure 1
|
Water Science & Technology
|
68.10
|
2013
Sample drawing illustrating temporary water volume balance during the process of water infiltration from the module.
METHODS Model characteristics
In a case where water is not flowing into the module, a decrease in the water level due to filtration is observed. The function of water level change in the module in the emptying phase will be then described by the following equation:
The water volume balance equation for infiltration module systems with an absorptive surface F, which is filled at a constant rate Q, without taking into account the impact of infiltration through side walls can be formulated as follows:
t t H(t) ¼ H0 exp Kf R 1 exp R R
F dH(t) ¼ Q dt F q(t) dt
(5)
where the left side of the equation describes the increase in water volume in the infiltration module and the right side describes the volume of water flowing into the reservoir, less the amount of water infiltrating into the ground. This is illustrated in Figure 1. Assuming that: H(t) q(t) ¼ þ Kf R
(6)
(9)
During the infiltration of water-containing suspension a significant decrease in the filtration coefficient of the soil’s top layer is observed. In this case, the value of the product of the Kf·R constants is close to 0, Equations (8) and (9) take the respective forms: t H(t) ¼ QF R 1 exp R
(10)
t H(t) ¼ H0 exp R
(11)
and introducing a parameter defined by the ratio: Experimental sites and methodology QF ¼
Q F
(7)
it is possible to determine the equation describing the changes in water level in an infiltration module for water flowing in at a constant rate: t t Kf R 1 exp H(t) ¼ QF R 1 exp R R
(8)
The experimental sites were constructed with the use of prefabricated openwork modules. The side walls of those modules contain apertures that enable the infiltration of inflowing water into the ground. Modules were wrapped in 1.6 mm thick geotextile made from polypropylene, characterised by perpendicular water permeability of 0.0026 m/s. The dimensions of the infiltration modules were 500 × 1,000 × 400 mm (length × height × width). These systems
E. Burszta-Adamiak & J. Łomotowski
2147
|
Modelling of percolation rate of stormwater in underground infiltration systems
are prepared for the management and infiltration of stormwater and are commonly used in engineering practice. Infiltration modules were placed in the ground according to the guidelines provided by the manufacturer. Prior to the beginning of the experiments, geotechnical tests had been conducted in the site where the modules were located. Below the bottom of experimental site no. 1, there was a deposit of sandy clay, covered by medium sand reaching up the soil’s surface. Experimental site no. 2 was located on cohesive clay, covered by sandy clay up to the depth of 0.4 m below the land’s surface. Above that depth, it was covered with a deposit of fine sand. In order to prepare the clogging agent (dispersion suspension), kaolin clay was used on both experimental sites. The kaolin clay suspension used for the tests was determined based on a 2-year study of the grain size distribution and concentration of suspensions present in rainfall, snowfall and roof runoff. These studies were conducted with the use of a laser particle sizer, Mastersizer 2000, manufactured by Instruments Ltd. Samples were collected both on the site where later studies on infiltration modules were conducted and in several other Polish cities. The preliminary studies showed that the average respective concentrations of suspensions in rainfall, snowfall and roof runoff samples equalled 0.0075% vol., 0.0082% vol. and 0.012% vol., respectively. By referring the obtained results to the concentration of kaolin clay suspension and the volume of suspension introduced into the test sites, it can be stated that one year of conducted analysis corresponds to approximately five years of exploitation in real conditions. The use of higher concentrations of kaolin clay suspension resulted from the need to intensify the clogging process which is much slower under real conditions. The particle size of the clogging agent fell within the range from 0.25 to 100 μm. These particles accounted
Figure 2
|
Water Science & Technology
|
68.10
|
2013
for approximately 60% of the pollutants present in rainwater. The selection of a model suspension characterised by a particle size composition similar to that of stormwater enabled us to model the processes occurring in nature in a more precise way. Modules were filled with 60 dm3 of suspension of 2.5 g/dm3 concentration each time. The suspension used for the tests was prepared after several days of soaking in tap water in order to eliminate the process of mineral expansion. The filling lasted for 8–10 min, and the infiltration time varied from 0.5 hours at the beginning of the test period, to 9 hours after a year of testing. The tests on the sites were conducted in the period from 18th June 2003 to 14th June 2004. Infiltration modules were filled once a week on average during that time. During tests, the duration of the experiment was measured, along with changes in the water level, with kaolin clay suspension in the modules in the filling and infiltration stages.
RESULTS AND DISCUSSION Due to a large number of factors influencing clogging and due to their significant variability over time, it is practically impossible to model the processes occurring in nature. The construction of a simplified experimental model, together with the application of a dispersion suspension characterised by an adequate concentration and size of particles, enables us to model processes that would take up to, or even more than, ten years in the real world in short periods of time. The impact of infiltration through side walls was omitted in calculations. The scanning photos of the sediment deposited on the surface and inside the geotextiles showed that the flow of suspension occurred mainly through the bottom (Figure 2). This is mainly the result of the sedimentation
Scanning photos of kaolin clay sediment deposited in the geotextile on the bottom of the module (left image) and on the side walls of the site (right image) magnified 1,000 times.
E. Burszta-Adamiak & J. Łomotowski
2148
|
Modelling of percolation rate of stormwater in underground infiltration systems
process mechanism, which is one of the specific processes occurring during the infiltration of polluted waters. In experimental site no. 1, as much as 88.4% of the total mass of kaolin clay sediments deposited in the module clogged at the bottom, while only 11.6% was deposited on the walls. A similar situation was observed in experimental site no. 2, where 90.1% of the total mass of kaolin clay was
Figure 3
|
Water Science & Technology
|
68.10
|
2013
deposited on the bottom in the form of sediment, while 9.9% was found in the geotextile covering the walls. Changes in the water level during filling and infiltration The short duration of the filling phase in the experiment conducted (only 8–10 min) is likely to influence the
Change comparison in water levels during filling and infiltration phases with a regression function described by the general models (10) and (11) (a, b, e, f) and value of Ho with the increase in the suspension volume at experimental sites no. 1 and no. 2 (c, d). (Notation: 1. 355 dm3; 2. 769 dm3; 3. 1597 dm3 suspension volume Vz in the experimental sites).
2149
E. Burszta-Adamiak & J. Łomotowski
|
Modelling of percolation rate of stormwater in underground infiltration systems
effectiveness in describing the physical phenomena. This results from the fact that rainfall occurring naturally, characterised by variable intensity (and thus by low predictability), which determines the duration and course of the filling phase, is very difficult to model on a semi-technological scale. Because of that, Equation (10), referring to the filling phase, may be less applicable in practice than Equation (11), describing the changes in water levels in the infiltration phase, which reflects the nature of the clogging phenomenon more accurately. The plots of the changes observed in the water levels in the infiltration modules in the filling and infiltration phases on selected measurement days, and changes in the water levels’ H0 for the whole period of the experiment are presented, respectively, in Figures 3(a), 3(b), 3(e), 3(f) and 3(c), 3(d). Along with the measurement data presented in Figures 3(a), 3(b), 3(e), 3(f), regression functions calculated with the STATISTICA 10.0 PL software described by the models (Equations (10) and (11)) are presented. While evaluating the water level changes during emptying the test infiltration modules, a transient infiltration rate acceleration was observed at the end of the infiltration phase. In the analysed test sites, the change in the gradient of transient infiltration rates occurred at water levels ranging from 4 to 6 cm. This phenomenon was observed for all measurements. It could be explained by an increase in the suction power of the soil located below the geotextile. At high transient infiltration rates, the thickness of the fully saturated zone stabilises. When the transient infiltration rate decreases, the thickness of the fully saturated zone starts to decrease as more water flows out than flows in from the land surface direction in the zone that has not been fully saturated with water. This causes a decrease in soil moisture and an increase of suction pressure below the surface of the sediments. The decrease in the thickness of the saturated layer, combined with an increase in the suction pressure, leads to an acceleration of the transient rate, as described by the Green-Ampt Equation (1). Calculation of hydraulic resistance Models (10) and (11) were based on the assumption that the observed hydraulic resistance is the same in the phases of filling and emptying the infiltration modules. Experience proves that these values differed significantly. The cause of this should be linked to the occurrence of variable depths of the wetting front during the experiment period. In low permeability soil (experimental site no. 2),
Figure 4
|
Water Science & Technology
|
68.10
|
2013
Substituted hydraulic resistance in the infiltration and filling phase ratio on experimental sites no. 1 (white points) and no. 2 (black points) (Figure 4(a)) and the comparison of hydraulic resistance of soil during the infiltrating phase on experimental sites no. 1 and no. 2 (Figure 4(b) and 4(c)) grouped in dimensionless parameters.
the variability of the hydraulic resistance observed in the infiltration phase was higher in comparison to the filling phase than in more permeable soil (experimental site no. 1) (Figure 4(a)). In order to generalise the results, and thus to increase their universality, Figures 4(b) and 4(c) present plots with dimensionless parameters (Vz/ (FHo) for the x-axis and R/tf for the y-axis). On both experimental sites, along with the increase in the filtered volume, an increase in resistance was observed. Changing this parameter proves the occurrence of the clogging phenomenon.
2150
E. Burszta-Adamiak & J. Łomotowski
|
Modelling of percolation rate of stormwater in underground infiltration systems
CONCLUSIONS The tests and analyses conducted lead to the following conclusions: 1. The practical value of the soil clogging models presented for the purposes of stormwater infiltration devices design is limited. It results from a higher number of factors influencing the rate of water infiltration into the soil. 2. Substitute hydraulic resistance R of the soil in the phase of filling the retention and infiltration modules is significantly lower than resistance in the emptying phase, and the differences between these values depend on the type of soil and the degree of geotextile clogging and the soil’s superficial layer. 3. Application of the model presented for the phase of water infiltration in infiltration modules enables us to estimate the degree of clogging in the modules studied. However, this method is more suitable for reservoirs embedded in more permeable soils than for those located in cohesive soils. 4. It seems reasonable to conduct further studies, which will constitute the basis for the formulation of more precise conclusions related to the phenomenon of clogging in retention and infiltration reservoirs. Future experiments should take into account additional variables fundamental for the clogging phenomenon, e.g. different intensity of filling, types of soils, etc.
REFERENCES Balades, J. D., Legret, M. & Madiec, H. Permeable pavements: pollution management tools. Water Science and Technology 32 (1), 49–56. Bouwer, H. Theory of seepage from open channels. Advances in Hydrosciences 5, 121–170. Bouwer, H. Artificial recharge of groundwater: hydrogeology and engineering. Hydrogeology Journal 10, 121–142. Burszta-Adamiak, E. Badania nad zastosowaniem geowłóknin do przeciwdziałania kolmatacji w procesie infiltracji. (Research on nonwoven geotextile use in order to counteract clogging process during infiltration into the ground). PhD Thesis, University of Technology, Wroclaw. Chua, L. H., Leong, M. C., Lo, E. Y., Reinhard, M., Robertson, A. P., Lim, T. T., Shuy, E. B. & Tan, S. K. Controlled field studies on soil aquifer treatment in a constructed coastal sandfill. Water Science and Technology 60 (5), 1283–1293. Dechesne, M., Barraud, S. & Bardin, J. P. Indicators for hydraulic and pollution retention assessment of stormwater infiltration basins. Journal of Environmental Management 77, 371–380.
Water Science & Technology
|
68.10
|
2013
Gautier, A., Barraud, S. & Bardin, J. P. An approach to the characterization and modeling of clogging in storm water infiltration facilities. In: Proc. the Eighth International Conference on Urban Storm Drainage (I. B. Joliffe & J. E. Ball, eds). August 30–September 3, The Institution of Engineers Australia, The International Association for Hydraulic Research, and The International Association on Water Quality, Sydney, Australia, 1007–1015. Geiger, W. & Dreiseitl, H. Nowe sposoby odprowadzania wód deszczowych; Poradnik retencjonowania i infiltracji wód deszczowych do gruntu na terenach zabudowanych (New ways to stormwater drainage. Guide for stormwater retention and infiltration into the ground in urban areas). Oficyna Wydawnicza Projprzem-EKO, Bydgoszcz. Iwasaki, T. Some notes on sand filtration. Journal of the American Water Works Association (AWWA) 29, 1591–1602. Marla, C. M. & Lee-Hyung, K. Long-Term Monitoring of Infiltration Trench for Nonpoint Source Pollution Control. Water, Air and Soil Pollution 212 (1–4), 13–26. Nivala, J., Knowles, P., Dotro, G., García, J. & Wallace, S. Clogging in subsurface-flow treatment wetlands: measurement, modeling and management. Water Research 46 (6), 1625–1640. Pedretti, D., Barahona-Palomo, M., Bolster, D., Fernàndez-Garcia, D., Sanchez-Vila, X. & Tartakovsky, D. M. Probabilistic analysis of maintenance and operation of artificial recharge ponds. Advances in Water Resources 36, 23–35. Pérez-Paricio, A. Integrated modelling of clogging processes in artificial groundwater recharge. PhD Thesis, Technical University of Catalonia (UPC), Barcelona. Raimbault, G., Nadji, D. & Gautier, C. Stormwater Infiltration and Porous Materiał Clogging. 8th ICUSD, Sydney, pp. 1016–1024. Rinck-Pfeiffer, S., Ragusa, S., Sztajnbok, P. & Vandevelde, T. Interrelationships between biological, chemical, and physical processes as an analog to clogging in aquifer storage and recovery (ASR) wells. Water Research 34 (7), 2110–2118. Taylor, S. W., Milly, P. C. D. & Jaffe, P. R. Biofilm growth and the related changes in the physical properties of a porous medium. 2. Permeability. Water Resources Research 26 (9), 2161–2169. Vandevivere, P., Baveye, P., Sanchez de Lozada, D. & Deleo, P. Microbial clogging of saturated soils and aquifer materials: evaluation of mathematical models. Water Resources Research 31 (9), 2173–2180. Williams, J. R., Ouyang, Y., Chen, J. S. & Ravi, V. Estimation of infiltration rate in vadose zone: Application of selected mathematical models. Report EPA/600/R-97/128b Vol. II. Ying, M., Guangyao, G., Zailin, H., Shaoyuan, F. & Dongyuan, S. Modeling water infiltration in a large layered soil column with a modified Green-Ampt model and HYDRUS. Computers and Electronics in Agriculture 71, 40–47. Zhuanxi, L., Tao, W., Meirong, G., Jialiang, T. & Bo, Z. Stormwater runoff pollution in a rural township in the hilly area of the central Sichuan Basin, China. Journal of Mountain Science 9 (1), 16–26.
First received 22 November 2012; accepted in revised form 15 July 2013. Available online 25 October 2013
Copyright of Water Science & Technology is the property of IWA Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.
© IWA Publishing 2013 Water Science & Technology
|
68.10
|
2013
Modelling of percolation rate of stormwater from underground infiltration systems Ewa Burszta-Adamiak and Janusz Łomotowski
ABSTRACT Underground or surface stormwater storage tank systems that enable the infiltration of water into the ground are basic elements used in Sustainable Urban Drainage Systems (SUDS). So far, the design methods for such facilities have not taken into account the phenomenon of ground clogging during stormwater infiltration. Top layer sealing of the filter bed influences the infiltration rate of water into the ground. This study presents an original mathematical model describing changes in the infiltration rate variability in the phases of filling and emptying the storage and infiltration tank systems, which
Ewa Burszta-Adamiak (corresponding author) Janusz Łomotowski Wroclaw University of Environmental and Life Sciences, Institute of Environmental Engineering, pl. Grunwaldzki 24, 50-365 Wroclaw, Poland E-mail: [email protected]
enables the determination of the degree of top ground layer clogging. The input data for modelling were obtained from studies conducted on experimental sites on objects constructed on a semitechnological scale. The experiment conducted has proven that the application of the model developed for the phase of water infiltration enables us to estimate the degree of module clogging. However, this method is more suitable for reservoirs embedded in more permeable soils than for those located in cohesive soils. Key words
| clogging, hydraulic resistance, modeling, storm water management, underground infiltration system
NOMENCLATURE
INTRODUCTION
F
bottom surface of infiltration facility (cm2)
H0
water level in the infiltration module at the end of the filling phase (cm)
H(t)
water level in the infiltration module at time t (cm)
Hs
water level above ground surface (cm)
hf
negative pressure head at wetting front (cm)
I(t)
accumulated water infiltration into the ground (cm)
Kf
wetting zone hydraulic conductivity (cm min1)
Q
infiltration flow (cm3 min1)
QF
variable
parameter
used
in
model
(cm min1) q(t)
infiltration rate at time t (cm min1)
Rsubstituted hydraulic resistance (min) t
time (min)
tf
the filling phase time (min)
Vz
suspension volume (dm3)
Z(t)
depth of wetting front (cm)
doi: 10.2166/wst.2013.467
(10)
Local infiltration and retention are matters of immense importance for stormwater management. Such tasks can be realised with the use of underground or surface storage and infiltration systems. In spite of a growing interest in such facilities’ application, they are still designed and operated without taking into consideration the ongoing process of clogging. Assumption of the lowest value of the filtration coefficient from the collection of results obtained during geotechnical studies – for calculation purposes – does not correspond to the dynamic changes in the filtration coefficient caused by the clogging process. An additional assumption, that the filtration coefficient remains constant throughout the operation period is not only incorrect, but also harmful, as with time the need to modernize existing elements of the system and to invest additional funds occurs. Numerous studies (Burszta-Adamiak ; Marla & Lee-Hyung ; Zhuanxi et al. ) have shown that water flowing into facilities contains a significant amount of pollutants, which deteriorate the infiltration parameters of the infiltration modules. Infiltration modules are becoming blocked over time.
2145
E. Burszta-Adamiak & J. Łomotowski
|
Modelling of percolation rate of stormwater in underground infiltration systems
This process is known as ground clogging. We distinguish between physical, biological and chemical clogging. In fact, the process is a very complex one, being a result of specific individual processes. Physical clogging during the infiltration of stormwater is caused by additives that remain in a suspended state. Physical clogging can also be caused by bubbles of gas exuded from water or from the soil. The development of biofilm in the sediment zone and in the layer adjacent to soil contributes to biological clogging. Chemical clogging takes place when suspensions or insoluble minerals are deposited on grains of soil (Nivala et al. ). The phenomenon of clogging occurs on infiltration water intakes, during the filtration of water through rapid and slow filters, in the course of exploitation of underground water intake facilities (clogging of well filters and drains), trickling filters, sand filters and subsurface wastewater infiltration systems (Rinck-Pfeiffer et al. ; Pedretti et al. ). Regardless of the type of the given infiltration system, clogging is an undesirable phenomenon (Gautier et al. ; Bouwer ; Lloyd et al. ). The phenomenon of clogging develops at various rates. Usually, during the first year of infiltration facilities operation, no significant decrease in water infiltration to the ground is observed, although in subsequent years such a decrease can reach up to even 50% of the initial permeability (Balades et al. ). On the other hand, Geiger & Dreiseitl () disagree, claiming that the phenomenon of clogging of stormwater infiltration facilities is most intense during the initial phase. The infiltration rate is the basic technological parameter that is taken into account during the process of artificial infiltration systems design. The first model of water infiltration into the ground was developed by Green and Ampt in 1911 (Williams et al. ; Ying et al. ). This simple model is based on the assumption that the water penetrates into the ground according to Darcy’s law, whereas the infiltration rate is determined by head loss in the saturated and wetted zones Z: q(t) ¼
Hs þ Z(t) hf Hs hf dI(t) þ Kf ¼ Kf ¼ dt Z(t) Z(t)=Kf
(1)
Assuming that: Z(t) ¼ Zconst
(2)
and introducing R parameter, defined by the ratio: R ¼ Zconst =Kf
(3)
Water Science & Technology
|
68.10
|
2013
Equation (1) could be rewritten as: q(t) ¼
Hs hf þ Kf R
(4)
The Green-Ampt model offers a good description of measurement results in cases characterised by stable inflow conditions, i.e. when a fixed depth of water remains on the ground surface. However, for variable inflow conditions (which are typical for stormwater infiltration facilities due to the random nature of rainfall) both the error in transient infiltration rate prognosis and accumulated infiltration of water into the ground increase. The Iwasaki equation (Iwasaki ) is a well-known model that describes the mechanical clogging of filter beds taking into account the changes in the concentration of suspended solids into liquid during the flow through porous media. Other examples of models describing chemical and biological clogging can be found in the works of Vandevivere et al. (), Taylor et al. () and PérezParicio (). In spite of the fact that numerous models that allow for a better understanding of the nature of phenomena occurring in porous media have been developed, the process of clogging in stormwater infiltration facilities is still typically analysed by means of the change evaluation in infiltration intensity during a given test period (Raimbault et al. ). The intensity of infiltration decreases gradually, until over time, a layer of low-permeable soil is created that does not meet design requirements (Balades et al. ). This can be illustrated by the equation used for the hydraulic evaluation of clogged infiltration systems performance, which was developed by Bouwer (). Numerous variations of the Bouwer model can be found. In the works of Dechesne et al. (), the model was applied for the purposes of infiltration basins evaluation with a clogged sediment layer on the bottom. Gautier et al. () tested and then described the process of water infiltration through absorptive surfaces, dividing the flow into infiltration through the clogged bottom and banks of the basin. The models used to evaluate the phenomenon of clogging in stormwater infiltration facilities presented in the literature have been developed based on the test results performed for surface infiltration systems. Due to the fact that the area designed for the construction of infiltration systems is often limited, it is quite often necessary to use underground infiltration systems. Underground storage facilities first store the collected inflow of stormwater and then
E. Burszta-Adamiak & J. Łomotowski
2146
|
Modelling of percolation rate of stormwater in underground infiltration systems
enable free infiltration of water into the ground. The evaluation of the clogging phenomenon in underground storage facilities progress is difficult due to the fact that the layer of suspended solids and/or biomass is deposited on the infiltration surface located below the land surface. Thus, in this type of facility, it is difficult or even impossible to perform any declogging activities that are traditionally performed in surface infiltration systems. Insufficient information concerning the hydraulic fundamentals of designing underground stormwater infiltration facilities lead to these experiments. The aim of the analysis was to develop simple models describing the changes in the infiltration rate in the phase of filling and emptying the retention and infiltration reservoirs and to test their applicability in clogging progress evaluation.
Figure 1
|
Water Science & Technology
|
68.10
|
2013
Sample drawing illustrating temporary water volume balance during the process of water infiltration from the module.
METHODS Model characteristics
In a case where water is not flowing into the module, a decrease in the water level due to filtration is observed. The function of water level change in the module in the emptying phase will be then described by the following equation:
The water volume balance equation for infiltration module systems with an absorptive surface F, which is filled at a constant rate Q, without taking into account the impact of infiltration through side walls can be formulated as follows:
t t H(t) ¼ H0 exp Kf R 1 exp R R
F dH(t) ¼ Q dt F q(t) dt
(5)
where the left side of the equation describes the increase in water volume in the infiltration module and the right side describes the volume of water flowing into the reservoir, less the amount of water infiltrating into the ground. This is illustrated in Figure 1. Assuming that: H(t) q(t) ¼ þ Kf R
(6)
(9)
During the infiltration of water-containing suspension a significant decrease in the filtration coefficient of the soil’s top layer is observed. In this case, the value of the product of the Kf·R constants is close to 0, Equations (8) and (9) take the respective forms: t H(t) ¼ QF R 1 exp R
(10)
t H(t) ¼ H0 exp R
(11)
and introducing a parameter defined by the ratio: Experimental sites and methodology QF ¼
Q F
(7)
it is possible to determine the equation describing the changes in water level in an infiltration module for water flowing in at a constant rate: t t Kf R 1 exp H(t) ¼ QF R 1 exp R R
(8)
The experimental sites were constructed with the use of prefabricated openwork modules. The side walls of those modules contain apertures that enable the infiltration of inflowing water into the ground. Modules were wrapped in 1.6 mm thick geotextile made from polypropylene, characterised by perpendicular water permeability of 0.0026 m/s. The dimensions of the infiltration modules were 500 × 1,000 × 400 mm (length × height × width). These systems
E. Burszta-Adamiak & J. Łomotowski
2147
|
Modelling of percolation rate of stormwater in underground infiltration systems
are prepared for the management and infiltration of stormwater and are commonly used in engineering practice. Infiltration modules were placed in the ground according to the guidelines provided by the manufacturer. Prior to the beginning of the experiments, geotechnical tests had been conducted in the site where the modules were located. Below the bottom of experimental site no. 1, there was a deposit of sandy clay, covered by medium sand reaching up the soil’s surface. Experimental site no. 2 was located on cohesive clay, covered by sandy clay up to the depth of 0.4 m below the land’s surface. Above that depth, it was covered with a deposit of fine sand. In order to prepare the clogging agent (dispersion suspension), kaolin clay was used on both experimental sites. The kaolin clay suspension used for the tests was determined based on a 2-year study of the grain size distribution and concentration of suspensions present in rainfall, snowfall and roof runoff. These studies were conducted with the use of a laser particle sizer, Mastersizer 2000, manufactured by Instruments Ltd. Samples were collected both on the site where later studies on infiltration modules were conducted and in several other Polish cities. The preliminary studies showed that the average respective concentrations of suspensions in rainfall, snowfall and roof runoff samples equalled 0.0075% vol., 0.0082% vol. and 0.012% vol., respectively. By referring the obtained results to the concentration of kaolin clay suspension and the volume of suspension introduced into the test sites, it can be stated that one year of conducted analysis corresponds to approximately five years of exploitation in real conditions. The use of higher concentrations of kaolin clay suspension resulted from the need to intensify the clogging process which is much slower under real conditions. The particle size of the clogging agent fell within the range from 0.25 to 100 μm. These particles accounted
Figure 2
|
Water Science & Technology
|
68.10
|
2013
for approximately 60% of the pollutants present in rainwater. The selection of a model suspension characterised by a particle size composition similar to that of stormwater enabled us to model the processes occurring in nature in a more precise way. Modules were filled with 60 dm3 of suspension of 2.5 g/dm3 concentration each time. The suspension used for the tests was prepared after several days of soaking in tap water in order to eliminate the process of mineral expansion. The filling lasted for 8–10 min, and the infiltration time varied from 0.5 hours at the beginning of the test period, to 9 hours after a year of testing. The tests on the sites were conducted in the period from 18th June 2003 to 14th June 2004. Infiltration modules were filled once a week on average during that time. During tests, the duration of the experiment was measured, along with changes in the water level, with kaolin clay suspension in the modules in the filling and infiltration stages.
RESULTS AND DISCUSSION Due to a large number of factors influencing clogging and due to their significant variability over time, it is practically impossible to model the processes occurring in nature. The construction of a simplified experimental model, together with the application of a dispersion suspension characterised by an adequate concentration and size of particles, enables us to model processes that would take up to, or even more than, ten years in the real world in short periods of time. The impact of infiltration through side walls was omitted in calculations. The scanning photos of the sediment deposited on the surface and inside the geotextiles showed that the flow of suspension occurred mainly through the bottom (Figure 2). This is mainly the result of the sedimentation
Scanning photos of kaolin clay sediment deposited in the geotextile on the bottom of the module (left image) and on the side walls of the site (right image) magnified 1,000 times.
E. Burszta-Adamiak & J. Łomotowski
2148
|
Modelling of percolation rate of stormwater in underground infiltration systems
process mechanism, which is one of the specific processes occurring during the infiltration of polluted waters. In experimental site no. 1, as much as 88.4% of the total mass of kaolin clay sediments deposited in the module clogged at the bottom, while only 11.6% was deposited on the walls. A similar situation was observed in experimental site no. 2, where 90.1% of the total mass of kaolin clay was
Figure 3
|
Water Science & Technology
|
68.10
|
2013
deposited on the bottom in the form of sediment, while 9.9% was found in the geotextile covering the walls. Changes in the water level during filling and infiltration The short duration of the filling phase in the experiment conducted (only 8–10 min) is likely to influence the
Change comparison in water levels during filling and infiltration phases with a regression function described by the general models (10) and (11) (a, b, e, f) and value of Ho with the increase in the suspension volume at experimental sites no. 1 and no. 2 (c, d). (Notation: 1. 355 dm3; 2. 769 dm3; 3. 1597 dm3 suspension volume Vz in the experimental sites).
2149
E. Burszta-Adamiak & J. Łomotowski
|
Modelling of percolation rate of stormwater in underground infiltration systems
effectiveness in describing the physical phenomena. This results from the fact that rainfall occurring naturally, characterised by variable intensity (and thus by low predictability), which determines the duration and course of the filling phase, is very difficult to model on a semi-technological scale. Because of that, Equation (10), referring to the filling phase, may be less applicable in practice than Equation (11), describing the changes in water levels in the infiltration phase, which reflects the nature of the clogging phenomenon more accurately. The plots of the changes observed in the water levels in the infiltration modules in the filling and infiltration phases on selected measurement days, and changes in the water levels’ H0 for the whole period of the experiment are presented, respectively, in Figures 3(a), 3(b), 3(e), 3(f) and 3(c), 3(d). Along with the measurement data presented in Figures 3(a), 3(b), 3(e), 3(f), regression functions calculated with the STATISTICA 10.0 PL software described by the models (Equations (10) and (11)) are presented. While evaluating the water level changes during emptying the test infiltration modules, a transient infiltration rate acceleration was observed at the end of the infiltration phase. In the analysed test sites, the change in the gradient of transient infiltration rates occurred at water levels ranging from 4 to 6 cm. This phenomenon was observed for all measurements. It could be explained by an increase in the suction power of the soil located below the geotextile. At high transient infiltration rates, the thickness of the fully saturated zone stabilises. When the transient infiltration rate decreases, the thickness of the fully saturated zone starts to decrease as more water flows out than flows in from the land surface direction in the zone that has not been fully saturated with water. This causes a decrease in soil moisture and an increase of suction pressure below the surface of the sediments. The decrease in the thickness of the saturated layer, combined with an increase in the suction pressure, leads to an acceleration of the transient rate, as described by the Green-Ampt Equation (1). Calculation of hydraulic resistance Models (10) and (11) were based on the assumption that the observed hydraulic resistance is the same in the phases of filling and emptying the infiltration modules. Experience proves that these values differed significantly. The cause of this should be linked to the occurrence of variable depths of the wetting front during the experiment period. In low permeability soil (experimental site no. 2),
Figure 4
|
Water Science & Technology
|
68.10
|
2013
Substituted hydraulic resistance in the infiltration and filling phase ratio on experimental sites no. 1 (white points) and no. 2 (black points) (Figure 4(a)) and the comparison of hydraulic resistance of soil during the infiltrating phase on experimental sites no. 1 and no. 2 (Figure 4(b) and 4(c)) grouped in dimensionless parameters.
the variability of the hydraulic resistance observed in the infiltration phase was higher in comparison to the filling phase than in more permeable soil (experimental site no. 1) (Figure 4(a)). In order to generalise the results, and thus to increase their universality, Figures 4(b) and 4(c) present plots with dimensionless parameters (Vz/ (FHo) for the x-axis and R/tf for the y-axis). On both experimental sites, along with the increase in the filtered volume, an increase in resistance was observed. Changing this parameter proves the occurrence of the clogging phenomenon.
2150
E. Burszta-Adamiak & J. Łomotowski
|
Modelling of percolation rate of stormwater in underground infiltration systems
CONCLUSIONS The tests and analyses conducted lead to the following conclusions: 1. The practical value of the soil clogging models presented for the purposes of stormwater infiltration devices design is limited. It results from a higher number of factors influencing the rate of water infiltration into the soil. 2. Substitute hydraulic resistance R of the soil in the phase of filling the retention and infiltration modules is significantly lower than resistance in the emptying phase, and the differences between these values depend on the type of soil and the degree of geotextile clogging and the soil’s superficial layer. 3. Application of the model presented for the phase of water infiltration in infiltration modules enables us to estimate the degree of clogging in the modules studied. However, this method is more suitable for reservoirs embedded in more permeable soils than for those located in cohesive soils. 4. It seems reasonable to conduct further studies, which will constitute the basis for the formulation of more precise conclusions related to the phenomenon of clogging in retention and infiltration reservoirs. Future experiments should take into account additional variables fundamental for the clogging phenomenon, e.g. different intensity of filling, types of soils, etc.
REFERENCES Balades, J. D., Legret, M. & Madiec, H. Permeable pavements: pollution management tools. Water Science and Technology 32 (1), 49–56. Bouwer, H. Theory of seepage from open channels. Advances in Hydrosciences 5, 121–170. Bouwer, H. Artificial recharge of groundwater: hydrogeology and engineering. Hydrogeology Journal 10, 121–142. Burszta-Adamiak, E. Badania nad zastosowaniem geowłóknin do przeciwdziałania kolmatacji w procesie infiltracji. (Research on nonwoven geotextile use in order to counteract clogging process during infiltration into the ground). PhD Thesis, University of Technology, Wroclaw. Chua, L. H., Leong, M. C., Lo, E. Y., Reinhard, M., Robertson, A. P., Lim, T. T., Shuy, E. B. & Tan, S. K. Controlled field studies on soil aquifer treatment in a constructed coastal sandfill. Water Science and Technology 60 (5), 1283–1293. Dechesne, M., Barraud, S. & Bardin, J. P. Indicators for hydraulic and pollution retention assessment of stormwater infiltration basins. Journal of Environmental Management 77, 371–380.
Water Science & Technology
|
68.10
|
2013
Gautier, A., Barraud, S. & Bardin, J. P. An approach to the characterization and modeling of clogging in storm water infiltration facilities. In: Proc. the Eighth International Conference on Urban Storm Drainage (I. B. Joliffe & J. E. Ball, eds). August 30–September 3, The Institution of Engineers Australia, The International Association for Hydraulic Research, and The International Association on Water Quality, Sydney, Australia, 1007–1015. Geiger, W. & Dreiseitl, H. Nowe sposoby odprowadzania wód deszczowych; Poradnik retencjonowania i infiltracji wód deszczowych do gruntu na terenach zabudowanych (New ways to stormwater drainage. Guide for stormwater retention and infiltration into the ground in urban areas). Oficyna Wydawnicza Projprzem-EKO, Bydgoszcz. Iwasaki, T. Some notes on sand filtration. Journal of the American Water Works Association (AWWA) 29, 1591–1602. Marla, C. M. & Lee-Hyung, K. Long-Term Monitoring of Infiltration Trench for Nonpoint Source Pollution Control. Water, Air and Soil Pollution 212 (1–4), 13–26. Nivala, J., Knowles, P., Dotro, G., García, J. & Wallace, S. Clogging in subsurface-flow treatment wetlands: measurement, modeling and management. Water Research 46 (6), 1625–1640. Pedretti, D., Barahona-Palomo, M., Bolster, D., Fernàndez-Garcia, D., Sanchez-Vila, X. & Tartakovsky, D. M. Probabilistic analysis of maintenance and operation of artificial recharge ponds. Advances in Water Resources 36, 23–35. Pérez-Paricio, A. Integrated modelling of clogging processes in artificial groundwater recharge. PhD Thesis, Technical University of Catalonia (UPC), Barcelona. Raimbault, G., Nadji, D. & Gautier, C. Stormwater Infiltration and Porous Materiał Clogging. 8th ICUSD, Sydney, pp. 1016–1024. Rinck-Pfeiffer, S., Ragusa, S., Sztajnbok, P. & Vandevelde, T. Interrelationships between biological, chemical, and physical processes as an analog to clogging in aquifer storage and recovery (ASR) wells. Water Research 34 (7), 2110–2118. Taylor, S. W., Milly, P. C. D. & Jaffe, P. R. Biofilm growth and the related changes in the physical properties of a porous medium. 2. Permeability. Water Resources Research 26 (9), 2161–2169. Vandevivere, P., Baveye, P., Sanchez de Lozada, D. & Deleo, P. Microbial clogging of saturated soils and aquifer materials: evaluation of mathematical models. Water Resources Research 31 (9), 2173–2180. Williams, J. R., Ouyang, Y., Chen, J. S. & Ravi, V. Estimation of infiltration rate in vadose zone: Application of selected mathematical models. Report EPA/600/R-97/128b Vol. II. Ying, M., Guangyao, G., Zailin, H., Shaoyuan, F. & Dongyuan, S. Modeling water infiltration in a large layered soil column with a modified Green-Ampt model and HYDRUS. Computers and Electronics in Agriculture 71, 40–47. Zhuanxi, L., Tao, W., Meirong, G., Jialiang, T. & Bo, Z. Stormwater runoff pollution in a rural township in the hilly area of the central Sichuan Basin, China. Journal of Mountain Science 9 (1), 16–26.
First received 22 November 2012; accepted in revised form 15 July 2013. Available online 25 October 2013
Copyright of Water Science & Technology is the property of IWA Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.
