Al-Rubaei, A.

Blecken, G.-T.

Hunt, W.

Merriman, L.

Viklander, M.

I-Revues

NOVATECH 2016 
1 
A survey of 30 Swedish stormwater ponds - 
evaluation of pond design, maintenance needs and 
implications for practice 
Inspection de 30 bassins de traitement des eaux 
pluviales en Suède - Evaluation de leur conception, leur 
maintenance, leurs besoins et enseignements pratiques 
 
Ahmed Mohammed Al-Rubaei*1, Laura S. Merriman**, William F. 
Hunt III**, Jiri Marsalek*, Maria Viklander*, Godecke-Tobias 
Blecken* 
 
*  Urban Water Engineering, Department of Civil, Environmental and Natural 
Resources Engineering, Luleå University of Technology, 971 87 Luleå, 
Sweden; ahmed@ltu.se; marvik@ltu.se; godble@ltu.se   
**  Biological and Agricultural Engineering, North Carolina State University, 
Campus Box 7625, Raleigh, NC 27695, USA; lsmerrim@ncsu.edu, 
wfhunt@ncsu.edu  
RÉSUMÉ 
L’inspection de 30 bassins d’eaux pluviales Suédois a révélé qu’approximativement la moitié de ceux-
ci ne respectait pas les recommandations de conception actuelles. 5 des 30 bassins n’étaient pas du 
tout accessibles et 4 autres n’étaient pas accessibles par engins motorisés. Il s’agit d’une menace 
pour le fonctionnement à long terme de ces équipements étant donné que leur conception empêche 
toute maintenance. La majorité des bassins n’avait pas de chambre de sédimentation et 11 des 25 
bassins accessibles avaient une faible efficacité hydraulique. Durant les inspections, plusieurs 
problèmes mineurs ont été détectés qui pourraient pourtant être réglés facilement par les 
municipalités. Des dépôts de sédiments ont été observés dans tous les bassins inspectés. Les 
concentrations de métaux dans ces dépôts étaient relativement basses, sûrement en raison des 
faibles quantités de fines retenues.  En accord avec des études précédentes, ces travaux montrent 
que négligence, manque de maintenance et conception incomplète sont des problèmes récurrents 
pour les infrastructures de gestion des eaux de pluie. La réglementation aurait besoin d’évoluer en 
conséquences pour inclure des coefficients de sécurité lors de la conception de ces infrastructures et 
en exigeant leur inspection régulière. Il est également nécessaire que les gestionnaires d’équipements 
de traitement des eaux pluviales soient présents lors de la réception de ces équipements après 
construction.  
ABSTRACT 
A survey of 30 Swedish ponds revealed that about half of the ponds was not designed according to 
up-to-date recommendations. 5 of 30 ponds were not accessible at all and further 4 ponds not 
accessible for machinery. This is a threat to the long term function of these ponds since maintenance 
is prevented by design. A majority if the ponds did further lack a forebay; and 11of 25 ponds showed a 
poor hydraulic efficiency. When inspecting the ponds on site, several minor issues were detected 
which, however, could be repaired easily by the municipalities. All ponds retained sediment. The metal 
concentrations in this sediment were relatively low, maybe due to the relatively little amount of fines 
retained. Similarly to other field studies which evaluate the function of SCMs in field, this study shows 
that neglect, under-maintenance and insufficient design is an important issue for a relatively high 
number of SCMs. Regulators should thus include factors of safety in SCM design, assess SCM 
function conservatively and require regular inspection. Stormwater asset managers should be present 
at the final inspection and acceptance of each construction project which includes stormwater 
facilities.  
KEYWORDS 
Stormwater control measures (SCM), wet ponds, long term function, maintenance 
SESSION 
2 
INTRODUCTION 
One of the mostly common stormwater control measure (SCM) in Sweden and worldwide are wet 
sedimentation ponds (VanLoon et al. 2000). The main treatment mechanism is sedimentation, but 
(depending on shape, depth, vegetation etc.) also other factors can contribute to the quality treatment. 
Forebays enable the deposition of coarse and gross solids as well as larger soil particles in a separate 
treatment step.  
As all SMCs, stormwater wet ponds can only work efficiently if they are inspected and maintained 
frequently (Blecken et al. in press). However, the (rather limited) available data indicates that 
maintenance is often lacking, which may jeopardise the ponds’ treatment performance (Starzec et al. 
2005).  
Visual inspections of inlet and outlet structures should be carried out regularly or after significant 
storms (EPA 1999). Malfunctions of the hydraulic structures, accumulation of debris and rubbish, 
erosion, and uncontrolled vegetation growth or die-off may cause problems (Blecken et al. in press). 
Maintenance includes especially the scheduled removal of the accumulated sediment (Heal et al. 
2006).  
The need to gather more data on the long term functionality and/or maintenance needs of existing 
SCMs rather than focusing exclusively on “showcase” pilot facilities has been highlighted (D'Arcy and 
Sieker 2015). Therefore, the main objective of this study were to survey the status of 30 wet ponds in 
5 Swedish municipalities. The survey involved also classifying the ponds’ design. 
MATERIAL AND METHOD 
30 Swedish municipal ponds between 3 and 33 years old, located in the Swedish cities of Malmö, 
Örebro, Växjö, Umeå, and Östersund were visited and inspected. Key characteristics of the ponds and 
their catchments were identified based on municipal data-sets.  
An inspection and maintenance checklist based on the WSUD Technical Design Guidelines for South 
East Queensland Waterways, 2006 was used to determine the ponds’ general status. The pond 
design was evaluated by checking if the ponds had a forebay, and estimating the ration between the 
pond area and the (impervious) catchment area as well as the ponds’ hydraulic efficiency.  
To determine the latter, an approach suggested by Persson et al. (1999) using a “λ-value” to describe 
the hydraulic efficiency (with λ ≤ 0.5 indicating poor hydraulic efficiency, 0.5 < λ < 0.7 indicating 
satisfactory hydraulic efficiency, and λ > 0.7 indicating a good hydraulic efficiency) was used. The 
shapes and inlet/outlet locations of the ponds were determined from maps and aerial photos and 
compared to the geometries of the 13 basins layouts studied by Persson et al. (1999), and the most 
appropriate efficiency values were adopted (or interpolated) from this reference. The hydraulic 
efficiency parameter λ indicates how evenly the flow velocities and tracer concentrations are 
distributed throughout the pond, and the occurrence of “dead” (recirculation) zones is avoided  
In August and September 2013, sediment samples were collected from bottom sediment to the inlets 
and outlets of 25 ponds (5 ponds were not accessible, see below). All sediment samples were 
analysed for particle size distribution (PSD) and total Cd, Cr, Cu, Ni, Pb and Zn concentrations using 
standard methods.  
RESULTS AND DISCUSSION 
Pond Design 
Maintenance accessibility 
Adequate maintenance and inspection access is a pre-requisite for securing a pond’s long-term 
function (Larm 2000). Swedish design recommendations also highlight the fundamental importance of 
maintenance access (Larm 2000). However, 9 of the 30 ponds were not readily accessible to 
inspectors or machinery. Five of the ponds could not even be accessed on foot, and therefore could 
not be inspected at all (Figure 1). This is a major problem because it means that 30% of the visited 
ponds were designed in a way that prevented maintenance. The large number of inaccessible ponds 
in the group chosen for study is thus somewhat surprising. 
 
NOVATECH 2016 
3 
 
Figure 1: Accessibility of the surveyed ponds 
 
Hydraulic efficiency 
The hydraulic efficiency model of Persson et al. (1999) describes the hydraulic efficiency of a pond 
using lambda (λ), which depends on the pond’s shape, the location of its inlet and outlet, and its flow 
distribution. Of the 25 accessible ponds, 11 ponds showed a poor hydraulic efficiency (λ ≤ 0.5), four 
ponds a satisfactory hydraulic efficiency (0.5 < λ < 0.7) and 10 ponds a good hydraulic efficiency (λ > 
0.7). The fact that relatively many ponds were improperly designed could reduce their effectiveness at 
treating sediments and associated metals. A principal component analysis indicated that the λ-values 
were clearly correlated with both the percentage fine sediment retained in the ponds and high metal 
concentrations (see below).  
Sediment forebay 
To fit a sediment forebay in pond designs is commonly recommended to improve their efficiency of 
pollutant removal because forebays effectively trap coarser (often less polluted) sediments and 
thereby facilitate sediment removal and disposal (EPA 1999, Larm 2000, Sansalone and Buchberger 
1997). However, only 7 of the 25 evaluated ponds had a forebay. Although the lack of a forebay does 
not necessarily impair a pond’s function, it does generally impede sediment removal. 
Survey Results 
Most of the accessible 25 ponds were in relatively good condition. About half of them was in need of 
(minor) maintenance, mainly due to sediment and litter accumulation at their inflow and outflow points. 
Only one major issue was detected: the outlet structure of one pond was found to be damaged and 
leaking. However, the municipality was aware of this leakage and it was repaired shortly after our 
inspection which underlines the need for municipalities to regularly inspect SCMs to detect damage as 
early as possible. The fact that the inspection survey revealed relatively few minor issues that could be 
easily corrected demonstrates the importance of relatively simple regular inspections serving to detect 
minor problems at an early stage, before they would seriously impact on pond functioning.  
The scope of the survey was limited by incomplete data available in municipal records. Besides some 
of the missing basic pond characteristics, some other relevant data, e.g. design criteria (detention 
time, multiple peak flow controls, sediment storage volume, sediment influx to the ponds, pond 
geometry including bathymetry, etc.) were not readily available from the municipal records. This lack of 
basic pond information implies risks for successful operation of, and maintenance planning for, these 
facilities. 
Sediment 
The distribution of sediment particle sizes varied widely among the 25 ponds. In most cases (16 of 25) 
only minor percentages of fines (silt and clay) were retained in the ponds. In contrast, in nine ponds, 
the percentage of these fines exceeded 50%.  
The metal concentrations measured in the pond sediment were assessed against the Swedish EPA 
guidelines (Swedish EPA 2000) for the protection and management of receiving water bodies and 
ecosystems. The Pb and Cd concentrations were in the class I range (i.e. very low concentrations), 
corresponding to levels that are not expected to cause adverse effects (Swedish EPA 2000). 
SESSION 
4 
Approximately 40% of the ponds had sediments with Cr, Cu, Ni, or Zn concentrations in the class III 
range, which can have adverse biological effects on the receiving waters, in the remaining ponds 
these concentrations were low (class I and II). It is clear that these ponds are having beneficial effects 
on their receiving bodies of water because they are retaining this relatively highly polluted sediment. 
The high number of ponds with relatively low metal concentrations may be due to the relatively coarse 
material given that heavy metals are most strongly associated with fine-grained fractions (Sansalone 
and Buchberger 1997).  
Correlation of percentage fines, sediment metal concentrations and pond 
design 
A PCA was performed to an overview of the significance and correlations of the factors affecting the 
ponds’ treatment performance indicators, namely the metal concentrations and proportion of fines in 
the accumulated sediments. The factors added were age, λ-value, and ratio pond area/impervious 
catchment area.  
The PCA reveals that the metal concentrations in the sediment close to the inlet, the ratio of pond 
area/ impervious catchment area, the Cr and Ni sediment concentrations at the outlet, the percentage 
of fines, and the λ-values are correlated (PC1). The Cd, Cu, Pb, and Zn sediment concentrations at 
the outlet and pond age are grouped together along the second component (PC2). The λ-values 
correlate with the fine sediment fractions and metal concentrations underlining the importance of 
careful design.  
 
Figure 2. Loading plot for principal components 1 and 2 obtained from a PCA of sediment metal concentrations 
(close to the inlet and outlet), sediment PSD, pond age, ratio of pond areas impervious catchment area, and 
lambda (λ). 
Conclusions 
In conclusion, although several minor issues were detected, the surveyed ponds were in satisfactory 
condition and all of the detected issues could be corrected with relatively little effort. However, none of 
the identified issues would have been rapidly detected during standard inspections due to the general 
lack of regular inspection schedules and inspection access. The need to regularly inspect and 
evaluate the functioning of existing SCMs that have been in operation for long time should be 
highlighted to improve and optimize the design of these systems. However, one alarming result of this 
study is that 1/3 of the ponds were designed and operated in a way that would prevent detection of 
these or other issues during a standard inspection because there was no regular inspection regime or 
no maintenance access.  
Given that this and other studies which show neglect, under-maintenance and/or insufficient design of 
SCMs (Al-Rubaei et al. 2013, Starzec et al. 2005, Wardynski and Hunt 2012, Lindsey et al. 1992), 
regulators should (i) include factors of safety in the SCM design allowing for partial failure of the 
systems, (ii) assess the SCM performance conservatively to avoid assuming too positive performance 
and (iii) clearly require regular (e.g. annual) inspection of the facilities (Blecken et al. in press). 
Stormwater asset managers should be present at the final inspection and acceptance of each 
construction project which includes stormwater facilities. Besides right design, long-term maintenance 
is crucial for SCM functionality and must be regarded if LID, SUDS or WSUD should be successful 
NOVATECH 2016 
5 
concepts in the long run.  
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D'Arcy, B.J. and Sieker, H. (2015). Germany’s long-term support for SUDS development. Water 21 17(1), 26-28. 
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Inspection de 30 bassins de traitement des eaux pluviales en Suède - Evaluation de leur conception, leur maintenance, leurs besoins et enseignements pratiques

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Inspection de 30 bassins de traitement des eaux pluviales en Suède - Evaluation de leur conception, leur maintenance, leurs besoins et enseignements pratiques

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