SEDIMENT REMOVAL IN STORMWATER MANAGEMENT PONDS
TABLE OF CONTENTS
Chapter:1 INTRODCUTION 2
1.1 General Information 2
1.1.1 Time Frame for Stormwater Management in Ontario…………………………………………..……….2
1.1.2 Types of Ponds………………………………………………………………………………………………………….3
1.2 Types of Pollutants 4
1.2.1 Suspended Solids………………………………………………………………………………………………………4
1.2.2 Heavy Metals…………………………………………………………………………………………………………..5
1.2.4 Other Constituents………………………………………………………………………………….………………..6
1.3 Problem Statement……………………………………………………………………………………………………………….6
1.4 Scope & Objectives………………………………………………………………………………………………………..…..7
Chapter:2 BACKGROUND AND LITERATURE REVIEW 8
2.1 Stormwater Management Ponds 8
2.2 Total Suspended Solids 9
2.3 TSS Impact on Pond Performance……………………………………………………………………………………….10
2.4 Long-Term Maintenance of Ponds………………………………………………………………………………………..11
Chapter: 3 SUSPENDED SOLIDS LOADING IN PONDS…………………………………………….14
3.1 Catchment Characteristics……………………………………………………………………………………………..……14
3.2 Previous Work…………………………………………………………………………………………………………….…….15
3.2.1 Best Management Practices……………………………………………………………………………………..16
3.2.2 Monitoring Studies…………………………………………………………………………………………………..16
3.2.3 Runoff from Land Uses……………………………………………………………………………………………17
Chapter: 4 TOTAL SUSPENDED SOLIDS REMOVAL IN PONDS……………………………….20
4.2 Quiescent Settling………………………………………………………………………………………………………………23
4.2.1 Empirical Models…………………………………………………………………………………………………….23
4.2.2 Sediment Trap Efficiency………………………………………………………………………………………….24
4.3 Pond Design Guidelines……………………………………………………………………………………………………….25
4.3.1 Stormwater Best Management Practice Design Guide (USEPA)………………………………….. 25
4.3.2 Stormwater Management Planning and Design Manual………………………………………………..29
Chapter: 5 MODELING OF SEDIMENT REMOVAL IN PONDS…………………………………33
5.1 Stormwater Management Model…………………………………………………………………………………………..33
5.2 System for Urban Water Treatment and Analysis Integration (SUSTAIN) ………………………………….34
Chapter: 6 CONCLUSIONS…………………………………………………………………………………………………..40
Chapter: 7 REFERENCES…………………………………………………………………………………………….41
1 CHAPTER: 1 INTRODUCTION
- General Information
It is evident that urban stormwater contributes to the degradation of waterways through adversely changing the stream morphology, water quality, aquatic habitat, and ecosystem. In urban regions, increasing impervious surface construction has hindered water percolation and groundwater recharge which often creates urban flooding and direct discharge of stormwater and associated pollutants into streams. In order to minimize these impacts, urban stormwater management practices are divided into two control categories: Lot-level and conveyance controls and End-of-pipe facilities. Lot-level and conveyance controls are enforced to maintain the natural hydrologic cycle to the greatest extent possible and end-of-pipe facilities are needed for flood, erosion control and water quality improvements. (Casey, Simon, & Atueyi, 2006). In this project, we are considering end-of-pipe controls particularly stormwater management ponds. These ponds are designed to capture and store the runoff from impervious surfaces, promote infiltration and retain sediments
- Time Frame of Stormwater Management in Ontario
The evolution of stormwater management happened in the early 1980’s in Ontario, the main objective was then to provide flood control although over the time there were drastic improvements in the management practices.
In early 90’s the management adopted the Master Drainage Plan focusing on floodplain management, runoff quality control, and erosion control. This resulted in the construction of dry and wet ponds for most effective solutions to these issues.
In today’s practices, there are number objectives covered under (OMOE, 2003) which not only includes the ones covered previously but also regulates the treatment of these ponds, fisheries protection, stream morphology and protection of ground water. One of best management practices for stormwater management are followed through water balance, use of green infrastructure and LID practices to accelerates the process of more surface runoff into stormwater ponds (Macmilan & Glen).
- Types of Ponds
According to U.S protection agency, the stormwater ponds are divided into two types depending on their different applications and characteristics.
- Dry Ponds:
Stormwater dry ponds are constructed to store temporarily excess stormwater, these ponds are not meant to hold stormwater for the longer period of time. The pollutants are allowed to settle down at the bottom of the basin and the water is allowed to slowly drain out to the adjoining land features including wetlands and streams. The dry detention ponds have two primary applications to be used for flood control, encourage water quality control (pollutant removal) and channel protection. Unlike wet ponds they do not have a permanent pool, however, they are sometimes used as an extra space to store stormwater in the case of flood emergencies. The design and costs of these ponds depend upon the drainage area, slope, soils/topography, and groundwater, however dry detention ponds are the least expensive Stormwater treatment practices.
- Wet Ponds:
Wet ponds are mostly identical to dry ponds except that they are meant to store stormwater for a longer period of time. They are constructed larger in size than dry ponds so that they can hold a specific large volume of stormwater. The basins of wet ponds have permanent pool throughout the year and nutrients uptakes happen due to biological activities. These ponds are most extensively applicable Stormwater treatment practice due to their broad rescue protection goals such as flood control, channel protection, groundwater recharge and pollutant removal. Although these ponds may require annual inspection and monthly maintenance to clean and remove debris from inlet and outlets.
The size and design parameters of wet and dry ponds are similar that depend upon the drainage area, slope, soils/topography, groundwater and also a number of impervious surfaces available. It is also considered that these ponds are most cost-effective stormwater management practices. Both the types of the basin are designed depending on specific time stormwater events (for example 10 years).
- Constructed Wetlands:
Wetlands are widely applicable in stormwater management practices and have similar structural design and functions as that of wet ponds. Wetlands have shallow zones (less than 0.5m) and incorporate wetland plants which most effective pollutant removal and also aesthetic value. The wetlands are used to achieve following resource protection goals such as flood control, channel protection, and pollutant removal.
- Types of Pollutants in Stormwater Ponds
The time when rainfall strikes the surface till it reaches to the stormwater ponds the water encounters various sources of contamination. The urban runoff is comprised of various contaminants depending upon the location of runoff (residential, commercial, industrial, agricultural etc.), time of contact and their level of concentrations present.
The pollutants can enter in water through various mechanisms in the runoff and these mechanisms all together can lead to severe water quality degradation. Some of these following mechanisms are: Atmospheric Scrubbing, Scour, and Erosion, Surface Wash off, Deposition and Transport and transformation etc. (Hamind & Tsihrintzis, 1997)
The following pollutants are typically found in stormwater runoff:
- Suspended Solids: These are the principal pollutant in urban runoff and the one we are considering our study for this project. The suspended are usually the street dust and eroded sediments which make the water turbid/cloudy and have the particle size greater than 0.4 μm. These are the pollutants which do not settle down in the water and comprises of organic (25-30% of TTS) and inorganic materials. (Hamind & Tsihrintzis, 1997).
The Table 1.1 below shows the concentrations of various pollutants present in urban stormwater runoff inflow to ponds in comparison to their acceptable provincial water quality standards (stormwater management planning and design mannual, 2003).
Table 1.1 Comparison of Urban Stormwater Runoff Concentrations with Provincial Water Quality Objectives
High levels of suspended solids in runoff (greater than 120 mg/L) present in the stormwater runoff tends to have serious impacts on the receiving water (Richard, 2010). The runoff which enters directly into the water without any treatment before is responsible for degrading the quality of receiving water by making it turbid, inhibiting plant growth and destroying aquatic species diversity (Shammaa & Zhu, 2013).
- Heavy Metals: The concentrations of heavy metals are proven to be found greater in stormwater than in sanitary sewage. Heavy metals such as cadmium, zinc, lead, and arsenic etc. are the pollutants from major contributors like automobiles, industrial and commercial land uses. The pollutants from these sources directly lead to in contact with runoff. Increased amount of these heavy metals above their threshold standard in water will possess serious contamination issues.
- Chlorides: high amount of chloride content are found in the water during the entire winter season, salts are applied all over the roads and pavements during the early phase of snowfall. U.S.A range salt quantities ranges from 180-550 kg per two-lane street per mile (Hamind & Tsihrintzis, 1997). The high amount of salts is washed off directly into runoffs and into ground water.
- Other Constituents: There are various other pollutants which are taken into consideration such as Oil, grease, and hydrocarbons which are added into runoff through highways. It is found that every year considerable amount (4.2 X109 liters) (Hamind & Tsihrintzis, 1997) of lubricants are lost into the environment from the automobiles and industries through spills. Hydrocarbons released from motor vehicles remains suspended in the air and then get scrubbed when rainfall occurs and finally making their way into the runoff.
- Problem Statement
The concept of stormwater ponds was first designed as simple storage facilities with well-defined hydraulic and hydrologic functions without any consideration towards the water quality standards (Buren, Watt, & Marasalek, 1997). Indeed human activities can have inevitable effects not only on drainage basin hydrology but also on the hydrological cycle. The Environmental Protection Agency (EPA) recently stated that urban runoff is the second most source of pollution to U.S lakes and rivers (USEPA, 1994).
For decades, these stormwater ponds and wetlands have been used to overcome these impacts, but sometimes these practices may be comprised due to several reasons such as improper design and siting, poor vegetation management, clogged inlets and outlets and inadequate access to maintenance.
The design and characteristics of each pond are different which depends on its operation, therefore, it is difficult to determine a standard stormwater management pond performance. Although number of studies have been conducted to define the efficiencies of these ponds in removing different types of pollutants but, there is always lack of long-term pollutant removal efficiency data which makes it complex for stormwater engineers and managers to understand.
The performance of stormwater management pond is majorly hindered when there is reduced storage capacity due to sediment accumulation when it exceeds the water quality storage requirements and regulations (OMOE, 2013).
Sediment accumulation is essential to indicate that the pond is successful in performing its functions of sediment and pollutant removal. In order to keep the stormwater management pond fully functional for the long-term basis, it is important to have a regular monitoring, maintenance, and sediment removal mechanism.
Furthermore, the approach is to provide a desired system-wide approach towards sediment settling efficiencies and long-term performance of total suspended solids as per OMOE standards from the stormwater management ponds by choosing within the range of potential computer models available.
- Scope & Objectives
The objective of this project is to investigate and report on issues relevant for sediment removal in stormwater management (SWM) ponds. The focus on the project will be specifically in the design of SWM ponds, sediment loading rates, removal mechanisms, and modeling of suspended solids capture.
The report will entail the following:
• Review of hydrologic and hydraulic concepts related to flows and sediment loadings of SWM ponds
• Review of design guidelines and standards for SWM ponds
• Review of theories related to discrete particle settling, and modeling approaches of the same that have been used to predict the sediment accumulation in SWM ponds
• Development and application of a USEPA SWMM model of a single catchment to simulate the long-term loading and removal of suspended solids
• Producing a final report summarizing the literature review and study results.
- CHAPTER: 2 BACKGROUND AND LITERATURE REVIEW
- Stormwater Management (SWM) Ponds
Stormwater is referred to rainfall and heavy snow that melts to flow over the land (roads, houses, parking lots, commercial and industrial properties) down into the water drains. These drains direct the flow of water into the nearby constructed pond known as SWM ponds.
Stormwater management ponds are widely used as best management practices in Australia, Canada, Northern Europe and the USA. These ponds are designed to provide flood control by holding runoff from impervious surfaces and water quality control by allowing the settling of sediments and associated pollutants reducing the need for multiple end-of-pipe facilities (MOEE 1994). As reported by USEPA, stormwater ponds are proven efficient to provide 90% removal of suspended solids (TSS) and other pollutants at considerable rates. (Marsalek & Marsalek, 1997). In addition to resource protection benefits of wet ponds, it has been found that these ponds also provide economic benefits by increasing property values because of “Pond Front”.
The SWM planning and design and manual (STORMWATER MANAGEMENT POND) states that the wet ponds are less land-intensive and are normally more reliable during adverse weather conditions (e.g. Winter/spring). These ponds are commonly designed individually, in response to recognized land use pattern, to provide aimed level of sediment removal (Harrell & Ranjithan, 2003).
The main purpose of the wet pond is to remove the various pollutants and sediments present in stormwater runoff before it is discharged into receiving water. In order to achieve that goal, the runoff is held in these ponds for a significant duration of time (detention time), and gravitational settling works to settle the particles at the bottom of the pond.
The wet ponds comprise a permanent pool and dynamic storage for the removal of suspended solids and are designed to detain a 2-year, 24-hour storm period and in most cases, the selected design storm ranges from 12.5 mm to 25 mm. Pond design constitutes of an inlet, sediment forebay, outlet and outfall (Chocat & Marsalek, 2002) which are explained further in details in section: 4.1 and 4.2.
- Total Suspended Solid Removal in Ponds
The focus of this report is on total suspended solids (TTS) accumulation rates in SWM ponds. The fine particles present in stormwater runoff often carried as suspended materials, however, a total load of these suspended particles all together is called total suspended solids (TTS).
TSS is considered as one of the major pollutants in the stormwater runoff with its harmful impacts on the acquiring water bodies by increasing turbidity, inhibiting plant growth, affecting river aquatic life and depleting dissolved oxygen. Every year around thousands of tons of TTS are released to the rivers with about 70% of this loading attributed to stormwater runoff (Shammaa Y. , Zhu, Gyurek, & Labatiuk, 2002). It is considered that total suspended solid is directly proportional to the degree of urbanization and therefore stormwater management ponds are one of the proactive source control practices.
The suspended solids accumulation rates in SWM ponds vary over a wide range, depending upon various factors such as catchment area characteristics, precipitation patterns and pond design and operation.
The settling of suspended solids in a pond is complex process encircling various sub-processes such as diffusion, advection, particle flocculation and disaggregation (by turbulence) and hence resulting into sediment disposition or scouring (Krishnappan, Marsalek, Watt, & Anderson, 1999).
(Shammaa Y. , Zhu, Gyurek, & Labatiuk, 2002) states that the TTS removal rate is a function of detention time, depth of pond and particle settling velocities. The suspended solids present in these ponds are available in different particle sizes and have their respective percentage in urban runoff and average settling velocities (MOEE 1994) see Table 2.1. The particle settling velocity is dependent on its size, shape and specific gravity (density).
Table 2.1 Settling velocities for different sizes of particles in stormwater (MOEE 1994)
From the above table 2.1, it can be concluded that the particles with greater size fraction have higher percentage particle mass in urban runoff and have more average settling velocities in comparison to the lighter particles.
In the STORMWATER MANAGEMENT POND design approach, the first step is to determine the protection level (Table 2.2) based on the long-term suspended solids removal of the pond and their lethal and chronic effects on aquatic life.
|Habitat Protection Level
|Target TSS Removal Rate
Table: 2.2 TSS Removal Criteria (stormwater management planning and design mannual, 2003)
- TSS Impacts on Pond Performance
The percentage of suspended solids removal is often used as the indicator of overall long term performance of stormwater quality control ponds.
The combination of increased imperviousness and increased peak flows severely affect the type and quantity of suspended solids inflow to the ponds. The long-term changes in composition and concentration of suspended solids beyond the provided acceptable standards (stormwater management planning and design mannual, 2003) can have potential cumulative effects on the pond performance (Packman , Comings, & Booth, 1999).
Every stormwater pond has designed active storage depth of providing quality control through the settling of particles which is usually about 2 meters for wet ponds. Due to lack of monitoring and maintenance of these ponds with years and years of sediment accumulations often results into when the suspended solids removal performance comes to a limit of design storage beyond which there is negligible evidence of an increase in the settling of suspended solids (stormwater management planning and design mannual, 2003). In conclusion, the stormwater ponds are designed to provide about 90% of sediment removal but the sediment accumulation reduces the effective storage volume and the long-term STORMWATER MANAGEMENT POND removal efficiency of suspended solids.
Thus to keep the ponds full functional all the time, they have to be properly maintained and regular sediment removal is essential (Marsalek & Marsalek, 1997).
- Long Term Maintenance of Ponds
Similar to other municipal infrastructures such as roads, water supply, sanitary sewers, water and wastewater treatment plants etc., for the effective long-term operation of stormwater ponds requires suitable maintenance strategies. One of the main reasons behind the failure and poor performance of stormwater ponds in the past is due to lack of maintenance which makes it mandatory for the STORMWATER MANAGEMENT POND designers to provide management facilities in order to evaluate the removal and disposal of accumulated sediments in stormwater ponds. (Stormwater management facility sediment maintenance guide , 1999)
In order to have long-term maintenance of the ponds, (stormwater management planning and design mannual, 2003) states that it is advisable to prepare an annual maintenance report which includes the following information annually:
- Inspection – which includes observations made from the inspection results such as hydraulic operations of the facility (detention time, the occurrence of overflows, conditions of vegetation, inlet and outlet function, spills and oil/grease contaminations, trash build-up.
- Measured sediment depths ( under/above-provided guideline standards)
- Monitoring results of flows
- Maintenance and operation activities
- Detailed recommendations and maintenance for upcoming years.
The sediment removal that accumulates in STORMWATER MANAGEMENT PONDS is dependent on many factors as:
- type of stormwater management ponds;
- design storage volume;
- characteristics of the catchment area (explained in detailed in section: 3.1)
- municipal practices
The maintenance frequency for sediment removal in the pond can be calculated theoretically based on the rate of performance reduction (removal frequency of sediments) with a loss in the storage volume, although there are some limitations of this performance-storage relationship in conditions such as upstream development and poor sediment/erosion control.
The way to assess the rate of sediment accumulation is through performing continuous simulations for end-of-pipe stormwater management facilities (i.e. SWM ponds).
These simulations indicate the total suspended solids removal efficiencies with the varying volume of storage and different levels of imperviousness. These removal efficiencies are then converted into volumes of sediment captured by the SWM ponds on an annual basis.
Once the protection level is established, the designed storage volumes for the wet pond can be determined from Table 2.3 for different impervious levels.
|STORMWATER MANAGEMENT POND Type
|Storage Volume (m³/ha) for
|Enhanced 80% long-term S.S. removal
|Normal 70% long-term S.S. removal
|Basic 60% long-term S.S. removal
Table 2.3 The volumetric water quality criteria for Wet ponds (stormwater management planning and design mannual, 2003)
In order to determine the corresponding minimum storage volume for each impervious level of wet ponds below which the sediment removal maintenance is essential can be done through interpolation (volume vs treatment) from MOEE STORMWATER MANAGEMENT POND (Stormwater management facility sediment maintenance guide , 1999) based on the recommended allowable 5% acceptable reduction in original design storage due to sediment accumulation Table 2.4
|STORMWATER MANAGEMENT POND Type
|Storage Volume (m³/ha) for
|Enhanced 80% long-term S.S. removal
|Normal 70% long-term S.S. removal
|Basic 60% long-term S.S. removal
Table 2.4 Minimum required STORMWATER MANAGEMENT POND storage volume prior to maintenance (Stormwater management facility sediment maintenance guide , 1999)
Therefore, according to the above two procedures (continuous simulation and sedimentation model) can be used to determine the average annual TSS removal efficiency of a stormwater management pond with a certain storage volume. Thus the required timeframe for the maintenance of the stormwater management pond can be evaluated based on annual sediment accumulation and the corresponding loss in the storage volume.
The techniques for predicting long-term suspended solids removal in wet detention ponds can be categorized into three groups: (a) Empirical models (b) statistical analyses (c) Deterministic models. These approaches are explained in details in section: 4.
The long-term performance of pond depends upon the sediment removal maintenance in order to restore the available storage and the treatment efficiency. The sediment accumulation rates can be estimated using the sediment trap efficiencies and field monitoring (MOEE SWMP manual, 2004).
The maintenance frequency can be evaluated by the modeling suspended solid loading rate and the pond sediment removal efficiency. The theoretical approach is applied using simple mass balance:
Maintenance frequency/sediment accumulation = Sediment Inflow – Sediment Outflow
The continuous accumulation of sediment over the years inside pond becomes an issue when the volume of the pond reduce to minimum storage volume (Table 2.4) after which maintenance is highly essential. There are several removal technologies available that can be used to restore the pond volume such as mechanical or hydraulic methods. Mechanical methods are usually either excavation or dredging whereas hydraulic methods are referred to suction dredging (Stormwater Management facility sediment maintenance guide, 1999).
- CHAPTER: 3 SUSPENDED SOLID LOADING IN PONDS
- Catchment Characteristics
In order to understand the hydrologic analysis of each subcatchment, it is considered that the characteristics of catchment influence the hydrologic properties. The characteristics of catchments are important for any analyses and depend upon the purpose and objective of analyses which in this report is directed towards to understand the various catchments may or may not have an effect on various aspects of hydrology such as sediment movement (Boughton, 1968). Climate variability and catchment characteristics interact together to describe hydrological responses (Carrillo, Troch , Sivapalan, & Wagener, 2011).
The catchment hydrology is used in evaluating increased frequency and intensity of rainfall and flooding events, higher runoff and peak flows, and more rapid peaking of storm flows. (Australian Guidelines for Urban Stormwater Management , 2000)
Rainfall and catchment characteristics are among the most vital influential factors in relation to urban stormwater quality. It has been recognized that the pollutant build-up and wash-off processes are influenced catchment characteristics.
Investigation of sediment accumulation rates requires catchment characteristics as one of the major measures to be considered. Catchment characteristics such as impervious surface fraction and land use, land use typically considered as major characteristics and pollutant-build loads are assigned accordingly. (Egodawatta, Liu, & Guan, 2013)
Land use data of the area is usually available while construction of pond from historic images and records available and these land uses are divided into various categories such as industrial, commercial and high/med/low residential. (Explained in section: 3.2.3). Catchment characteristics influence sediments loading (pollutants) in the pond and can be used to enhance the modeling adopted.
- Previous Work
- Best Management Practices (BMP) Database
According to USEPA, best management practices cover a broad perspective of various activities like maintenance procedures, prohibition of certain practices, and physical treatments aiming common target of removing and reducing pollutants in stormwater runoff and ultimately protecting receiving water sources (Kneip & Callaway, 2017). The BMPs for stormwater management can be divided into two categories: 1) Structural BMPs and 2) Non-structural BMPs. The structural BMPs are the engineered built systems to provide the desired quality and flood control. These are generally referred to the physical treatment practices such as stormwater management ponds, porous pavements, rainwater harvesting, bio-retentions, green-roofs etc. whereas the non-structural BMPs includes a variety of pollution control measures, education, management practices.
It has been considered that for the removal of pollutants such as total suspended solids, phosphorus and heavy metals, physical “treatment practices” are best to be implemented. It is often recommended to choose combinations of BMPs to be used in order to achieve the three primary objectives that are flood prevention and water quality control and stream morphology protection (Martin, Ruperd, & Legret, Urban stormwater drainage management: The development of multicriteria decision aid approach for best management practices , 2006).
In this project we are considering stormwater ponds as the best management practice for the removal of suspended solids from the stormwater runoffs however, they do not alleviate other hydrologic impacts of development which have adverse effects on ecosystem such as inadequate base flow (Damodaram, Giacomoni, Khedun, & Holmes , 2010).
The BMP performance can be evaluated with respect to six criteria targeting technical and hydrological capabilities, environmental impacts, social perception with considering the maintenance and economic aspects (Martin, Ruperd, & Legret , 2006). Technical performance is conducted in respect of design frequency and implementation problems and hydraulic performance include BMP adequateness in reducing flooding risks and peak discharges and mitigating runoff flow rates. The environmental criteria assess the effectiveness of BMP in stormwater pollution control and protecting stream quality. The social criteria indicate the lifestyle and environmental improvement, landscape and leisure facilities and public awareness.
The operation and maintenance indicator focus on the maintenance problems and explanation about BMP dysfunctional.
- Monitoring Studies:
The (Urban stormwater BMP performance Monitoring, 2002) provides guidance for approaches and techniques that are essential and useful for monitoring BMPs. There are many methods available to monitor the basic flow and water quality and are expected to be advanced with emerging technologies. The BMP selection can be improved with the help of monitoring programs as it will provide collection, storage, and analysis of data.
The efficiency of the BMP systems e.g. stormwater management ponds can be determined with help of available data and can be expressed in terms of sediment control/removal and achievement in flood control.
There are the existing set of database and protocols available which aim towards improving the consistency of BMP monitoring information.
Several monitoring strategies could be applied to direct BMP effectiveness such as mentioned in the journal paper (Strecker & Quigley, 2001) : a) Input/output concentrations comparisons can be applied to new BMPs in new development areas to evaluate the inflow load versus outflow loads b) Retrofitting of the existing BMPs (before/after approach) for better results of water quality protection c) comparison between structural and non-structural BMP however, input/output load/concentrations monitoring is the common approach applied.
- Historical Approaches: There are variety of pollutant removal methods available which has been applied in monitoring studies to assess the BMP efficiency such as Efficiency ratio, summation of loads, regression of loads, mean concentrations, and efficiency of individual storm loads however, these methods provides efficiency removal for a particular pollutant and also lacks in providing information about the differences in inflow and outflow water quality measures significantly.
- New approaches: In analyzing new and existing monitoring studies following methods are favorable to be used: a) Effluent Probability method: It is a more straight-forward method and directly provides results about BMP effectiveness and outflow water quality. Under this approach, a standard probability plot is made for both inflow and outflow pollutants/load.
- Runoff from different land uses
If a rain falls over a grassy and forest area the runoff is expected to be less as most of the water will get percolated in the soil, however, if the same rain falls over an impervious area such as parking lots it is expected to have more surface runoff. The changes in land use over time have caused the significant increase in impervious areas and ultimately resulting in increased surface runoffs by reducing biofiltration capacity (Harbor, 2007 ) .
The constituents of stormwater runoff are highly influenced by the urban land uses, which is a significant contributor to water quality problems worldwide since stormwater runoff from urban areas is contaminated with serious pollutants as discussed earlier in section: 1.2. To future predict the runoff characteristics of a particular region it is important to understand the historical land use transformation of that area.
It is evident that with the continuous increase in population there will be more and more land transformation in upcoming years and hence it essential to understand the crucial impacts it will be leading to. Under such circumstances, understanding the historical land use transformation will provide better application and design of new ponds for future purposes and to retro-fit present ones.
Different types of urban land use provides varying range of pollutants in the runoff which is represented in the table below Table: 3.1
Table 3.1 Pollutant loading from runoff by urban land use (lbs/acre-yr)
While observing the above table, it can be concluded that the maximum amount of suspended solid are being released in the stormwater runoff are from construction sites around (6000 lbs/acre-yr) followed by commercial sectors (1000 lbs/acre-yr), freeways (880 lbs/acre-yr), and industrial sector (860 lbs/acre-yr) respectively.
The United States environmental protection agency (USEPA) states that the pollutant loadings found in urban stormwater runoff are 200 times more than that found in domestic water. The concentration of TSS in urban runoff ranges between (20-2890 mg/L) however, the guidelines states the acceptable concentration of TSS is around (120-150 mg/L) (stormwater management planning and design mannual, 2003).
- CHAPTER: 4 SUSPENDED SOLIDS REMOVAL IN PONDS
The operation of stormwater management ponds can be described with the help of mathematical models that can be used to predict the performance of the ponds. The conceptual model’s approaches are based on (a) settling of suspended solids in the pond (Sedimentation Models) (b) removal of particle fractions from dissolved contaminants by the physical, chemical and biological process (Pond water quality models). (Wanielista & Yousef , 1993 )
- Hydraulics ( Models for sediment removal efficiency)
The design of ponds requires an integrative approach towards various aspects involving biological and ecological sciences, hydrological engineering and flow hydraulics (Persson & Wittgren, 2003). In a stormwater management ponds, the residence time will be influenced primarily by two factors: hydrology and hydraulics. Hydrology is basically the temporal distribution of the inflow and hydraulics is the flow patterns that occurs in the basin during an event (Walker, 1998 ).
The Performance of the constructed pond is often insufficient if the pond is designed with unsatisfactory hydraulic control. In order to obtain high hydraulic efficiency, the pond design must include the well-defined shape and depth of the pond, length-to-width ratio, baffling, types and location of inflow and outflow structures, and vegetation.
Hydraulic efficiency of the pond depends upon two major abilities in the hydrodynamics of the facility. The first ability is to provide efficient utilization of the detention storage available in the pond to have well-distributed stormwater inflow. The second ability is the amount of mixing or re-circulation i.e. Plug flow. (Persson, Somes, & Wong, 1999). The water pollution control federation (WPCF, 1990) recommends that the constructed ponds should have plug flow since plug flows are considered as optimal flow and are preferred because all fluid elements settle around nominal residence (Kadlec & Knight, 1996). The removal rate of TSS increases with the loading rate, which makes plug flow more suitable.
The above two behavior can be understood more closely by modeling them as continuously stirred tank reactors (CSTRs). The substance removal in each tank is based on first-order kinetics, where N is number of tanks, N=1 defines completely mixed reactor (Persson & Wittgren, 2003)
The nominal residence time ( tn ) can be defined as:
The nominal residence time increase with the increase in total volume or a decrease of the hydrological load. The removal of pollutants such as suspended solids can be understood with the help of above modeling approach and the removal efficiency of suspended solids is defined by (USEPA, 1993) as dynamic sediment trap efficiency, E.
The above equation represents that the removal efficiency of suspended solids is dependent on the particle velocity ( ), settling basin area (A), and pond settling performance constant (n).
The above equations have been applicable when the residence time is equal to nominal residence time under stationary conditions.
However, in reality, the actual residence time (tm) is always less than nominal residence time (tn) in the case of ponds. For such conditions, the hydraulic performance can be analyzed by tracer test (Wong, Fletcher, & Duncan, 2005). By injecting a tracer (Rhodamine WT) in the pond inlet and monitoring the concentration of the tracer at the outlet over time, for different water systems will give various residence time functions (RTD). The average time that tracer particles spend in the water system is defined as mean detention/residence time (tm).
The response of tracer concentration at the outlet plotted relative to time gives the hydraulic detention time distribution curve.
Figure: 1 and 2 Shows distribution of tracer concentration over time (Persson, Somes, & Wong, 1999)
The above figure 1 shows is an illustration of tracer concentration time distribution, where tn is the nominal residence time, tp is the peak time of the curve and tm is the mean residence time.
The mean residence time represented as the centroid of the RTD function f (t), this behavior is explained by (Kadlec & Knight, 1996).
The hydraulic efficiency (λ) is designed to reflect the effective volume of the storage and the hydraulic residence time. The value of hydraulic efficiency ranges between zero and one, with the help of these values it’s easier to understand the hydrodynamic conditions of the pond. The values near to zero represent poor flow hydrodynamic conditions and values closer one reflects near plug-flow conditions.
The hydraulic efficiency (λ) can be represented as:
Where hydraulic efficiency (λ) ranges from zero to unity, ev is the effective volume ratio (ranging from zero to 1). (Thackston, 1987) Developed a model to calculate effective volume ratio linked to width and length of the pond;
Using the data evaluated from tracer study, various hydraulic parameters (Mean detention time, dispersion number, time to start of short-circuiting etc.) can be determined (Shilton , 2005 ).
- Quiescent settling
The aim of the designed stormwater management ponds is to provide effective sediment removal without being extremely large in size. To do this, it requires information about the size of particles in stormwater runoff reaching to the pond and their fall velocities (Davies, 2009 ).
The fall velocity/settling rate of a particle is defined as settling of particles by its own weight, when the frictional and buoyant force balances the gravitational forces.
The settling of suspended solids in stormwater management ponds is dependent on the quiescent and dynamic settling. The quiescent settling occurs when particles settle in still water, whereas, dynamic settling occurs when particles settles in flowing water. There are possibilities of both quiescent and dynamic settling conditions existing simultaneously in different zones of the stormwater management ponds during an event (Bradford & Gharabaghi , 2004).
The settling rate of particles under quiescent settling can be calculated using
Where Vs is the fall velocity/ settling rate, r is the Stroke’s radius, ρs and ρf are particle and fluid density respectively and µ is the fluid dynamic viscosity and g is acceleration due to gravity.
The (USEPA, 2004) suggests that total suspended solids removal efficiencies for quiescent conditions can be estimated through:
Where, R = Quiescent removal rate in m3/day for particles
V = Settling velocity in m/sec for particles
A = Surface area in m2 for the permanent pool
The removal ratio, RR is estimated using;
Where: R is the quiescent removal rate
V is the mean runoff volume in m3
T is the average time interval between storms
- Empirical Models: These are the traditional methods available to predict sediment trap efficiency, first developed in 1943 by Brown. He explained trap efficiency based on reservoir characteristics, by plotting a curve between the ratios of reservoir storage capacity against catchment area. In 1953, Brune described the correlation between trap efficiency and ratio of reservoir storage capacity and annual inflow (Gu, Dai , & Zhu, 2016 )
- Sediment Trap Efficiency (STE)
It is defined as the fraction of sediments that enters the pond and gets settled at the bottom of the pond. There are numbers of theoretical models developed since 1945 over time, which are used to predict the sediment removal efficiency. The most used models are Camp’s model (1945), CSTRS (1984), BASIN (1985), SWMM (1988) and STEP (2001).
Camp’s (1945), ideal trapping efficiency (TE) in rectangular pond,
min 100%wswc.100% (8)
Where ws is the settling velocity and wc is critical velocity
The above equation explains that, for the particles with ws > wc all the sediments will be able to settle and trap efficiency achieved will be 100%, whereas, if ws < wc then trap efficiency is defined as the ratio of settling velocity and critical velocity.
(Chen , 1975), formulated sedimentation efficiency for dynamic settling in stormwater management ponds by modifying Camp’s model (1945). The sedimentation efficiency (TET), is expressed as a function of surface loading rate for various turbulent flow conditions (Guo & Adams , 1994);
In SWMM model (1998), a dimensionless turbulence α is defined which is used as a weighting factor to achieve overall trap efficiency from quiescent to turbulent conditions;
(10) Where µ* is the shear velocity and n is Manning’s coefficient.
For quiescent conditions α = 1 and turbulent conditions α = 0.01
The above models are the most widely used and known, however, they carries some limitations when it comes to complex hydraulic conditions.
(STEP, 2001) is defined as sediment trap efficiency model for small ponds. This model is similar to most of the theoretical models and like CSTR’s, BASIN therefore, for STEP the pond is subdivided into several sections (equal surfaces not equal volumes). STEP uses two mass balance equations, one is used for routing water and the other for routing sediment through the pond (Verstraeten & Poesen , 2001 ).
- Pond Design Guidelines
A provincial stormwater management guidance manual provides the benefit of providing solutions to the demands of technical guidance. The pond design guidelines will help stormwater managers to achieve goals such as (a) planning and design principles of stormwater management ponds (b) proper maintenance and management operations for long-term performance.
Stormwater management practices have subsequently been widely applied across USA, Canada (Ontario) and Australia and European countries. The pond design guidelines used in some of these countries are explained below:
- Stormwater Best Management Practice Design Guide (USEPA, (Clar, Barfield, & Connor, 2004) )
The most widely used manual all over the world is developed to provide assistance for more detailed understading of stormwater management system. The guidebook provide design principles for various best management practices, maintenance considerations, and retrofit oppertunites.
The guidebook suggests that the stormwater pond must be designed to have permanent pool throughout the practice. The pond should be efficient to provide channel protection, flood control and water quality control.
Stormwater pond varies with their difference in design e.g., wet ponds, micro pool ED pond, wet ED ponds, and, multiple pond systems.
Figure: 3 the plan view of a Wet Pond (USEPA, 2004)
Figure 4: Profile of Wet Pond (USEPA, 2004)
- Design Approach: The design criteria for water quality control in wet detention ponds are categorized into two separate approaches. First approach is based on sedimentation of solids and other treats the pond as lake with controlled levels of eutrophication and their mechanisms for nutrient removal. Both approchaes are applied to provide pollutant removal efficiencies to hydraulic residence time. The wet pond designed to provide flood control (stormwater management) in addition to water quality improvement.
- Design Parameters: Since the primary removal mechanism of pollutants in the wet pond through sedimentation, therefore the pond must be designed to achieve maximum setlling of solids within the permanent pool. The pool volume of pond is replaced by runoff during storm events and is treated during the dry period until the next storm event. Some of the important design parameters are discussed below:
Table: 4.1 Hydrologic & Hydraulic Design Criteria for Wet Pond Systems (USEPA, 2004)
- Pool Volume: It is one of the most critical parameters to consider during the sizing
of the wet pond as the flood control storage and ability to remove pollutant depend on it. The permanent pool is described in relation to the drainage area or runoff volume. The detention time (T) is dependent on the volume of the permanent pool, the volume of runoff and number of runoff events.
where T is detention time in years, VB is the volume of permanent pool, VR is the volume of runoff for an average storm and n is the number of runoff events per year.
Some local regulations provides extended detention of specified runoff volume as surcharge above the permanent pool in order to prevent short circuiting and improve suspended solids sedimentation. The manual states that to acheieve 80 to 90% suspended solids removal, the minimum detention time required is of two weeks and minimum VB/VR of 4.
Therefore, it is recommeneded to provide extended surcharge volume whenever the VB/VR is less than 2.5
Table 4.2 : Recommended Design Vaules for Wet Pond (USEPA, 2004)
- Pool Depth: The depth of the pool is considered a significant design parameter since it influences solids settling. Pool depth should be adequate enough to ensure aerobic conditions and prevent thermal stratification or resuspension of previously settled solids through turbulence generated by wind or storms.
The manual suggests an average depth of 3 to 6 ft is sufficient to maintain the environment within the pool.
- Minimum Surafce Area of Permanent Pool: It is recommended in the manual that for On-site wet pond, the designed minimum pool surface area should be 0.25 acres. The surface area of pool is influenced by the local topography, minmum depth and solid settling guidelines.
- Minmum Drainage Area and Pool Volume: The drainage should provide adequate of base flow to restrict excessive detention times and also to prevent severe drawdown of the permanent pool during dry seasons. The drainage area is determined through water balance calculation which requires data such as local runoff, evapotrnpiration, exfiltration and base flow.
The manual recommends a standard minimum drainage area of 20 acres is required to maintain the dry weather inflow.
- Side Slopes: The slopes along the shoreline of the wet pond should be 4:1 (Horizontal:Vertical).
- Pond Configuration: Length to width ratio should be as large as possible to enhance several benefits such as reducing short-circuiting, increasing sedimentation and to prevent stratification inside the pool. The minimum length to width ratio of 2:1 is recommended for the permanent pool.
Another design parameter includes construction of sediment forebay near the inlet to facilitate major cleanout activities.
- Outlet: The oulet should be desgined in such a way to provide desired flood control performace. The outlet consists of a riser with hood and the riser should be sized to drain out the permamnent pool within 48 hours so that sediments can be removed mechanically when required.
- Stormwater Management Planning and Design Manual (Ontario, 2014)
The manual provides guidance to design the various volumetric elements of wet pond to achieve operational success.
Figure: 5 Plan of Wet Pond (STORMWATER MANAGEMENT POND Manual)
- Drainage Area: The manual recommends drainage area for wet pond should be equal or less than10 hectares. Wet ponds requires atleast 5 heactares of drainage area to maintain permanent pool.
- Detention Time: The detention time is targeted to be 24 hours in all conditions however, the detention time reduces to a minimun of 12 hours due to clogging in outlet.
The detention time is calculated using drawdown time based on surface area and pond depth ;
2ApCAo2g0.5(h10.5 – h2 0.5 ) (14)
t = drawdown time in seconds
Ap = Surface area of the pond (m2)
C = Discharge Coefficient ( typically 0.63)
Ao =Corss-sectional area of the orifice (m2)
g = Gravitational acceleration constant (9.81 m/s2)
- Minimum Orifice Size: The manual provides 75 mm as the smallest diameter orifice accepted by most municipalties to make secure that clogging does not occur. It is recommended to use minmum orifice size of 100 mm for exposed outlet designs. For perforated riser outlet, the minumum orifice size of 50 mm is acceptable.
- Length:Width Ratio: In order to provide the longest flow path through, the inlet of the pond should be loacted as far distant from the outlet possible. Therefore the flow length of the pond is described by the length:width ratio. The favored length to width ratios ranges between 4:1 to 5:1.
Berms/Serpentine designs in pond to change the flow path at certain elevations and lengthen the effective flow path. These designs are beneficial for ponds as they prevent short-circuiting.
- Permanent Pool and Active Storage Depths: The average permanent pool depth in a wet pond is should be around 1 to 2 meters. The maximum restricted depth to pond is between 2.5 to 3 meters, ponds deeper than that found to have reduced oxygen content creating anoxic conditions and also become stratified.
Active storage depth is provided above the permanent pool and recommended to have maximum depth of upto 1.5 to 2 meters.
- Grading: Grading and landscaping is provided near the pond edges to increase the functionality of pond and ensure public safety. A minimum slope of 5:1 is reccomeneded to provide of about 3 meters one or the side of the permanent pool.
Slopes for extended detention section of pond should be within 3:1.
- Inlet Configuration: The stormwater runoff transport system have one discharge location into the wet pond. The inlets can be of two types i) Submerged Pond Inlet ii) Non-submerged Pond Inlet.
- Outlet Confirguration: The outlet of the pond must be located at the embankment for ease of operations, maintenance and aesthetics. The two mostly used design types for ponds are reverse sloped outlet pipe and perforated riser outlet pipe.
Table: 4.3 Summary of Design Guidance (STORMWATER MANAGEMENT POND and manual)
Table: 4.3 Summary of Design Guidance (STORMWATER MANAGEMENT POND and manual)
- Sediment Forebay: It is designed to facilitate maintenace and to trap larger particles near the inlet of the pond (improves pollutant removal). The sizing of the sediement forebay is based on inlet configuration and the depth should be atleast 1 m to prevent re-suspension. The adequate sizing of sediment forebay depends upon various calculation such as settling calculation, dispersion length, and clearout frequency.
- CHAPTER: 5 MODELING OF SUSPEDNED SOLIDS IN PONDS
There are varieties of water quality models available to investigate the loading of of pollutants in stormwater runoff. These mathematical models asses the pollution sources and provide adequate water quality solutions (Hamind & Tsihrintzis, 1997).
The models which are widely used in USA and Canada for pollutant modeling are listed below:
- SWMM (Stormwater Management Model 5.1.012)
SWMM is a dynamic hydrology-hydraulic water quality simulation model, it can be used for single event or long-term contious simulations. The simulations are based on runoff quality and quality coming from various land uses of urban areas. SWMM is capable of calculating the prodcution of pollutant loads present in stormwater runoff.
SWMM models up to ten pollutants ( BOD, COD, total nitrogen, total phosphorus, suspended solids, settelable solids, oil/grease and many other user specified pollutant). It defines collection of subcatchments as pervious and impervious areas. For impervious areas, different types of buildup models are available: linear, power, exponential and Michaelis-Menton (Obropta & Kardos, 2007).
Input of buildup and wahsoff functions are required to determine the quality of stormwater runoff from each land use category for each subcatchment, after simulations the output are observed in the form of pollutographs. It is designed to perform frequency anaylsis for any time series and evaluate BMP’s and associated control stratergies.
However, there are some limitations in model performance related to uncertainity of water quality simulations as it lacks to understand true physical, chemical and biological processes that occur in nature.
SWMM model was not specifically designed to estimate the pollutant removal efficiency of a wet detention pond but could also be used for that purpose if adequate data are available (Youn & Pandit, 2012).
- PCSSWMM is widely used for sizing the detention facilities and to achieve the two main objectives of stormwater managaement i.e. flood contorl and water quality control. The model has the capability to simulate the long-tem behavior of watershed and detention systems and it is used to predict the suspended solids removal .
Various functions of PCSSWMM are listed below:
- It instantly determine the required storage volume with thee help of storage optimizer tool to counteract the peak flows.
- It account for all the LID (low impact development) practices
- It helps to model the pond according to various activites such as evaporation, infiltration, exfiltration etc.
- Verify polliutant removal and also seasonal behavior through continous simulations.
- System for Urban Stormwater Treatment and Analysis Integration (SUSTAIN)
This model is developed by the United States Environmental Protection Agency (USEPA) to evaluate the perfromace of different LID practices. The simulation results are used to rank the areas for nonpoint sources pollution and helps to provide adequate LID practices. There are fourteen different types of LID practices (wetlands, wet ponds, dry ponds, biodetention cells, rain barrels, sand filters and infiltration basins etc.) available to select and are classifiedon the basis of point, linear and area (Chen, Sheng, & Chang, 2014).
The SUSTAIN model consists three major elements:
- Framework manager
- Five simulation (siting tool, a land module, a LID module, a conveyance module and optimization module)
The land and conveyance module are applied to simulate water quantity and quality parameters. The climatic data are available from the GIS data of that region.
The input parameters such as runoff and pollutants prodcued from land simulation modules are applied to LID facilities, and all the other environmental processes such as infiltration, evapotranspiration and pollutant removal are then simulated to have the outputs in the form of runoff control and poluution reduction from the individual LID designs.
The optimization module helps to select the most productive and cost-effective LID design and location (Nix, Heaney, & Huber , 1998).
- WinSLAMM (The Source Loading and Management Model)
It was developed in the mid-1970’s by United State Environmental Protection Agency (USEPA) as part of their of their receiving water projects. The aim of the model is to recognize sources of urban stormwater pollutants and to evaluate the efficiency of control practices. The model was designed for management options such as street cleaning, infiltration swales, grass swales and wet detention ponds.
Over the years, WinSLAMM has been extensively developed to extend wide range of capabilities. Some of the following are mentioned below:
- The model can evaluate a long-series of rainfall events
- The model is based on actual field data.
- Uncertainities of modeling parameters ate represented by Monte-Carlo simulations
- Cost of control practices can be caluclated depending upon the model run (batch processor analyzes the runoff volume and pollutants and comine unit cost data to identiy cost effective control options)
- The model can be interfaced with other models for more detailed receiving water evaluations (with SWMM )
- It also provide modeling for municipal pollutant loadings and discharge reductions
- Evaluate the annual/seasonal pollutant loads and identify sources of pollutants for different rain conditions for a specific envinronment.
- WinDETPOND (v 8.4.2) was developed to perform contionus simulations of stomwater wet detention ponds. WinWETPOND was verfified using the detention pond data collected from the Monroe detention pond monitored by WDNR (Wisconsin Dept. of Natural Resources) and USGS (United States Geological Survey) .
Some important input parameters to conduct wet detention pond modeling are listed below;
- Inflow hydrograph shape factors/ actual hydrographs
- Intital stage conditions
- Particle size distribution
- Stage-Area information
- Outlet options ( including hydraulic outlets, seepage, evaporation, pumped outlet etc.)
There modeling with WinSLAMM has some limitations as it unstaisfactory to predict peak flows and flood analyses and doesn’t include constrcutuon erosion and snowmelt in evaluations (Water Quality Assessment and Data Results , 2009).
- CHAPTER: 6 CONCLUSIONS
The purpose of this study was to develop an understanding towards sediment removal mechanism in stormwater management ponds. Stormwater management ponds provide the significant improvement in the water quality and flood control. In order to achieve long-term efficiency of this facility, it is necessary to understand various parameters influencing the performance of these ponds. The report discussed these factors such as runoff from different land uses, suspended solids particle sizes and their respective settling velocities which are essential to determine pond performance.
The main emphasis is paid towards the understanding of removal mechanism of sediment in the stormwater pond which is dependent on the hydrology, hydraulics and settling in the pond.
The hydraulics focus on the hydraulic residence time for the runoff in relation to sediment trap efficiency. It has been evident that longer residence time corresponds to increased sediment trap efficiencies. This influencing factor is used during the design of ponds to increase the residence time by elongating the pond and introducing baffles. The sediment trap efficiencies are understood with the help of different empirical and theoretical models developed overtime.
Stormwater management ponds have become one of the trending stormwater management practice over past 10 years therefore, the report has reviewed various pond design parameters for different countries like USA and Canada.
The last section contributes towards the application of the variety of USEPA model available for water quality modeling. These models are continuously applied in practice to evaluate annual sediment removal efficiencies in stormwater ponds.
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