Assessing total suspended sediment and total phosphorus removal efficiency of permittee owned/operated constructed stormwater ponds

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This page provides guidance related to assessing the total suspended sediment (TSS) and total phosphorus (TP) removal efficiency of permittee owned/operated ponds constructed and used for the collection and treatment of stormwater. Four (4) evaluation strategies are discussed.

• Evaluation of Minnesota Pollution Control Agency (MPCA) stormwater pond design criteria
• Stormwater pond inspection/assessment
• Stormwater pond pollutant removal modeling
• Stormwater pond water quality monitoring

The TSS and TP removal efficiency of constructed stormwater ponds degrades over time due to the loss of storage volume to sedimentation and/or sediment phosphorus release. For this reason, it is critical that stormwater ponds be sized correctly for their contributing drainage area and that pond inspection and assessments be performed routinely to monitor sedimentation and identify potential maintenance needs. In addition to evaluating pollutant removal efficiency through comparison to design standards and evaluation of sedimentation, the water quality performance of stormwater ponds can be evaluated using various water quality modeling programs or measured directly through water quality monitoring.

Guidance presented will assist MS4s (Municipal Separate Storm Sewer System) evaluate the TSS and TP treatment effectiveness of ponds post-construction and over their design life. The adjacent table provides a summary of the four (4) TSS and TP removal efficiency evaluation strategies discussed within this memorandum.

TSS and TP Removal Efficiency Evaluation Strategies
Link to this table

Pollutant Removal Assessment Strategy	Description	Relative effort	Relative accuracy
Evaluation of MPCA stormwater pond design criteria	Evaluation of pond sizing criteria against MPCA stormwater pond design standards to produce a relative evaluation of pond performance	Low	Low
Stormwater pond inspection/assessment	Guidance related to scheduling and performing routing visual inspections and less-frequent assessments of pond sedimentation depth	Medium/high	NA1
Stormwater pond pollutant removal modeling	Evaluation of the pollutant reduction achieved by stormwater ponds through the use of empirically-based or physically-based water quality models	Low/medium	Medium
Stormwater pond water quality monitoring	Evaluation of the pollutant reduction achieved by stormwater ponds through direct monitoring of pollutant concentrations into and leaving the pond	High	High

¹Stormwater pond inspection/assessment does not inherently provide an estimate of TSS/TP removal. However, inspection/assessment efforts are critical to ensuring a stormwater pond is performing as originally designed.

Evaluation of MPCA Stormwater Pond Design Criteria

The MPCA Minnesota Stormwater Manual contains detailed design criteria for many water quality best management practices (BMPs), including constructed stormwater ponds. In addition to outlining construction stormwater pond requirements stipulated by the MPCA Construction General Permit (CGP), the Minnesota Stormwater Manual’s Design Criteria for Stormwater Ponds contains guidance and recommendations related to many aspects of stormwater pond design and construction, from grading and site layout, to overflow spillway design and development of a landscaping plan. Although guidance within the Design Criteria for Stormwater Ponds is primarily focused on requirements related to construction of design of stormwater ponds for new development, elements within the guidance related to sizing of the pond permanent pool volume and live storage water quality volume can be used to evaluate (a) the impact of sedimentation over time and (b) the impact of development and changing land use over time on the water quality performance of existing stormwater ponds. The following subsections outline how design criteria can be used to evaluate the water quality treatment efficiency of existing stormwater ponds and how design criteria can be used to estimate pollutant load reduction.

Estimating Water Quality Performance of Existing Stormwater Ponds

Constructed ponds and constructed wetlands are examples of sedimentation practices. (Source: CDM Smith).

As discussed in Section 2.0, the Minnesota Stormwater Manual’s Design Criteria for Stormwater Ponds contains guidance and requirements related to the sizing of pond permanent pool volume (Vpp) and live storage water quality volume (Vwq). As defined by the Minnesota Stormwater Manual, the permanent pool (aka, “dead storage”) is the volume of water below the pond outlet, and the water quality volume (aka, “live storage”) is the storage volume between the pond outlet and the pond overflow elevation as shown in Figure 1.

The Minnesota Stormwater Manual’s Design Criteria for Stormwater Ponds outlines minimum requirements for permanent pool volume (Vpp) and water quality volume (Vwq) as outlined by the CGP. Narrative descriptions and resulting equations used to evaluate minimum volume required are outlined below.

The Required minimum permanent pool volume, or dead storage (Vpp), below the outlet elevation), is 1800 cubic feet of storage below the outlet pipe for each acre that drains to the pond

\(V_{pp} = 1800A\)

where

A = the drainage area of the stormwater pond (acres)

or

\(V_{pp} = 0.0417 A \)

where

A = the drainage area of the stormwater pond (square feet)

The equations and definitions, above, were created for designing and constructing a stormwater pond to treat runoff from new development. Existing stormwater ponds may have larger Vpp than the minimum required by the CGP, or may have larger or smaller Vwq than required. To estimate the water quality performance of existing stormwater ponds, methodology outlined in the Minnesota Stormwater Manual’s MIDS Calculator documentation for stormwater ponds requires the user to evaluate the tributary area to the pond and volume dimension of the pond to determine the “design level” (e.g., Design Level 2) of the pond, and recommends assumed pollutant removal efficiency values based on the design level (e.g., 84% TSS removal for Design Level 2). Criteria for each MIDS stormwater pond design level are summarized in Table 2.

MIDS Calculator stormwater pond design level criteria related to pond volume
Link to this table

MIDS Stormwater Pond Design Level1	Permanent Pool Volume (Vpp), ft3	Water Quality Volume (Vwq), ft3	Pollutant reduction (%)2
			TSS	TP	PP	DP
Design Level 1	≥ 1,800 ft3 per acre of tributary area	<= 1 inch from impervious area	60	34	62	0
Design Level 2		<= 1 inch from impervious area	84	50	84	8
Design Level 3		<= 1.5 inch from impervious area	90	60	90	23

¹From MIDS Calculator documentation for stormwater ponds. Note: the table summarizes design-level criteria related to permanent pool volume and water quality volume. The complete list of criteria for each design level is summarized on the MIDS calculator website linked above.
² TSS = total suspended solids; TP = total phosphorus; PP = particulate phosphorus; and DP = dissolved phosphorus. Pollutant reduction values cited assume no upstream treatment within tributary area to pond (i.e., untreated urban runoff).

Steps for summarizing the estimating water quality performance of existing stormwater ponds using methodology outlined in the Minnesota Stormwater Manual’s Design Criteria for Stormwater Ponds and MIDS Calculator documentation for stormwater ponds are outlined, below.

1) Determine the permanent pool volume (Vpp) of the pond – the VPP can be determined through a number of sources, including record drawings, as-builts, and bathymetric survey. Note: before using record drawing or as-built data, a pond assessment (Section 3.0) should be conducted to determine the extent to which sedimentation has reduced the Vpp. If estimating volume from bathymetric contour data, the following equation can be used to calculate volume between any two bathymetric contours. The total bathymetric volume can then be calculated by summing the volume between all available bathymetric contours

\( 𝑽_{𝟏−𝟐} = (𝑨_𝟏 + 𝑨_𝟐)/2 × (𝑬_𝟐 − 𝑬_𝟏) \)

Where,

V_1-2 = the volume between contours 1 and 2;

A₁ and A₂ = the area of contours 1 and 2, respectively; and

E₁ and E₂ = the elevation of contours 1 and 2, respectively

After calculating the volume between each bathymetric contour, the total bathymetric volume can be calculated by summing the volume calculated between each set of contours

\( ∑ 𝑽_𝒊 = (𝑨_{𝒏+𝟏} + 𝑨_n)/𝟐 × (𝑬_{𝒏+𝟏} − 𝑬_𝒏) \)

If only the area at the bottom of the pond (A_pp) and the area at the permanent pool of the pond (A_bot) is known, the bathymetric volume can be calculated using the simplified equation

\( ∑ 𝑽_{bathymetric} = (𝑨_{pp} + 𝑨_{bot})/𝟐 × (𝑬_{pp} − 𝑬_{bot}) \)

Where

V_bathymetric = bathymetric volume;

A_pp = area at the permanent pool of the pond;

A_bot = area at the bottom of the pond;

E_pp = elevation at the bottom of the pond; and

E_bot = elevation of the bottom of the pond.

2) Determine the water quality volume (Vwq) of the pond – as shown in Figure 1, the Vwq is the volume between the ponds permanent pool and the natural or designed overflow elevation. The Vwq can be determined through a number of sources, including record drawings, as-builts, survey data, and surface LiDAR data. A rough estimate of Vwq can be calculated by determining the permanent pool area and the area at the natural or designed overflow elevations. Equation 3, above, can then be used using these two elevations and areas.

3) Evaluate the Vpp of the pond – determine the CGP required Vpp based on the total drainage area to the stormwater pond using Equation 1, above (i.e., 1,800 ft3 per acre of drainage area). If the Vpp is greater than 1,800 ft3, proceed to step 4. If the Vpp of the pond is less than 1,800 ft3 per acre of drainage area, guidance within the Minnesota Stormwater Manual suggests that the pond should not be included in site pollutant removal calculations, as the pond is unlikely to provide adequate treatment. To estimate the water quality performance of a stormwater pond not meeting minimum Vpp requirements, calculations in the following steps can proceed by using only the area for which the Vpp is sized to adequately treat (i.e., Vpp ÷ 1,800 ft3 /acre = treated area (acres)). The remaining portion of the total drainage area to the pond would then be assumed to bypass (i.e., 0% treatment). Alternatively, water quality performance of undersized stormwater ponds can be evaluated through modeling (Section 4.0) or monitoring (Section 5.0).

4) Evaluate the tributary impervious area to the pond – for small sites (e.g., developments less than two acres, etc.), impervious area can be determined through manual evaluation of site impervious cover from record drawings or site plans. For larger drainage stormwater ponds with larger drainage areas (e.g. regional stormwater ponds with drainage areas greater than five acres), land use datasets can be used to estimate total impervious area within the ponds drainage area. The Minnesota Geospatial Information Office (MnGeo) maintains a database of current and historic land use which can be used to evaluate land use and estimate impervious area. Additionally, the University of Minnesota (UMN) provides land cover and impervious data at varying resolution statewide and for specific regions throughout Minnesota (e.g. Twin Cities Metro, Duluth, Rochester, etc.).

5) Determine the impervious area treatment depth in the pond Vwq – using the pond Vwq (Step 2) tributary impervious area (step 4), calculate the impervious area treatment depth using Equation 4, below. Note: Equation 4 is the same as Equation 2 but rearranged to calculate the impervious area treatment depth provided by the pond Vwq.

\( 𝑫_𝒊 = 𝑽_𝒘 𝑨_𝒊 × 𝟏_{𝒊𝒇} \)

Where

Dimp = impervious area treatment depth (inch);

Vwq = water quality volume in cubic feet (ft3); and

Aimp = tributary impervious area (ft2).

6) Determine the MIDS pond design level and corresponding pollutant reduction (%) – after confirming the Vpp is greater than 1,800 ft3 per tributary acre (Step 3) and determining the impervious area treatment depth in the Vwq (Step 5), reference Table 2 to determine the MIDS pond design level (e.g., Design Level 2) and corresponding pollutant reduction (e.g. 84% TSS reduction). Note: pollutant reduction values (%) included in Table 2 assume no upstream water quality BMPs in the tributary area to the stormwater pond (i.e., untreated stormwater runoff). If BMPs within the watershed to the stormwater pond provide significant treatment (e.g., 50% of the tributary area passes through a large infiltration basin before discharging to the stormwater pond), water quality performance should instead be evaluated through modeling (Section 4.0) or monitoring (Section 5.0).

7) Determine influent pollutant loading and pollutant load reduction (lbs) – after determining the pond level design pollutant removal efficiency (%) from Table 2, annual pollutant mass removal (e.g., pounds to TSS removal per year) can be determined by applying the pollutant removal efficiency (%) to the annual influent pollutant mass load. Methodology for determining the annual influent pollutant mass load to the stormwater pond and calculating the pollutant mass removal within the stormwater pond is discussed in Section 2.2.

Estimating Annual Pollutant Load Reduction Existing Stormwater Ponds

To estimate the pollutant mass reduction (e.g., pounds of TSS removal per year) in an existing stormwater pond, it is first critical to determine the annual pollutant mass load from the tributary watershed to the stormwater pond. One method of estimating annual pollutant export associated with runoff from a watershed is the Simple Method (Schueler, 1987; CWP & CSN, 2008). The Simple Method is utilized by many annualized water quality models (e.g., the MPCA Simple Estimator spreadsheet model, see Section 4.0) and is a recommended method for calculating credits for stormwater ponds in the Minnesota Stormwater Manual. The Simple Method equation is shown below (Equation 5), followed by steps for determining Simple Method parameter inputs, calculating annual pollutant loading, and calculating annual pollutant reduction

\( 𝑳_{annual} = 𝟎.𝟐 × 𝑨 × 𝑷 × 𝑷_𝒋 × 𝑹_𝒗 × 𝑬MC_{𝒑ollutant} \)

Where

L_annual = annual pollutant load to the stormwater pond (e.g., pounds of TSS per year, lbs TSS/yr);

A = drainage area to stormwater pond (acres);

P = annual precipitation depth (in);

P_j = fraction of rainfall events that produce runoff (default value of 0.9);

R_v = runoff coefficient (see discussion in Step 1, below);

EMC_pollutant = the flow-weighted event mean concentration (EMC) of pollutant in runoff (mg/L, see discussion in Step 1, below); and

0.227 = unit conversion factor.

1) Determine Simple Method input parameters – the following defines each Simple Method input parameter and provides a summary of how to determine or estimate each parameter:

• Drainage Area (A) – the total drainage area to the pond (acres).
• Annual Precipitation (P) – annual average precipitation depth (inches). Can be determined from local long-term rainfall records (e.g., 10-year average precipitation from local airport).

Note: average annual precipitation depth within the state of Minnesota by zip code can be determined using the MIDS Calculator.

• Rainfall Fraction (P_j) – fraction of rainfall events which produce runoff (unitless). This Simple Method assumes some fraction of annual rainfall is delivered in small, low-intensity rainfall events that do not produce runoff. Typically, a PJ value of 0.9 is assumed.
• Runoff Coefficient (R_v) – the runoff coefficient is the fraction of annual rainfall that is converted into runoff. Runoff coefficient can be calculated as a function of site impervious area using the equation, below. Note: a description of how to determine site impervious area

and impervious fraction is provided in Section 2.1, Step 4. Alternatively, the area-weighted watershed Rv value can be calculated using the land use-based Rv values from the MPCA Simple Estimator shown in Table 3.

\( 𝑹_𝒗 = 𝟎.𝟎5 + 𝟎𝟎9 × 𝑰 \)

Where,

I = impervious area percentage (i.e., if 75% impervious, I = 75).

• Pollutant Concentration (EMC_pollutant) – the flow-weighted average pollutant EMC (mg/L). Because localized monitoring of runoff pollutant EMCs is typically not available, standard literature values for pollutant EMC can be used to estimate pollutant loading. The MIDS Calculator suggests typical urban runoff EMC values of 54.5 mg/L and 0.3 mg /L for TSS and TP, respectively. Land used based EMC values from the MPCA Simple Estimator can be used to calculate a land use-based area weighted TSS and TP EMC based on land use within the drainage area to the stormwater pond. These values are based on an extensive literature review.

MPCA Simple Estimator: R_v, TSS EMC, and TP EMC Values for Land Use Types
Link to this table

Land use	Runoff coefficient (Rv)1	Event mean concentration (EMC)(mg/L)
		Total phosphorus (TP)	Total suspended solids (TSS)
Commercial	0.8	0.200	75
Industrial	0.8	0.235	93
Institutional	0.75	0.25	80
Mixed use	0.5	0.290	76
Open space	0.2	0.190	21
Residential	0.4	0.325	73
Transportation	0.8	0.280	87

¹Runoff coefficients vary with soil and slope. Link here

2) Calculate annual pollutant load reduction – after calculating the annual pollutant loading to the stormwater pond (Step 1), the stormwater pond annual pollutant mass load reduction (e.g., pounds of TSS removed per year) can be calculated using the equation, below

\( 𝑹_{annual} = 𝑳_{annual} × 𝑷R_{pollutant} \)

Where, R_annual = annual pollutant load reduction (e.g. pounds of TSS removed per year, lbs TSS/yr); L_annual = annual pollutant load to the stormwater pond (e.g., pounds of TSS per year, lbs TSS/yr); and PR_pollutant = pollutant reduction efficiency of the stormwater pond (%). Note: determination of pollutant reduction efficiency is discussed in Section 2.1.

Limitation of MPCA Stormwater Pond Design Criteria Methodology

The MPCA stormwater pond design criteria described in Section 2.0 is a simplified methodology used to provide an estimate of stormwater pond water quality performance when other, more accurate methods (see methods listed in Table 1) are not feasible. The following list summarizes limitations of the MPCA stormwater pond design criteria methodology:

• Input sensitivity: because the methodology produces an annualized estimate of pollutant reduction, input assumptions can have a significant impact on pollutant reduction calculations. For example, assumed TSS pollutant event mean concentrations from Table 3 could impact TSS influent loading by ± 100%. For this reason, input parameters should be carefully evaluated based on site-specific and best-available information. The methodology is especially sensitive to the following parameters:
  • Directly connected imperious fraction (Section 2.1);
  • Pollutant event mean concentration (Section 2.2); and
  • Water quality and permanent pool volume of the pond (Section 2.1).
• Upstream treatment: this methodology assumes no water quality treatment in the tributary area to the stormwater pond. Because upstream, tributary BMPs have the potential to impact the pollutant loading and pollutant particle scale distribution, this methodology should not be used for stormwater ponds with significant upstream water quality treatment.
• In-pond dynamics: this methodology does not account for in-pond dynamics such as:
  • Internal phosphorus loading (i.e., the release of bound phosphorus from pond sediment);
  • Sediment resuspension (i.e., scour of previously-settled sediment during large inflow events);
  • Inlet/outlet short-circuiting (i.e., inlet flow moving directly to outlet, limiting the flow detention time);
  • Macrophyte growth (i.e., the growth and life cycle of aquatic plants and algae).

Stormwater Pond Inspection and Assessment

As discussed in Section 1.0, the pollutant removal efficiency of constructed stormwater ponds degrades over time due to the loss of storage volume to sedimentation. Additionally, routine maintenance issues (e.g., pond outlet trash rack clogged with debris after storm; sand bar formation at inlet(s); etc.) can significantly reduce the hydraulic and water quality performance of stormwater ponds. For this reason, the municipal separate storm sewer system (MS4) General Permit requires permittees to: a) Perform routine visual inspection of structural BMPs (i.e., inspection), and b) Develop procedures and a schedule to determining the total suspended solids (TSS) and total phosphorus (TP) treatment effectiveness of all municipally owned/operated stormwater ponds (i.e., assessment) (MS4 General Permit Part III.D.6.), including evaluation of sedimentation.

For the purposes of evaluating the pollutant removal efficiency of stormwater ponds, “inspection” is defined as all components of routine visual inspection (Section 3.1). This typically involves walking the pond perimeter, inspecting outlets, and looking for signs of sedimentation and potential maintenance needs (e.g., clogged outlet).

A stormwater pond “assessment” encompasses all activities related to determining the total suspended solids (TSS) and total phosphorus (TP) treatment effectiveness of permittee owned and operated stormwater ponds (Section 3.2). As outlined by the MS4 General Permit, this involves developing procedures to evaluate TSS and TP treatment effectiveness, including development of a schedule for completing assessments of all municipal owned and operated stormwater ponds. Because the pollutant removal efficiency of a stormwater pond can be reduced as permanent pool volume is lost to sedimentation, a pond assessment should include an evaluation of sediment accumulation within the pond. Guidance in Sections 2.0, 4.0, and 5.0 provides methods for estimating, modeling, and directly monitoring the TSS and TP removal efficiency of ponds, respectively, but do not provide guidance on evaluating the permanent pool volume lost to sedimentation. For this reason, the assessment subsection (Section 3.2) provides guidance on how to estimate and directly measure sedimentation volume.

The following subsections provide guidance and recommendations related to the development of inspection and assessment procedures for stormwater ponds.

Inspection

Although the MS4 General Permit requires annual inspection of structural BMPs (Part III.D.E), the permit makes special exception for stormwater ponds, requiring only one (1) inspection of all ponds and outfalls prior to the expiration date of the permit. Due to critical hydraulic, water quality, and flood protection functions of stormwater ponds, it is recommended that inspection plans be developed to ensure that that:

1) Visual inspection of all municipal stormwater ponds and associated inlets and outlets occurs at least once per year; and that 2) Additional visual inspections are performed as needed in response to large storms (e.g., a rainfall event greater than 2 inches).

Visual Inspection SOP and Checklist

Developing a visual inspection standard operating procedure (SOP) for stormwater ponds is critical for ensuring that visual inspections are carried out in a standardized and repeatable fashion. Standardization allows results of inspections from different ponds to be compared to assess relative priority of inspection and maintenance needs, and repeatability allows results of inspections of the same pond to be tracked year to year to evaluate how condition of the stormwater pond has changed. In addition to allowing for evaluation of inspection prioritization (discussed further in Section 3.1.2), an inspection SOP checklist also increases the efficiency and effectiveness of inspectors / municipal operators while performing routing visual inspections.

Although there are many examples for visual inspection SOP checklists which can be used as templates for designing a stormwater pond inspection SOP (see example in Appendix A), it is recommended that an individual use these documents as templates and revise as needed based on conditions within the MS4 (e.g., number of stormwater ponds managed, staff availability and skills, etc.). The following list outlines specific recommendations that should be included or considered in the development of a visual inspection SOP for stormwater ponds:

• Electronic documentation: tracking inspection electronically, rather than relying on paper files, allows for more efficient analysis / tracking of pond inspections. If tracked with paper files in field, include instructions to scan and enter notes electronically within one (1) day of completing inspection. If possible, consider tracking inspection notes electronically in field using a laptop or tablet.
• Quantitative metrics: whenever possible, include quantitative (i.e., numerical) metrics, as being quantitative allows for tracking of maintenance needs over time and relative comparison of maintenance needs between ponds. For example, if including a checklist item for outlet clogging consider using a numerical scale (e.g., “Is the outlet clogged? Rate 1-5, where 1 indicates 0% clogged, and 5 indicates ≥ 90% clogged).
• Photo documentation: include photos of site in visual inspection protocol. As needed, be instructive regarding photos (e.g., “include photograph of inlet #1, inlet #2, pond outlet structure, and emergency overflow berm”). Photos can be useful in tracking evolving conditions over time (e.g. formation of sand bar near pond inlet).
• Immediate action protocols: include protocols / instructions for addressing maintenance needs requiring immediate action (e.g., blocked/obstructed inlet, pipe failure, etc.).
• Infrastructure inventory: include record drawing (e.g., as-built, aerial imagery with locations circled, sketch, etc.) for each pond indicating where critical infrastructure is located (e.g., inlets, outlets).
• Field staking / marking / GPS coordinates: include instructions related to field marking e.g., place stake with orange ribbon to indicate inlet, orange spray paint to indicate structure damage such as joint operation, etc.) and/or recording the GPS coordinates of critical structures. This can greatly increase efficiency of maintenance and future site inspections.
• Visual inspection of sedimentation: include instructions to evaluate visual signs of sedimentation (e.g., formation of sand bars in pond, bank and channel erosion, bank failure, outlet silted in / buried, etc.). Direct assessment / measurement of pond sedimentation is typically not conducted during routine visual assessment, but signs of sedimentation/changes in sedimentation observed during visual inspection can indicate need to perform an assessment of pond sedimentation (Section 3.2).
• Visual inspection for short-circuiting: include instructions to evaluate proximity of inlets to outlets. If inlets are located near to outlets, flow into the pond can “short-circuit” directly to the outlet, allowing for little residence time and sedimentation of influent particles, greatly reducing pollutant removal efficiency from the affected inlet(s). If short-circuiting is occurring, inlets may be realigned or baffles may be installed to prevent bypass of pollutants.

An example of a stormwater pond visual inspection SOP checklist from the EPA’s Stormwater Wet Pond and Wetland Management Guidebook (USEPA, 2019) is included in Appendix A. In addition to providing a detailed inspection checklist, the guidebook outlines detailed recommendations related to recommended frequency of pond inspection and maintenance. Adapted from the guidebook (USEPA, 2019), Table 5 and Table 6 outline the inspection operator skill level required and recommended frequency for various inspection tasks. Note: some inspection tasks outlined in Table 6 are in excess of the once annual visual inspection recommendation outlined in this memorandum, and are included to provide context and frequency recommendations for a wide range of inspection actions which should be considered based on factors unique to the MS4 (e.g., number of ponds managed, staff availability and qualifications, etc.) when developing a stormwater pond inspection SOP.

In addition to establishing a standardized, repeatable methodology for performing routing visual inspections, it is critical to inventory and rank the relative inspection priority of stormwater ponds to ensure ponds with higher likelihood of requiring maintenance are inspected with higher frequency. Development of an inspection prioritization system is discussed further in Section 3.1.2.

Inspection skill level descriptions (Source (adapted): USEPA Stormwater Wet Pond and Wetland Management Guidebook (USEPA, 2019).)
Link to this table

Inspection skill level	Definition
0 (low)	No special skills or prior experience required, but some basic training via manual, video, or other materials is necessary.
1	Inspector, maintenance crew member or citizen with prior experience with ponds and wetlands
2	Inspector or contractor with extensive experience with pond and wetland maintenance issues
3 (high)	Professional engineering consultant required.

Inspection action recommendations (Source (adapted): USEPA Stormwater Wet Pond and Wetland Management Guidebook (USEPA, 2019).
Link to this table

Frequency Recommendation	Inspection items
Monthly to Quarterly or After Major Storms (>1”)	Inspect low flow orifices and other pipes for clogging Check the permanent pool or dry pond area for floating debris, undesirable vegetation Investigate the shoreline for erosion Look for broken signs, locks, and other dangerous items
Several Times per Hot/Warm Season	Inspect stormwater ponds for possible mosquito production
Semi-annual to annual	Identify invasive plants Ensure mechanical components are functional
Every 1 to 3 years	Complete all routine inspection items above Inspect riser, barrel, and embankment for damage Inspect all pipes Monitor sediment deposition in facility and forebay
2 to 7 years	Monitor sediment deposition in facility and forebay
5 to 25 years	Remote television inspection of reverse slope pipes, underdrains, and other hard to access piping

Inspection Prioritization

Due to the need for routine inspection, if an inventory of all stormwater ponds is available (as required by MS4 General Permit Part III.C.2), it is recommended that MS4s develop an inspection prioritization list for all municipal stormwater ponds. The purpose of an inspection prioritization list is to help ensure that ponds likely to have maintenance needs are inspected annually, and to help identify ponds with lower maintenance needs which may be inspected less frequently (e.g., once every two years). Note: this recommendation is targeted at MS4s responsible for inspection of many stormwater ponds and wetlands, where annual inspection may not be feasible for all ponds. For MS4s with a small number of stormwater ponds or staff availability and resources to perform annual inspection on all stormwater ponds, annual inspection is recommended and inspection prioritization may not be necessary.

After establishment of an, inspection program using a standardized stormwater pond inspection SOP checklist (see Section 3.1.1), inspection prioritization can be ranked using results of inspection SOP worksheets, including quantitative metrics used to rank maintenance needs. An example of ranking categories and associated inspection frequency is shown in Table 7. Note: ranking categories and the inspection frequency assigned to each can be highly dependent on conditions unique to the MS4 (e.g., number of stormwater ponds managed, staff availability, etc.). For this reason, the categories and recommendations provided in Table 7 are meant to serve only as an example of one method of inspection prioritization.

Example of inspection prioritization categories
Link to this table

Prioritization category	Inspection frequency goal
1 (high priority)	Perform visual inspection of 100% of rank 1 ponds annually.
2	Perform visual inspection of 50% of rank 2 ponds annually.
3 (low priority)	Perform visual inspection of 25% of rank 3 ponds annually.

Prior to the establishment of a routine visual inspection program (Section 3.1.1), other metrics related to the potential pollutant loading and hydraulic function of MS4 stormwater ponds can be used to create an inspection prioritization list. Prior to establishing a database of visual inspection metrics, it is recommended that any or all of the following metrics be used to create a ranked prioritization list, as available:

• Institutional knowledge (e.g. municipal operator experience, resident complaints, etc.): it is recommended that ponds with known maintenance issues (e.g., high water levels, sedimentation issues, etc.) be assign high inspection priority.
• Drainage area: it is recommended that ponds with larger drainage areas be assigned higher priority than those with smaller drainage areas. Note: drainage areas for small ponds may be determined from development plans, while determining drainage areas for larger, regional ponds

may require delineation using available stormsewer infrastructure data and topography.

• Pond surface area: if pond drainage areas are not known, it is recommended that ponds with larger surface area be assigned high priority than those with smaller surface area. Prioritization strategies and ranking methodology will be highly dependent on (a) what data is available, and (b) conditions unique to the MS4 (e.g., number of ponds managed, institutional knowledge of municipal operators, etc.). An example of an inspection prioritization methodology developed by the City of Oakdale is available on the Minnesota Stormwater Manual’s stormwater pond assessment page.

Assessment

As outlined in Section 3.0, stormwater pond “assessment” encompasses all activities related to determining the total suspended solids (TSS) and total phosphorus (TP) treatment effectiveness of permittee owned and operated stormwater ponds. Additionally, the MS4 General Permit, Part III.D.6. Requires MS4s to develop a schedule based on measurable goals and priorities established by the permittee for completing assessments of all MS4 stormwater ponds. Because the pollutant removal performance of a stormwater pond can be greatly reduced as permanent pool volume is lost to sedimentation, an assessment plan must include an evaluation of the sedimentation volume within the stormwater pond. Because methods of estimating, modeling, and directly monitoring the TSS and TP removal efficiency of ponds are outlined in Sections 2.0, 4.0, and 5.0, respectively, this subsection focuses on guidance related to evaluating pond sedimentation volume (Section 3.2.1) and developing a pond assessment plan and schedule (Section 3.2.2).

Evaluating Pond Sedimentation Volume

Determining the sedimentation volume within a stormwater pond requires the following: a)Determining the original bathymetric volume (design volume, if constructed) of the stormwater pond; and b)Determining the existing bathymetric volume of the stormwater pond.

The difference between (a) and (b) is the permanent pool volume lost to sedimentation (i.e., sedimentation volume). Determining the original bathymetric volume (e.g., constructed bathymetric volume, design bathymetric volume) typically requires gathering available background information. If the pond is a constructed feature, bathymetric volume should be determined from best-available record drawings or best available information (e.g., as-built drawings, design drawings, design calculations, etc.). If the pond is not a constructed feature or if design records are not available or were not maintained, original bathymetric volume may be estimated by determining the sedimentation depth using the survey rod transition or sediment core methods, described below.

Determining the existing bathymetric volume and sedimentation volume can be determined through the methods described below. Methodology is organized from most accurate and most labor intensive to lease accurate and least labor intensive:

• Bathymetric volume: a bathymetric survey can be used to obtain a direct measurement of the existing bathymetric volume. Several methods of surveying bathymetric volume are described and compared, below. Additionally, Table 8 provides a summary of the relative accuracy and relative cost of each method:
  • Grid Survey – Relative Depth: If water level is at a known elevation or there is a relative survey benchmark in the area, bathymetric survey can be performed by determining depth to pond bottom at points throughout the pond (relative depth, i.e., 2.3 feet deep). It is recommended that X,Y grid spacing be established to create a representative depth surface. Once digitized, the existing bathymetric surface can be compared to the design or original bathymetric volume to determine the sedimentation volume. Note: if comparing a bathymetric survey to design or record drawings, make sure the same benchmark reference is being used in both dataset (e.g., the outlet elevation, benchmark in area, etc.) or adjust the volume calculations accordingly to obtain an accurate calculation of sedimentation volume.
  • Grid Survey – Total Station (TS), Real Time Kinematic (RTK) survey: similar to the “relative depth” method described above, but rather than using relative depth measurements to water surface or a known benchmark, uses a TS or RTK station to measure bathymetric elevations. By establishing an X,Y grid or shooting many elevations at representative points within the pond, a bathymetric elevation model can be created and used to calculate an accurate estimate of bathymetric volume.
  • Continuous Survey: Sonar: there are many “fish finder” sonar depth measurement devices available on the market today capable of recording continues depth measurements from a fixed position (e.g., a boat, kayak, etc.). Collected sonar data can be sent directly to cloud-based data processing services (e.g., C-MAP) which generate digital bathymetric elevation models form the collected data. Processing services have various pricing models, with some charging a fixed price per data set (i.e., a fixed price per pound). Although this method has the advantage of producing a continuous record of bathymetric depth and elevation, disadvantages include:
    • Horizontal GPS accuracy: the horizontal accuracy of various “fish finder” devices may not be sufficient for every application and may need to be supplemented with a more accurate GPS device.
    • Depths less than 2-feet & excessive vegetation: many sonar technologies are incapable of accurately measuring depths less than two feet (based on limitations related to the speed of sound through water). Additionally, many sonar technologies will not produce accurate depth measurements through dense vegetation.

Comparison of Bathymetric Survey Methods
Link to this table

Pollutant Removal Assessment Strategy	Description	Relative accuracy	Relative cost
Grid Survey - Relative Depth	Measuring relative depths along an X,Y grid of points or at monitored GPS locations. This method relies on determining the water surface elevation on the day of survey, either through a known benchmark or known pond outlet elevation.	Low	Low
Grid Survey - Total Station (TS), Real Time Kinematic (RTK) Survey	Similar to "relative depth" method, but utilizes TS or RTK survey to measure pond depths directly	Medium/high1	Medium/high1
Continuous survey - sonar	Continuous monitoring of pond depth using sonar. Many "fish finder" style sonar devices can be used for this application. Collected data can be sent to cloud processing companies to develop bathymetric volumes directly form collected data	Medium/high2	High

¹Accuracy dependent on number of points collected, and cost dependent on if MS4 owns and has trained staff to operate TS/RTK survey equipment.
²Accuracy dependent on pond depth and vegetation (lower accuracy if less than 2 feet deep and/or highly vegetated

Sedimentation volume: sediment core: collection of sediment core(s) at several locations can be used to determine depth of accumulated sediment. Review of soil composition throughout the profile of the sediment core can help determine the transition from accumulated material (e.g., plant biomass, coarse sediment and sand, etc.) to native soil texture (e.g. fine grain soil texture such as silt and clay). If multiple cores are collected, a sediment depth surface can be created to calculate sedimentation volume. Alternatively, the average sediment depth can be assumed across the entire pond bathymetric surface, although this method will produce less-accurate results as sediment accumulation is typically concentrated at pond inlet locations.

• Sedimentation volume: survey rod transition: the simplest method of estimating depth of accumulated sediment is to manually push the survey rod point into pond sediment and to feel for a transition from soft, accumulated sediment to harder native material (i.e., the design pond bottom). Because this method is reliant on the accuracy of the surveyor to record the transition point and is inherently subjective, it is the least accurate method of estimating sedimentation volume. However, recording the depth at the top of accumulated sediment to the top of native material (the transition point) can provide an estimate of accumulated sediment depth which may be sufficient for determining when pond sediment management is required. If depth is estimated at many points throughout the pond, a sediment depth surface can be created to calculate sedimentation volume. Alternatively, the average sediment depth can be assumed across the entire pond bathymetric surface.

Note: the existing bathymetric volume, not the design volume, should be used as bathymetric volume referenced in Sections 2.0 and 4.0 for estimating the current, existing conditions pollutant removal efficiency of stormwater ponds. Although recommendations related to sediment management (i.e., dredging) are not included in this memorandum, the MCPA Stormwater Manual suggests that sediment management should occur every 25 years or once fifty percent (50%) of the design permanent pool volume has been lost to sedimentation.

Developing a Pond Assessment Plan and Schedule

The MS4 general permit (Part III.D.6.d) requires permittees to (a) assess the TSS and TP treatment effectiveness of all permittee owned/operated stormwater ponds and (b) develop a schedule (which may exceed the permit limit) based on measurable goals and priorities established by the permittee for completing assessments of all MS4 stormwater ponds. As discussed in Section 3.2.1, the TP and TSS removal effectiveness of stormwater ponds can be estimated or evaluated, or directly measured using methodology outlined in Sections 2.0, 4.0, and 5.0, respectively. A stormwater pond assessment plan and schedule can take many forms, as the schedule is not specified in the general permit and the measureable goals and priorities are established by the permittee. The Minnesota Stormwater Manual Stormwater Pond Assessment page contains examples of assessment plans and schedules developed by MS4s and approved by the MPCA. The City of West St. Paul’s Assessment Plan (City of West St. Paul, 2016) is included in Appendix B to provide an example of how an assessment plan and schedule can be structured.

Case Study: RWMWD inspection and assessment SOP

The Ramsey-Washington Metro Watershed District (RWMWD) has developed a stormwater pond inspection and assessment SOP document for its member municipalities and MS4s to insure stormwater pond inspection, assessment, and maintenance procedures are conducted using a standardized methodology, and to insure that ponds are inspected frequently and maintained as needed. The RWMWD SOP provides examples of inspection and assessment procedures, as well as guidance related to:

• Schedule (i.e., recommended schedule and frequency of inspection and assessment efforts);
• Visual inspection procedures;
• Pond assessment procedures;
• Bathymetric survey procedures;
• Information collection and recording procedures;
• Sediment characterization procedures;
• Pond sediment management procedures;
• Contracting and construction oversight; and
• Staff training and documentation.

The RWMWD inspection and assessment SOP document is included in Appendix C of this document.

Stormwater Pond Pollutant Removal Modeling

A common method of estimating the TSS and TP removal efficiency of stormwater ponds as well as other water quality best management practices (BMPs) is water quality modeling. There are a large number of water quality models that can be used to assess pollutant removal efficiency of BMPs, ranging from complex, physically-based models which simulate the transport of sediment particles and the transport, decay, and ultimate fate of associated pollutants, to simplified spreadsheet-based models which use empirical relationships to estimate the pollutant loading and removal. The following subsections provide a summary of available and recommended water quality models, as well as a case study which highlights how water quality modeling can be used to evaluate the pollutant removal efficiency of managed stormwater ponds, and how modeling results can be used to help inform and prioritize pond inspection and assessment efforts.

Available Water Quality Models

The Minnesota Stormwater Manual maintains a comprehensive list of available water quality models and provides guidance related to selecting a model based on a variety of criteria (see Available stormwater model and selecting a model). The online database contains a narrative summary of many commonly used water quality models as well as a tabular database summarizing general information for sixty (60) models. Using the tabular data regarding model capabilities, a user can select and filter the list of models to those that meet specific criteria (e.g., is the model public access? Does the model include build-in BMPs? Does the modeling include TSS and TP pollutant modeling? Runoff reduction and infiltration modeling? Etc.).

Due to the large, comprehensive nature of this database, even when filtering based on several criteria, there will typically be many models (e.g., greater than ten models) that meet a specified set of criteria. To help inform the selection of a water quality model, the MPCA has developed a TMDL Modeling Package (Objective 1, Task A of the TMDL Toolkit) which provides background information and modeling guidance for four (4) water quality models commonly used in Minnesota (Section 4.2).