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.
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.
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.
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
or
\(V_{pp} = 0.0417 A \)
where
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,
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 (App) and the area at the permanent pool of the pond (Abot) is known, the bathymetric volume can be calculated using the simplified equation
\( ∑ 𝑽_{bathymetric} = (𝑨_{pp} + 𝑨_{bot})/𝟐 × (𝑬_{pp} − 𝑬_{bot}) \)
Where
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
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.
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
Pj = fraction of rainfall events that produce runoff (default value of 0.9);
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:
Note: average annual precipitation depth within the state of Minnesota by zip code can be determined using the MIDS Calculator.
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,
MPCA Simple Estimator: Rv, 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, Rannual = annual pollutant load reduction (e.g. pounds of TSS removed per year, lbs TSS/yr); Lannual = annual pollutant load to the stormwater pond (e.g., pounds of TSS per year, lbs TSS/yr); and PRpollutant = pollutant reduction efficiency of the stormwater pond (%). Note: determination of pollutant reduction efficiency is discussed in Section 2.1.
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: