Minnesota Stormwater Manual test page 5 Revision as of 19:58, 16 November 2020 by Mtrojan (talk | contribs) (→Estimating Water Quality Performance of Existing Stormwater Ponds)
Contents
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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
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
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
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
\( 𝑽_{𝟏−𝟐} = (𝑨_𝟏 + 𝑨_𝟐) × (𝑬_𝟐 − 𝑬_𝟏) \)
Where,
- V1-2 = 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
\( ∑ 𝑽_𝒊 = (𝑨_{𝒏+𝟏} + 𝑨_{𝒏𝟐}) × (𝑬_{𝒏+𝟏} − 𝑬_𝒏) 𝒏𝒊=𝟏 </math) If only the area at the bottom of the pond (App) and the area at the permanent pool of the pond (Abot) is known, the bathymetric volume can be calculated using the simplified equation, below <math> 𝑽_{𝒃athymetric} = (𝑨_{𝒑p} + 𝑨_{𝒃ot}) × (𝑬_{𝒑p} − 𝑬_{𝒃ot}) \)
Where
- V bathymetric = bathymetric volume;
- App = area at the permanent pool of the pond;
- Abot = area at the bottom of the pond;
- Epp = elevation at the bottom of the pond; and
- Ebot = 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
- Lannual = 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);
Pj = fraction of rainfall events that produce runoff (default value of 0.9);
- Rv = runoff coefficient (see discussion in Step 1, below);
- EMCpollutant = 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 (Pj) – 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 (Rv) – 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.
𝑹𝒗 = 𝟎.𝟎5 + 𝟎𝟎9 × 𝑰 [8]
Where,
- I = impervious area percentage (i.e., if 75% impervious, I = 75).
- Pollutant Concentration (EMCpollutant) – 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 (Table 3) 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. Additional literature values for typical TSS and TP EMC values are provided in Table 4.
MPCA Simple Estimator: Rv, TSS EMC, and TP EMC Values for Land Use Types
TSS and TP EMC Literature Values
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 (see Table 2).
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).
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