Pre-Treatment Bioretention Cells Bioswales IOWA STORMWATER MANAGEMENT MANUAL DECEMBER 16, 2015

Transcription

1 Pre-Treatment Bioretention Cells Bioswales IOWA STORMWATER MANAGEMENT MANUAL DECEMBER 16, 2015

2 Urban Runoff Background How we got here What Problem?? Provenance of the Problem Unified Sizing Criteria What to Require / Design For Pretreatment Options / Design The First (and Often Missing) Step in Treatment

3 How We Got Here Natural Ecosystem Drainage The natural ecosystem channel forming flow is typically the ~1.5-year discharge. This is nature s design flow for channels

4 Urban Stream Hydrology More runoff, more frequently Increase in magnitude and frequency of runoff events of all sizes More surface water, less groundwater Delivery of more of the stream s annual flow as surface runoff rather than base flow or interflow Rapid bounce (stage and discharge) Due to quick delivery of runoff to streams. Stream flow velocities increase leading to bank erosion, downcutting, etc. Increased risk of flash flood events.

5 Urban Stream Hydrology Constant flow Less maintenance Overgrowth Increase flow + reduction in surface cover = Infrastructure impacts and property damage Suspended sediments and other pollutants

6 Urban Stream Hydrology Channel Evolution Model Look Familiar? Source: riverrestoration.wikispaces.com

7 Streambank Erosion

8 Streambank Erosion

9 Streambank Erosion

10 Pre- and Post-development Discharge

11 Stormwater Management Goals Traditional standard: Peak rate control (flooding) Need standards that: Address effects from development to improve water quality and control the maximum rate of flow to prevent downstream flooding. Provide a more sustainable flow pattern that urban streams can handle.

12 Hydraulic Alteration after Traditional Methods Q p after development Q Q in developed area after conventional detention Time

13 Unified Sizing Criteria Groundwater recharge (Rev) Stormwater runoff quality (WQv) Stream channel protection (CPv) Overbank flood protection (Qp) Extreme flood protection (Qf) Downstream analysis

14 Uniform Sizing Criteria Groundwater Protection Volume Recharge Volume Address the most commonly occurring events, which deliver most pollutants to urban streams.

15 Uniform Sizing Criteria Water Quality Standard (WQv) Address the most commonly occurring events, which deliver most pollutants to urban streams. In numerical terms, it is equivalent to the 90% cumulative frequency rainfall depth multiplied by the volumetric runoff coefficient (Rv) and the site area.

16 Uniform Sizing Criteria Channel Protection Standard (CPv) Address slightly larger, yet still common events that cause rapid bounce in water levels in streams

17 Uniform Sizing Criteria Overbank Flood Protection Address the volume and peak of runoff generated during minor storms to reduce potential surcharge of local storm sewer systems and/or overbank flooding.

18 Uniform Sizing Criteria Extreme Flood Protection Address the volume and peak of runoff generated during rarely occurring, major storms to reduce potential for infrastructure damage from major flooding.

19 Stormwater Quality Management Capturing and treating runoff from smaller storms should capture the largest percentage of the runoff events and runoff volume from the urban landscape. A BMP capable of capturing these smaller storms would also capture the first flush volume of the larger, more infrequent runoff events. Design for water quality BMPs should be centered on the Water Quality Volume required to capture and retain the runoff for the smaller storms from a given site.

20 Water Quality Volume References: ISWMM Section 2B-1, 2C-6 Water Quality Volume (WQv): Volume of runoff expected to be generated from a given area from the 90% percentile storm event (1.25 rainfall) On average, 90% of storms in Central Iowa are equal to or less than 1.25 in depth. So if we can capture and treat runoff from such events, we will be treating runoff from the vast majority of storm events. Studies have also shown that treating to this level often results in 80% removal of suspended solids NPDES requirement.

21 100.0 Frequency of 24-hr Precipitation Events Cedar Rapids percent Precipitation Range (inches) Source: Ray Wolf Science and Operations Officer NOAA / National Weather Service Davenport (Quad Cities), Iowa

22 Source: Ray Wolf Science and Operations Officer NOAA / National Weather Service Davenport (Quad Cities), Iowa

23 Rainfall Frequency Comparison Period Greater than The amount of rain that occurs with the heaviest 1% of events is NOT increasing, BUT the frequency at which the top 1% events are occurring IS increasing. Source: National Climate Assessment Report (Chapter 3)

24 Determining Water Quality Volume Simple Method (Schueler) 1. Determine volumetric runoff coefficient Rv = (I) where I = site imperviousness in % (if 50% use 50 not 0.50) 2. WQv = P x Rv where P = rainfall (inches) WQv = Water Quality Volume in watershed inches

25 Imperviousness and Runoff Coefficient Schueler, 1994

26 Water Quality Volume Volumetric Runoff Coefficient (Rv): Rv = (I) where I = percent impervious cover (%) ISWMM 2C-6, Eq. 1, pg. 2 Water Quality Volume (WQv): WQv = Rv x (1.25 ) x A / 12 A = site area in acres WQv in acre-feet (multiply by 43,560 to get cubic feet) Equation 2 on same page

27 WQv Calculation (TR-55 Method) NRCS TR-55 method is assumed to underestimate flow from small storms (< 2 ) To compensate, calculate an adjusted curve number CN to use in TR-55 to generate a similar volume of runoff as predicted by Equation 2. CN = 1000 / [ P + 10Q a 10(Q a *Q a *P) 0.5 ] Qa = 1.25 * Rv (For 1.25 storm) P = rainfall in inches (1.25 for WQv) Equation 3

28 Small Storm Hydrology Percent Impervious Area Rv WQv (per acre) (CF) Adjusted CN 1.25 event 0% % % % % % % % % % %

29 Peak Flow Calculation Procedure Step 1. Calculate adjusted CN for the 1.25 storm. Step 2. Calculate Tc as previously described. Step 3. Complete TR-55 procedure with adjusted CN and rainfall depth of Needs to be a separate model from other storm events, since CN has been adjusted Don t use these adjusted curve numbers for larger storms.

30 WQv Calculation Example Example: 10 acre development with 50% pavement, 50% lawn in good condition Rv = (I ) = (50) = 0.50 Qa = Rv * P = 0.50 * 1.25 = WQv = 1.25 * * 10 acres * / 12 = 22,688 CF Adjusted CN : CN = 1000 / [ (1.25 ) + 10Q a 10{Q a *Q a *(1.25 )} 0.5 ] CN = 93 Run TR-55 with Tc = 10 minutes (typically calculated, not assumed) Q = 9.4 cfs

31 Soil Specific Recharge Factors Fraction of WQv, depending on pre-development soil hydrologic group.

32 Channel Protection Storage Volume (1-year, 24-hour event) Channel Protection Volume Procedure listed in Section 2C-6 is used to estimate: Channel Protection Volume (CPv) or the extended detention volume required to capture and slowly release runoff from the 1-year, 24-hour storm event. Detention routing is complicated USE SOFTWARE! WINTR-55 (Free and relatively easy) HydroCAD (Cheap and relatively easy, simple models) XPSWMM (Expensive, detailed models, 2D surfaces)

33 Urban Stream Channel Stability

34 Channel Protection Storage Volume (1-year, 24-hour event) Note the following items when applying the step-by-step procedure for estimating CPv: Step 1. Calculate the NRCS curve number and time of concentration for a given watershed or subwatershed based on previous sections. Step 2. The 1-year, 24-hour storm depth in Central Iowa is 2.34 inches (use NOAA Atlas 14) Step 3. When reviewing the output from a TR-55 (or TR-20) analysis, know the following: If the software used to run the analysis provides total runoff volume in cubic feet, the runoff volume in inches can be calculated as below: Qa = Runoff Volume (cf) x 12 (in/ft) / [43,560 (sf / ac) x Watershed Area (ac)] The unit peak discharge can be calculated as below: qu = Peak discharge (cfs) / [Watershed Area (sq. mi) x Qa (inches)]

35 Channel Protection Storage Volume (1-year, 24-hour event) Step 4. Draw a line up from qu in Figure 1, then over to the left to find the ratio (q o / q i ). Step 5. Solve for the peak release rate from the extended detention basin during a 1-year event. For this equation, Q i is the Peak discharge from the TR-55 model output (in cfs). Step 6. Solve for the estimated ratio of extended detention storage required compared to the runoff volume from the study area during the storm event.

36 Channel Protection Storage Volume (1-year, 24-hour event) Step 7. Solve for the estimated extended detention storage volume required. This is an estimate for initial basin sizing to be used for preliminary site design. Software packages may give results for runoff volume in either inches or cubic feet. Note the required conversions for desired volume measurement. Equation 4, 2C-6 pg. 4 Step 8. When a preliminary site design has been developed that accommodates the storage above, solve for the preliminary size of the control outlet. Note that q o comes from Step 5, and ho depends on the design of the basin and the depth of storage required to achieve the required extended detention volume. Equation 5, same page. Step 9. A perforated riser pipe may be required in lieu of an orifice of 4-inches in diameter or smaller (or other means applied to prevent clogging of the basin outlet). Step 10. Use preliminary basin design to develop stage-storage-discharge relationships for flow routing. Then perform an actual reservoir routing calculation (see Section 2C-10) to verify that the initial design means the release peak rate requirements (from step 5) and an extended drawdown of the basin can be observed (24- to 48-hour drawdown).

37 Overbank and Downstream Flooding (Q p - Q f ) Protect downstream conveyance systems from overbank flooding or surcharge from postdevelopment peak flows Design approach: Control post-development peak discharge rate at no more than the pre-development rates (pre-settlement) Meadow in good condition Some jurisdictions may want to set additional limits for runoff to 5-year runoff rate from existing conditions (agriculture). Rationale for this criterion is that most urban conveyance systems are designed to convey 5-year runoff temporary storage of excess runoff volume is typically required. Look at full spectrum of events WQv (1.25 ), CPv (1-yr) Qp: 2-yr, 5-yr, maybe 10-yr [ Overbank] Qf: 10-yr, 25-yr, 50-yr, 100-yr [Extreme Flood]

38 Extreme Flood Protection (Q f ) Evaluate the effects of storms up to the 100-year storm on the stormwater management system, adjacent property, and downstream facilities and property. Manage the impacts of the extreme storm event through detention controls and/or floodplain management.

39 Pre-Treatment Bioretention Cells Bioswales IOWA STORMWATER MANAGEMENT MANUAL DECEMBER 16, 2015

40 PRE-TREATMENT Grass swale as part of a pre-treatment train upstream of a constructed wetland. Source: Nilles Associates

41 Pre-Treatment Removes the heaviest loadings of materials that can cause the primary practice to fail. Forebay prior to a bioswale. Source: Amy Foster, City of Coralville

42 Pre-Treatment Grass swales (not bioswales) Filter strips Forebays Mechanical separators

43 Pre-Treatment Provide a defined location where coarser sediments, trash and debris can be collected and removed (easily). Filter strip pre-treatment of flow off of adjacent road prior to entry into a bioretention cell in Ankeny. Source: Jennifer Welch, Polk SWCD

44 Pre-Treatment Provide a defined location where coarser sediments, trash and debris can be collected and removed (easily). Filter strip pre-treatment of flow off of adjacent road prior to entry into a bioretention cell in Ankeny. Source: Jennifer Welch, Polk SWCD

45 Pre-Treatment Provide a defined location where coarser sediments, trash and debris can be collected and removed (easily). Filter strip pre-treatment of flow off of adjacent road prior to entry into a bioretention cell in Ankeny. Source: Jennifer Welch, Polk SWCD

46 Pre-Treatment Provide a defined location where coarser sediments, trash and debris can be collected and removed (easily). Filter strip pre-treatment of flow off of adjacent road prior to entry into a bioretention cell in Ankeny. Source: Jennifer Welch, Polk SWCD

47 Pre-Treatment Manufactured water quality devices Some types can separate multi-phase detritus from hotspot runoff floaters Trash fuels, oils and grease sinkers heavy debris sediment

48 Pre-Treatment Manufactured water quality devices Some types can separate multi-phase detritus from hotspot runoff floaters Trash fuels, oils and grease sinkers heavy debris sediment Image courtesy of Contech ES

49 Pre-Treatment Missing Here If pretreatment cannot be provided (or is not provided), more intensive maintenance of a practice will be necessary.

50 Shall provide a volume of 0.1 inches of runoff for the impervious watershed area they drain. Pre-Treatment Forebay upstream of a small pond. Source: Nilles Associates

51 Pre-Treatment Recommendation: Practices that are adequately sized may receive a credit of 10% of the total WQv requirement (due to pre-treatment volume )for a given site from an approving jurisdiction or funding agency.

52 Pre-Treatment Pretreatment practices which are sized by a velocity or rate of flow usually have a maximum value to allow suspended solids to deposit or other materials to be screened effectively (Stokes Law). Image courtesy of web.deu.edu.tr For Grass Swales - ~ 1fps

53 Pre-Treatment Caveat Often lack: Volume to provide for extended detention of larger storm events and / or re-suspension. Maintenance

54 Pre-Treatment Velocities at critical points such as inflow and outflow points, and changes in grade or cross section, should not exceed the values in the table.

55 Pre-Treatment Regular maintenance must be completed to remove collected sediments and minimize the potential for re-suspension.