Appendix H. Storm Water Management

Transcription

1 Appendix H Storm Water Management

2 R.J. Burnside & Associates Limited 292 Speedvale Avenue West Unit 20 Guelph ON N1H 1C4 CANADA telephone (519) fax (519) web Technical Memorandum Date: Project Name: Niska Road Improvements Class EA SWM from Ptarmigan Drive to 900m west Client Name: City of Guelph Submitted To: Philip Rowe Submitted By: Tony Elias Reviewed By: Nicholle Smith, Youssef Haroon 1.0 Background Drainage along Niska Road is currently conveyed by open ditches towards the Speed River. While ditches offer environmental benefits such as opportunities for infiltration, higher hydraulic roughness and bio-filtration, maintenance of open ditches can be a burden on the municipality. In this particular case, sections of the ditch are heavily scoured, devoid of vegetation, and attacking the road embankment. As part of the background review for the EA, Burnside acquired drainage plans and reports for lands that may be contributing to the Niska Road ditch. The City s information provided details on the stormwater management facility located at the corner of Niska Road and Ptarmigan Drive, referred to as the Block 177 Facility, which outlets to the south side of Niska Road. The pond was designed in 1992 and services almost 22ha of residential development, providing water quality treatment and flood storage for a 100 Year storm event. (Further details on the pond are available in the report entitled Kortright Hills Phase III Subdivision, Victoria Woods Development Corporation Inc., Stormwater Management Report dated September 1992, prepared by the City of Guelph.) The pond has been identified as a significant contributor to the erosion problem, due to the area of lands being serviced and the volume of water being discharged. It is noted that Burnside met with GRCA in September 2015 to discuss a potential diversion of flows from the SWM pond away from the Niska Road ditch and across lands presently owned by the GRCA. Burnside was informed that the GRCA lands under consideration (presently being farmed) are currently in the process of being re-designated. GRCA will soon begin a Master Plan, for this reason, the GRCA could not consider a new flow path across the property.

3 Technical Memorandum Page 2 of Rural Cross-section vs. Urban Cross-section 2.1 Rural Cross-section In general, it can be argued that ditches are preferred for their environmental benefits (infiltration, roughness, bio-filtration) and there are many locations along Niska Road where the existing open ditches are stable and could remain as open ditches. However, there are also areas that are unstable. Outlined in this section are some drainage concepts to consider should the preference for Niska Road be a rural section. Roadside ditches allow natural velocity attenuation and infiltration but may be prone to erosion. Ditch design must consider flow velocities and provide appropriate erosion protection (Ex. rock check dams spaced according to ditch slope, natural vegetation, and grade controls such as terracing). Ditches and culverts must be designed to capacitate and convey flooding from storm events. In general, drainage patterns would be maintained to manage runoff volume and velocity (such as, reconstruction and maintenance of ditch which veers from roadside and empties into adjacent wetland). The Storm Management Facility at the intersection of Niska Road and Ptarmigan Drive discharges to the roadside ditch which increases the volume of runoff in the ditch and associated erosion. This discharge volume would need to be accounted for in the ditch design to prevent ongoing maintenance. Currently, this ditch is fairly steep and is being eroded by the continuous discharge from the pond there is clear evidence that this ditch was never meant to accept the volumes or rates being discharged from the pond. If we were to pursue a rural cross-section it would be strongly recommended that a stormsewer be used through this section to intercept discharge from the pond and convey it to the nearest outlet (that is not private property). Today, where the ditch is most unstable a considerable effort would be required to stabilize the area where the Ptarmigan Drive SWM Pond outlets to the road ditch. Consideration should be given to the quality of runoff entering Speed River and whether additional treatment is required. Grasses in ditches in a rural cross-section provide treatment and increase the quality of runoff. If required, additional treatment may be provided by ponding or bio-filtration and these options should be explored. Mitigation measures will be required to minimize permanent or temporary impacts along the road corridor and to adjacent lands, (including replanting of native vegetation). Floodplains associated with the Speed River are located within the Study Area and PSW are located on adjacent GRCA lands. These lands subject to GRCA O.Reg. 150/06. Regulation of Development, Interference with Wetlands and Alterations to Shoreline and Watercourses approval will be required prior to construction. This option has the highest potential for effects to the natural features based on the potential footprint size. The width required to reconstruct the gravel shoulder and the restoration of the roadside ditches large enough to convey the anticipated volume of stormwater

4 Technical Memorandum Page 3 of Urban Cross-Section Road designs must account for drainage in order to avoid issues with erosion, water quality and flooding. The means to address these potential impacts differs depending on whether the design conforms to a rural, or urban, cross-section. Outlined below are some drainage concepts to consider should the preference for Niska Road be an urban section. Urban cross-sections are generally more efficient in conveying stormwater through the use of curb and gutter systems and stormsewers. Further erosion is largely abated though efficiency can often result in erosion or flooding at the outlet due to the velocity of the discharge. However, the efficiency, and the associated increases in velocity, can often result in erosion or flooding at the outlet. Therefore, consideration will be given to out letting the stormsewer a distance from the Speed River to allow mitigation prior to discharging to the river. If not possible, installation of a low dissipation structure may be used alternatively to reduce erosion and promote infiltration. Consideration must be given to property ownership as stormwater should not be directed onto private property, or as in this case GRCA lands. As noted above, inlets into the stormsewer will be required to accommodate discharges from the Stormwater Management Facility at the intersection of Niska Road and Ptarmigan Drive. Even though water quality should not be measurably worse in a storm sewer than in a ditch, consideration must be given to improving water quality as part of the design. Measures to consider include bio-filters, plunge pools, perforated pipes, infiltration galleries, oil/grit separators. Also, with an urban section there is an opportunity to prevent roadside sedimentation currently impacting roadside vegetation and trees. To adequately abate the ongoing erosion and maintenance issues, it is recommended that Niska Road be re-constructed as an urban section. The urban road section would include a stormsewer to accept discharge from the SWM pond, as well as local runoff generated by the road. The concrete stormsewer will be better suited to deliver the runoff to the Speed River, despite the steep gradients along Niska Road, due to its durability. The urban cross-section provides the greatest opportunity to create the smallest cross-section footprint. Additional cross-sections that further reduce the width of the road and trees impacts and encroachment onto wetland boundary can also be explored. 2.3 Urban Cross-Section Pre-design Considerations As noted above, the urban cross-section option along this corridor has the lowest potential to result in encroachment into the natural features within the study area, as the sidewalks and curbs abut the road; and the grassed boulevard naturalized quickly as it meets the existing terrain, therefore a smallest road cross-section footprint can be achieved.

5 Technical Memorandum Page 4 of 12 The overall stormwater management design must accommodate all existing drainage, not just the road drainage. In this case, the existing SWM facility at the corner of Ptarmigan and Niska Road must be routed into the new sewer, which will significantly increase the size of sewer required for the entire length of the project (as a side note, Burnside met with GRCA on September 1, 2015 to discuss the potential of re-directing pond discharge away from Niska Road and, instead, across lands owned by GRCA. GRCA explained that the subject lands are being re-designated for development and cannot be encumbered by a drainage swale or pipe). The conversion to an urban cross-section improves conveyance and eliminates the problems associated with ditch erosion (which is extensive on the south side of Niska between the storm pond outlet and the wetland). However, flow energy is much higher in a stormsewer and the volume is typically higher because infiltration opportunities offered by open ditches have been eliminated. As a result, some form of mitigation must be introduced prior to discharging either into the Speed River or other outlet. Also, since contaminants are picked up on the road and can be more easily and efficiently delivered to the river within a sewer; mitigation is also required to address water quality, prior to discharge to the natural environment. Efforts have been made by the municipality to armour the critical sections, but the degradation continues. Figure 1 illustrates the condition of the ditch on the south side, just west of Pioneer Trail. Figure 1 : Niska Road ditch (south side), just west of Pioneer Trail

6 Technical Memorandum Page 5 of 12 Ditches that are approaching the Speed River, experience a number of grade change and runoff is directed onto lands currently owned by the Grand River Conservation Authority (GRCA). Due to the ongoing erosion within the ditch, sediment is being deposited within the forested area (see Figure 2 below). Figure 2: Alluvial deposits from Niska Road ditch

7 Technical Memorandum Page 6 of Stormwater Modelling Stormwater modelling was conducted to; understand the potential volume of stormwater in a 5 year storm event in conjunction with stormwater flowing from Block 177 SWM pond; determine the size of storm sewer pipe; and, determine the potential size of a plunge pool or receiving/grit separator tank In order to quantify flows from the Block 177 SWM pond and the local runoff from Niska Road, a hydrologic model was developed. The model was used to demonstrate the following: the 5 Year peak flow rate generated by the Block 177 SWM facility in combination with Niska Road runoff. The model can account for the delayed response (to rainfall) from the pond as well as the immediate response from the road. If the timing of the different hydrograph peaks were ignored, the stormsewers would be over-sized; flow velocities exiting the stormsewer, adjacent to the Speed River. Volume and velocity information is required to ensure that mitigation measures (ie. plunge pools or stilling basins) are effective at reducing velocity and capturing sediment, prior to discharge to the Speed River. Previous reports provided by the City supplied much of the necessary input for the hydrologic model. As a result, the model accounts for runoff from the Ptarmigan Place subdivision (Kortright Hills Phase III) to the Block 177 SWM facility for a 5 Year event, the corresponding discharge from the pond, and the addition of local drainage along Niska Road. The results of the preliminary modelling (in PCSWMM format) are summarized in Table 1. Table 1: Preliminary Hydrologic Modelling Results Design Storm Peak Flow at Speed River Flow Velocity at Speed River 5 Year 0.83 m 3 /s 1.8 m/s The modelling indicates that a 675 mm diameter concrete stormsewer will be sufficient to convey the 5 Year storm, for most of the study area. The pipe size increases at the base of the hill where the slope decreases. The discharge from the Ptarmigan Drive stormwater management facility requires a large starting diameter. We have determined that the storm pipe sizes start at 675 mm dia and increase to 1200 mm dia at the plunge pool.

8 Technical Memorandum Page 7 of Plunge Pool We investigated the plunge pool option as this allow for effective management of both water quality and rate of discharge into the Speed River. It is also a preferred SWM tool given the naturalized area. Equations from the MOE Stormwater Management Planning and Design Manual (March 2003) were referenced to design a stilling basin which would cushion the flows and promote sediment deposition, prior to release to the Speed River. The Dispersion Length refers to the length of fluid required to slow a jet discharge (ie. pipe discharge) to a desired velocity. For this basin, the target velocity for flows leaving the basin is 0.5 m/s for a 5 Year design storm: Dispersion Length is calculated as follows : 8Q dv f where Q = inlet flowrate (m 3 /s); d = depth of permanent pool (m); V f = desired velocity at the forebay berm (m/s) Also, the minimum Bottom Width of the basin is described by: Bottom Width: Dispersion Length / 8 Substituting in the values from Table 1, and assuming a 1m deep permanent pool, the following dimensions were derived: Dispersion Length: 13.3m Bottom Width: 1.7m The proposed Plunge Pool is shown on Figure 3. The actual dimensions are: Design Length: 15m (bottom) + 2:1 sideslopes and end slopes Design Bottom Width: 5m An additional check of the average velocity in the forebay which should be less than or equal to 0.15 m/s, works out to be m/s. This ensures that erosion will not occur within the basin. The Plunge Pool would need to be located on lands owned by GRCA (south east corner of the bridge. Preliminary discussions with GRCA indicate that these lands may be suitable for the facility as they were previously disturbed and because there were no plans to re-purpose the property. It is noted that, in the interest of tree preservation, the stormsewer is meant to enter the GRCA property through an existing gate/driveway. Further, if the GRCA agrees to the location of the Plunge Pool, an easement, in favour of the municipality, will need to be secured to allow for inspection and maintenance of the facility by City staff. The approximate dimensions of the easement are shown on Figure 3.

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10 Technical Memorandum Page 9 of 12 Inlet Plunge Pool Outlet Figure 4 Plunge Pool Location *Addition concept sketches can be found attached It would be imperative that the City is granted permanent access to the outlet (all the way to the Speed River), to allow for regular inspection, maintenance and repairs. If the City does not own the lands where the outlet is located, access is typically secured by means of an easement in favor of the City. Mitigation Measures Measures to consider for velocity mitigation include: check dams, rip rap aprons, bioengineering (vegetative linings), flow spreaders, etc. Other measures to consider for water quality treatment include: bio-filters, infiltration galleries, and oil/grit separators. If space permits, a partial rural cross-section upstream of the bridge with outlet to the stormsewer into the ditch will be considered to allow for energy dissipation prior to discharging to the Speed River. This rural ditch could consist of a heavily-armoured, flat, open ditch intended to reduce flow velocity and promote sediment deposition. This partial rural cross-section is show in Figure 5.

11 R.J. Burnside & Associates Limited 292 Speedvale Avenue West Unit 20 Guelph ON N1H 1C4 CANADA telephone (519) fax (519) web Figure 5 - Partial Rural Cross-Section

12 Technical Memorandum Page 11 of 12 Forms of treatment that are available for a stormsewer outlet include: o Oil/Grit separators: These chambers are designed to remove sediment from the runoff and particles attached to the sediment. Periodic clean-out is required. Can be used if space limited. o Bio-swales: These are basically enhanced ditches with wide bottoms and dense vegetation that will reduce flow velocity and promote settling of sediment. They require some space in order to effectively reduce velocity and provide sediment storage. The GRCA has indicated preference for bio-swales. This preference will be considered during the detailed design phase of the EA. Maximization of the use of open channels over curb and gutter systems is ideal, but areas of high velocity make open channels impractical. o Rip Rap Channel: Similar to a bio-swale but uses heavy armouring to dissipate energy and prevent erosion. Vegetation will typically get established naturally to improve filtration. Options available for naturalized collector tank if necessary; periodic clean-out is required. o Opportunity to increase elevation of road to allow for capacity for regional flood overflow. Other Considerations Aquatics curb and gutter could potentially increase intensity of runoff directed to Speed River during precipitation events and spring melt, increasing flows and water volume within the River. Water quality could be potentially impacted due to curb and gutter related road runoff (potentially containing contaminants) toward the River. Construction processes could potentially impact aquatic habitat through runoff, spills, seepage, etc. Appropriate erosion and sediment control and mitigation measures will be required to minimize potential impacts to site and adjacent lands during construction. Mitigation measures will be required to minimize permanent or temporary impacts to the road corridor and adjacent lands, including replanting of native vegetation. Floodplains associated with the Speed River are located within the Study Area and PSW are located on adjacent GRCA lands. Development on these lands is subject to GRCA O.Reg. 150/06. Regulation of Development, Interference with Wetlands and Alterations to Shoreline and Watercourses approval will be required prior to construction. Proposed location and design for SWM were recommended in areas determined to be outside of the footprint of wetland boundaries within the study area, with the consideration of minimizing potential adverse effects to wetland features and functions within the study area. Mitigation measures will be fully explored during the detail design process and all stormwater management construction stipulations, requirements and environmental recommendations will be carried forward in the design drawings and tender document.

13 Technical Memorandum Page 12 of Inspection and Maintenance Requirements The installation of a stormsewer on Niska Road, from Ptarmigan Drive to the outlet at the Speed River, will eliminate erosion within the south ditch and, therefore, reduce maintenance obligations for the City. However, periodic inspection of the Plunge Pool and outfall to the Speed River will be required to ensure that: All components of the Plunge Pool (inlet, outlet, etc.) are adequately protected; There are no significant blockages; Sideslopes are stable, vegetation is thriving, and sediment accumulation is less than 1/5 to 1/4 overall depth of basin (sediment will be largely from de-icing activities). At the same time, catchbasins along Niska Road can be inspected for sediment accumulation; Outfall to the Speed River is stable (no evidence of erosion, vegetation is established and providing good coverage, no signs of vandalism); Security fence is intact. An Operations and Maintenance Manual can be provided for municipal staff if necessary to monitor time and frequency of inspections, to note any deficiencies, etc.

14 PROPOSED BRIDGE TO BE DETERMINED SIB SIB HP 15.0 m 5.0 m NISKA ROAD 5.0 m TO SPEED RIVER ARMORED SPILLWAY OF CITY OF GUELPH 25.0 m EASEMENT IN FAVOUR SEE PLUNGE POOL DETAIL EXISITNG GROUND PROP. STORM SEWER MIN. 1.5 m PLUNGE POOL SECTION SCALE: 1:200 RIP RAP APRON AND PLANTINGS PLUNGE POOL DETAIL SCALE: 1:250 SPEED RIVER 10.0 m TO SPEED RIVER 3 1 N - Client CITY OF GUELPH Figure Title NISKA ROAD PLUNGE POOL Drawn KDR Checked Date TE 2015/10/08 Scale Project No. N.T.S Figure No. FIG3 File: C:\Users\krooyakkers\Desktop\Niska Road PCSWMM\Figure.dwg Date Plotted: October 8, :37 AM