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Thursday , 29 July 2021

Town of Elk Point: Proposed Stormwater Drainage Improvements

As part of a comprehensive drainage study for the Town of Elk Point, existing drainage issues have been analyzed to identify the corresponding improvements to reduce flooding and drainage issues. The analysis was carried out based on information gathered through an open house session with the Town residents, GIS dataset, LiDAR and field surveys and computer modeling. A number of locations that are potentially experiencing drainage issues have been studied. Improvements are proposed (Figure 1) at both local and drainage infrastructure network levels. These improvements are:

  1. Construction of dry ponds in the Highway 41 green space;
  2. Upgrade of the 47th Street culvert;
  3. Construction of a berm/swale structure beside roadways near the intersection of Highway 41 and Railway Avenue;
  4. Upgrade of the Iron Horse Trail culvert;
  5. Upgrade of the Railway Avenue culvert;
  6. Construction of curb and gutter at 46 Street and 50th Avenue intersection, and upgrade of stormwater pipe and inlets;
  7. Upgrade of the 50th Avenue culvert;
  8. Cleanup of the existing ditch;
  9. Reconstruction of the ditch along south perimeter of the school sports field; and
  10. Construction of a new ditch north of the A G Ross

 

The construction of dry ponds, upgrade of some culverts and cleanup of existing ditch (improvements 1, 4, 5, 7 and 8) will improve the drainage network performance by providing safe temporary storage of excess stormwater and removing hydraulic bottlenecks on existing drainage channels. As a result, the industrial area near the Town’s south boundary will see a significant reduction in flooding frequency and severity.

Improvements 2, 3 and 6 provide additional protections to the affected properties by directing stormwater through controlled flow paths to avoid flooding of private property.

Improvements 9 and 10 will eliminate the safety hazard posed by steep side slope of existing drainage ditch.

 

Detailed descriptions of the drainage issues and improvements are provided below.

Figure 1

 

 

Drainage issue: uncontrolled stormwater spill from 47th Street culvert inlet (Figure 2)

 

The stormwater generated from the industrial area north of 54th Avenue is intercepted by the shallow curbside gutter that drains west along 54th Avenue and toward 47th Street. Due to the limited capacity of the 550 mm culvert crossing under 47th Street, after an intense rainfall, uncontrolled stormwater flow would spill over 54th Avenue to the adjacent private property and pose a risk of flooding to the property.

Improvement 1 and 2: Involves the construction of three dry ponds areas in the green space between Highway 41 and 47th Street and upgrade of the 47th Street culvert with two parallel 600 mm (2 ft) diameter culverts. The duplicate 600 mm culverts would have enough capacity to allow stormwater to flow west under 47th Street. The two dry ponds further north will reduce peak flow rate from the upstream industrial area, allowing more stormwater coming from the new 600 mm (2 ft) culverts to be temporarily stored in the 54th Avenue dry pond. The improvements would reduce the risk of uncontrolled overland spill, allowing stormwater to safely pass through the area in a controlled flow path. The ponds will have a gentle 4:1 side slope, and a maximum depth of 1 metre (~3 ft), measuring from the south edge of the ponds. They will be filled with stormwater during and immediately after a rainfall, otherwise, the ponds will remain dry.

Figure 2

 

Drainage Issue: Backups upstream of the creek and ditch confluence near the town boundary due to undersized culverts (Figure 3)

The two existing culverts crossing Iron Horse Trail, as well as the 900 mm culvert crossing Railway Avenue are considered undersized. These hydraulic bottlenecks lead to high water surface elevations in the creek and ditch channels after intense rainfalls events, resulting in further backups in upstream drainage systems, and potential flooding of the nearby properties.

Improvement 4: Upgrade of the Iron Horse Trail culverts. Two 1200 mm (4ft) CSP culverts will be added to the crossing. Along with the existing 1200 mm culvert, three culverts in parallel will provide enough capacity for stormwater flow and eliminate the hydraulic bottleneck. The existing 900 mm culvert is assumed to be removed.

Improvement 5: Upgrade of the Railway Avenue culverts. Two 1200 mm CSP culverts will be installed at the crossing, replacing the existing 900 mm culvert.

 

Figure 3
Drainage Issue: Overland flooding risk at 4601 Railway Avenue (Figure 2&3)

 

Property on the southeast corner of the Railway Avenue and Buffalo Trail is in the overland flow path and is at risk of flooding during intense rainfall events. To reduce the flood risk, a combination of upstream, local and downstream improvements is being proposed.

 

 

Improvement 1: Three dry ponds are proposed upstream to attenuate flow and reduce downstream peak flow rate.

Improvement 3: Site grading to form a shallow swale and berm structure along the north and west perimeter of the property to direct flow away from the building. The grading will be designed with mild slopes to allow vehicles to easily access the adjacent business.

Improvement 4: upgrade of the Iron Horse Trail culverts. Upgrading the culvert will remove the hydraulic bottleneck on the creek, lower the water surface elevation and allow stormwater to flow downstream more efficiently.

 

Drainage issue: Flooding near 46th Street and 50th Avenue (Figure 4)

 

Limited capacity of the drainage infrastructure near the intersection of 46th Street and 50th Avenue could result in flooding into the adjacent private property during intense rainfall events.

 

Figure 4

Improvement 6: Two stages have been proposed at this location:

 

Drainage Issue: Flooding due to undersized culvert near the intersection of 50th Avenue and Railway Avenue (Figure 5).

The culvert serves a large drainage area, including both the industrial area and the ranch land nearby. When the culvert is overwhelmed by the amount of stormwater draining toward it, the excess flow will spill over 50th Avenue toward the ditch on the south. Backups in the culvert inlet also leads to water ponding at the southeast corner of 4530 50th Avenue.

Improvement 7 and 8: Upgrade of the culvert to two parallel 900 mm culverts from the existing 600 mm. Cleanup of the existing ditch to allow unobstructed stormwater flow.

Stage 1 – The proposed improvement is the construction of a curb and gutter along the south side of 50th Avenue, and the upgrade of stormwater pipes and inlets. The upgraded pipes and inlets will increase the sewer capacity and reduce the amount of stormwater flowing overland. The curb and gutter would intercept and direct the overland stormwater flow away from 4601 50th Avenue and into the ditch west of the building, where it could pass through safely. An alternative to curb and gutter could involve site grading to direct the flow spilling across 50 Ave into the adjacent ditch to the west.

 

 

Stage 2 – This stage involves constructing curbs and gutter along 51 Ave starting from the intersection of 47th Street and 51st Ave to direct flow to a new storm sewer constructed along 51st Avenue. This would reduce the potential for flood flows to enter the adjacent properties to the south. The storm system would intercept flow form 51 Ave and 46 Street, reducing erosion of the road ditch along 46 Street and reduce flooding at the 50 Ave intersection. Constructing curb and gutter along 46 Street (or just in a vicinity of the 46 Street and 51 Ave intersection) would allow reduced erosion to occur along 46 Street.

Figure 5
Drainage Issue: steep side slopes of the school ditch and spills from downstream culvert inlet (Figure 6).

 

In addition, stormwater frequently spills from the downstream culvert inlet due to limited capacity of the culvert.

Improvement 9: Reconstruction of the ditch with gentler 3:1 side slopes.

 

Improvement 10: Construction of a ditch north of the A G Ross Arena parking lot. The ditch extends west to direct stormwater generated from drainage area north of it to the existing drainage channel east of Circle Drive. The total volume of stormwater draining toward the ditch near the south boundary of the school will reduce by half approximately, resulting in a significant reduction in spill frequency from the culvert inlet.

Figure 6

 

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