Municipal sanitary sewer systems need to be designed to specifically account for large rainfall events, otherwise there is a potential for wastewater flooding during these storms. In other words, sanitary systems need to be designed with storm drainage in mind in order to prevent flooding.
Sanitary maintenance holes are designed to account for some storm water drainage inflow and infiltration (I/I) through pick holes and defects. But they can’t allow too much or else they pose a risk for sewage flooding.
To that end, storm drainage must be considered when designing sanitary sewer systems and conducting sanitary sewer systems analysis.
Urban Storm Drainage Standards
The typical standard for storm water drainage sewers in North American urban areas will require that storm sewers be sized to capture and convey 5-year storm peak flow.
Streets are designed to convey any excess flow above the five-year peak to meet a one in a 100-year storm peak flow standard. Put simply, storm sewers have to be prepared for a fairly common level of stormwater flows, while street drainage needs to be designed in such a way that in the event of a large storm, acceptable levels of overland flow are maintained.
In Ontario, standards normally require that the flow be kept within the public right-of-way or up to a maximum of 300 mm depth measured from the bottom of the gutter. Depending on the road type and a 2% cross-slope, this represents various depths of submergence at the crown of the road.
Design standards mandate that your sanitary sewer capacity analysis include the occurrence of inflow and infiltration. When designing sanitary sewer systems, you need to be able to determine the following:
- Which sanitary maintenance holes (MH) are submerged during storm events
- How much water enters through pick holes and small defects when submerged
- If the threat of inflow and infiltration requires that these submerged sanitary maintenance holes be sealed completely
- How to identify these maintenance holes most prone to submergence
Maintenance hole inspections by experts can help predict which MHs are most likely to become flooded during overland flooding events, ensuring that your sanitary sewer system design can account for this.
Furthermore, Civica has developed a methodology to quantify inflows with a flood testing technique where the area on top of MH is flooded to measure inflow and recommend remediation measures in advance of the next major storm event.
Ministry of Environment, Conservation and Parks (MECP) Road Drainage Guidelines
Stormwater management has to conform to standards, including both municipal and provincial as required.
In the MECP’s Stormwater Management Plan and SWMP Design Manual (MECP, 2019), roads are considered flow channels and are referred to as the major system. While the ultimate standards for using roadways as floodways are usually set by the local municipality, the general design criteria recommended by the MECP includes:
- On arterial roads, the depth at the crown shall not exceed 0.15 m
- On local roads, flow should not overtop the curbs
- On collector roads, one lane should be left free from flooding
- On arterial roads, one lane should be left free from flooding in each direction
- Flow should not cross roads except for minor storms
- Low points along the road grade should not exist unless the low points are conveying flow to the major system
- The product of flood depth at the gutter multiplied by the flow velocity shall be less than 0.65 m²/s
- At regular intervals along the road, major storm runoff should be conveyed to a watercourse or a major channel
A Review of Municipal Design Standards
Let’s now go over a sample of design standards in Ontario that are typically replicated across municipalities in Canada and North America.
What’s important to remember is the role that streets play in your stormwater management. They act as overland channels to deal with excess inflow, as mentioned above. Assuming you have a MH lid with two 1-inch x 1-inch (2.5 cm x 2.5 cm) pick hole openings, the variables affecting inflows are summarized for eight Ontario municipalities in the table below:
Table 1: Municipal Design Standards Review
|1. From each municipality’s design standards/guidelines for local/minor road type.|
|2. Based on recommended street grading and allowable MH distance from centre line.|
|3. Using orifice equation and pick-hole dimensions of 25mm x 25mm.|
The average stormwater inflows allowed through submerged pick holes is concerning. This becomes even more significant when considering the various MH inflow pathways for stormwater into the sanitary sewers. Gaps between the cover and supporting frame, and leaks below the frame between extension rings, bricks, connections between the rungs and the walls, and along joints between and within prefabricated riser segments are all vulnerable to excess inflow.
Maintenance hole inspections should consider the surrounding pavement, as a sunken MH is at risk of allowing excess inflow, and deteriorated pavement will create gaps and fractures that can allow further infiltration.
Dual-drainage modelling and MH flood testing is critical to identify these at-risk MHs before a severe event can occur, pre-empting basement flooding and sewage overflows.
Contact Civica Infrastructure to Get Assistance with Your Sanitary Sewer Systems and Sanitary Maintenance Holes Inspection
If you need modelling and flow monitoring consulting support, we’re here to help. Civica is a leader in water management solutions. Our expertise spans across sanitary and storm sewer systems as well as natural watershed asset planning and asset management. Contact us today for a free consultation.
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