In the previous article, we discussed how storm drainage design standards can impact wastewater systems.
In this article, we’ll be looking more in-depth at stormwater drainage, specifically risk factors for sanitary maintenance holes (and by extension sewage flooding and basement flooding), as well as provide some recommendations on how to prepare your sanitary sewer systems so that these worst-case scenarios do not occur.
Factors Affecting Sanitary MH Submergence
If sanitary maintenance holes (MH) are becoming submerged, that’s a warning sign that your sanitary sewer systems may be experiencing higher inflow and infiltration (I/I). Examining the factors that can make MHs vulnerable to submergence is important to prevent flooding and ensure wastewater systems are equipped to deal with large storm events:
Hydrologic and Hydraulic Factors
Hydrologic factors are the typical rainfall-runoff response variables such as the size, slope, and shape of the catchment area, the impervious areas, the pervious areas, soils, vegetation, etc. Hydraulic factors include the street geometry (e.g., longitudinal slopes, cross-sections, surface roughness), spacing, size and configuration of inlets to the storm sewer and the inlet blockage. Both need to be considered when preparing stormwater management plans.
Inlet blockages and their consequences are recognized in many municipal standards, some of which require the assumption of 50% inlet blockage during street drainage design. However, design mandates in Ontario do not specify how the blockage factors are to be used when specifying inlet spacing and types along streets. Designers are left with the responsibility to verify that the amount of stormwater to be captured into storm sewers meets the design standards. Municipal-owned inlets typically have operational requirements for regular inlet inspection and cleaning.
Changing Conditions During a Storm
The changing conditions during a storm and along the length of the street drainage system result in varying depths of flows and flooding within a street. Depending on their position within the street cross section, some sanitary MHs are more susceptible to submergence than others. This is particularly the case during intense storms that convey significant debris and during winter conditions when inlet grates are blocked by ice. In all cases, the position of the sanitary maintenance holes within the street and the elevation is a factor.
Basement Flooding and Environmental Spills Problem
Flooding of basements and environmental spills are a large problem in most urbanized areas. Sanitary MH submergence can produce inflows greater than six times the sanitary sewer capacity. The factors that limit basement flooding and spills include:
- Storm drainage design standards that allow sanitary MH submergence to represent the upper limits of municipal drainage designs
- The temporal and spatial variability of large storms is such that very large storms tend not to re-occur at the same place
- The routing and storage-attenuation in sewers
- The depth of sanitary sewers and the hydraulic gradeline elevation before reaching connected basements or surface spill levels.
However, the changing temporal and spatial variability of large storms also brings to light the potential impacts associated with climate change.
Findings and Reports of Basement Flooding and Sewage Spills
Aside from routine maintenance hole inspection, there is another tool that can be used to prevent these floods: a modified dual-drainage analysis technique that is able to identify flood-vulnerable MHs. These vulnerable MHs are typically a primary culprit for the significant I/I into sanitary sewer systems. Addressing these vulnerable MHs goes a long way towards storm water drainage preparedness.
The approach to identify flood vulnerable MHs accounts for the specific topographic, hydrologic, and hydraulic settings at and upstream of each MH location and can be used to quantify the reduction in flood potential through the implementation of a selective MH sealing program.
The technique can be applied to identify the vulnerable MHs and resulting excessive I/I in existing urban areas and as part of the new subdivision design. Both peak inflow rates and volumes can be determined.
MH flood testing is being used to quantify the amount of water allowed through each MH under various flood conditions. When combined, the modified dual-drainage analysis and the MH flood testing can be used to analyze the amount of I/I reductions and the cost-benefit of selective MH sealing.
Combined with flow monitoring, and you can more adequately prepare your development or municipality for these storm events.
Conclusion and Recommendations
- Identify and seal vulnerable MHs (can include a provision for storm sewer inlet blocking)
- Use dual-drainage system in addition to typical maintenance hole inspection to identify at-risk MHs
- Clearer analysis specifications by the municipalities and other system owners can be implemented, ensuring that developers can more readily determine storm drainage vulnerabilities
- Flood testing where specific inflow reductions require quantification
- Regular inspection to identify degradation
- Eliminate I/I sources when found (even if minimal, this can make the difference between and heavy rain and a flood)
Consult Civica for Designing Storm Drainage Systems
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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