During heavy rains and snowmelt, streets are designed specifically to channel water into ponding areas. In some circumstances, however, the amount of rainfall can be such that maintenance holes become submerged, exceeding the sanitary sewer capacity and causing floods. An inflow and infiltration inspection can help quantify storm water inflows through submerged maintenance holes, allowing the municipality or developer to make the necessary alterations to protect sanitary sewers.
When conducted as part of a larger sanitary sewer analysis, municipalities can be better prepared for these high intensity rainfall (surface flooding) events and can increase the sanitary sewer capacity (or divert water) as needed. This is a critical element of any wastewater asset management strategy.
Introduction to Maintenance Hole (MH) Submerging Problems in Ontario
Aside from maintenance holes in low-lying areas (obviously vulnerable to flooding), it is difficult to determine which maintenance holes are most vulnerable to flooding.
Some studies have suggested the use of simplified assumptions. For example, the American Society of Civil Engineers (ASCE, 2009), suggests that MH Inflow and Infiltration (I&I) should be quantified by evaluating the structural conditions and by “estimating” the flow rate. Forty-seven (47) other references have been obtained and reviewed which recognize the possible problems, yet they either over-simplify the solution or “skirt” a solution for the identification and quantification of sanitary maintenance hole inflow and infiltration.
The lack of a consistent methodology for identifying and quantifying inflow and infiltration in maintenance holes has led to the use of dual drainage modelling.
Methodology for Identifying and Quantifying Maintenance Hole (MH) Inflow
The analysis below takes a deeper look at dual drainage modelling, the traditional approach to this methodology, and additional resources that can be leveraged to perform wastewater system analysis and otherwise improve stormwater management.
The term “dual-drainage” refers to the hydrologic (storm water flow rate quantification) and hydraulic (conveyance analysis) processes whereby urban surface storm water runoff is analyzed, and the flow is then “divided” between the road surfaces and the storm sewer pipes. The method of dividing the flow uses hydraulic methods for storm inlets to calculate the “captured” flow into the storm sewers and the “carry-over” flow remaining on the surface.
The analysis answers the question of the distribution between overland flow (major system) and the underground sewer flow (minor system). Prior to dividing the flow, the approach involves modelling rainfall runoff entering the street from direct rain, from boulevards and lots, and other areas, and routing it along the street.
Traditional Dual-Drainage Approach
The adjustments to the traditional dual-drainage approach refer to the detail and quality of input data. In addition to using Light Detection and Ranging (LiDAR) data to develop a digital elevation model (DEM) of the tributary area and streets, the enhanced approach verified this data by surveying street cross-sections at sanitary MH locations, storm inlets, and changes in longitudinal slopes and road cross-section shapes. Each cross-section provided 3D points of the gutter, sanitary MH elevations, and the crown of the road. The difference in elevation between the gutter (channel bottom) and sanitary MH elevation defined the “offset” used when calculating the depth of flow and predicts MH submergence.
Selecting Dual-Drainage Analysis Computer Software
The selection of an appropriate dual-drainage analysis computer software is an important step. The key is to reliably predict the depth of flows on the streets at various times during and after the storm. For analysis of existing areas, it was also important that the tool accept mass points of surface topography to generate digital elevation model (DEMs) of “as-built” conditions.
For new designs, the tool should allow for coding of the various proposed areas, street cross-sections, intersections, etc. In both cases, tools to facilitate the analysis of storm inlets is an advantage.
Building a Dual Drainage Model
Site Specific Flood Analysis
For robust, site-specific analysis (e.g., to quantify inflows at each MH susceptible to submergence), Civica has also developed a method for quantifying inflows using a “MH Flood Test” technique. This technique, performed in the field, floods the area on top and around each MH while measuring the inflows inside the MH structure.
The flood test produces the I&I capture curve which is then used to quantify the peak and duration of inflow under various storm (flooding depth) conditions. The purpose of MH flood testing is to further enhance the understanding of sanitary MH inflow and to determine how much water is actually allowed into the sanitary sewers. Both inflows through pick holes and from other pathways such as the frame-cover interface and any additional surficial defects (holes, cracks, etc.) around the MH are measured. The output from this field test is a “Depth-Inflow” curve that represents the inflow rate allowed into the sanitary sewers as a function of surface flow depths.
Contact Civica to Learn More about Inflow and Infiltration (I&I)
Civica is a leader in municipal wastewater management solutions and collection system flow monitoring. Our expertise spans across sanitary and storm sewer systems as well as natural watershed asset planning and asset management. We offer consulting on inflow and infiltration inspection services, maintenance hole inspection, sanitary sewer capacity analysis, CCTV sewer inspection, sewage water management, flood analysis, stormwater management consulting, and more. Contact us today for a free consultation.
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