How to Model Loads and Patterns in InfoSewer
In addition to physical components, InfoSewer employs four types of informational objects to describe the behavior and operational aspects of a sewer collection system. The informational objects are loads, curves, patterns, and controls.
Sanitary sewer system flow has two main components: sanitary or dry-weather loads and wet-weather loads. These loads are based on knowledge of the area land use patterns, wastewater generation characteristics, industries, inflow and infiltration characteristics, external flows, etc.
Sanitary or dry-weather flow results from human activity and is defined as the flow that exists in the sewer collection system during rainless periods. This flow is composed of domestic, commercial, industrial, and institutional waste. The sanitary loads are the basic data required for any hydraulic computation.
Wet-weather flow is related to rainfall activity and consists of groundwater infiltration (extraneous flow entering the sewer system because of poor construction, corrosion of the pipe, ground movement or structural failure through joints, porous walls or breaks) and structure inflow (extraneous flow entering the sewer system through manhole covers, basement drains and other sources).
For steady-state modeling, manhole loads can be either unpeaked or peaked as follows:
· Unpeakable Flow type – The corresponding load for each manhole is modeled as a direct flow into the sewer system.
where Qbase represents the average base flow (in flow units).
· Peakable Base Flow – InfoSewer uses a general form of the Federov’s formula as follows:
where K and r are peaking factor parameters.
Default values are K = 2.4 and r = 0.89. Values of K and r can be modified.
· Peakable Coverage Flow – InfoSewer uses the following formula which can describe both the Harman and Babbitt equations:
where P represents the population and a, b, c, d and e are peaking parameters. The default values for these parameters are: a = 5; b = 0; c = 0.2 , d = 0, and e = 1 which represents Babbitt equation (Babbitt and Baumann 1958). For the Harman equation (Babbitt and Baumann 1958): a = 14; b = 4; c = 0.5, d = 1 and e = 1.
For an extended period simulation, no peaking formula is used, instead, the multiplication factors from the diurnal pattern are used to adjust (multiply) all types of loads before they are aggregated. An example peaking-factor pattern is shown below.
Infiltration and inflow affect the operation of a sanitary sewer system and pumping, treatment, and overflow regulators facilities.
Infiltration occurs in gravity pipes while inflow occurs at manholes and wet wells. Infiltration loads refer to the volume of groundwater entering the sewer system from the soil through defective joints, broken or cracked pipes, improper connections, or manhole walls. Accurately determining infiltration is generally difficult as these loads depend on soil type, soil moisture conditions, system size and integrity, water table level, and the number of illegal connections. They are normally computed by subtracting base flow from total metered flow during dry weather or by compiling flow isolation measurements. Infiltration can be defined as proportional to the pipe length; proportional to the pipe length and to the pipe diameter; proportional to the pipe surface area (pipe length multiplied by its perimeter); proportional to the number of defects in the pipe (count-based); or as a pattern load/hydrograph (flow vs. time).
Inflow loads refer to stormwater or other drainage water and wastes (extraneous water) entering the sewer system through manhole covers. Inflow is measured during wet weather conditions and is determined by subtracting base flow and infiltration from data recorded during wet weather conditions. Inflows can be specified as pattern loads/hydrographs (flow vs time) for any manhole.
Patterns are used to represent temporal variations within the system. They consist of a collection of multipliers (multiplication factors) that are applied to a base load to allow it to vary over time during an extended period simulation. The time interval used in all patterns is a fixed value set by the user. Although all patterns must utilize the same time interval, each can have a different number of periods. If the duration of a pattern is less than the total duration of the simulation, then the pattern will repeat itself and will wrap around to its first period again.
Two options are available for representing a pattern: stepwise or continuous (linear). A stepwise pattern is one that assumes a constant multiplication factor for each pattern time period. Within each time period a quantity remains at a constant level equal to the product of its nominal value and the pattern’s multiplier for that time period. A continuous (linear) pattern is one that linearly interpolates for the multiplication factors between two adjacent time periods.
Different patterns can be applied to individual manholes or groups of manholes to accurately represent actual loading categories (e.g., low density residential, commercial, and industrial).
As an example of how patterns work consider a manhole with an average load of 2.0 CFS. Assume that the pattern time interval has been set to 4 hours and with the following multipliers:
Then during the simulation, the actual load collected for this manhole will be as follows: