What Is a Subnet?
A subnet (sub network) is a logical drainage group within a network.
Think of it like:
“All storm inlets that drain to Pond A”
“All sanitary laterals that connect to Lift Station 2”
Each subnetwork must:
Contain pits (sources)
Contain at least one sink (outlet)
Be internally connected
Storm and Sanitary subnetworks are independent of each other
Why Subnetworks Exist
They allow you to:
Separate drainage basins
Define multiple discharge locations
Create trunk vs local hierarchy
Prevent cross-routing between service zones
Control phased development
Without subnetworks, everything would drain to one location.
Within a subnetwork:
All pits must connect to one of its sinks.
Sinks define the drainage boundary.
Pits cannot connect to sinks in other subnetworks.
If Subnetwork 2 has pits but no sink → model error.
If Subnetwork 1 has two sinks → Service AI decides which pits go to which sink based on routing cost.
What Is a "Subnet In" / Pit?
A subnet in / pit is a location where flow enters the network.
Think of it as:
“The point where water (or wastewater) gets collected and handed off to the pipe system.”
It is always an inflow point.
🌧️ In Storm Systems
A pit could be:
A curb inlet
A catch basin
A manhole
A parcel
A building
Ground control (configurable polygon)
What makes it a pit?
It receives runoff from surface catchment
It has upstream contributing area
It needs to connect to something downstream
Service AI calculates how much flow enters each pit during catchment analysis.
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🚽 In Sanitary Systems
A pit is typically:
A building
A parcel
Ground control (configurable polygon)
It represents where wastewater enters the gravity system.
There is no surface catchment logic for sanitary — but the concept is identical:
It’s a source node that must be routed to a downstream outlet. Sanitary pits do not need an area draining to them, they are more of just a straight load to the pit.
Key Characteristics of a Pit
Has flow entering
Has no downstream assignment yet
Must be allocated to a sink
Belongs to a specific subnetwork (
SAI_SUBNET_OUT)
If a pit exists without a sink in its subnetwork → routing fails.
What Is a "Subnet Out" / Sink?
A subnet out / sink is where the network discharges.
Think of it as:
“The final receiving point for a group of pits.”
It is always an outflow point.
🌧️ In Storm Systems
A sink could be:
A detention pond
A headwall
A storm outfall
A manhole
A pump station
Storm water flows downhill toward sinks.
🚽 In Sanitary Systems
A sink might be:
A lift station
A connection to an existing trunk sewer
A treatment plant connection
A regional manhole
Sanitary pipes must slope toward a sink that can accept flow.
Key Characteristics of a Sink
Has an invert elevation
Has capacity to receive flow
Receives flow from multiple pits
Belongs to a specific subnetwork (
SAI_SUBNET_IN)
Every active subnetwork must have at least one sink
Assets That Are Both a Pit and a Sink
Some objects can be both a pit and a sink:
A detention pond.
It is a sink for local storm runoff.
It is also a pit for the downstream trunk system.
A manhole
Receive flow from one subnet
Contribute flow to another subnet
A sanitary lift station
Neighborhood draining to a trunk sewer manhole
Trunk sewer flowing to a lift station
Lift station pumping to treatment
That’s why ponds/manholes often have:
"Subnet (In)" → local drainage subnet
"Subnet (Out)"→ trunk subnet
Example — Storm Subnetwork Structure
Imagine:
Subnetwork 1 → drains to Pond A
Subnetwork 2 → drains to Pond B
Each pond must have:
"Subnet (In)" set to its subnetwork ID
Valid invert elevation
Capacity sufficient for upstream flow
If Subnetwork 2 has pits but no sink → model fails.
How to set Subnets on assets
To set up assets, click on an Allsite object and configure its subnet. If hte value is left blank it is automatically set to subnet 1. Set the subnet to 0 to leave it disconnected and ignored from Service AI analysis.
Note - for roads, if using corridors, set the subnet for the curb on the corridor region curb sub assembly extended data
Ponds can be set as a pit (in) and a sink (out).
Manholes can be set as a pin (in) and a sink (out)
🚨 Common Pit/Sink Mistakes
Sink without invert elevation
Service AI cannot calculate downstream grade.
Pits assigned to subnet 3, but no sink exists in subnet 3
Routing fails immediately.
Sink exists but is outside servicing extent
Catchments don’t reach it.
Everything left null
Everything defaults to subnet 1