As indicated on page 112 of the Planning Manual, constraints are basically a restriction to planning based on limiting resources, laws and regulations, and the need to not do harm to a system. A good plan will avoid undesirable consequences of any actions.
To help in thinking about this part of Project Development, consider the following:
Availability of time, people, money, data, GIS capabilities
Form and degree of public interest and support or opposition
Views of the other governmental agencies in the area
Conflicting ideas on what the future conditions of the area should be
Existing physical structures, e.g., highways, dense housing
Existing administrative structures, e.g., land set-asides, Advanced Identification and Special Area Management Plans
Planned physical and administrative structures, e.g., population growth areas
Legal and regulatory provisions, e.g., the Endangered Species Act, the Clean Water Act
Presence of significant natural and cultural resources that could be affected
Possible disturbances to the system, such as hurricanes, changes upriver
Any other red flags