Project file used by HydroCAD, a program that models stormwater activity for water drainage solutions; contains all essential data for the structure and setup of a HydroCAD project; stored in ASCII text format. HCP files are used with several other HydroCAD files that comprise a HydroCAD project, including .HCE, .HUH, .HCR, .HCI, .HCU, and .HCC files.