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This web page addresses only software programs referenced in the Surface Water Design Manual (SWDM).

Important Note:

As of the 2016 SWDM, King County Runoff Time Series (KCRTS) is no longer an approved model. KCRTS may only be used for projects vested to earlier versions; it is required methodology projects vested under the 1998, 2005, and 2009 SWDM versions, unless otherwise approved to use WWHM 2012 or MGS Flood.

Approved models for the 2016 SWDM are WWHM 2012 (Western Washington Hydrology Model 2012 from the Washington State Department of Ecology) and MGS Flood (MGS Engineering Consultants, Inc.). See Chapter 3, Section 3.2 for more information regarding the approved models for the 2016 SWDM.

For approved models for the 2021 SWDM see Reference 6-D: Supplemental Modeling Guidelines. Any changes in approved models will be posted on the most recent SWDM web page.

At this time, the King County Backwater (KCBW) program may be used for any version of the SWDM.

WKCRTS Hydrologic Model Program

King County Runoff Time Series (KCRTS) Windows (WKCRTS) version 6.0.

For sizing detention and infiltration flow control facilities.

The WKCRTS program is bundled with KCBW in the zip file that can be downloaded from the link below. Documentation is included in the zip file.

WKCBW Hydraulic Model Program

King County Backwater (KCBW) Windows (WKCBW) version 6.0.

A standard step backwater analysis program for pipe networks, culverts, and simple open channels. 

The WKCBW program is bundled with WKCRTS in the zip file that can be downloaded from the link below. Documentation is included in the zip file.

WKCRTS and WKCBW software package

WKCRTS and WKCBW Version 6.0. After downloading, before installing the software open the zip file and read the "! Read First ! KCRTS & KCBW" PDF file.

Pond volume calculation spreadsheet

This Excel spreadsheet can be used to estimate pond volumes for typical land cover conversions on till soils only.


King County provides no warranty of any kind as to the accuracy of this spreadsheet on specific projects. All facility volumes should be verified using WKCRTS, WWHM 2012, or MGS Flood, whichever is the approved model for the project.


SeaTac and Landsburg runoff data files for WKCRTS and WKCBW are included in the software package zip file. Both WWHM 2012 and MGS Flood likewise include runoff data files. Consequently no separate data files are posted for download.

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