The Full Backwater Analysis is used to compute a simple backwater profile (hydraulic grade line) for a proposed or existing conveyance system for the purposes of verifying capacity.
The Full Backwater Analysis begins at the downstream end of the conveyance system, when the analysis reaches a trunk main, or at the last downstream maintenance hole of a ¼ mile analysis and is computed back through each upstream pipe segment and structure.
For discharges to tidally influenced areas, the tailwater elevation shall not be lower than 4.64 feet (which is a conservative estimate of the mean high tide) using current City of Tacoma datum.
3.4.1 Full Backwater Analysis Design Criteria
Conveyance systems shall be modeled as if no onsite detention is provided upstream.
Conveyance systems shall be analyzed for fully developed conditions. The fully developed conditions shall be derived from the following percentages of impervious area:
In commercial areas, the percent impervious shall be 85%.
In industrial areas, the percent impervious shall be 70%.
In residential areas, the percent impervious shall be 60%.
The fully developed conditions shall apply to both the offsite and onsite basins.
For discharges to tidally influenced areas, the tailwater elevation shall not be lower than 4.64 feet (which is a conservative estimate of the mean high tide) using current City of Tacoma datum.
3.4.2 Pipe System Design Events
All new conveyance systems shall be designed using the Full Backwater Analysis. The design events for pipe systems are as follows.
For privately maintained systems:
For the 10-year, 24-hour design storm, assuming a Type 1A rainfall distribution (3.0-inches) using a 10-minute timestep, there shall be a minimum of 0.5 feet of freeboard between the water surface and the top of any maintenance hole or catch basin.
For the 100-year, 24-hour design storm, assuming a Type 1A rainfall distribution (4.1-inches) using a 10-minute timestep, overtopping of the pipe conveyance system may occur, however, the additional flow shall not extend beyond half the lane width of the outside lane of the traveled way and shall not exceed 4 inches in depth at its deepest point.
For the 100-year, 24-hour design storm assuming a Type 1A rainfall distribution (4.1-inches) using a 10-minute timestep, off-channel storage on private property is allowed with recording of the proper easements. When this occurs, the additional flow over the ground surface is analyzed using the methods for open channels described in Section 3.4.4 Open Channel Design Events. Per Joint Administrative Policy and Procedure Directive No. 2021-02-001, Environmental Services/Site Development Group will evaluate and determine the acceptability of this type of localized flooding.
For publicly maintained systems:
For the 25-year, 24-hour design storm, assuming a Type 1A rainfall distribution (3.5-inches) using a 10-minute timestep, there shall be a minimum of 0.5 feet of freeboard between the water surface and the top of any maintenance hole or catch basin.
For the 100-year, 24-hour design storm, assuming a Type 1A rainfall distribution (4.1-inches) using a 10-minute timestep, overtopping of the pipe conveyance system may occur, however, the additional flow shall not extend beyond half the lane width of the outside lane of the traveled way and shall not exceed 4 inches in depth at its deepest point.
The starting tailwater elevation to be used in the backwater analysis for pipe systems is the water surface elevation of the next downstream pipe at an assumed depth of 90% full.
For discharges to tidally influenced areas, the tailwater elevation shall not be lower than 4.64 feet (which is a conservative estimate of the mean high tide) using current City of Tacoma datum.
For the 100-year, 24-hour design storm assuming a Type 1A rainfall distribution (4.1-inches), if overtopping of the system occurs, the applicant shall show the extent of the impacts on neighboring properties and the right-of-way. The full extent of flooding shall be shown. The applicant may be required to provide mitigation for localized flooding. Per Joint Administrative Policy and Procedure Directive No. 2021-02-001, Environmental Services/ Site Development Group reserves the right to make the determination of final mitigation requirements.
3.4.3 Culvert Design Events
All new conveyance systems shall be designed using the backwater analysis. The design events for culverts systems are as follows.
Culverts shall convey the peak volumetric flowrate calculated using a 10-minute timestep from a Type 1A, 25-year, 24-hour frequency storm (3.5 inches) using a single event model, without submerging the culvert inlet.
For culverts 18-inch diameter or less, the maximum allowable headwater elevation for the 100-year, 24-hour design storm, assuming a Type 1A rainfall distribution (4.1-inches) using a 10-minute timestep, (measured from the inlet invert) shall not exceed 2 times the pipe diameter or arch-culvert-rise.
For culverts larger than 18-inch diameter, the maximum allowable headwater elevation for the 100-year, 24-hour design storm, assuming a Type 1A rainfall distribution (4.1-inches), (measured from the inlet invert) using a 10-minute timestep shall not exceed 1.5 times the pipe diameter or arch-culvert-rise.
The maximum headwater elevation at the 100-year, 24-hour design flow, assuming a Type 1A rainfall distribution (4.1-inches) using a 10-minute timestep, shall be below any road or parking lot subgrade except as allowed per BMP F140: Parking Lots for Additional Detention.
The starting tailwater elevation to be used in the backwater analysis for culverts systems is the water surface elevation of the next downstream culvert at an assumed depth of 100% full.
For discharges to tidally influenced areas, the tailwater elevation shall not be lower than 4.64 feet (which is a conservative estimate of the mean high tide) using current City of Tacoma datum.
3.4.4 Open Channel Design Events
All new conveyance systems shall be designed using the backwater analysis. The design event for open channels is as follows:
Constructed and natural channels shall contain the 100-year, 24-hour storm event, assuming a Type 1A rainfall distribution (4.1-inches) using a 10-minute timestep.
A minimum of 6 inches of freeboard is required for all open channels.
The Direct Step Backwater Method can be used to compute backwater profiles on prismatic channel reaches (i.e. reaches having uniform cross section and slope) where a backwater condition or restriction to normal flow is known to exist. The method can be applied to a series of prismatic channel reaches in succession beginning at the downstream end of the channel and computing the profile upstream.
Calculating the coordinates of the water surface profile using this method is an iterative process achieved by choosing a range of flow depths, beginning at the downstream end, and proceeding incrementally up to the point of interest or to the point of normal flow depth. This is best accomplished by the use of a table or computer programs. Provide documentation of the equations and assumptions used in the analysis.
The Standard Step Backwater Method is a variation of the Direct Step Backwater Method and can be used to compute backwater profiles on both prismatic and non-prismatic channels. In this method, stations are established along the channel where cross section data is known or has been determined through field survey. The computation is carried out in steps from station to station rather than throughout a given channel reach as is done in the Direct Step method. As a result, the analysis involves significantly more trial-and-error calculation in order to determine the flow depth at each station.