(A) General. Water surface profiles may be estimated using normal depth or backwater modeling methods, as appropriate. Both methods are discussed in the following sections.
(B) Normal depth.
(1) If the depth and direction of flow in an open channel are nearly constant with respect to time and space (i.e. steady, uniform flow conditions), the flow regime is said to be “normal”. Under such conditions, the hydraulic characteristics of a channel can be evaluated by using the Manning’s equation, which is described in such hydraulic texts as Open-Channel Hydraulics, by V.T. Chow (1959) and the Handbook of Hydraulics, by E.F. Brater and H.W. King (1982).
(2) When delineating natural floodplains using “normal depth” calculations, it is important to check that the energy grade line slopes continuously in the downhill direction. The energy grade line is defined as a line connecting points of known total head or total specific energy, H, as computed by:
H = Y + V2/2g (Eqn. 4.1)
Where:
H = Total specific energy, in feet.
Y = Depth of flow, in feet.
V = Average flow velocity, in feet per second.
g = Gravitational constant = 32.2 ft/sec2.
(3) In those cases where the slope of the energy grade line does not closely approximate the channel-bed slope, the assumption of uniform flow is not valid. In such instances, backwater calculations must be performed, rather than the much simpler analysis based on “normal depth” assumptions.
(C) Backwater modeling.
(1) If the assumptions of uniform-flow cannot be satisfied, then a water-surface profile will need to be developed using the Standard Step Backwater Method. This is especially true if the channel cross-section geometry changes significantly along the reach being analyzed or there are hydraulic structures or controls such as culverts, bridges, weirs, and/or flow contractions/expansions, which affect hydraulic conditions.
(2) The basic procedure for performing Standard Step Backwater calculations is presented in a number of readily available references including Open-Channel Hydraulics, by V.T. Chow (1959) and Applied Hydraulics in Engineering, by H. M. Morris and J. M. Wiggert (1972). However, due to the extensive calculations associated with performing Standard Step Backwater method, computer programs are usually employed for this purpose. The COE HEC-2 - Water Surface Profiles (1991) (HEC-2) or the COE HEC-RAS - River Analysis System (HEC-RAS) (2001) programs are the most widely used and accepted computer programs in use for this type of analysis. The most current version of the COE software shall be utilized for computer backwater modeling on projects that are to be reviewed by the town. Submittals of HEC-2 or HEC-RAS shall include detailed output files and computer diskettes or CD-ROMs containing input/output files, in conjunction with summary tables that present pertinent hydraulic parameters. In addition, cross-section plots for representative cross-sections and cross-sections at critical locations shall be included within the hydraulic reports.
(3) One advantage of using the Standard Step Method is that if the computation is started at an assumed elevation that is inaccurate for a given discharge, the resulting flow profile will converge to the correct flow depths with each succeeding cross-section evaluated along the reach. Therefore, if a starting water-surface elevation cannot be established at the beginning of the reach, an assumed elevation may be established far enough away from the “starting” cross-section to correct for any initial error.
(4) The step computations should proceed in an upstream direction for subcritical profiles and in the downstream direction for supercritical profiles. For flow regimes that contain both subcritical and supercritical flows, the computations should be performed both in the upstream direction and in the downstream direction and the appropriate water-surface elevations determined. The subcritical profile shall be used for 100-year floodplain delineations.
(Res. 1637, passed 2-28-02)