Tuflow and estry manual Version 3

3.14.22D Flow Constriction (FC) Attributes

Flow constriction details are entered using either the traditional fixed field text line entries, using MapInfo tables or as a combination of these two methods. These are presented in Table 4 .15. The information required for fixed field input are shown in grey. An example of how to apply 2D FCs to a bridge structure is shown in Figure 4 .7
Table 4.15 Flow Constriction (FC) Attribute Descriptions

GIS Attribute	Description	Cols in Text File	Type
N/A	Flag Identifier “FC”	01-02	T
type	Secondary flag identifier where: Blank for general (does not include allowances for any vertical walls or friction from underside of deck. “BC” for Box Culverts (Note: At this stage, BC only available if Manning’s n bed resistance option specified.) “BD” for Bridge Deck “FD” for Floating bridge Deck	06-07	T
N/A	N grid coord	11-15	I
N/A	M grid coord	16-20	I
N/A	Second N grid coord (See Note below)	21-25	I
N/A	Second M grid coord (See Note below)	26-30	I
invert	Invert of constriction (m above datum). Mandatory for box culverts (type = “BC”). If not a box culvert, and you wish to leave the Zpt levels unchanged (ie. no invert constriction), enter a value greater than the obvert level (see below).	31-40	N
obvert_or_BC_height	type = blank or “BD”: Obvert of constriction (m above datum) type = “BC”: Height of box culvert (m). Values less than 0.01 are set to 0.01. type = “FD”: Floating depth (m) of the deck (ie. depth below the water line). Build 2004-04-AD. Enter a sufficiently high value (eg. 99999) if there is no obvert constriction.	41-50	N
u_width_factor	Flow width constriction factor in the X-direction (ie. the flow width perpendicular to the X-direction). For example, a value of 0.6 sets the flow width at the left hand and right hand sides of the cell to 60% of the cell width. Values less than 0.001 are set to 1. Use a value of 1.0 to leave the flow width unchanged. Values greater than 1 can be specified.	51-60	N
v_width_factor	Width constriction factor in the Y-direction. See description above for u_width_factor.	61-70	N
add_form_loss	Form loss coefficient. Used for modelling fine-scale contraction/expansion losses (eg. bridge pier losses, vena-contracta losses, etc) not picked up by the change in the 2D domain’s velocity patterns. Can be used as a calibration parameter. The form loss coefficient is applied as an energy loss based on the dynamic head equation below where is the add_form_loss value. The form loss coefficient is applied 50/50 to the right and left sides (u-points) of the cell, and similarly to the v-points.	71-80	N
Mannings_n	Manning’s n value. For box culverts (BC), the Manning’s n of the culverts (typically 0.011 to 0.015) should be specified. This overwrites any previously specified Manning’s n values at the cell. If set to less than 0.001, a default value of 0.013 is used. For bridge decks (BD and FD), the percentage contribution to the bed resistance by the deck’s underside is set equal to “fc_n”/“Bed_n”/2. Ignored for “Blank” type FC’s.	81-90	N
no_walls_or_neg_width	Number of vertical walls per grid cell, or, if a negative number, the width of one culvert. Applicable to Box Culverts only. Not used by other types of FCs.	91-100	N
blocked_sides	Indicates whether any of the walls of the constricted cell(s) are blocked off (ie. no flow across/through the side wall). Specify one or more of the following letters in any order with in the field to indicated which wall(s) are blocked: “R” – block right hand side wall “L” – block left hand side wall “T” – block top side wall “B” – block bottom side wall Note: the quotes should be omitted.	101-110	T
invert_2	leave blank (not used as yet)	111-120	T
obvert_2	leave blank (not used as yet)	121-130	T
Comment	General comment or note for own use – not used.	n/a	T



Figure 4.7 Setting FC Parameters for a Bridge Structure

3.14.32D Upstream Controlled Flow
(Weirs and Supercritical Flow)

Where flow in the 2D domain becomes upstream controlled, TUFLOW automatically switches between either weir flow and/or upstream controlled friction flow.

If Supercritical is set to ON (the default as of Build 2002-11-AD) the following rules apply. Note: the bed slope at ZU and ZV points is determined as the slope from the upstream ZC point to the ZU or ZV point in the direction of positive flow.

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  Where the bed slope at a ZU or ZV point is in the same direction as the water surface slope, tests are carried out to determine whether the flow is upstream controlled or downstream controlled. The adopted flow regime is determined by comparing the upstream and downstream controlled regime flows (preference to the lower flow) and whether the Froude No exceeds 1 (unless changed by Froude Check). The equation used for upstream controlled flow is the Manning equation with the water surface slope set to the bed slope. The Froude No check was introduced at Build 2002-11-AD – models using upstream controlled flow switch prior to this build can use the “PRE 2002-11-AD” switch for Supercritical. It is recommended that the Froude No check be used (which is the default setting from Build 2002-11-AD onwards) as it provides more accurate switching. A further check was incorporated in Build 2003 01 AF that phases out the Froude Check as the water surface approaches the horizontal (otherwise in some situations, the flow would remain in the upstream controlled regime). This check can be disabled for backward compatibility using Froude Depth Adjustment.
  
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  Weir flow only occurs if the bed slope is adverse (different direction) to the water surface slope. Weir flow across 2D cell sides is modelled by first testing whether the flow is upstream or downstream controlled. If upstream controlled, the broad-crested weir flow equation is used to replace the calculations for downstream controlled (sub-critical) flow conditions. Weir flow maybe switched off using the Free Overfall options.
  

TUFLOW produces an increase in water level at transitions from supercritical flow to subcritical flow as occurs with a hydraulic jump. It does not, however, model the complex 3D flow patterns that occur at a hydraulic jump, as it uses a 2D horizontal plane solution. Results in areas of transition should be interpreted with caution. It is also important to be careful presenting results in areas of supercritical flow as complex flows (such as surcharging against a house) may occur that would yield higher localised water levels – it is good practice to also view the energy levels when providing advice on flood planning levels.

If Supercritical is set to OFF, and Free Overfall is set to ON (the default), weir flow may occur on both adverse and normal bed slopes.

The weir flow switch may be varied spatially over the grid by setting a weir factor of zero where there is to be no automatic weir flow using Read MI WrF. The weir factor also allows calibration or adjustment where the broad-crested weir equation is applied. The broad-crested weir equation is divided by the weir factor. Therefore, a factor of 1.0 represents no adjustment, while a factor greater than one will decrease the flow efficiency. Note: the weir factor is not the broad-crested weir coefficient. For further information, refer to Syme 2001b.