weirbos.broadcrest {weirs}R Documentation

Compute Open-Channel Flow over Broad-Crested Weir by Bos (1989)

Description

Compute open-channel flow (discharge) over a broad-crested weir in accordance with Bos (1989) [BOS] with extension into the coefficients k_c, k_R, and k_s from Hulsing (1967). The weir crest of opening (width) b in feet is P feet above the channel bottom and L feet long in the flow direction. A rectangular approach channel is specified by width B, but the area of the channel (and hence rectangular assumption) can be bypassed by function arguments, although B is used in the contraction ratio b/B unless this ratio is superceded. For the weirbos.broadcrest() function, the computations are exclusively based on the foot-second unit system and careful adherance by the user is required.

The discharge equation for an acceptable tail-water condition h_t is

Q = k_c k_R k_s C_v C b \frac{2}{3} \sqrt{\frac{2}{3}g} H^{1.5}

where Q is discharge in cubic feet per second, k_c is the contraction coefficient that also is a function of the abutment rounding r, k_R is the approach rounding coefficient that is a function of the approach crest rounding R, k_s is the downstream embankment slope coefficient, C_v is the approach velocity coefficient, C is the discharge coefficient, b is the width in feet of the weir crest, and H is total head in feet on the weir, which is computed by

H = h + v_o = h + \alpha v^2/2g

where h is static head in feet on the weir, v_o is velocity head in feet in the approach section, v is mean velocity in feet per second in the section computed by v=Q/A for cross section area A in square feet, which by default is computed by A=(h + P)B, but can be superceded. The quantity g is the acceleration of gravity and is hardwired to 32.2 feet per square second. The dimensionless quantity \alpha permits accommodation of a velocity head correction that is often attributable to cross section subdivision. The \alpha is outside the scope of this documentation, is almost always \alpha=1, and is made available as an argument for advanced users.

The discharge equation is solved for two conditions that consider the approach velocity head. First, the equation is solved as shown above for H through iteration and C_v=1. Second, the equation is solved using h and C_v, which is determined by nomograph lookup.

The weirbos.broadcrest() function is vectorized meaning that optional vectors of h can be specified along with an optional and equal length vector h_t. The function assumes rectangular approach conditions to compute approach area A if not superceded by the optional A argument, which also can be a vector.

The weirbos.broadcrest() function also permits optional vectors of L and b/B (by the argument contractratio) so that tuning of the weir-computed discharge to a measured discharge potentially can be made. The crest length L can be used to increase discharge slightly by shortening in say the circumstances of a slightly downward sloping crest. (Such potential “crest” sloping is distinct from the downstream embankment slope—do not confuse the two.) The b/B can be used to decrease discharge by decreasing k_R in say the circumstance of an inlet that is rougher or has asperities that slightly increase the expected contraction and reduce flow efficiency. To clarify, the fact that L and b/B can be vectorized as optional arguments shows a mechanism by which tuning of the computational results to measured Q values can occur without replacing the fundamental C and C_v nomographs of BOS and nomograph and lookup tables for k_c, k_R, and k_s of TWRI3A5 (Hulsing, 1967). In all cases, these coefficients can be superceded by user-specified scalars or vectors in various combinations.

Usage

weirbos.broadcrest(h, ht=NULL, b=NULL, B=NULL, P=NULL, L=NULL,
                   R=0, r=0, A=NULL, alpha=1,
                   slopeus="vertical", slopeds="vertical",
                   kc=NULL, kr=NULL, ks=NULL, C=NULL,
                   contractratio=NULL,
                   hhptest=TRUE, extended=TRUE,
                   header="",
                   flowdigits=2, coedigits=3,
                   verbose=FALSE, eps=0.001, maxit=20)

Arguments

h

Mandatory scalar or vector of static heads h in feet on the weir;

ht

Optional scalar or vector of tail water heads h_t in feet on the weir;

b

Mandatory scalar width of weir crest b in feet normal to flow;

B

Mandatory scalar width (or top width) of approach channel B in feet. Technically, it is possible with arguments contractratio and A to bypass any computations normally involving B. This would be the mechanism to bypass the B as a scalar restriction;

P

Mandatory scalar height of weir crest P in feet above channel bed;

L

Optional scalar or vector of lengths L in feet of broad-crested weir in direction of flow;

R

Optional scalar radius of curvature R in feet of vertical upstream face;

r

Optional scalar radius of curvature r in feet on the vertical abutments at inlet of weir crest;

A

Optional scalar or vector of approach cross-section area A in square feet for each h that supersedes the rectangular channel computation A=(h+P)B;

alpha

Optional scalar or vector of velocity head correction term \alpha dimensionless. The default is unity (\alpha=1), which is most certainly appropriate for the vast majority of weir computations;

slopeus

String signifying the approach embankment slope in the format “hz:vt”, thus, slope is defined as the ratio of the horizontal hz to vertical distance vt. (This is opposite of the more common convention for the trigometric function tan().) The string “vertical” must be provided as the value for slopeus for vertical slopes;

slopeds

String signifying the downstream embankment slope in the format “hz:vt”, thus, slope is defined as the ratio of the horizontal hz to vertical distance vt. (This is opposite of the more common convention for the trigometric function tan().) The string “vertical” must be provided as the value for slopeds for vertical slopes;

kc

Contraction coefficient k_c, if provided, supercedes nomograph lookup and interpolation by h/P and b/B. Optionally, this coefficient may be a vector;

kr

Rounding coefficient k_R, if provided, supercedes tabular lookup and interpolation by R/h. Optionally, this coefficient may be a vector;

ks

Downstream embankment slope coefficient k_s, if provided supercedes tabular lookup and interpolation by h/L and downstream slope slopeds. Optionally, this coefficient may be a vector;

C

Discharge coefficient, if provided, supercedes nomograph lookup and interpolation by h/L and slopeus. Optionally, this coefficient may be a vector;

hhptest

Logical for test of h/(h+P) \le 0.35 to follow Bos (1989) to determine C, but the test can be ignored with this logical argument;

contractratio

Optional vector of user specified contraction ratios, if provided, supercedes use of b/B. For example, b.over.B[i] <- contractratio[i];

extended

A logical that controls the contents of the data frame on return;

header

A string (usually) or any other content to add to the attributes() of the returned data frame under the non-original label name of header;

flowdigits

The number of digits to report on flow, velocity head, and total head;

coedigits

The number of digits to report on weir coefficients;

verbose

A logical controlling intermediate messages. This might be reserved for development work and no verbose output in a released version of weirs could occur;

eps

An absolute error of discharge for convergence in cubic feet per second; and

maxit

Maximum number of iterations for the computation of the total head from summation of static and velocity head H = h + \alpha v^2/2g for the final Q_H in item flow of the returned data frame.

Value

An R data.frame() is returned and the extended=TRUE version is described below:

head

Echoed h on the input in feet;

flow

Flow Q_H in cubic feet per second based on total head H;

delta

First order difference of Q_H;

flowo

Flow Q_h in cubic feet per second based on static head h;

flowcv

Flow Q_{cv} in cubic feet per second based on C_v \ne 1 nomograph lookup by h/L and static head h;

error

Absolute convergence error \epsilon of Q_H in cubic feet per second;

velhead

Velocity head v_o = v^2/2g = (Q_H/A)^2/2g in feet;

H

Total head H = h + v_o;

ht

Echoed h_t on the input in feet;

L

Echoed L in feet;

b.over.B

Echoed b/B;

h.over.L

Echoed h/L;

h.over.P

Echoed h/P;

C

Discharge coefficient C;

Cv

Approach velocity coefficient;

kc

Contraction coefficient k_c;

kr

Rounding coefficient k_R;

ks

Downstream slope coefficient k_s;

message

Messages concerning the computation of Q for each value of h; and

source

weirbos.broadcrest.

The extended=FALSE version is restricted to the most salient items including Q_H, Q_h, Q_{cv}, v_o, C, C_v, k_c, k_R, and k_s.

Note

The weirbos.broadcrest() function will stop() under conditions of unspecified or implausible L, B, and P as well as incompatibility of b and B, such as B<b. This function will also stop() if the length of the vector arguments or optional vector arguments do not match the length of h. The only exception is that if h_t is not specified, then internally it is treated a vector of length h having values of zero. There are other conditions that will cause the function to stop and consultation of the if() statements at the beginning of the function is recommended.

When the weirbos.broadcrest() function encounters non-stopping errors or warnings, it silently continues with error reporting in the message item in the returned data frame. This behavior is considered a feature and necessary to support the return of the data frame. The message states are:

  1. If h_t is too large, then submergence is assumed and NA is returned for all items. The evaluation of submergence is made if h_t/h \ge 0.66;

  2. If h/(h+P) \le 0.35 then C can be determined and computation progress, otherwise NA is returned for all items;

  3. If h is zero, then zero is returned for Q_H, Q_h, Q_{cv}, \epsilon, and v_o and NA is returned for others;

  4. If a given h/L tests as too high (h/L < 1.5) for broad-crested weir flow and hence the weir is functioning as sharp-crested, then NA is returned for all items. Weirs with h/L < 0.08 are treated as low head, then NA is returned for all items;

  5. If the contraction ratio b/B is too small (b/B < 0.20), then too much contraction is concluded and NA is returned for all items;

  6. If the downstream embankment slope is too shallow (>5, then k_s is indeterminant and only the values for C, k_c, and k_R are returned;

  7. If nonconvergence occurs or estimated Q goes to infinity (supercritical approach or choking), then NA is returned for all Q, \epsilon, and v_o, but the estimated C, k_c, k_R, and k_s are returned, and;

  8. If no problems were detected, then ok is the message.

The influence of abutment rounding by the ratio r/b > 0 on k_c is accommodated by prorating between (1) k_c from h/P and b/B or user-specified k_c and (2) k_c = 1 unless r/b > 0.10 for which k_c = 1.

Nomograph and tabular lookup and interpolation is made throughout the computations. The linear interpolating approx() function is used for all interpolation. Most commonly, a form of bilinear interpolation is made. First, the two bounding curves for a given condition are interpolated in the horizontal direction and then the resulting two values are interpolated in the vertical. The horizontal interpolation by approx() explicitly uses the rule=2, which means that extrapolation to the left and right using the respective end point is made. In other words, the nomographs (and tables) are flat lined when extrapolation is needed. Within the code, the horizontal interpolations can be identified by rule=2 and the vertical interpolations lack the rule argument. Finally, the nomographs are in the hashed environment .weir.nomographs, which sources from the file ‘sysdata.rda’ of the package. The file ‘./inst/Nomographs4R/nomographs.R’ is used to create the ‘sysdata.rda’ file.

Author(s)

W. Asquith with digitizing of nomograph contributions by W. Miller

References

Bos, M.G., 1989, Discharge measurement structures: International Institute for Land Reclamation and Improvement Publication 20, Wageningen, The Netherlands, 401 p. http://content.alterra.wur.nl/Internet/webdocs/ilri-publicaties/publicaties/Pub20/pub20.pdf

Hulsing, Harry, 1967, Measurement of peak discharge at dams by indirect methods: U.S. Geological Survey Techniques of Water-Resources Investigations, Book 3, Chapter A5, 29 p., http://pubs.usgs.gov/twri/twri3-a5/

See Also

weir.broadcrest

Examples


# Simple, scalar inputs and results
weir.broadcrest(type="BOS", 0.5, b=8, B=11, P=6, L=3.25);

# Vector of heads
weir.broadcrest(type="BOS", c(0.5,0.4,0.3), b=8, B=11, P=6, L=3.25);

# Now compare two solutions by weirbos.broadcrest() to weir3a5.broadcrest()
h <- seq(0.01,1.5, by=0.001)
Qtank     <- weir3a5.broadcrest(h, b=7.97, B=10.97, P=1.125, L=3.76, flowdigits=4)
QtankbosA <- weirbos.broadcrest(h, b=7.97, B=10.97, P=1.125, L=3.76, flowdigits=4)
QtankbosB <- weirbos.broadcrest(h, b=7.97, B=10.97, P=1.125, L=3.76,
                                   hhptest=FALSE, flowdigits=4)
plot(Qtank$flow, h, type="l", col=8, lwd=6,
     xlab="FLOW, CFS", ylab="HEAD, FEET")
lines(QtankbosA$flow, h, col=2, lwd=4) # see the trunction in the red line
lines(QtankbosB$flow, h, col=3, lwd=2)

# See examples for weir3a5.broadcrest() for additional examples that
# explore higher complexity of argument settings, which test the
# nomographs for k_c, k_R, and k_s
[Package weirs version 0.25 Index]