weir3a5.sharpcrest: Compute Open-Channel Flow over Broad-Crested Weir by TWRI3A5

Description

Compute open-channel flow (discharge) over a sharp-crested weir in general accordance with Hulsing (1967) [TWRI3A5]. 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can be bypassed by function arguments, although $B$ is used in the contraction ratio $b/B$ unless this ratio is superceded.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 weir3a5.sharpcrest() function, the computations are exclusively based on the foot-second unit system and careful adherance by the user is required as not all “coefficients” are dimensionless.

The discharge equation for an acceptable tail-water condition $h_t$ is $$Q = k_c k_t C b H^{1.5}$$ where $Q$ is discharge in cubic feet per second, $k_c$ is the contraction coefficient that also is a function of the abutment rounding $r$, $k_t$ is the submergence adjustment coefficient, $C$ is the discharge coefficient, $b$ is the width in feet of the weir crest, and $H$ is total free-flow head in feet on the weir assuming $h_t = 0$, 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computed by $v=Q/A$ for cross section area $A$ in square feet, which by default is computed by $A=(h + P)B$, but can be superceded.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 weir3a5.sharpcrest() 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 The function assumes rectangular approach conditions to compute approach area $A$ if not superceded by the optional A argument, which also can be a vector.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 weir3a5.sharpcrest() 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. The $b/B$ can be used to decrease discharge by decreasing $k_c$ 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 nomographs and lookup tables of TWRI3A5 for $k_c$, $k_t$, and $C$. In all cases, these coefficients can be superceded by user-specified scalars or vectors in various combinations.

Usage

weir3a5.sharpcrest(h, ht=NULL, b=NULL, B=NULL, P=NULL, L=NULL, r=0, A=NULL, alpha=1, slopeus="vertical", kc=NULL, kt=NULL, C=NULL, contractratio=NULL, extended=TRUE, header="", resetkts=TRUE, flowdigits=2, coedigits=3, verbose=FALSE, eps=0.001, maxit=20)

Arguments

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

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

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

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 requirement;

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

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

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

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;

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

Coefficient for submergence adjustment, if provided, supercedes nomograph lookup and interpolation by $H/P$ and $h_t/P$. Optionally, this coefficient may be a vector;

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

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;

resetkts

A logical controlling whether interpolated $k_t > 1$ values are reset to $k_t = 1$ and so diverges slightly from TWRI3A5 (fig.4);

flowdigits

The number of digits to report on flow, velocity head, total head, computed $h_t/H$, and computed $H/P$;

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

head: Echoed $h$ on the input in feet;
flow: Flow $k_t Q_H$ in cubic feet per second based on total head $H$;
delta: First order difference of $k_t Q_H$;
flowfree: Flow $Q_H$ in cubic feet per second using free flow conditions, $h_t = 0$;
flowo: Flow $Q_h$ in cubic feet per second using free flow conditions, $h_t = 0$ based on 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;
Hfree: Total head $H = h + v_o$ in feet;
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 $h/P$;
ht.over.H: Computed $h_t/H$;
H.over.P: Computed $H/P$;
C: Discharge coefficient $C$;
kc: Contraction coefficient $k_c$;
kt: Coefficient to adjust for submergence $k_t$. Note that interpolated values $k_t > 1$ are set to $k_t = 1$, if resetkts=TRUE, because weir3a5.sharpcrest() uses iteration to determine $H$. This practice diverges slightly from TWRI3A5 (fig.4);
message: Messages concerning the computation of $Q$ for each value of $h$; and
source: weir3a5.sharpcrest.

References

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/

Examples

Run this code


weir3a5.sharpcrest(0.45, L=0.125, P=0.32, b=5.81, B=5.81)


h <- seq(0.15,0.64,by=.01)
Qo <- weir3a5.sharpcrest(h, L=0.125, P=0.32, b=5.81, B=5.81)
print(Qo)

ht <- seq(0.15,0.64,by=.01)/2
weir3a5.sharpcrest(h, ht=ht, L=0.125, P=0.32, b=5.81, B=5.81)

plot(Qo$flow, Qo$head, type="l", log="xy")
Q <- weir3a5.sharpcrest(h, ht=0.21*h, L=0.125, P=0.32, b=5.81, B=5.81)
lines(Q$flow, Q$head, lty=2)
Q <- weir3a5.sharpcrest(h, ht=0.4*h, L=0.125, P=0.32, b=5.81, B=5.81)
lines(Q$flow, Q$head, lty=2)
Q <- weir3a5.sharpcrest(h, ht=0.6*h, L=0.125, P=0.32, b=5.81, B=5.81)
lines(Q$flow, Q$head, lty=2)
Q <- weir3a5.sharpcrest(h, ht=0.8*h, L=0.125, P=0.32, b=5.81, B=5.81)
lines(Q$flow, Q$head, lty=2)