FLOOD05393

• County: Statewide
• Basin: Statewide
• Title: Flood-Hydrograph Analyses and Computations
• Date: 8/31/1959
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

e

e

e

EM 1110--2-1405
App IV
31 Aug 59

DEFINITION AND EXPLANATION OF TERi\Itl FOR PERTINENT DATA SHEET
(ENO. FOR:\I 1821)

Ohf;.{~r\-'ed Unit Hydrog-raphs'-Unit Hydrographs calcll-
latl'd for tht: indicated I-'torrns for the observed period of
duration of rainfall excess.
I)ata from {)bservcd Ullit JJydrographs:
Column l-lJatc of OCClUf('nCC of rainfall associatr:d with
the observpd unit hydrog;raph shown.
Column 2-0b.'>I'fvcd Hnit hydrog;raph graphical symbol
associated with storm dale.
Column :~--P=raillfu]1 cxpn:sscd as av('rag-e depth in
inches occurring: over contributing dmillagc
area. Computed by method of Thipsscn
Polygons and adjusted to conform with the
isohydal patt('rn,
Column 4--'{u=timc ill hOllrs from beginning to end of
cornputl'd, significant rainfall excess.
Column Er---Nc=thc rainfall quantity in inches on the
drainagp area available for surface run-off
Jtlillll~ computed losses, tind equal to the
surface run-off ddf'rmined from stream flow
at tbe stream gage.
Col1lmn t)-LrP=dh;t,ance in mi1{'~ measured along the
prillciple channel from the stream gage to the
computed center of mass of rainfall. L~p
j,..; computed as summation of products of the
t'stimatcd aVI'rage rainfall volume 011 each
Thi('sSPIl Polygon area, and its chaT!ncl diFl-
tance from th(~ stream gage, divided by the
total rainfall volume, giving C"onsidcration to
trihutary drainage divides. If Lcp is not
measured the storm c(~nter has b(~en in-
dicated as "Upstream" "Downstream" or
"Uniform."
Column 7-J3a:-;is of stream /1;age record: Continuous
stagl'-recorder record (Ree,), or manually
observed stage gage read once daily (1 D).
twice daily (2D), etc,

Column 8-QpR'=pcak value of observed ullit hydrograph
ill c.Ls.
Column 9-Qp=pl~ak value of unit hydrograph in c,f.s.
aftc'r adju:-;tment to specifiN1 unit rainfall-
exce:-;s duration.
Column lO-tpH=interval in hours from center of mass
(centroid) of rainfall excess to the time of
ob:;ern-d unit hydrograph peak.
Column 11~tp=illterval in hours from midpoint of unit
rainfall duration, t~, to time of peak of unit
hydro~raph correspondinK to specified "tr."
Column 12-t.=interval in hours from center of lnas>;
(centroid) of rainfall exces..<; to time 50 per-
cent of rUll-off has occurred.
C I 13 E ., I . C t.R
...0 umn - < mplnca coeffiCient, IR,= (LL".,) .3

Column 14-Empirical coefficient, C,,640= tpR.D~A~PR,

Column 15--K =thc ratio of change in channel storage to
change in discharge. It is computed from
the recession ~idc of the observed flood
hydro~raph.
K..=the minimum ratio which is normally
found at the point of inflection.

(Q,+Q,) I
K=,---"X--.
2 (Q,-Q,)

where

1= time interval in hou~ between Q, and Q2
in c.Ls.

Column 16--T~=the interval in hours from the end of
rainfall excess to the time of inflection of the
recession hydrograph.

57

PLATE NO, 17 (Sheet a of :l)