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

EM 1110-2-(405
App IV
31 ..\ug 59

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EXPLANATION OF ITEMS ON UNIT HYDROGRAPH BASIC DATA SHEET

(1) Plot hyetographs, that is, average rainfall rate in time hlo(~ks for the watershed, and for zones of the
wat(,fshcd if rainfall is non-uniform. Indicate hy solid shading the rainfall exc<,ss for each period.
Show assuffiN} ullit duration of rainfall excess. Auxiliary tabulated data for t.he watl~rsll('d and
zon('s should indudc tit(' area, average prpcipitation, and rainfall ('xc<"ss.
(2) Plot hydrogmphs of total observed flow, surface runoff, unit hydrograph, and base flow on the same
time seale as thl' hyetographs. Each hydrograph should be lahel('d. Auxiliary tabubit('d data
should inchHh~ OJ(' lag, the p<,ak di~chargc of observed hydrogrllph, volumt' of direet"runofT, and
assuffiPd timl' of beginning of r~infall ('xct'ss.
(;~) Plot cumulative curves of rainfall at all or a f('pr('sC"ntativp numlwf of rain gages in and adjae<"nt
to tlH' wat('islH'cI. Recording records can be plotted directly, and the IIlass eurv('s for non-f('cording
gag('s C'stimah'd from the daily amounts and tlH\ shap(' of tho n'cording g~e mass curves.
(4) R('cord }wre any unusual features of the analysis or stfC'am flow and pfPcipitatioll fecords, sueh as
large extensions of diseharge rating curv(', partial nocord, etc. State type of stn'am gage records
on which obs('rved hydrograph is haspel.
(5) Pres('nt a Inap of the waU'rslll'd to a suitable scal('. Locate the various items giv('Jl in the It'gend
using symbols indicated.
(6) Pr~parl' titlc block in ac~ordance with agency requir~m('nts.
(7-8) Self-explanatory.
(9) Office of the agency making unit hydrograph d~termination.
(10) Area us(.d in eomputing inches of direct runoff.
(II) Length along longest watercourse from gaging station to head of stream.
(12) Lengt.h along wat('rcoursc from gaging station to ccntt'f of areR.
(l:l) Self-,'xplanatory.
(14) Average depth of rainfall occurring over thc waterslU'd during thc storm associakd with the
hydrograph und~r analysis.
(I5) Observed duration of the rainfall excess. a
(16) Average depth of direct runoff over thc wat~rshed (rainfall excess). ,.,
(17) P~ak discharge of unit hydrograph (for observed duration).
(18) Peak discharge of unit hydrograph in cfs per square mill' (for obs~rved duration).
(19) Peak discharge of unit hydrograph (adjusted to common duration).
(20) Time from center of rainfall cxeess to peak dischargc (for observcd duration of storm).
(21) Timc from ccnter of rainfall cxc~ss to peak dischargc (unit hydrograph adjustcd to common
duration) .
(22) Time from center of rainfall exe-css until 50 percent of runoff volume has occurrcd.

(23) Snyder Coefficient. C'R- (Ll:)"

(24) Snyd~r Coefficicnt. C.640=q'~PR
(i!5) Intervals of time so seleCted that discharges will adcquately dcfine tlw hydrograph. Time intervals
may be uniform or may be varied.
(26) Total str('smflow including base flow, recession from previous storm, and dinoct runoff from storm
under study, but excluding subsequent runoff. If valu('s shown arc meant for p('riod, so indicate
in column .heading with proper adjustment also to Column 25.
(27) Includes base flow and recession from previous storm.
(28) Column (26) minus column (27).
(29) Values in column (28) adjusted to I inch of runoff. Ins,'rt obserV<'d duration of rainfall excess in
column heading.
(30) Observcd unit hydrograph reduced to duration spl'cified in "olumn headin~.
(31) Rcconstitution of original total hydrograph usin~ unit hydr()~rap", rainfall excess and base flow.
(To be filled in where duration of adjusted unit hydrograph differs substantially from duration
of ob""rved storm.)
(32-33) Leave blank.

I'LATg NO. 18 (SI"'et :l of :l)

lJ. S. GOVU1~,\]J;XT PIHNTlNG OFFICE: 1970 0 - 377-564

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