Streamflow and Runoff The character, amount, and timing of discharge from a basin tells a lot about flow paths within the basin Therefore, important to hydrogeologist to identify how to create stream hydrographs from discharge measurements Measurement of discharge :
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The character, amount, and timing of discharge from a basin tells a lot about flow paths within the basin
Therefore, important to hydrogeologist to identify how to create stream hydrographs from discharge measurements
Measurement of discharge:
Method used depends on type of study, size of river and flow, data requirements, etc.
For small scale field studies discharge is measured over a crosssection at the point of interest using:
 time surface float from point a to point b
 average velocity is 0.85*surface velocity
 mean velocity*xsectional area = discharge
 measurement distance should be ~10x stream width
 very rough estimate
b) stream gage
 average velocity in a stream occurs at 0.368*depth above the bottom
 therefore standard to measure along a crosssection at 0.4 depth off bottom
 for more accurate estimates, averaging and depth measurements can be made..
 good method for turbulent streams
 need to use tracer that is a) easily soluble, b) have no or very low natural concentrations in stream, c) be conservative, d) easily detectable at low concentrations, e) ecofriendly, f) affordable
 sodium chloride (NaCl) often used, and concentration/conductivity relation established
 by introducing tracer upstream and measuring rate of concentration change downstream we can calculate discharge for the stream reach
for a gulp injection
where CT is the initial tracer concentration being introduced
Cb is the background concentration in the stream
and the integral in the denominator is evaluated graphically from a C vs. t plot..
 can be used in small streams where it is possible to focus all flow into a collector
More “Permanent” installations can be made using:
a)
 Dam (plate) with a vnotch (typically) that allows for controlled discharge through the notch.
 discharge can be related to the height of the backwater
 problems with sedimentation
Q = 3.33(L0.2H)H3/2
90o Vnotch
Where: Q is discharge (cfs)
L is length of weir crest (ft)
H is head of backwater above crest (ft)
See text for metric equations
 artificial stream reaches that conduct flow through through a constricted cross section that has a fixed stagedischarge relation (rating curve)
 height measured in flume relates directly to discharge
 sedimentation problems minimal

often useful to calibrate with stream gaging over a range of flows (both weirs and flumes). Rating curve
c) stage measurement at natural crosssections
establish rating curve for natural stream crosssection
 use pressure transducer or float in stilling well to continuously measure stage (and therefore discharge)
 best sites are:
a) more still area just up from fast water reach
b) area that is geomorphologically stable
Often in an ungaged area we are interested in assessing the discharge of a recent flood event, or we need discharge where measurements of velocity can’t be made
 If evidence of the highest stage is marked on the stream banks can be approximated by measuring two crosssections (more can be measured) numbered i=1 for the upstream section and i=2 for the downstream section
K is called the conveyance for each cross section
g is gravitational acceleration
uc is a unit conversion factor ( )
Yi is the average depth for each section
Ai is the crosssectional area for each section
Xi is the xsectional water surface width
C is called Chézy’s C and is a factor describing the crosssection’s resistance to flow
Um is another unitconversion factor
n is “Manning’s Roughness Coefficient” (values from tables), generally between 0.00.2
ΔZ = diff. Between high water marks of two sections
L is distance between two sections
k is an eddy coefficient (if downstream xsect area is bigger than upstream k = 0.5, k=0 is opposite is true
This method gives values within 1020% of reality
Manning Equation crosssection’s resistance to flow
 can be used for past flows or flows under conditions where direct measurements are impossible
Where R is the hydraulic radius (ratio of xsectional area/wetted perimeter)(wetted perimeter is the crosssectional length of the stream bed), S is the slope of the channel (rise over run, so head gradient). Velocity is in meters/sec
Wetted perimeter is a+b+c
c
a
b
Hydrograph can be discharge over any period of time, but usually we analyze hydrographs before during and after specific precipitation events
Rising portion of a hydrograph is known as the concentration curve
Region in the vicinity of the peak is called the crest segment
Falling portion is the
The shape of individual hydrographs will depend on the precipitation pattern characteristics and the basin properties
Hydrographs can be separated into two major components: quickflow (or event flow, storm runoff, direct runoff) and baseflow..