Stream morphology and hydrology

Hydraulic geometry

B = aQb

h = cQf

U = kQm

Typical at a station values (midwestern streams)

b = 0.25

f = 0.4

m = 0.35

Typical downstream values

b = 0.5

f = 0.4

m = 0.1

Hydrology short overview

• Stream records

• Flow duration curves

• Flood frequency curves

• Importance of short-lived (<< 1 day) rare events (“flashiness”)

Example: Anoka discharge record

Flow duration curve

Note: daily Q values do not capture true maxima

For small basins (< 100 km^2), 15 min max Q can be ~10x annual daily max Q

Flow duration curve

Flood frequency (return period)

Note: Find annual max on water year basis

If gage data are not available…

• USGS regional curves: multiple regression of flood frequency on basin area, other variables

• Gage analysis: estimation based on discharge records from nearby basins, correcting for basin area

• Literature: “prediction in ungaged basins”

USGS regional curves

USGS regional curves

USGS regional curves

USGS regional curves

Qn = discharge with return interval n in years (cfs)

DAREA = drainage area (sq. miles)

SLOPE = channel slope (feet / mile)

LAKE = area covered by lakes (as % of drainage area, 0=1%)

ROFF = generalized mean annual runoff (in)

Minnesota Region D: Lorenz et al. USGS 2009-5250

Note: analogous reports also available for estimating low-flow statistics

Q1.5 = 0.892 DAREA0.778 SLOPE 0.422 (LAKE+1)-0.29 ROFF 1.144

Q2 = 1.58 DAREA0.785 SLOPE 0.436 (LAKE+1)-0.328 ROFF 1.011

Q5 = 4.62 DAREA0.795 SLOPE 0.453 (LAKE+1)-0.392 ROFF 0.78

Q10 = 7.98 DAREA0.799 SLOPE 0.456 (LAKE+1)-0.419 ROFF 0.671

Q50 = 19.9 DAREA0.803 SLOPE 0.455 (LAKE+1)-0.458 ROFF 0.507

Q500 = 48.9 DAREA0.803 SLOPE 0.449 (LAKE+1)-0.488 ROFF 0.376

Mean annual runoff

Estimating flow duration curve in ungaged basins

• Assuming there is a gaged basin nearby (similar climate, vegetation, rock type, etc):

• Plot Q/Q2 vs % time exceeded for the gaged basin

• Estimate Q2 for the ungaged stream assuming Q2 = bankfull Q

• Estimate other % time exceeded for a given Q by multiplying the Q/Q2 value from the gaged curve by the Q2 value for the ungaged stream

Example Q/Q2