Stream morphology and hydrology

Hydraulic geometry B = aQ b h = cQ f U = kQ m 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

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 Note: analogous reports also available for estimating low-flow statistics Q1.5 = DAREA SLOPE (LAKE+1) ROFF Q2 = 1.58 DAREA SLOPE (LAKE+1) ROFF Q5 = 4.62 DAREA SLOPE (LAKE+1) ROFF 0.78 Q10 = 7.98 DAREA SLOPE (LAKE+1) ROFF Q50 = 19.9 DAREA SLOPE (LAKE+1) ROFF Q500 = 48.9 DAREA SLOPE (LAKE+1) 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