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COMET University Faculty Hydrometeorology Course June 2000. Dennis L. Johnson. Dennis L. Johnson, Asst. Professor Juniata College Environmental Science & Studies (814) 641-5335 (Phone) (814) 641 – 3685 (Fax) Johnson@juniata.edu (Email) Http://www.Juniata.edu/~johnson/. Usual Houghton.
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Dennis L. Johnson
Dennis L. Johnson, Asst. ProfessorJuniata CollegeEnvironmental Science & Studies(814) 641-5335 (Phone)(814) 641 – 3685 (Fax)Johnson@juniata.edu (Email)Http://www.Juniata.edu/~johnson/
….and If a Flood Does Occur in an Overland Situation – Does the Nearest Stream Even Feel It?
Flood--A general and temporary condition of partial or complete inundation of normally dry land areas from:
Overflow of inland or tidal waters.
The unusual and rapid accumulation or runoff of surface waters from any source.
Mudslides (i.e., mudflows) which are proximately caused by flood, as defined above, and are akin to a river of liquid and flowing mud on the surface of normally dry land areas, as when earth is carried by a current of water and deposited along the path of the current.
The collapse or subsidence of land along the shore of a lake or other body of water as a result of erosion or undermining caused by waves or currents of water exceeding the cyclical levels which result in flood, as defined above.
Long term and short term
… An Earth Science. It Encompasses the Occurrence, Distribution, Movement, and Properties of the Waters of the Earth and Their Environmental Relationships." (Viessman, Knapp, Lewis, & Harbaugh, 1977 - Introduction to Hydrology, Harper & Row Publishers, New York)
Generalized effect of routing
Hydraulic Grade Line
Special Thanks, Credit, and Recognition to Don Cline
National Operational Hydrologic Remote Sensing Center
Mean Annual Blowing Snow Sublimation
Loss in mm SWE over 1 km
Snow Depth for
One Inch Water
10 to 30
98” to 33”
Ordinary new snow immediately
after falling in still air
50 to 65
20” to 15”
70 to 90
14” to 11”
Average wind-toughened snow
Hard wind slab
New firn snow
400 to 550
2.5” to 1.8”
Advanced firn snow
550 to 650
1.8” to 1.5”
Thawing firn snow
600 to 700
1.6” to 1.4”
Usually T < 0oC, but can occur at any temperature up to 0oC. Little tendency for snow grains to stick together.
T = 0oC. The water is not visible even at 10x magnification. Has a distinct tendency to stick together.
T = 0oC. The water can be seen at 10x magnification by its miniscus between grains, but cannot be pressed out by squeezing snow (pendular regime).
T = 0oC. The water can be pressed out by squeezing snow, but there is an appreciable amount of air (funicular regime).
T = 0oC. The snow is flooded with water and contains a relatively small amount of air.
Preferential Flow Paths
Melt and rain water are
lagged and attenuated as they move through the snow cover.
Function of depth, density, ice layers, grain size, and refreezing.
Snake River Valley, Idaho
1.6 micron Channel
… Storms that have high intensity levels may also cause excess precipitation because the intensity (inches per hour) may exceed the current infiltration capacity (inches per hour).
… periods of low rainfall or no rainfall will allow the soil to "recover" and increase the capacity to infiltrate water.…
Infiltrated water replenishes soil moisture and groundwater reservoirs. Infiltrated water may also resurface to become surface flow.
… attempt to account for infiltration by estimating excess precipitation (the difference between precipitation and excess being considered infiltration), for example, the Soil Conservation Service (SCS) runoff curve number method
…water may move via several paths.
…subsurface flow can be evaporated if there is a well maintained transfer mechanism to the surface. This is particularly true for areas of high ground water table (the free water surface of the groundwater) which is within the limits of the capillary action or transport abilities.
…Vegetation may also transpire or use the water.
…The subsurface flow may also continue to move with the groundwater table as a subsurface reservoir, which the natural system uses during periods of low precipitation.
… Overland flow or surface flow is that precipitation that either fails to penetrate into the soil or that resurfaces at a later point due to subsurface conditions.
… often referred to as "sheet" flow.
… for the purposes of this discussion, overland flow (sheet and surface flow, as well) is considered to be the flow that has not had a chance to collect and begin to form gullies, rills, swales
… will eventually reach defined channels and the stream system.
… may also be infiltrated if it reaches an area that has the infiltration capacity to do so.
… Overland flow distances are rather limited in length - National Engineering Handbook (1972) - overland flow will concentrate into gullies in less than 1000 feet.
… Other (Seybert, Kibler, and White 1993) recommend a distance of 100 feet or less.
... sheet flow or overland flow will soon concentrate into gullies and rills in the process of flowing towards the stream network. The location of these gullies and rills may vary from storm to storm, depending on storm patterns, intensities, current soil and land use conditions.
… swales are of a more constant or permanent nature.
… do not vary in location from storm to storm.
… Swales are a natural part of the landscape or topography that are often more apparent than gullies and rills.
… Flow conditions and behaviors in swales are very close to that which is seen in channels.
… Excess precipitation ultimately reaches the stream channel system.
… the stream system is generally more defined, it is by no means a constant or permanent entity.
… The stream bed is constantly changing and evolving via aggredation and degradation.
… Stream channels convey the waters of the basin to the outlet and into the next basin.
… attenuation of the runoff hydrograph takes place.
… Stream channel properties (flow properties) also vary with the magnitude of the flow.
… Channels are commonly broken into main channel areas and overbank areas.
… overbank areas are often referred to as floodplains.
… Stream gaging stations are used to determine flows based on elevations in the channel and/or floodplain.
… Bank full is often thought of as flood stage although more rigorous definitions are more applicable as they pertain to human activity and potential loss of life and property.
… It is worth noting that the 2-year return interval flow is often thought of as "bank-full".
Basin Process Representation
We must begin to think of the basin as a “whole”
Excess Precip. Model
Excess Precip. Model
CN ranges from 1 to 100 (not really!)
Land use and treatment Hydrologic soil group
practice condition A B C D
Straight row ---- 77 86 91 94
Straight row Poor 72 81 88 91
Straight row Good 67 78 85 89
Contoured Poor 70 79 84 88
5-day antecedent rainfall, inches
Dormant Season Growing Season
I Less than 0.5 Less than 1.4
II 0.5 to 1.1 1.4 to 2.1
III Over 1.1 Over 2.1
Free - Primary
Free - SupplementalSAC-SMA
Duration of excess precipitation.
Uniform loss rate of
0.2 inches per hour.
The final shape of the Snyder unit hydrograph is controlled by the equations for width at 50% and 75% of the peak of the UHG:
The 645.33 is the conversion used for delivering 1-inch of runoff (the area under the unit hydrograph) from 1-square mile in 1-hour (3600 seconds).
Comes from the initial assumption that 3/8 of the volume under the UHG is under the rising limb and the remaining 5/8 is under the recession limb.
where : Tlag = lag time in hours
L = Length of the longest drainage path in feet
S = (1000/CN) - 10 (CN=curve number)
%Slope = The average watershed slope in %
McCuen (1989) and SCS (1972) provide values of k for several flow situations (slope in %)
Sorell & Hamilton, 1991
R - The linear reservoir routing coefficient can be estimated as approximately 0.75 times the time of concentration.
Synthetic time-area curve - The U.S. Army Corps of Engineers (HEC 1990)
Weaknesses, strengths, etc…
Not all of the watershed is contributing during an event......