Our Hazardous Environment GEOG 1110 Dr. Thieme Drainage Basins, River Channels, Streamflow

Drainage Basin the fundamental landscape unit for collection and distribution of water and sediment. separated from the adjacent basin by a divide.

FIGURE 6.9a

FIGURE 6.9b

Gradient - the vertical drop over a certain horizontal distance “rise over run”

Drainage Pattern

Channel Pattern

Sinuosity reduces grade. 100’ The concept of sinuosity, or degree of meandering can be explained by looking at these three types of channels. If three streams start from an elevation of 100 ft and drop to an elevation of 90 ft, which will have the steepest grade. Clearly, the straightest channel will have the steepest slope. If the stream is allowed to meander, it will reduce its grade by meandering. If the energy entering the reach is very high, there will be bank cutting and deposition, that starts the meandering process. Generally a stream in a steep valley floor is more prone to meander than a stream with little grade. 90’ Steep grade Medium grade Low grade

Sinuosity reduces velocity More deposition will occur in a sinuous stream. If the channel is blocked, it may change course or widen to a “braided” condition. Excessive sinuosity may result in severe degradation of the stream. Meandering will continue until the grade is reduced so far that deposition blocks the stream. At that point the channel’s course may change, increasing the grade by cutting to the next meander, or it may widen out. Thus there is a limit to how wide a stream’s meander can go. A wide shallow stream is highly unstable and generally has poor aquatic and riparian habitat.

Braided Rivers The following conditions favor the braided channel pattern: erodible banks abundant coarse sediment supply rapid fluctuations in discharge

Base Level is the lower limit to which to which a river can downcut its channel.

A meander increases in amplitude by erosion of the cutbank and deposition on the point bar. A cut-off meander forms an oxbow lake.

Meander Scrolls indicate historical migration of meander bends. Meanders rapidly abandoned by avulsion become oxbow lakes.

Features of a Meandering River Valley: Cutbanks Point Bars Floodplains Oxbow Lakes (Cut-off Meanders) Natural Levees Yazoo Streams Backswamps Terraces

Figure 4.7

Flood natural process of overbank flow can be characterized by a discharge (cfs or m3s-1) at the point where water overflows the banks also by a stage (feet or meters) or depth of flow

FIGURE 6.10

Discharge (Q) is the amount of water passing a given cross-section of a river in a given unit of time.

Continuity Equation Similar to Principle of Conservation of Mass and Energy in Physics cross-section time Q = w * d * v discharge width depth velocity

Hydraulic Geometry channel width, depth, and velocity all increase in the downstream direction Depth increases at the fastest rate Why does velocity increase?

Mountain streams have turbulent flow Water molecules and sediment particles are moving at many angles which are not parallel to streamflow Boulders or large woody debris in a channel introduce more frictional drag

Turbulent and Laminar Flow Deep, narrow channel at upstream end. More of the water comes in contact with channel walls. Velocity is decreased by frictional drag. Wide, shallow channel at downstream end has laminar flow. Most of the resistance to flow comes from shear between water planes.

Streams and Erosion base level – the lowest level to which a stream can erode local base level – controlled by local features such as a larger river or a lake or a resistant bed ultimate (absolute) base level – sealevel

Graded Stream has just the velocity required for the load supplied from the drainage basin Stream adjustments toward equilibrium or “graded” condition: Raising base level causes deposition Lowering base level causes erosion Increasing gradient causes meandering

River Terraces abandoned floodplains formed when the river flowed at a higher level than at present

Terrace (T-2) Terrace (T-1) Floodplain (T-0) This picture shows all the parts of a meandering stream. The high cut bank on the right shows where erosion occurs. In the very center of the picture on the left side of the channel there is a point bar. This whole channel is entrenched in a flood plain. In the distance you can see a terrace where a flood plain occurred in the past. Floodplain (T-0)

Valley Cross-Section Terrace Terrace Cut Floodplain bank Cutting a slice across the channel, we can see all the parts. Normal flow is way down in the bottom of the channel. Only occasionally, on average once every two years, flow gets up to the flood plain level. Less frequently it will get out of the channel into the flood plain. It is important there be a flood plain to keep the water contained, reduce its energy, and protect areas out of the flood plain from damage. The Terrace shown at the limits of the flood plain show where sediment was deposited in a flood plain long ago. Flood flow channel “Normal” flow Channel

Stream Gage measure discharge in cubic feet per second (cfs) or cubic meters per second (cumecs)

FIGURE 6.C

Figure 5.D

Rating Curve - used to convert water height (stage) to discharge

Hydrograph a graph showing changes through time in river discharge (cfs or m3s-1) water depth (feet or meters) stage (feet or meters) relative to some datum

FIGURE 6.E Recurrence Interval - Average number of years within which a flood of a given magnitude occurs in the period of record

N + 1 R.I. = _______ M N = total number of years of record M = rank of magnitude of flood

FLOODPLAIN MAPPING Based on contours that correspond to depths of gaged floods Used for land use planning Used for determining eligibility for flood insurance Subject to uncertainties in the statistical analysis of flood frequency

Figure 4.8a Morning Streamflow

Figure 4.8b Afternoon Meltwater Flood

Flash Floods typically occur in the upper part of a drainage basin. generally produced by intense rainfall of short duration falling over a small area.

Santa Elena Canyon of Big Bend National Park March 20, 2004 over 2 m (6 ft) high wall of water surprised group of five canoeists from the University of Wisconsin-Whitewater

Big Thompson Canyon in the Colorado Front Range July, 1976 triggered by system of thunderstorms up to 25 cm (9.8 in) of rain fell in a few hours

Downstream Floods cover a wide area generally produced by storms of long duration that saturate the soil and produce increased runoff.

Downstream Movement of a Flood Crest: Chattooga River near Clayton, GA Savannah River near Calhoun, S.C. Savannah River near Clyo, Ga.