Hydrology and drainage Water on the landscape

Water and the Landscape: Hydrologists are concerned with how the landscape influences the water, whereas Geomorphologists are concerned with how the water influences the landscape. Geomorphic Questions: How does rainfall influence hillslope shape? What determines where water accumulates in the landscape? Why are channels where they are? How does water run off the landscape? What shear stresses are created and how does that influence sediment transport? What sets the location of channel heads, thereby setting the drainage density in a landscape? When do pore pressures become high enough to initiate landslides?

Drainage Patterns Reflect the topographic fabric of a landscape

Drainage Density The drainage density is the sum of the length of the channels in a basin divided by the area of the basin. Basins with high drainage density have much shorter hillslopes, than those of low drainage density.

Stream Ordering

Horton’s Law of Stream Order

Channel initiation in the landscape "What sets the upstream end of the channel network? The beginnings of the channels. Ok, so where do channels begin? At the channel head. Ok, so what sets a channel head?” Uphill of the channel head, there is a slightly concave up bowl. The distance from the drainage divide to the channel head is inversely proportional to the drainage density.

Horton’s Idea Horton's idea was that a threshold is exceeded at some point downslope where the thickness of water flow is sufficient to begin eroding. This can be seen on a diagram of distance down the hillslope from the divide, x vs. sheet flow thickness, h, and the associated shear stress, t. At the flow thickness where the entrainment shear stress, tcrit, is exceeded, hillslope sediment is entrained a channel begins to incise.

Why do we care where channel heads are? 1. Smaller upstream areas (zero-order basins) required to create a channel head, leads to much higher drainage densities. 2. Because water flows through the landscape much more swiftly and efficiently in channels than traveling over hillslopes and through the shallow subsurface. 3. Channels and hillslopes are totally different passageways for water, because of the rate at which water flows as a result of its thickness – double thickness and the discharge increases by a factor of 10. 4. The ability to erode increases dramatically with water depth for two reasons: a. because sed. entrainment is a thresholded process b. because above the threshold, sed. transport is non-linearly related to discharge. 5. Hence, anywhere in the landscape where water is focused, sediment is preferentially entrained. 6. Hillslopes are very inefficient at moving water, so the shorter the hillslope, the faster the rainfall event will be felt down stream.

Water Balance R Ab ET Q

Rainfall Intensity

Precipitation and Discharge: Lag to Peak

Monitoring flash floods Badlands of Utah

Infiltration to Vadose Zone and Saturated Zone

Runoff Mechanisms Chief Elements of the Runoff System: The system begins with rainfall intercepted by the landscape, which either runs off or infiltrates - depending on the transient infiltration capacity during a rainfall event. Overland flow, appearing when rainfall intensity exceeds infiltration capacity, delivers water most swiftly to the channel because of the lack of frictional resistance. Subsurface Storm Flow moves through the vadose zone, where there is wide variability of the hydraulic conductivity. Groundwater Flow, through the saturated zone, is the slowest contributor to stream flow and comprises the bulk of the baseflow in the hydrograph. Saturated Overland Flow occurs from a seepage face, where the ground water table is elevated above the stream level.

Variable Source Area Concept

Characteristic Hydrographs for Landscapes Dominated by Various Runoff Mechanisms

Where do Channels Begin? Slope-Area Relationship: Montgomery & Dietrich (1988)