Deep Seated Landslides/Earthflows

Step Wise Guide

Erosion

· How can I identify large landslides and earthflows in my watershed?

In many landscapes, large landslides and earthflow terrain can be readily identified using aerial photography, indicated by certain types of topography (large head scarps, lobate toes, areas of exposed soil). However, many large landslide features may be difficult to detect, since they might be dormant. NetMap contains several tools that can be used, singly or in combination, to identify large landslides and earthflows. Large landslides can create channel and valley floor habitats but they also can represent a risk to important resources since they can bury habitats and impact roads and other structures.

Refer to the "Warning" button in the tool interface. Follow up any remote sensing work, including using NetMap, with field work to verify environmental conditions and landslide and debris flow risk. There is never zero risk, there is only degrees of risk, which often is best considered on a relative basis.

Step 1: Go to NetMap Erosion Tools > Deep Seated Landslides/Earthflows. There you will find a reference to several tools in NetMap that can be used to help identify and locate large deep-seated landslides and earthflows.

Step 2: Go To Basic Tools > Profile Tools > Longitudinal Profile. This tool allows you to select any channel attribute and create a longitudinal profile of any length in any channel. Of particular interest are the attributes of elevation, gradient and floodplain width, all of which can be affected by large landslides entering valleys. Large landslides commonly increase channel gradients at and near their downstream toe and cause decreasing gradients upstream of them – this is reflected in the elevation and gradient attributes. In addition, floodplain and valley widths are often wider upstream of large landslides and earthflows.

Step 3: Go To Fluvial Morphology Tools > Floodplain Mapping. NetMap digital watersheds contains a floodplain width attribute calculated at 2x bankfull depth. This is in the form of an attribute in the reach shape file and a polygon that can be loaded from the Floodplain mapping tool. You can create other floodplain attributes by increasing (or decreasing) the bankfull depth criteria, 3x, 4x etc. up to 10x to create maps of valley widths. Where floodplain widths abruptly increase (either using the reach attribute or the polygon) for distances on the order of a kilometer or more, a deep seated landslide might be the cause (or not).

Step 4: Go To Basic Tools > Profile Tools > Cross Section Profile. This tool allows you to create cross sectional profiles of any topographic surface in your watershed. Using it from ridge to valley or across hillslopes at a single elevation may reveal a deflation in the topography, which may include a prominent headscarp. These are characteristics of a deep seated landslide and or an earthflow. The ridge to valley cross section may also reveal hummocky surfaces or benches (sometimes with lakes or ponds), indicators of dormant or active deep seated landslides and earthflows.

Step 5: Go To Erosion > Generic Erosion Potential. Generic erosion potential, or GEP for short, uses a topographic index of hillslope steepness and convergence. High values of GEP (e.g., 0.7 – 1) reveal steep and highly dissected slopes, often susceptible to shallow landslides and gully erosion. Low values of GEP reflect low gradient topography. Deep seated landslides and earthflows may have distinct patterns of GEP, very high at head scarps and high near their toes in valleys and low in between. Thus, an analyst can use the GEP index to search for such patterns.

Step 6: Go To Erosion > Channelized Mass Wasting > Debris Flows. High debris flow potential requires steep headwater streams. Deep seated landslides and earthflows often create large areas of low gradient topography. This in turn will create a set of headwater streams with low gradients, and correspondingly low debris flow potential, except near steeper headscarps. Thus, keep a sharp out eye for suspicious patterns in GEP and debris flow potential. Isolated areas of lower values, within a watershed of mostly higher values, could be used to pinpoint the location of dormant or active deep seated landslides and earthflows.

Step 7: Keep a look out for anomalous shifts in channel and valley orientation. A large deep seated landslide or earthflow commonly causes abrupt shifts in stream direction along their toes.