Bank Erosion Prediction (BEHI, NBS)

The prediction of stream bank erosion rates uses the "Bank Assessment for Non-point source Consequences of Sediment" (BANCS) method. This method as published by Rosgen (2001a) utilizes two bank erodibility estimation tools: the Bank Erosion Hazard Index (BEHI), and Near Bank Stress (NBS). The application involves evaluating the bank characteristics and flow distribution along river reaches and mapping various risk ratings commensurate with bank and channel changes. An estimate of erosion rate is made, and then multiplied times the bank height times the length of bank of a similar condition, providing an estimate of cubic yards and/or tons of sediment/year. This information can be compared to the sediment yield data to apportion the amount of sediment potentially contributed by streambanks.

The relationships developed to convert measurements of streambank variables into risk categories are shown along with bank erosion and bank angle illustrations in Figures 112 to 114 (Rosgen, 2001a). A sketch of a streambank and some of the variables surveyed and calculated is shown in Worksheet 20 (PDF, 38 kb, 1 p.). The use of channel materials, bank stratification and all of the variable ratios and ranges are summarized in the Bank Erosion Hazard Index (BEHI) form (Worksheet 21, PDF, 40 kb, 1 p.).

Figure 112. Streambank erodibility criteria used for the BEHI rating (Rosgen 1996, 2001a)

Figure 113. Illustrated examples of the five BEHI criteria

Figure 114. Common bank angle scenarios

The Near-Bank Stress (NBS) variables used in the prediction methodology indicate potential disproportionate energy distribution in the near-bank region (1/3 of channel cross-section associated with the bank being evaluated). Changes in near-bank stress can accelerate streambank erosion. The initial criteria for NBS are summarized in Rosgen (1996, 2001a) and shown in Table 18. Additional criteria, shown in Worksheet 22 below, were established to assist in the field determination of NBS for a variety of inventory levels. Use Worksheet 22a (PDF, 39kb, 1 p.) to complete an NBS rating using the methods (1-7) appropriate for the available data. Use the conversion table in Worksheet 22a to determine and record the final NBS rating for impaired and reference reaches.

It is imperative to also plot the BEHI and NBS ratings on a photo or map in order to identify specific locations in need of mitigation, restoration or changed riparian management.

The combination of BEHI and NBS risk ratings were used to develop the relations shown in Figure 115 (PDF, 303 kb, 1p.) (Colorado data) and in Figure 116 (PDF, 507 kb, 1p.) (Yellowstone National Park data.) These relations indicated a statistically valid way of predicting annual streambank erosion rate in feet/year using BEHI and NBS ratings (Rosgen 1996, 2001a). This allows the user to predict annual streambank erosion rates to help apportion annual sediment yields to various sources. The relations for Colorado (sedimentary and metamorphic) and Yellowstone (volcanism and alpine glaciation) are not intended to be universal for alluvium, but rather provide a framework for other to develop similar relations or validate these relations locally. The application of these relations allows the field practitioner to obtain BEHI and NBS ratings along river reaches of varying condition. It is imperative to plot these ratings also on a photo or map, in order to identify specific locations in need of mitigation, restoration, or changed riparian management.