S ' 0.0041 ( A &0.365

(6.4)

where: S

= the stable slope; and

A

= the drainage area in square miles.

One factor to consider is the drainage area size in developing this type of empirical relationship.

As shown in Figure 6.13, only one reach less than 8 square miles was included in the original data,

whereas, a later sample of watersheds in the same vicinity are primarily in the range of 2 to 10 square miles.

Figure 6.14 is a comparison of the DEC monitoring reach energy slope data shown as CEM types, and

the GDM No. 54 slope-area curve. For the portion of the slope-area curve greater than 10 square miles,

most of the reaches are CEM 4 or CEM 5, indicating a reasonable degree of stability. For drainage areas

less than 10 square miles, the slope-area curve is defined by CEM 2 or CEM 3, generally unstable

reaches. The CEM 4 data less than 10 square miles in drainage area are below the regression relationship

(Eq. 6.4).

Figure 6.15 is similar to Figure 6.13, with the following exceptions: a) 1995 DEC monitoring reach

data for only CEM 4 and CEM 5 reaches are plotted; and b) these data exclude reaches that are ponded

as a result of grade control construction. Ponding was not included in the original conception of the CEM.

A new regression was made of the plotted data and the following relationship was plotted using a solid line

(Figure 6.15):

S ' 0.0018 ( A &0.145

(6.5)

Parameters are as previously noted. The GDM No. 54 relationship is shown above as the dash line. The

primary reasons for lowering and flattening of the relationship is that the sediment supply to the reaches has

been reduced by upstream grade control structures and other measures emplaced by the DEC Project.

Therefore, the prior empirical relationship (Eq. 6.4) is now invalid due to the effectiveness of erosion

control measures.

Stability, as defined by the CEM criteria, includes a balance between sediment supply and sediment

transport capacity. As the sediment supply has been reduced, the stable slope must also be reduced.

Therefore, although the slope-area curve is a useful benchmark for comparison of reaches, the curve will

require updating as success occurs in reducing sediment supply. Consideration should be given to using

design procedures that explicitly include sediment supply and transport capacity.

Unfortunately the empirical slope-area regional stability curve, although useful, does not explicitly

include sediment yield or sediment transport capacity. The relationships only implicitly include the sediment

yield of the stable channels used in the data base. Figure 6.16 depicts the relationship between the energy

slope and the computed sediment concentration in the DEC monitoring reaches. A regression expression

for the sediment concentration data is:

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