Use the U.S. Army Corps of Engineers HEC-RAS computer code to solve Example 5.4.4.EXAMPLE...

Use the U.S. Army Corps of Engineers HEC-RAS computer code to solve Example 5.4.4.

EXAMPLE 5.4.4

For the data in Figure 5.4.2, start with the known water surface elevation at river mile 1.0 and determine the water surface at river mile 1.5 (adapted from Hoggan, 1997).

SOLUTION

Computations are presented in Table 5.4.1.

Table 5.4.1 Standard Step Backwater Computation

*

**ho = Ce|Δ(αV2/2g)| for Δ(αV2/2g) < 0 (loss due to channel expansion); h0 = Cc|Δ(αV2/2g)| for Δ(αV2/2g) > 0 (loss due to channel contraction).

***W2 = W1 + Δ(αV2/2g) + hL + ho = 125.0 + (−0.06) + 0.111 + 0.02 = 125.066 ≅ 125.07.

Source: Hoggan (1997).

Figure 5.4.2 Cross-section and reach length data for example 5.4.3 (from Hoggan (1997)).

EXAMPLE 5.4.3

For the compound cross-section at river mile 1.0 shown in Figure 5.4.2, determine the energy correction factor α. The discharge is Q = 11,000 cfs and the water surface elevation is 125 ft.

SOLUTION

Step 1 Compute the cross-sectional areas of flow for the left overbank (L), channel (C), and right overbank (R):

Step 2 Compute the hydraulic radius for L, C, and R:

Step 3 Compute the conveyance factor for L, C, and R:

Step 4 Compute totals At and K1:

Step 5 Compute K3/A2 and :

Step 6 Use equation (5.4.24) to compute α: