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I I I I I I I I I I I I I I I I I I I Methodoloav Values of Manning's roughness coefficient "n" were estimated for the channel and the left and right overbanks for each of the study reaches. Estimates were based on field observations and were computed using the procedures outlined in the Federal Highway Administration publication "Guide for Selecting Manning's Roughness Coefficient for Natural Channels and Floodplains". This method estimates Manning's "n" by assigning a base value for a particular soil type and applying adjustments for the degree of irregularity of the channel, the variations in channel cross-sections, the effects of obstructions, the amount of vegetation, and the degree of meander of the channel. Manning's "n" used in the hydraulic model for channels varied from 0.020 to 0.035. The values for the overbanks varied from 0.020 to 0.050. The delineation of the tOO-year floodplain on Upper Lena Gulch and its tributaries was accomplished using the U.S. Army Corps of Engineers HEC-2 Water Surface Profiles computer model. The HEC-2 Water Surface Profiles model was developed for use in computing water surface profiles in natural or man-made channels. The computational procedure used in the model is the Standard Step Method. A complete description of model capabilities is beyond the scope of this report, however some basic assumptions used by the program are: t) flow is steady; 2) flow is gradually varied; 3) flow is one dimensional; and 4) channel slopes are relatively small. The hydraulic model assumes the channel has rigid boundaries, which means that the cross- section shape does not vary with time or flow rate to account for erosion or bank deterioration. This is generally appropriate for the tOO-year storm. Any changes in channel geometry are transient and are not likely to significantly affect the floodplain limits. Bridges were modeled in HEC-2 using the normal bridge or special bridge options. Normal bridge methods assume channel friction is the principal loss through the structure. This is most applicable for very long structures or when the section is not obstructed by a large number of pier bents. The normal bridge method is also most appropriate for highly irregular openings which defy representation by a simple trapezoidal section. Special bridge options are appropriate when momentum loss computations best represent bridge hydraulics or when pressure flow is expected. Flow in the channel was assumed to be sub-critical. This implies that flow is controlled by backwater effects and is a function of downstream energy and losses in the intervening reach. This is a common assumption in the delineation of floodplains and the determination of flood profiles. Because of the sub-critical assumption, depths of flow below critical depth are not mathematically possible. This assumption produces a conservative estimate of the flood profile since it is deeper than a computed super-critical depth. This assumption also represents the long-term physical condition in which super-critical flow is unlikely due to local anomalies such as scour-holes and debris snags. All bridges were modeled with the possibility of overflows. A weir coefficient of 2.6 was used and an overflow section was defined. In cases of extreme undersizing, weir overflows often accounted for a significant amount of conveyance. Where roadway overtopping occurred but flows did not leave the main channel, the bridge overflow options were used without further manipulation. t t