FLOOD11726 - Laserfiche WebLink

Home Browse Search

The fldodway is the channel of a stream, plus any adjacent flood plain areas, that must be kept free of encroachment in order that the 100-year flood be carried without sub- stantial increases in flood height. As minimum stan- dards, the Federal Insurance Administration limits such increases in flood heights to 1.0 foot, provided that hazardous velocities are not produced. The flqodways computed for the streams in this study were determined using both the criteria of flood plain en- croachment and conveyance reduction as well as considera- tion of hazardous velocities and depths within the flood plain. The tributary streams to the St. Vrain Creek are genera~ly steep and severely encroached upon by urban development. Further encroachment or conveyance ability reduction would result in increased velocities above already hazardous velocity conditions. Hazardous veloci- ties are considered to be average overbank velocities in excess of 2 to 3 feet per second. As shown on the Floodway Map (Exhibit 3) the floodway limits for each of the streams except Dry Creek, were plotted at the cross section locations; between the cross sections, the floodway boundaries have been drawn to represent streamlined limits characteristic of floodway limits. The flOodway for Dry Creek was contained within the chan- nel, and therefore was not shown on the map and table. Many factors affect the determination of the encroach- ment limits on each stream, including sand and gravel mining operations, overtopped bridges, present develop- ment w~thin the flood plain, and divided flow conditions. To determine the final floodway boundary at each cross section, characteristic velocities were considered, along with encroachment limits which cause no more than the maximu~ 1 foot rise in the flood height. Where appropri- ate, an equal degree of encroachment was permitted on each side of the main channel. The degree of conveyance reduction on each side of the channel was determined by hydraulic limitations. The results of these floodway computations are tabulated at selected cross sections for St. Vrain Creek, Spring Gulch, and Left Hand Creek (Table 1). The ar~a between the floodway and the boundary of the 100-year flood is termed the floodway fringe. The flood- way fringe thus encompasses the portion of the flood plain that could be completely obstructed without increa~ing the water-surface elevation of the 100-year 19