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N~erous homes are located on the North St. Vrain Creek t lood P I a I n u ps tream 0 I Lyon s. ~~any 0 f the homes wOJ I d be flood ed by the lr<te""6diateflegioMI Flooc:; more would betloodoo by the Standard Project Flood. From Reference Points Ib to 6b the areaS flooded by the two potential fl~s vary significantly, averaging 200 feet wider for the larger flood Which would inundate to an average width of 550 feet. From Reference Poir<t 6b to Lyons the flooded araas of the two floocs ~ore closely coincide, '11th excep- tions for short reachos at Refer$nce Points 91.> and 12b. The area 'iUbjoct to floodlr<g Or< Sooth St. Vraln Croek Is wlder t>ut th(wo ilppaarstol.>ef"werhOlles In the flood plain there. Ir<Lyons, virtually all the town area between the two br<;ln<:h streams is sub- ject to flooding. This vulnerability has been demonstrated to a degree during past floods. On the left bank of North St. Vroin Creek little flooding would OCCur upstreMlof Referen<:e Point 20b, whereas downstream of this polnt tha flooded area widens to from 400 to 650 feet from the stream bank. DownstrOaM from Lyons the two potential floods cover about the same areas. ThG area subject to flooding expands drastlcally downstream from Reference Polnt 12. At this location the creek flews along the left edge of the flood plain. T~o right bank flood pl~in is lower thar< the flood plain adjacent to the channel. The flood waters separato and a portion flo><sdown the rlght sIde of tho flood plain leaving islandsln the center. Because the flows on each slde of the flood plain are somewhat separated from each othor two different flood profiles are produced. An ~dltlonal sot of reference points are provided to show flood elevations on the right side of the flood plain. Cross section 9-9 ~nd 10-10 on plate 10 show the different levels of flooding. The relatIonship of the flows between the left and rlght sides of tha flood plain Is in a delicate balance. ~ew r~ds, structures or other modifications to the flood plair< could change tMtlOOd pattarnand flood land that is not nOw tlood9d. New construction should be flood proofed several feet "bove the Flooded areas and flood damages - The areas along St. Vrain Creek, both rural and inLyons,thatwOJld be floodad by the Inter- medIate Reglonal and Standard Project Floods are Shown On plates 2 through 5. Water surface profiles for these floods were determir<ed uslng bllckwatercomputatlonsemplovirogtheStand"rd Step Mvthod. The profiles, on plates 6 through 8, show the depths of flooding in the channel and the elevation of the water surfaces oyer the adjacent flood pL"in. He~ losses at bridges were computed using data in "Hydrau I rcs of Brldge Waterways" publ ished by the U. S. Dep~rtment of Ccmnerce. Flood bOJndarles were plotted On the detai led topo- graphic mapping and then transferred to aerlal photogr~phs ir< this report. Plates 9 and 10 portray cross sections across the channel and valley and show ground elevations and depths of overbank flood- ing. Reference points havo been provided to locate flood elevations at intervals along St. Vraln Creek. The reference points coincide with the location of the surveyed cross sections used in the study. The location of the reference points are shown on both the "Flooded Area" plates and the "Profile" plates. T(lbles5throogh8show reference point data including the elevations of the streambed and the Intermediate Reglonal and Standard Project Floods. A bearing is alSO provided showing the orlentatlon ot the cro~s section through the reference point. Depths of flow for the Intermediate Regional and Stand~rd Project Floods can be obtained trom the prof lies. cross sections, or roference t~bles. For any speci f ic situation where accuracy of floodoo areadelineatlon is needed it will be necessary to take the appropriate flood elevation from one of thfJse sources and locate by survey thlocorrespondlng grOJnd elevation on tho flood plain. 20 "