FLOOD03288 - Laserfiche WebLink

Home Browse Search

predicted stage at a downstr<~am loca'tion. Previous corps experience 1,dth flood warning systems in Ne- braska and Iowa has shown that: a combination of stage and the rate of change in stage at the! 1Ilarning gaqe has more bearing on flood severity at downs1:re!am locations, than the stage alone at the warning gage. Flat:e o:E rise relates directly to the shape of the incident hydrograph. By using 2 or more up- stream gages capable of defining rate of rise, it is possible to better evaluate an approaching flood wave. Factors which are important to the flood coordinators dl~cision making pro- cess, such as flood wave at1::emuation, can be evaluated from rate of rise and stage data at 2 gages. This can be done while the flood is still within the ,:::hannl~l banks, making it unnecessary to rate the entire valley cr(JSS section to pro- vide flood warning. Rates of rise were computed using a HEC'-l model of the sin. Those rates of rise w'~re applied to the hydraulic ings computed for bClth gage si t:es. The initial alarm tings for each gage were established" based upon hydraulics and local condi ticms. 'rhe pr(Jposed initial tings are listed in ~~able 2. ba- rat- set- site set- TABI,E 2. INITIAL FLOOD ALARM I,EVELS stages in Feet of Eleva1:ion above Mean Sea I,evel Discharges in Cubic Fee't Per Second FLOOD WARNING GAGE WARNING LEVEL STAGE DISCHARGE (msl) (cfs) Highway 52 (Upstream Gage) Upper 4772.0 1,400 * Middle 4770.0 775 L01,e,r 4768.0 315 County Road (Downstream Gage:1 Uppe,r 461.3.0 6,250 * Middle 4611.5 4,150 Lowe,r 4609.0 7.50 * The middle float stage is flexible, bu1: it mus1: be set so as not to interfere with the upper or lower floats. The rates of rise between 1:he lower and upper warning lev- els, which are characteris1:ic of th,: rising limbs of floods of 10, 50, 100 and 500 year intensity, are listed in Table 3. Note that the rise time bet:ween floats, listed for the 500-year flood at the downstre,am gag':, is much shorter than the rise time for the 100-year flood, as a result of a two peaked hydrograph. Between t:he peaks, the water level falls below both the upper and middle floats. Rise time is mea- sured on the first rise, which occurs from local runoff, be- fore the larger flood arrivl,s from upstream portions of the basin. A second high stagE' alarm will bl~ generated with the Eo