Hydrologic hazards can be considered in three categories; An indication of flooding potential for areas located

Transcription

1 4.0 General Hydrologic Design Critera 4.1 Hydrologic Hazards Hydrologic hazards can be considered in three categories; flooding, channel migration, and tributary stajility. An indication of flooding potential for areas located adjament to the proposed alignment is shown on the valley flat mapping presented in Section As previously discussed, the evaluation of flooding susceptibility is based on field characteristics such as elevation, soil development, litter cover and vegetation. The map has been prepared with a conservative bias, however major debris jams, extensive flooding and channel movement by tributary streams, or the channelization of water along road or pipeline right-of-ways may result in flood waters reaching unexpected areas. The mapping does not indicate the potential depth of flooding, except that the smaller numbered units f ~ x (for example, a "2" versus a "3") can be expected to have a deeper cover of water under conditions of maximum flow. Some additional information will be available once the recommended surveys to determine the water elevations associated 1 with the December 27, 1980 flood are completed.. Historic rates of channel migration on Coldwater River, discussed in Section 2.4.4, are substantial. Future channel displacements can be expected to occur at lower recurrence l. %currcnce interval years at the station Coldwater River near Brookmere. &Y,\lpsx?&m

2 interval discharges within the wetland areas and smaller numbered units shown on the valley flat mapping. On the basis of erosion rates calculated after the December 27, 1980 flood, x channel displacements due to a xx~qe single event can be expected to exceed 50 m along reaches bounded by wetland areas or units coded as a r2". Rates of channel migration of up to or more than 20 m can be expected to occur adjacent to units coded fi2/3@, Q3/2&, *(2 + 3)*, or M3A'. Migration rates in units coded as *3/4.', L4/34, or w4w will be much lower, and would generally be expected to be less than 5 m. These erosion rates are representative of conditions associated with a 20 to 25 year recurrence interval discharge, and thus erosion rates can be expected to exceed these values during higher discharge events. These erosion rates are sufficiently high that sections of the proposed alignment which are not now situated adjacent to the river, can be expected to come under fluvial attack over the lifetime of the proposed highway. However as channel displacement is primarily by meander progression and the occasional cut-off, areas of attack are reasonably predictable over a short term period. For this reason the site specific training works discussed in Volume 1 have been extended to include areas which will likely be attacked by fluvial activity in the near future. This allows rip-rap to be easily keyed in below potential depths of scour and for training works to he constructed "in the dry", which is a less environmentally

3 damaging procedure. t is, however, uneconomic to protect the entire alignment in this manner and it is therefore recommended that the periodic assessment of channel migration be made on the basis of current air photography, and remedial action be undertaken prior to fluvial attack. The stock piling of suitably sized r$p-rap material while the initial construction equipment is available would also be of significant benefit. The stability of streams tributary to Coldwater River will pose some difficulty as the alignment generally crosses them on their famwhere they are both laterally and vertically unstable. Specific design recommendations for straightening or channelizing the major (non-f ish bearing) streams are discussed in the site specific recommendations presented in Volume 11. Smaller tributaries can also be expected to result in localized maintenance problems and site specific drainage designs should be a considered in the vicinity of all the fluvial &ilk ole. fans or active channels indicated on Figure n 'L. as *ng upslope of the proposed alignment.

4 4.2 Design Discharges Upstream basin areas and design discharges for the major creeks, river crossings and diversion sites are shown in Table Daily discharge values for drainage basins over 20 miles in area have been estimated from the regional flood frequency analyses shown in Figures to Daily discharges for areas less than 20 miles2 in size have been estimated on the basis of the envelope curves about unit runoff values shown in Figures and Due to the uncertainties in the position of this curve, values have only been estimated for the 2 year and 200 year recurrence interval discharger, based on C E\ the Creager coefficient 4 equal to 2 and 4 respectively. Within the uncertainties of the methodology, these values correspond to ~lsc r.-o\ ti- a -.cr -rr. estimates of an average and a large,%discharge. nstantaneous flows have been calculated on the basis of the regionally observed ratios of instantaneous to mean daily flows discussed in Section Average values of this ratio are in the range of 1.2 (see Figure ) and,this figure has been used in the calculation of the instantaneous flow associated with the two year recurrence interval daily discharge. nstantaneous -S\O w 5 discharqes associated with the 100 and 200 year events have been based on the maximum observed ratios, shown in Figure

5 n -' 9 rt w N N m -' / 3 0 Ln 4 N N O N N N N N N L n N N ~ m m t w m e w w N ~ ~ - o N ~ ~ N w L ~ N ~ w o N N N w m w m ~ n ~ o m 1 ~ m N 2. N m c 0 m rt N N N N N P P w cc L n L n w m W w m 4 m ~ ~ ~ ~ ~ w o w ~ w w w P W 4 m O m - O w m e

6 , & \ - - availability of bedrock suitable for use as 1 t o 1 2 8' diameter rip-rap varies over the proposed route. 1 Observations on the behavior of rock in existing revetments indicates that the commonly occurring ~\\o<\tc s c\?\ \+ borr\ L 4 sedimentary rocks (particularly greenstone) are h 1 highly susceptivle to fracturing and form poor, i construction materials. Thus surveys to locate high quality burrow sites (especially of non-foliated igneous rocks) should be undertakenv. 2 t

7 & Cattle Path Wherever the highway fills encroach on the stream, a pathway is required for cattle to get past these poi~lts gf impass. To insure that there will be no impact on the river and fish habitat a standard design has been provided for these paths (Figure ). f the elevation of the path is too low, then they could be flooded quite often resulting in reduced effectiveness of boulders providing habitat alany the rip-rap, also increasing the chance of debris accumulation on the ~ath~requiring maintenance of the paths. To avoid both these potential problems the paths should be above the 100 year flood level.