Page:Final Report - The Columbia River Interstate Bridge.pdf/63

 (6) Embankments:

Embankments, especially where resting on soft subsoils, and against which there may be overﬂow, are subject to deterioration or damage through several years and often do not ﬁnally consolidate for a long time. Your approach embankments have now passed through two seasons of high water of more than ordinary height with comparatively and actually small amount of damage. We believe this substantially perfect condition due to the manner in which the embankments were built, and we do not think you need be apprehensive of any very general or serious subsidence. For the protection of the slopes of embankments from erosion concrete slabs were constructed over parts of the slopes. In the main part these have held satisfactorily, but portions of the slabs on the Vancouver embankment washed down in the high water of 1917 and were replaced by rip-rap stone. The setting up of pronounced current with eddies might cause similar difﬁculties at the ends of the embankments on Hayden Island or at the slough crossings. However, the present cheaper construction may hold indeﬁnitely and there is no need to place more stone now. For several years at times of extreme high water you should have embankments patrolled throughout, giving particular attention to the ends, so that cutting or washing may be stopped in incipient stages. These embankments are not unlike river levees and during their ﬁrst ﬁve or six years should receive the same sort of patrolling during high waters. We consider it signiﬁcant that the damage done by the 1917 high water was on a portion of embankment not constructed at the time of the previous high water, and that no damage was caused by the 1917 high water at the points where the chief damage was done in 1916.

Considerable expense has been entailed by the fertilizing and seeding of the slopes of the embankments, in order to provide a sod to prevent erosion and weathering of the slope. The results so far accomplished indicate that the securing of a compact sod is possible, but it may be necessary to refertilize the slopes and replant parts of the same from time to time for several years to secure satisfactory results. Attention should be given to this planting and also the roses and broom planted along the approach railings.

Evergreen blackberries were set out along the lower portions of the embankment slopes of the approach embankments with a view to developing in time a hedge sufficiently thick to keep cattle from climbing the sides of the embankments. Most of these bushes (lid not survive their ﬁrst high water and it was concluded