Scour resistance of farm dam spillways with grass dormant

Abstract

This report is the fourth in a series dealing with the use of natural grasses and bituminous and soil admixtures for surfacing farm dam bywash spillways. When the work reported here was completed, the test channels were demolished to make way for new construction, and no further tests of this nature are contemplated. The previous report (Yong and Stone, 1967) dealt with the scour resistance capacity for grasses having a dense even cover, tested at the peak of their growing season. This report covers winter tests with the grass dormant. Once they have established a good root system, the grases withstand high velocities amazingly well, even when dormant, and there is no need to reduce permissible design velocities as given previously for grasses subject to winter floods. Three channels were available for each grass type, so a test was also made on one channel of each grass of the extra protection that might be afforded by anchoring a layer of chicken wire on the spillway at or near the soil surface and letting the grass grow up through it. In fact, none of the spillways failed hydraulically, failure due to the structural inadequacy of the channels preceding scour failure in every case, and no definite conclusion could be reached as to the added benefit of the chicken wire.