Using Soil Erodibility Factor and Saturated Hydraulic Conductivity to Assess Coast and Beach Geomorphology for the Coastal Road Erosion Susceptibility Index (CRESI) on Oahu, Hawaii, USA

Takahashi, Chandelle
Francis, Oceana
Civil Engineering
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Soils play an important role in the coast and beach regions’ susceptibility to erosion. Many of Hawaii’s state roads were aligned along the coasts and are currently experiencing erosion. The Coastal Road Erosion Susceptibility Index (CRESI) was developed as a tool to assist road agencies in identifying and ranking specific road locations that are susceptible to erosion. Two of the eleven parameters in CRESI are coast geomorphology and beach geomorphology, which are defined qualitatively in the current index. Typically, the coast is composed of volcanic geology and the beach is composed of non-cohesive soils such as sand. Therefore, this study aimed to relate erodibility to the materials found in the CRESI coast and beach regions and introduce more quantitative criteria to the two parameters. Although all of the coast and beach soil samples in this study can be broadly classified as sand according to USDA and as sand with fractions of silt and clay according to USCS, each sample had different compositions where its associated properties were considered when relating it to the soil’s erodibility. For the coast geomorphology parameter, physico-chemical parameters of soil texture, structural class, permeability class, and organic matter were used to calculate the soil erodibility factor, K, for a representative coast soil sample. The K-factor was calculated using an altered nomograph equation derived from various rainfall simulations. A series of 1 hour and two 0.5 hour rainfall simulations with an intensity of 6.35 cm/hr and combinations of thirteen 6.35 cm rains on moderately dry soil, four 3.18 cm rains on wet soil, and three 6.35 cm rains on wet soil were performed. A higher K-factor indicated that the soil was more susceptible to erosion while a lower K-factor indicated that the soil was less susceptible. The K-factors obtained in this study were used to create a proposed improved criteria for the CRESI coast geomorphology parameter. The results of this study also found that a coast soil sample’s erodibility is highly dependent on the silt and rock fragment fractions. Silt particles are easily detached due to its lower particle mass and lack of cohesive properties, while rock fragment fractions, such as vegetation or coarse aggregates, provide a protective layer to the soil’s surface. Organic matter, clay particles, and coarse sand particles also help to reduce the soil’s erodibility. For the beach geomorphology parameter, the physical property of effective diameter was used to calculate a representative beach soil sample’s saturated hydraulic conductivity, ksat. A higher ksat-value indicated that the soil was less susceptible to erosion while a lower ksat-factor indicated that the soil was more susceptible to erosion. The ksat-factors obtained in this study were used to create a proposed improved criteria for the CRESI beach geomorphology parameter that only considered saturated hydraulic conductivity. For beach soil samples, a greater erosion susceptibility was attributed to larger grain sizes, which created a matrix with larger voids between the particles. The larger voids enhanced the sand’s infiltration capacity and ability to absorb and dissipate the water flow’s energy. With smaller grain sizes, the pore water pressure needed to detach the sand particle is more easily reached, increasing the sand’s erosion susceptibility.
Civil engineering, Soil sciences, beach, coast, erodibility, erosion, K-factor, saturated hydraulic conductivity
247 pages
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