Role of geomorphic and hydraulic parameters in governing pore water seepage from salt marsh sediments

Abstract

A numerical boundary integral equation model has been used to study the effects of marsh geomorphology and hydraulic parameters on the seepage of pore water into tidal channels and its subsequent recharge by tidal inundation. In general, the results show that regardless of the geomorphic configuration the locus of maximum seepage flux occurs at or near the intersection of the creek bank and the channel water surface. Over a tidal cycle, typically two thirds of the total seepage discharge occurs through the creek bank with only about a third discharging from the channel bottom. In most cases the volume of seepage discharging through the creek bank exceeds by a factor of two or more that emanating from the channel bottom. Of the creek bank discharge, up to half may occur through the seepage face that develops above the tide line at tidal stages below mean tide. The ratio of creek bank to bottom seepage only approaches unity when the thickness of the aquifer beneath the bottom is similar to or greater than the width of the channel bottom and the ratio of conductivity to specific yield exceeds 0.5. For given hydraulic parameters, total seepage volume varies directly with creek bank slope and aquifer thickness, whereas these factors are inversely related to the ratio of the two seepage components. Channel bottom width, on the other hand, has no influence on total seepage volume but greatly increases the component ratio as the width approaches zero. Since most of the seepage discharge is derived from sediments within several meters of the creek bank, variations in seepage discharge driven by variations in marsh geomorphology might play a role in the productivity of creekside cord grass. Several hypotheses are offered and discussed regarding the effects of geomorphology on creekside productivity. These results also indicate that placement of seepage meters only on the channel bottom will not give samples or measures representative of the total seepage.