Behavior of the groundwater in a riverine-type mangrove forest

Abstract

The behavior of groundwater and physical properties of bottom sediment in a riverine-type mangrove forest which is composed of a tidal creek and fringing mangrove swamps were investigated through field observations at Iriomote Island, Japan. After the tidal water ebbed from the swamp surface to the creek, groundwater levels at swamp sites near the creek fell by up to 15 cm by the next flood tide, although the fall was negligible at sites far from the creek and at the open coast outside the mangrove forest. The amount of groundwater discharged to the creek from the swamp depended strongly both on the tidal range and the presence of the steep bank which separates the tidal creek from the fringing mangrove swamp. Based on the fall of groundwater level, the bulk hydraulic conductivity of the swamp was estimated to be 1.5×10⁻² cm/s. This value is two to three orders of magnitude larger than that measured in a laboratory using small scale sediment core samples collected in the swamp. These results suggest that the presence of crowded, intricate and large animal burrows as well as sediment layers rich in mangrove humus increases permeability in the mangrove swamp. Further, it is suggested that the mangrove topography with the steep bank of the tidal creek plays an important role which enhances material exchanges through groundwater between the mangrove swamp and the adjacent offshore waters.

Appendices

Appendix A

Cintron and Novelli (1984) have classified the mangrove landform into three types based on their topographical features, i.e. the riverine forest, the fringe forest and the basin forest.

The riverine forest (R-type) is defined as a floodplain along a river drainage or a tidal creek, which is inundated by most high tides and dry up at most low tides. Almost tidal creeks meander sharply and intertwine with each other. Sea waves are hardly found in the swamp, because of reduction due to bottom resistance through the long tidal creek. The fringe forest (F-type) is defined as a shore facing an open sea and is directly exposed to the action not only of tidal water but also of sea waves. The bottom slope of this area continues gradually to the adjacent open coast. The basin forest (B-type) is defined as a partially impounded depression, which is inundated by few high tides during dry season, but high tides during wet season. During dry season, the water level in the basin like a pond continues to decrease very slowly caused by the groundwater flow discharging to open sea due to water level difference between the basin and the open sea.

Appendix B

The specific yield in this study was determined in the laboratory as follows.

At a given site in the field, a cylindrical sediment core was sampled by a pipe sampler (ca. 7 and 15 cm in diameter and height, respectively) so as to be undisturbed and unconsolidated (see Figure B1(a)). Since the specific yield generally varies in time, the core samples above the water table in the field were collected just before the flood tide comes again. The volume (v) of the sample wrapped by a thin film was measured by immersion in a water tank and noting the change in water level from a ₀ to a ₁ (Figure B1(b)). After the air in the sample was removed, the volume (v′) of the sample was measured by noting the water level a ₂ (Figure B1(c)). The specific yield, γ_E, is calculated by Equation (B1).

$$ \gamma _{\rm E}=\frac{v-v^{\prime}}{v}=\frac{a_1 -a_2} {a_1 -a_0} $$

({\rm B}1)

Appendix C

When a soil is uniform, the following formulae for hydraulic conductivity are well known (Kikkawa 1976).

The hydraulic conductivity by Zunker’s formula: K _Z

$$K_{\rm Z} =\frac{1.10}{\nu} \left(\frac{\varepsilon} {1-\varepsilon} \right)^2D_{10}^2$$

({\rm C}1)

The hydraulic conductivity by Slichter’s formula: K _S

$$K_{\rm S} =\frac{10.2}{\mu _t} \left(\frac{1}{\chi} \right) D_{10} ^2 $$

({\rm C}2)

The hydraulic conductivity by Terzaghi’s formula: K _T

$$K_{\rm T}=800\frac{\mu _{10}} {\mu _t} \left(\frac{\varepsilon -0.13}{\sqrt[3]{1-\varepsilon} } \right)^2 D_{10}^2 $$

({\rm C}3)

where ν is the kinematic viscosity in cm²/s, ɛ the porosity, D ₁₀ the grain-size in cm at which 10% by weight of the grain particles are finer and 90% are coarser, μ _t the viscosity in g/cm/s at groundwater temperature (t °C), and χ is given as a function depending on ɛ.