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Abstract: Impact of Spring Sapping and Surface Runoff on Microtidal Beaches: Erosional and Depositional Processes. Landforms Related to Groundwater, Surface Sheetwash, Drainage Channel Formation, Associated with Significant Rain Events

OTVOS, ERVIN G., Gulf Coast Research Laboratory, USM Institute of Marine Sciences, Ocean Springs, MS 39566-7000

Summary

While shore bluff retreat through runoff erosion and piping is often discussed, the important role of the sapping and surface erosion effects of emerging ground water on beach foreshores has received little or no attention. “Spring(head) sapping” involves concentrated seepage flow that dislodges clast particles along subsurface planes or linear pathways of weakness. Sapping occurs both in rocks (e.g., in valleys of Hawaii, the Colorado Plateau, and apparently also on Planet Mars) and in cohesive and cohesionless sediments, including California beach sands (Higgins, 1984; Baker, 1990). It results in an array of landforms and associated surface features.

Essential features of sapping-created landforms, in contrast with runoff-dominated drainage networks often include:

(1) (amphi)theater-headed, abrupt valley heads

(2) vertical valley walls, flat valley floors

(3) alcoves at valley wall toe

(4) hanging tributary valleys face trunk valley

(5) fewer downstream tributaries; higher junction angles

(6) high valley relief in valley

(7) low drainage density, variable channel trend

(8) lesser dissection, flat surfaces of interfluve area

(9) smaller surface catchment area

(10) subsurface pipe networks that conduct water and sediment particles

Microtidal Mississippi Sound beaches, underlain by low-permeability humate sand lenses develop multiple shallow groundwater horizons, sapping-related miniature landforms and sedimentary structures. Correlation of dry periods and significant rain events with formation of sapping-related landforms, intensive foreshore/backshore runoff channels, and the fine-textured, common dendritic rills on the lower foreshore indicate that the groundwater beneath the beach face responsible for these structures is not of salt water but of fresh water origin, in the investigated south Mississippi beaches.

Disintegration of miniature slump blocks, grainflow and wave swash action during falling tide expand the miniature valleys through headward erosion and seaward growth. This is accompanied by seaward evacuation of the eroded sand by miniature creeks downslope on the flat valley flood. Miniature alluvial fans accumulate at the channel terminals.

Formation of descending miniature terrace sets often accompany valley incision. The next incoming tide eliminates the gullies and flattens the foreshore. As long as the impact of the rain event lasts and the ground water horizons are recharged, the cycle is repeated with each outgoing tide.

Episodes of intensive rain (usually 2-to-6 inches within the span of one-to-two days), most prevalent in the late fall-to-early spring period on the northeastern Gulf of Mexico result in erosion of de significant amounts of sand in offshore direction from the broad, artificially maintained Harrison County beach through quickly developing wide gullies.

Falling tide level, caused by strong offshore-directed winds during recurring cold/cool weather fronts of the late fall-to-early spring period steepens gullly gradients increases the gully gradients, thus enhancing channel runoff and thus beach erosion. Similar, precipitation- linked beach erosion processes were observed on other, northeastern Gulf beaches as well and are expected to occur worldwide in areas of significant rainfall. These erosion processes are expected to be active particularly along wide beaches, backed by high bluffs that transmit and emit groundwater. Groundwater at such localities is periodically discharged along the bluff toeline and across the beachface.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah