John E. Costa

Physical Geomorphology of Debris Flows

Debris flows claim hundreds of lives and cause millions of dollars of property damage throughout the world each year. In Japan alone, an average 90 lives are lost annually from debris flows (Takahashi 1981). In 1970 a debris avalanche (a rapidly moving form of debris flow) triggered by an earthquake, completely destroyed the city of Yungay, Peru, killing an estimated 17,000 people and burying the whole city under 5 m of mud and debris (Plafker and Erickson 1978). Some countries with chronic losses from debris flows include Japan (Okuda et al. 1980); United States (Committee on Methodologies for Predicting Mudflow Areas, 1982; Scott 1972; Cummans 1981; Scott 1971; Flaccus 1958; Williams and Guy 1973; Woolley 1946; Morton and Campbell 1974); Indonesia (Scrivenor 1929); Tanzania (Temple and Rapp 1972); Scandinavia (Rapp and Stromquist 1976); Costa Rica (Waldron 1967); China (Li and Luo 1981; Chinese Society of Hydraulic Engineering 1980); Brazil (Jones 1973); Ireland (Prior et al. 1968); Romania (Balteanu 1976); India (Starkel 1972); Bangladesh (Wasson 1978); New Zealand (Selby 1967; Pierson 1980a, b); and the Soviet Union (Gol’din and Lyubashevskiy 1966; Niyazov and Degovets 1975; Gagoshidze 1969).

Effects of Agriculture on Erosion and Sedimentation in the Piedmont Province, Maryland

A study of surficial deposits in the Piedmont province of Maryland has made it possible to construct a balance equation of sediment production and deposition since erosive agricultural land use began in the 1700s. Truncated Piedmont upland soil profiles imply approximately 0.15 m of soil erosion. In Western Run, a basin 155 km2, consisting of 85 percent Chester and Manor soils, this amounts to 130 × 10−3 hm3/km2 of eroded sediment. Reservoir sedimentation rates imply that 34 percent of the eroded sediment has been carried out of the system. The rest of the sediment remains in the watershed as alluvium in the upper 1 m of flood plains and as colluvium and sheetwash deposits on hillslopes. Agricultural sediment stored in flood plains constitutes 14 percent of the estimated soil erosion. The sediment was deposited mostly by overbank deposition at rates as high as 1.6 cm/yr. The remaining 52 percent of the eroded sediment occurs as colluvium and sheetwash deposits on hillslopes and as fan-shaped colluvial-alluvial deposits at junctures of headwater tributaries. Wood from such deposits was radiocarbon dated at 290 ± 100 yr. A buried junk pile in a flood plain yielded license plates whose dates imply that after 1925 the dump was buried by overbank deposition at rates as high as those for a basin undergoing urbanization. Piedmont basins less than 26 km2 have statistically longer bankfull recurrence intervals than streams with drainage areas greater than 26 km2. This suggests that since the decline of agricultural land use in the early 1900s, small upland tributaries have adjusted to decreased sediment loads by entrenchment into and erosion of sediment deposited since the initiation of colonial agricultural land use.

Stratigraphic, Morphologic, and Pedologic Evidence of Large Floods in Humid Environments

Because of the difficulties faced by engineers, planners, and geomorphologists involved in the prediction of very large floods, the use of stratigraphic, morphologic, and pedalogic evidence for recognition of large floods in humid environments is helpful. Coarse gravel lenses in fine-grained alluvium, inverse grading, and clay balls are stratigraphic records of large floods, best preserved in valley reaches with wide cross sections. Alluvial fans and stepped or benched flood plains are landforms observed to form in lowlands following catastrophic floods. Mappable alluvial soils delineate only areas affected by moderate floods with recurrence intervals of less than 50 yr.

Colorado Big Thompson flood: Geologic evidence of a rare hydrologic event

Conventional engineering hydrologic analysis of the 1976 Colorado centennial Big Thompson flood indicates that the flood event, while catastrophic, was not extremely unusual. The geologic evidence in the area immediately below the area of maximum runoff, however, indicates that a flood of this magnitude had not occurred in the main channel for several thousand years. Alluvial-fan deposits extensively eroded by the flood waters were dated by observations of hue oxidation, coring of trees, and 14 C dating of buried charcoal. Some eroded deposits were as old as 7,000 to 10,000 14 C yr. Where usable, geologic evaluations of extreme flood events may yield more accurate estimates of the risk of large, rare floods than conventional methods.

The dilemma of flood control in the United States

In spite of increasing annual expenditures for flood control, losses from flooding continue to rise in the United States. This seeming contradiction arises from overdependence on federally supported structural solutions to flood problems. Nonstructural controls are initiated reluctantly at local levels of government because of constitutional questions, restrictions of local tax bases, lack of federal subsidies for nonstructural solutions, and the high costs of delineating flood hazard areas. The success of the National Flood Insurance Program is doubtful since only about five percent of the flood-prone communities in the United States have qualified for the regular program. Future reduction of flood losses is dependent upon increasing popular awareness of flood hazards and altering federal subsidy policies to reduce the impact of local land-use regulations.