The hydrology of areas of low precipitation - L'hydrologie des régions à faibles précipitations
(Proceedings of the Canberra Symposium, December 1979; Actes du Colloque de Canberra,
décembre 1979): IAHS-AISH Publ. no. 128.
Long-term flood frequency analysis using geological data
VICTOR R. BAKER and R. CRAIG KOCHEL
University of Texas at Austin, USA
PETER C . PATTON
Wesleyan University, Connecticut, USA
Abstract. Because extreme value theory encounters problems with the negatively skewed annual
flood distributions and short gauge records that occur in many arid regions, we here demonstrate
a geological approach to palaeoflood analysis. The most useful data for geological flood frequency
analysis come from confined bedrock canyons in which relatively small discharge variations
produce relatively large changes in stage. Slack-water deposits up to the maximum stage height of
the largest floods may be preserved at protected locations along such reaches. The dating of
various flood events is possible by radiocarbon analyses of appropriate materials intercalated with
the slack-water deposits. The Lower Pecos River of western Texas contains several sites with
correlative slack-water sequences for 1 0 - 1 2 major flood events. The Arenosa locality preserves a
record of the Pecos flood history over the past 9500 years. The data show that the Pecos flood
of 1954, which was nearly an order of magnitude larger than any other in 40 years of record, had
a recurrence interval of 2000+ years.
Analyse de la fréquence des crues sur de longues périodes à partir de données géologiques
Résumé. La Théorie de la valeur extrême s'appliquant difficilement aux distributions des crues
annuelles avec coefficient d'asymétrie négative et aussi par suite de la brièveté des relevés de stations
de jaugeage dans les régions arides, nous présentons ici une méthode géologique pour l'analyse des
crues sur des bases géologiques se rencontrent dans des gorges rocheuses à lit très étroit, dans
crues sur des bases géologiques se rencontrent dans des gorges rocheuses à lit très étroit, dans
lesquelles des variations assez modestes du débit produisent des changements assez considérables
de niveau. Les dépôts apportés par l'eau à la hauteur maximum des grandes crues peuvent
subsister dans les endroits protégés le long de ces gorges: Il est possible de vérifier l'âge des
différentes crues par l'analyse radio-carbone des matériaux les plus convenables mêlés aux
délaissés de crues. Dans la partie inférieure du Pecos River, à l'ouest du Texas, on trouve plusieurs
sites avec des séries de délaissés de crues comportant 10 à 12 grandes crues! La région d'Arenosa
conserve l'histoire des crues du Pecos depuis 9500 ans. Les données montrent que l'inondation
du Pecos de 1954, laquelle fut d'un ordre de grandeur plus élevé que toutes les autres pour des
relevés s'étendant sur 40 ans, a eu une période de retour de 2000+ ans.
INTRODUCTION
The arid and semiarid regions of the world are subject to pronounced changes in rainfall regimen over time scales of decades and centuries. Conventional flood frequency
analyses of measured streamflow in such areas may provide misleading results if the
period of record coincides with a period of abnormal rainfall—runoff conditions.
A related problem arises when attempting to estimate the recurrence in arid regions for
rare catastrophic floods of extremely long return period. Hydrologists are increasingly
becoming aware that, despite its statistical elegance, extreme value theory encounters
great difficulty in treating the distributions of annual floods that have substantial
tails, i.e. large negative skew. Such distributions often characterize arid regions (Baker,
1977). The problem is compounded because these same regions usually lack sufficiently long records to provide a suitable data base. Various hydrogeomorphic techniques have been advocated for flood studies in such areas (Baker, 1976; Patton and
Baker, 1976).
We report here on a seldom used technique for assessing long-term flood frequency
analysis and subsequent palaeohydrologic interpretation of 'slack-water deposits'.
3
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Victor R. Baker étal.
These are generally fine-grained flood sediments that accumulate at flood stage in
channel expansions and in backflooded tributaries where current velocity is reduced.
We have discovered especially useful sequences of such sediments in the deeply
entrenched river valleys of southwest Texas and south—central Utah. The slack-water
sequences are not as spectacular as those described for the Pleistocene Missoula floods
of eastern Washington (Bretz, 1969; Baker, 1973), but they are much more pronounced than those recognized on humid region flood plains (Costa, 1978; Jahns,
1947). Slack-water deposits are difficult to interpret on rivers with broad flood plains
because a very large increase in overbank discharge (occurring every year or two) will
only produce a slight increase in stage. Narrow bedrock valleys, which are adjusted to
scour by relatively infrequent floods (Baker, 1977), provide the best study sites. In
the confined bedrock valleys of central Texas we have found that great floods commonly fill the valley from bedrock wall to bedrock wall (Baker, 1975).
In our central Texas studies we have found that buried charcoal from surficial fires,
buried soil A horizons, and archaeological materials can provide sufficiently detailed
time stratigraphy to establish flood recurrence intervals. The buried soils are particularly common and result from the fact that floods with recurrence intervals in excess
of several hundred years allow sufficient time for incipient A horizon development
prior to burial by the sedimentation of a brief flood event. However, if the slack-water
deposit is not sufficiently elevated above more frequent flood stages, then sedimentation by the more frequent events will be so rapid that soil profile development is
inhibited. The ideal locations for producing these buried A horizons are those which
have accumulated very thick slack-water sequences (certainly to an elevation in excess
of 100-year flood stage) along streams which have highly right-skewed flood frequency
distributions, and which flow in deep, narrow valleys.
PECOS RIVER
As a measure of the variability of flood flows, Beard (1975) calculated the standard
deviation of the logarithms of the annual peak series for 2900 stations in the US. The
Pecos River at Comstock, Texas provides a striking example of the high flash flood
potential, as measured by this criterion, in arid regions where physiographic factors
(Patton and Baker, 1976) combine with climatic ones to induce pronounced flood
variability. The 39 annual flood peaks until 1954 provide no indication of the potential
for the flood that came in that year (Fig. 1). From 24—29 June 1954, an extratropical
disturbance resulting from Hurricane Alice migrated far inland and produced rainfall
of up to 1067 mm on the Pecos-Devils River divide (Fig. 2). This storm produced the
largest flood on record in Texas, which peaked at 27440m 3 /s near Comstock, Texas,
nearly eight times the previous recorded maximum discharge. The contributing area
for this flood was only 9300 km2 (Patton and Baker, 1977) because 200 km upstream
at Sheffield, Texas, the maximum discharge was only 475 m 3 /s.
The gauged flood frequency record of the Pecos clearly presents a dilemma to the
water resources planner. Extrapolating the frequency curve of the first 39 years (Fig. 1,
curve c) the expected recurrence interval for the 1954 flood would be on the order of
millions of years, way beyond the probability limits that would be accepted as being
satisfactory for most flood control projects. If the curve is drawn through the point
based on the 40 years of record (Fig. 1, curve a) the flood has a recurrence interval of
only 41 years. The steep slope on the curve (high skew) predicts extreme magnitudes
for floods having a recurrence interval greater than 100 years, which could result in
the gross overdesign of water control structures. Moreover, the various hydrological
methods for 'correcting' such 'outliers' are no more than guesses for the real flood
frequency distribution, since the observed distribution is a sample over too short a
time base. It is simply not possible to accurately estimate the frequency of the 1954
Pecos River flood from the existing hydrological data set.
Long-term flood frequency analysis using geological data
10
100
Recurrence interval (years)
1000
10000
FIGURE 1. Flood frequency curves for the Pecos River near Comstock, Texas. Curve (a)
shows the frequency curved based on the 1954 flood and the other 39 years of recorded
annual flood peaks. Curve (c) is the extrapolated flood record, excluding the 1954 event.
Curve (b) is a palaeoflood frequency curve estimated from the alluvial stratigraphy at
Arenosa Shelter (site A, Fig. 2). Vertical bars for the Arenosa data represent the estimated
error in determining flood discharge from the sediment elevations. The horizontal bars are
the standard deviations of the radiocarbon dates.
We were able to estimate the recurrence interval of the 1954 flood from a spectacular alternating sequence of cultural habitation layers and flood slack-water deposits
at the Arenosa Rock Shelter (Fig. 2) on the Pecos River near its confluence with the
Rio Grande (Patton, 1977; Patton and Baker, 1977; Patton and Dibble, 1978).
Arenosa Shelter is a bedrock overhang that began filling with flood sediment approximately 9500 years ago, following the diversion of the Pecos River channel away from
the western bedrock wall. Analysis of the alluvial stratigraphy and radiocarbon dating
of the intercalated cultural horizons provided the means of establishing a palaeoflood
frequency record for the Pecos River (Patton, 1977). Sediment deposited by the 1954
flood buried a 1300-year old surface and indicates that the flood had a recurrence
interval in excess of 2000 years (Patton, 1977; Patton and Baker, 1977).
The alluvial stratigraphy of Arenosa Shelter provided real data on the flood peaks
with recurrence intervals between 100 and 1000 years (Fig. 1, curve b). Thus, one key
slack-water site can be used to extend conventional hydrological flood frequency
curves to accommodate rare, large floods indicated in the slack-water sedimentation
record.
We have recently studied additional slack-water sedimentation sites along the Lower
Pecos River (Fig. 3) to refine the method and to establish its regional applicability.
Although each Pecos River tributary canyon contains slack-water sediment accumulations, not all sites are suitable for palaeoflood determinations. Several tributary canyon
characteristics seem to favour the preservation of useful flood slack-water sequences:
(1) an intermediate sized drainage area and relatively low channel gradient to prevent
periodic flushing of canyon floor sediments by tributary floods; (2) protected sites of
accumulation, such as in the lee of bedrock protrusions or on the inside of meander
bends; (3) a sufficiently large junction angle with the main stream to allow substantial
6
Victor R. Baker et al.
FIGURE 2. Index map showing the West Texas study area. The letter A shows the location
of Arenosa Shelter, and the numbers refer to slack-water sedimentation sites shown in
Fig. 4. The Comstock gauging station is at the US90 bridge over the Pecos River. (Based on
the US Geological Survey Del Rio and Sonora, Texas, 1: 250 000 scale maps.)
back flooding; and (4) minimal vegetative cover of the deposits to reduce bioturbation
and to allow easy trenching of the site.
The slack-water deposits of the Lower Pecos River tributary canyons are generally
preserved as terraces along the margins of the canyons. The slack-water sediment
mineralogy of predominantly quartz sand requires a source from the Pecos River,
since all of the tributaries are underlain by carbonate rocks. The sediments are wellstratified layers of silt, sand, and organic debris (Fig. 4). Horizontal laminations are
occasionally present and are most common in the sandy layer. In addition to layers of
leaves and other fine-grained organic material, logs are found interbedded with the
sediment. We are currently investigating the radiocarbon dating of these materials.
Five of the best preserved Pecos canyon tributary sites reveal a correlative stratigraphy of 10 to 12 distinct layers that apparently record the same flood events (Fig.
4). No erosional unconformities are present in the sequences, each of which rests on
up to several metres of bouldery sand and gravel derived from the local tributaries.
Incipient soil development on many of the buried alluvial and colluvial deposits
includes root mottling, oxidation, formation of sand-sized calcium carbonate nodules,
colour changes, and minor clay translocation. Parametric plots of grain size parameters
generally discriminate between soils developed on colluvium and alluvium. The
incipient soil development indicates considerable time spans between the deposition
of the sediment containing the soil and the next flood of comparable magnitude.
Grain size (Mz of Folk, 1974) displays variable vertical trends within a single flooddepositional unit; usually no systematic changes in grain size are present (Fig. 4). Apart
Long-term flood frequency analysis using geological data
7
FIGURE 3. Details of the Lower Pecos River study region, showing the locations of slackwater sediment accumulations short distances up the mouths of Pecos River tributaries.
Note the relatively short lengths of the tributaries and the low regional drainage density.
(Based on US Geological Survey Mouth of Pecos, Shumla, and Langtry 15' maps.)
from occasional horizontal laminations, sedimentary structures are generally absent,
perhaps indicating very rapid fallout from suspension during these backflooding
surges. Average grain size for each slack-water section decreases with increasing distance
from the Pecos River. Sorting appears to improve slightly with increasing distance from
the Pecos River.
DEVILS RIVER
Although the Devils River basin is adjacent to the Lower Pecos River study sites (Fig.
2), its record of Holocene flood sedimentation is very different from that observed
along the Pecos. Figure 5 illustrates the stratigraphy of a site on the west bank, 1.5 km
south of Miller Canyon (Fig. 2, site 8). The section shows a basal layer of mottled and
gleyed massive clay, probably a pond or swamp deposit. This is overlain by limestone
cobbles and boulders emplaced by a flood down the Devils River tributary containing
the deposit. Slack-water sediments overlying the gravel show two prominent buried
soils. Their properties include a thin, dark brown partial A horizon, a gradual increase
in clay content and intensity of reddish hues in the B horizon, which averages 2 0 40 cm in thickness, and a basal relatively-clay-poor brown horizon (25-30 cm thick).
Victor R. Baker et al.
FIGURE 4. Stratigraphie sections at the five Pecos River slack-water sedimentation sites
located on Fig. 3.
Bga COBBLES a
E H BOULDERS
[ H SAND
H
SILT
j § SILT a CLAY
II BURIED SOIL
ccc
CALCIUM CARBONATE-RICH LAYERS
V
GLEYED a
9
GASTROPOD-RICH LAYER
6%
MOTTLED
FIGURE 5. Stratigraphie section at a small tributary to Devils River (locality 8, Fig. 2)
showing complex Holocene alluvial stratigraphy typical of the Devils River basin.
Long-term flood frequency analysis using geological data
9
These buried soils probably required considerable periods of time for their formation
and indicate significant time lapses between large magnitude flood events during this
interval of deposition. At the surface, a large side-channel gravel bar of the Devils
River overrides the entire sequence, prograding southward perpendicular to the
exposed section.
Our initial reconaissance work on the Devils River tributary valleys suggests that
the flood record preserved in each valley is unique and non-correlative. The differences
between the Pecos and Devils River slack-water records probably result from the
differing basin physiographies. The Lower Pecos River basin is characterized by poorlydeveloped tributary networks and broad, relatively flat un dissected interfluves. The
result is a relatively low drainage density, and the flood runoff is dominated by the
main stem, while the influence of downstream tributaries is probably minor. The
apparent uniformity of the number of flood layers in the Pecos slack-water sediments
may indicate fairly uniform flooding over the reach studied. On the other hand, Devils
River tributaries have developed significantly higher drainage densities. As a result of
their greater runoff efficiencies, the Devils River tributaries may display a much greater
flood response to local intense rainstorms. Evidence for the large contribution of
tributary water to the flood regimen of this river is provided by the large quantities
of coarse tributary sediment characteristically found at the mouths of most of the
tributaries of the Lower Devils River. We suggest that intense isolated summer
thunderstorms yield tremendous runoff in the Devils River tributaries. Thus, each
tributary contains a unique record of tributary floods of varying frequencies interbedded with periodic backflood deposits from the Devils River.
Acknowledgements. Our work has been supported by the Division of Earth Sciences, National
Science Foundation, NSF Grant EAR 77-23025. Publication support was provided by the Geology
Foundation, University of Texas at Austin.
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