Introduction
Floodplains are relatively low and flat portions of land adjacent to rivers subject
to periodic inundation by river water (Wolman and Leopold, 1957). They have great
socio-economic and ecological importance, ranging from intense inhabitation and
industrial use, through high-productivity agricultural land to sites of extraordinary
biodiversity and biological productivity that have suffered little management or other
human intervention (Philippi, 1996). During late Holocene human activity has brought
about rapid changes in many floodplains through, for example, land use change and
channel engineering (Lewin, 1978; Alexander and Marriott 1999). They provide flat and
fertile ground that is very productive and easy for farming (SAST, 1994; Philippi, 1996).
Business and industry benefit from the inexpensive access to the river transportation.
Rivers provide a source of water and a place to dump waste. Floodplain homes and
recreational facilities benefit from the beauty of streams and lakes, recreation in and on
the water and access to wild life hunting, fishing etc (Knight, 1989, Philippi, 1996). All
this activity is disrupted, sometimes disastrously, by flooding.
Under natural conditions, flooding just happens. Annual floods are a normal part
of natural water cycle when excessive amount of water flow out of the channels and
across the floodplains. It is an ordinary and natural event resulting from uncontrollable
meteorological forces (Philippi, 1996). Floodplain land essentially belongs to the river
itself. Rivers tend to claim the portion of that land whenever it needs to accommodate the
excess amount of water being carried in the channel. The portion of floodplain land that
1
river uses annually for flooding for a short duration is also used by human being for the
rest of the period as it has great economic advantages. Flooding is essential for healthy
functioning of the ecosystem and for maintaining fertility of floodplain land but at the
same time it becomes a problem when it causes economic damages on the same land
area. In general flooding causes economic damages and ecological benefits.
It is important to manage river floodplains to reduce the economic damages and
protect the ecological benefits. One of the most important goals of floodplain
management is to prevent the disruption of the human activity on floodplains and
damages of homes and human property over the floodplains. Management of floodplains
is an important issue today, specially in the view of severe conflicts between the
environmental and economic interest in the use of floodplains (Philippi, 1996).
During the early 1800’s when the cities began to grow near the floodplain of
Mississippi and Missouri Rivers to take advantage of the transportation network over the
rivers, it was realized that either the cities will have to tolerate flooding or some type of
protection must be constructed to protect the cities from flooding (IFMRC, 1994). After
much debate regarding levees and other methods of flood protection, a policy that led to
construction of local protection was developed (Humphrey and Abbot, 1861). When the
levees alone failed to provide sufficient protection, flood control reservoirs were
constructed upstream to store floodwaters and release them slowly. The purpose of flood
control work was to regulate flood flows and thus prevent flood damage (SAST, 1994).
2
US Army Corps of engineers has played an important role in the shaping of
Mississippi and Missouri Rivers into its present state. The corps of engineers is
responsible for flood control, river stabilization, and channel maintenance and navigation
control over these rivers.
Missouri River is the longest river in North America. It originates in the
southwestern Montana and flows 3678 km to the east and southeast before it joins the
Mississippi River near St. Louis, Missouri. It drains an area of 1,354,564 km 2, which is
approximately one sixth of the contiguous United States (Patrick, 1995). Missouri River
floodplain system has been altered dramatically by human activity. The two most
important agents of change are the series of main stem reservoirs in the upper river and
the channelization of the lower river. The Missouri river today is very different from the
wild anastomosed river seen by Lewis and Clark. The six main stem impoundments
constructed between 1938 and 1963 in the upper Missouri River have substantially
reduced turbidity, regulated its flow and degraded the formerly alluvial channel. The
extensive floodplain of Missouri River throughout its length has been either inundated by
the impounded river water or developed for agricultural, industrial and urban use. From
downstream of Gavins Point Dam the river has been channelized for navigation for about
735 miles. US Army corps of engineers has constructed channel maintenance structures
such as wing dikes, spur dikes and revetments. These structures use the force of the river
to scour a narrower and deeper channel that facilitates navigation. Flood control
structures constructed by AEC have transformed the Missouri River from a wild
anastomosing river to stable meandering river.
3
Lower Missouri River floodplain is used for agricultural proposes to a great
extent. Schmudde (1963) classified bottomlands of lower Missouri River floodplain as
loop bottoms and long bottoms. Long bottoms are land areas enclosed by river bends,
which are much longer in the down valley direction than they are in cross-valley
direction. Loop bottoms are areas enclosed by river bends that are nearly the same size in
long and cross-valley directions. Loop bottoms have high surface gradients and higher
sinuosity reaches as compared to the long bottoms.
These bottomlands are protected to varying degrees from floodwater by flood
control levees (SAST, 1994). The present system of flood control levees along the lower
Missouri River floodplain is aggregate of levee network constructed by different agencies
depending upon the magnitude and frequency with which the area in inundated during
floods. Depending upon the type of construction and these levees protect farmlands, from
floods of 2 – 10 year frequency. These farmlands are inundated and damaged by very big
flood events from time to time despite the strong network of flood control structures built
by corps of engineers, when flood control system is overwhelmed by the magnitude of
the flood event, as in the case of Great flood of 1993.
During the summer of 1993, the Missouri River experienced the largest recorded
flood in terms of river stage, daily discharge, and total discharge (SAST, 1994).
Agricultural Damages from the flood had two primary causes; excessive moisture, which
prevented planting and reduced yields in upland and actual flooding that destroyed crops
4
and severely damaged many acres of fertile crop land. Damage from scouring and
deposition severely affected ~1,800 km2 of agricultural land. Distinct scoured, stripped
and depositional zones associated with as much as +/- 5 m of localized erosion and
deposition characterized patterns of erosion and deposition on the floodplain. In all some
300 km2 of floodplain land was covered with sand deposits ~60 cm thick. For restoring
the damaged cropland either deep plowing or removal of sand was required. The costs to
remove this sand and restore soil productivity was estimated to be ~$1.2 million per km 2
(IFMRC, 1994). Local and federal management agencies require rapid post-event
measurements of the location and depth of erosional/depositional features to access the
total damage to the cropland. These assessments are critical for recovery and mitigation
of the cropland to restore it to productive state.
Lisbon Bottoms (Figure 1), a loop bottom on the Missouri River that was too
severely impacted by the 1993 flood to be restored (Scientific Assessment and Strategy
Team, 1994), is representative of that experienced elsewhere along the lower Missouri
River floodplain. Lisbon Bottoms consists of about 875 ha of river bottom along the
Missouri River in Howard County, Missouri, from approximately river mile 213 to river
mile 219 (Humburg and Burke, 1999).
After the Great flood of 1993 devastated Lisbon Bottom’s farmland and network
of levees, it was acquired by USGS and became part of the Big Muddy Fish and Wildlife
Refuge (Humburg and Burke, 1999). Since the land area of Lisbon Bottoms experienced
extreme damage, and then became a part of Big Muddy Wild life Refuge, there has been
5
only limited post-flood agricultural activity, thus preserving the nature and extent of
levee breaks and associated erosion and deposition. Therefore Lisbon bottoms can be an
ideal site to estimate net erosion and deposition on floodplains that would be
representative of conditions immediately after the great flood.
Figure 1 - Location Map.
6
There is number of ways by which estimation of net erosion and deposition of
sediment over the floodplain can be done, which includes traditional land survey,
electromagnetic induction measurements (Kitchen et al., 1996) and use of remote sensing
data like SIR-C and TOPSAR radar data (Izenberg et al., 1996). These methods are either
expensive, time consuming (land survey) or less precise (aerial photograph/radar
interferometry). GIS based technique, which involves comparison of a pre-flood and a
post flood DEM to quantify net sediment deposition could be used to make accurate
estimations. A Digital Elevation Model (DEM) provides a digital representation of a
portion of the earth's terrain over a two dimensional surface. The basic data for a DEM is
based on terrain elevation observations that are derived commonly from these sources:
digitized contours, photogrammetric data capture (including aerial photography and
digital satellite imagery), aerial and land surveying.
NASA's Airborne Topographic Mapper is a laser based remote sensing instrument
used to derive very high-resolution topographic elevation observations in digital format
(Krabill et al., 1984, Ritchie et al., 1996). Airborne laser altimeters offer the potential to
accurately measure land surface features and properties over large areas quickly and
easily (Krabill et al., 1984, Schreier et al., 1985, Weltz et al., 1994). The NASA ATM is
a conically scanning airborne laser altimeter system capable of acquiring a swath of
highly precise topographic measurements ~ 250 m wide (depending on altitude), with
typical spot spacing of 1-3 m, and vertical precisions of 10-15 cm.
7
For precise quantification of sediment erosion/deposition during large-scale
riverine floods, good vertical resolution is required to effect accurate elevation difference
mapping. DEMs prepared using conventional remote sensing techniques, such as aerial
photographs or radar interferometry usually exhibit vertical and spatial resolution that
lack the ability to discriminate topographic change that is of the order of +/- 1 meter. By
contrast, DEMs generated using ATM data can provide highly precise horizontal (~ 1 m)
and vertical (~100 - 150 mm) spatial measurements that would permit us to characterize
morphological change and patterns of erosion and deposition.
This project aims to assess net topographic change by comparison between a preflood DEM and a post-flood DEM to obtain details of topographic change (elevation) to
the order of 10 - 15 cm. The results obtained from this comparison would give better
understanding of patterns of erosion and deposition due to 1993 and 1996 floods and
calculation of precise amounts of net sediment deposition on any portion of the
floodplain represented in both DEMs. Successful completion of this project would give
insight to the erosional and depositional processes during large floods and estimation of
sedimentation in a cost effective and timely manner.
8