Channel evolution of Des Moines Lobe till drainage ditches in southern Minnesota (USA)

ORIGINAL ARTICLE

Channel evolution of Des Moines Lobe till drainage ditchesin southern Minnesota (USA)

Joe Magner • Brad Hansen • Tim Sundby •

Geoff Kramer • Bruce Wilson • John Nieber

Received: 8 November 2010 / Accepted: 11 April 2012 / Published online: 4 May 2012

� Springer-Verlag 2012

Abstract Some drainage ditches in the intensively man-

aged row-crop agricultural region of southern Minnesota

evolved from a trapezoidal form to multi-staged channel

forms similar to natural streams. Older ditches constructed

in cohesive sediment of the Des Moines Lobe till tend to

follow a channel evolution model developed by Simon and

Hupp. Site cross sections, longitudinal water and bed

profiles and bed material particle size were determined

according to Harrelson and others at 24 older ditch reaches,

5 newly constructed ditch reaches and 13 natural stream

reaches. Morphological features were hypothesized to

change from trapezoidal form to flat bench banks, similar

to benches found in natural stream channels. All data were

statistically analyzed with respect to drainage area using

regression, because channel form is directly related to

drainage area for a given climate, geology and land use.

Results show similar regression slope and intercept for

bankfull channel width and bankfull cross-sectional area

(CSA) of older ditches and natural streams compared to

typical trapezoidal designed ditches. Evolved ditches

developed a small floodplain bench above the ditch bed and

adjusted their bankfull widths similar to natural stream

channels with respect to drainage area. Old ditches showed

a relatively strong R2 (0.82, 0.68) for channel CSA and

width, a weaker R2 (0.45) for water surface slope, and little

to no correlation with bed particle size. Channel form

appears to have adjusted more quickly than bed facets and/

or bed particle size distribution. However, stepwise

regression determined that D84, width/depth ratio and mean

bankfull depth explained 83 % of the variability of channel

features across varying drainage areas. Findings suggest a

possible reduction of long-term maintenance costs if older

ditches are allowed to evolve over time. A stable ditch

form similar to natural streams is typically self-sustaining,

suggesting that prior to a scheduled clean-out, the ditch

should be examined for hydraulic capacity, sediment

transport and bank stability.

Keywords Drainage ditches � Channel bench/stage �Channel evolution � Channel stability

Introduction

Drainage of land in Minnesota from the mid-1800s onward

was necessitated by the desire to provide arable land for a

growing population in the north-central region of the USA

and for food worldwide. Much of the drainage required the

construction of open-channel ditches to convey water from

agricultural areas within a catchment to the watershed

outlet. Issues of polluted runoff, including sediment and

nutrients, lack of aquatic habitat and long-term channel

sustainability are common problems associated with dit-

ches. Though ditch management is a concern worldwide,

this manuscript will focus on the rural agricultural ditches

associated with the unique geology of the Des Moines

Lobe till (DMLT) located in southern Minnesota (Fig. 1a).

The DMLT has been extensively drained over the past

century (Lenhart et al. 2009). Ditch development has made

the landscape economically viable by allowing landowners

Electronic supplementary material The online version of thisarticle (doi:10.1007/s12665-012-1682-3) contains supplementarymaterial, which is available to authorized users.

J. Magner (&) � B. Hansen � T. Sundby � G. Kramer �B. Wilson � J. Nieber

Department of Bioproducts and Biosystems Engineering,

University of Minnesota, 1390 Eckles Ave.,

St. Paul, MN 55108, USA

e-mail: [email protected]

123

Environ Earth Sci (2012) 67:2359–2369

DOI 10.1007/s12665-012-1682-3

http://dx.doi.org/10.1007/s12665-012-1682-3

to grow high-value agronomic crops such as corn and

soybeans. However, extensive drainage development has

led to adverse environmental impacts to down gradient

lakes, streams and rivers. One of the issues that the Min-

nesota Pollution Control Agency (MPCA) must address is

excessive fine-grained sediment detachment and transport

that causes violations of the state water quality standards

for turbidity in southern Minnesota streams and rivers.

Reducing excessive turbidity from channel sources will

require smarter management of agricultural ditches in

landscapes that contain silts and clays.

The environmental impact of drainage ditches can be

significant in terms of nutrient and sediment loss from the

catchment. Drainage systems by design move water off the

Fig. 1 a The location and

extent of the Des Moines Lobe

till (DMLT) region across

southern Minnesota into Iowa.

Source: Iowa Geological and

Water Survey (Quade 1998).

b The location of Minnesota

within the USA and the study

area of southern Minnesota with

respect to municipalities and

specific study sites labeled by

numbers found in Table 1

2360 Environ Earth Sci (2012) 67:2359–2369

123

landscape as quickly as possible. Drainage systems (open

ditch and subsurface drainpipes) by-pass natural ecosystem

processes of sediment and nutrient attenuation and

decrease the upland hydraulic residence time. Alexander

et al. (2000) have shown that higher order (larger) streams

have limited ability to attenuate nutrients. Smaller, low-

order streams offer more channel contact capable of

attenuating excess nutrients in runoff, with up to a 50 %

reduction during periods of high biological activity (Pet-

erson et al. 2001). Riparian zone buffer systems are

essential for trapping sediment and sediment attached

phosphorous and providing nitrogen transformation

(Osborne and Kovacic 1993; Magner and Alexander 2008).

Many kilometers of smaller low-order streams have been

converted to drainage ditches in the DMLT plain of

southern Minnesota (Quade 2000). Because drainage dit-

ches are relatively straight channels that are periodically

cleaned out or maintained for hydraulic capacity and sed-

iment transport, they have limited ecological value (Smiley

and Dibble 2005). Biological systems of vegetative and

animal communities connected between riparian zone and

the channel bed are essential for managing and buffering

adverse influences of upland polluted runoff (Stewart et al.

2001; Magner et al. 2008).

Agricultural ditch maintenance in southern Minnesota is

costly to landowners and the general public. Generally,

straight ditches fail to adequately transport sediment,

resulting in the loss of hydraulic capacity via sediment

aggradation. Aggradation will typically flatten the hydraulic

gradient and increase channel hydraulic residence.

Increased hydraulic residence will increase the depth of

flow and backup water in drainpipes. If water fails to flow

adequately in drainpipes, then the soil profile will remain

saturated and limit corn and or soybean crop yield (Blann

et al. 2009). When landowners petition the drainage

authority (county government in southern Minnesota),

drainage inspectors will view the ditch and determine if a

clean-out is required to restore hydraulic continuity. If the

inspectors agree that a clean-out is needed, then a sub-

stantial private and public financial commitment will be

required from the landowners and the drainage authority.

Recently, the Minnesota Drainage Ditch Inspectors Asso-

ciation estimated that *$12 million dollars per year in

Minnesota was spent on drainage ditch maintenance. Blue

Earth County (Fig. 1b) alone spent $650,000 in 2005 and

$1.2 million in 2006 for maintenance of open-channel dit-

ches and subsurface drainpipes (Hansen et al. 2006). In

Ohio, it was estimated that an average of $450/mile is spent

annually on open-channel ditch maintenance (Hansen et al.

2006).

Natural fluvial processes, if allowed to proceed without

human intervention, appear to produce a multi-stage ditch

form. Trapezoidal channel features of a Swift County ditch

unravel over time as a direct result of channel over-wid-

ening; the channel evolved to a two-stage form (Christner

et al. 2004). Powell et al. (2007a) have suggested that a

new design and maintenance approach is needed to reduce

adverse environmental impacts and the cost of ditch

maintenance. Current ditch designs use recurrence intervals

of 10–100 years. Channel-forming flows are normally

associated with a 1.0- to 2.0-year recurrence intervals

(Simon et al. 2004) and may be as frequent as 0.5-year for

some intensively drained catchments (Powell et al. 2007a).

Typical over-designed channels appear to require channel

adjustment based on the channel-forming or bankfull flow

(Rosgen 1996). An underlying design assumption used for

ditches in the DMLT was that a flat landscape would have

low unit stream power and the cohesive soil would provide

flow resistance allowing the channel to remain stable. Ward

et al. (2004) suggested that a two-stage design, based on

the principles of fluvial geomorphology, will require less

frequent and less substantial maintenance. Given less dis-

turbance, such a design would also likely encourage

healthier aquatic ecosystem by limiting excess sediment

disturbance associated with a ditch clean-out (Powell et al.

2007b).

Given private and public desire to reduce adverse

environmental impacts and ditch maintenance costs in

southern Minnesota, baseline morphologic information was

needed to assess the current channel evolution of southern

Minnesota streams and ditches and the potential usefulness

of a more natural and stable ditch design (Ward et al. 2004;

Powell et al. 2007a). Therefore, the objectives of the study

were to: (1) develop empirical relationships between

channel morphological variables with respect to drainage

area, (2) determine if ditch age significantly influenced

channel morphology and (3) determine if the morphology

of ditches was significantly different from the morphology

of unmanaged natural streams found in the DMLT.

Methods

Description of the DMLT

The DMLT was created by multiple advances and retreats

of the Laurentide Ice Mass. The landscape left behind was

relatively flat, including historic lacustrine plains, more

rolling ground moraine with some hillier stagnation mor-

aines. Tills deposited by the Des Moines Lobe was the

result of lithologically distinct till sheets corresponding to

unique ice-stream source areas and the varying dominance

of nearby and competing ice streams and their tributaries

(Lusardi et al. 2011).

In general, however, a dense blue-gray calcareous loamy

till underlies weathered less dense olive-brown clay loam

Environ Earth Sci (2012) 67:2359–2369 2361

123

subsoil. In the western portion of Minnesota, the till is

underlain by Cretaceous deposits, mostly sandstone,

whereas sites east and south of the Minnesota River bend

near Mankato, Minnesota are underlain by layers of

Cambrian and Ordovician sandstone, limestone and shale.

Given the relatively flat terrain underlain by relatively

poorly drained soils, drainage network development was

estimated to be ‘‘immature’’ compared to other regions of

the USA (Quade 2000) as the land was developed for

agricultural use. The historic hydrologic budget was

dominated by precipitation and evapo-transpiration with

limited interflow through the upper olive-brown till into

prairie lakes and wetlands (Leach and Magner 1992). Deep

groundwater recharge into bedrock aquifers was limited by

the dense blue-gray till. Though the till is generally con-

sidered saturated, the low hydraulic conductivity

(*10-8 cm/s) and concordant anisotropy force subsurface

water to primarily move laterally (Magner et al. 1993).

Depending on the landscape terrain, the weathered or

ablation till functioned as a shallow aquifer where water

primarily moved through preferential fractures or mac-

ropores. In the upper two meters of soil, weathering has

allowed the development of secondary porosity such that

water today drains rapidly to drainage outlets, given a

hydraulic head. The twenty-first century drainage network

of agricultural land found throughout southern Minnesota

and northern Iowa would not exist if the upper till was not

fractured, allowing rapid preferential movement of runoff

water. The creation of artificial channels (ditches) and

enlarged preferential fractures (subsurface drainpipes)

represent the current day ‘‘matured’’ drainage system found

in DMLT. Land use management coupled with a mature

drainage network has resulted in ecologically impaired

water quality (Lenhart et al. 2009; Minnesota Pollution

Control Agency 2010).

Field site selection and metric measurement

To develop the specific hydraulic relationships associated

with watershed characteristics, it was decided to use only

ditches that were old enough (not maintained within

10 years) to potentially start forming a bankfull bench. The

formation of a bench indicates that the ditch has started to

develop a floodplain similar to the evolved incised natural

stream stage V as defined by Simon and Hupp (1986). Five

ditches were defined as new, \5 years old with a trape-

zoidal form. Thirteen natural channel reaches were mea-

sured and served as reference reaches for typical DM LT

region as suggested by Rosgen (1996, 2001). Data were

collected across the intensively managed row-crop terrain

over varying scales from 29 ditch reaches and 13 natural

stream reaches from 2003 to 2005 (Fig. 1b). For relatively

easy access, most sites were located up- or downstream of a

bridge or road culvert. Crossing effects were noted and

sites were located in a fashion that minimized their influ-

ence upon the collection of survey data. Surveys were

conducted using techniques outlined in Harrelson et al.

(1994), including channel cross sections, channel bed and

water surface longitudinal profiles, and a particle size

distribution of bed sediment.

The number of cross sections measured ranged from 1 to

3 depending on reach complexity. For each study reach, a

longitudinal profile survey was completed over a channel

length equal to at least 20 bankfull channel widths.

Regional hydraulic geometry curves developed Magner

and Steffen (2000) for natural streams in southern Min-

nesota were used to estimate initial channel bankfull width

and cross-sectional area. To facilitate the identification of

the channel-forming or bankfull stage, sharp bends in the

ditch were examined for depositional features. Addition-

ally, these sharp bends often developed a bankfull bench on

the inside bend; depositional flats and subtle bank angle

changes were used to verify bankfull stage by comparing

them to the regional hydraulic geometry curves developed

by Magner and Steffen (2000). Magner and Steffen (2000)

developed relationships for discharge, cross-sectional area,

mean bankfull depth and mean bankfull width from field-

observed bankfull elevations with staff gage heights at the

US Geological Survey (USGS) gaging stations on natural

streams. The underlying assumption is that fluvial pro-

cesses over time will shape and form any channel whether

it began as a ditch or natural channel. Ditch benches could

form at different rates, be higher or lower than natural

stream benches, but over time, a channel-forming geometry

will adjust to its contributing drainage area; thus, the

contributing drainage is typically defined as one of the

dominant controlling factors of channel form (Lawlor

2004).

Bed material particle size was generally measured via

the pebble count method (Harrelson et al. 1994); however,

if the bed consisted mostly of fine-grained silt and clay, a

sample of the bed material was collected at ten locations

over the reach and a particle size analysis was conducted in

the laboratory. Data from these surveys were entered into

Mecklenburg’s Version 2.2 spreadsheet (Mecklenburg and

Ward 2004), which calculated the pertinent hydraulic

relationships necessary to assess channel hydraulics.

Data analysis

Regressions were developed between drainage area and the

bankfull channel metrics of cross-sectional area, mean

width, mean depth, percent channel slope and particle size

(D50 and D84). Statistical differences in the power rela-

tionships between streams and ditches were examined

using the methods given by Kleinbaum et al. (2008).


123

Differences in intercepts and slopes were determined

independently using t tests. Differences in the coincidence

of the two regression lines were assessed using indicator

variables and the F tests. Lastly, a stepwise regression

(SPSS, version 14.0 software, SPSS Inc., Chicago, IL,

USA) was performed on the eight metrics presented in

Table 1. The objective of the stepwise regression was to

determine which variables could explain the most vari-

ability between channels measured in the study.

Results

Hydraulic geometry and statistical relationships

Table 1 presents the mean values for bankfull channel

cross-sectional area (CSA), mean bankfull width (Wbkf),

mean bankfull depth (dbkf), width/depth (W/d) ratio, per-

cent channel slope (S), D50, (cumulative particles finer than

50 or 84 %) and D84. Data are illustrated in Figs. 2, 3, 4, 5,

6 and 7. Sites in Figs. 2, 3 and 5 illustrate similar trends

between the older evolved ditches and natural stream

reaches. Sites 38–42 (new ditches) illustrate the influence

of the trapezoidal design, similar CSA with drainage area

and a uniform channel slope as designed. Though the old

ditch trend line remained above that of the stream trend

line, both old ditches and streams showed similar regres-

sion slopes (0.6, ditch) (0.56, stream) and similar intercepts

(0.26, ditch) (0.21, stream). Evidence of a channel-forming

flow was not found in new ditches, because the banks were

relatively new and had not shifted yet. New ditch data

shown in Fig. 4 show no significant difference between old

ditches, whereas natural streams show the weakest corre-

lation. Nevertheless, all three trend lines converge upon

similar intercepts (0.23, 0.24, and 0.29 for old, new and

streams, respectively). Stream bankfull depth will vary

depending on channel type; whereas most ditches will have

a laterally confined channel type by design.

Based on the methodology of Kleinbaum et al. (2008)

described above, using a 5 % level of significance, there

were no significant differences in the intercepts and slopes

for the variables of width, depth, cross-sectional area and

slope. However, there were significant differences in the

coincidence of the regression lines for depth and cross-

sectional area; data support the findings that lines are

parallel but do not coincide with each other. A significant

difference was found between the slopes of the regression

relationships for D50 and the intercepts for D84. Both D50

and D84 were significantly different in the coincidence of

regression lines.

Figures 6 and 7 illustrate the difference between historic

fluvial processes during drainage network development

following the retreat of the DMLT glaciers. Water washing

over the till, given time with concordant boundary shear,

produced an increase in particle size with increased

drainage area. D50 for natural streams showed a steeper

regression slope and stronger R2 compared to the slope and

R2 for the D84. The particle sizes found in new ditches were

all silt and clay with no trend due to minimal sediment

transport. Particle sizes found in old ditches showed little

to no correlation with relatively flat and slightly reversed

trends compared to natural stream reaches. Both the

intercept for D50 (p = 0.054) and the slope for D84

(p = 0.093) were statistically significantly different at the

10 % level.

These data likely reflect the flat gradient nature of dit-

ches and lack of stream power to transport sediment.

Table 2 presents the results of stepwise regression

modeling. Stream channel variables are largely driven by

present-day geology, climate and land use. Generally, these

factors are relatively constant across the study area. This

means that the predominant dependent variable becomes

drainage area; the larger the drainage area, the larger the

channel. Using the eight metrics presented in Table 1, a

best fit predictor model of channel variables across drain-

age area suggests that D84, mean dbkf and W/d ratio

explained 83 % of the variability.

Discussion

The results of this study suggest that at least some

unmaintained ditches will evolve toward natural stream

channels with respect to bankfull channel width and cross-

sectional area. This suggests that fluvial processes driven

by climate, geology and land use over time will influence a

trapezoidal ditch to adapt and adjust form with respect to

drainage area. Based on close examination of the data

scatter in both Figs. 2 and 3, both old ditches and natural

streams in the DMLT became more uniform after

*44 km2. At scales less than 44 km2, channels appear to

be less equilibrated, perhaps due to changes in climate and/

or land use. For example, the two smallest streams used in

this study have relatively large differences in precipitation;

generally, precipitation increases from west to east across

Minnesota. In Fig. 3 at scales less than 100 km2, there are

data points that could be considered outliers both above

and below the old ditch and stream trend lines.

Mean bankfull depth is typically not visible in new

ditches and can only be estimated based on the use of

regional hydraulic geometry curves. Lateral migration will

always be limited by the incised nature of ditches com-

pared to the typically wider valley walls associated with

natural streams. Clearly, meander pattern extent in evolved

ditches will not approach that of natural channels. Valley

type, (natural valley wall compared to a ditch wall) and


123

Table 1 Data collected at 42 sites identified in Fig. 1 (A missing data)

Drainage

area (km2)

Cross section

area (m2)

Bankfull

width (m)

Mean bankfull

depth (m)

Width/depth

ratio

Percent

slope

D50

(mm)

D84

(mm)

Old ditch

1 5.85 0.66 1.86 0.40 4.7 0.175 1.5 26

2 6.55 1.01 2.07 0.49 4.3 0.29 0.13 1.5

3 7.56 0.63 2.03 0.30 6.3 0.6 1 50

4 7.67 1.02 2.53 0.40 6.3 0.24 1.1 10.5

5 15.54 2.32 4.91 0.46 10.9 0.035 0.33 0.83

6 20.72 3.73 6.64 0.56 11.82 0.01 0.12 0.57

7 28.49 3.28 5.12 0.64 7.9 0.025 0.33 1

8 33.38 2.10 4.02 0.52 7.7 0.1 6.7 10.7

9 34.68 1.47 3.69 0.40 9.4 0.22 25.2 41

10 38.33 1.30 2.50 0.52 4.9 0.23 0.5 8

11 43.98 2.14 3.87 0.55 7 0.36 0.5 8

12 51.83 1.89 3.84 0.49 7.8 0.098 1.5 5

13 56.62 3.47 5.64 0.61 9.1 0.07 0.2 10.5

14 69.93 3.10 2.13 0.61 8.6 0.031 0.2 0.42

15 88.06 4.50 5.94 0.76 7.9 0.027 A A

16 113.96 3.13 6.19 0.52 12.2 0.17 11 51

17 118.62 3.47 6.46 0.55 12 0.032 0.3 0.6

18 119.14 3.99 6.10 0.65 9.32 0.07 0.30 0.60

19 160.58 4.68 5.73 0.82 7.01 0.09 0.25 0.54

20 162.83 5.81 6.28 0.91 6.8 0.01 0.5 28

21 195.29 5.75 6.22 0.91 6.8 0.005 0.2 21

22 207.20 8.64 9.27 0.94 9.9 0.03 0.45 0.85

23 341.36 15.24 12.98 1.16 11.2 0.01 0.5 4

24 580.16 15.61 13.39 1.16 11.5 0.0044 A A

Stream

25 6.35 1.02 2.04 0.52 4.2 0.2 0.4 2

26 6.50 0.22 1.07 0.21 5 0.17 A A

27 15.46 2.44 3.78 0.64 5.9 0.04 0.2 16

28 44.19 2.03 4.33 0.49 9.3 0.29 A A

29 58.22 0.87 2.68 0.33 8.24 0.11 3.20 170.00

30 71.56 2.73 6.31 0.43 14.57 0.48 4.50 32.00

31 101.19 1.68 3.75 0.46 8.5 0.0019 0.4 2.9

32 123.54 6.14 8.32 0.74 11.28 0.07 A A

33 145.04 2.62 6.68 0.39 17.01 0.08 2.80 7.20

34 145.40 2.61 6.89 0.40 18.1 0.0017 1.4 5.4

35 170.94 5.14 9.14 0.56 16.27 0.46 38.00 120.00

36 214.97 3.73 6.64 0.56 11.82 0.03 18.00 93.00

37 300.44 6.35 8.29 0.77 10.83 0.00 45.00 190.00

New ditch

38 2.75 0.98 3.47 0.27 12.67 0.1 s/c 0.3

39 4.51 1.00 3.32 0.30 10.90 0.07 s/c 1

40 16.99 0.82 2.74 0.30 9.00 0.14 s/c s/c

41 18.34 1.39 3.87 0.37 10.58 0.06 s/c s/c

42 21.24 1.80 4.60 0.40 11.62 A s/c s/c


123

y = 0.2637x0.6045

R² = 0.8237y = 0.2099x0.5599

R² = 0.6099y = 0.7912x0.1661

R² = 0.2436

0.10

1.00

10.00

100.00

1.00 10.00 100.00 1000.00Cro

ss-S

ecti

on

al A

rea

(Sq

-M)

Drainage Area (Sq-Km)

Drainage Area Vs Cross-Sectional AreaDitch Stream NewPower (Ditch) Power (Stream) Power (New)

Fig. 2 Power function regressions of drainage area in square

kilometers plotted against the bankfull cross-sectional area in square

meters of channels presented as Ditch (numbers 1–24, Table 1),

Stream (numbers 25–37, Table 1) and New (numbers 38–42, Table 1)

y = 1.1308x0.357

R² = 0.6781y = 0.7435x0.4382

R² = 0.7699y = 3.1377x0.0546

R² = 0.0717

1.00

10.00

100.00

Ban

kfu

ll W

idth

(m

)


Drainage Area Vs Bankfull WidthDitch Stream NewPower (Ditch) Power (Stream) Power (New)

1.00 10.00 100.00 1000.00


kilometers plotted against the bankfull width in meters of channels

presented as Ditch (numbers 1–24, Table 1), Stream (numbers 25–37,

Table 1) and New (numbers 38–42, Table 1)

y = 0.2338x0.2379

R² = 0.756y = 0.2909x0.1174

R² = 0.1873y = 0.2429x0.1303

R² = 0.662

0.10

1.00

10.00

Ban

kfu

ll D

epth

(m

)


Drainage Area Vs Bankfull DepthDitch Stream NewPower (Ditch) Power (Stream) Power (New)

1.00 10.00 100.00 1000.00


kilometers plotted against the bankfull depth in meters of channels



y = 1.0365x-0.728

R² = 0.4512

y = 0.7726x-0.655

R² = 0.1746

y = 0.0858x0.0097

R² = 0.0006

0.001

0.01

0.1

1

Ch

ann

el S

lop

e (%

)

Drainage Area (Sq Km)

Channel Slope Vs Drainage AreaDitch Stream NewPower (Ditch) Power (Stream) Power (New)

1.00 10.00 100.00 1000.00


kilometers plotted against the percent channel slope of channels



y = 0.9665x-0.111

R² = 0.0092

y = 0.0166x1.1876

R² = 0.5653

Par

ticl

e S

ize

(mm

)


Particle Size D50 Vs Drainage Area

Ditch Stream Power (Ditch) Power (Stream)

0.1

1

10

100

1 10 100 1000


kilometers plotted against the cumulative percent of bed particles

finer than 50 % of the total particle size analysis for channels

presented as Ditch (numbers 1–24, Table 1) and Stream (numbers25–37, Table 1)

y = 10.919x-0.239

R² = 0.0298

y = 1.0156x0.7139

R² = 0.2504

Par

ticl

e S

ize

(mm

)


Particle Size D84 Vs Drainage Area

Ditch Stream Power (Ditch) Power (Stream)

1

10

100

1000

1 10 100 10000.1


kilometers plotted against the cumulative percent of bed particles

finer than 84 % of the total particle size analysis for channels

presented as Ditch (numbers 1–24, Table 1) and Stream (numbers25–37, Table 1)


123

stream type will influence the plot of geomorphic data,

particularly when DA is plotted against dbkf. Therefore,

caution should be exercised when using region hydraulic

geometry curves to estimate dbkf. Channel water surface

slope appears to have adjusted with time even in the flat

terrain DMLT given similar climate, geology and land use.

Constructed ditch channel slopes do not have facets and

follow a uniform slope similar to a run. Yet, with the

passage of time, facets (e.g., riffles and pools) will develop

because of changes in sediment supply and transport.

Because of the cohesive nature of the soils, fine-grained

material (silt, clay and fine sand) will dominate the particle

sizes typically found in DMLT ditches. Though the data in

Fig. 5 show relatively similar slopes and intercepts for the

old ditches and streams, the scatter of the data is relatively

large. These results may be influenced by varying degrees

of channel instability; both scour and aggradation.

Bank and bed erosion in the older ditches likely accu-

mulated downgradient with increasing drainage area

resulting in the negative trends presented in Figs. 6 and 7.

Yet, the ditches are connected to larger streams and rivers

and if base-elevation lowering occurs in the future, knick-

points could likely migrate up into the ditches driving a

new round of channel evolution. As channels cut deeper

into the dense blue-gray till, sand and gravel size material

will likely be introduced into the ditch bed. Currently, the

data indicated that most ditch beds sit upon resistant fine-

grained till, yielding small particle size material to the bed

compared to natural channels, which over time have

transported fine-grained material downstream leaving more

sand and gravel in the bed. It is important to note that

natural channels form as basin denudation occurs over

time, whereas ditches were created to provide a link

between prairie wetlands that would minimize the areal

extent of flooding in the DMLT.

From a water quality management perspective, flat gra-

dient ditches will have smaller particle sizes compared to

more evolved streams that have evolved over longer time

periods. Therefore, is it realistic to ask DMLT ditches to

meet the same water quality standards as DMLT natural

streams? The trapezoidal design limits the lateral extent of

water movement; however, over time the data suggest ero-

sion has occurred either due to hydraulics or geotechnical

failure from groundwater seepage. Given this design fea-

ture, excessive fine-grained sediment transport will likely

occur in a ditch and may violate state water quality standards

more often compared to an evolved natural stream. Fine-

grained sediment storage is likely to occur, but how much

and will it back up unacceptable amounts of water in agri-

cultural fields? Agricultural producers become concerned

when the soil vadose zone remains saturated for more than

36 h. The results of this study have both water quantity and

quality management implications; the following questions

are central to how the results of this study can be applied:

1. Should new ditches be designed to manage not only

water, but also future sediment inputs?

2. How often should a ditch be cleaned or should

‘‘regular’’ ditch maintenance be conducted?

3. Should more science and ecological engineering be

used to make ditch management decisions?

4. Can ditch maintenance be conducted in an ecologically

friendly manner or should ditches be self-maintaining?

Today, drainage management must consider not only the

movement of water, but also the environment (Smiley and

Dibble 2005). Because ditches were primarily designed to

drain water from the surrounding landscape, ecological

function was typically not a design consideration. However,

the design of a ditch may hold the key for future attainment

of water quality standards. Options for mitigating issues of

excessive sediment and nutrients in ditches and streams

include the incorporation of best management practices

(BMPs), which include structural and nonstructural controls

and operating procedures designed to prevent or reduce non-

point source pollution. Drainage BMPs include, but are not

limited to, grassed side-inlet waterways, energy dissipation

devices/structures and riparian vegetation/buffers, buried

rock inlets, erosion control fabric and constructed wetlands

(Yates et al. 2007). However, in landscapes drained by

subsurface drainpipes, water can by-pass constructed wet-

lands and buffer strips, unless they are located within an

Table 2 Stepwise regression of variables in Table 1 where W/d ratio,

mean dbkf and D84 were found to correlate best with drainage area for

streams and ditches measured in southern Minnesota

R value R2 Adjusted

R2Std. error Durbin–

Watson

Model summary

0.918 0.843 0.826 36.87 1.46

Sum of squares df Mean square F Sig.

ANOVA

Regress 203,689 3 67,896.2 49.93 0.000

Residual 38,074.9 28 1,359.82

Total 241,763 31

Unstandardized coefficients Standardized coefficients

B Std. error b t Sig.

Coefficients

Constant -220.564 27.818 -7.93 0.000

D84 0.595 0.14 0.325 4.236 0.000

Mean dbkf 346.509 34.12 0.766 10.16 0.000

W/d ratio 10.697 1.873 0.437 5.711 0.000

Dependant variable: drainage area

Best fit predictors: W/d ratio, mean dbkf and D84


123

active floodplain. Therefore, another potential way to miti-

gate sediment and nutrient pollution is by an alternative ditch

design that more closely resembles a natural stream; this idea

addresses question 3 above. Historic work by Wolman and

Miller (1960) defined the term ‘‘effective discharge’’ as the

streamflow that transports the greatest amount of sediment

over time, forming and shaping the bed and banks. Channel

shape is a function of the sediment supply and transport rate

and frequency of occurrence for a given streamflow dis-

charge. Base-flow discharges occur continuously, but are not

effective in transporting large amounts of sediment. Extreme

events, which have the greatest power to transport the most

sediment, occur too infrequently. Thus, the effective dis-

charge for natural non-urbanizing streams in the midwestern

USA is generally associated with the discharge at the

bankfull stage, which has a recurrence interval between 1.4

and 1.6 years (Annable 1994), although caution is required

because certain landscapes in the upper midwest can fall

outside of this range. Midwestern regions that contain rela-

tively large portions of wetland and lake storage will not

have a concordant linear relationship between drainage

area and channel cross-sectional area.

Christner et al. (2004) point out that traditional agri-

cultural ditches are designed to carry their maximum

anticipated flow when they are filled to 80 % of their

design depth. Return intervals corresponding with these

discharge values occur infrequently and are typically

greater than 50 years. Traditional ditch design does not

allow for the effective movement of sediment, typically

resulting in accumulation of sediment requiring periodic

clean-out maintenance. This design also lacks an active

floodplain, which limits the interaction of water and veg-

etation and any concordant nutrient attenuation.

The two-stage ditch design has the potential of improv-

ing both sediment continuity and ecological function (Ward

et al. 2004; Powell et al. 2007a). This design is based on the

natural fluvial processes that occur in response to the con-

struction of an oversized trapezoidal channel (Simon stage

IV and V), in which a small effective discharge channel is

formed by building an active floodplain within the ditch

itself (Ward et al. 2004; Powell et al. 2007a). The channel is

sized to convey the effective or bankfull discharge with

benches that serve as active floodplains for frequent flood

peaks; however, the ditch geometry will still confine the

more infrequent flood peaks (Fig. 8).

One of the processes by which an active floodplain is

formed within ditch geometry was the channel response to

deliberate over-widening (Christner et al. 2004): perhaps,

an ecologically acceptable approach to channel disturbance.

Judicial Ditch #8 in central Minnesota was over-widened to

accommodate the protection of a new bridge. The new and

wider channel allowed for the development of a low-flow

channel that meandered and establish its own dimension,

pattern and profile, along with an active floodplain within

the ditch geometry. The new effective discharge channel

allowed for the natural movement of sediment and the

growth of vegetation, and improved channel development

which created a combination of riffles, runs, pools and

glides, all of which created better habitat for fish. As a

result, Judicial Ditch #8 had the second highest fish Index of

Biological Integrity score of all streams measured in the

Minnesota River basin in 2003. The same ditch also dem-

onstrated the third highest qualitative habitat evaluation

index score and some of the lowest values of nitrogen

(nitrite ? nitrate), total phosphorus and total suspended

solids of all the channelized streams surveyed in 2003 in the

Minnesota River basin by the MPCA (Anderson 2008).

A second approach to the development of an active

floodplain within ditch geometry is bank excavation above

a designed bench (Powell et al. 2007a). In 2009, a joint

project between the MPCA, University of Minnesota,

Mower County Soil and Water Conservation District and

Fig. 8 A typical two-stage

ditch design geometry as

proposed by the United States

Department of Agriculture,

Natural Resources Conservation

Service National Engineering

Handbook


123

the Nature Conservancy designed and constructed a multi-

stage ditch near the Iowa border in southern Minnesota:

multi-stage because in some locations, the low-flow

channel contained an inner berm based on more coarse bed

material. Over 1.5 km of ditch was re-shaped (removal of

upper bank material) to create a ditch similar to that shown

in Fig. 8 in reaches of more cohesive soils. If this approach

works similar to Judicial Ditch #8 or two-stage ditches in

Ohio and Indiana (Powell et al. 2007b), this new ditch

should be self-maintaining assuming no catastrophic

events. Additional research over time will be required to

answer the fourth question above for the DMLT.

Despite initial construction costs and the increased width

of the ditch system, which would require the surrender of

agricultural land, the benefits of a multi-stage ditch are

several-fold. The initial function of water conveyance is not

negatively impacted, the multi-stage channel is more

capable of transporting sediment more effectively than the

traditional design, and overall ditch stability is improved,

thereby reducing the need for costly maintenance. Fur-

thermore, because the multi-stage channel is more likely to

retain its design shape, it is easier to predict its flood pro-

tection performance. In addition, there is a potential to

improve habitat due to increased vegetation on the benches,

water depth variation and improvements to the substrate due

to the ability of the stream to transport sediment. Finally,

the multi-stage ditch may also be useful for improving

water quality due to nutrient assimilation of vegetation on

the benches (Ward et al. 2004; Powell et al. 2007b). The

results of this study suggest that ditches in the DMLT have

to some extent evolved away from the trapezoidal form to a

form that contains at least one stage or bench depending on

soil cohesion and upstream sediment supply.

Conclusions

Older ditches in the DMLT have adjusted over time to more

closely mimic similar scaled natural streams in the region

compared to the designed trapezoidal form. The results

from this study suggest that formation of a bench within the

ditch geometry is possible and construction of two-stage

ditches within the DMLT would likely be beneficial for

long-term ditch channel stability. Reducing costs associated

with aggraded sediment removal offers a cost savings to

landowners who are required to pay for ditch maintenance.

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Channel evolution of Des Moines Lobe till drainage ditches in southern Minnesota (USA)

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