Stream Crossings: Effects on Streams at Fort Riley Military Installation
Gilbert Malinga1, James Steichen1, Stacy Hutchinson1 ,Phillip Woodford2 , Tim Keane3, and Amanda Pollock4
1 Dept. of Biological and Agricultural Engineering, Kansas State University. 2 Integrated Training Area Management, Fort Riley.3 Dept. of Landscape Architecture, Kansas State University. 4 Dept. of Architecture, Kansas State University
Background
Hardened LWSC Design and Construction
Current Research
Military maneuvers involve effectively moving soldiers and equipment across
Fort Riley military installation training areas, and this sometimes involves
crossing streams. Prior to 1992, the military randomly selected where they
would cross a stream or constructed earthen fords to cross. During or after
high-flow events, both the randomly selected sites and earthen fords posed a
safety issue for soldiers and equipment (Fig.1 and 2). Furthermore, use of the
randomly selected sites and earthen fords caused tremendous degradation
to the streams through tearing of stream banks and generation of excessive
amounts of sediment, exceeding Total Maximum Daily Load (TMDL) limits for
water quality downstream.
Design of hardened LWSCs depends on a number of site characteristics, and
these include: topography, soil type, area draining into stream crossing and
stream channel characteristics.
Overview
Current research is focused at assessing impacts of stream crossings on
stream morphology.
Construction Procedures
Objectives:
• Assess impacts of stream crossings on stream morphology.
• Make recommendations for design and construction of LWSCs.
• Based upon lessons learned studying stream crossings, make
recommendations for site selection for LWSCs.
In 1992, a Low Water Stream Crossing (LWSC) project was initiated at Fort
Riley to address problems related to use of earthen fords and randomly
selected crossing sites. New designs were developed. Selected stream
crossing sites were modified by hardening stream beds and approach roads
with rock and gravel (Fig.3).
By 2002, the LWSC project was generally considered a success. Project
achievements realized were: provided safer training conditions for military,
improved access to additional training areas, and alleviated some of the
environmental impacts related to crossing streams.
• Cut or fill approaches to LWSC site to a grade not exceeding 12%, minimum
width of approach road shall not be less than 5.5m (Fig.4).
• Stream bed at crossing shall be excavated to a depth of 1.2 m or until a firm
surface is reached. Minimum width and length of excavation shall not exceed
6.1 m and stream width plus 3 m respectively (Fig.5). Caution should be
exercised not to over-modify stream channel dimensions.
• Geotextile material shall be laid over excavated bed area and filled with
46-61 cm diameter rock and compacted until original bed elevation is reached.
• Layer of geotextile shall be laid on surface of graded approach road, and a
layer of 30 cm high of 20-30 cm rock placed above the geotextile. An
additional layer of 30 cm high rock is placed to fill voids in the larger rock and
also act as a wearing surface (Fig.6)
• Drainage ditches constructed on the sides of approach roads shall be
graveled with riprap.
• BMPs shall be employed during construction of LWSCs so as to minimize
potential environmental impacts.
Delineation of drainage areas
Surveys of watershed areas serving the stream crossing sites has been
conducted using a blimp (Fig. 10). Future surveys will conducted using Light
Detection And Ranging (LIDAR) technology. Digital Elevation Model (DEM)
developed from resulting data will give us a picture of the changed size of the
watershed areas.
Methods used include:
• Stream mapping.
• Sediment sampling.
• Road mapping.
• Delineation of drainage areas serving stream crossings.
Stream Mapping.
Techniques employed include:
1. Cross section ( Fig. 7) and longitudinal profile surveys (Fig. 8):
These surveys are conducted annually and after high flow events. The resurveys provide a means of monitoring lateral and vertical migration rates of
stream channel.
Fig.7 Cross
section survey
across a riffle on
Silver Creek.
FR2 Profile
100.5
Elevation (m)
100
Fig.1 Military tank stuck in an unimproved stream
crossing
Fig.4 Cross Section of Approach Road (drawing courtesy of
Sample,1996).
Road Mapping and Design
Sediment transported from upland areas through approach roads and
deposited into streams is a major concern to stream stability. Mapping of
slopes, soil types and vegetation on approach roads will be conducted to
develop a better understanding of erosion dynamics on approach roads and
potential for sediment delivery into streams through stream crossing sites.
99.5
Bankfull
99
Left Bank
98.5
Right Bank
98
Thalweg
97.5
97
0
20
40
60
80
100
120
140
160
Fig.10 Blimp taking aerial photos of watershed areas serving a stream
crossing site.
Lessons Learned
Based upon lessons learned, we have observed that:
• Design and construction of LWSC is working well, but the major concern is
site selection for stream crossings. Fig.11 and 12 show a stream crossing site
where a stream is creating a meander cutoff. Questions raised are; was this a
good location for a stream crossing and is the crossing the cause of the
meander cutoff or is it simply aiding the creation of the meander cutoff. Riffles
are best locations for LWSCs, avoid pools, meander bends (Fig.13.) and
tributary entry locations on streams.
• Sediment transported from approach roads is another major concern.
Gravelling of roads, at least up to 200 ft on either side of LWSC will reduce
amount of sediment detached from the roads.
• Creation of water bars (built across roads) to divert storm runoff to riparian
management zones will reduce amount of sediment from upland areas
delivered through approach roads into the streams.
180
Distance (m)
Fig.8 Longitudinal profile (across reach on Three Mile Creek)
looking downstream.
Fig.9 Site on
Wind Creek
instrumented
with erosion
pins.
Fig.2 Ruts created by military tank at an unimproved
crossing site. (Photo Courtesy of Sample, 1996)
Fig.5 Plan view of hardened low water stream crossing
(drawing courtesy of Sample,1996).
2. Bank profile surveys (Fig. 9):
Sites have been instrumented with erosion pins, which will be resurveyed
annually to estimate and also validate annual erosion rates predicted using
Bank Erosion Hazard Index (BEHI) and Near Bank Stress (NBS) methods
(Rosgen,2005).
3. Bed material characterization:
Wolman pebble count (1954) and bar sample analysis (Rosgen,2005) are
conducted annually and after high flow events. Data collected from these tests
are essential for understanding sediment transport characteristics of streams.
4. Scour chains:
Scour chains have been installed at designated locations along the streams.
The chains are resurveyed annually and after high flow events to determine
scour or deposition depth and entrainment sizes of bed material.
Fig.3 Hardened LWSC under construction.
After a decade of operation, a need has arisen to re-evaluate performance
of LWSCs and their impact on stream stability. Current research is aimed
at investigating potential impacts of stream crossings on stream
morphology.
Fig.6 Profile of hardened LWSC (drawing courtesy of
Sample,1996).
Fig. 11 and 12 stream creating meander cutoff at stream
crossing site
Suspended Sediment Sampling
Sampling of suspended sediment at locations along the stream, above and
below LWSC is being conducted using ISCO automatic water samplers. Goal
of the sampling is to estimate amount of sediment entering the streams at
stream crossing sites.
Fig .13 Stream depositing sediment at stream crossing site
Conclusion and Future Work
• Stream crossings at Fort Riley exhibit a potential of causing stream
instability. In order to develop a better understanding of stream dynamics,
stream monitoring needs to be conducted on a continuous basis.
• LWSCs designs and construction procedures work well, but more attention
should be paid to LWSC site selection and conditions of approach roads.
• Some stream crossings work well and others do not. Future work is
focused on developing a better understanding of how stream crossings affect
stream dynamics.