1. Stream Crossings: Effects on Streams at Fort Riley Military Installation Gilbert Malinga 1, James Steichen 1, Stacy Hutchinson 1,Phillip Woodford 2, Tim Keane 3, and Amanda Pollock 4 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 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. 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. Hardened LWSC Design and Construction 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. Construction Procedures 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. Current Research Overview Current research is focused at assessing impacts of stream crossings on stream morphology. 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. 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 re- surveys provide a means of monitoring lateral and vertical migration rates of stream channel. 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. 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. 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. Fig.4 Cross Section of Approach Road (drawing courtesy of Sample,1996). Fig.6 Profile of hardened LWSC (drawing courtesy of Sample,1996). Fig.7 Cross section survey across a riffle on Silver Creek. Fig.8 Longitudinal profile (across reach on Three Mile Creek) looking downstream. 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. 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. 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. Fig.1 Military tank stuck in an unimproved stream crossing Fig.2 Ruts created by military tank at an unimproved crossing site. (Photo Courtesy of Sample, 1996) Fig.3 Hardened LWSC under construction. Fig.5 Plan view of hardened low water stream crossing (drawing courtesy of Sample,1996). Fig. 11 and 12 stream creating meander cutoff at stream crossing site Fig.13 Stream depositing sediment at stream crossing site 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.9 Site on Wind Creek instrumented with erosion pins.