Flood Risk Perceptions and Overdevelopment in the Floodplain1

ABSTRACT: There is a long standing hypothesis that overdevelopment has occurred in the nation's floodplains due to imperfect information about the potential flood hazard, an expectation of disaster relief and anticipation of future structural protection. This hypothesis is investigated with multiple regression analysis of data for a case study area. In particular the question of whether floodplain residential property values are fully discounted for expected flood damages is addressed by considering the impact of the National Flood Insurance Program on property values. The extent to which flooding risk perceptions are based on low cost information such as distance from and elevation above the river is also considered. Finally, implications for floodplain management policy are discussed.     

FLOODPLAIN DELINEATION: AN ECONOMIC APPROACH1

ABSTRACT: Errors inherent in the conputation of water surface profiles are magnified by the shallow lateral slopes typical of most flood-plains, producing significant floodplain mapping errors. An economic optimization procedure which weighs the cost of overstating the location of a floodplain boundary against the cost of understanding the location provides a means for establishing the boundary location. In most cases the cost of overstating the boundary location exceeds the cost of understating the location by several magnitudes so that the boundary location should coincide with the minimum water surface elevation which would be expected to result from a flood of specified recurrence interval.     

HYDROLOGIC CONSIDERATIONS OF THE UPPER ST. JOHNS RIVER BASIN,FLORIDA1

ABSTRACT: Since the early 1900's the Upper St. Johns River Basin, Florida, has been experiencing rapid agricultural development, through reclamation of the low-lying marshland by organized drainage districts, private land owners and corporations. These low-lying marsh areas, which constitute the floodplain, have been altered by the creation of levees, thereby reducing the natural storage capacity and permanently altering the regime of the system. This floodplain encroachment has resulted in both higher and lower water levels occurring in the upper basin area, and an increase in hydroperiod. This paper discusses the history and background of the basin accompanied by a hydrologic discussion, and suggests recommendations for a water management plan that could improve the overall operation and management of the Upper St. Johns River basin.     

RIPARIAN COMMUNITIES ASSOCIATED WITH PACIFIC NORTHWEST HEADWATER STREAMS: ASSEMBLAGES,PROCESSES, AND UNIQUENESS1

Riparian areas of large streams provide important habitat to many species and control many instream processes — but is the same true for the margins of small streams? This review considers riparian areas alongside small streams in forested, mountainous areas of the Pacific Northwest and asks if there are fundamental ecological differences from larger streams and from other regions and if there are consequences for management from any differences. In the moist forests along many small streams of the Pacific Northwest, the contrast between the streamside and upslope forest is not as strong as that found in drier regions. Small streams typically lack floodplains, and the riparian area is often constrained by the hillslope. Nevertheless, riparian‐associated organisms, some unique to headwater areas, are found along small streams. Disturbance of hillslopes and stream channels and microclimatic effects of streams on the riparian area provide great heterogeneity in processes and diversity of habitats. The tight coupling of the terrestrial riparian area with the aquatic system results from the closed canopy and high edge‐to‐area ratio for small streams. Riparian areas of the temperate, conifer dominated forests of the Pacific Northwest provide a unique environment. Forest management guidelines for small streams vary widely, and there has been little evaluation of the local or downstream consequences of forest practices along small streams.     

Community strategies to improve flood risk communication in the Red River Basin, Manitoba, Canada

More than a decade after the 1997 Red River Flood, vulnerability to future flooding exists due to a lack of risk communication. This study identifies risk communication gaps and discusses the creation of strategies to enhance information-sharing, bottom-up activity and partnership development. The objectives were achieved using mixed methods, including interviews, a floodplain-wide survey, and a decision-makers' risk management workshop. The results highlight a number of external pressures exerted by regional floodplain policies and procedures that restrict risk communication and affect social vulnerability in the rural floodplain. The failures of a top-down approach to floodplain management have impacted on communities' abilities to address floodplain risks, have amplified local risks, and have decreased community cooperation in floodplain management initiatives since the 1997 'Flood of the Century'. Recommended policies promote the establishment of community standards to compensate for gaps in risk communication and the development of partnerships between floodplain communities.     

The State of Protected Forest Communities in the European Forest–Steppe Zone of Russia and Prospects for Their Reconstruction: A Case Study of Specially Protected Areas of Penza Oblast

The structure of most forest communities in the European forest–steppe, including protected areas, has been disturbed. The adverse consequences of these disturbances include the incomplete age structure of the populations of cenosis-forming species and the irreversibility of successions. The conservation of biodiversity in the structurally disturbed communities is impossible without human intervention, which must be aimed at restoring their age heterogeneity with regard to their specific ecological features.     

CHANNEL MORPHOLOGY EVOLUTION AND OVERBANK FLOW IN THE GEORGIA PIEDMONT1

ABSTRACT: Historic changes in stream channel morphology were investigated in the Georgia Piedmont to better understand the hydrologic processes and functioning of the region's riverine systems. USGS gaging station data and channel geomorphology data were collected from thirty study sites in the Upper Oconee River Basin for flood frequency analysis. Historic and modern (i.e., present-day) channel capacity discharge (i.e., overbank flow) was calculated using Manning's equation and historic channel cross-section records. The recurrence interval for overbank flow was estimated for each site from flood frequency data. Results indicate that channel expansion has occurred throughout the basin, especially in upper reaches. Recurrence intervals for modern overbank events were variable and generally high ranging from < 2 to > 500 years for first to third order streams. They were less variable and lower for fourth and fifth order streams, ranging from < 2 to 3 years. Potential depositional thresholds were identified that exemplify the complex response of sediment distribution patterns throughout the basin. Results indicate overbank flows occur less frequently now than they once did due to historic accelerated sedimentation and subsequent channel expansion. One application of these findings is that these basin processes are likely applicable across the region and may impact the hydrologic functioning of associated Piedmont riverine wetlands that depend on flooding regimes.     

HARVEST INFLUENCES ON FLOODWATER PROPERTIES IN A FORESTED FLOODPLAIN1

ABSTRACT: Floodplain forests directly influence water quality by serving as sinks, sources, or transformers of nutrients. Increases in the demand for timber raise the question of how silvicultural disturbance may affect this function. The objective of this research was to compare biogeochemical relationships between undisturbed vs. disturbed conditions in a floodplain forest. A randomized complete block design consisting of three blocks and two treatments (partial harvest and undisturbed) was installed on the Flint River floodplain, Georgia. The partial cut was conducted during September-October 1993. Automated water samplers were situated to sample during flood events as sheet flow entered and exited treatment plots during the 1994, 1995 and 1996 flood seasons. Prevs. post-contact comparisons indicated that the undisturbed floodplain has minimal influence on water chemistry at this scale of measurement. Although the partial harvest on an 8-ha scale had minimal effect upon sheetfiow water chemistry for three years following harvest, the data suggest that harvests may stimulate a minor increase in Ca and K sink activity.     

The Effect of Channelization on Floodplain Sediment Deposition and Subsidence Along the Pocomoke River,Maryland1

Kroes, Daniel E. and Cliff R. Hupp, 2010. The Effect of Channelization on Floodplain Sediment Deposition and Subsidence Along the Pocomoke River, Maryland. Journal of the American Water Resources Association (JAWRA) 46(4): 686-699. DOI: 10.1111/j.1752-1688.2010.00440.x Abstract: The nontidal Pocomoke River was intensively ditched and channelized by the mid-1900s. In response to channelization; channel incision, head-cut erosion, and spoil bank perforation have occurred in this previously nonalluvial system. Six sites were selected for study of floodplain sediment dynamics in relation to channel condition. Short- and long-term sediment deposition/subsidence rates and composition were determined. Short-term rates (four years) ranged from 0.6 to 3.6 mm/year. Long-term rates (15-100+ years) ranged from −11.9 to 1.7 mm/year. ¹³⁷Cs rates (43 years) indicate rates of 0.24 to 7.4 mm/year depending on channel condition. Channelization has limited contact between streamflow and the floodplain, resulting in little or no sediment retention in channelized reaches. Along unchannelized reaches, extended contact and depth of river water on the floodplain resulted in high deposition rates. Drainage of floodplains exposed organic sediments to oxygen resulting in subsidence and releasing stored carbon. Channelization increased sediment deposition in downstream reaches relative to the presettlement system. The sediment storage function of this river has been dramatically altered by channelization. Results indicate that perforation of spoil banks along channelized reaches may help to alleviate some of these issues.     

Urban Land Use,Channel Incision,and Water Table Decline Along Coastal Plain Streams,North Carolina1

Abstract: This study evaluates the effects of urban land use on stream channels and riparian ground‐water levels along low‐order Inner Coastal Plain streams in North Carolina. Six sites with stream catchments of similar size (1.19‐3.46 km²) within the Tar River Basin were selected across an urban land use gradient, as quantified by a range of catchment total impervious area (TIA; 3.8‐36.7%). Stream stage and ground‐water levels within three floodplain monitoring wells were measured manually and using pressure transducers from May 2006‐June 2007. Channel incision ratio (CIR), the ratio of bank height to bankfull height, was also measured at each monitoring site and along stream reaches within the study area (12 urban and 12 rural sites). Riparian ground‐water levels were inversely related to catchment TIA (%). As TIA (%) and stormwater runoff increased, the degree of stream channel incision increased and riparian ground‐water tables declined. In urban floodplains (>15% TIA), the median ground‐water level was 0.84 m deeper than for the rural settings (<15% TIA). This has resulted in a shift to drier conditions in the urban riparian zones, particularly during the summer months. CIR was found to be a reliable surface indicator of “riparian hydrologic drought” in these settings.