A Sediment Budget for an Urbanizing Watershed, 1951‐1996, Montgomery County, Maryland,U.S.A.1

Abstract: Despite widespread interest, few sediment budgets are available to document patterns of erosion and sedimentation in developing watersheds. We assess the sediment budget for the Good Hope Tributary, a small watershed (4.05 km²) in Montgomery County, Maryland, from 1951‐1996. Lacking monitoring data spanning the period of interest, we rely on a variety of indirect and stratigraphic methods. Using regression equations relating sediment yield to construction, we estimated an upland sediment production of 5,700 m³ between 1951 and 1996. Regression equations indicate that channel cross‐sectional area is correlated with the extent of development; these relationships, when combined with historical land use data, suggest that upland sediment yield was augmented by 6,400 m³ produced by enlargement of first‐order and second‐order stream channels. We used dendrochronology to estimate that 4,000 m³ of sediment was stored on the floodplain from 1951‐1996. The sediment yield from the watershed, obtained by summing upstream contributions, totals 8,100 m³ of sediment, or 135 tons/km²/year. These results indicate that upland erosion, channel enlargement, and floodplain storage are all significant components of the sediment budget of our study area, and all three are approximately equal in magnitude. Erosion of “legacy” floodplain sediments originally deposited during poor agricultural practices of the 19th and early 20th Centuries has likely contributed between 0 and 20% of the total sediment yield, indicating that these remobilized deposits are not a dominant component of the sediment yield of our study area.     

锡林河河漫滩湿地植物群落特征及物种多样性分析

内蒙古高原草原区湿地具有不同于典型湿地的特征,同时还兼具草原的一些特征,在保持生物多样性和珍惜物种资源方面具有不可替代的重要作用.通过样方调查,以重要值为数量特征,采用指示种分析法和多样性指数测定,对内蒙古高原典型内陆河流——锡林河中游河漫滩湿地植被群落特征及物种多样性进行了系统分析和研究.结果表明:(1)锡林河中游河漫滩湿地植被依微地形由高河漫滩向低河漫滩可分为芦苇+羊草,黄花苜蓿+无茫雀麦,灰脉苔草+早熟禾,水甜茅群落,小糠草+蒙古扁穗草等5个群落,以禾本科、菊科、豆科三大草本为主,建群种有别于典型湿地和典型草原群落.(2)植物水分生态类型以湿生、中生为主且无水生类型,具有典型湿地和草原的双重特征;(3)湿地植物群落物种多样性、均匀性和丰富度表现出相似的变化趋势,多样性和丰富度较草甸草原低但高于典型草原;(4)锡林河河漫滩湿地植物群落是典型河流湿地和典型草原之间的过渡类型.表6,参28.     

The Role of Floodplain Soils as a Barrier to Radionuclide Migration: An Example of the Techa–Iset' River System

The contents of ⁹⁰Sr and ¹³⁷Cs and the pattern of their redistribution in the soil and plant cover of floodplain ecosystems have been assessed. It is shown that the radionuclide distribution across the floodplain and along the river flow is determined by the formation of a barrier to their migration near the river channel, at which less mobile ¹³⁷Cs accumulates. The soil and plant cover of the central floodplain are enriched with ⁹⁰Sr. Differences in radionuclide migration in floodplain soils and their input into plants are determined by the relationship between the processes of their immobilization and migration with soil water.     

VENTURA RIVER FLOOD OF FEBRUARY 1992: A LESSON IGNORED?1

ABSTRACT: On February 12, 1992, a portion of the Ventura River, California, flowed through the Ventura Beach RV Resort which had recently been constructed across a major historically active distributary of the Ventura River delta. State and local land-use planners recognized the flood hazards associated with the site, but decision-makers relied on analytical methods developed by the U.S. Army Corps of Engineers and flood-hazard categories developed by the Federal Emergency Management Agency, which did not adequately reflect the mobile nature of the Ventura River channel and distributaries. A better understanding of the historical behavior of the Ventura River could have averted the flood damages experienced in 1992. Low intensity recreational, agricultural, or habitat restoration use of the site would minimize potential flood damages and obviate the need for structural flood protection that would impact the river's natural resources. Continued operation of the recreational vehicle park could result in additional flood damages in the relatively near future; recognizing the limitations of the flood-modeling methodologies used for the Ventura Beach RV Resort could prevent similar miscalculations of flood potential in comparable situations.     

NATIONAL FLOOD INSURANCE AND THE COASTAL ZONE: A CASE STUDY OF HURRICANE ELOISE1

ABSTRACT: This paper presents evidence that currently published flood insurance premiums may be insufficient to cover expected losses in coastal areas subject to hurricane. The problems of developing flood premiums in coastal zones are discussed and Federal Insurance Administration (FIA) reaction to these problems analyzed. Flood losses in the coastal zone of Bay County, Florida due to hurricane Eloise are compared with losses which would be predicted by the FIA. This comparison raises important questions concerning the adequacy of flood premiums in coastal zones and the undesirable indirect effects that underpricing flood risk will have on location decisions in the coastal floodplain.     

EFFECT OF URBANIZATION ON ALTERNATIVE FLOOD CONTROL STRATEGIES1

ABSTRACT: The effect of urbanization on alternative flood control strategies was investigated for a large developing watershed in Texas. Urban and rural areas were modeled separately using a geographically-referenced data base and the U.S. Corps of Engineers HEC-1 and HEC-2 programs, and results yielded a double-peaked hydrograph. Hydrograph input parameters were modified to predict the effects of a wide range of management alternatives including on-site storage, reservoirs, channelization, and development controls. Results indicated a combination of alternatives is required to protect existing and future developments.     

FLOODPLAIN ANALYSIS,SIMPLIFIED VS. COMPLEX METHODS: A CASE STUDY1

ABSTRACT: The accurate and reliable determination of floodplains, floodway boundaries, and flood water elevations are integral requirements of Flood Insurance Studies. These studies are intended to be used for determining the flood insurance rates. Therefore, the accuracy of the water surface profiles are important. To ensure the high degree of accuracy, the HUD Flood Insurance Administration has developed standards which must be met in the analysis of water surface profiles. A somewhat less accurate study is required for the preparation of Flood Emergency Plans. As part of the flood insurance studies of eight locations in the State of North Dakota, various flood hazard and floodplain information reports were reviewed. The hydrologic and hydraulic analyses, especially the computation of the 100-year water surface profiles, were completed using both simplified and complex hydraulic computation methods. Significant differences were found (1 to 3 feet) between the profiles computed by the SCS simplified method and those computed by HEC-2 computer program. However, the floodplain boundaries determined by both methods were found to be similar. Approximate methods are recommended for rapid determination of the floodplain, floodway boundaries, and inundation area mapping, while sophisticated computer programs (HEC-2) are recommended to be used for developing areas where the 100-year flood elevation has a significant impact on the cost of land development.     

美国的洪泛平原管理

美国开展洪泛平原管理已有约40年历史，洪水保险计划是美国泛平原管理的主要措施。了解并借鉴他们的思路与具体策略，将有助于我国洪泛平原管理政策的制定。     

APPLICATION OF THE U.S.G.S. DIFFUSION HYDRODYNAMIC MODEL FOR URBAN FLOODPLAIN ANAYSIS1

ABSTRACT: The two-dimensional Diffusion Hydrodynamic Model, DHM, is applied to the evaluation of floodplain depths resulting from an overflow of a leveed river. The environmental concerns of flood protection and high flow velocities can be better studied with the help of the two-dimensional DHM flow model than by use of the one-dimensional modeling techniques. In the test case, some of the predicted flood depth differences between the DHM and the one-dimensional approach (i.e., HEC-2) are found to be significant. Although the DHM generates considerable information, it is easy to use and does not require expertise beyond that required for use of the one-dimensional approaches.     

A COMPARISON OF SINGLE‐CELL AND MULTICELL CULVERTS FOR STREAM CROSSINGS1

ABSTRACT: Single‐barrel culverts are a common means of roadway crossings for smaller streams. While this culvert design provides an economical solution for a crossing, the adverse effects of conveying the stream through a single opening can be far reaching. The single‐barrel culvert is typically sized for a design storm much greater than the channel forming discharge. This oversizing causes an interruption of the normal flow patterns and sediment transport for the system. Shallow depths at low flow in the pipe and perching at the outlet can impede fish passage. Multicell culverts (where the main culvert at the channel invert is sized for bankfull discharge, and additional pipes are placed at the floodplain elevation to convey overbank flow up to the design discharge) have been recommended as a best management practice to minimize erosion and improve fish passage. This flume study scaled a prototype single‐barrel culvert to both a single‐cell model, and a multicell design to compare outlet scour and flow depths within the culvert. The results provide designers and planners with evidence of the benefits of multicell culverts to justify the higher cost of installation compared to single‐barrel culverts.