FLOOD06271 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I Over the years, the conversion of once-active floodplain to agricultural production fields through berm construction has considerably narrowed the area available for flood flows, Where discharges around 3,000 cfs once could spread over a large overbank area with minimal depth, they are now confined in extent and exhibit greater depth, This greater depth is primarily responsible for many of the adjacent groundwater saturation problems, When the river is at a high stage for an extended period, the local groundwater level rises and the adjacent farmland becomes saturated, either from hampered drainage or direct wetting by groundwater, An extreme example of this is shown in Figure 7 at the center,pivot irrigation plot, At this location, the 3,000-cfs water surface in the river is roughly two feet higher than the adjacent farmland, As alluded to, the duration of the high river stage has a direct effect on the severity of the saturation problem. This was apparent in 1995 when a relatively high release extended over several weeks (Figure 4). In addition to farmland, berms and their subsurface zones become saturated. If berms are not engineered for this saturated condition, failure through piping and sloughing can occur, When evaluating these effects, it is worth bearing in mind that a reduction in the flow released from John Martin Reservoir directly translates to an increase in duration for a given volume of water which must be evacuated. Over the years, the safe flood control release from John Martin Reservoir has been decreased, The current 3,000-cfs level is the minimwn necessary for effective flood damage reduction operations, A theoretical reduction below this level would result in further channel adjustment, preswnably, by narrowing, When upstream rainfall events initiated flood control operations, river stages would remain high for an even longer period of time than the current operation entails, The price paid for reduction in operating capacity were effectively illustrated in 1995, This is why the maintenance of the current 3,000 cfs level, at a minimwn, is essentiaL An increase in operational channel capacity would reduce saturation problems because it would shorten durations; but achievement of this may be difficult, given the extent of encroachment that has already occurred, And increasing the capacity would have to be balanced with availability of flows of this magnitude on a frequent enough basis to maintain the channel at this capacity, 3.3 ECOLOGICAL RESOURCES Historic and current ecological conditions were determined from scientific literature, government reports, consultation with private individuals and agency representatives, and site visits, Prowers County lies within the Southwestern Tablelands ecoregion which is transitional between the Southern Rocky Mountain and Western High Plains ecoregions (Bailey 1976), The native plant community outside the Arkansas River floodplain is comprised of short, prairie grasses, Common species include blue grama, side,oats grama, buffalo grass, galleta, alkali sacaton, sand dropseed, western wheatgrass, and three-awn, Throughout the lower Arkansas River valley, agricultural and range lands predominate, often directly abutting the river channeL 25