The complex nature of low flow mixing in natural channels has been investigated using both laboratory experiments and the numerical solution of a proposed mathematical model that is based on a set of mass balance equations describing the mixing and mass exchange mechanisms. Laboratory experiments, which involved collection of channel geometry, hydraulic, and dye dispersion test data, were conducted in a model of four pool and riffle sequences in a 49-m long tilting flume. The experimental results show that flow over the model pool-riffle sequences is highly non-uniform. Concentration-time curves are significantly skewed with long tails. Comparison between measured and predicted concentration-time curves shows good agreement in the general shape, peak concentration and time to peak. The proposed model shows significant improvement over the conventional one-dimensional dispersion model in predicting natural mixing processes in open channels under low flow conditions through pools and riffles.

Denitrification activity of microbial communities in sand-gravel sediments and biofilms developing on a concrete riverbed of an urban small stream, Nogawa River, was measured monthly by laboratory incubations after the acetylene inhibition technique during une year from December 1987. The annual mean denitrification rate of the biofilm samples, 1.5 mgN m-2 h-1. was higtrer than the rate ut the sandgravel sediment samples. 0.8 mgN m-2 h-1. Contrary, ratios of the denitrified nitrogen to the total inorganic nitrogen fluxes was lower at the stretch where an attached microbial community developed un the concrete riverbed the stretch with sand-gravel riverbed. The longer residence times of water masses ut the site with a sand-gravel riverbed is responsible for the above relationship. Based on the average denitrifieation rates, if all the channel of Nogawa River were covered with concrete, the proportion of denitrified nitrogen to the input nitrogen should decrease to 1.3 %, whereas the ratio is 2.5% when the riverbed is composed of sands and gravel. The results suggest that the efficiency of nitrogen removal by benthic denitrification is strongly affected by the structure of a channel of the river.

Water samples were collected every one hour during July 23-24 1987 from the Minami-Asakawa River and sewage effluents into it. Mearldaily loading and budgets of some bioelements, such as carbon, nitrongen and phosphorrs, were estimated. the fluxeds of suspended matter, organic carbons and ammoioum appeared to remove by self-purification pocesses. On the other hand, those of nitrate, nitrite and chlorophyll a, which were considered to be produced in situ, increased with the river flow. From these results, it was suggested that the river is not only the place for transportation of bioelements but also that for their metabolism.

본(本) 연구(硏究)는 대구시(大邱市)의 주요 하천(河川)인 이천(梨川), 범어천(汎魚川), 칠성천(七星川), 신천(新川), 달서천(達西川) 및 공단천(工團川)의 오염상태(汚染狀態)를 조사(調査)하여 도시하수(都市下水)의 효과적(效果的)인 처리(處理)와 금호강(琴湖江)의 오염방지책(汚染防止策) 수립에 필요한 기초자료(基礎資料)를 제공(提供)코자 하였다. 시기별(時期別) 각(各) 하천(河川)의 pH, DO, COD, 질산염 및 인산염을 분석(分析)한 결과(結果)는 다음과 같았다. 1) 각(各) 지점별(地點別) 평균치(平均値)는 pH 7.3~8.2, DO 흔적~6.5ppm, COD 20.4~116.9ppm, T-N 23.2~31.7ppm, -N 18.3~27.7ppm, -N 0.08~1.89ppm, -N 0.19~1.51ppm, -P 2.50~17.28ppm이었다. 2) 지점별(地點別) 오염도(汚染度)가 가장 높은 곳은 공단천(工團川)이었으며 12개월(個月)동안의 평균치(平均値)는 pH 8.2, DO 흔적, COD 116.9ppm, T-N 23.2ppm, -N 18.3ppm, -N 1.89ppm, -N 1.51ppm, -P 17.28ppm이었다. 3) 시기별(時期別)로 비교(比較)하면 pH, DO, COD, T-N, -N의 함량(含量)과 pH는 겨울철이 여름철보다 높았으며 -N and -P의 함량(含量)은 여름철이 다소 높았다.