Evaluating appropriateness of the design methodology for urban sewer system

도시 하수관거 설계 방법의 적정성 평가

Abstract

The objective of this study is to evaluate the appropriateness of methodology for designing urban sewer system using a rational method-based model, Makesw and an urban runoff model, SWMM. The Gunja basin was selected as a study area and precipitation, runoff, vegetation, soil, imperviousness data were used to estimate floods. The appropriateness of methodology was evaluated based on comparison analysis between floods estimated from Makesw and SWMM. The comparison analysis was conducted between floods estimated from Makesw and SWMM, which were simulated using design rainfall and measured rainfall from past inundation events. The comparison results showed that in the case of design rainfall, the rational method-based floods were larger than that based on SWMM in all main lines. However in several branch lines, the rational method-based floods were smaller than thoes based on SWMM. In addition, for the case of measured rainfall from past inundation events, it was easily to find the main and branch lines where the rational method-based floods were smaller than SWMM based ones. Especially, the lines where rational method-based floods were underestimated, were mostly main, $1^{st}$, $2^{nd}$ lines. It was concluded that the rational method-based results were not conservative. Based on rational method (steady flow analysis) and SWMM (unsteady flow analysis), the more conservative results the method provides, the more highly it is recommended to use in designing an urban sewer system.

본 연구의 목적은 합리식 기반의 하수관거 설계모델 Makesw와 도시유출해석모델 SWMM을 활용한 국내 하수관거 설계 방법의 적정성 평가를 수행하는 것이다. 대상유역으로는 군자배수구역을 선정하였으며, 강수량, 유량, 토지피복도, 토양도, 불투수도, 수치지형도의 자료를 수집하여 홍수량 산정에 활용하였다. 적정성 평가는 SWMM 기반 홍수량을 기초로 Makesw 기반 홍수량과 비교하여 수행하였다. 홍수량 비교분석은 설계강우와 실제 침수강우사상 기반의 Makesw와 SWMM의 홍수량에 대하여 수행하였다. 설계강우 기반 홍수량 비교 결과 유역의 간선에서는 합리식 설계홍수량이 SWMM 설계홍수량보다 과대모의 하였으나, 유역 내 지선에서는 합리식 설계홍수량이 과소모의되는 관거가 다수 나타났다. 또한 실제 침수사상 적용 결과, 간선 및 지선에서 합리식 모델의 홍수량이 SWMM의 홍수량보다 과소모의되는 관거가 다수 발견되었다. 특히, 유역 중심에 위치하는 간선, 1차 2차지선에서 유량 차이가 큰 관거가 집중되었다. 따라서 합리식 설계방법은 보수적인 설계방법이라고 말하기 어려우며, 하수관거 설계시 정상류해석 방법인 합리식과 부정류 해석방법인 SWMM을 활용한 설계를 병행하고 안정적인 해석결과를 활용하여 설계를 할 필요가 있다.

Fig. 1. Study procedure

Fig. 2. Study area and builded drainage system

Fig. 3. Calibration result of runoff simulation at Gunja sub-catchment (2005, 2006)

Fig. 4. Verification result of runoff simulation at Gunja sub-catchment(2007)

Fig. 5. Results comparison of the design flood between Makesw and SWMM

Fig. 6. Results comparison of real flood between Makesw and SWMM (Event6)

Fig. 7. Each case of overflowed manhole result and vulnerable discharge capacity of sewer

Table 1. Observed rainfall and discharge data

Table 2. Calibration results of major parameters in SWMM

Table 3. Quantitative results of SWMM parameter calibration and verification

Table 4. Number of conduit results of SWMM over design flood by rational method model

Table 5. Number of conduit results of SWMM over real flood by rational method about all cases