Dimensionering af forsinkelsesbassin i naturfølsomt område i Aalestrup

Abstract

Dette projekt tager udgangspunkt i en problemstilling fra Aalestrup, hvor der i forbindelse med separatkloakering af byen skal anlægges et forsinkelsesbassin til overfladevandet. Grundet topografien i området skal bassinet placeres i et naturbeskyttet område ned mod Simested Å. Naturbeskyttelsen medfører, at der ikke er plads til at anlægge et bassin med tilstrækkelig volumen. COWI A/S har lagt op til at der anlægges et bassin med et volumen på ca. 1/3 af det foreskrevne. Ved kraftige regnhændelser vil der ske overløb fra bassinet, hvilket medfører en overrisling af det omkringliggende engområde. Denne overrisling vurderes at medføre et fugtigere klima for plantelivet, hvilket resulterer i øget biodiversitet. I dette projekt arbejdes videre med at optimere COWI A/S’ løsning. Ud fra en 2D hydrodynamisk overflademodel undersøges forskellige bassinudformninger med henblik på at finde en løsning, hvor en stor del af engområdet overrisles, samtidig med at en stor del af overløbsvandet infiltrerer i området. Den bedste mulighed i forhold til at optimere overrislingen vurderes at være, at overløbskanten placeres i det nordøstlige hjørne af bassinet samt at udjævne en grøft øst for bassinet. Overfladevandets indhold af tungmetaller og næringssalte undersøges, og der konkluderes at bassinet bør have så god en stoftilbageholdelsesevne som muligt. Stoftilbageholdelsesevnen for den valgte bassinudformning undersøges for suspenderet stof med en partikeltransportmodel. Her findes frem til, at bassinet har en tilbageholdelsesevne af SS på 100 % og at ind- og udløb bør placeres i den sydlige ende af bassinet. Resultatet kan dog muligvis være anderledes, hvis der anvendes en 3D model, og vindpåvirkningen medregnes. Dette blev fravalgt pga. tidsmangel. For den udvalgte bassinudformning undersøges også, om overrislingen forårsager erosion. Modelberegningen viser, at der kun er meget begrænsede problemer med erosion.

This project is based on a problem from Aalestrup where a detention pond for surface water must be built in connection with the transformation of the old combined sewer system to a separate sewer system. Due to the topography of the area the detention pond must be built in a protected area adjacent to the stream Simested Å. Since the detention pond must be built in a protected area there is no room to construct the detention pond with a sufficient volume. COWI A/S has suggested to construct a detention pond with a volume of 1/3 of the required. During heavy rain events the detention pond will overflow, leading to an irrigation of the surrounding meadowland. This irrigation is estimated to result in a more humid environment for plant life, resulting in increased biodiversity. In this project, further work to optimize the solution proposed by COWI A/S will be conducted. By means of a 2D hydrodynamic surface model, different detention pond designs are studied in order to find a solution that results in irrigation of a large area while also ensuring that most of the irrigation water infiltrates in the meadowland. The best option in terms of optimizing irrigation is considered to be a weir located at the northeast corner of the detention pond and to smooth out a ditch east of the detention pond. The surface water content of heavy metals and nutrients is studied, and it is concluded that the removal efficiency of the detention pond must be as high as possible. By means of a particle transport model, the removal efficiency of suspended solids is studied for the chosen detention pond design. Here it is concluded that the detention pond will remove 100 % of the SS and that the in- and outlet should be placed in the southern end of the detention pond. The result may however be different if a 3D model, and wind effects are taken into account, which was deselected due to lack of time. For the chosen detention pond design it is examined if the irrigation leads to erosion of the bottom in the meadowland. Model calculations show that there are only very limited problems with erosion.

A master thesis from Aalborg University

Dimensionering af forsinkelsesbassin i naturfølsomt område i Aalestrup

[Dimensioning of retention basin in a protected nature area in Aalestrup]